Arabidopsis SHY2/IAA3 inhibits auxin-regulated gene expression.

In Arabidopsis, SHY2 encodes IAA3, a member of the auxin-induced Aux/IAA family. Gain-of-function mutations in SHY2/IAA3 cause enlarged cotyledons, short hypocotyls, and altered auxin-regulated root development. Here we show that the gain-of-function mutation shy2-2 decreases both the induction and repression of auxin-regulated genes, suggesting that SHY2/IAA3 acts as a negative regulator in auxin signaling. shy2-2 affects auxin induction of many previously characterized primary response genes, implying that it might repress primary auxin responses. In addition, shy2-2 also affects expression of multiple auxin-nonresponsive genes. Light regulates expression of SHY2/IAA3, suggesting a possible link between light and auxin response pathways.