binding of Ca^2 to CaM (10^5 to 10^6 M) results in a conformational change in such a way that the hy-
drophobic pockets of CaM are exposed in each globular end, which can then interact with proteins (affin-
ity in the nanomolar range) and regulate the activity of several unrelated target enzymes. The hydropho-
bic pockets that are exposed upon Ca^2 binding are believed to interact with hydrophobic regions along
the amphipathic helix of CaM target proteins [209,210]. In addition to CaM, studies indicate the presence
of CaM-like proteins in plants [100,106,167,202,211–213]. However, the function of these proteins in the
Ca^2 signaling pathway(s) is not fully characterized as compared with that of CaM. These nonconserved
CaM-like or CaM-related proteins differ from the conserved CaM in containing more than 148 amino
acids and EF hand motifs (three to six) showing limited homology with CaM [213]. Hence, it is likely that
these proteins are functionally distinct and are involved in controlling different Ca^2 -mediated cellular
functions.
- Effect of Stress Signals on the Expression of Calmodulin and Calmodulin-Related
Proteins
In plant cells, CaM and CaM-like proteins are highly responsive to physical and hormonal signals such
as touch, wounding, light, and auxin [100,214] that are known to raise free [Ca^2 ]cytlevels. In Ara-
bidopsis, the expression of CaM and CaM-like proteins is rapidly induced by several stimuli such as
touch, wind, rain, and wounding [100,211]. Signal-induced changes in CaM and CaM-like proteins may
play a significant role in cell growth and physiology. Small changes in CaM levels have been shown to
affect drastically the progression of the cell cycle in animal cells [215] (see Chapter 11). Touch and wind
signals have been shown to cause rapid and transient increases in free [Ca^2 ]cytthat occur prior to ob-
served changes in target gene expression [3,26,37,100,211,212]. Hence, it is likely that signal-induced
changes in the [Ca^2 ]cytlevel could be involved in the expression of CaM and CaM-like proteins to acti-
vate downstream targets and to modify the cellular response. In several plant systems, there are multiple
CaM genes that code for identical proteins or contain a few conservative changes [106,108,167,216,217].
These small changes in amino acid composition of CaM isoforms may contribute to differential interac-
tion of each CaM isoform with target protein. For instance, a kinesin-like CaM-binding protein (KCBP)
interacts differentially with different CaM isoforms from Arabidopsis[218]. Although the Arabidopsis
CaM isoforms differ in few amino acids, our studies revealed that the ArabidopsisCaM2 isoform has a
twofold higher affinity toward KCBP [218]. This study with CaM and target protein (KCBP) from the
same system provides a clue to the differential affinity of CaM isoforms and CaM-related proteins with
target proteins. However, these CaM isoforms may or may not differ much in their function or affinity to-
ward Ca^2 as compared with CaM-related proteins that may be involved in diverse cellular processes.
Differential expression of CaM and CaM-related genes in response to different stimuli is also an impor-
tant and convenient mechanism for cells for tuning Ca^2 -mediated stress signal transduction cascades.
Although the results are preliminary, studies of CaM and CaM-related gene expression in response
to different stimuli indicate that different CaM isoforms are involved in mediating a specific signal
[108,211,216]. Three of the six Arabidopsis Camgenes (Cam1,-2, and, -3) are inducible by touch stim-
ulation [108,216], indicating the presence of different cis-regulatory elements in their promoters. Another
example is the expression of TCHgenes (Cam-related genes from Arabidopsis) in response to various me-
chanical, chemical, and environmental stimuli [211]. These authors also provided direct evidence that in-
creased [Ca^2 ]cytlevels in response to various abiotic stimuli are a prerequisite for the up-regulation of
theTCHgenes. Braam and his coworkers have hypothesized two schemes for the TCHgene expression
in response to various stimuli. Divergent stress signals induce TCHgene expression via different cascades
involving various receptors in the signal pathway and interaction with different cis-regulatory elements
onTCHgenes. Alternatively, these different cascades may converge at one point before the onset of TCH
gene expression and induce the TCHgene expression through a common cis-regulatory element of the
TCHgenes. Further studies should help understand the function of the TCHgenes in Ca^2 -mediated
stress signal transduction cascades in plants.
The presence of multiple CaM isoforms in plants adds further complexity to the Ca^2 -mediated net-
work and points to their differential sensitivity to elevated [Ca^2 ]cytlevels in response to different stress
stimuli. In potato, only one of eight CaM isoforms (PCaM1) is inducible by touch [103]. A striking ex-
ample of differential regulation of CaMs comes from studies with soybean CaM isoforms. In soybean,
five CaM isoforms (SCaM1 to -5) have been identified. SCaM1, -2, and -3 are highly conserved com-
pared with other plant CaM isoforms including ArabidopsisCaM isoforms, whereas SCaM4 and -5 are
708 REDDY AND REDDY