Arabidopsis Aux/IAA genes are involved in brassinosteroid‐mediated growth responses in a manner dependent on organ type

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Abstract
Summary: We examined whether auxin/indole‐3‐acetic acid (Aux/IAA) proteins, which are key players in auxin‐signal transduction, are involved in brassinosteroid (BR) responses. iaa7/axr2‐1 and iaa17/axr3‐3 mutants showed aberrant BR sensitivity and aberrant BR‐induced gene expression in an organ‐dependent manner. Two auxin inhibitors were tested in terms of BR responses. Yokonolide B inhibited BR responses, whereas p‐chlorophenoxyisobutyric acid did not inhibit BR responses. DNA microarray analysis revealed that 108 genes were up‐regulated, while only eight genes were down‐regulated in iaa7. Among the genes that were up‐ or down‐regulated in axr2, 22% were brassinolide ‐inducible genes, 20% were auxin‐inducible genes, and the majority were sensitive neither to BR nor to auxin. An inhibitor of BR biosynthesis, brassinazole, inhibited auxin induction of the DR5‐GUS gene, which consists of a synthetic auxin‐response element, a minimum promoter, and a β‐glucuronidase. These results suggest that Aux/IAA proteins function in auxin‐ and BR‐signaling pathways, and that IAA proteins function as the signaling components modulating BR sensitivity in a manner dependent on organ type.