A similar approach has been used to measure elevated [Ca^2 ]cytlevels in Arabidopsis[36,42,73] and
in the moss plant Physcomitrela patens[40]. Furthermore, cold-induced Ca^2 elevation is dependent
on microtubule organization [74]. Although the precise Ca^2 stores that release Ca^2 in response to
cold stress are not well defined, a plasma membrane–associated Ca^2 channel was found to be stimu-
lated by cold treatment [75,76] and vacuolar Ca^2 store release was found to be via the inositol 1,4,5-
triphosphate (IP 3 ) channel [36]. The contribution and sensitivity of different Ca^2 stores in elevating
levels of [Ca^2 ]cytin response to cold stress might explain the dynamic regulation of Ca^2 levels by
internal stores.
Monitoring of signal-induced changes in the presence of Ca^2 channel blockers or channel openers
of plasma membrane or organelles indicated that different signals use distinct Ca^2 stores in elevating
[Ca^2 ]cyt. For instance, cold-induced Ca^2 increase is inhibited by plasma membrane channel blockers
but is not affected by organellar channel blockers. However, wind-induced Ca^2 increase is blocked by
organellar Ca^2 channel blockers whereas plasma membrane channel blockers did not have any effect
[37,39]. These studies indicate that the extracellular Ca^2 contributes to cold-induced elevation of Ca^2
and internal Ca^2 stores contribute to wind-induced increase in [Ca^2 ]cyt.
CALCIUM IN STRESS SIGNAL TRANSDUCTION 699
TABLE 1 Effect of Environmental, Chemical, Hormonal, and Biotic Stress Stimuli on Changes in
Cytosolic Calcium Levels in Higher Plants
Methods used to Effect
measure free on
Signal cytosolic Ca^2
a
[Ca^2 ]cytb Responsec ReferenceCold 1, 2 ↑ CORgene expression, 32–34
proline synthesis, changes
in membrane lipid profile
and cold acclimatization
Drought 1, 2 ↑ Gene expression, synthesis 31,35,36
of osmoprotectants, and
osmotolerance
Wind 1, 2 ↑ Morphogenesis 37,38,39
Touch 1, 2 ↑ Thigmomorphogenesis, 26,40–44
cytoskeletal organization
Heat shock 1, 2 ↑ Thermotolerance 45,46
Hypoosmotic stress 2 ↑ Osmoadaptation 47–50
Red light 1 ↑ Photomorphogenesis 51
Abscisic acid 1, 2, 3 ↑,↓ Stomatal closure 20,52–55
Gibberellic acid 2 ↑ -Amylase secretion 25,56
Auxin 1 ↑ Cell elongation and cell 57
division
Jasmonic acid 1 ↑ Induction of wound- 58
regulated genes
expression
NaCl 1, 2, 3 ↑ Gene expression and 26,31,59–63
osmolyte synthesis,
Kuptake
Ozone stress 2 ↑ Production of AOSs 64
Oxidative stress (H 2 O 2 ) 1, 2 ↑ Production of AOSs and 65–67
HR and cell death
Pathogens and elicitors 1, 2 ↑ Phytoalexin biosynthesis 26,66–69
and induction of HR
NOD factors 1 ↑ Nodular formation and root 70
hair curling
aCytosolic free calcium levels are measured using injection of fluorescent indicator dyes (1), transgenic plants expressing ae-
quorin (2), or cameleon (3).
bIncrease (↑) or decrease (↓) in cytosolic free calcium levels.
cCOR, cold-regulated; AOSs, active oxygen species; HR, hypersensitive response; and [Ca (^2) ]cyt, cytosolic free calcium.