Annals of Botany 2009 103(6):v; doi:10.1093/aob/mcp061
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John Bryant takes a closer look at some of this month's Original Articles
J. A. Bryant, Professor
University of Exeter, UK
E-mail j.a.bryant{at}exeter.ac.uk
Human touch leads to pallid plants and lumpy lesions
The inability of
plants to ‘run away’ has implications for the ways
in which they interact with their environment. One feature that
is encountered by all plants is mechanical perturbation; in
plants growing on land this may include wind and rain action,
herbivore activity, encounters with other objects (such as stones,
rocks and even other plants) and touch. It is this latter factor
that has been investigated by
Porter et al., in Hawai'i (pp. 847–858).
They have focused especially on changes in plant growth and
morphology (thigmomorphogenesis) in response to a touch treatment
in the tropical fruit tree,
Carica papaya. This species was
chosen because its small genome has been sequenced, it is readily
transformed by GM techniques and it grows rapidly. Touch treatment
consisted of gently bending seedling stems to 45° forward,
back, left and right. This was done three times during the day.
After 25 days of this treatment, seedlings were stunted (46
% decrease in height) but the hypocotyls were 36 % broader.
A very striking feature was the loss of pigmentation. Chlorophyll
content was reduced by more than 50 % while anthocyanins (which
normally colour the hypocotyl dark purple) were almost completely
absent. By contrast, lignin content increased by 12 % and the
authors suggest that the dramatic reduction in anthocyanins
may be caused by diversion of precursors from the anthocyanin
to the lignin pathway. Touch treatment also led to the development,
on the abaxial side of the petioles, of novel periderm (cork)
outgrowths that contained suberin. Genetic analysis showed that
Carica has fewer touch-inducible (
TCH) and mechano-sensitive
channel of small conductance-like (
MSL-like) genes than
Arabidopsis,
possibly reflecting the relative sizes of their genomes. Study
of
TCH gene expression in
Carica revealed that two homologues
of the
Arabidopsis TCH1 genes were only slightly up-regulated
by the touch treatment but were significantly up-regulated by
a different mechanical perturbation, namely spraying with water.
Copper arrests root growth and provokes small protein responses
Rice is arguably
the most important crop in the world with half the world's population
relying on it. Increased understanding of how rice plants withstand
environmental stress may well contribute to world food security.
Thus,
Zhang et al. (Nanjing, China and Tokyo, Japan, pp. 923–930) have investigated the effects of Cu on germinating rice embryos.
Like many micronutrients, Cu is toxic in higher concentrations.
The authors have focused especially on metallothioneins (MTs),
low-molecular-weight, cysteine-rich, metal-binding proteins.
Studies of plant MTs at the protein level have been limited,
but more is known about MT genes and gene expression. For example,
expression of type-2 MTs is associated with Cu tolerance in
Arabidopsis. Rice seeds were germinated in the presence of 0–200 µ
M CuSO
4; radicle lengths were measured at 4, 6 and 8 days. At
4 days, only 200 µ
M Cu had any inhibitory effect,
but by day 6 there was some inhibition at Cu concentrations
of 50 µ
M and greater. Proteins were extracted from
6-day germinating embryos and fractionated by 1-D and 2-D gel
electrophoresis. The latter revealed that 13 protein spots in
the 5–25 kDa molecular-weight range were up-regulated
(by up to 5·3-fold) by Cu treatment, while three were
down-regulated. These 16 protein spots were digested with trypsin
and subjected to peptide-mass fingerprinting using a MALDI-TOF
mass spectrometer. Data were then used to search several protein
databases. This showed clearly that the protein exhibiting the
greatest extent of up-regulation was an MT-like protein. Other
up-regulated proteins included a putative cell-wall-associated
protein kinase, two pathogenesis-related proteins and a small
GTP-binding protein. One of the down-regulated proteins was
a small cytochrome P450 (CYP90D2). The authors state that this
is the first time that proteomic evidence has been obtained
that MT and CYP90D2 are Cu-responsive proteins in plants. From
a wider perspective, the results also increase our knowledge
of Cu tolerance mechanisms in this important crop.
[Data] Mining for Mn – it's a pressing matter
Long before computer-based
resources such as biological databases were even thought of,
a different type of botanical data bank, the herbarium, was
in existence. Herbaria, collections of preserved dried plants,
often dating back several centuries, contain a wealth of data
waiting to be ‘mined’.
Fernando et al. (Melbourne and Toowong, Australia, pp. 931–939) have thus used herbarium specimens to search for Mn hyperaccumulators.
Hyperaccumulators, which comprise less than 0·2 % of
angiosperm species, in general have the ability to take up and
sequester amounts of particular metals at levels many times
greater than would normally be toxic, even to tolerant species.
They have been suggested as agents for bio-remediation of contaminated
soils and even as sources of metals for commercial extraction.
Mn hyperaccumulators are relatively scarce with only about 11
species known, most of which are native to New Caledonia. The
authors rationale was that eastern Australia is geographically
close to New Caledonia and is home to ecologically equivalent
species to the Mn hyperaccumulators. Leaf fragments were taken
from herbarium specimens representing 47 species in seven genera
spread across three families; fragments were assayed for Mn,
Ni, Fe, Al and Ti. In the family Myrtaceae, five
Gossia species
were definitely identified as Mn hyperaccumulators (including
G. bidwillii, which was already known to accumulate Mn to high
levels); one of these,
G. fragrantissima, also accumulated Ni.
It was confirmed that
Macadamia species (family Proteaceae)
accumulate either Mn or Al. In the family Celestaceae,
Maytenus cunninghamii was shown for the first time to be a hyperaccumulator.
Interestingly, this species exhibits considerable variation
in its ability to accumulate Mn, and at the two ends of the
range the plants are morphologically distinct from each other,
suggesting the existence of subspecies. Overall, the authors'
rationale has been supported by the results. What is now needed
is assay of plants growing in the field.
Organs move closer to do it for themselves
The evolutionary
advantages of sexual reproduction have been discussed widely
since Darwin's time. Despite these advantages, sex is expensive
and if suitable mates are in short supply, may fail completely.
Reliance or partial reliance on asexual reproduction, such as
happens in many plants, may therefore be regarded as a form
of reproductive ‘safety net’. However, there is
a half-way situation between asexual reproduction and full-scale
sexual reproduction, namely self-fertilization. This process,
which has evolved from outbreeding, is widespread in plants
and enables seed set when suitable mates and/or pollination
agents are scarce or absent. At the molecular and cellular level
it involves loss of self-incompatibility, whilst at the morphological
level changes in floral architecture occur.
Vallejo-Marín and Barrett at Toronto (pp. 951–962) have studied a species,
Eichornia paniculata, an aquatic annual,
in which there is a wide variety in the extent of inbreeding,
associated with variation in floral architecture. It therefore
looks as if evolution of selfing is actually in progress in
this species. The authors are interested to know whether changes
in floral architecture occur together or arise independently
during this evolutionary process. Plants from 31 Brazilian populations
were grown under uniform conditions. The main focus was on herkogamy
(stigma–anther separation) and flower size in relation
to stylar morphology (long, medium or short styles). Reduction
in herkogamy (i.e. stigma and anther being closer together),
a feature of inbreeding plants, occurred only in the mid-styled
morph and was independent of changes in flower size. Herkogamy
was clearly under genetic control but was also subject to environmental
influences, leading to phenotypic plasticity; moreover, there
was genetic variation in the degree of sensitivity to environmental
factors. A further interesting feature of the results was the
existence of individual plants with both normal and reduced
herkogamy – a clear indication of the status of this species
as an intermediate form.
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