Molecular and physiological characterization of arabidopsis GAI alleles obtained in targeted Ds-tagging experiments.

Bioactive gibberellin (GA) is an essential regulator of vascular plant development. The GAI gene of Arabidopsis thaliana (L.) Heynh. encodes a product (GAI) that is involved in GA signalling. The dominant mutant gai allele encodes an altered product (gai) that confers reduced GA responses, dwarfism, and elevated endogenous GA levels. Recessive, presumed loss-of-function alleles of GAI confer normal height and resistance to the GA biosynthesis inhibitor paclobutrazol. One explanation for these observations is that GAI is a growth repressor whose activity is opposed by GA, whilst gai retains a constitutive repressor activity that is less affected by GA. Previously, we described gai-t6, a mutant allele which contains an insertion of a maize Ds transposable element into gai. Here we describe the molecular and physiological characterization of two further alleles (gai-t5, gai-t7) identified during the Ds mutagenesis experiment. These alleles confer paclobutrazol resistance and normal endogenous GA levels. Thus the phenotype conferred by gai-t5, gai-t6 and gai-t7 is not due to elevated GA levels, but is due to loss of gai, a constitutively active plant growth repressor.