Mutation in domain II of IAA1 confers diverse auxin‐related phenotypes and represses auxin‐activated expression of Aux/IAA genes in steroid regulator‐inducible system

Abstract

Summary: Most of Aux/IAA genes are rapidly induced by auxin. The Aux/IAA proteins are short‐lived nuclear proteins sharing the four conserved domains. Domain II is critical for rapid degradation of Aux/IAA proteins. Among these gene family members, IAA1 is one of the earliest auxin‐inducible genes. We used a steroid hormone‐inducible system to reveal putative roles and downstream signaling of IAA1 in auxin response. Arabidopsis transgenic plants were generated expressing fusion protein of IAA1 (IAA1‐GR) or IAA1 with a mutation in domain II (iaa1‐GR) and the glucocorticoid hormone‐binding domain (GR). IAA1‐GR transgenic plants did not exhibit any discernable phenotypic differences by DEX treatment that allows nuclear translocation of the fusion protein. In contrast, diverse auxin‐related physiological processes including gravitropism and phototropism were impaired by DEX treatment in roots, hypocotyls, stems, and leaves in iaa1‐GR transgenic plants. Auxin induction of seven Aux/IAA mRNAs including IAA1 itself was repressed by DEX treatment, suggesting that IAA1 functions in the nucleus by mediating auxin response and might act as a negative feedback regulator for the expression of Aux/IAA genes including IAA1 itself. Auxin induction of Aux/IAA genes in the presence of cycloheximide can be repressed by DEX treatment, showing that the repression of transcription of the Aux/IAAs by the iaa1 mutant protein is primary. Wild‐type IAA1‐GR could not suppress auxin induction of IAA1 and IAA2. These results indicate that inhibition of auxin‐activated transcription of Aux/IAA genes by the iaa1 mutant protein might be responsible for alteration of various auxin responses.