Annals of Botany 2009 103(5):iii; doi:10.1093/aob/mcp042
© 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
Tree of trees confirms complexity of colour conundrum
People will pay good
money to travel in order to see dying leaves. Put like that
it sounds crazy but actually the colours on display during the
autumn in some parts of the world are spectacular. However,
autumn colours are not an inevitable by-product of leaf senescence.
It is not a universal feature that leaves produce brilliant
colours; indeed it is not even the norm. In the northern hemisphere,
the forests of New England, with a high proportion of
Acer species,
show us that it is particular types of tree, sometimes aided
by weather conditions, that provide the autumn colours. We can
distinguish two main types of colour producers. In predominantly
yellow leaves, the loss of chlorophyll allows the accumulated
carotenoid pigments to be seen. In predominantly red leaves,
the colour is caused by the synthesis of anthocyanins specifically
in the autumn. What, then, is the selective advantage of autumn
colours? This question has led
Marco Archetti at Oxford (pp. 703–713) to examine the phylogenetic relationships between the tree species
that produce predominantly red or predominantly yellow colours
during autumn. The data are fascinating. Of the 2368 species
(representing 400 genera of trees from the temperate regions),
only 209 (spread among 70 genera) produced red coloration and
378 species (in 97 genera) produced yellow coloration independently
of red. The distribution of colour in the phylogenetic ‘tree’
strongly suggests that the production of autumn colours, whether
red or yellow, has evolved many times. The distribution within
lineages of species with autumn colours shows that colour production
may be gained and then lost during evolution. Any selective
advantage of having coloured leaves in autumn is not at all
clear but the author considers photoprotection or a co-evolutionary
interaction, and his detailed phylogenetic analysis of different
colorations may distinguish between some of the hypotheses.
Whatever the outcome, the phenomenon remains for us to enjoy,
aesthetically as well as scientifically.
Message from L1L prepares the way for epiphyllous embryos
Induction of embryogenesis
is an important factor in plant propagation from tissue culture
and in obtaining whole plants from transgenic cells or explants,
but many developments have been empirical with little scientific
understanding. It is very interesting that some plants, e.g.
Bryophyllum daigremontianum, naturally produce plantlets on
their leaves. Another example is a particular clone (EMB-2)
of the hybrid between
Helianthus annus and
H. tuberosus, studied
by a multi-locus Italian group,
Chiappetta et al. (pp. 735–747).
EMB-2 produces normal leaves (NEP leaves) and leaves that produce
epiphyllous embryos on their adaxial surface (EP leaves). The
authors were interested in the possible roles of the
LEAFY COTYLEDON 1-LIKE (
L1L) gene and of auxin in establishing embryogenic competence.
Expression of
L1L in EP leaves was readily detected by RT–PCR;
in situ hybridization showed that expression was localized to
the groups of cells that give rise to the epiphyllous embryos.
L1L mRNA was not detected in NEP leaves unless they were induced
to form embryos during
in vitro culture; nor was the mRNA detected
in leaves from the clone A2, which are incapable of embryogenesis.
The situation with auxin was not so clear. Comparison of EP,
NEP and A2 leaves showed no obvious correlation between auxin
amounts and embryogenic potential. In EP leaves, auxin was concentrated
in groups of cells, the position of which predicted the sites
of embryogenesis. This pattern of auxin distribution was not
seen in A2 or NEP leaves. Confusingly, induction of embryogenesis
in NEP leaves led to a reduction in the amount of auxin, although
there were indications that its distribution within leaves began
to be more localized. Nevertheless, addition of auxin to cultured
NEP leaves led to a 25 % increase in regeneration and a slight
increase in
L1L expression. Overall, it is clear that localized
L1L expression and IAA accumulation are involved in epiphyllous
embryogenesis and represent early events in the pathway to somatic
embryogenesis.
Genes lost in the woods
In this year of Darwin
we are all very aware of the genetic phenomena associated with
isolated populations. Factors such as founder effect, genetic
drift and natural selection will lead to divergence from other
isolated populations derived from the same taxon. For Darwin,
it was of course the finches of the Galapagos Islands that were
a key to his thinking about natural selection, although we now
know of many more examples. Thus,
Jacquemyn et al. (University of Leuven and Institute for Agricultural and Fisheries Research, Belgium, pp. 777–783) have studied genetic variation in founding populations of
Primula elatior. This is a long-lived, obligately out-breeding perennial
that grows in deciduous forests. Older plants are readily distinguished
from younger plants by the number of rosettes. The authors located
a recently established population of
P. elatior in a 20-year-old
forest ‘fragment’ separated from other suitable
habitats by at least 400 m. The plants were classified
as ‘young’ (1 rosette), ‘medium-aged’
(2–5 rosettes) and ‘old’ (more than 5 rosettes).
The latter were presumed to be the initial colonizers. AFLP
analysis was used to estimate genetic diversity within each
age range; 122 usable bands were detected and each of the three
classes was scored for band richness and expected heterozygosity.
The results were very clear. The founding population had the
highest band richness and expected heterozygosity; both values
declined through the subsequent generations. Parentage analysis
indicated that the young and medium-aged plants had mostly arisen
from the founding population, although there was evidence for
a small amount of gene flow from outside. This will have ameliorated
slightly the loss of band richness. The data thus show that
genetic diversity can decline quickly in isolated habitats.
This has implications for conservation and for the genetic fitness
of the population, although in this example the changes are
regarded as too small to affect population viability.
Desiccation does not disturb developmental potential of Digitalis seeds
Seeds are remarkable
structures, containing an embryo in which further development
is suspended by desiccation. Desiccation actually starts before
the seed is mature but is completed after the formation of the
abscission layer. In order to obtain seed for conservation or
commerce, collection must occur before the seeds are shed from
the plant, but if they are collected too early subsequent vigour
and viability may be affected. Thus,
Butler et al. (Wellesbourne, Kew and Reading, UK, pp. 785–794) collected seeds of
Digitalis purpurea in the post-abscission,
pre-shedding phase; seed moisture content was just below 50
%. Seeds were subjected to a complex range of relative humidity
(RH) and drying treatments, from which the authors obtained
a large data set. We focus here on a selection. Harvesting the
seeds did not inhibit further desiccation, which continued to
completion within 4–8 d at RHs between 15 % and 95 %,
although the actual equilibrium moisture content varied with
RH. Seeds placed immediately at RH 15 % were less long-lived
than seeds held at 30–95 % RH. Thus, from 15–80
% RH, increased post-harvest RH led to increased longevity,
although at 95 % RH longevity began to decline again. For germination,
the relationship between equilibrium moisture content (which
was correlated with the RH at which the seeds were kept) and
germination percentage was linear for seeds kept at between
15 and 80 % RH for 4 or 8 d. However, the highest germination
rate was seen in seeds kept at 95 % RH for 8 d. For nearly all
treatments, the germination rate improved if seeds were dried
at 15 % RH between the initial humidity treatment and the germination
assay. The authors conclude that seeds continue to mature, reflected
in increased longevity, after they have been harvested. Further,
based on these data and others not discussed here, maturation
can continue if seeds are returned to a moist environment after
exposure – even long-tem exposure – to 15 % RH.
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Ann Bot 2009 103: i.
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