Annals of Botany 2009 104(1):iii; doi:10.1093/aob/mcp145
© The Author 2009. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
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
NR regulation? Just say NO
Nitric oxide (NO)
is one of the more recently discovered signalling molecules
but has been shown already to participate in a wide range of
plant processes. One example is the assimilation of N via nitrate
reductase (NR), the subject of a study by
Jin et al. (Zhejiang University, Hangzhou, China and La Trobe University, Bundoora, Australia, pp. 9–17).
Tomato seedlings were grown hydroponically under low nitrate
(0.5 m
M) or high nitrate (5.0 m
M) conditions. After
2 weeks' growth under these conditions, plants were treated
with a NO donor (sodium nitroprusside, SNP, or diethylamine
NONOate sodium, NONOate) or with a NO scavenger (cPTIO), or
were untreated (controls). In the untreated plants, NR activity
was approx. 3-fold higher under high-N than under low-N conditions.
NO, supplied either via SNP or NONOate,
increased NR activity
in low-N plants but
decreased it in high-N plants. The reverse
was true for the NO scavenger, cPTIO. A specific study of the
effects of SNP showed that it did not affect either NR gene
transcription or the amount of NR protein. Its effects were
thus mediated at the post-translational level. The authors also
tested the direct effects of NO on enzyme extracts. Addition
of NO stimulated NR activity in extracts from low-N plants while
addition of the NO scavenger inhibited enzyme activity. In high-N
plants low concentrations of NO stimulated NR activity but higher
concentrations inhibited it, as did, once again, the NO scavenger.
Interpretation of these results is made more complicated by
the fact that NR activity itself produces NO. Indeed, the authors'
in situ measurements of NO in low-N and high-N roots is consistent
with this. Nevertheless, it is clear that there is an interaction
between NO and N-supply in the observed effects on NR activity.
Further, the effects of NO may be mediated by direct interaction
with the protein, as has been shown previously for interactions
with haemoglobin.
The seedier side of germplasm conservation
Seeds are so familiar
to us that we often forget what remarkable structures they are:
embryonic organisms, dried down to a very low water content
and capable of surviving in that state for months or even many
years. They are therefore excellent subjects for germplasm conservation.
However, even under ideal storage conditions there is extensive
variation in seed life span between species, as discussed by
Probert et al. (Wakehurst Laboratory, Royal Botanic Gardens, Kew, pp. 57–69).
They collected, from a very wide range of habitats, seeds of
195 species belonging to 71 different families. In order to
make observations within a realistic time-frame, seeds were
subjected to rapid ageing (45 °C/60 % RH, or 60 °C/60 %
RH). Seeds were sampled regularly for germination tests; time
taken for viability to fall to 50 % (
p50) was determined.
As under more ideal storage conditions, there was a very wide
range in seed life span. At 45 °C/60 % RH,
p50 ranged from 0·1 days to 771 days. There was no obvious
relationship between life span and taxonomic position: most
orders exhibited a wide range of longevities. Order Myrtales
contained the longest-lived species,
Calothamnus rupestris,
and order Liliales contained the shortest-lived,
Narthecium ossifragum. Seed life span was correlated significantly with
the absence of endosperm (mean
p50 of 20·3 with endosperm
and 65·7 days without endosperm). The climate at the
place of a seed's origin was shown to be important: life span
was positively correlated with mean annual temperature and negatively
correlated with total annual rainfall. This leads to the conclusion
that seeds without endosperm from plants growing in hot, dry
environments are likely to be long-lived. Early angiosperms
had endospermic seeds with small embryos and probably lived
in moist environments. The authors suggest that these early
flowering plants would have had short-lived seeds and that seed
longevity evolved either as an adaptation to climatic drying
or in association with invasion of hot, dry environments.
Flower of youth, bloom of old age
The work of
Henk van Dijk at Université Lille I (pp. 115–124) reminds us that there are some important projects that simply
cannot be fitted into a typical pattern of short-term grant
funding. In a very well-planned and carefully executed long-term
study, he investigated flowering and seed production in relation
to plant age in sea beet (
Beta vulgaris ssp.
maritima). Plants
grown from seed collected at different locations in Western
Europe showed marked differences in mean life span between populations.
In addition to these natural populations, the author established
a synthetic population, based on plants originating from 93
different wild populations. All plants were grown in a greenhouse
under natural day length at ‘temperatures slightly warmer
than external temperatures’. Date of flowering, seed set
and root diameter were determined each year until plant death.
Pooled populations from different regions exhibited mean life
spans ranging from 2·2 years (inland France) to 7·1
years (NW Brittany); the synthetic population showed a mean
of 5·7 years. There was a clear effect of age on the
date of flowering which, right across the age range, was 1·31
days later per year. Later flowering was correlated with a decline
in seed production, probably because the period for seed and
fruit ripening was shorter. In the year before death, these
trends were especially evident, with flowering occurring 3·3
days later than in plants that had at least one more year to
live. Seed production also declined further, as did investment
in root biomass. These data thus suggest two different mechanisms
(or sets of mechanisms) operating during a plant's life span,
one involving a slow, longer-term decline and one involving
a more catastrophic decline towards the end of life. The reader
is referred to the paper for further discussion; at this point
I simply wish to acknowledge again the commitment and patience
involved in carrying out this important and interesting investigation.
Conifers wued in Arctic role
During its long history
the earth has experienced many climatic changes. In some of
the warmest periods, characterized by very high atmospheric
CO
2 concentrations, coniferous forests extended into the polar
regions. How then did the trees respond to the elevated CO
2 concentrations in regions where there is 24 h of daylight
in high summer?
Llorens et al., at Sheffield (pp. 179–188) have used ‘living fossil’ conifers in experiments
aimed at answering this question.
Sequoia sempervirens,
Metasequoia glyptostroboides and
Taxodium distichum are all members of genera
represented in the Mesozoic Arctic forests and are thus appropriate
experimental models. Plants were grown for 3 years under conditions
that mimicked the Cretaceous Arctic climate, including the seasonal
variations in daylight hours and temperature. CO
2 concentrations
were either elevated (800 µmol mol
–1) or ambient
(400 µmol mol
–1). In general it is expected
that elevated CO
2 concentrations will lead to an increase in
water-use efficiency (WUE). However, the authors suggest that
this effect will be negated by the very long daylight hours
of summer in which there is little or no time to ‘recover’
from day-time conditions. This hypothesis was apparently supported
by the data from
Sequoia and
Metasequoia, in which improvements
in leaf WUE were only observed early and late in the growing
season. However, in
Taxodium the effect persisted throughout
the season. Further, when measured at whole-plant level and
integrated over the whole growing season, WUE of all three species
was stimulated by elevated CO
2. Analysis of the components that
make up WUE showed that, surprisingly, transpiration rates were
little affected by elevated CO
2, such that there was no significant
effect on total plant water use. The major effect was on increased
rates of photosynthetic CO
2 uptake. This occurred despite a
lower carboxylation efficiency (as indicated by isotope discrimination
measurements), but was probably aided by a reduction in photorespiration
at the elevated CO
2 concentrations.
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