SPINDLY, A TETRATRICOPEPTIDE REPEAT PROTEIN INVOLVED IN GIBBERELLIN SIGNAL-TRANSDUCTION ARABIDOPSIS

Gibberellins (GAs) are a major class of plant hormones that control ma ny developmental processes, including seed development and germination , flower and fruit development, and flowering time, Genetic studies wi th Arabidopsis thaliana have identified two genes involved in GA perce ption or signal transduction. A semidominant mutation at the GIBBERELL IN INSENSITIVE (GAI) locus results in plants resembling GA-deficient m utants but exhibiting reduced sensitivity to Gk Recessive mutations at the SPINDLY (SPY) locus cause a phenotype that is consistent with con stitutive activation of GA signal transduction, Here we show that a st rong allele of spy is completely epistatic to gal, indicating that SPY acts downstream of GAI. We have cloned the SPY gene and shown that it encodes a new type of signal transduction protein, which contains a t etratricopeptide repeat region, likely serving as a protein interactio n domain, and a novel C-terminal region, Mutations in both domains inc rease GA signal transduction, The presence of a similar gene in Caenor habditis elegans suggests that SPY represents a class of signal transd uction proteins that is present throughout the eukaryotes.