Re-evaluation of the cytokinin receptor role of the Arabidopsis gene GCR1

GCR1 has been tentatively identified in Arabidopsis thaliana as the first p lant G-protein coupled receptor (GPCR) (Josefsson and Rask 1997) implicated in the cytokinin sensory pathway (Plakidou-Dymock et al. 1998). A protein fusion of GCR1 and green fluorescent protein has been expressed in Arabidop sis and shown GCR1 to be located on the plasma membrane. Studies of plants with altered GCR1 expression have led us to question GCR1's involvement in cytokinin signaling. Transgenic Arabidopsis plants containing sense and ant isense constructs for GCR1 have been produced and over- and under-expressio n confirmed. The analysis of 12 antisense and 17 sense lines has failed to reveal the previously reported "Dainty" phenotype or altered cytokinin sens itivity. We have used the "Gauntlet" approach to test the plants' response to various plant hormones although this has not yet identified a mutant phe notype. The yeast-two hybrid system has been used and so far there is no ev idence to suggest GCR1 interacts with heterotrimeric G proteins. Before GCR 1 can be identified as genuine G-protein coupled receptor, the identificati on of a ligand and a proof of association with heterotrimeric G-proteins sh ould be obtained.