Annals of Botany 2009 103(3):iii; doi:10.1093/aob/mcp002
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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
Root recognition reflects parasite preferences
Plant-on-plant parasitism
occurs widely amongst angiosperms.
Fernández-Aparicio et al. (Córdoba and Palos de la Frontera, Spain, pp. 423–431) cite papers indicating that this life-style has arisen independently
in 17 angiosperm families, giving rise to approx. 4000 parasitic
species. Amongst these there is variation in the extent to which
the parasite depends on the host and in the degree of specialization
in host preference. For all parasitic plants there is the problem
of detecting the host; for the specialist parasites this means
detecting only the correct host. The authors have focussed on
one aspect of this, namely the effects of root exudates of host
and non-host plants on the germination of seeds of specialist
and of more generalist parasites in the broomrape genera
Orobanche and
Phelibanche. Root exudates were collected from 41 different
angiosperms and tested on the seeds of each of nine broomrape
species. A general germination stimulant, GR24, was used as
a positive control with water as a negative control. At one
end of the range were the specialist parasites. Seeds of
O. gracilis were only stimulated to germinate by exudates from
its host, those of
O. hederae by exudates from its host and
one non-host species, while
O. densiflora was stimulated by
exudates from its host and from six non-host species. Seeds
of all three failed to germinate in the presence of GR24. By
contrast, seeds of the weedy non-specialist species
O. minor,
P. aegyptiaca and
P. ramosa germinated in the presence of exudates
from most of the plants tested, including some non-host species.
Other weedy, generalist broomrapes showed intermediate germination
behaviour but seeds of all non-specialist species germinated
in the presence of GR24 (albeit at a low percentage in
O. foetida).
Thus in general, the germination responses reflect the behaviour
of parasites in the field, emphasizing that detection of specific
chemical signals is part of the host-recognition mechanism.
Fruit makes light work of vitamin C synthesis
It has been known
for several decades that the level of irradiance affects the
accumulation of ascorbic acid (vitamin C). The positive correlation
in leaves between ascorbate content and soluble carbohydrate
content, as shown in Smirnoff's laboratory (very close to where
I am writing), suggests in turn a relationship between ascorbate
accumulation and photosynthetic metabolism. Considering the
role of sugars in ascorbate synthesis, this is perhaps not unexpected.
However, in fruits the situation may not be so straightforward.
For example, increased irradiance leads to increased ascorbate
levels in tomatoes ripened
after detachment from the plant.
Gautier et al. (Avignon and Montfavet, France, pp. 495–504) have therefore studied the effect of reduced irradiance on ascorbate
content in tomato fruits ripening on the plant. Leaves or developing
fruits, or both, were shaded; ascorbate was assayed at different
stages of ripening. First, it was clear that shading only the
fruit led, at the ‘breaker’ stage of ripening, to
a very marked reduction in fruit ascorbate content; shading
only the leaves had no effect. Secondly, it was shown that this
effect of shading the fruit was apparent at all stages of ripening
from green through to red. The main effect of leaf shading was
to slow ripening but at any particular colour stage did not
affect fruit total ascorbate content. However, leaf shading
did lead to changes in the ratio of oxidized to reduced ascorbate
at different stages of ripening. Thus, despite the lack of effect
on total ascorbate accumulation, leaf metabolism can affect
fruit ascorbate metabolism. In control plants the increasing
ascorbate content in fruit was correlated with increasing content
of soluble carbohydrates. This relationship was broken both
in plants in which only fruit were shaded (low ascorbate, normal
levels of soluble carbohydrates) and in which leaves were shaded
(normal ascorbate levels, reduced soluble carbohydrate content).
The relationship between leaf and fruit metabolism is thus complex.
Salt-lover has no need to mind its Ps and Qs
Many metabolic events
are mediated by assemblages of proteins that are truly beautiful
both in their complexity and their function. Good examples are
the complexes that participate in photosynthesis, such as the
photosystem II (PSII) complex studied by
Pagliano et al. (Padova, Italy and Szeged, Hungary, pp. 505–515).
As discussed by the authors, the oxidation of water is a multi-step
process that requires Ca
2+ and Cl
– to function properly.
The roles of most of the protein subunits in the complex have
been elucidated. However, there are two, termed here PsbQ and
PsbP, for which the functions are as yet unknown, although evidence
is building that they may be involved in modulating the levels
of Cl
– (and possibly Ca
2+). The authors reasoned that
in an extreme halophyte such as
Salicornia veneta, with much
higher leaf Cl
– concentrations than non-halophytes, these
proteins may be redundant. From their very detailed study, we
focus on the key results. First,
S. veneta possesses
PsbQ and
PsbP genes. Despite this, PsbQ protein is completely absent
from the thylakoids and its mRNA cannot be detected. The
PsbP gene is transcribed but at a low level. Thus PsbP protein is
present in
S. veneta at only approx. 30 % of the level seen
in the reference plant,
Spinacea oleracea. For all other PSII
proteins,
S. veneta is similar to
S. oleracea. Further, PSII
function in the halophyte is not at all impaired by these differences
in protein content. PsbQ must therefore be regarded as non-essential
in
S. veneta. The situation with PsbP is more problematic. It
is present in
S. veneta in sub-stoichiometric concentrations
compared with other PSII components and thus cannot have an
essential role in PSII function in this species. One possibility
suggested by the authors is that it has a role as an assembly
factor or chaperone, an idea that is supported by data from
Arabidopsis thaliana and from cyanobacteria.
Plentiful P prevents Clusia CAM capabilities
For several years
I had an active research interest in plants that can switch
from C
3 photosynthesis to crassulacean acid metabolism (CAM)
and thus my attention was caught by the paper by
Maiquetía et al. (Caracas, pp. 525–532).
They have worked with the C
3–CAM plant
Clusia minor and
point out that factors other than water potential can affect
the switch to CAM in this and other species. One of those factors
is P supply, especially important in
C. minor which often grows
in acidic, P-deficient soils. In such soils, a plant's P supply
may be improved by the presence of mycorrhizae, which may in
turn influence the switch from C
3 to CAM. The authors grew
C. minor seedlings under three conditions: in unsterilized forest
soil containing native mycorrhizae (‘Nat’); in sterilized
forest soil inoculated with a ‘foreign’, very effective
mycorrhizal fungus,
Scutellospera fulgida (‘SF’);
and in sterilized forest soil supplemented with P (‘Ph’).
Growth parameters and nutritional status were monitored for
10 months, after which watering was ceased for 21 days. From
an extensive data set, we focus here on nutrient status and
on responses to water deficit. The Nat plants had the lowest
P content; Ph and SF treatments were equally effective in ameliorating
P deficiency. The highest N contents were observed in SF plants
and the lowest in Nat plants. SF and Ph plants also had a greater
ability to maintain leaf water potential and water content during
water deficit and did not exhibit the large diel leaf acid fluctuations
typical of CAM. Nat plants by contrast had clearly switched
into CAM by day 7 of water deficit, although by day 21 diel
acid fluctuations had returned to the levels seen in well-watered
plants. Thus, in
C. minor, P and N deficiencies play a major
role, alongside water deficit, in the induction of CAM.
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Related articles in Ann Bot:
- Recognition of root exudates by seeds of broomrape (Orobanche and Phelipanche) species
- M. Fernández-Aparicio, F. Flores, and D. Rubiales
Ann Bot 2009 103: 423-431.
[Abstract]
[Full Text]
- Regulation of tomato fruit ascorbate content is more highly dependent on fruit irradiance than leaf irradiance
- Hélène Gautier, Capucine Massot, Rebecca Stevens, Sylvie Sérino, and Michel Génard
Ann Bot 2009 103: 495-504.
[Abstract]
[Full Text]
- The extreme halophyte Salicornia veneta is depleted of the extrinsic PsbQ and PsbP proteins of the oxygen-evolving complex without loss of functional activity
- Cristina Pagliano, Nicoletta La Rocca, Flora Andreucci, Zsuzsanna Deák, Imre Vass, Nicoletta Rascio, and Roberto Barbato
Ann Bot 2009 103: 505-515.
[Abstract]
[Full Text]
- Mycorrhization and phosphorus nutrition affect water relations and CAM induction by drought in seedlings of Clusia minor
- M. Maiquetía, A. Cáceres, and A. Herrera
Ann Bot 2009 103: 525-532.
[Abstract]
[Full Text]
