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Glucosinolates in Insect and Pathogen Defense |
Glucosinolates are a large group of
amino acid-derived secondary metabolites found in the Brassicaceae and
related families. Dozens of different types of these N- and
S-containing compounds can co-occur in a given ecotype. Tissue
disruption by herbivores or pathogens brings glucosinolates, which
alone are harmless, into contact with thioglucosidases (myrosinases),
thereby producing unstable aglycones. These aglycones, in turn,
generate numerous compounds (thiocyanates, nitriles, and
isothiocyanates) that serve as defense compounds against insects and
pathogens. In this issue, Kliebenstein et al. (pp. 811-825)
examine the quantitative and qualitative variation in 34 types of
glucosinolates in the leaves and seeds of 39 Arabidopsis ecotypes.
Based on their individual glucosinolate profiles, the authors
grouped the 39 Arabidopsis ecotypes into seven classes that could
be traced to variation at only three of the five identified gene loci
involved in determining glucosinolate type and titer. The authors
propose that there is a small set of polymorphic loci that generate
modular alterations in the glucosinolate profiles of Arabidopsis. This
genetic mechanism may allow for rapid evolution in response to changing
selective pressures such as altered herbivore or pathogen abundance.
In a related work, Jander et al. (pp. 890-898) examine the
differential susceptibility of various Arabidopsis ecotypes to feeding
damage by the larvae of the cabbage looper (Trichoplusia ni), a generalist herbivore that can complete its entire life cycle on Arabidopsis. The Columbia ecotype is much more susceptible to
feeding by cabbage loopers than is the Landsberg erecta
ecotype (Fig. 1). The authors trace a
good part of this difference to a quantitative trait locus,
TASTY, that occurs on chromosome 1. This locus, the function
of which remains unknown, apparently affects neither glucosinolate
titer nor trichome density. When identified, the protein product of
TASTY may lead to the discovery of new mechanisms of insect
resistance in plants.
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Figure 1.
The Columbia ecotype of Arabidopsis is much more
susceptible to attack by cabbage loopers than is the Landsberg
erecta ecotype.
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The roles of glucosinolates in plant defense, however, are not just
limited to repelling herbivore attacks: Glucosinolates also serve to
deter pathogen invasions. In this issue, Brader et al. (pp.
849-860) report that cell wall elicitors produced by inoculation
with Erwinia caratovara, a pathogenic bacterium that causes
soft rot in a wide variety of crops, causes a 3-fold increase in the
indolylmethylglucosinolate content of Arabidopsis. This increase is
mimicked by methyl jasmonate and does not occur in a mutant that is
insensitive to jasmonate. Two other major players in plant defense,
namely ethylene and salicyclic acid, do not appear to be involved. The
authors also demonstrate that the breakdown products of
indolylmethylglucosinolate and other glucosinolates inhibit the
growth of E. carotovora in culture.
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Pollen That Is Raring-to-Go |
As an aid to dispersal, pollen usually becomes strongly dehydrated
at maturity. Rehydration in nature generally only occurs when pollen
grains acquire water from the female, thus enabling pollen tubes to
grow. In this issue, Johnson and McCormick (pp. 685-695)
identify a gametophytic Arabidopsis mutant raring-to-go, which has pollen tubes that germinate precociously within the anther
when the plant is placed in a humid environment. Pollen tubes are
easily visualized by staining for callose with aniline blue.
Raring-to-go mutants are unusual in that they stain for callose prior to anther dehiscence (Fig.
2). The authors describe a new technique
for screening pollen in bulk, and have isolated several other mutants
with unusual callose staining patterns. These mutants will be helpful
in elucidating the molecular processes underlying pollen hydration and
tube growth.
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Figure 2.
The pollen grains of raring-to-go
Arabidopsis mutants have unusual callose staining patterns (blue), and
germinate precociously.
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Flavonoid Biosynthesis Mutants and Auxin Transport |
Flavonoids, perhaps best known as the major red and purple
pigments in flower petals, comprise a diverse group of
phenolic compounds that serve a variety of ecological and
physiological functions. Winkel-Shirley (pp. 485-493)
brings the reader up-to-date on the numerous advances that have
been made in recent years concerning the molecular biology of flavonoid
biosynthesis, particularly in Arabidopsis. A unique and useful feature
of using Arabidopsis in this regard is that all but one of the enzymes involved in central flavonoid metabolism are encoded by a single-copy gene. The genetic loci involved in flavonoid biosynthesis, therefore, can be readily discerned by the study of transparent testa
(tt) mutants that lack the ability to
produce seed coat pigments. Arabidopsis apparently does not use
flavonoids in all the same ways that other species do. For example,
there is little evidence that Arabidopsis uses flavonoids in plant
defense. Nevertheless, these flavonoid biosynthesis mutants of
Arabidopsis are helping to define the roles of flavonoids in other
essential processes such as UV protection and the regulation of auxin
transport. Brown et al. (pp. 524-535) take advantage of
tt mutants to provide new insights into the role of
flavonoids in regulating the polar movement of auxin in vivo. The
authors report that tt mutants that lack functional chalcone
synthase have three times more secondary inflorescence stems,
reduced height, decreased stem diameter, and more and longer secondary roots than do wild type. Auxin transport is elevated in this
tt mutant, and bypassing the block in flavonoid biosynthesis by treatment with the flavonoid precursor naringenin restores the
normal phenotype. Further evidence for a role for flavonoids in the
regulation of auxin transport comes from Peer et al. (pp.
536-548), who use a variety of biochemical and visualization techniques to localize sites of flavonoid accumulation in Arabidopsis. In mature plants, flavonoid accumulation was localized in cauline leaves, pollen, stigmata, floral primordia, and in the stems of elongating inflorescences. The flavonoid precursor accumulation patterns were similar in tt mutants, suggesting that
flavonoids accumulate in the cells in which they are synthesized.
Another developmental abnormality of tt mutants was noted,
namely early flowering under low-light intensity. In toto, the results
suggest aglycone flavonols are important regulators of growth and development.
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Alcohol Dehydrogenase (ADH) on Earth and in Heaven |
Plants undergo pronounced biochemical, physiological, and
anatomical changes in response to hypoxic conditions such as they might
encounter in waterlogged soils, for example. Chief among the
biochemical changes is a switch from glycolysis to fermentation. This
change is accompanied by marked increases in the expression of proteins
involved in fermentative metabolism, including ADH. Anatomical changes
are also induced by hypoxia, most notably the formation of aerenchyma
tissue by the lysis of root cortical cells. This response appears to be
mediated by ethylene. In this issue, Peng et al. (pp.
742-749) provide evidence that ethylene is also involved in the
hypoxic induction of ADH in Arabidopsis. By means of chemical
inhibitors and ethylene signal transduction mutants, the authors
establish that two signaling pathways, one ethylene dependent and one
ethylene independent, are involved in the hypoxic induction of ADH. The
ethylene-dependent pathway, however, appears to be
involved only in the later stages of the response to hypoxia.
Flooded soils are the not the only place where plants face low oxygen
conditions: Hypoxia, arising from a lack of convection-driven gas
movements under conditions of microgravity, is also a stress encountered during space travel. This stress must eventually be overcome if humans are to incorporate plants into biologically based
life support systems for use in future long-term space travel or
extraterrestrial colonization. Paul et al. (pp. 613-621) sent transgenic Arabidopsis plants containing the ADH gene
promoter linked to the 
-glucuronidase reporter
gene into space for 5 d aboard the space orbiter
Columbia. Under hypoxic conditions on Earth, ADH was induced
in the roots and shoot apex, but in space, ADH was induced only in the
roots. These results suggest that caution is necessary in extrapolating
from hypoxic conditions on Earth to those in space. Although the cause
of this discrepancy is uncertain, the authors discuss the possibility
that calcium gradients may be disrupted during space flight, an
intriguing hypothesis given that hypoxia leads to increases in
cytoplasmic calcium. Much like the environment of space, the calcium
blockers ruthenium red and gadolinium inhibited ADH production in the
shoots of plants expressing ADH in their roots.
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Glucosinolates in Insect and...
Pollen That Is Raring-to-Go