The Regulation of Brassinosteroid Biosynthesis in Arabidopsis

Brassinosteroids (BRs) are steroid hormones that affect virtually every physiological process of plants throughout their life cycle. In contrast to the rapid progress in our understanding of BR signaling pathways, little is known about the regulatory mechanisms underlying BR homeostasis, particularly the upstream signals that regulate BR biosynthesis and inactivation. BR biosynthesis occurs through network pathways and basically is regulated at the transcriptional level of BR biosynthetic genes. When the BR signal is activated, BR-specific transcription factor, BZR1, inhibits transcription of BR biosynthetic genes through feedback downregulation mechanisms. Moreover, BR biosynthesis is also affected by other hormones such as auxin. This review focuses on recent progress in our understanding of the regulation of BR biosynthesis, with an emphasis on the transcriptional mechanisms that regulate this process, the effect of other hormones, exogenous signals, and inactivation of BRs.