Annals of Botany 2008 102(4):NP; doi:10.1093/aob/mcn176
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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
CHS in CMS – the initial effect of orf138?
Cytoplasmic male
sterility (CMS) is a very useful tool in plant breeding, eliminating
the possibility of selfing when outcrossing is required. For
example, in radish (
Raphanus sativa), CMS is caused by the mitochondrial
(‘cytoplasmic’) gene
orf128 but fertility may be
restored by the nuclear
Rf gene. This system, known as the Ogura
cytoplasm, is the only CMS system available in cultivated radish
and is thus widely used in its breeding and that of other members
of the Brassicaceae. Despite this, relatively little is known
about the way in which
orf128 affects pollen development or
viability and it is this deficiency that has been addressed
by
Yang et al. at Kyoto (pp. 483–489). Light and electron
microscopic investigation of gametogenesis revealed that pollen
development was normal until the tetrad stage. However, at this
point things started to go wrong. Cell division continued in
tapetal cells causing over-proliferation of the tapetum. This
effectively inhibited further development of the microspores.
The microspores did not gain colour and exine formation was
perturbed. The failure in colour development was confirmed by
the absence of UV-absorbing compounds, consistent with a deficiency
in biosynthesis of flavanoid compounds. Assay by real-time PCR
of mRNAs encoding enzymes involved in synthesis of flavanoids
showed that most were unaffected in CMS plants. However, one
– that encoding chalcone synthase (CHS), a key regulatory
enzyme in flavanoid biosynthesis – was very markedly reduced
in abundance. Thus the nuclear
CHS gene was down-regulated in
the presence of the mitochondrial gene
orf128. Further, normal
patterns of
CHS expression were observed in plants in which
fertility had been restored by the nuclear
Rf gene.
CHS has
previously been shown to be involved in nucleus-based male sterility
in radish and is known to be essential for pollen development
in several other species, but now we know that
CHS is involved
in sterility in a CMS plant.
Study of submergence shows that small is beautiful
It is fascinating
to see how the basic tenets of molecular biology have been modified
over the past 30 years and, in particular, the discovery that
different types of RNA act in the regulation of gene expression.
The rise to prominence of microRNA (miRNA), a class of RNA that
targets mRNA for degradation, has been especially spectacular.
The first miRNA was discovered in 1993 but it was not until
2000 that a second was found. However, since then, the list
has grown to several thousand (see
http://microrna.sanger.ac.uk).
In plants they are involved in regulation of gene expression
in a range of developmentally or environmentally induced events.
This picture has been added to by the work of
Zhang et al. (Baoding, Wuhan and Jingzhou, China, pp. 509–519).
They used a miRNA microarray to investigate miRNA populations
in maize roots, showing that expression of 39 different miRNAs
is affected by submergence, falling into four different patterns.
A group of five miRNAs show early up-regulation; the target
mRNAs for four of these encode transcription factors. Thus these
transcription factors, mostly involved in root growth and morphogenesis,
are in turn down-regulated, most likely leading to an inhibition
of elongation and a promotion of adventitious roots. A fifth
early up-regulated miRNA targets starch synthase and aminotransferase
mRNAs, leading to reduction in starch and amino acid synthesis.
The latter is reciprocated by the second pattern of miRNA changes
in which down-regulation occurs in the first 24 hours, followed
by a later re-induction of synthesis. The target messenger RNAs
of these miRNAs thus show early up-regulation. They include
mRNAs encoding transcription factors and enzymes involved in
polysaccharide breakdown and enzymes involved in eliminating
the products of anaerobic metabolism. The overall pattern of
changes is thus seen to enable the root to respond appropriately
to submergence at all levels from morphogenetic to metabolic.
Salt, sorbitol and superoxide – revelations about roots and ROS
The role of reactive
oxygen species (ROS) in the life of a plant is somewhat ambivalent.
On one hand, active growth is associated with ROS production
and may indeed be dependent on those ROS, especially superoxide,
O
2•– that is produced by apoplastic NADPH oxidase.
On the other hand, ROS may be very damaging and plant cells
possess efficient mechanisms to detoxify or remove them. Further,
ROS are often produced when plants are exposed to stress. What
then happens in a growing organ that is exposed to stressful
conditions? To investigate this further,
Bustos et al. at Códoba, Argentina (p. 551–559) have exposed maize roots to hyper-osmotic stress, a situation
in which oxidative damage is known to occur in some plant species.
Roots treated for 3 h with either 150 m
M NaCl or 300 m
M sorbitol elongated at half the rate of roots in control (Hoagland
solution) conditions. Assays of ROS showed that in control roots,
the positions of the highest growth rates and the highest O
2•–coincided.
However, in stressed roots the situation was more complex. NaCl
treatment led to reduced O
2•–production, but the
reduction mainly occurred closer to the tip than the zone of
growth. In sorbitol-treated roots, O
2•–levels increased
rather than decreased and again this change took place much
nearer the tip than the growth zone. No ROS-mediated damage
was reported. Roots maintained in the osmotica for 2 d and then
returned to higher water potentials showed a recovery in their
growth rates. This recovery was inhibited if roots were treated
with the O
2•–scavengers MnCl
2 and Tiron, indicating
a dependence on O
2•–. Further, the highest levels
of O
2•– now coincided once again with the highest
growth rates. The complexity of these responses suggests that
O
2•–and possibly also other ROS have more than one
role in root growth, at least under hyper-osmotic conditions.
Genome size and growth rates – grass goes against the trend
As biologists we
hold two contrasting ideas. The first is that genomes are stable
and that organisms possess mechanisms to ensure that stability.
The second is that genomes are dynamic and have clearly changed
over evolutionary time. Plants are one of the groups that exhibit
the latter especially clearly: closely related species, even
within the same genus, may possess genomes of very different
sizes, based particularly on variation in the amount of non-coding
DNA. If such changes occur over evolutionary time, might we
not expect to observe in the present time, ‘changes in
progress’, such as subtle differences in genome size between
individuals that may represent an early stage of divergence?
The answer, at least for the grass
Festuca pallens, is clearly
Yes.
marda et al. (Brno, Czech Republic, pp. 599–607) have used flow cytometry to measure DNA amounts in 562
F. pallens seedlings derived from 17 maternal plants from a single population.
To eliminate artefactual differences resulting from experimental
error, they checked by co-processing in pairs plants whose genome
sizes had been shown to differ. Only if this resulted in a double
peak in the flow cytometer were the differences regarded as
genuine. The data thus verified were very clear. Amongst all
the seedlings there was a 1·188-fold variation in genome
size; even for the progeny of one maternal plant the variation
was 1·119-fold. There was, however, also a tendency to
the mean, probably caused by the variation in the genome sizes
of the paternal plants pollinating an individual maternal plant.
Nevertheless, the data also indicated a possible selective advantage
of larger genomes, contrasting with the accepted view that larger
genomes are associated with slower growth rates. The authors
found a positive correlation between growth rate and genome
size, possibly explaining why this population in its particular
habitat had a higher mean genome size than a nearby population.
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Related articles in Ann Bot:
- Inhibition of chalcone synthase Expression in Anthers of Raphanus sativus with Ogura Male Sterile Cytoplasm
- Soojung Yang, Toru Terachi, and Hiroshi Yamagishi
Ann Bot 2008 102: 483-489.
[Abstract]
[Full Text]
- Submergence-responsive MicroRNAs are Potentially Involved in the Regulation of Morphological and Metabolic Adaptations in Maize Root Cells
- Zuxin Zhang, Liya Wei, Xilin Zou, Yongsheng Tao, Zhijie Liu, and Yonglian Zheng
Ann Bot 2008 102: 509-519.
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