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A Novel LOX-Dependent Defense Mechanism |
In soybean, lipoxygenases (LOXs) are a multigene family
consisting of at least eight members. LOXs are also multifunctional; in
some cases, they act as storage proteins; in other cases, they catalyze
the addition of molecular oxygen to polyunsaturated fatty acids
an
important step in the pathway leading to the biosynthesis of various
defense signal molecules including jasmonic acid. In two companion
papers, Dubbs and Grimes (pp. 1269-1279,
1281-1288) identify the roles and locations of four of these LOXs
(VLX A-D) in soybean pod walls and announce their discovery of a novel
and potentially important defense mechanism in soybeans (Fig.
1). In their first contribution, the
authors provide evidence that VLXD accumulates transiently in the
endocarp just prior to seed filling, suggesting that it may act as a
temporary storage protein. In the second paper, Dubbs and Grimes report
that three other isoforms (VLXA, VLXB, and VLXC) immunolocalize to a
single layer of cells in the soybean pod wall (the mid-pericarp layer
or MPL). The cells of this undistinguished layer turn out to be quite
extraordinary in that extensive regions of their cell walls are so thin
that the protoplasm of one cell will, in some cases, bulge into the adjacent cell and occasionally rupture, apparently allowing some of the
cellular contents to mix. Conceivably, this merging of cytoplasms in
the MPL during mechanical stress could bring latent enzymes, such as
LOX, into contact with previously unavailable substrates, leading to a
burst in the production of defense signaling molecules.
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Life and Death at the Edge |
The hypersensitive response (HR) is a major form of defense
initiated by plants to check the spread of many types of invading pathogens. The basic strategy underlying the HR is to localize the
pathogen at the site of inoculation by means of the rapid necrosis of
the surrounding tissue. Although the basic underlying strategy of the
HR has long been recognized, much remains to be learned about its
actual mechanism. Moreover, an even more fundamental question has
arisen concerning whether cell death is a cause or a consequence of
resistance. There are accumulating reports of cases where pathogen
spread is checked even without cell death. Wright et al.
(pp. 1375-1385) bring to bear a large arsenal of
microscopic techniques upon the question of what is happening to the
cells along the edge of the developing necrotic lesion (Fig.
2). Using tobacco mosaic virus (TMV)
tagged with green fluorescent protein, the authors were able to
determine the precise localization of TMV-infected cells in relation to
the developmental progression of HR lesions. They conclude that the HR
of tobacco leaves is a two-phase process. The first stage culminates
with the collapse and dessication of most of the infected area. During
the second stage, the remaining infected cells along the edge of the
lesion are progressively eliminated. The authors propose that
diffusible signals, possibly salicylic acid or ethylene, might prevent
the surviving viruses along the edge of the initial lesion from
spreading further.
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Secrets of a Checkered Leaf and a Ghostly Tomato |
Carotenoids, such as 
-carotene and xanthophyll in the
chloroplast, and lycopene in the tomato chromoplast, use phytoene as a
precursor in their respective biosyntheses. Desaturases are involved in
many steps in the biosynthesis of carotenoids, and several of them
appear to be linked to a quinol:oxygen oxido-reductase. It has been
found that inactivation of the immutans gene in
Arabidopsis results in reduced phytoene desaturation and, consequently,
reduced carotenoid content. The checkered appearance of the leaves of this phytoene-accumulating mutant are a manifestation of its
constitutively low levels of carotenoids, and consequent susceptibility
to photooxidative damage during early chloroplast differentiation (Fig.
3). Josse et al. (pp.
1427-1436) report that the immutans gene product, upon
expression in Escherichia coli, confers a detectable
cyanide-resistant electron transport component to isolated membranes.
The sensitivity of this activity to n-propyl gallate, an
inhibitor of the cyanide-resistant alternative oxidase (AOX) of plant
mitochondria, as well as many structural similarities to AOX, suggest
that the immutans gene product is a plastidic AOX homologa
plastid terminal oxidase (PTOX)that serves as a
quinol: oxygen oxidoreductase during carotenoid synthesis.
Genetic evidence also indicates a role for PTOX in determining the
phenotype of the ghost mutant of tomato. This mutant also exhibits
checkered leaves like immutans, but has pale petals and
fruits. This, and the finding that the expression of PTOX parallels the
expression of two desaturases involved in lycopene synthesis, suggests
that PTOX also plays a role in carotenoid accumulation in petal and
fruit chromoplasts.
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Al Catcher in the Rye |
In acid soils, which comprise over 40% of the world's arable
lands, Al ions are soluble in the soil solution and are toxic to root
growth and function. Understanding the physiological bases underlying
Al phytotoxicity is, therefore, an important step toward developing
cultivars that can thrive in acid soils. A major way by which
Al-tolerant species detoxify Al is by secreting organic acids (e.g.
malate, citrate, and oxalate) into the soil. These organic acids, in
turn, form stable complexes with Al, thereby preventing its intra- and
extracellular binding to the roots. In this issue, Li et al.
(pp. 1537-1543) examine the mechanism(s) by which Al
triggers these tolerant plants to secrete organic acids. They compared
the response of rye, one of the more Al-tolerant graminaceous crops,
with that of an especially tolerant cultivar of wheat (cv Atlas 66).
These two plant types revealed very different mechanisms of organic
acid secretion. The wheat cultivar showed a rapid release of malate in
response to Al, possibly due to the opening of an anion channel. In
contrast, rye roots released both malate and citrate in response to Al,
but only after a lag phase of many hours. Al exposure also led to an
increase in citrate synthase activity in rye but not in wheat. These
differences indicate that these two grasses arrived at the same
solution to the problem of Al phytotoxicity (organic acid secretion) by
convergent evolution.
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Phosphatidic Acid May Be a New Lipid Signal |
Plants recognize the presence of pathogenic microbes by
perceiving chemical elicitors released by the pathogen directly or produced during the breakdown of fungal or plant cell walls. Evidence is emerging for a role for phospholipid signaling in the responses of
plants to elicitors. Several types of elicitors have been found to
activate phospholipase C in plants, thereby causing
polyphosphoinositide turnover and the release of inositol trisphosphate
(IP3) and diacylglycerol (DAG). Although a role for IP3 in releasing
intracellular calcium is now well established in plants, the role of
DAG in plants is less clear (in animal cells, it activates protein
kinase C). Recent studies, however, have shown that DAG in plants is
rapidly phosphorylated to phosphatidic acid (PA) by DAG kinase and that
PA, in turn, is phosphorylated to diacylgycerolphosphate (DGPP). PA, of
course, can also be produced directly by the activation of
phospholipase D, but van der Luit et al. (pp.
1507-1515) report that nearly all of the PA produced by tomato
suspension cells in response to three types of elicitors results from
DAG kinase activity. Citing recent studies indicative of a role
for PA in such diverse processes as algal
deflagellation, 
-amylase secretion, and stomatal closure, the
authors put forth the provocative argument that PA itself may serve as
a signaling molecule in plants and that DGPP production may be a means
of attenuating the PA signal.
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