The DELLA motif is essential for gibberellin-induced degradation of RGA

Abstract

RGA and GAI are homologous genes that encode putative transcriptional regulators that repress gibberellin (GA) signaling in Arabidopsis . Previously we showed that the green fluorescent protein (GFP)-RGA fusion protein is localized to the nucleus in transgenic Arabidopsis , and expression of this fusion protein rescues the rga null mutation. The GA signal seems to derepress the GA response pathway by degrading the repressor protein RGA. The GA-insensitive, semidominant, semidwarf gai - 1 mutant encodes a mutant protein with a 17-amino acid deletion within the DELLA domain of GAI. It was hypothesized that this mutation turns the gai protein into a constitutive repressor of GA signaling. Because the sequences missing in gai-1 are identical between GAI and RGA, we tested whether an identical mutation ( rga -Δ 17 ) in the RGA gene would confer a phenotype similar to gai - 1 . We demonstrated that expression of rga -Δ 17 or GFP -( rga -Δ 17 ) under the control of the RGA promoter caused a GA-unresponsive severe dwarf phenotype in transgenic Arabidopsis . Analysis of the mRNA levels of a GA biosynthetic gene, GA4 , showed that the feedback control of GA biosynthesis in these transgenic plants was less responsive to GA than that in wild type. Immunoblot and confocal microscopy analyses indicated that rga-Δ17 and GFP-(rga-Δ17) proteins were resistant to degradation after GA application. Our results illustrate that the DELLA domain in RGA plays a regulatory role in GA-induced degradation of RGA. Deletion of this region stabilizes the rga-Δ17 mutant protein, and regardless of the endogenous GA status rga-Δ17 becomes a constitutively active repressor of GA signaling.