 |
Pharmacology of Calcium Spiking Induced by Nod Factors |
The initial events in the establishment of
the N2-fixing Rhizobium-legume symbiosis
involve reciprocal signaling between the plant and its prospective
bacterial partners. Legume roots exude a variety of flavonoid compounds
that activate the transcription of bacterial nod genes.
The products of many nod genes direct the synthesis of a
class of modified lipochitooligosaccharide signaling molecules (Nod
factors). The perception of Nod factors by the prospective host elicits
a range of responses in the root epidermis that includes
periodic, transient increases in cytosolic Ca2+ levels
(Ca2+ spiking). Activation of Ca2+ spiking
shows specificity for Nod factor structures produced by compatible
symbiotic bacteria and is not observed in non-nodulating plant mutants.
In this issue, Engstrom et al. (pp. 1390-1401) report on
their screening of a variety of compounds that modulate the activity of
enzymes known to be components of Ca2+ signaling
in mammalian systems for their ability to alter Nod factor-induced
Ca2+ spiking in Medicago truncatula
root hairs. Their results suggest that
IP3-mediated Ca2+ release
is a conserved feature of Ca2+ spiking in both
mammalian and plant systems.
| |
Different Roles for Catechin Enantiomers Secreted into
Rhizosphere |
The spotted knapweed (Centurea maculosa) is an
economically destructive exotic invader in western North America. It is
a noxious weed that secretes an allelochemical that inhibits the growth of nearby plants. Knapweed roots, growing in culture, also secrete this
allelochemical (Fig. 1). The exudate from
these cultured roots causes the death of a wide variety of plants
within 14 d. In this issue, Bais et al. (pp.
1173-1179) present evidence that the toxic allelochemical
secreted by spotted knapweed roots is (
)-catechin. Although spotted
knapweed roots secrete (±)-catechin, only ()-catechin is
phytotoxic. So why do spotted knapweed roots secrete both
enantiomers? The authors demonstrate that (+)-catechin is
inhibitory to soil-borne bacterial pathogens, whereas the
phytotoxic ()-catechin enantiomer was without effect on any of the
soil-borne pathogens tested.
View larger version (122K):
[in this window]
[in a new window]
|
Figure 1.
The effects of catechin enantiomers on plant
growth. A, ()-Catechin produces a "ring of death" around a
spotted knapweed growing in the wild. B, Neither enantiomer affects the
growth of spotted knapweed in culture. D, Unlike the (+)-enantiomer,
the ()-enantiomer is highly toxic to toadflax (Linaria
dalmatica) growing in culture.
|
|
| |
Mechano-Sensitivity of Ethylene-Insensitive Mutants |
The growth of many plant species is extremely sensitive to
touch. Scientists have identified several components involved in mechanical signal transduction, but there is great uncertainty concerning how these individual links interact in planta. An increase in cytoplasmic Ca2+ is one event that occurs within seconds
following mechanical stimulation, and this increase has been implicated
in the up-regulation of specific touch (TCH)
genes, including TCH3, which encodes for a
calmodulin-like protein. Older studies, however, revealed that many plant species also respond to mechanical stimulation by producing ethylene. There is considerable uncertainty as to how the
Ca2+/TCH branch of the signal transduction
pathway interacts, if at all, with the ethylene branch. It has been
found TCH3 is up-regulated by ethylene independently of
mechanical stimulation, but the simple interpretation of these results,
is confounded by the discovery that the physiological responses to
mechanical stimulation, including the up-regulation of
TCH3 expression, are completely normal in ethylene-insensitive Arabidopsis mutants (ein2 and
etr1). In this issue, Wright et al. (pp.
1402-1409) confirm the fact that a great many
ethylene-insensitive mutants, with the notable exception of
ein6 (an uncloned gene), respond normally to mechanical
stimulation in respect to the up-regulation of TCH3. They
also confirm that ethylene-overproducing mutants and constitutive
triple response mutants respond normally to mechanical stimulation.
These results reveal the necessity of EIN6 for the transduction of
mechanically stimulated TCH3 expression in Arabidopsis.
Evidence is also presented that suggests a role for protein
phosphorylation in controlling the up-regulation of TCH3 expression.
| |
Phosphorylation of a C3 Leaf Pyruvate,
Orthophosphate (Pi) Dikinase |
Pyruvate,Pi dikinase (PPDK) is an important rate-limiting enzyme
of the C4 photosynthetic pathway where it catalyzes the
ATP- and Pi-dependent formation of phosphoenolpyruvate
(PEP) from pyruvate. In C4 plants, PPDK activity is
regulated in a reversible, light-dependent manner that enables the
overall pathway to function optimally. PPDK regulatory protein (RP) is
an unusual, bifunctional kinase/phosphatase that catalyzes this
light-dependent cycle of phosphorylation and dephosphorylation. Whereas
PPDK and RP are found in the chloroplast stroma of C4
plants, PPDK is only present, and at low concentrations, in the
cytosol of C3 plants. Although C3 PPDK is
highly homologous to its C4 counterpart, it is not believed
to function in photosynthesis. Whatever its function, the conversion of
PPDK from a non-photosynthetic role in C3 plants to a
photosynthetic one in the mesophyll chloroplasts of
C4 leaves was a transition repeated independently in
a wide range of angiosperm families during the course of C4
evolution. This implies a more or less common evolutionary
pathway for C4 photosynthesis facilitated by the
pre-existence of the homologs of the C4 cycle enzymes in
C3 plants. In this issue, Chastain et al. (pp.
1368-1378) demonstrate that C3 PPDK in the
leaves of several angiosperms and in isolated intact
spinach (Spinacia oleracea) chloroplasts undergoes
light/dark-induced changes in phosphorylation in a manner similar to
C4 PPDK. They also present evidence that an
RP-like activity mediates the light/dark modulation of the PPDK
phosphorylation state in C3 leaves and likely
presents the ancestral isoform of this unusual and key
C4 regulatory "converter" enzyme.
| |
A Role for Mitochondria in Programmed Cell Death
(PCD) in Plants |
In animal cells, mitochondria play an important role in
triggering programmed cell death (i.e. apoptosis) in response to
diverse stimuli. A key step in this process is the opening of
mitochondrial permeability transition pores (MTPs), which allows for
the release of apoptosis-inducing factor and the translocation of
cytochrome c into the cytosol. In contrast, the evidence
for a role for mitochondria in PCD in plants is less clear. In this
issue, Tiwari et al. (pp. 1271-1281) explore the
question of whether mitochondrially derived
H2O2, the dismutation
product of reactive oxygen species (ROS), is involved in plant PCD.
Increasing evidence points to H2O2 as a major factor in
PCD in plants. H2O2 not
only induces PCD in cultured soybean (Glycine max) and
Arabidopsis cells, but has also been shown in soybean cells to cause
the activation of Cys proteases, enzymes that also play a crucial role
in animal cell apoptosis. Tiwari et al. report that oxidative stress
increases mitochondrial electron transport in non-photosynthetic,
cultured Arabidopsis cells, resulting in the amplification of
H2O2 production, depletion
of ATP, and cell death. The increased generation of H2O2 also causes opening of
MTPs and the release of cytochrome c from mitochondria.
A Ser/Cys protease inhibitor prevents the release of cytochrome
c and the induction of cell death. In short, the evidence
suggests that oxidative stress-induced PCD in non-photosynthetic Arabidopsis cells is remarkably similar to the process of apoptosis of
animal cells.
| |
Polycomb Group (PcG) Genes in Maize (Zea mays) |
During development, the change in the pattern of a gene's
expression is often stably maintained through many mitotic cell divisions even though the transcriptional regulator that effected the
change is present only transiently. In fruitfly (Drosophila melanogaster), PcG proteins help maintain the transcriptional repression of homeotic genes throughout development. All examples of
PcG protein-based repression appear to operate through the formation of
repressive chromatin structures. It is of great interest, therefore,
that there have been an increasing number of reports concerning the
presence of PcG protein-encoding genes in plants. The range of
their activities
from regulating endosperm formation to the induction
of flowering by vernalizationattests to their central importance in
plant development. In this issue, Springer et al. (pp.
1332-1345) present the results of their search for
PcG-like genes in maize. Using the 11 cloned PcG
gene sequences from fruitfly, as well as two PcG homologs
from Arabidopsis, the authors present evidence for the occurrence and
differential expression of three classes of PcG homologs in maize. The
authors propose that the main function of plant PcG proteins may be to
maintain the gene expression patterns determined by developmental
decisions. This type of repression must be reset at meiosis
each generation, whereas repression mediated by DNA methylation
provides a meiotically heritable mechanism for gene silencing.
TOP
Pharmacology of Calcium Spiking...
Different Roles for Catechin...