InVigna radiata, touch stimulus differentially affects the expression of CaM isoforms, suggesting
the specificity of the signal transduction pathway [105]. Several CaM and CaM-like proteins (CLPs) were
identified in Arabidopsis[100,106,107]. Of these, transcripts of AtCaM1,-2,-3, and touch (TCH) genes
are inducible severalfold by mechanical stimuli [100,108]. The expression of TCHgenes is enhanced by
touch, wind, and darkness [41,42,109,110], suggesting a role for Ca^2 in transducing these signals in Ara-
bidopsis. The increased activity of CaM and CLPs may serve two important functions in thigmomorpho-
genesis: sequestration of increased [Ca^2 ]cytand regulation of the cytoskeletal network by associating
with microtubule-associated proteins [43,111].
The strength of artificial wind application on transgenic aequorin seedlings correlated with the
[Ca^2 ]cytlevels [112]. Mechanical stimuli enhance the [Ca^2 ]cytin various tissues including meristem-
atic, differentiated, and root cap zones [113]. Although the subcellular contribution of the Ca^2 signal is
not understandable at this time, research indicates that mitochondrial, endoplasmic reticulum (ER), cell
wall, and vacuolar stores contribute to [Ca^2 ]cyt[37,38,114]. In tobacco, expression of one specific CaM
isoform is inducible by cold and wind. By targeting aequorin to cytoplasm (aequorin) and nuclear (nu-
cleoplasmin-aequorin) organelles, Van der Luit et al. [39] showed that distinct cellular Ca^2 pools re-
spond to wind and cold stimuli in the expression of the NpCaM1gene. Wind and cold stimuli induce
[Ca^2 ]nand [Ca^2 ]cyt, respectively. These results suggested that different Ca^2 transients employ distinct
signal pathways in NpCaM1gene expression [39].
Soil containing high sodium chloride causes osmotic stress resulting in dehydration of the plant cell
and interferes with the nutritional uptake of other components and water [115] essential for growth and
development. Calcium has been implicated in drought and salinity stress. Calcineurin, a Ca^2 /CaM-de-
pendent protein phosphatase, is involved in salinity tolerance in plants, suggesting a role for Ca^2 in salt
tolerance [116]. Evidence for the involvement of Ca^2 in salt stress also comes from the Salt-Overlay-
Sensitive3 (SOS3) mutant of Arabidopsis. It is sensitive to sodium and lithium ions but not to osmotic
stress. But high levels of Ca^2 in the medium support the growth and development of these mutants, sug-
gesting the important role of Ca^2 in salt stress [117]. The SOS3gene was cloned and found to encode a
protein similar to calcineurin B, a regulatory subunit of Ca^2 -dependent protein phosphatase [63]. These
results suggest that SOS3 acts as a Ca^2 sensor in salt tolerance in Arabidopsis. Salt stress induces sev-
eral Ca^2 -regulated signal components such as Ca^2 -ATPase in tobacco cells [71] and tomato [118] and
expression of the AtCP1[119],phosphatidylinositol-specific phospholipase C[82], and certain CDPKs
[120] in Arabidopsisand mung bean [121].
Treatment of corn root protoplasts with sodium chloride resulted in elevated [Ca^2 ]cyt(1.1 to 1.8
M) [61]. Using^45 Ca^2 , it has been shown that NaCl-treated algae, such as CharaandDunaliella, ele-
vated their [Ca^2 ]cytand the magnitude of elevation was in direct proportion to the concentration of NaCl
[86,122]. The increased [Ca^2 ]cytin sodium chloride–treated barley protoplasts and wheat aleurone cells
was measured with the help of the Ca^2 -binding fluorescent dyes indo-1 and fluo-3 [62,123]. Further ev-
idence for the elevated [Ca^2 ]cytin response to NaCl was obtained with tobacco [26] and Arabidopsis[31]
seedlings expressing aequorin. The same group provided evidence for the vacuolar origin of elevated
[Ca^2 ]cytin response to NaCl stress.
Plants produce active oxygen species (AOSs) (O• 2 , OH•, and H 2 O 2 ) when they are exposed to dif-
ferent stresses such as anoxia, UV-B radiation, or ozone [124]. The AOSs damage cellular macro-
molecules and cause severe irreversible damage in plants. To prevent such oxidative damage, plants el-
evate certain enzymatic activities including superoxide dismutase, catalase, peroxidase, glutathione
synthase, and alcohol dehydrogenase as defense mechanisms to remove AOSs and to increase oxida-
tive tolerance [65,124–126]. The pollutant gas ozone induces [Ca^2 ]cytin pinto bean leaves [127]. Us-
ing transgenic aequorin Arabidopsisplants, the effect of O 3 gas on [Ca^2 ]cythas been studied [64]. In
these plants, biphasic [Ca^2 ]cyttransients were reported. Further, with the help of lanthanum chloride
and EGTA, the activity of AOS-induced glutathione synthase was found to require a second [Ca^2 ]cyt
transient peak [64]. Using fura-2, McAinsh et al. [128] showed that the levels of [Ca^2 ]cytwere ele-
vated in the guard cells of Commelina communistreated with H 2 O 2 and methyl viologen, which pro-
duce AOSs in these plants. Transgenic tobacco seedlings expressing aequorin also revealed that H 2 O 2
induced [Ca^2 ]cytlevels [65].
Deprivation of O 2 in flooded soils causes the plants to activate anaerobic respiration and alcohol de-
hydrogenase [129,130]. It was demonstrated that anoxia induced^45 Ca^2 uptake in maize roots and Ca^2
chelators inhibited this uptake. Therefore, survival of maize seedlings in anoxia stress is dependent on
CALCIUM IN STRESS SIGNAL TRANSDUCTION 701