Function of α subunit of heterotrimeric G protein in brassinosteroid response of rice plants

Abstract

The alpha subunit of heterotrimeric G-proteins (G alpha) is involved in a broad range of aspects of the brassinosteroid (BR) response, such as the enhancement of lamina bending. However, it has been suggested from epistatic analysis of d1 and d61, which are mutants deficient for G alpha and the BR receptor BRI1, that G alpha and BRI1 may function via distinct pathways in many cases. In this study, we investigated further the genetic interaction between G alpha and BRI1. We report the analysis of transformants of T65d1 and T65d1/d61-7 into which were introduced a constitutively active form of G alpha, Q223L. The application of 24-epi-brassinolide (24-epiBL) to T65d1 expressing Q223L still resulted in elongation of the coleoptile and, in fact, it was enhanced over the wild-type plant (WT) level in a concentration dependent manner. In T65d1/d61-7 expressing Q223L, the seed size was enlarged over that of d61-7 due to activation of G alpha. These results suggest that Q223L is able to augment the BR response in response to 24-epiBL and also that Q223L functions independently of BRI1 in the process of determining seed morphology, given that Q223L was functional in the BRI1-deficient mutant, d61-7.

Brassinosteroid Regulates Cell Elongation by Modulating Gibberellin Metabolism in Rice

“…We cannot exclude the possibility that BR has other branched pathways, which are independent of GA biosynthesis and signaling, that promote cell elongation. Several studies have suggested that there is another BR signaling pathway independent of the BRI1 receptor, possibly involving components such as D1, BU1, and TAIHU DWARF1/ERECT LEAF1 (TUD1/ELF1) Oki et al, 2009b;Tanaka et al, 2009;Hu et al, 2013;Sakamoto et al, 2013). Interestingly, D1 has also been suggested to be involved only in the high-sensitivity response of GA (i.e., in response to low concentrations of GA) (Ueguchi- Tanaka et al, 2000).…”

Heterotrimeric G protein signalling in the plant kingdom

“…Finally, gpa1 and agb1 mutants are hyposensitive to BR [21,26,128,129,133] in BR promotion of seed germination [26,73], and in BR inhibition of hypocotyl and root elongation [26]. agb1 mutants are hyposensitive to methyl jasmonate (MeJA) inhibition of root elongation and seed germination [18].…”

Brassinosteroid Signaling and Application in Rice

“…Thus, the role of D1/RGA1 in BR signaling pathway might not be specific and D1/RGA1 could be the factor acting at a very upstream position of many different signaling pathways. Double mutant analysis between d1 and d61-7 also suggests that D1/RGA1 should be involved in an alternative BR signaling pathway independent of OsBRI1 (Oki et al, 2009). …”

“…Interestingly, in d1 mutant, OsBU1 is obviously less induced by BR, indicating OsBU1 is downstream of D1/RGA1 in BR response (Tanaka et al, 2009). As D1 and OsBRI1 have been implicated function independent of the upstream BR signaling pathways (Oki et al, 2009), OsBU1 could be regarded as a much downstream integrator of BR responses.…”

“…However, the specifically shortened second internode, the obviously decreased seed size and the defective skotomorphogenesis strongly suggest that it should also be a BR-related mutant Oki et al, 2009). D1 encodes an alpha-subunit of rice heterotrimeric G protein, RGA1.…”