Annals of Botany 2008 101(5):NP; doi:10.1093/aob/mcn037
© The Author 2008. 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
Dyeing to escape
As I glance out of
my window at the rain being driven by 100-kph winds it is difficult
to imagine ecosystems where fire is an essential factor. In
such ecosystems, one of the effects of fire is the breakage
of seed dormancy of many fire-dependent species. This phenomenon
is discussed by
Briggs and Morris, University of Western Sydney, Australia (pp. 623–632).
Our knowledge of the mechanisms by which fire breaks seed dormancy
is very ‘patchy’. For some species, for example,
it is clear that smoke alters a seed-coat barrier so that a
germination inhibitor may escape, but for many other species
no obvious mechanism has been described. One such is
Grevillea linearifolia, a native of eastern Australia. Dormancy is imposed
by the seed coat; high germination rates are achieved in the
absence of fire if the seed coat is removed. In the authors'
experiments, 23 % of whole, untreated seeds germinated.
This increased to 50 % in seeds exposed to heat, and to
67 % in seeds exposed to smoke. Seeds treated with both
heat and smoke exhibited 80 % germination. The approach
to detection of cell wall permeability changes was simple and
effective, making use of the dye Lucifer Yellow (LY). The dye
was applied either to the outside of the seed coat of whole
seeds or to the inside of seed coats of partly dissected seeds.
Permeation of the dye was observed by light microscopy in seed
coats from control seeds and from seeds exposed to heat and/or
smoke. The results were clear: smoke and/or heat treatment did
not make the complex seed coat of
G. linearifolia permeable
to LY. The authors have therefore turned their attention to
the possibility that heat and/or smoke alter the physical properties
of the seed coat cells, as suggested by the ‘mechanical
constraint’ model of seed coat dormancy.
Monkeyflowers, bees – but no birds
Working in a university
department with a long-standing research interest in
Mimulus,
I noticed immediately the paper by
Cooley et al.(Durham, USA and Santiago, Chile (pp. 641–650) who are interested in speciation mechanisms and reproductive
isolation. In a recent paper (Hybridization as an invasion of
the genome.
Trends in Ecology and Evolution 20: 229–237;
2005), Mallet states that at least 25 % of plant species,
mostly younger in evolutionary terms, are involved in hybridization.
How then do reproductive barriers actually arise? Cooley
et al. carried out a very careful, well-designed and thorough study
based on the possibility that pollinator preference plays a
role in floral diversification. They worked on Chilean populations
of four
Mimulus (sub)-species,
M. luteus luteus,
M. l. variegatus,
and
M. naiandinus and
M. cupreus. The latter has orange flowers
rather than the classic yellow with variable red spotting of
the ‘monkeyflower’. In the field, pollinator visits
were studied for large numbers of plants. Laboratory-based observations
included floral morphology, floral anthocyanins and nectar contents.
The three yellow (sub)species were all visited almost exclusively
by the same generalist pollinator, a bumble bee,
Bombus dahlbomii,
which made no overall distinction between flowers based on the
amount of red anthocyanin pigment spots when species were growing
separately. However, observations in an area where both
M. l. luteus and
M. naiandinus (and hybrids) grow revealed that individual
pollinators had different preferences but, overall, there was
a bias towards
luteus-type flowers.
Mimulus cupreus differs
from the other three, both in overall plant morphology and in
possessing orange flowers. It received very few pollinator visits
and, indeed, it exhibits a high degree of selfing. Low pollinator
visitation was not caused by discrimination against orange flowers:
a rare yellow morph was equally neglected. The authors have
thus shown that the appearance of red-pigmented flowers in ‘yellow
monkeyflower’
Mimulus species has not led to changes in
pollinators and that we need to know more about non-pollinator
mechanisms in the generation of floral diversity.
To boldly grow
In a recent book
(
Propitious esculent: the potato in world history. Heinemann,
Oxford, 2008) John Reader suggests that in manned flights to
Mars, the spacecraft should be able to accommodate a small stand
of potatoes in order to feed the astronauts and to help maintain
the CO
2/O
2 balance on board. However, if reliance is to be placed
on plants grown in spacecraft then we need to know that plant
growth and development are not unduly disrupted by the microgravity
environment. Previous experiments to investigate plant growth
in space have given conflicting results, as explained by
De Micco et al. (Naples, Italy and Grenoble, France, pp. 661–669).
In attempting to avoid some possible sources of variation, e.g.
the intense gravitational changes that occur during re-entry,
the authors set up an experiment in which soybean germination,
seedling growth and then fixation of the seedlings after 5 d
of growth were all performed automatically under microgravity
during a 16-d orbit in the Foton-M2 capsule. After return to
earth, seedlings were examined for general anatomical features
and also by light, fluorescence and transmission electron microscopy.
Particular attention was focused on the xylem and on the structure
of xylem cell walls. Comparison was made with plants grown on
earth but under otherwise identical conditions. Interestingly,
growth under microgravity, albeit for only 5 d, had rather little
effect on the seedlings. Clear effects were only seen at LM
and EM levels: some xylem cell walls were thinner and there
were perturbations in the orientation of cellulose microfibrils
in the early stages of wall growth. However, this was not seen
in the later stages of cellulose deposition when microfibrils
assembled into normal lamellae. By the time secondary thickening
occurred, xylem cell walls were similar (although sometimes
thinner) to those in control plants. Returning to our opening
theme, all this is good news for cosmic potato growers!
In the cold light of day
Many green plants
are damaged by exposure to cold, sunny conditions. Not only
is there the likelihood of freezing damage but also of damage
to the photosynthetic machinery, such as photo-bleaching. The
linkage between cold and light damage is discussed by
Rapacz et al., Kraków and Czgestochowa, Poland (pp. 689–699) in relation to cereals. They point out that the cold-response
gene
COR14b is also involved with chloroplast redox metabolism:
COR14b mRNA increases following cold treatment but accumulation
of the protein is also regulated post-transcriptionally by the
redox state of chloroplast plastocyanin. The authors thus investigated
the effects of cold-acclimation on both cold-tolerance and tolerance
to high light in four barley (
Hordeum vulgare) cultivars. Acclimation
at 2 °C for 14 d significantly increased the freezing tolerance
of all four cultivars, such that the temperature causing 50 %
lethality dropped in 14-d acclimation to –12 °C in
the least cold-tolerant cultivar and to –15 °C in
the most cold-tolerant. Cold-acclimation also led to increased
tolerance of high light and an increased photosynthetic capacity
when exposed to high light conditions after cold acclimation.
The authors went on to study the expression of several proteins
involved in cold acclimation and/or in photosynthetic metabolism;
here we focus on Cor14b. In cultivars that had greater basic
freezing tolerance and less basic light tolerance,
COR14b mRNA
and Cor14b protein accumulated quickly during acclimation but
then declined (although not back to control levels). In cultivars
with less basic freezing tolerance and greater basic light tolerance,
mRNA and protein accumulated more slowly but higher levels were
maintained through 14-d acclimation. We may conclude that the
results obtained by the authors are symptoms of a network of
interacting events enabling the plant to link together different
aspects of its activity in response to environmental stresses
– a nice example of what is now fashionably called systems
biology.
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Related articles in Ann Bot:
- Seed-coat Dormancy in Grevillea linearifolia: Little Change in Permeability to an Apoplastic Tracer after Treatment with Smoke and Heat
- Candida L. Briggs and E. Charles Morris
Ann Bot 2008 101: 623-632.
[Abstract]
[Full Text]
- Is Floral Diversification Associated with Pollinator Divergence? Flower Shape, Flower Colour and Pollinator Preference in Chilean Mimulus
- A. M. Cooley, G. Carvallo, and J. H. Willis
Ann Bot 2008 101: 641-650.
[Abstract]
[Full Text]
- Xylem Development and Cell Wall Changes of Soybean Seedlings Grown in Space
- Veronica de Micco, Giovanna Aronne, Jean-Paul Joseleau, and Katia Ruel
Ann Bot 2008 101: 661-669.
[Abstract]
[Full Text]
- The Effects of Cold Acclimation on Photosynthetic Apparatus and the Expression of COR14b in Four Genotypes of Barley (Hordeum vulgare) Contrasting in their Tolerance to Freezing and High-light Treatment in Cold Conditions
- Marcin Rapacz, Barbara Wolanin, Katarzyna Hura, and MirosLaw Tyrka
Ann Bot 2008 101: 689-699.
[Abstract]
[Full Text]
