Bioactive gibberellins (GAs) are important plant growth hormones that promote seed germination, leaf expansion, stem elongation and flower development in higher plants. The dwarf cultivars of wheat and rice developed during the 'Green Revolution' in the 60s and 70s remarkably increased grain yields. Recent studies revealed that these cultivars are modified in their GA production or GA response, illustrating the important role of GA in regulating plant development relevant to agriculture.
The GA signaling pathway is inhibited by repressor proteins. Growth and development occur through inactivation of these repressor proteins by the GA signal. Two such repressors, RGA and GAI, have been identified in the model plant Arabidopsis. Similar proteins (homologs of RGA/GAI) with conserved function have also been isolated in crops, including rice, wheat, corn, barley and grape. In fact, the dwarf wheat cultivars (part of the Green Revolution) carry specific mutations in the RGA/GAI homolog in wheat (Rht). Therefore, studies on RGA/GAI proteins and GA signaling in Arabidopsis may have a broad impact in improving the quality of agricultural crops.
The amino acid sequences of RGA and GAI are over 80% identical, and they have partially redundant function in repressing GA signaling in Arabidopsis. The major role of RGA and GAI is to inhibit GA-stimulated stem growth, leaf expansion and floral induction. However, they do not control seed germination and flower development, suggesting that additional repressors must modulate these processes. Three candidate Arabidopsis genes RGL1, RGL2 and RGL3 (for RGA-LIKE) were identified by DNA sequence comparison. To determine which RGL gene(s) play a major role in modulating flower development, the phenotypes of mutants that are defective in these genes will be characterized. GA de-represses its signaling pathway by causing degradation of the RGA protein. Expression patterns of RGL genes and the RGL protein stability in response to GA will be examined to elucidate the molecular mechanisms by which RGLs modulates GA signaling.
RGA/GAI proteins are thought to repress expression of downstream genes in the GA response pathway. To identify the target genes of RGA and GAI, the new GeneChip technique will be employed to survey alterations in expression profiles of the whole Arabidopsis genome after GA treatment and by mutations in RGA and/or GAI genes. Identification of GA-response genes and RGA/GAI targets will help to dissect GA signal transduction pathway. Because the homologs of RGA/GAI are highly conserved in plants, the knowledge gained from this project will also facilitate strategies in future manipulation of crops.
