A previous paper (Annals of Botany 103: 673–685) described formation of clayey pavements in lateral root catchments of eucalypts colonizing a recently formed sand dune in south-west Western Australia. Here chemical and morphological aspects of their formation at the site are studied.
Chemical and physical examinations of soil cores through pavements and sand under adjacent heath assessed build-up of salts, clay and pH changes in or below pavements. Relationships of root morphology to clay deposition were examined and deposits subjected to scanning electron microscopy and energy-dispersive X-ray analysis. Xylem transport of mineral elements in eucalypt and non-eucalypt species was studied by analysis of xylem (tracheal) sap from lateral roots.
The columns of which pavements are composed develop exclusively on lower-tier lateral roots. Such sites show intimate associations of fine roots, fungal filaments, microbiota and clay deposits rich in Si, Al and Fe. Time scales for construction of pavements by eucalypts were assessed. Cores through columns of pavemented profiles showed gross elevations of bulk density, Al, Fe and Si in columns and related increases in pH, Mg and Ca status in lower profiles. A cutting through the dune exhibited pronounced alkalinity (pH 7–10) under mallee woodland versus acidity (pH 5–6·5) under proteaceous heath. Xylem sap analyses showed unusually high concentrations of Al, Fe, Mg and Si in dry-season samples from column-bearing roots.
Deposition of Al–Fe–Si-rich clay is pivotal to pavement construction by eucalypts and leads to profound chemical and physical changes in relevant soil profiles. Microbial associates of roots are likely to be involved in clay genesis, with parent eucalypts supplying the required key mineral elements and carbon sources. Acquisition of the Al and Fe incorporated into clay derives principally from hydraulic uplift from ground water via deeply penetrating tap roots.
Dry fruits remain around the seeds at dispersal in a number of species, especially the Brassicaceae. Explanations for this vary, but usually involve mechanisms of innate dormancy. We speculate that, instead, a persistent fruit may give additional protection through control of dehydration, to species growing in arid or Mediterranean environments where water is sporadic.
X-rays and weight measurements were used to determine the extent to which Raphanus raphanistrum seeds within mature fruits imbibe water, and germination tests determined the roles of the fruit and seed coat in seed dormancy. Rates of water uptake and desiccation, and seedling emergence were compared with and without the fruit. Finally, germinability of seeds extracted from fruits was determined after various periods of moist conditions followed by a range of dry conditions.
Most seeds rapidly take up water within the fruit, but they do not fully imbibe when compared with naked seeds. The seed coat is more important than the dry fruit wall in maintaining seed dormancy. The presence of a dry fruit slows emergence from the soil by up to 6–8 weeks. The fruit slows the rate of desiccation of the seed to a limited extent. The presence of the fruit for a few days during imbibition somehow primes more seeds to germinate than if the fruit is absent; longer moist periods within the pod appear to induce dormancy.
The fruit certainly modifies the seed environment as external conditions change between wet and dry, but not to a great extent. The major role seems to be: (a) the physical restriction of imbibition and germination; and (b) the release and then re-imposition of dormancy within the seed. The ecological significance of the results requires more research under field conditions.
Positive selection in the -crystallin domain (ACD) of the chloroplast small heat shock protein (CPsHSP) gene was found in a previous study and was suggested to be related to the ecological adaptation of Rhododendron species in the subgenus Hymenanthes. Consequently, it was of interest to examine whether gene duplication and subsequent divergence have occurred in other sHSP genes, for example class I cytosolic sHSP genes (CT1sHSPs) in Rhododendron in Taiwan, where many endemic species have evolved as a result of habitat differentiation.
A phylogeny of CT1sHSP amino acid sequences was built from Rhododendron, Arabidopsis thaliana, Oryza sativa, Populus trichocarpa, Vitis vinifera and other species for elucidation of the phylogenetic relationships among CT1sHSPs. Phylogenies of Rhododendron CT1sHSP nucleotide and amino acid sequences were generated for positive selection and functional divergence analysis, respectively. Positively selected sites and amino acid differences between types of Rhododendron CT1sHSPs were mapped onto the wheat sHSP16·9 protein structure. Average genetic distance (Dxy) and dN/dS ratios between types of Rhododendron CT1sHSP genes were analysed using sliding window analysis. Gene conversion was also assessed between types of Rhododendron CT1sHSPs.
Two types of Rhododendron CT1sHSP were identified. A high level of genetic similarity and diversity within and flanking the ACD, respectively, between types of Rhododendron CT1sHSP were found. Main differences between the two types of Rhododendron CT1sHSPs were: (1) increased hydrophobicity by two positively selected amino acid sites and a seven-amino-acid insertion in the N-terminal arm; and (2) increased structural flexibility and solubility by a seven-amino-acid insertion in the N-terminal arm and one positively selected amino acid site in the C-terminal extension.
Functional conservation of the ACD of Rhododendron CT1sHSP genes was inferred because of strong purifying selection. However, sequence variations flanking the ACD in Rhododendron CT1sHSP gene duplicates may have resulted in functional divergence and played important roles in chaperon function enhancement.
The study of rapid evolution in invasive species has highlighted the fundamental role played by founder events, emergence of genetic novelties through recombination and rapid response to new selective pressures. However, whether rapid adaptation of introduced species can be driven by punctual changes in genome organization has received little attention. In plants, variation in genome size, i.e. variation in the amount of DNA per monoploid set of chromosomes through loss or gain of repeated DNA sequences, is known to influence a number of physiological, phenological and life-history features. The present study investigated whether change in genome size has contributed to the evolution of greater potential of vegetative growth in invasive populations of an introduced grass.
The study was based on the recent demonstration that invasive genotypes of reed canarygrass (Phalaris arundinacea) occurring in North America have emerged from recombination between introduced European strains. The genome sizes of more than 200 invasive and native genotypes were measured and their genome size was related to their phenotypic traits measured in a common glasshouse environment. Population genetics data were used to infer phylogeographical relationships between study populations, and the evolutionary history of genome size within the study species was inferred.
Invasive genotypes had a smaller genome than European native genotypes from which they are derived. This smaller genome size had phenotypic effects that increased the species' invasive potential, including a higher early growth rate, due to a negative relationship between genome size and rate of stem elongation. Based on inferred phylogeographical relationships of invasive and native populations, evolutionary models were consistent with a scenario of genome reduction by natural selection during the invasion process, rather than a scenario of stochastic change.
Punctual reduction in genome size could cause rapid changes in key phenotypic traits that enhance invasive ability. Although the generality of genome size variation leading to phenotypic evolution and the specific genomic mechanisms involved are not known, change in genome size may constitute an important but previously under-appreciated mechanism of rapid evolutionary change that may promote evolutionary novelties over short time scales.
The carnivorous plant Venus flytrap (Dionaea muscipula) produces a rosette of leaves: each leaf is divided into a lower part called the lamina and an upper part, the trap, with sensory trigger hairs on the adaxial surface. The trap catches prey by very rapid closure, within a fraction of a second of the trigger hairs being touched twice. Generation of action potentials plays an important role in closure. Because electrical signals are involved in reduction of the photosynthetic rate in different plant species, we hypothesized that trap closure and subsequent movement of prey in the trap will result in transient downregulation of photosynthesis, thus representing the energetic costs of carnivory associated with an active trapping mechanism, which has not been previously described.
Traps were enclosed in a gas exchange cuvette and the trigger hairs irritated with thin wire, thus simulating insect capture and retention. Respiration rate was measured in darkness (RD). In the light, net photosynthetic rate (AN), stomatal conductance (gs) and intercellular CO2 concentration (ci) were measured, combined with chlorophyll fluorescence imaging. Responses were monitored in the lamina and trap separately.
Irritation of trigger hairs resulted in decreased AN and increased RD, not only immediately after trap closure but also during the subsequent period when prey retention was simulated in the closed trap. Stomatal conductance remained stable, indicating no stomatal limitation of AN, so ci increased. At the same time, the effective quantum yield of photosystem II (PSII) decreased transiently. The response was confined mainly to the digestive zone of the trap and was not observed in the lamina. Stopping mechanical irritation resulted in recovery of AN, RD and PSII.
We put forward the first experimental evidence for energetic demands and carbon costs during insect trapping and retention in carnivorous plants, providing a new insight into the cost/benefit model of carnivory.
Rhododendron (Ericaceae) is a large woody genus in which hybridization is thought to play an important role in evolution and speciation, particularly in the Sino-Himalaya region where many interfertile species often occur sympatrically. Rhododendron agastum, a putative hybrid species, occurs in China, western Yunnan Province, in mixed populations with R. irroratum and R. delavayi.
Material of these taxa from two sites 400 km apart (ZhuJianYuan, ZJY and HuaDianBa, HDB) was examined using cpDNA and internal transcribed spacer (ITS) sequences, and amplified fragment length polymorphism (AFLP) loci, to test the possibility that R. agastum was in fact a hybrid between two of the other species. Chloroplast trnL-F and trnS-trnG sequences together distinguished R. irroratum, R. delavayi and some material of R. decorum, which is also considered a putative parent of R. agastum.
All 14 R. agastum plants from the HDB site had the delavayi cpDNA haplotype, whereas at the ZJY site 17 R. agastum plants had this haplotype and four had the R. irroratum haplotype. R. irroratum and R. delavayi are distinguished by five unequivocal point mutations in their ITS sequences; every R. agastum accession had an additive pattern (double peaks) at each of these sites. Data from AFLP loci were acquired for between ten and 21 plants of each taxon from each site, and were analysed using a Bayesian approach implemented by the program NewHybrids. The program confirmed the identity of all accessions of R. delavayi, and all R. irroratum except one, which was probably a backcross. All R. agastum from HDB and 19 of 21 from ZJY were classified as F1 hybrids; the other two could not be assigned a class.
Rhododendron agastum represents populations of hybrids between R. irroratum and R. delavayi, which comprise mostly or only F1s, at the two sites examined. The sites differ in that at HDB there was no detected variation in cpDNA type or hybrid class, whereas at ZJY there was variation in both.
Dalbergia nigra is one of the most valuable timber species of its genus, having been traded for over 300 years. Due to over-exploitation it is facing extinction and trade has been banned under CITES Appendix I since 1992. Current methods, primarily comparative wood anatomy, are inadequate for conclusive species identification. This study aims to find a set of anatomical characters that distinguish the wood of D. nigra from other commercially important species of Dalbergia from Latin America.
Qualitative and quantitative wood anatomy, principal components analysis and naïve Bayes classification were conducted on 43 specimens of Dalbergia, eight D. nigra and 35 from six other Latin American species.
Dalbergia cearensis and D. miscolobium can be distinguished from D. nigra on the basis of vessel frequency for the former, and ray frequency for the latter. Principal components analysis was unable to provide any further basis for separating the species. Naïve Bayes classification using the four characters: minimum vessel diameter; frequency of solitary vessels; mean ray width; and frequency of axially fused rays, classified all eight D. nigra correctly with no false negatives, but there was a false positive rate of 36·36 %.
Wood anatomy alone cannot distinguish D. nigra from all other commercially important Dalbergia species likely to be encountered by customs officials, but can be used to reduce the number of specimens that would need further study.
Game theoretical models predict that plants growing in dense stands invest so much biomass in height growth that it trades-off with investment in other organs such as the leaves, leading to decreased plant production. Using the stoloniferous plant Potentilla reptans, we tested the hypothesis that genotypes investing more in the petioles in response to increased density show a greater decrease in total plant mass. We also tested whether a greater increase in mother ramet investment would lead to a greater decrease in investment in vegetative propagation.
To uncouple costs and benefits of investments in petioles, ten genotypes that were known to differ in their response to shading signals were grown in monogenotypic stands at two different densities.
Genotypes differed in their increase in petiole investment in response to an increase in density, but not in their decrease in total plant mass or root mass. Total lamina area per plant did not differ significantly between the densities, nor did the mass invested in the laminae per unit of total plant mass. Genotypes differed considerably in the change in vegetation height and petiole investment, but this was not significantly negatively correlated with the change in total plant mass. The genotypes did differ in the change of mass investment in the mother ramet: a greater increase in investment in the mother ramet was correlated to a greater decrease in vegetative propagation.
While a greater increase in height investment did not lead to a greater decrease in biomass production, it did lead to a decrease in vegetative propagation. This ability to change allocation towards the mother ramets may imply that competition within a stand of stoloniferous plants does not necessarily result in lower total biomass production due to increased height investment.
Flood-tolerant tree species of the Amazonian floodplain forests are subjected to an annual dry period of variable severity imposed when low river-water levels coincide with minimal precipitation. Although the responses of these species to flooding have been examined extensively, their responses to drought, in terms of phenology, growth and physiology, have been neglected hitherto, although some information is found in publications that focus on flooding.
The present review examines the dry phase of the annual flooding cycle. It consolidates existing knowledge regarding responses to drought among adult trees and seedlings of many Amazonian floodplain species.
Flood-tolerant species display variable physiological responses to dry periods and drought that indicate desiccation avoidance, such as reduced photosynthetic activity and reduced root respiration. However, tolerance and avoidance strategies for drought vary markedly among species. Drought can substantially decrease growth, biomass and photosynthetic activity among seedlings in field and laboratory studies. When compared with the responses to flooding, drought can impose higher seedling mortality and slower growth rates, especially among evergreen species. Results indicate that tolerance and avoidance strategies for drought vary markedly between species. Both seedling recruitment and photosynthetic activity are affected by drought,
For many species, the effects of drought can be as important as flooding for survival and growth, particularly at the seedling phase of establishment, ultimately influencing species composition. In the context of climate change and predicted decreases in precipitation in the Amazon Basin, the effects of drought on plant physiology and species distribution in tropical floodplain forest ecosystems should not be overlooked.
Over a season, plant condition, amount of ongoing reproduction and biotic and abiotic environmental factors vary. As flowers age, flower condition and amount of pollen donated and received also vary. These internal and external changes are significant for fitness if they result in changes in reproduction and mating.
Literature from several fields was reviewed to provide a picture of the changes that occur in plants and flowers that can affect mating over a season. As flowers age, both the entire flower and individual floral whorls show changes in appearance and function. Over a season, changes in mating often appear as alteration in seed production vs. pollen donation. In several species, older, unpollinated flowers are more likely to self. If flowers are receiving pollen, staying open longer may increase the number of mates. In wild radish, for which there is considerable information on seed paternity, older flowers produce fewer seeds and appear to discriminate less among pollen donors. Pollen donor performance can also be linked to maternal plant age. Different pollinators and mates are available across the season. Also in wild radish, maternal plants appear to exert the most control over paternity when they are of intermediate age.
Although much is known about the characters of plants and flowers that can change over a season, there is less information on the effects of age on mating. Several studies document changes in self-pollination over time, but very few, other than those on wild radish, consider more subtle aspects of differential success of pollen donors over time.
Although many hypotheses have been proposed to explain variation in leaf size, the mechanism underlying the variation remains not fully understood. To help understand leaf size variation, the cost/benefit of twig size was analysed, since, according to Corner's rule, twig size is positively correlated with the size of appendages the twig bears.
An extensive survey of twig functional traits, including twig (current-year shoots including one stem and few leaves) and leaf size (individual leaf area and mass), was conducted for 234 species from four broadleaved forests. The scaling relationship between twig mass and leaf area was determined using standardized major axis regression and phylogenetic independent comparative analyses.
Leaf area was found to scale positively and allometrically with both stem and twig mass (stem mass plus leaf mass) with slopes significantly smaller than 1·0, independent of life form and habitat type. Thus, the leaf area ratio (the ratio of total leaf area to stem or twig mass) decreases with increasing twig size. Moreover, the leaf area ratio correlated negatively with individual leaf mass. The results of phylogenetic independent comparativeanalyses were consistent with the correlations. Based on the above results, a simple model for twig size optimization was constructed, from which it is postulated that large leaf size–twig size may be favoured when leaf photosynthetic capacity is high and/or when leaf life span and/or stem longevity are long. The model's predictions are consistent with leaf size variation among habitats, in which leaf size tends to be small in poor habitats with a low primary productivity. The model also explains large variations in leaf size within habitats for which leaf longevity and stem longevity serve as important determinants.
The diminishing returns in the scaling of total leaf area with twig size can be explained in terms of a very simple model on twig size optimization.
Concomitant increases in O2 and irradiance upon de-submergence can cause photoinhibition and photo-oxidative damage to the photosynthetic apparatus of plants. As energy and carbohydrate supply from photosynthesis is needed for growth, it was hypothesized that post-submergence growth recovery may require efficient photosynthetic acclimation to increased O2 and irradiance to minimize photo-oxidative damage. The hypothesis was tested in two flood-tolerant species: a C3 herb, Alternanthera philoxeroides; and a C4 grass, Hemarthria altissima. The impact of low O2 and low light, typical conditions in turbid floodwater, on post-submergence recovery was assessed by different flooding treatments combined with shading.
Experiments were conducted during 30 d of flooding (waterlogging or submergence) with or without shading and subsequent recovery of 20 d under growth conditions. Changes in dry mass, number of branches/tillers, and length of the longest internodes and main stems were recorded to characterize growth responses. Photosynthetic parameters (photosystem II efficiency and non-photochemical quenching) were determined in mature leaves based on chlorophyll a fluorescence measurements.
In both species growth and photosynthesis recovered after the end of the submergence treatment, with recovery of photosynthesis (starting shortly after de-submergence) preceding recovery of growth (pronounced on days 40–50). The effective quantum yield of photosystem II and non-photochemical quenching were diminished during submergence but rapidly increased upon de-submergence. Similar changes were found in all shaded plants, with or without flooding. Submerged plants did not suffer from photoinhibition throughout the recovery period although their growth recovery was retarded.
After sudden de-submergence the C3 plant A. philoxeroides and the C4 plant H. altissima were both able to maintain the functionality of the photosynthetic apparatus through rapid acclimation to changing O2 and light conditions. The ability for photosynthetic acclimation may be essential for adaptation to wetland habitats in which water levels fluctuate.
Optimization of osmotic dehydration in different plant cells has been investigated through the variation of parameters such as the nature of the sugar used, the concentration of osmotic solutions and the processing time. In micro-organisms such as the yeast, Saccharomyces cerevisiae, the exposure of a cell to a slow increase in osmotic pressure preserves cell viability after rehydration, while sudden dehydration involves a lower rate of cell viability, which could be due to membrane vesiculation. The aim of this work is to study cytoplasmic vesicle formation in onion epidermal cells (Allium cepa) as a function of the kinetics of osmotic pressure variation in the external medium.
Onion epidermal cells were submitted either to an osmotic shock or to a progressive osmotic shift from an osmotic pressure of 2 to 24 MPa to induce plasmolysis. After 30 min in the treatment solution, deplasmolysis was carried out. Cells were observed by microscopy during the whole cycle of dehydration–rehydration.
The application of an osmotic shock to onion cells, from an initial osmotic pressure of 2 MPa to a final one of 24 MPa for <1 s, led to the formation of numerous exocytotic and osmocytic vesicles visualized through light and confocal microscopy. In contrast, after application of a progressive osmotic shift, from an initial osmotic pressure of 2 MPa to a final one of 24 MPa for 30 min, no vesicles were observed. Additionally, the absence of Hechtian strand connections led to the bursting of vesicles in the case of the osmotic shock.
It is concluded that the kinetics of osmotic dehydration strongly influence vesicle formation in onion cells, and that Hechtian strand connections between protoplasts and exocytotic vesicles are a prerequisite for successful deplasmolysis. These results suggest that a decrease in the area-to-volume ratio of a cell could cause cell death following an osmotic shock.
Polyamines are small polycationic molecules found ubiquitously in all organisms and function in a wide variety of biological processes. In the past decade, molecular and genetic studies using mutants and transgenic plants with an altered activity of enzymes involved in polyamine biosynthesis have contributed much to a better understanding of the biological functions of polyamines in plants.
Spermidine is essential for survival of Arabidopsis embryos. One of the reasons may lie in the fact that spermidine serves as a substrate for the lysine -> hypusine post-translational modification of the eukaryotic translation initiation factor 5A, which is essential in all eukaryotic cells. Spermine is not essential but plays a role in stress responses, probably through the modulation of cation channel activities, and as a source of hydrogen peroxide during pathogen infection. Thermospermine, an isomer of spermine, is involved in stem elongation, possibly by acting on the regulation of upstream open reading frame-mediated translation.
The mechanisms of action of polyamines differ greatly from those of plant hormones. There remain numerous unanswered questions regarding polyamines in plants, such as transport systems and polyamine-responsive genes. Further studies on the action of polyamines will undoubtedly provide a new understanding of plant growth regulation and stress responses.
The pattern of callose deposition was followed in developing stomata of the fern Asplenium nidus to investigate the role of this polysaccharide in guard cell (GC) wall differentiation and stomatal pore formation.
Callose was localized by aniline blue staining and immunolabelling using an antibody against (1 -> 3)-β-
In post-cytokinetic stomata significant amounts of callose persisted in the nascent ventral wall. Callose then began degrading from the mid-region of the ventral wall towards its periphery, a process which kept pace with the formation of an ‘internal stomatal pore’ by local separation of the partner plasmalemmata. In differentiating GCs, callose was consistently localized in the developing cell-wall thickenings. In 2-DDG-, tunicamycin- and CPA-affected stomata, callose deposition and internal stomatal pore formation were inhibited. The affected ventral walls and GC wall thickenings contained membranous elements. Stomata recovering from the above treatments formed a stomatal pore by a mechanism different from that in untreated stomata. After coumarin or dichlobenil treatment, callose was retained in the nascent ventral wall for longer than in control stomata, while internal stomatal pore formation was blocked. Actin filament disintegration inhibited internal stomatal pore formation, without any effect on callose deposition.
In A. nidus stomata the time and pattern of callose deposition and degradation play an essential role in internal stomatal pore formation, and callose participates in deposition of the local GC wall thickenings.
In hypocotyls of flax (Linum usitatissimum) cadmium-induced reorientation of growth (i.e. an increase in expansion and a decrease in elongation) coincides with marked changes in the methylesterification and cross-linking of homogalacturonans within various cell-wall (CW) domains. The aim of the present study was to examine the involvement of pectin methylesterase (PME) and peroxidase (PER) in this cadmium-induced CW remodelling.
CW proteins were extracted from hypocotyls of 10- and 18-d-old flax that had been treated or not treated with 0·5 m
In cadmium-treated seedlings, PME activity increased as compared with controls, particularly at day 10. The increased activity of PME was accompanied by increased abundance of both a basic protein isoform (B2) and a particular transcript (Lupme5). In contrast, induction of PER activity by cadmium was highest at day 18. Among the four reported PER genes, Flxper1 and 3 increased in abundance in the presence of cadmium at day 18.
The temporal regulation of Lupme and Flxper genes and of their respective enzyme activities fits the previously reported cadmium-induced structural changes of homogalacturonans within the CWs. After PME-catalysed de-esterification of homogalacturonans, their cross-linking would depend on the activity of PERs interacting with calcium-dimerized blocks and reinforce the cell cohesion during the cadmium-induced swelling.
Potassium (K) fertilizers are used in intensive and extensive agricultural systems to maximize production. However, there are both financial and environmental costs to K-fertilization. It is therefore important to optimize the efficiency with which K-fertilizers are used. Cultivating crops that acquire and/or utilize K more effectively can reduce the use of K-fertilizers. The aim of the present study was to determine the genetic factors affecting K utilization efficiency (KUtE), defined as the reciprocal of shoot K concentration (1/[K]shoot), and K acquisition efficiency (KUpE), defined as shoot K content, in Brassica oleracea.
Genetic variation in [K]shoot was estimated using a structured diversity foundation set (DFS) of 376 accessions and in 74 commercial genotypes grown in glasshouse and field experiments that included phosphorus (P) supply as a treatment factor. Chromosomal quantitative trait loci (QTL) associated with [K]shoot and KUpE were identified using a genetic mapping population grown in the glasshouse and field. Putative QTL were tested using recurrent backcross substitution lines in the glasshouse.
More than two-fold variation in [K]shoot was observed among DFS accessions grown in the glasshouse, a significant proportion of which could be attributed to genetic factors. Several QTL associated with [K]shoot were identified, which, despite a significant correlation in [K]shoot among genotypes grown in the glasshouse and field, differed between these two environments. A QTL associated with [K]shoot in glasshouse-grown plants (chromosome C7 at 62·2 cM) was confirmed using substitution lines. This QTL corresponds to a segment of arabidopsis chromosome 4 containing genes encoding the K+ transporters AtKUP9, AtAKT2, AtKAT2 and AtTPK3.
There is sufficient genetic variation in B. oleracea to breed for both KUtE and KUpE. However, as QTL associated with these traits differ between glasshouse and field environments, marker-assisted breeding programmes must consider carefully the conditions under which the crop will be grown.
Salt stress has become a major threat to plant growth and productivity. Arbuscular mycorrhizal fungi colonize plant root systems and modulate plant growth in various ways.
This review addresses the significance of arbuscular mycorrhiza in alleviation of salt stress and their beneficial effects on plant growth and productivity. It also focuses on recent progress in unravelling biochemical, physiological and molecular mechanisms in mycorrhizal plants to alleviate salt stress.
The role of arbuscular mycorrhizal fungi in alleviating salt stress is well documented. This paper reviews the mechanisms arbuscular mycorrhizal fungi employ to enhance the salt tolerance of host plants such as enhanced nutrient acquisition (P, N, Mg and Ca), maintenance of the K+ : Na+ ratio, biochemical changes (accumulation of proline, betaines, polyamines, carbohydrates and antioxidants), physiological changes (photosynthetic efficiency, relative permeability, water status, abscissic acid accumulation, nodulation and nitrogen fixation), molecular changes (the expression of genes: PIP, Na+/H+ antiporters, Lsnced, Lslea and LsP5CS) and ultra-structural changes. Theis review identifies certain lesser explored areas such as molecular and ultra-structural changes where further research is needed for better understanding of symbiosis with reference to salt stress for optimum usage of this technology in the field on a large scale. This review paper gives useful benchmark information for the development and prioritization of future research programmes.
The polygalacturonase (PG) gene family has been found to be enriched in pollen of several species; however, little is currently known about the function of the PG gene in pollen development. To investigate the exact role that the PG gene has played in pollen development and about this family in general, one putative PG gene, Brassica campestris Male Fertility 9 (BcMF9), was isolated from Chinese cabbage (Brassica campestris ssp. chinensis, syn. B. rapa ssp. chinensis) and characterized.
RT-PCR, northern blotting and in situ hybridization were used to analyse the expression pattern of BcMF9, and antisense RNA technology was applied to study the function of this gene.
BcMF9 is expressed in particular in the tapetum and microspore during the late stages of pollen development. Antisense RNA transgenic plants that displayed decreased expression of BcMF9 showed pollen morphological defects that resulted in reduced pollen germination efficiency. Transmission electron microscopy revealed that the homogeneous pectic exintine layer of pollen facing the exterior was over-developed and predominantly occupied the intine, reversing the normal proportional distribution of the internal endintine layer and the external exintine in transgenic pollen. Inhibition of BcMF9 also resulted in break-up of the previously formed tectum and baculae from the beginning of the binucleate stage, as a result of premature degradation of tapetum.
Several lines of evidence, including patterns of BcMF9 expression and phenotypic defects, suggest a sporophytic role in exine patterning, and a gametophytic mode of action of BcMF9 in intine formation. BcMF9 might act as a co-ordinator in the late stages of tapetum degeneration, and subsequently in the regulation of wall material secretion and, in turn, exine formation. BcMF9 might also play a role in intine formation, possibly via regulation of the dynamic metabolism of pectin.
Knowledge of pollen dispersal patterns and variation of fecundity is essential to understanding plant evolutionary processes and to formulating strategies to conserve forest genetic resources. Nevertheless, the pollen dispersal pattern of dipterocarp, main canopy tree species in palaeo-tropical forest remains unclear, and flowering intensity variation in the field suggests heterogeneity of fecundity.
Pollen dispersal patterns and male fecundity variation of Shorea leprosula and Shorea parvifolia ssp. parvifolia on Peninsular Malaysian were investigated during two general flowering seasons (2001 and 2002), using a neighbourhood model modified by including terms accounting for variation in male fecundity among individual trees to express heterogeneity in flowering.
The pollen dispersal patterns of the two dipterocarp species were affected by differences in conspecific tree flowering density, and reductions in conspecific tree flowering density led to an increased selfing rate. Active pollen dispersal and a larger number of effective paternal parents were observed for both species in the season of greater magnitude of general flowering (2002).
The magnitude of general flowering, male fecundity variation, and distance between pollen donors and mother trees should be taken into account when attempting to predict the effects of management practices on the self-fertilization and genetic structure of key tree species in tropical forest, and also the sustainability of possible management strategies, especially selective logging regimes.
The gametophytes of most homosporous ferns are cordate–thalloid in shape. Some are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. The aim of the present study was to clarify the morphological evolution of the strap-shaped gametophyte of microsoroids (Polypodiaceae) by precise analysis of their development.
Spores of Colysis decurrens collected in Kagoshima, Japan, were cultured and observed microscopically. Epi-illuminated micrographs of growing gametophytes were captured every 24 h, allowing analysis of the cell lineage of meristems. Light microscopy of resin-sections and scanning electron microscopy were also used.
Contrary to previous assumptions that strap-shaped Colysis gametophytes have no organized meristem, three different types of meristems are formed during development: (1) apical-cell based – responsible for early growth; (2) marginal – further growth, including gametophyte branching; and (3) multicellular – formation of cushions with archegonia. The cushion is two or three layers thick and intermittent. The apical-cell and multicellular meristems are similar to those of cordate gametophytes of other ferns, but the marginal meristem is unique to the strap-shaped gametophyte of this fern.
The strap-shaped gametophytes of C. decurrens may have evolved from ancestors with a cordate shape by insertion of the marginal meristem phase between the first apical-cell-based meristem and subsequent multicellular meristem phases. Repeated retrieval of the marginal meristem at the multicellular meristem phase would result in indefinite prolongation of gametophyte growth, an ecological adaptation to epiphytic habitats.
The slippery waxy zone in the upper part of pitchers has long been considered the key trapping structure of the Nepenthes carnivorous plants; however, the presence of wax is reported to be variable within and between species of this species-rich genus. This study raises the question of the adaptive significance of the waxy zone and investigates the basis for an ontogenetic cause of its variability and correlation with pitcher shape.
In Brunei (Borneo) the expression of the waxy zone throughout plant ontogeny was studied in two taxa of the Nepenthes rafflesiana complex, typica and elongata, which differ in pitcher shape and size. We also tested the adaptive significance of this zone by comparing the trapping efficiency and the number of prey captured of wax-bearing and wax-lacking plants.
In elongata, the waxy zone is always well expanded and the elongated pitchers change little in form during plant development. Wax efficiently traps experimental ants but the number of captured prey in pitchers is low. In contrast, in typica, the waxy zone is reduced in successively produced pitchers until it is lost at the end of the plant's juvenile stage. The form of pitchers thus changes continuously throughout plant ontogeny, from elongated to ovoid. In typica, the number of captured prey is greater, but the role of wax in trapping is minor compared with that of the digestive liquid, and waxy plants do not show a higher insect retention and prey abundance as compared with non-waxy plants.
The waxy zone is not always a key trapping structure in Nepenthes and can be lost when supplanted by more efficient features. This study points out how pitcher structure is submitted to selection, and that evolutionary changes in developmental mechanisms could play a role in the morphological diversity of Nepenthes.
Gene flow and genetic variability within and among alpine plant populations can be greatly influenced by the steep environmental gradients and heterogeneous topography of alpine landscapes. In this study, the effects are examined of natural isolation of alpine habitats on genetic diversity and geographic structure in populations of C. thyrsoides, a rare and isolated European Alpine monocarpic perennial with limited seed dispersal capacity.
Molecular diversity was analysed for 736 individuals from 32 populations in the Swiss Alps and adjacent Jura mountains using five polymorphic microsatellite loci. Pollen flow was estimated using pollen grain-sized fluorescent powder. In addition, individual-based Bayesian approaches were applied to examine population structure.
High within-population genetic diversity (HE = 0·76) and a relatively low inbreeding coefficient (FIS = 0·022) were found. Genetic differentiation among populations measured with a standardized measure was considerable (G'ST = 0·53). A significant isolation-by-distance relationship was found (r = 0·62, P < 0·001) and a significant geographic sub-structure, coinciding with proposed postglacial migration patterns. Altitudinal location and size of populations did not influence molecular variation. Direct measures of pollen flow revealed that insect-mediated pollen dispersal was restricted to short distances within a population.
The natural isolation of suitable habitats for C. thyrsoides restricts gene flow among the populations as expected for a monocarpic species with very limited seed dispersal capacities. The observed high within-population genetic diversity in this rare monocarpic perennial is best explained by its outcrossing behaviour, long-lived individuals and overlapping generations. Despite the high within-population genetic diversity, the considerable genetic differentiation and the clear western–eastern differentiation in this species merits consideration in future conservation efforts.
Coastal development has led to extensive habitat destruction and the near extinction of the beach clustervine, Jacquemontia reclinata (Convolvulaceae), an endangered, perennial vine endemic to dune and coastal strand communities in south-eastern Florida. We examined the breeding system of this rare species, and observed visitors to its flowers, as part of a larger effort to document its status and facilitate its recovery.
Reproductively mature experimental plants were grown from seed collected from wild plants in two of the largest remaining populations. Controlled hand pollinations on potted plants were conducted to determine the level of compatibility of the species and to investigate compatibility within and between populations. Seeds from the hand pollinations were planted in soil, and they were monitored individually, recording time to seed germination (cotyledon emergence). Wild plants were observed in several of the remaining populations to determine which species visited the flowers.
Hand pollination and seed planting experiments indicate that J. reclinata has a mixed mating system: flowers are able to set fruit with viable seeds with self-pollen, but outcross pollen produces significantly greater fruit and seed set than self-pollen (≥50 % for crosses vs. <25 % for self-pollinations). Visitors included a wide array of insect species, primarily of the orders Diptera, Hymenoptera and Lepidoptera. All visitors captured and examined carried J. reclinata pollen, and usually several other types of pollen.
Remnant populations of beach clustervine will have greater reproductive success not only if floral visitor populations are maintained, but also if movement of either pollen or seed takes place between populations. Restoration efforts should include provisions for the establishment and maintenance of pollinator populations.
Many plants reproduce both clonally and sexually, and the balance between the two modes of reproduction will vary among populations. Clonal reproduction was characterized in three populations of the wild strawberry, Fragaria virginiana, to determine the extent that reproductive mode varied locally between sites. The study sites were fragmented woodlands in Cook County, Illinois, USA.
A total of 95 strawberry ramets were sampled from the three sites via transects. Ramets were mapped and genotyped at five variable microsatellite loci. The variability at these five loci was sufficient to assign plants to clones with high confidence, and the spatial pattern of genets was mapped at each site.
A total of 27 distinct multilocus genotypes were identified. Of these, 18 genotypes were detected only once, with the remaining nine detected in multiple ramets. The largest clone was identified in 16 ramets. No genets were shared between sites, and each site exhibited markedly different clonal and sexual recruitment patterns, ranging from two non-overlapping and widespread genets to 19 distinct genets. Only one flowering genet was female; the remainder were hermaphrodites.
Local population history or fine-scale ecological differences can result in dramatically different reproductive patterns at small spatial scales. This finding may be fairly widespread among clonal plant species, and studies that aim to characterize reproductive modes in species capable of asexual reproduction need to evaluate reproductive modes in multiple populations and sites.
Soluble sugars are involved in responses to stress, and act as signalling molecules that activate specific or hormone cross-talk transduction pathways. Thus, exogenous sucrose treatment efficiently induces tolerance to the herbicide atrazine in Arabidopsis thaliana plantlets, at least partially through large-scale modifications of expression of stress-related genes.
Availability of sugars in planta for stress responses is likely to depend on complex dynamics of soluble sugar accumulation, sucrose–starch partition and organ allocation. The question of potential relationships between endogenous sugar levels and stress responses to atrazine treatment was investigated through analysis of natural genetic accessions of A. thaliana. Parallel quantitative and statistical analysis of biochemical parameters and of stress-sensitive physiological traits was carried out on a set of 11 accessions.
Important natural variation was found between accessions of A. thaliana in pre-stress shoot endogenous sugar levels and responses of plantlets to subsequent atrazine stress. Moreover, consistent trends and statistically significant correlations were detected between specific endogenous sugar parameters, such as the pre-stress end of day sucrose level in shoots, and physiological markers of atrazine tolerance.
These significant relationships between endogenous carbohydrate metabolism and stress response therefore point to an important integration of carbon nutritional status and induction of stress tolerance in plants. The specific correlation between pre-stress sucrose level and greater atrazine tolerance may reflect adaptive mechanisms that link sucrose accumulation, photosynthesis-related stress and sucrose induction of stress defences.
In the UK, the flowers of fruit-bearing hedgerow plants provide a succession of pollen and nectar for flower-visiting insects for much of the year. The fruits of hedgerow plants are a source of winter food for frugivorous birds on farmland. It is unclear whether recent declines in pollinator populations are likely to threaten fruit-set and hence food supply for birds. The present study investigates the pollination biology of five common hedgerow plants: blackthorn (Prunus spinosa), hawthorn (Crataegus monogyna), dog rose (Rosa canina), bramble (Rubus fruticosus) and ivy (Hedera helix).
The requirement for insect pollination was investigated initially by excluding insects from flowers by using mesh bags and comparing immature and mature fruit-set with those of open-pollinated flowers. Those plants that showed a requirement for insect pollination were then tested to compare fruit-set under two additional pollination service scenarios: (1) reduced pollination, with insects excluded from flowers bagged for part of the flowering period, and (2) supplemental pollination, with flowers hand cross-pollinated to test for pollen limitation.
The proportions of flowers setting fruit in blackthorn, hawthorn and ivy were significantly reduced when insects were excluded from flowers by using mesh bags, whereas fruit-set in bramble and dog rose were unaffected. Restricting the exposure of flowers to pollinators had no significant effect on fruit-set. However, blackthorn and hawthorn were found to be pollen-limited, suggesting that the pollination service was inadequate in the study area.
Ensuring strong populations of insect pollinators may be essential to guarantee a winter fruit supply for birds in UK hedgerows.
The stem growth habit, determinate or indeterminate, of soybean, Glycine max, varieties affects various plant morphological and developmental traits. The objective of this study is to identify the effect of stem growth habit in soybean on the stomatal conductance of single leaves in relation to their leaf morphology in order to better understand the ecological and agronomic significance of this plant trait.
The stomatal conductance of leaves on the main stem was measured periodically under favourable field conditions to evaluate gmax, defined as the maximum stomatal conductance at full leaf expansion, for four varieties of soybean and their respective determinate or indeterminate near isogenic lines (NILs). Leaf morphological traits including stomatal density, guard cell length and vein density were also measured.
The value of gmax ranged from 0·383 to 0·754 mol H2O m–2 s–1 across all the genotypes for both years. For the four pairs of varieties, the indeterminate lines exhibited significantly greater gmax, stomatal density, numbers of epidermal cells per unit area and total vein length per unit area than their respective determinate NILs in both years. The guard cell length, leaf mass per area and single leaf size all tended to be greater in the determinate types. The variation of gmax across genotypes and years was well explained by the product of stomatal density and guard cell length (r = 0·86, P < 0·01).
The indeterminate stem growth habit resulted in a greater maximum stomatal conductance for soybean than the determinate habit, and this was attributed to the differences in leaf structure. This raises the further hypothesis that the difference in stem growth habit results in different water use characteristics of soybean plants in the field. Stomatal conductance under favourable conditions can be modified by leaf morphological traits.
Reduction in female fitness in large clones can occur as a result of increased geitonogamous self-fertilization and its influence through inbreeding depression. This possibility was investigated in the self-compatible, bee-pollinated perennial herb Aconitum kusnezoffii which varies in clone size.
Field investigations were conducted on pollinator behaviour, flowering phenology and variation in seed set. The effects of self-pollination following controlled self- and cross-pollination were also examined. Selfing rates of differently sized clones were assessed using allozyme markers.
High rates of geitonogamous pollination were associated with large display size. Female fitness at the ramet level decreased with clone size. Fruit and seed set under cross-pollination were significantly higher than those under self-pollination. The pre-dispersal inbreeding depression was estimated as 0·502 based on the difference in seed set per flower between self- and cross-pollinated flowers. Selfing rates of differently sized clones did not differ.
It is concluded that in A. kusnezoffii the negative effects of self-pollination causing reduced female fertility with clone size arise primarily from a strong early-acting inbreeding depression leading to the abortion of selfed embryos prior to seed maturation.
Myco-heterotrophic plants are partly or entirely non-photosynthetic plants that obtain energy and nutrients from fungi. These plants form a symbiosis with arbuscular mycorrhizal, ectomycorrhizal or saprotrophic fungi to meet their nutrient demands.
This Botanical Briefing summarizes current knowledge about myco-heterotrophy, discusses its controversial aspects and highlights future directions for research.
Considerable recent progress has been made in terms of understanding the evolutionary history, germination and nutrition of myco-heterotrophic plants. Myco-heterotrophic plants: (1) are diverse and often ancient lineages that have coevolved with fungi, (2) often demonstrate unusually high specificity towards fungi during germination and maturity, and (3) can either cheat common mycorrhizal networks supported by neighbouring photosynthetic plants to satisfy all or part of their energetic and nutritional needs, or recruit free-living saprotrophic fungi into novel mycorrhizal symbioses. However, several fundamental aspects of myco-heterotrophy remain controversial or unknown, such as symbiotic costs and physiology.
In the Florida Everglades, the expansion of cattail (Typha domingensis) into areas once dominated by sawgrass (Cladium jamaicense) has been attributed to altered hydrology and phosphorus (P) enrichment. The objective of this study was to quantify the interactive effects of P availability and soil redox potential (Eh) on the growth and nutrient responses of Typha, which may help to explain its expansion.
The study examined the growth and nutrient responses of Typha to the interactive effects of P availability (10, 80 and 500 µg P L–1) and Eh level (–150, +150 and +600 mV). Plants were grown hydroponically in a factorial experiment using titanium (Ti3+) citrate as a redox buffer.
Relative growth rate, elongation, root-supported tissue/root ratio, leaf length, lateral root length and biomass, as well as tissue nutrient concentrations, were all adversely affected by low Eh conditions. P availability compensated for the negative effect of low Eh for all these variables except that low P stimulated root length and nutrient use efficiency. The most growth-promoting treatment combination was 500 µg P L–1/ + 600 mV.
These results, plus previous data on Cladium responses to P/Eh combinations, document that high P availability and low Eh should benefit Typha more than Cladium as the growth and tissue nutrients of the former species responded more to excess P, even under highly reduced conditions. Therefore, the interactive effects of P enrichment and Eh appear to be linked to the expansion of Typha in the Everglades Water Conservation Area 2A, where both low Eh and enhanced phosphate availability have co-occurred during recent decades.
Wetlands are species-rich habitats performing valuable ecosystem services such as flood protection, water quality enhancement, food chain support and carbon sequestration. Worldwide, wetlands have been drained to convert them into agricultural land or industrial and urban areas. A realistic estimate is that 50 % of the world's wetlands have been lost.
This paper reviews the relationship between wetlands and agriculture with the aim to identify the successes and failures of agricultural use in different types of wetlands, with reference to short-term and long-term benefits and issues of sustainability. It also addresses a number of recent developments which will lead to pressure to reclaim and destroy natural wetlands, i.e. the continuous need for higher production to feed an increasing world population and the increasing cultivation of energy crops. Finally, attention is paid to the development of more flood-tolerant crop cultivars.
Agriculture has been carried out in several types of (former) wetlands for millennia, with crop fields on river floodplain soils and rice fields as major examples. However, intensive agricultural use of drained/reclaimed peatlands has been shown to lead to major problems because of the oxidation and subsidence of the peat soil. This does not only lead to severe carbon dioxide emissions, but also results in low-lying land which needs to be protected against flooding. Developments in South-East Asia, where vast areas of tropical peatlands are being converted into oil palm plantations, are of great concern in this respect. Although more flood-tolerant cultivars of commercial crop species are being developed, these are certainly not suitable for cultivation in wetlands with prolonged flooding periods, but rather will survive relatively short periods of waterlogging in normally improved agricultural soils. From a sustainability perspective, reclamation of peatlands for agriculture should be strongly discouraged. The opportunities for agriculture in naturally functioning floodplains should be further investigated. The development and use of crop cultivars with an even stronger flood tolerance could form part of the sustainable use of such floodplain systems. Extensive use of wetlands without drastic reclamation measures and without fertilizer and pesticides might result in combinations of food production with other wetland services, with biodiversity remaining more or less intact. There is a need for research by agronomists and environmental scientists to optimize such solutions.
According to the Intergovernmental Panel on Climate Change (IPCC) 2007, natural wetlands contribute 20–39 % to the global emission of methane. The range in the estimated percentage of the contribution of these systems to the total release of this greenhouse gas is large due to differences in the nature of the emitting vegetation including the soil microbiota that interfere with the production and consumption of methane.
Methane is a dominant end-product of anaerobic mineralization processes. When all electron acceptors except carbon dioxide are used by the microbial community, methanogenesis is the ultimate pathway to mineralize organic carbon compounds. Emergent wetland plants play an important role in the emission of methane to the atmosphere. They produce the carbon necessary for the production of methane, but also facilitate the release of methane by the possession of a system of interconnected internal gas lacunas. Aquatic macrophytes are commonly adapted to oxygen-limited conditions as they prevail in flooded or waterlogged soils. By this system, oxygen is transported to the underground parts of the plants. Part of the oxygen transported downwards is released in the root zone, where it sustains a number of beneficial oxidation processes. Through the pores from which oxygen escapes from the plant into the root zone, methane can enter the plant aerenchyma system and subsequently be emitted into the atmosphere. Part of the oxygen released into the root zone can be used to oxidize methane before it enters the atmosphere. However, the oxygen can also be used to regenerate alternative electron acceptors. The continuous supply of alternative electron acceptors will diminish the role of methanogenesis in the anaerobic mineralization processes in the root zone and therefore repress the production and emission of methane. The role of alternative element cycles in the inhibition of methanogenesis is discussed.
The role of the nitrogen cycle in repression of methane production is probably low. In contrast to wetlands particularly created for the purification of nitrogen-rich waste waters, concentrations of inorganic nitrogen compounds are low in the root zones in the growing season due to the nitrogen-consuming behaviour of the plant. Therefore, nitrate hardly competes with other electron acceptors for reduced organic compounds, and repression of methane oxidation by the presence of higher levels of ammonium will not be the case. The role of the iron cycle is likely to be important with respect to the repression of methane production and oxidation. Iron-reducing and iron-oxidizing bacteria are ubiquitous in the rhizosphere of wetland plants. The cycling of iron will be largely dependent on the size of the oxygen release in the root zone, which is likely to be different between different wetland plant species. The role of the sulfur cycle in repression of methane production is important in marine, sulfate-rich ecosystems, but might also play a role in freshwater systems where sufficient sulfate is available. Sulfate-reducing bacteria are omnipresent in freshwater ecosystems, but do not always react immediately to the supply of fresh sulfate. Hence, their role in the repression of methanogenesis is still to be proven in freshwater marshes.
Understanding the role of different components of hydrology in structuring wetland communities is not well developed. A sequence of adjacent wetlands located on a catenary sequence of soils and receiving the same sources and qualities of water is used to examine specifically the role of water-table median position and variability in affecting plant and microbial community composition and soil properties.
Two replicates of three types of wetland found adjacent to each other along a hydrological gradient in the New Jersey Pinelands (USA) were studied. Plant-community and water-table data were obtained within a 100-m2 plot in each community (pine swamp, maple swamp and Atlantic-white-cedar swamp). Monthly soil samples from each plot were analysed for soil moisture, organic matter, extractable nitrogen fractions, N mineralization rate and microbial community composition. Multivariate ordination methods were used to compare patterns among sites within and between data sets.
The maple and pine wetlands were more similar to each other in plant community composition, soil properties and microbial community composition than either was to the cedar swamps. However, maple and pine wetlands differed from each other in water-table descriptors as much as they differed from the cedar swamps. All microbial communities were dominated by Gram-positive bacteria despite hydrologic differences among the sites. Water-table variability was as important as water-table level in affecting microbial communities.
Water tables affect wetland communities through both median level and variability. Differentiation of both plant and microbial communities are not simple transforms of differences in water-table position, even when other hydrologic factors are kept constant. Rather, soil genesis, a result of both water-table position and geologic history, appears to be the main factor affecting plant and microbial community similarities.
Nitric oxide (NO) is a signalling and physiologically active molecule in animals, plants and bacteria. The specificity of the molecular mechanism(s) involved in transducing the NO signal within and between cells and tissues is still poorly understood. NO has been shown to be an emerging and potent signal molecule in plant growth, development and stress physiology. The NO donor S-nitrosoglutathion (GSNO) was shown to be a biologically active compound in plants and a candidate for NO storage and/or mobilization between plant tissues and cells. NO has been implicated as a central component in maintaining iron bioavailavility in plants.
Iron is an essential nutrient for almost all organisms. This review presents an overview of the functions of NO in iron metabolism in animals and discusses how NO production constitutes a key response in plant iron sensing and availability. In plants, NO drives downstream responses to both iron deficiency and iron overload. NO-mediated improvement of iron nutrition in plants growing under iron-deficient conditions represents a powerful tool to cope with soils displaying low iron availability. An interconversion between different redox forms based on the iron and NO status of the plant cells might be the core of a metabolic process driving plant iron homeostasis. Frataxin, a recently identified protein in plants, plays an important role in mitochondria biogenesis and in maintaining mitochondrial iron homeostasis. Evidence regarding the interaction between frataxin, NO and iron from analysis of frataxin knock-down Arabidopsis thaliana mutants is reviewed and discussed.
Convective gas flow in helophytes (emergent aquatic plants) is thought to be an important adaptation for the ability to colonize deep water. In this study, the maximum depths achieved by seven helophytes were compared in 17 lakes differing in nutrient enrichment, light attenuation, shoreline exposure and sediment characteristics to establish the importance of convective flow for their ability to form the deepest helophyte vegetation in different environments.
Convective gas flow development was compared amongst the seven species, and species were allocated to ‘flow absent’, ‘low flow’ and ‘high flow’ categories. Regression tree analysis and quantile regression analysis were used to determine the roles of flow category, lake water quality, light attenuation and shoreline exposure on maximum helophyte depths.
Two ‘flow absent’ species were restricted to very shallow water in all lakes and their depths were not affected by any environmental parameters. Three ‘low flow’ and two ‘high flow’ species had wide depth ranges, but ‘high flow’ species formed the deepest vegetation far more frequently than ‘low flow’ species. The ‘low flow’ species formed the deepest vegetation most commonly in oligotrophic lakes where oxygen demands in sediments were low, especially on exposed shorelines. The ‘high flow’ species were almost always those forming the deepest vegetation in eutrophic lakes, with Eleocharis sphacelata predominant when light attenuation was low, and Typha orientalis when light attenuation was high. Depths achieved by all five species with convective flow were limited by shoreline exposure, but T. orientalis was the least exposure-sensitive species.
Development of convective flow appears to be essential for dominance of helophyte species in >0·5 m depth, especially under eutrophic conditions. Exposure, sediment characteristics and light attenuation frequently constrain them to a shallower depth than their flow capacity permits.
Iron is an essential element for both plant productivity and nutritional quality. Improving plant iron content was attempted through genetic engineering of plants overexpressing ferritins. However, both the roles of these proteins in plant physiology, and the mechanisms involved in the regulation of their expression are largely unknown. Although the structure of ferritins is highly conserved between plants and animals, their cellular localization differs. Furthermore, regulation of ferritin gene expression in response to iron excess occurs at the transcriptional level in plants, in contrast to animals which regulate ferritin expression at the translational level.
In this review, an overview of our knowledge of bacterial and mammalian ferritin synthesis and functions is presented. Then the following will be reviewed: (a) the specific features of plant ferritins; (b) the regulation of their synthesis during development and in response to various environmental cues; and (c) their function in plant physiology, with special emphasis on the role that both bacterial and plant ferritins play during plant–bacteria interactions. Arabidopsis ferritins are encoded by a small nuclear gene family of four members which are differentially expressed. Recent results obtained by using this model plant enabled progress to be made in our understanding of the regulation of the synthesis and the in planta function of these various ferritins.
Studies on plant ferritin functions and regulation of their synthesis revealed strong links between these proteins and protection against oxidative stress. In contrast, their putative iron-storage function to furnish iron during various development processes is unlikely to be essential. Ferritins, by buffering iron, exert a fine tuning of the quantity of metal required for metabolic purposes, and help plants to cope with adverse situations, the deleterious effects of which would be amplified if no system had evolved to take care of free reactive iron.