demonstrated. Accumulating and ongoing research activities from different laboratories indicate the pres-
ence of several such proteins in plants. Identification of the biological function of Ca^2 and Ca^2 /CaM-
binding proteins is a necessary step in further elucidating the role Ca^2 . Calcium-mediated stress signals
have been shown to modulate various components located downstream in the signal pathway in order to
evoke specific cellular activity. In most cases, the identity of the full cascade (beginning from changes at
the cytosolic level to the final physiological response) has not been investigated. The Ca^2 signaling cas-
cade in plants seems to work as a complex grid and interact with several components in many possible
combinations.
The present understanding of the Ca^2 signlaing cascade in stress signal transduction is depicted in
Figure 4. The resting [Ca^2 ]cytlevel is about 100 nM. The calcium stores such as plasma membrane, vac-
CALCIUM IN STRESS SIGNAL TRANSDUCTION 721
Figure 4 Schematic illustration of calcium-mediated signal pathways in stress signal transduction. The trans-
port of calcium into and out of the cellular compartments (plasma membrane, vacuole, ER, and mitochondria)
in response to a stress signal is indicated with arrows (the arrows point to the direction of calcium flow). The
IP 3 (inositol-1,4,5-triphosphate) system and its sensitive channels in vacuole and ER are shown. The estimated
concentration of calcium in different organelles is indicated in mM [4,6,326,353]. The vertical arrows (↑) in-
dicate increased levels of respective components. The question mark (?) denotes lack of information. [Ca^2 ]cyt,
free cytosolic calcium; ER, endoplasmic reticulum; Mt, mitochondria; Plast, plastids; Nuc, nucleus; R, an
unidentified receptor on plasma membrane; PLC, phospholipase C; PIP2, phosphatidylinositol-4,5-bisphos-
phate; IP3, inositol-1,4,5-triphosphate; DG, diacyl glycerol; PKC, protein kinase C; GE, gene expression;
AOSs, active oxygen species; GABA, -aminobutyric acid; PRGE, pathogen-related gene expression; CDPK,
calcium-dependent protein kinase; CaM, calmodulin; CBLs, calcineurin B–like proteins; EF-hands, EF hand
motifs containing proteins; GAD, glutamate decarboxylase; ACA2, ArabidopsisCa^2 -ATPase; cNGCs, cyclic
nucleotide gated channels; TCBP48, tobacco calmodulin-binding protein 48; and CBP, calmodulin-binding
protein.