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Liverwort Isoprenoid Production |
Among the Bryophytes, only the
liverworts possess oil bodies containing large quantities of essential
oils. Liverwort oil bodies are cytoplasmic structures bound by a single
membrane that originates from the dilation of ER cisternae. They have
no structural equivalent in the seed plants, the oil bodies of which
accumulate mostly acyl lipids and are surrounded by a monolayer of
phospholipids containing basic proteins. Biochemical
studies have indicated that the isoprenoid biosynthetic pathways in
liverworts are similar to those of the seed plants, but the isolation
and purification of intact liverwort oil bodies is difficult and
precludes direct analysis of their enzyme component. Suire et al.
(pp. 971-978) have been able to circumvent this problem using a
cyto-immunological approach employing antibodies for several key
enzymes involved in isoprenoid biosynthesis. The immunolocalization of
these enzymes in Marchantia polymorpha indicate that the
enzymes equivalent to the plastid and cytosolic enzymes involved in
isoprenoid biosynthesis in seed plants are found in the oil bodies of
liverworts (Fig. 1). The
liverwort oil body, therefore, represents a unique structure in which isoprenoid production and storage are combined.
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Transgenic Proteins from the Tap |
Transgenic protein isolation may someday be as easy as
placing a beaker in a growth chamber, going home for a nice dinner, and
returning the next morning to find the beaker filled with a simple
solution rich in the protein of your choice. At least the possibility
is raised by the results of Komarnytsky et al. (pp.
927-933), who report upon their initial success in getting
transgenic tobacco plants to excrete three heterologous proteins by
means of leaf guttation. The heterologous proteins (ER signal peptides
fused to bacterial xylanase, jellyfish green fluorescent protein, or
human placental alkaline phosphatase) accounted for almost 3% of the
total soluble protein in the guttation fluid. This
technology has the potential of enhancing the efficiency of recombinant
protein production technology by increasing yield, abolishing
extraction, and simplifying purification. Tobacco may not even be the
best species for this purpose: there exist even weepier species in
which collection might be more efficient.
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Genomics of the Glutathione S-Transferase (GST) Gene
Family |
The GSTs are a huge multigene family present at every stage
of plant development. Although they presumably function in nature to
protect the cell from oxidative damage, they have been most intensively
studied in relation to their ability to sequester xenobiotics. Several
major classes of herbicides are rendered innocuous after conjugation
with glutathione. In this issue, McGonigle et al. (pp.
1105-1120) report on their use of BLAST searches to identify 25 soybean and 42 maize GST sequences
probably the majority of GSTs
expressed in these species. Plant GSTs are typically divided into
three types. Types I and III are similar to the mammalian theta class
and type II is similar to the mammalian zeta class. Type
II GSTs turn out to be poorly represented both in variety of
individual genes (two in maize and one in soybean) and in absolute expression levels. Sequence comparisons of 66 GST sequences from both
maize and soybean revealed some notable features. There are three amino
acids that are absolutely conserved in each case, most notably a Ser in
the active site of the enzyme. Type III GSTs also have four
conserved sequence patches mapping to distinct structural features
(Fig. 2). Using DNA microarray analysis, the authors observed increased
expression among the type III GSTs in maize in response to seven
different inducer treatments. Different genes responded differently to
different treatments. A GST active with one substrate
generally exhibited some activity with all the others, suggesting broad
individual enzyme substrate specificity.
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Polyamine Regulation of Guard Cell K+
Channels |
Polyamines have been implicated in the responses of
plants to a wide variety of abiotic stresses, including
K+ deficiency, drought, salt stress, and air
pollution. Despite numerous reports of a correlation between stress and
polyamine titer, the physiological rationale for stress-induced
polyamine accumulation remains unknown. In other organisms, there is
accumulating evidence that polyamines may play a role in ion channel
regulation. In animal cells, certain types of inward-rectifying
K+ channels,
N-methyl-D-aspartate receptors, and
voltage-activated Ca2+ channels have been shown
to be affected by intracellular polyamine levels. Recent studies have
also implicated periplasmic polyamine levels in the regulation of porin
activity in the outer membranes of Escherichia coli. In this
issue, Liu et al. (pp. 1315-1325) extend these findings to
include the voltage-dependent inward K+ channels
in the plasma membrane of Vicia faba guard cells. Patch clamp recordings reveal that polyamines inhibit the guard cell K+ channel from the cytoplasmic side. Consistent
with this finding, the authors report that polyamines prevent
light-induced stomatal opening. The authors also found that polyamines
block the inward K+ current induced in tobacco
mesophyll cells by the expression of the Arabidopsis KAT1 gene (a
putative guard cell K+ channel). This model
system should prove most useful in elucidating the mechanism of
polyamine inhibition.
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Xylem Embolisms: An Un-Canny Interpretation of Cryo-Scanning
Electron Microscopy (Cryo-SEM) |
Not too along when plants were still essentially a "black
box," plant biologists could at least take some pride in the fact that they understood the ascent of sap. The
"Cohesion-Tension" theory, first proposed in 1894, seemed almost
unassailable. A century later, however, Martin Canny put forth an
intrepid new hypothesis that threatened to overturn long-held notions
concerning plant water relations. Much of Canny's arguments were based
upon results garnered by two recently developed techniques: the direct measurement of sap pressure with xylem pressure probes and direct cryo-SEM observations of xylem vessels. Canny's hypothesis did what
any good hypothesis does: it sent scientists back to the lab.
Unfortunately, the new studies that have emerged have not been
favorable to Canny's hypothesis. Previously, in this journal, Wei et
al. (1999)
provided evidence that called into question the anomalous
xylem pressure probe studies. Now, in this issue, Cochard et al.
(pp. 1191-1202) turn their attention to the anomalous cryo-SEM
investigations. They confirm the earlier cryo-SEM findings that
seemingly indicate, contrary to the predictions of the
"Cohesion-Tension" theory, that a large percentage of vessels become transiently embolized during the diurnal peak in transpiration. Cochard et al., however, find no evidence for a
corresponding diurnal trough in the hydraulic conductivity of the
petioles that would be expected if these embolisms were real. One
of the techniques has to be wrong. The authors present their case
that artifactual cavitation may occur in vessels when their sap is
frozen under high negative pressure such as occurs during cryo-SEM preparation.
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Both Ethylene and ABA Involved in High Auxin Growth
Inhibition |
It has long been known that high concentrations of auxin, both
natural and synthetic, give rise to enhanced ethylene production. Epinastic growth abnormalities and the inhibition of root and shoot
growth are some of the symptoms induced by high auxin. Previously, Klaus Grossman and his colleagues have called attention to the fact
that this auxin-induced increase in ethylene production is soon
followed by a massive accumulation of ABA in the shoot, and that
ethylene inhibitors prevent this increase in ABA. Since ethylene and
ABA are both growth inhibitors, the question arises as to which of the
hormones underlie the high auxin-induced inhibition of growth? In this
issue, Hansen and Grossman (pp. 1437-1448) shed light on
this question by comparing the effects of high auxin on the inhibition
of shoot growth in two tomato mutants: the
ethylene-perception mutant never-ripe and the ABA-synthesis
mutant flacca. Both mutants showed less auxin-induced
growth inhibition than did the wild types, suggesting a role for both
hormones in auxin-induced growth inhibition. As expected, the two
mutants also did not show the usual large increase in ABA or its
precursor xanthoxal in response to high auxin treatment.
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Liverwort Isoprenoid Production
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