| Gibberellins are a large class of tetracyclic diterpenoid plant hormones that affect many aspects of plant growth and development. Arabidopsis SPINDLY (SPY) negatively regulates gibberellin signal transduction. SPY has ten copies of tetratricopeptide repeats (TPR) and shows significant similarity to animal O-Linked N-acetylglucosamine transferase (OGT).; To investigate the functional role of SPY's TPR domain in GA signal transduction, fifteen spy alleles were sequenced. Mutations of multiple independent alleles were found to cluster in TPR 6, or TPR 8 and 9, suggesting that these TPRs are critical for GA signaling. TPR motif from other proteins mediates protein-protein interactions. To test if SPY's TPR domain functions to assemble protein complexes, the N-terminus of SPY was overexpressed. Consistent with the hypotheses that SPY's TPRs mediate the formation of SPY-SPY complexes or the association of SPY with other proteins, the wild-type transformants exhibited weak spy-like phenotypes. In addition, SPY's TPRs were found to mediate SPY-SPY interactions in yeast and in in vitro.; The yeast two-hybrid system was used to identify SPY-interacting proteins. Four major classes of Arabidopsis proteins were identified through the screening. GIGANTEA (GI) is a novel nuclear protein that specifically interacted with SPY, both in yeast and in in vitro binding assays. GI promotes flowering in long days, and affects hypocotyl elongation in red light. Phenotypic characterizations of a spy-4/gi-2 double mutant revealed that spy suppressed the hypocotyl elongation under red light, and the late flowering of gi, indicating that SPY acts with GI in regulating both developmental processes. spy plants also exhibited a novel long hypocotyl phenotype under far-red light, suggesting a role for SPY in phyA signaling. Therefore, SPY may function to integrate the GA signal transduction and light sensing pathways. |
