Auxin regulation of embryonic root formation.

The plant hormone auxin was initially identified as the bioactive substance that induces roots in plant tissue culture. In the past decades, mechanisms for auxin action, including its transport and response, have been described in detail. However, a molecular and cellular description of its role in root initiation is far from complete. In this review, we discuss recent advances in our understanding of auxin-dependent embryonic root formation. During this process, a root meristem is initiated in a precise and predictable position, and at a stage when the organism consists of relatively few cells. Recent studies have revealed mechanisms for local control of auxin transport, for cellular differences in auxin response components and cell type-specific chromatin regulation. The recent identification of biologically relevant target genes for auxin regulation during embryonic root initiation now also allows dissection of auxin-activated cellular processes. Finally, we discuss the potential for hormonal cross-regulation in embryonic root formation.