The <i>Arabidopsis GAI</i> gene defines a signaling pathway that negatively regulates gibberellin responses

TLDR

The Arabidopsis gai mutant allele reduces gibberellin responsiveness. We cloned the GAI gene and its close homolog GRS. GAI encodes a nuclear transcriptional co‑activator that represses GA responses; the mutant gai lacks a 17‑amino‑acid segment, making it resistant to GA, and mutations in SPY and GAR2 suppress the gai phenotype, revealing a negative GA signaling pathway that modulates growth through derepression rather than direct stimulation.

Abstract

The Arabidopsis gai mutant allele confers a reduction in gibberellin (GA) responsiveness. Here we report the molecular cloning of GAI and a closely related gene GRS. The predicted GAI (wild-type) and gai (mutant) proteins differ only by the deletion of a 17-amino-acid segment from within the amino-terminal region. GAI and GRS contain nuclear localization signals, a region of homology to a putative transcription factor, and motifs characteristic of transcriptional coactivators. Genetic analysis indicates that GAI is a repressor of GA responses, that GA can release this repression, and that gai is a mutant repressor that is relatively resistant to the effects of GA. Mutations at SPY and GAR2 suppress the gai phenotype, indicating the involvement of GAI, SPY, and GAR2 in a signaling pathway that regulates GA responses negatively. The existence of this pathway suggests that GA modulates plant growth through derepression rather than through simple stimulation.