Chemistry Reference
In-Depth Information
stomatal closure, CK and IAA promote stomatal opening. And we mentioned before, NO
acts as a key intermediate in the ABA-mediated signaling network in stomatal closure.
Moreover BR-mediated signaling was regulated by ABA, and in turn, ABA was also shown
to inhibit BR-induced responses under abiotic stress (Divi U et al., 2010). And it is not hard
to deduce that there are other tricky relationships between different hormone-involved
pathways. Cross-talk between the phytohormones results in synergetic or antagonist
interactions, which is crucial for plants in abiotic stress responses. To characterize the
molecular mechanisms regulating hormone synthesis, signaling, and action means a lot to
modificate hormone biosynthetic pathways to develop transgenic crop plants with
promoted tolerance to abiotic stress.
2.3. Ca
2+
as an intermediate signal molecule
So far, we have already touched some grounds related to Calcium (Ca
2+
) functioning in
signal transduction. Among all ions in eukaryotic organisms, it is likely to be the most
versatile one who almost links to all aspects of plant development, not to mention many
regulatory processes. The reason why it is so powerful may root in its flexibility in
exhibiting different coordination numbers and complex geometries, and this ability makes it
easily form complexes with proteins, membranes, and organic acids. However we won't
detect high cytosolic or organelle Ca
2+
concentrations resulting from the tight management
of various Ca
2+
pumps and transporters. The reason why the concentration needs to be
controlled is that higher Ca
2+
concentrations can chelate negatively charged molecules in the
cell leading to cytotoxicity. Interestingly, all the secondary signaling molecules we
mentioned above may activate transient increases in cytosolic Ca
2+
, and transient elevations
in cytosolic Ca
2+
concentration have been documented to have relationship with a multitude
of physiological processes linking to abiotic stress responses. So we may wonder
concentration control probably will help us tell the story of another famous second
messenger--- Ca
2+
in signal transduction for plants under abiotic stress.
Earlier in 1982, research on the green algae Chara told us the cytosolic Ca
2+
concentration
change predicted Ca
2+
might work as a second messenger in plants (Williamson and Ashley,
1982). Based on later reports, it has been found various stimuli will spur their own special
Ca
2+
responses differing in where and how changes happen (Johnson et al., 1995; Tracy et al.,
2008), which exactly supports the former concept of Ca
2+
signature. For plants, to maintain
Ca
2+
homeostasis, they need the help from Ca
2+
channels, pumps, and exchangers (carriers)
to make specific adaptation to every kind of stimulus (Kudla et al., 2010). Later, cellular Ca
2+
signals are decoded and transmitted by Ca
2+
-binding proteins that relay this information
into downstream responses. Major Ca
2+
signal transduction routes contain Ca
2+
-regulated
kinases mediating phosphorylation events and regulation of gene expression via Ca
2+
-
regulated transcription factors and Ca
2+
-responsive promoter elements.
Generally speaking, Ca
2+
signaling comprises three phases: generation of a Ca
2+
signature,
sensing the signature and transduction of the signal (Reddy and Reddy, 2004). Having
discussed above, we are informed that the concentration change are always triggered by
