Role of Ca²⁺/calmodulin in plant response to abiotic stresses: A review

Calcium ions (Ca²⁺) play an important role as a second messenger in mediating plant response to abiotic stresses. Cytosolic Ca²⁺ signals are translated to biochemical processes via Ca²⁺ modulated proteins such as calmodulin (CaM). We have taken a molecular approach to clone cellular targets of Ca²/CaM. A ³⁵S-labeled recombinant CaM was used as a probe to screen various cDNA expression libraries. One of the isolated clones from petunia codes for the enzyme glutamate decarboxylase (GAD) which catalyzes the conversion of glutamate to y-aminobutyric acid (GAB A). The activity of plant GAD has been shown to be dramatically enhanced in response to cold and heat shock, anoxia, drought, mechanical manipulations and by exogenous application of the stress phytohormone ABA in wheat roots. We have purified the recombinant GAD by CaM-affinity chromatography and studied its regulation by Ca²/CaM. At a physiological pH range (7.0-7.5), the purified enzyme was inactive in the absence of Ca²⁺ and CaM but could be stimulated to high levels of activity by the addition of exogenous CaM (K₀₅ = 15 μM) in the presence of Ca²⁺ (K₀₅ = 0.8 μM). Neither Ca²⁺ nor CaM alone had any effect on GAD activity. To assess the role of CaM binding to GAD in vivo, transgenic tobacco plants expressing a mutant petunia GAD lacking the CaM-binding domain, or transgenic plants expressing the intact GAD were prepared and studied in detail. We have shown that the CaM-binding domain is necessary for the regulation of glutamate and GABA metabolism and for normal plant development. Moreover, we found that CaM is tightly associated with a 500 kDa active GAD complex. This tight association of CaM with its target is likely to be important for rapid modulation of GAD activity by Ca²⁺ signaling in response to stresses.