Handbook of Plant and Crop Physiology

uole, ER, and mitochondria contain Ca^2 in the mM range. To maintain such a steep electrochemical gra-
dient between cytoplasm and organelles, the membranes are equipped with pumps and channels that open
and pump Ca^2 (10^6 Ca^2 ions/sec) to elevate [Ca^2 ]cytin response to the signal and maintain Ca^2 
homeostasis (Figure 4). Further, these channels and pumps are gated by other messengers such as IP 3 ,
cADP-ribose, or voltage that regulate the Ca^2 release in response to a specific stress stimulus [4,326].
Once the [Ca^2 ]cytlevels are raised, an array of high-affinity Ca^2 -binding proteins sense and initiate a
specific signal cascade via their cellular targets to generate appropriate responses to cope with the stresses
(Figure 4) [4,7,19,167,353]. These sensors include CDPKs, CaMs, calcineurin B–like proteins, and other
EF hand–containing proteins that modulate downstream target components. For example, CaMs regulate
the activity of CaM-binding proteins, which in turn participate in stress tolerance mechanisms (Figure 4).

ACKNOWLEDGMENTS

Research in our laboratory is supported by grants from the National Science Foundation, Agricultural Ex-
periment Station, USDA, NASA, Plant Biotechnology Laboratory, Colorado Biotechnology Research In-
stitute, and Colorado RNA Center. We thank Dr. Irene S. Day and Dr. Farida Safadi for comments on the
manuscript.

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