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NADP Malic Enzymes in C3 and C4
Flaveria Species |
The genus Flaveria
is unusual in containing C3, C4, and
C3-C4 intermediate species. This diversity
makes Flaveria an interesting subject for studying the
molecular events that have accompanied the evolutionary transition from
C3 to C4 photosynthesis. In the most common
C4 pathway for C fixation, NADP-malic enzyme (NADP-ME) is
involved in decarboxylating malate in the chloroplasts of bundle sheath
cells. Isoforms of plastidic NADP-ME are encoded by two genes in all
species of Flaveria, including C3,
C4, and C3-C4 intermediate types.
In this issue, Lai, Wang, and Nelson (pp. 125-139) report
that only one of these genes (ChlME1) encodes for the
isoform involved in C4 photosynthesis. A
comparison of the expression patterns of ChlMe1 and
ChlME2 genes in developing leaves of
C3 and C4 Flaveria
species revealed that in C4 species, ChlMe1 is expressed non-specifically early in leaf
development and becomes bundle sheath-specific as leaves mature (Fig.
1). In C3 species,
however, ChlMe1 is only transiently expressed early in leaf
development. In contrast, ChlMe2 expression occurs only transiently during chloroplast development in both
C3 and C4 species, possibly
serving to provide a burst of NADPH and pyruvate for protein and lipid
synthesis during chloroplast biogenesis. These results indicate that
during the course of C4 evolution, the expression pattern of ChlMe2 remained constant, while the expression
pattern of ChlMe1 changed markedly.
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Figure 1.
Expression of one of the forms of chloroplastic
NADP-ME in the bundle sheath cells of the mature leaves of a
C4-type Flaveria species indicates its
involvement in C4 photosynthesis.
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A companion paper by Lai, Tausta, and Nelson (pp. 140-149)
examines the role of cytosolic NADP-ME in Flaveria. They show that the gene CytMe encodes for cytosolic NADP-ME in
all Flaveria species regardless of the species' mode of
photosynthesis. Based on the expression pattern of CytMe,
the authors propose that cytosolic NADP-ME has several distinct roles
in plants, including the supplying of NADPH for cytosolic metabolism,
the balancing of cellular pH in illuminated leaves, and in providing
reducing agents and carbon metabolites during wound repair.
CytMe transcripts of different size appear to be involved in
these three different processes.
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Nitric Oxide: A Key Link in Abscisic Acid-Induced
Stomatal Closure |
The process by which abscisic acid (ABA) induces stomatal
closure has been intensively studied, but despite great effort, our
understanding of the ABA signal transduction mechanism in guard cells
is far from clear. Perhaps some important pieces of the puzzle are
provided by Neill et al. (pp. 13-16), who present
pharmacological evidence that nitric oxide (NO) plays a critical role
in ABA-induced stomatal closure in pea (Pisum sativum). NO
causes stomatal closure, and both inhibitors of NO synthesis and NO
scavengers block ABA-induced stomatal closure. Neill et al. also
employed diaminofluorescein diacetate (DAF-2 DA), a fluorescent
indicator probe to visualize NO levels in guard cells under various
pharmacological treatments. The application of ABA increased DAF-2 DA
fluorescence in pea guard cells, and this increase was prevented by
pretreatment with either a NO scavenger or an inhibitor of NO synthesis
(Fig. 2). Because NO signaling commonly
involves the production of the second messengers cyclic GMP and
cADP-Rib, the authors also studied the respective effects of a specific
inhibitor of NO-sensitive guanylate cyclase and an antagonist of
cADP-Rib on stomatal aperture. Neither of these inhibitors alone had an
effect on stomatal aperture, but both inhibited ABA- and NO-induced
stomatal closure. The authors propose that NO is a key link in
ABA-induced stomatal closure, and that ABA- and NO-induced stomatal
closures require the synthesis and action of cyclic GMP and
cADP-Rib.
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Figure 2.
The fluorescent indicator probe DAF-2 DA
reveals NO synthesis in pea guard cells under different pharmacological
conditions: a, control; b, ABA; c, ABA and NO scavenger; d, ABA and NO
synthase inhibitor.
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New Light on the Functions of Phytochromes |
The perception of red (R) and far-red (FR) light by various
phytochromes affects the growth and development of a plant throughout its life. In Arabidopsis, phytochrome is a small gene family consisting of five members, PHYA through PHYE.
Individual phytochrome family members have both partially overlapping
and distinct functions. In the case of the photoregulation of hypocotyl
elongation, phyB mediates the classic R/FR photoreversible low fluence
response (LFR). In contrast, phyA mediates two other types of
responses: the high irradiance response (HIR) that requires sustained
exposure to FR, and the very-low-fluence response (VLFR) that is
mediated by brief exposures to radiation between 300 and 780 nm. In
this issue, Luccioni et al. (pp. 173-181) report on their
studies of the relative magnitudes of HIR, VLFR and LFR responses in
different accessions of Arabidopsis. Their analysis reveals a
significant negative correlation between VLFR and LFR or HIR. The
authors also provide tantalizing evidence that brassinosteroids may be part of the "switch" mechanism that adjusts plant sensitivity to
light by means of these different phytochrome responses. A mutant that
displays an enhanced VLFR but reduced HIR and LFR was found to be
allelic to a brassinosteroid biosynthesis mutant. The enhancement of
VLFR by this mutation was lost in seedlings not expressing functional
phyA. The authors suggest that brassinosteroids may play a role in fine
tuning a plant's repertoire of phytochrome-mediated responses to best
suit the growth and development of the plant under the light conditions
it encounters.
In contrast to our insight into the functions of phyA and phyB, much
less is known about the function of other phytochromes. In this issue,
Hennig et al. (pp. 194-200) report that phyE plays a role
in controlling photo-induced seed germination in Arabidopsis. Previous
studies have shown that both phyA and phyB mediate the photo-induction
of seed germination by R light whereas the induction of seed
germination by FR light is mediated only by phyA. However, a role for
other phytochrome members in this process was indicated by the fact
that phyA phyB double mutants still demonstrated
R/FR-reversible induction of seed germination. Hennig et al. employed a
set of photoreceptor mutants to test whether phyD or phyE or both can
control photo-induced germination. Their results indicate that only
phyB and phyE participate directly in R/FR reversible germination, but
that phyE, unlike phyB, does not inhibit phyA-mediated germination. In
fact, phyE is required for germination of Arabidopsis seeds in HIR
conditions. This interaction of phyE with phyA, however, is not
observed in other HIR responses, including the induction of cotyledon
unfolding or agravitropic growth.
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A Papain Ortholog Expressed in Differentiating Xylem Elements |
Tonoplast rupture releases vacuolar contents into
the cytoplasm of differentiating tracheary elements and is rapidly
followed by cell death. Hydrolytic enzymes released during this process continue the post-mortem digestion of the cell. A Cys peptidase (XCP1)
that is homologous to papain has previously been detected in
Arabidopsis, and it is localized exclusively in the xylem. To determine
whether XCP1 could be involved in tracheary element autolysis,
Funk et al. (pp. 84-94) investigated the localization of XCP1 using XCP1 promoters fused to 
-glucuronidase
and immunofluorescent confocal microscopy. Their results indicate that
XCP1 is localized in the in the vacuole, consistent with it playing a
role in tracheary element differentiation. The ectopic expression of
XCP1 resulted in a range of phenotypes, with the most
severely affected lines exhibiting stunting, increased anthocyanin
levels, and early leaf senescence. The authors also present an
intriguing hypothesis that the differentiation of laticifers may simply
be a variation of the emerging model of tracheary element
differentiation. They point out that differentiating tracheary elements
and laticifers have many features in common, including their
occurrence in the xylem, their accumulation of high levels of
hydrolytic enzymes, and their formation of intercellular connections
through end-wall perforations. Within the laticifer protoplast,
however, only vesicles of ER origin are retained as the rest of the
internal organelles, including the vacuole, become broken down. In
laticifers, papain is localized in the ER vesicles, not in the central
vacuole. Perhaps because of this, the complete autolysis of laticifers
is prevented, and enzymes identical or paralogous to those used to
catalyze the final steps of tracheary element autolysis are employed in laticifers as part of a pressurized defense network that is poised for
the quick release of defensive peptidases.
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Systemic Induction of a Ca2+-Dependent Protein
Kinase (CDPK) |
Plants undergo systemic physiological changes in response to
local injuries caused by insects, pathogen attack, or mechanical wounding. The systemic wound-induced response is regulated by chemical
factors including abscisic acid, jasmonic acid, oligosaccharides and
the octadecapeptide systemin, and by physical signals such as hydraulic
variation potentials and electrical activation potentials. An important
step in the signal transduction pathways of many of these chemical and
physical factors is a transient increase in cytoplasmic
Ca2+ levels, and the activation of CDPKs. In this issue,
Chico et al. (pp. 256-270) report upon their isolation of a
cDNA clone (LeCDPK1) from tomato (Lycopersicon
esculentum) that encodes for a CDPK. LeCDPK1 was rapidly and
transiently enhanced in detached tomato leaves treated with pathogen
elicitors or H2O2.
Moreover, a systemic increase in LeCDPK1 mRNA was
detected upon wounding, and this was correlated with an increase in the
activity of a soluble CDPK. These results suggest that the
up-regulation of LeCDPK1 is an integral part of tomato's defense
against both biotic and abiotic attacks.
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NADP Malic Enzymes in...
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