cific genes by Ca^2 is due to regulation of phosphorylation of transacting factors by Ca^2 /CaM-depen-
dent protein kinase II (CaM K II) [291] or Ca^2 /phospholipid-dependent protein kinase (protein kinase
C) [295]. Calcium influences translation at the initiation as well as elongation steps. One of the elonga-
tion factors (eEF-2, eurkaryotic elongation factor-2) is a substrate for a Ca^2 /CaM-dependent protein ki-
nase III (CaM K III) and phosphorylation of eEF-2 makes it inactive, resulting in the inhibition of trans-
lation [296–298]. Depletion of Ca^2 results in a decrease in the rate of initiation of protein synthesis
[299–301]. Inhibition of protein synthesis by Ca^2 depletion is correlated with dephosphorylation of a ri-
bosome-associated protein (26 kDa) [302].
Although there is a great deal of information on the involvement of Ca^2 in regulating various phys-
iological processes [12,13], very little is known about the role of Ca^2 in regulating gene expression in
plants. Manipulation of [Ca^2 ]cytby various means is shown to affect the expression of specific genes in
plants. Lam et al. [303] presented some evidence indicating that Ca^2 and CaM mediate the induction of
expression of chlorophyll a/bbinding (cab) genes. Ten percent of light-induced cab mRNA could be in-
duced in the dark by increasing the intracellular level of Ca^2 by ionomycin [303]. Microinjection stud-
ies with tomato phytochrome mutant (aurea) clearly show that Ca^2 is involved in the expression of spe-
cific genes [91]. In the aureamutant, light-regulated genes are not expressed. Injection of a plasmid
construct containing cab promoter, a light-responsive promoter, fused to GUS reporter (cab-GUS) into
cells of the aureamutant showed no expression of GUS gene. However, coinjection of cab-GUS with
Ca^2 or Ca^2 -activated CaM resulted in the expression of the GUS gene. These results suggest the in-
volvement of Ca^2 and CaM in regulating the cab promoter activity [91]. Partial development of chloro-
plasts in the aureamutant, which requires the expression of several genes, could be obtained by mi-
croinjection of Ca^2 and CaM into these cells [304]. These results indicate that Ca^2 regulates the
expression of genes involved in chloroplast development.
InArabidopsis, CaM and CaM-related genes (TCH1,TCH2,TCH3, and TCH4) are strongly induced
by various mechanical stimuli such as touch and wounding [100]. It has been demonstrated that increased
external Ca^2 or heat shock rapidly induced the expression of touch-induced CaM-related genes (TCH2,
TCH3, and TCH4), whereas the TCH1gene, which codes for CaM, is not significantly induced [212].
Heat shock, in the presence of EGTA, a Ca^2 chelator, did not show induction of TCHgenes. This EGTA
effect is reversed by Ca^2 replenishment. Based on these results, it was suggested that heat shock elevates
[Ca^2 ]cytlevels, which in turn regulates the expression of TCH2,-3, and -4genes. Heat shock is known
to increase cytosolic Ca^2 in animal cells [305–307]. Evidence for the involvement of Ca^2 in heat shock
stress in plants has been obtained from various studies [45,46,92,93]. The calcium effect on touch-in-
duced CaM-related genes is specific because magnesium, another divalent ion, did not have any effect.
Furthermore, increased Ca^2 did not affect the expression of the heat shock–induced gene. In the case of
Arabidopsis, the expression of one of the Camgenes (TCH1) is not significantly affected by an increase
in external Ca^2 . There are multiple Camgenes in Arabidopsisand it is not known whether any of these
genes are affected by changes in [Ca^2 ]cytlevels [108,216,217].
Studies show that Ca^2 is involved in stress-induced (both abiotic and biotic) gene expression. Plant-
pathogen interaction, chemical elicitors, and a number of other stress factors have been shown to stimu-
late ethylene production in plants [308]. Ethylene is involved in the expression of some of the pathogen-
esis-related proteins including a chitinase. Depletion of Ca^2 by Ca^2 chelator blocked ethylene-induced
chitinase synthesis, whereas artificial elevation of cytosolic Ca^2 with Ca^2 ionophore (ionomycin) or an
inhibitor of microsomal Ca^2 -ATPase induced chitinase synthesis in the absence of ethylene [309]. These
results indicate that Ca^2 mediates the induction of the chitinase gene by ethylene. More recently, it has
been shown that ethylene induced the phosphorylation of specific proteins although it is not known
whether Ca^2 is involved in this ethylene-regulated protein phosphorylation [310]. Using inhibitors of
protein kinases and protein phosphatase, it was concluded that protein phosphorylation is one of the in-
termediate events involved in ethylene signal transduction. Fungal elicitors that are known to induce
pathogenesis-related proteins have been shown to elevate cytosolic Ca^2 [26].
In some plants, freezing tolerance can be developed by exposing them to nonfreezing low tempera-
ture [10]. This process, which is known as cold acclimation or cold hardening, is associated with changes
in gene expression [10,195,311,312]. Expression of some of the cold-regulated genes is positively corre-
lated with the ability of plants to develop freezing tolerance [195,312]. Earlier studies have shown eleva-
tion of cytosolic Ca^2 in response to cold shock [26,27]. In alfalfa, low temperature–induced freezing tol-
erance is completely abolished by the Ca^2 channel blocker lanthanum and verapamil and partially by
CALCIUM IN STRESS SIGNAL TRANSDUCTION 713