Deleting 17 amino acids (named the DELLA motif) in the N-terminal region of either GAI or RGA confers a GA-unresponsive dwarf phenotype in Arabidopsis ( Koornneef et al., 1985 Peng et al., 1997 Dill et al., 2001). This domain is involved in modulating the activity of the RGA and GAI proteins in response to GA ( Peng et al., 1997 Dill et al., 2001). GAI and RGA are distinguished from other GRAS family members by an N-terminal DELLA domain ( Peng et al., 1997 Silverstone et al., 1998). ![]() GAI and RGA belong to the DELLA subfamily within the GRAS family of plant regulatory proteins ( Pysh et al., 1999). Interestingly, seed germination and floral development are not restored by removing both RGA and GAI functions in the ga1-3 background, suggesting that neither RGA nor GAI plays a major role in controlling these two GA-dependent processes ( Dill and Sun, 2001). Although both RGA and GAI regulate these processes, the contribution of RGA is greater than that of GAI ( Dill and Sun, 2001). leaf expansion, apical dominance, abaxial trichome development, and stem elongation) as well as floral initiation ( Dill and Sun, 2001 King et al., 2001). Through studies of the rga-24 and gai-t6 null alleles in a ga1-3 background, RGA and GAI proteins were demonstrated to have overlapping functions in repressing many vegetative growth processes (e.g. Thus, RGA and GAI may function as transcriptional regulators that directly or indirectly repress the expression of GA-induced genes. Fusions of either RGA or GAI to the green fluorescent protein (GFP) are localized to the nuclei of cells in transgenic Arabidopsis ( Silverstone et al., 2001 Fleck and Harberd, 2002). Loss-of-function mutations in RGA (repressor of ga1-3) and GAI (GA insensitive) can suppress some of the effects of GA deficiency, suggesting that RGA and GAI negatively regulate a subset of GA responses in Arabidopsis ( Peng et al., 1997 Silverstone et al., 1997). Also, ga1-3 seeds cannot germinate without the application of GA ( Koornneef et al., 1983). Under long-day conditions, floral initiation in ga1-3 is delayed, and the mutant flowers are male-sterile ( Koornneef et al., 1983 Wilson et al., 1992). The ga1-3 plant also exhibits defects in all the developmental processes regulated by GAs: This mutant is impaired in root growth and trichome initiation, has reduced apical dominance, and fails to flower under short-day conditions ( Wilson et al., 1992 Chien and Sussex, 1996 Silverstone et al., 1997 Fu and Harberd, 2003). The large reduction in bioactive GAs in ga1-3 leads to a GA-deficient phenotype characterized by dark green leaves and severe dwarfism ( Koornneef and van der Veen, 1980 Silverstone et al., 2001). This mutant contains a large deletion in the GA1 gene, which encodes ent-copalyl diphosphate synthase, the enzyme catalyzing the first committed step in GA biosynthesis ( Sun and Kamiya, 1994). The importance of GAs is clearly illustrated by the Arabidopsis ga1-3 mutant. Seed germination, vegetative growth, and floral and seed development, for example, all require GAs ( Davies, 1995). We showed that RGL2 protein in imbibed seeds is rapidly degraded by GA treatment and that the F-box protein SLY1 is required for RGL2 degradation to occur.īioactive gibberellins (GAs) are phytohormones that are essential for many processes throughout the life of a plant. Interestingly, RGL2 expression is regulated not only at the transcript level. We further provided evidence that RGA, RGL1, and RGL2 are all involved in modulating floral development. Using the newly isolated rgl1, rgl2, and rgl3 T-DNA insertion mutants, we demonstrated that RGL2 is the major repressor in seed germination. RGA and, to a lesser extent, GAI mRNAs were expressed ubiquitously in all tissues, whereas RGL1, 2, and 3 transcripts were present at high levels only in germinating seeds and/or flowers and siliques. To investigate further the function of the RGL genes, we examined the expression profiles of all 5 DELLA protein genes by real-time PCR. ![]() Previous studies provided evidence that RGL2 and possibly RGL1 control seed germination. Three additional DELLA proteins RGL1, RGL2, and RG元 are present in Arabidopsis. ![]() RGA and GAI belong to the DELLA subfamily within the GRAS family of plant regulatory proteins. However, rga and gai in combination cannot rescue seed germination or floral development in ga1-3. ![]() The rga and gai null alleles interact synergistically to rescue vegetative growth and floral initiation in ga1-3, indicating that RGA and GAI are major repressors for these processes. The GA-deficient mutant ga1-3 is a nongerminating, extreme dwarf that flowers late and produces male-sterile flowers. RGA (repressor of ga1-3) and GAI (gibberellin insensitive) are negative regulators of plant hormone gibberellin (GA) signaling in Arabidopsis.
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