plementation. Plants harboring the antisense CAX1or vector alone showed a wild-type pattern without
any symptoms, but these symptoms could be induced by Ca^2 depletion in the growth medium. Further,
consistent with these findings, constitutive antiport activity was observed in sense transgenic tobacco
plants compared with control plants. These results indicate that the hyperantiport activity of the CAX1
gene causes severe depletion in free cytosolic Ca^2 levels due to pumping of Ca^2 from cytosol to vac-
uole. This nonavailability of cytosolic Ca^2 impairs mitigating the stress-related signal transduction cas-
cades and related cellular adjustments in the transgenic tobacco plants expressing the CAX1gene [343].
To investigate the role of distinct Ca^2 pathways in mitigating cold and wind stress stimuli, trans-
genic tobacco plants expressing aequorin in cytoplasm (MAQ 2.4) and nucleus (MAQ 7.11) were con-
structed [39]. These transgenic plants allowed the authors to monitor Ca^2 changes in different cellular
(cytoplasm and nucleus) compartments in response to specific stimulus. Using these transgenic plants,
these authors have measured the Ca^2 -mediatedNpCaM1gene expression and corresponding increase in
Ca^2 changes in cytosol and nuclear compartments in response to cold and wind shocks [39]. Compari-
son of Ca^2 dynamics and Ca^2 -inducedNpCaM1gene expression revealed that two independent Ca^2
signaling pathways are evolved in perceiving and transducing wind and cold stress stimuli. One is spe-
cific to wind and operates through nuclear Ca^2 and the other is specific to cold and operates through cy-
toplasmic Ca^2 . The results obtained from this transgenic approach to measure raises in Ca^2 levels es-
tablished the occurrence of distinct Ca^2 signal cascades and Ca^2 respones to specific stress stimuli in
plants.
As already discussed, the primary event in cellular activity in response to drought, cold, and salt
stress is elevated [Ca^2 ]cytlevels. Once the signal elevates [Ca^2 ]cyt, Ca^2 -sensing proteins are activated,
leading to expression of several target genes. The target genes include rd29A,kin1,cor.6.6,cor47,
cor15a, and erd10and their gene products are believed to be protective in response to various stresses and
718 REDDY AND REDDYTABLE 2 Summary of Genes Used in Developing Transgenic Plants to Decipher Their Role(s) in
Calcium-Mediated Signaling Pathways in Stress Signal Transduction
Transgenic
Gene Gene product plant/promoter Biological function ReferenceAtCax1 Ca^2 Hantiporter Tobacco/CaMV35S Pumping of Ca^2 to 343
vacuoles
AtCbf1 CRT/DRE-binding Tobacco/CaMV35S Induction of CRT/DREacis 321
factor element containing gene
expression (CORgenes)
AtDreb1a DRE-binding factor 1A Tobacco/RD29A Induction of CRT/DRE cis 344
element containing gene
expression (CORgenes)
BjGly I Glyoxalase I Tobacco/CaMV35S Tolerance to NaCl, 244
methylglyoxal
PhGad Glutamate decarboxylase Tobacco/CaMv35S Synthesis of GABA and 232
stress tolerance
NtCbp4 Membrane-bound cation Tobacco/CaMV35S Ni^2 tolerance 258
channel
GmCam4 Soybean CaM 4 isoform Tobacco/CaMV35S PR gene expression and 220
biotic stress tolerance
GmCam5 Soybean CaM5 isoform Tobacco/CaMV35S PR gene expression and 220
biotic stress tolerance
VU-3 Mutated form of Tobacco/CaMV35S Activation of NAD kinase 29
synthetic CaM and production of AOSs
(K115R)
PhCam53 Petunia hybridaCaM53 Transient and ectopic Importance of prenylation 203
isoform expression in domain and cellular
tobacco and localization
Arabidopsis
aCRT/DRE, C repeat/drought responsive element; COR, cold-regulated genes; GABA, -aminobutyric acid; PR, pathogen-
related; and AOSs, active oxygen species.