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Glyphosate-Resistant Goosegrass (Eleusine indica) |
Glyphosate is the active
ingredient of Roundup, the most extensively used foliar-applied,
broad-spectrum herbicide. This herbicide is effective against the
majority of annual and perennial grasses and broad-leaved weeds.
Glyphosate competitively inhibits the enzyme
5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which catalyzes an important step in the shikimic acid pathway. Recently, a
glyphosate-resistant (R) biotype of goosegrass was identified in
Malaysia that exhibited 2- to 4-fold greater resistance to glyphosphate than did a sensitive (S) biotype collected from the same region. This is an alarming development because goosegrass (Fig.
1), a problem weed in 46 different crop
species in more than 60 countries, is considered to be one of the
world's five worst weeds. In this issue, Baerson et al. (pp.
1265-1275) compare the inhibition of EPSPS activity by glyphosate
in extracts prepared from the R and S biotypes, and provide compelling
evidence that the site of glyphosphate resistance is the EPSPS enzyme
itself. Sequence comparisons of the predicted EPSPS mature protein
coding regions from both biotypes revealed four single-nucleotide
differences, two of which result in amino acid changes. One of these
changes, a Pro-106 to Ser-106 substitution in the R biotype,
corresponds to a substitution previously identified in a
glyphosate-insensitive EPSPS enzyme from Salmonella
typhimurium. EPSPS-deficient strains of Escherichia
coli expressing the mature EPSPS enzyme from the R biotype
exhibited an approximately 3-fold increase in glyphosate tolerance
relative to strains expressing the mature EPSPS from the S biotype.
These results provide the first evidence for an altered EPSPS enzyme as
an underlying component of evolved glyphosate resistance in any plant
species.
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Genomics of a Model Diatom |
Diatoms are major components of marine phytoplankton, and are
important for the biogeochemical cycling of minerals such as silica,
and for global carbon fixation. However, very little is known about the
cell biology of diatoms or their genomic structure. Phaeodactylum tricornutum (Fig.
2) has been proposed as a model diatom
species because of its ease of culture, short generation time, and
amenability to genetic transformation. Scala et al. (pp.
993-1002) generated approximately 1,000 expressed sequence tags
(ESTs) in order to examine its genome composition. They present evidence that the genome of P. tricornutum is small (<20
Mb). Although more than 60% of the sequences could not be
identified by similarity to sequences in the databases, approximately
20% had high similarity with a range of genes defined functionally at
the protein level. It is interesting that many of these sequences are
more similar to animal than to plant counterparts. For example, diatom
ESTs could be found with similarity to all the major components of the
extracellular matrix of mammalian cells. These results are likely a
reflection of the different phylogenetic histories of diatoms and
higher plants, and suggest that much of the repertoire of diatom genes
derives from the ancestral heterotrophic flagellate that was the host
for the secondary endosymbiosis.
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Figure 2.
The model diatom species P. tricornutum
can be genetically engineered to produce foreign proteins such as green
fluorescent protein. ©Anton Montsant and Fabio Formiggini.
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Chemical Induction of Gene Expression
by Ethanol Vapor |
Many gene expression systems rely on induction by
chemicals such as tetracycline and dexamethasone. Such systems allow
gene activity to be induced in the plant at defined times during
development, thereby avoiding problems that may be associated with
constitutive overexpression. Although extremely useful, these systems
can have adverse effects in some plant systems. The alc
gene expression system, which is based on a regulon from
Aspergillus nidulans, uses ethanol as an inducer. The
induction of gene expression after the uptake of ethanol via the roots
and after foliar sprays has been extensively studied using the
alc system. In this issue, Sweetman et al. (pp.
943-948) report that low concentrations of ethanol vapor
efficiently induce the alc gene expression system in tobacco
(Nicotiana tabacum), potato (Solanum tuberosum),
and oilseed rape (Brassica napus). Ethanol vapor treatment
may be preferable because it avoids exposing plants to comparatively high concentrations of ethanol from foliar sprays or root drenches. By
exposing different parts of a transgenic plant to ethanol vapor, the
authors show that it is possible to target expression of the alc system to particular organs. Labelingexperiments show
that ethanol is not readily translocated within the plant. Confined vapor treatment provides a powerful technique for inducing
organ-specific gene expression. The alc system may prove to
be a powerful technology for fundamental research and has the potential
for applied uses. Ethanol is inexpensive and biodegradable, and its
vapor may be particularly useful in controlling gene expression in
postharvest crops such as potato, cut flowers, or fruit.
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A Role for Nitric Oxide (NO) in Adventitious
Rooting |
Although auxins have long been
used to stimulate the adventitious rooting of plant cuttings, the
molecular mechanisms by which auxins promote this process are poorly
understood. In this issue, Pagnussat et al. (954-956)
present pharmacological evidence that NO may be a component of the
auxin signal transduction pathway involved in the adventitious rooting
of excised cucumber (Cucumis sativus) hypocotyls.
They report that the NO donors sodium nitroprusside (SNP)
and S-nitroso, N-acetyl penicillamine (SNAP)
mimic the effects of auxin in inducing adventitious rooting. These
NO-mediated effects were prevented by the simultaneous addition of a
specific NO scavenger. Electron paramagnetic resonance measurements
revealed that a transient increase of NO could be measured in explants
after 24 h of treatment with IAA, whereas the level of endogenous NO
dramatically shut down at 24 h in control explants. Endogenous NO
was also measured by means of an NO-specific fluorescent probe. The
auxin-treated explants displayed 4-fold more fluorescence
than did control explants. Although a direct effect of NO cannot be
discarded, these findings indicate that NO may be involved in mediating
auxin-induced rooting in cucumber. The involvement of NO in this
auxin-signaling pathway opens a wide field of research for all auxin effects.
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mRNA Gradients in Acetabularia acetabulum |
The unicellular green alga Acetabularia,
because of its large size and distinctive polar morphology, has long
been used as a model system for cellular studies. The giant cell forms
an elongated tube (a stalk of up to 60 mm in length), which at its
apical pole makes whorls of hairs, followed by a cap. At its basal
pole, the cell extends into a rhizoid wherein the single nucleus is
positioned. Classical grafting experiments led to the conclusion that
"morphogenetic substances" are released from the nucleus into the
cytoplasm where they elicit cellular morphogenesis. A major feature of
these morphogenetic substances was their extreme stability, because
enucleated cells could perform stalk, hair, and cap formation for
several weeks after removal of the nucleus. Based on the
differential morphogenetic capacity of anucleate apical, middle, and
basal cell fragments, the existence of apical/basal gradients of these
morphogenetic substances was postulated. Subsequent experiments favored
the hypothesis that the morphogenetic substances of A.
acetabulum are mRNAs. Vogel et al. (pp. 1407-1416)
studied the levels of specific mRNAs in the apical, middle, and basal
regions of A. acetabulum. Some mRNA classes are uniformly
distributed, others show development-specific patterns of distribution,
and still others show apex-to-base or base-to-apex gradients.
Surprisingly, calmodulin-4 shows an apex-to-base gradient, whereas
calmodulin-2 shows a gradient in the opposite direction.
Pharmacological evidence suggests that actin microfilaments are
involved in the directed transport and/or anchoring of these mRNAs.
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Genomic Imprinting and Cold Germination and
Desiccation Tolerance in Maize (Zea mays) |
Northern maize-growing regions have a short
crop-growing season that requires early planting and early
harvesting to avoid frost injury. Therefore, it is important for maize
seed to tolerate desiccation when harvested at high seed moisture and
to retain the ability to germinate under cold conditions for better
field stand establishment. Extensive efforts have been devoted to
understanding the genetics and physiology of the cold germination and
desiccation tolerance traits in maize. Previous studies have
shown that several endosperm genes are affected by genomic imprinting;
that is, their exposure depends on whether the allele is inherited from
the male or female parent. Kollipara et al. (pp. 974-992)
report on the differential expression of imprinted genes that may be involved in conferring cold germination and desiccation tolerance. The
authors screened recombinant inbred lines for cold germination and
desiccation tolerance. Reciprocal F1 hybrids were
made from divergent recombinant inbred lines. The reciprocal
F1 hybrids that showed differential phenotypes
for cold germination and desiccation tolerance, an indication of
imprinting, were profiled for mRNA and protein expression using
open-ended platforms. An mRNA profiling method identified 336 of 32,496 and 656 of 32,940 cDNA fragments that showed ~1.5-fold change in
expression between the reciprocal F1 hybrids for
cold germination and desiccation tolerance, respectively. Based on
protein-profiling analyses by PEM, a subset of these proteins was
identified by tandem mass spectrometry followed by a database query of
the spectra, and the differentially expressed genes/proteins were
classified into various functional groups. These results provide
valuable insights into the genes involved in stress response during
seed germination and maturation.
TOP
Glyphosate-Resistant Goosegrass...
Genomics of a Model...
one of the world's five worst weeds.
©Weed Science Society of America.