Rice dwarf mutant d1, which is defective in the alpha subunit of the heterotrimeric G protein, affects gibberellin signal transduction

Previously, we reported that the rice dwarf mutant, d1, is defective in the LY subunit Of the heterotrimeric G protein (Col), In the present study, gi bberellin (GA) signaling in dl and the role of the G alpha protein in the C A-signaling pathway were investigated. Compared with the wild type, GA indu ction of alpha-amylase activity in aleurone cells of dl was greatly reduced . Relative to the wild type, the GA(3)-treated aleurone layer of dl had low er expression of Ramy1A, which encodes alpha-amylase. and OsGAMYB. which en codes a GA-inducible transcriptional factor, and no increase in expression of Ca2+-ATPase. However, in the presence of high CA concentrations, or-amyl ase induction occurred even in dl. The CA sensitivity of second leaf sheath elongation in dl was similar to that of the wild type in terms of dose res ponsiveness, but the response of internode elongation to GA was much lower in dl. Furthermore, Os20ox expression was up-regulated, and the GA content was elevated in the stunted internodes of dl. All these results suggest tha t dl affects a part of the GA-signaling pathway, namely the induction of al pha-amylase in the aleurone layer and internode elongation. In addition, a double mutant between dl and another CA-signaling mutant, sir, revealed tha t SLR is epistatic to the DI, supporting that the Cor protein is involved i n GA signaling. However, the data also provide evidence for the presence of an alternative CA-signaling pathway that does not involve the G alpha prot ein. It is proposed that GA signaling via the Got protein may be more sensi tive than that of the alternative pathway, as indicated by the low CA respo nsiveness of this G alpha-independent pathway.