SummaryHeterotrimeric G-proteins transduce signals from activated G-protein-coupled receptors (GPCR) to appropriate downstream effectors and thereby play an important role in signaling. A role of G-proteins in salinity and heat stress tolerance has not heretofore been described. We report isolation of cDNAs of two isoforms of Ga (Ga1, 1152 bp; Ga2, 1152 bp), one Gb (1134 bp), two isoforms of Gc (Gc1, 345 bp; Gc2, 303 bp) and a GPCR (1008 bp) from Pisum sativum, and purification of all the encoded recombinant proteins (Ga, 44 kDa; Gb, 41 kDa; Gc, 14 kDa; GPCR, 35 kDa). The transcript levels of Ga and Gb were upregulated following NaCl, heat and H 2 O 2 treatments. Protein-protein interaction studies using an in vitro yeast two-hybrid system and in planta co-immunoprecipitation showed that the Ga subunit interacted with the pea Gb subunit and pea phospholipase C (PLCd) at the calcium-binding domain (C2). The GTPase activity of the Ga subunit increased after interaction with PLCd. The GPCR protein interacted with all the subunits of G-proteins and with itself. Transgenic tobacco plants (T 0 and T 1 ) constitutively over-expressing Ga showed tolerance to salinity and heat, while Gb-over-expressing plants showed only heat tolerance, as tested by leaf disk senescence assay and germination/growth of T 1 seeds/seedlings. These findings provide direct evidence for a novel role of Ga and Gb subunits in abiotic stress tolerance and possible cross-talk between PLC-and G-protein-mediated signaling pathways.
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