Using luminescence-imaging technology, the spatial and temporal pattern of elevated [Ca^2 ]cytin re-
sponse to low temperature has been demonstrated in transgenic plants expressing aequorin [32]. These
authors showed that a cold-induced signal was transmitted from root to aerial tissues with a lag period of
3 min. Proline, a small imino acid, accumulates in cold-stressed cells and acts as an osmoprotectant [77].
Using Ca^2 chelators such as EGTA and channel blockers such as lanthanum, it has been shown that Ca^2
is necessary for cold-induced accumulation of proline in Amaranthus[78] and tomato seedlings and cell
cultures [79]. Furthermore, -aminobutyric acid (GABA) is also accumulated in response to several
stresses. In asparagus cells, cold stress–induced Ca^2 levels regulate the activity of L-glutamate decar-
boxylase activity synthesizing GABA [80].
Drought stress induces the activity of several Ca^2 -signaling components, suggesting a role of Ca^2
in protecting plants from water loss [35,81]. In Arabidopsis, dehydration induces the accumulation of IP 3 ,
which in turn causes the release of Ca^2 from internal stores [82]. Further, transgenic Arabidopsis
seedlings in which aequorin is targeted to the cytoplasmic face of the tonoplast membrane [36] showed
release of vacuolar Ca^2 in response to mannitol treatment [31]. These results indicate the involvement
of IP 3 in increasing levels of [Ca^2 ]cytand that Ca^2 is released from vacuolar source [31]. Indirect evi-
dence has been obtained for the role of Ca^2 in mannitol-treated rice cell cultures [83] and Arabidopsis
seedlings [31]. These authors tested the mannitol-induced expression of RABandAtP5CS1genes in the
presence Ca^2 channel blockers such as verapamil or lanthanum or the Ca^2 chelator EGTA. The ex-
pression of these genes in treated cultures and plants is less than that of untreated counterparts, indicating
a role of Ca^2 in drought tolerance. With the help of pharmacological antagonists, elevated [Ca^2 ]cythas
been observed in response to hypoosmotic stress in the alga Nitella flexilis[84,85],Dunaliella salina[86],
Lamprothamium[87],Fucuszygote [47,48], and suspension cultures of Nicotiana tabacum[49,50].
Harmful high-intensity light and ultraviolet (UV) light cause severe damage in plants. The photore-
ceptors (primarily phytochrome, cryptochrome, and UV-B light photoreceptor) sense the light quantity
and activate downstream signal transduction pathways. Using a pharmacological approach combined
with the measurement of ultraviolet light–induced CHSgene expression, Christie and Jenkins [88] pro-
vided evidence for the involvement of elevated levels of [Ca^2 ]cytin response to UV-A/blue and UV-B
light in Arabidopsiscell cultures. Although these two light-induced signal transduction pathways differ
in their transducing downstream pathways, both signaling pathways induced elevated levels of [Ca^2 ]cyt.
Further work from the same laboratory confirmed the presence of Ca^2 involvement in UV-A/blue or
UV-B light signal transduction pathways [89]. Involvement of Ca^2 in phytochrome-controlled signal
transduction mechanisms has also been reported [90,91].
Presoaking of maize seeds before germination or maize seedlings in CaCl 2 solution greatly enhanced
thermotolerance of these seedlings after they were exposed to a higher temperature (50°C). Conversely,
the seedlings treated with Ca^2 chelator (EGTA) and channel blockers (lanthanum, verapamil) and CaM
inhibitors (CPZ and W7) had significantly reduced thermotolerance. Further, seedlings treated with the
same inhibitors showed enhanced thermotolerance when supplemented with Ca^2 . These results indicate
the requirement of Ca^2 in thermotolerance [46,92,93]. Using the fluorescent dye indo-1, Bisyaseheva et
al. [45] showed a fourfold increase in the levels of [Ca^2 ]cytin response to heat shock in pea mesophyll
protoplasts. Further evidence for the involvement of Ca^2 in heat stress has been obtained from Ca^2
measurement studies using transgenic seedlings expressing aequorin [46]. These measurements showed
increased levels of [Ca^2 ]cytin tobacco seedlings in 5 to 35 min after treatment at elevated temperatures
(39, 43, or 47°C) [46]. The expression of TCHgenes in heat shock–treated cultured Arabidopsiscells re-
vealed that their expression is Ca^2 dependent.
Mechanical stimuli such as wind, touch, wound, and rain perturb plant growth and development.
Plants respond to these stimuli and accordingly alter their morphogenesis (e.g., stunted growth, stem
thickening) [94,95]. Studies on thigmotropism in plants revealed that the changes in Ca^2 levels play a
role in plant responses to touch. The calcium chelating agent EGTA and the Ca^2 channel blocker lan-
thanum inhibited the rubbing- and touch-induced growth in soybean stems [96] and Mimosa pudica[97].
The direction of maize root growth changes upon mechanical stimulation. This thigmomorphogenic ef-
fect can be inhibited by treatment with gadolinium, an inhibitor of the stretch-activated Ca^2 channel in
maize roots [98]. Touch-stimulated coiling in Bryonica diociawas inhibited by treatment with the Ca^2
adenosinetriphosphatase (ATPase) inhibitor erythrosine B, suggesting the involvement of Ca^2 [99]. Me-
chanical stimulation induced the expression of several Ca^2 -binding proteins such as CaM or CaM-re-
lated proteins in Arabidopsis[100],Brassica napus[101],Bryonia[102], potato [103], and tomato [104].
700 REDDY AND REDDY