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Function of a Futile Cycle |
Met
S-methyltransferase (MMT) and homo-Cys
S-methyltransferase (HMT) are involved in a futile
cycle in which Met is converted to S-Methyl-Met (SMM) by
an S-adenosyl-Met (Ado-Met)-dependent methylation
reaction, and SMM is reconverted to Met by HMT. Two hypotheses have
been advanced to justify this seemingly wasteful cycling. The first is
that the SMM cycle prevents overshoots in Ado-Met synthesis from
depleting the free Met pool that is critical for protein synthesis. The
second hypothesis is that the SMM cycle provides a means whereby plants
can control Ado-Met levels in the absence of the feedback loops between
Ado-Met and the enzymes involved in its synthesis that occur in other
eukaryotes. Controlling the levels of Ado-Met and
S-adenoyslhomo-Cys (AdoHcy) is considered crucial to the
many methyl transfer reactions that take place in cells: AdoHcy
is a potent competitive inhibitor of methyltransferases so that the
Ado-Met:AdoHcy ratio (the methylation ratio) determines the activity of
these enzymes. In this issue, Kocsis et al. (pp.
1808-1815) investigated the function of SMM and its cycle by
isolating and characterizing insertional knockout mutants of MMT in
Arabidopsis and maize (Zea mays), both of which have single MMT genes. Both mutants lacked the capacity to produce SMM
and thus had no SMM cycle. Despite the loss of SMM, neither Arabidopsis nor maize mutants appeared to differ from wild-type (WT) plants in
morphology or fertility. Since SMM had been implicated in S transport,
the authors also measured S contents of mutant versus WT seeds but
found no difference. Free Met and thiol pools were unaltered in this
mutant, although there were moderate decreases (of 30% to 60%) in
free Ser, Thr, Pro, and other amino acids. Eliminating SMM did,
however, cause an increase in Ado-Met levels, a decrease in AdoHcy
levels, and a 45% increase in the methylation ratio. These
data indicate that the SMM cycle contributes to regulation of Ado-Met
levels rather than in preventing the depletion of free Met.
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Are Heterotrimeric G Proteins Involved in Phytochrome
Responses? |
Previous studies using pharmacological or
gain-of-function approaches have implicated heterotrimeric G proteins
in phytochrome-mediated red (R) and far-red (FR) light signal
transduction. In analogy to light perception in animals, the results of
these indirect studies were interpreted as supporting the idea that a
heterotrimeric G protein was positioned downstream of phytochrome in
the light signal transduction pathway and upstream of a cGMP-mediated
step. In this issue, Jones et al. (pp. 1623-1627), based on
their examination of the light sensitivities of G protein null mutants of Arabidopsis, cast serious doubt on this popular model of phytochrome signal transduction. Arabidopsis has a single 
-subunit of a
heterotrimeric G protein (GPA1), a single 
subunit (AGB1), and
possibly two 
-subunits. At a superficial level, gpa1 and
agb1 appear to be photomorphogenic mutants because their
hypocotyls are transiently shorter than wild type, and the hooks are
partially open in the dark. However, these phenotypic features can be
traced entirely to a defect in cell division, rather than to a defect
in cell elongation as is the case in other constitutive
photomorphogenic mutants. Single- and double-null mutants for the
GPA1 and AGB1 genes have wild-type sensitivity to
R and FR. Because loss-of-function in the single-copy genes encoding
the - and -subunits of heterotrimeric G-protein does not result
in altered R and FR sensitivity, the predominant theory of the last
decade that phytochrome control of seedling photomorphogenesis involves
a heterotrimeric G protein is not supported.
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A MADS Box Gene Affects Apical Dominance and
Tuberization |
MADS box genes are a family of highly conserved
transcription factors that have diverse roles in floral and vegetative
plant development. In this issue, Rosin et al. (pp.
1613-1622) demonstrate that Potato MADS box 1 (POTM1), a member of the SQUAMOSA-like family of
plant MADS box genes, is most abundant in vegetative meristems of
potato (Solanum tuberosum), accumulating specifically in the
tunica and corpus layers of the meristem, the procambium, the lamina of
new leaves, and newly formed axillary meristems (Fig.
1). To elucidate the function of POTM1,
transgenic plants with suppressed levels of POTM1 mRNA
expression were generated. The suppression of POTM1 mRNA
accumulation produces a phenotype exhibiting reduced apical dominance,
increased lateral growth, induced formation of shoot clusters on the
stem, increased starch accumulation in new leaves, and a reduction in
tuber formation. The POTM1 suppression phenotype has many
similarities to mutants that overproduce cytokinin and, indeed, the
authors show that POTM1 suppression is accompanied by a 2- to 3-fold increase in cytokinin content. The authors also report that
the axillary buds from POTM1 suppression lines produce a
proliferation of shoots in a model tuber system, whereas the axillary
bud from wild-type cuttings produces a single tuber. These results
suggest that POTM1 may be involved in regulating the balance
of growth in vegetative meristems, favoring the development of a
dominant sink organ.
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Figure 1.
A MADS box gene (POTM1) that influences
the balance of growth between vegetative meristems in potato is
expressed in the shoot apical meristem (AP), the axillary meristem
(AX), and the procambium (P).
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Stress Tolerance of Mannitol-Accumulating Wheat (Triticum
aestivum) |
Mannitol, which is not universally produced by plants, has
been reported to mitigate abiotic stress in several dicot species. In
this issue, Abebe et al. (pp. 1748-1755) introduced an
Escherichia coli gene for a mannitol-synthesizing
enzyme into wheat, a monocot species that normally does not synthesize
mannitol. Their purpose was to evaluate whether the transgenically
introduced ability to produce mannitol would improve the
tolerance of wheat to water stress and salinity. MtlD
encodes for mannitol-1-phosphate dehydrogenase, an enzyme that
catalyzes the conversion of Fru-6-phosphate to mannitol-1-phosphate
that is, in turn, converted to mannitol via nonspecific phosphatases.
Their results demonstrate that low levels of mannitol improve the
growth of transgenic wheat under water stress and salinity both at the
callus and whole-plant levels. The amounts of mannitol that accumulated
in the calli and mature leaves, however, were too small to protect
against stress through osmotic adjustment. They conclude that the
improved growth performance of mannitol-accumulating calli and mature
leaves was due to other stress-protective functions of mannitol such as
the OH· scavenging and/or improved stability of
macromolecular structures. The plant line used in their study
accumulated only 0.7 µmol g
1 fresh weight
mannitol in the flag leaf under unstressed conditions. This was the
highest amount of mannitol accumulated without causing any noticeable
side effects in transgenic wheat. Plants that accumulated higher
mannitol had severe abnormalities including sterility, stunted growth,
twisted heads, and curled leaves. These results point to the need to
carefully optimize the use of existing osmoprotectant-based mechanisms
and to explore the development of alternative engineering strategies,
such as the use of stress-inducible expression systems for stress
tolerance determinants, which lack potential detrimental effects on growth.
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Immunolocalization of Fucosylated Xyloglucan in
mur1 |
The Arabidopsis cell wall mutant mur1 was
originally isolated in a screen of leaves for changes in
monosaccharide composition. Chemical analyses of the
mur1 walls detected only trace amounts of Fuc
(Fuc) in the shoots, whereas Fuc levels in the roots were reduced by
40% compared with wild-type plants. In this issue, Freshour et
al. (pp. 1602-1612) examine the immunolocalization of a
monoclonal antibody called CCRC-M1 in mur 1 mutants. CCRC-M1 recognizes a Fuc-containing epitope found principally in the cell wall
polysaccharide xyloglucan. In a previous study, they demonstrated that
this epitope is present in almost all cell walls of wild-type seedlings. Here, they report that the localization of CCRC-M1 in
mur1 mutants is cell and tissue specific. The pattern of
CCRC-M1 labeling in mur1 plants reflects genetic redundancy
in the de novo synthesis of L-Fuc. The mutant
mur1 carries a missense mutation in the GMD2
gene, which results in the expression of a nonfunctional form of
GDP-D-Man 4,6-dehydratase, an enzyme required for
the biosynthesis of Fuc. A second gene, GMD1, with
significant sequence similarity to GMD2, is also present in
the Arabidopsis genome and has been shown recently to encode a second
isoform of GDP-D-Man 4,6-dehydratase. Thus,
the pattern of Fuc incorporation into xyloglucan in mur1
plants as recognized by the CCRC-M1 antibody likely results from the
expression of GMD1 activity in specific cells and at specific times
during the plant's growth and development. GMD1 could be functionally
redundant with GMD2 to ensure fucosylation of cell wall polymers that
are critical for the cell wall function of specific cell types within
the plant. In this regard, it is interesting to note the tissues in
which GMD1 expression is apparent in mur1 plants.
These include the two meristematic tissues present in roots, (the
apical meristems and the pericycle), and the only two cell types in
plants known to expand via tip growth (root hair cells and pollen
tubes). Perhaps the unique characteristics of these tissues and cells
require the fucosylation of one or more of the macromolecules that make
up their walls.
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