Annals of Botany 2009 104(2):iii; doi:10.1093/aob/mcp187
© 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
Ethylene in Lotus position for nodulation
Land-based biological
N-fixation amounts to approx. 150 million tonnes per year, of
which the symbiosis between legumes and rhizobia accounts for
about 25 %. We have made great progress in understanding the
biochemistry of N-fixation and in elucidating some aspects of
the relationship between the plant host and bacterial symbiont.
The tools of modern genetics have been very useful in this research,
as is illustrated by the work of
Lohar et al., at the Universities of Missouri, USA and Queensland, Australia (pp. 277–285).
It is already known that ethylene inhibits nodule formation
while ethylene inhibitors may promote it. To ‘dissect’
further the role of ethylene in nodule formation, the authors
have transferred the
Arabidopsis thaliana dominant
etr-1-1 allele
(which encodes a mutated non-functional ethylene receptor) into
Lotus japonicus. This resulted in plants that were clearly ethylene-insensitive,
as shown both by the absence of the well-known ‘triple
response’ and by their ability to grow and nodulate in
the presence of the ethylene precursor ACC. However, the degree
of ethylene-insensitivity varied between different transgenic
lines. Those with highest levels of insensitivity exhibited
spectacular effects on nodulation with up to 3·5 times
as many nodules/nodule foci as wild-type plants. This increase
in nodulation was ascribed both to increased infection and to
a seven-fold increase in nodules developing between the xylem
poles, a location in which nodule formation is usually very
low. This suggests that in wild-type plants ethylene has a role
in mediating positional information. Further, there was evidence
that ethylene may affect the activity of the symbiont itself:
transgenic plants had on average 1·7 times as many bacteroids
per ‘symbiosome’ as wild-type. Despite these major
differences between the wild-type and the ethylene-insensitive
plants, it was also clear that the latter had retained the auto-regulation
and the nitrate inhibition of nodulation. These two aspects
of regulation thus lie outside the influence of ethylene.
Come out from under the blanket – it's warm outside
As I write this,
the governments of the world's most powerful nations are being
urged to take drastic steps in order to keep global warming
to less than +2 °C. We hope that they (and we) succeed but
in the meantime the ranges of several species of animals (and
to a lesser extent of plants) are moving slowly towards the
poles. So, which factors will actually affect the ability of
plants to become established?
Milbau et al. (University of Antwerp, Belgium and Umeå University, Sweden; pp. 287–296) have asked this question in respect of sub-arctic regions. Summer
soil temperatures will be higher but winter soil temperatures
will be lower because the snow cover is likely to be thinner.
The authors stratified seeds at ‘real’ sub-arctic
winter soil temperatures that occur under a thick or a thin
blanket of snow. The seeds were then germinated either under
optimum conditions or at real sub-arctic spring/early temperatures,
or at real temperatures plus 2·5 °C. Firstly, to
state the obvious, seeds germinated most rapidly and attained
the highest germination percentage under optimum conditions.
Secondly, in comparisons between present and probable future
conditions, germination of most species was faster in the warmer
future conditions than in current conditions; germination percentages
were unaffected. Thirdly, comparison of the two stratification
conditions gave less clear-cut results. In ten species, exposure
to lower winter soil temperature caused delayed germination;
in four of those species there was also a reduction in germination
percentage, most marked (–33 %) in
Vaccinium uiliginosum.
By contrast, the beautiful
Silene acaulis showed a 19 % increase
in germination percentage and a decrease of 2 days in mean germination
time when stratified at lower temperature. Despite the negative
effects for some species of the lower winter soil temperatures,
the authors consider that earlier germination and longer growing
seasons will have an overall benefit for seedling establishment
in sub-arctic regions.
Biomass boost for wormwood wonder drug
I first became aware
of artemisinin in the 1980s when reading a student dissertation
on traditional Chinese medicine. It was already clear that the
compound, synthesized by
Artemesia annua (sweet wormwood), was
effective against malaria. Since then it has been characterized
as a sesquiterpene lactone and has entered mainstream medicine,
especially for use against
Plasmodium that is resistant to other
drugs. It thus joins the large number of plant-derived products
that comprise about 25 % of drugs prescribed in Western medicine.
However, there are problems in maintaining an adequate supply
of artemisinin, not least because the yields from the source
plant are relatively low and somewhat variable. There is now
therefore a programme of breeding and agronomic research aimed
not only at obtaining higher yields but also at producing varieties
of
A. annua that perform well in new areas, including the temperate
regions of northern Europe. The work described by
Davies et al. (a large UK-based multi-centre group; pp. 315–323) forms part of this research. Plants were raised in a glasshouse
and then, whilst still in pots, transferred outside. Rain was
excluded. Pots were supplied with different concentrations of
N and K; plant biomass and artemisinin concentrations were measured.
Biomass increased with increasing N supply up to 106 mg
N L
–1 (although leaf N continued to increase with higher
concentrations of applied N). Total leaf artemisinin content
also increased significantly with increasing N and, as with
biomass, plateaued at 106 mg N L
–1. However, because
of the increased biomass the actual concentration of artemisinin
on a dry weight basis decreased slightly. Plant biomass was
also increased following application of K at concentrations
up to 153 mg L
–1, but without added N there was no
increase in artemisinin content. Artemisinin accumulation in
leaves is thus linked with N-induced growth and it will be both
fascinating and important to explore this linkage further.
Little Mo helps to pull up ZIP during cold response
In human society
it is often said that problems rarely come in ones. This saying
can equally well be applied to the environmental stresses that
are experienced by plants, as is illustrated by the work of
Xuecheng Sun et al., at Wuhan, China (pp. 345–356). They
have studied the interaction between molybdenum (Mo) deficiency
and cold-responsiveness in two cultivars of winter wheat (
Triticum aestivum). One cultivar was Mo-efficient and one was Mo-deficient
and both were grown with and without added Mo. Cold stress was
applied by transferring plants from 15/12 °C to 5/2 °C.
The authors’ analyses of the responses to Mo application
and cold were very comprehensive and here we present a brief
overview of their data. In the absence of added Mo the Mo-efficient
cultivar was more freeze-tolerant than the Mo-deficient cultivar.
Addition of Mo improved the freeze-tolerance of both cultivars,
and in cold-stressed plants led to an increase in Mo-dependent
aldehyde oxidase activity. This enzyme is involved in the synthesis
of ABA, and thus the increase in ABA content during cold-stress
was much higher in plants supplied with Mo than in those not
supplied. This was again true of both cultivars. ‘Downstream’
from ABA, exposure to cold led to transient increases in transcription
of genes encoding bZIP-type transcription factors and in transcription
of ABA-dependent cold-response (
COR) genes. As before, these
responses were greater in the Mo-fed plants. This was also seen,
although to a lesser extent, later in the sequence of responses,
in transcription of the genes encoding CBF/DREB transcription
factors and of the ABA-independent
COR genes. The greater effect
of Mo on the ABA-dependent pathway may possibly be ascribed
to the metal ion's direct effect on synthesis of ABA via its
role as a co-factor for aldehyde oxidase. Direct interactions
in the ABA-independent pathway are not so clear, but nevertheless
the interplay between cold-tolerance, photosynthesis and Mo
provides food for thought.
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Ann Bot 2009 104: i.
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