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Cow Flops, Compost, and Proton Pumping in Maize (Zea
mays) |
Earthworm (Eisenia
foetida) compost enhances soil fertility by increasing the
availability of nutrients and improving the soil's structure and
water-holding capacity (Fig. 1). In the
process of hastening decomposition, earthworms also produce several
bioactive humic substances that exhibit auxin-like activity and which
improve plant nutrition and growth. Humic acids (HAs), macromolecules consisting mainly of long alkyl chains containing aromatic groups, comprise one of the major fractions of humic substances. The mechanism underlying their auxin-like activity remains unsettled. Auxins stimulate plant growth by inducing an increase in the amount of plasma
membrane H+-ATPase. Activation of the H+-ATPase
also improves plant nutrition by enhancing the electrochemical proton
gradient that drives ion transport across the cell membrane via
secondary transport systems. Could HAs be acting by a similar mechanism? In this issue, Canellas et al. (pp. 1951-1957) investigate the effects of HAs isolated from cattle manure earthworm compost on the earliest stages of lateral root development (an auxin-regulated process) and on the plasma membrane
H+-ATPase activity. HAs are found to enhance the
root growth of maize seedlings in conjunction with a marked
proliferation of sites of lateral root emergence. They also stimulate
the plasma membrane H+-ATPase activity,
apparently by promoting the expression of this H+-pumping protein. In addition, structural
analyses reveal the presence of exchangeable auxin groups in the
macrostructure of the earthworm compost HA. The stimulatory effect of
HA may be triggered by an association of HAs with specific receptors on the cell surface or may involve the release of small bioactive molecules from the HA macrostructure.
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Insights into a "Mystery Organelle" |
Researchers have long been aware of a mysterious organelle,
the endoplasmic reticulum (ER) body, in the cells of various
Brassicaceae, including Arabidopsis (Fig.
2). Fluorescent imaging of ER bodies by
green fluorescent protein with an ER retention signal has provided new
insights into the distribution and possible roles of these rod-shaped
structures that are characteristically 5 µm long and 0.5 µm
wide. Electron microscopic studies revealed that ER bodies have a
fibrous pattern inside, are derived from ER, and are surrounded by
ribosomes. Immunocytochemical analysis showed that ER bodies contain
precursors of two Cys proteinases, RD21 and 
VPE. Both RD21 and
VPE are vacuolar proteinases that are induced by environmental stresses. During salt-induced cell death, ER bodies fuse with each
other and with lytic vacuoles, thereby mediating the delivery of the
proteinase precursors directly into the vacuoles. These findings
suggest that ER bodies are a proteinase-sorting system that assists
Brassicacean cells under various stress conditions. In this issue,
Matsushima et al. (pp. 1807-1814) report that wounding and
treatment with methyl jasmonate (MeJa) induce many ER bodies in the
rosette leaves of Arabidopsis, which have no ER bodies under normal
conditions. The induction of ER bodies by MeJa is suppressed by
ethylene. An experiment using coi1 and etr1-4
mutant plants shows that the induction of ER bodies was strictly
coupled with the signal transduction of MeJa and ethylene. These
results suggest that the formation of ER bodies is a novel and unique
type of endomembrane system in the Brassicaceae and that it is involved
in the responses of certain cells to environmental stress.
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Figure 2.
Fluorescent imaging of Arabidopsis ER reveals
unique, rod-shaped bodies (ER bodies) that are produced in response to
stress.
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Gene Expression Analysis of Al Stress in Rye (Secale
cereale) |
Al is one of the most important limiting factors for crop
production on acid soils such as those that often occur in the tropics. With the goal of identifying novel genes regulated by Al stress, Milla et al. (pp. 1706-1716) employed an expressed sequence tag (EST) approach to analyze Al-induced changes in gene expression in
roots of an Al-tolerant rye (Secale cereale) cultivar. In
total, 1,968 sequences were obtained, and ESTs showing
significant homology to proteins of known function were
functionally classified. Comparison of the data sets from
non-stressed and stressed libraries revealed many previously reported
Al-responsive genes as well as 13 genes which hitherto have not been
implicated in Al stress. Among the more significant findings from this
study were the rapid down-regulation by Al stress of tonoplast
aquaporins (involved in cell elongation), and the late induction of the
ubiquitin-like protein SMT3 gene (involved in the control and recovery
of the cell cycle). In addition, the results suggest that
glutathione-dependent systems provide the most important antioxidant
defense in root apices under Al stress, whereas ascorbate-related
systems seem to be negatively affected. The strong and complex effects
of Al stress on several genes involved in the control of Fe uptake and
homeostasis suggest an important role of this mechanism during the
response of plants to Al, possibly in connection to oxidative stress.
Other novel Al-induced genes (pathogenesis-related protein 1.2, heme
oxygenase, and epoxide hydrolase) require further research to determine
where they fit into the overall response of Al-tolerant plants to toxic levels of Al.
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Gene Expression during Maize Pollen Maturation |
Generally, in male-sterile plant mutants, including the S-type
cytoplasmic male sterility (S-CMS) of maize, pollen inviability is
associated with starch deficiency. Pollen maturation appears to be
prematurely terminated if starch levels remain lower than a certain
threshold. The pollen (the gametophyte) itself controls starch deposition. Datta et al. (pp. 1645-1656) took
advantage of the failure of S-CMS pollen to "fill" during the last
stages of maturation to help elucidate, by comparison with wild-type pollen, what genes are expressed during this critical stage of pollen
development. As expected, male-fertile pollen, but not the CMS
starch-deficient genotypes, showed changes in the expression patterns
of a large number of genes during the "filling" stage of pollen
development. In addition to a battery of housekeeping genes necessary
for carbohydrate metabolism, they observed changes in hexose
transporter, plasma membrane H+-ATPase,
ZmMADS1, and 14-3-3 proteins. Collectively, the data suggest that
the combined effects of both reduced levels of sugars and their reduced
flux into starch biosynthesis may lead to the observed temporal changes
in gene expressions, and ultimately to pollen sterility.
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How Grass Disease Resistance (R) Genes Evolve |
Plants employ hundreds of disease resistance genes to defend
against a wide variety of pathogens and pests. The most abundant class
of R genes encodes proteins with a nucleotide-binding site and a
Leu-rich repeat (LRR) region. The LRR region apparently enables the
protein to recognize specific pathogen gene products and to initiate
several defense pathways. Because pathogens can easily become virulent
by loss of features that allow their recognition, R genes must evolve
rapidly to keep pace with evolving pathogen populations. From an
evolutionary perspective, it would be interesting to perform a
comparative analysis of DNA sequences from orthologous, R
gene-containing chromosomal regions from two closely related species.
Such studies might shed light on the mechanisms by which R genes are
able to evolve so quickly, and whether these same mechanisms are used
by different species. One well-studied R gene is Rp1, a
complex disease resistance locus in maize that confers resistance to
common leaf rust (Puccinia sorghi). In this issue, Ramakrishna et al. (pp. 1728-1738) sequenced a contiguous 268 kb of the Rp1-orthologous region in sorghum
(Sorghum bicolor) to determine the structural variation of
an R gene cluster that has diverged since the ancestral divergence of
maize and sorghum about 15 to 20 million years ago. The maize and
sorghum orthologous Rp1 regions share numerous structural
features, but all involve events that occurred independently in each
species. The data suggest that complex disease resistance gene clusters
are unusually prone to frequent internal and adjacent chromosomal
rearrangements of several types.
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Cell Wall Oligosaccharide Fingerprinting |
The selection of mutant plants with altered cell wall
composition provides new opportunities to study the functions of cell wall polysaccharides and to identify the gene(s) encoding enzymes involved in the biosynthesis of cell wall components. In this issue,
Lerouxel et al. (pp. 1754-1763) describe the use of
various enzymatic-fingerprinting methods that allow the rapid and
detailed structural analysis of cell wall polysaccharides. To
demonstrate the feasibility of the techniques, the researchers focused
on the analysis of xyloglucans, the most abundant hemicellulose present
in the primary cell walls of most plants. Of the three techniques
tested, the most impressive results were obtained using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The spectra generated by this technique take less than a minute to make, allowing for the rapid screening of a
vast number of plants. The authors were able to record reproducible MALDI-TOF MS spectra on xyloglucan fragment pools released from a
single Arabidopsis seedling. Applied to mur mutants, the
automated analysis was able to select not only strongly affected
xyloglucan mur1-mur3 mutants harboring side-chains
alterations, but also less obviously affected mutants such as
mur11. The automated and high-throughput methods presented
can easily be adopted for the analysis of other wall polysaccharides.
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