Annals of Botany 2008 102(6):NP; doi:10.1093/aob/mcn221
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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
RAMP speeds up metallic traffic
Investigations of
‘model’ plants such as
Arabidopsis thaliana give
us a lot of good information about plant genetics, biochemistry
and physiology. Despite this it is often necessary to carry
out similar investigations with specific crop plants in relation
to specific local situations. An example is provided by the
work of
Xiao et al. at Beijing (pp. 881–889), focusing
on a strongly cold-tolerant apple species,
Malus baccata, widely
used as a rootstock in north China. Iron deficiency is a widespread
problem, especially for trees grown on calcareous soils. The
authors have therefore investigated at the molecular genetic
level the trafficking of Fe together with Mn and Cd. NRAMPS
(natural resistance-associated macrophage proteins) are highly
conserved proteins, present in both prokaryotes and eukaryotes,
that are implicated in the transport of metal ions. Based on
sequence data from NRAMPS in different plant species, the authors
have used RT–PCR and RACE to isolate a full-length cDNA
encoding an NRAMP protein in
M. baccata. Southern blotting showed
that the
MbNRAMP1 gene exists as a single copy in the
M. baccata genome. The gene was most strongly expressed in roots and the
level of expression increased under Fe deficiency, as indicated
by northern blots. A direct demonstration of the role of MbNRAMP1
in trafficking of metal ions was achieved by transferring the
gene into yeast mutants deficient for Fe and Mn uptake. Both
strains were rescued by the
MbNRAMP1 gene, indicating that uptake
of both ions had been restored. Expression of
MbNRAMP1 also
increased Cd uptake in yeast, rendering the cells more sensitive
to Cd toxicity. As in
M. baccata itself, expression was strongly
influenced by iron status. Further, evidence obtained with GFP-tagged
MbNRAMP protein suggests that yeast cells can also regulate
the subcellular location of the protein in response to changes
in iron status.
Bitter harvest of ancient aubergines
Information about
domestication and early breeding of crop plants comes from many
sources, including archaeology, genetics, ethno-botany and agriculture.
Such sources have, for example, given us a picture of domestication
of cereals in the fertile crescent some 10,000 years ago. However,
for many non-staple crops, domestication was much more recent
(and indeed is still in progress), giving us further sources
of information on the process. These sources include fine arts
(e.g. Janick J, Paris HS. 2006. The Cucurbit images (1515–1518)
of the Villa Farnesina, Rome.
Annals of Botany 97: 165–176)
and literature, the latter typified by the work of
Wang et al. (Beijing and London, pp. 891–897).
Their focus is on
Solanum melongena (aubergine/eggplant), an
important cash crop in the family Solanaceae. This is believed
to have originated from a sub-tropical wild species,
S. incanum,
with domestication centred on south-east Asia; however, both
the documentary evidence and the genomic data are fragmentary.
For example, DNA sequence analyses have not included Chinese
eggplant cultivars nor the wild
Solanum species of that region.
The authors have taken advantage of the vast and extensively
archived range of ancient Chinese literature, including botanical
books, encyclopaedias and non-scientific literature. Their detailed
and painstaking analysis has come up with the earliest reliably
documented record of eggplant cultivation as 59
BC in the Chengdu
province, thus challenging the accepted view. According to descriptions
at the time, the fruits were very small and round and tasted
very bitter (described as ‘not palatable’); weedy
species present today in the same region also have small, very
bitter fruits. The literature provides evidence for selective
breeding from the 7th century
AD onward, leading to much sweeter
and larger fruits of a variety of shapes. For the latter two
traits we now have a clear understanding of their genetic basis.
Nevertheless, a question remains. Why was such a plant with
small and very bitter fruit, unsuitable for any modern-day moussaka,
cultivated in the first instance? Did it perhaps have a use
in ancient medicine?
Populations prove predominant in making sense of scents
In my lectures on
plant metabolism I have often pointed out the importance of
reactions and pathways that are common to the synthesis of several
different classes of compound. An example is the shikimic acid
pathway, leading to compounds as diverse as aromatic amino acids,
anthocyanins and the aromatic components of plant scents. It
is this common use of the shikimate pathway that has led
Majetic et al. at Pittsburgh, Colombia (SC) and Cornell (pp. 911–922) to propose that in
Hesperis matronalis, polymorphisms in anthocyanin-based
floral pigments may be linked to variations in floral scents.
They studied five populations located in southern Canada, north-west
Pennsylvania and northern Virginia. In each population, scent
composition and emission rates were analysed by GC-MS for 10
purple and 10 white individuals. This analysis revealed the
presence of 39 volatile compounds, a mixture of aromatics and
terpenoids, all of which were scored as present or absent for
each plant. Statistical analysis showed that the five populations
differed significantly from each other in terms of scent composition
and emission rates. Within each population the means represented
quite wide variation in both these features, but this was not
correlated with colour polymorphism. There were no differences,
either for aromatics or for terpenoids, between the purple and
white morphs. Thus, the hypothesis that shared biochemistry
leads to a linkage between colour and scent must be rejected.
Differences between plants are thus most likely based on population-level
phenomena. In small, recently established populations a founder
effect may be important, although if it is, then the intra-population
variation suggests multiple introductions (i.e. of several different
genotypes) into the same area. Other possibilities include differences
in pollinator species and their preferences, and differences
in stress factors, including herbivores and pathogens. As the
authors themselves suggest, experiments with known biochemical
mutants in ecologically relevant environments are likely to
provide valuable further information.
Defence budget allocations do not limit growth
Condensed tannins
(CTs) are amongst the range of plant defence chemicals. Some
CTs have antibiotic properties and as a class they are known
to deter herbivores. However, it is not clear whether they are
synthesized constitutively or are induced by enemy attack.
Häring et al. (Zurich, pp. 979–987) have used
Onobrychis viciifolia as a model plant in the study
of the role of specific elicitors in the synthesis of CTs. Elicitors
are specific chemicals that enable a plant to detect enemy attack
and can thus be used to mimic such attacks in the absence of
the invading organism. Plants were grown at 0·0027, 0·075,
0·67 or 2·0 m
M P (minimal to supra-optimal);
growth parameters and foliar CT concentrations were measured.
Plants were then given one of three treatments: control, sterile
mechanical wounding and sterile mechanical wounding plus the
application of one of three specific elicitors. The fungal elicitor
was a solution of hydrolysed chitin plus a chemically uncharacterized
compound from
Penicillium chrysogenum. The bacterial elicitor
was a suspension of lyophilized
Micrococcus lysodeikticus in
a solution of peptides derived from bacterial flagellin and
from a prokaryotic elongation factor. The insect elicitor was
the saliva of
Spodoptera littoralis caterpillars, previously
raised on
Onobrychis viciifolia plants. Relative growth rates
were highest at 0·67 m
M P. Foliar CTs were synthesized
at all P concentrations, but the concentrations were negatively
correlated with P concentration. Mechanical wounding led to
a decrease in CT concentrations. Application of fungal and bacterial
elicitors caused maintenance of CTs at control concentrations
whereas the insect elicitor induced significantly higher CT
concentrations. Thus, the data provide evidence for both constitutive
and inducible synthesis in the same plant, with the strength
of induction depending on which elicitor is present. Furthermore,
growth was unaffected by either mechanical wounding or by elicitor
application, indicating an absence of trade-offs between CT
synthesis.
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Related articles in Ann Bot:
- The Iron-regulated Transporter, MbNRAMP1, Isolated from Malus baccata is Involved in Fe, Mn and Cd Trafficking
- Haihua Xiao, Liping Yin, Xuefeng Xu, Tianzhong Li, and Zhenhai Han
Ann Bot 2008 102: 881-889.
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- Ancient Chinese Literature Reveals Pathways of Eggplant Domestication
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