25001:

Repression of the gibberellin signal transduction pathway by RGA, a transcriptional regulator.

Authors:	Silverstone, Aron, L.(A)Sun, Tai-ping(A)
Affiliations:	(A): Duke University, Department of Botany
Presenter:	Silverstone, Aron L., aron@acpub.duke.edu

Bioactive gibberellins (GAs) stimulate a variety of plant growth and developmental responses, including seed germination, stem elongation, and flower development. These events are controlled by changes in the GA concentration and/or by changes in tissue responsiveness to GA. Recent work in our lab and others has identified several key components of the GA signal transduction pathway. The RGA gene of Arabidopsis was identified by the ability of mutant rga alleles to partially suppress phenotypic defects of the GA biosynthetic mutant ga1-3. All the mutant alleles are recessive indicating that RGA likely encodes a negative regulator of GA response. The developmental processes affected by RGA include flowering time, stem growth and leaf abaxial trichome initiation. The SPINDLY (SPY) gene encodes another repressor of GA response, and the spy mutant is also able to partially suppress defects of the ga1-3 mutant. Interestingly, the combination of rga and spy mutations appears to almost completely activate the GA response pathway. We cloned the RGA locus by genomic subtraction and found that the RGA protein is a member of the GRAS family of regulatory proteins, which includes GAI, a third repressor of GA response. We have determined the mutations in 24 rga alleles by DNA sequence analysis to identify important conserved amino acids among the GRAS proteins. The RGA protein has 3 conserved domains named after an amino acid motif in each region: VHIID and RVER, which are present in all GRAS proteins and DELLA, which is present only in RGA, GAI and RGL (a GRAS protein of unknown function). The gai mutant has an in-frame deletion in the DELLA domain that turns the gai protein into a constitutive repressor of GA response. Therefore, the DELLA domain is likely to be a target for modification by the GA signal to alter RGA protein activity. We are isolating putative interactors of RGA using the yeast two hybrid system. Several motifs present in transcriptional regulators are found in RGA, including homopolymeric serines and threonines, leucine heptad repeats and a putative nuclear localization signal. A fusion protein between the green fluorescent protein (GFP) and RGA has been made to study the intracellular localization of the RGA protein. When both transiently expressed in onion epidermal cells and stably expressed in Arabidopsis, the GFP-RGA protein is nuclear localized. We are investigating whether the GA status of the plant changes its localization.