In plants, GABA is involved in normal plant growth and development. The detailed molecular anal-
ysis of GAD using the transgenic approach by Fromm and coworkers is discussed later in this chapter. In-
duction of GABA synthesis in response to various environmental stimuli [including cold shock, mechan-
ical stress, anoxia, water stress, heat shock, hormonal treatment, and TMV (tobacco mosaic virus)
infection] has been reviewed [263,264]. Soybean leaves produce 9-, 11-, and 18-fold increased levels of
GABA in response to touch, rolling, and crushing, respectively, within 30 sec [265] and GABA levels are
also inducible by cold treatment [266]. These environmental stresses also elevate the [Ca^2 ]cytlevel, and
its participation in stress signal transduction is known. The concomitant increased [Ca^2 ]cytlevels and
GABA synthesis via GAD activity in response to environmental stresses raise the possibility of the in-
volvement of GAD in Ca^2 -mediated stress signal cascades in plants.
Another Ca^2 /CaM-regulated enzyme is NAD kinase. Although the gene encoding NAD kinase has
not been cloned it has been shown that its activity depends on the activated CaM [248,249]. The NAD ki-
nase catalyses the conversion of NAD to NADP, an energy-generating molecule. The role of NAD kinase
activity is vital to living organisms, especially when energy is in demand under stress conditions. It has
been reported that NAD kinase plays a role in the oxidative burst and in the formation of active oxygen
species (AOSs) [29]. Production of AOSs, which are known for their role against invading pathogens, is
a costly event in plants and requires the input of NADPH molecules. A detailed analysis of the action of
mutated CaM in the production of AOSs in transgenic tobacco plants is discussed later in this chapter.
Quantitative retention of superoxide dismutase activity on a CaM-Sepharose affinity column has also
been reported [267]. However, these authors did not show its regulation by Ca^2 /CaM. A unique micro-
710 REDDY AND REDDYFigure 3 Schematic representation of calmodulin-binding proteins involved in stress tolerance in plants.
Numbers denote transmembrane domains. The location of calmodulin-binding domain (CBD) in NtCBP4,
NtCB48, and BjGLY I has not been identified (CBD?). The myristoylation motifs in BjGLY I are indicated by
asterisks (*). PhGAD (500 aa), Petunia hybridaglutamic acid decarboxylase [232]; NtCBP4 (708 aa), Nico-
tiana tabacumcalmodulin-binding protein 4 [258]; NtCB48 (499 aa), Nicotiana tabacumcalmodulin-binding
protein 48 [246]; BjGLY I (185 aa), Brassica junceaglyoxalase I [244]; and ACA2 (1014 aa), Arabidopsis
Ca^2 -ATPase 2 [229].