Overexpression of a GRAS protein lacking the DELLA domain confers altered gibberellin responses in rice

The rice SLR1 (SLENDER RICE 1) gene encodes a DELLA protein that belongs to a subfamily of the GRAS protein superfamily and that functions as a repressor of gibberellin (GA) signaling. Based on the constitutive GA response phenotype of slr1 mutants, SLR1 has been thought to be the sole DELLA-type protein suppressing GA signals in rice. However, in rice genome databases we identified two sequences homologous to SLR1: SLR1-like1 and -2 (SLRL1 and -2). SLRL1 and SLRL2 contain regions with high similarity to the C-terminal conserved domains in SLR1, but lack the N-terminal conserved region of the DELLA proteins. The expression of SLRL1 was positively regulated by GA at the mRNA level and occurred preferentially in reproductive organs, whereas SLRL2 was moderately expressed in mature leaf organs and was not affected by GA. Transformation of SLRL1 into the slr1 mutant rescued the slender phenotype of this mutant. Moreover, overexpression of SLRL1 in normal rice plants induced a dwarf phenotype with an increased level of OsGA20ox2 gene expression and diminished the GA-induced shoot elongation, suggesting that SLRL1 acts as a repressor of GA signaling. Consistent with the fact that SLRL1 does not have a DELLA domain, which is essential for degradation of DELLA proteins, a level of SLRL1 protein was not degraded by application of gibberellic acid. However, the repressive activity of SLRL1 against GA signaling was much weaker than a truncated SLR1 lacking the DELLA domain. Based on these characteristics of SLRL1, the functional roles of SLRL1 in GA signaling in rice are discussed.