Branching the Auxin Signaling; Multiple Players and Diverse Interactions

In contrast to its short signaling pathway, auxin plays a wide spectrum of biological roles during plant growth and development. The nuclear auxin signaling pathway can be described in a single step; auxin/receptor-mediated ubiquitination/degradation of transcriptional repressors that affect the auxin responsive genes. However, this simple signaling step generates not only diverse but also seemingly contrary signaling outputs, such as activation/repression of gene expression and promotion/inhibition of cell growth. This diversity of the nuclear auxin signaling is likely to derive from multiple and diverse players of the signaling components. The major nuclear auxin signaling components are receptors, repressors, DNA-binding transcription factors, and co-repressors. In most land plants, each component of the auxin signaling consists of multiple isoforms with somewhat diversified structures; and thus with diverse molecular functions. The diversity of members of each signaling component is anticipated to contribute to the diverse auxin responses. This study reviews the recent results regarding the diverse molecular structures and interactions among the nuclear auxin signaling players.