The Arabidopsis dwarf mutant shi exhibits reduced gibberellin responses conferred by overexpression of a new putative zinc finger protein

shi (for short internodes), a semidominant dwarfing mutation of Arabidopsis caused by a transposon insertion, confers a phenotype typical of mutants d efective in the biosynthesis of gibberellin (GA). However, the application of GA does not correct the dwarf phenotype of shi plants, suggesting that s hi is defective in the perception of or in the response to GA. In agreement with this observation, the level of active GAs was elevated in shi plants, which is the result expected when feedback control of GA biosynthesis is r educed, Cloning of the SHI gene revealed that in shi, the transposon is ins erted into the untranslated leader so that a cauliflower mosaic virus 35S p romoter in the transposon reads out toward the SHI open reading frame. This result, together with mRNA analysis, suggests that the phenotype of the sh i mutant is a result of overexpression of the SHI open reading frame. The p redicted amino acid sequence of SHI has acidic and glutamine-rich stretches and shows sequence similarity over a putative zinc finger region to three presumptive Arabidopsis proteins. This suggests that SHI may act as a negat ive regulator of GA responses through transcriptional control.