Annals of Botany 2009 103(1):iii; doi:10.1093/aob/mcn250
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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
Knowing our onions – experiments aid understanding of ethylene effects
Onion,
Allium cepa,
is an important cash crop, generally sold as dormant bulbs.
When dormancy is broken, the leaves around the shoot apex start
to elongate and the bulb ‘sprouts’. It is commercially
important to inhibit sprouting and ethylene has been used for
this purpose. However, as discussed by
Gebhard Bufler (Stuttgart; pp. 23–28),
the reported effects of ethylene on sprouting are somewhat variable,
depending on onion genotype, storage temperature and time of
application. The author used a cultivar, ‘Copra’,
which is dormant for several weeks after harvest. Bulbs were
stored at 18 °C; sprouting, as indicated by leaf-blade
elongation, started after 6 weeks of storage. Sprouting was
accompanied by an increase in the activity of sucrose synthase
(involved in sucrose mobilization) and in ATP content. Sprouting
was almost completely prevented in bulbs exposed throughout
storage to saturating concentrations of ethylene; increases
in sucrose synthase and ATP were also strongly inhibited, although
CO
2 output was increased. These effects were maintained for
at least 12 weeks after the natural period of dormancy. At first
sight, this suggests that ethylene is involved in maintaining
bulb dormancy; however, it is not so straightforward. Bulbs
transferred from ethylene to air during the dormant period sprouted
at the same time as control bulbs; in bulbs transferred to air
after the end of the natural dormancy period, sprouting occurred
almost immediately. This suggests that exposure to ethylene
inhibits sprout growth rather than dormancy
per se. The inhibition
of sprout elongation when ethylene was supplied after dormancy
breakage is consistent with this idea. Further, onion bulbs
under control conditions produce ethylene, albeit at very low
concentrations. Treatment of control bulbs after 4 weeks of
storage with the ethylene-action inhibitor 1-MCP led to a rapid
initiation of sprouting. This suggests that low levels of endogenous
ethylene are involved in maintaining dormancy, while the higher
concentrations of supplied ethylene inhibit sprout growth.
Just add salt to see modified patterns of multiple maize miRNAs
In October 2008,
I commented on the dramatic increase in research on microRNAs.
In that issue, a Chinese research group
1 had demonstrated the
role of miRNAs in controlling responses to flooding in
Zea mays.
Now, the same group reports their research on the regulation
of gene expression in response to salt stress, also in
Z. mays (Ding et al., Wuhan and Baoding, China; Cold Spring Harbor, USA; pp. 29–38).
Plants of a salt-tolerant and a salt-sensitive cultivar were
grown hydroponically. Salt treatment (200 m
M) started at
the three-leaf stage and RNA was extracted from roots 0·5,
5 and 24 h later. miRNAs were detected and assayed with
the µparaflo
TM chip containing 877 miRNAs from 17 plant
species. From the extensive data set we focus on a selection
of results. A total of 98 different miRNAs were shown to exhibit
different expression profiles in response to salt treatment.
This decreases in some miRNAs and increases in others; 18 of
the up-regulated miRNAs were only expressed in the salt-tolerant
cultivar. Further, in the salt-sensitive cultivar, up-regulation
of 25 miRNAs was delayed in comparison to the expression profile
in the salt-tolerant cultivar. In general, however, miRNA function
was initially enhanced in the sensitive cultivar and reduced
in the tolerant cultivar, suggesting a more widespread down-regulation
of miRNA-regulated genes in the sensitive cultivar. Looking
just at the miRNAs up- or down-regulated at the 0·5 h
point, the authors begin to build a network of early responses
in gene expression that correlate with salt tolerance. These
include a reduction in expression of genes involved in synthesizing
miRNAs, enhancement of auxin-response gene function, a modulation
of leaf development and changes in redox-metabolism, including
greater ability to scavenge reactive oxygen species. Obviously
some detail is still lacking but nevertheless the picture is
clear: as in flooding, so in salt-stress – post-transcriptional
regulation via miRNAs is an important component of the response.
Invasive alien encounters an enemy
Why do some alien
species such as Japanese knotweed (
Fallopia japonica) in the
UK or purple loosestrife (
Lythrum salicaria) in the USA become
invasive pests while others co-exist with the native biota without
becoming a nuisance? This is the question discussed by
Prider et al. at Adelaide (pp. 107–115).
They and other authors suggest that the invasive species are
successful because of absence of natural enemies – the
enemy release hypothesis (ERH). By contrast, less successful
aliens are limited by biotic pressures including susceptibility
to generalist enemies and competition from native species –
the biotic resistance hypothesis (BRH). The authors have studied
Cytisus scoparia (broom), a European leguminous shrub that has
become invasive in several countries where it has been introduced,
including Australia. This is attributable to the absence of
herbivorous insects that limit its growth in its native range
– a clear example of ERH. However, it has been recently
observed that in South Australia,
Cytisus is a good host for
the generalist parasitic vine
Cassytha pubescens. This has provided
an opportunity to compare the effects of
Cassytha parasitism
on
Cytisus and on a native shrub of similar habitat and habit,
Leptospermum myrsinoides. Observation of the number of plants
of each species that were infected by
Cassytha suggested that
the parasite had a slight preference for the native host. However,
it was very clear that the alien species was much more affected
by infection than the native species, with greater reduction
in photosynthesis and biomass accumulation and with a much higher
likelihood of host death. Indeed, in
Leptospermum plants, the
presence of ‘some dead biomass’ could not be attributed
to the parasite. The parasite itself also benefited more from
an association with
Cytisus, exhibiting much greater growth
rates on this host than on the native host. Thus, there is a
possibility for use of
Cassytha as a biological control agent
for
Cytisis.
C4 to C3 – getting back to basics in the forest clades
In most student
textbooks of plant physiology and biochemistry, C
4 photosynthesis
is described as being an adaptation to warmer, drier climates.
However, this is not a complete picture. There are certainly
some C
4 grasses whose native habitat is far from warmer or drier
and thus there may be additional selective pressures that have
led to the development of the C
4 pathway. In a recent review
(Sage RF. 2004. The evolution of C
4 photosynthesis.
New Phytologist 161: 341–370) it was estimated that C
4 photosynthesis
has evolved independently in at least 48 plant lineages. This
is a particularly astonishing figure when we consider that this
is a complex biochemical pathway that is a relatively recent
arrival in plant evolutionary history. The complexity of the
situation is further illustrated by the grass
Alloteropsis semialata,
which has both C
4 and C
3 subspecies, as described by
Ibrahim et al. (Sheffield, UK and Grahamstown, SA; pp. 127–136).
They have constructed a molecular phylogeny of the
Alloteropsis genus and other panicoid grasses. This was achieved by sequencing
a chloroplast gene,
ndhF, in all five
Alloteropsis species (including
both subspecies of
A. semialata) and adding these data to those
sequences of the same gene already known from approx. 150 other
members of the tribe Paniceae (which contains C
3 and C
4 species).
The phylogenetic tree constructed from the sequences shows very
clearly that the two
A. semialata subspecies are the most recently
diverged taxa in this group. Further, since that divergence
is much later than the evolution of C
4 photosynthesis in the
genus, the C
3 subspecies has undergone a reversion from C
4 to
C
3. The data similarly suggest that reversion has happened with
the divergence of
Panicum mertensii (now C
3) from
P. caricoides (C
4). The selective pressure that led to reversion is not clear,
but we share with the authors their wonder that such ‘extraordinary
lability’ can occur in ‘a character as complex as
the photosynthetic pathway’.
FOOTNOTES
1 Zhang Z, Wei L, Zou X, Tao Y, Liu Z, Zheng Y. 2008. Submergence-responsive microRNAs are potentially involved in the regulation of morphological and metabolic adaptations in maize root cells. Annals of Botany 102: 509–519. 
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