Multiple Roles of Brassinosteroid Signaling in Vascular Development

Brassinosteroids (BRs) are plant steroid hormones that control growth and stress responses. In the context of development, BRs play diverse roles in controlling cell differentiation and tissue patterning. The vascular system, which is essential for transporting water and nutrients throughout the plant body, initially establishes a tissue pattern during primary development and then dramatically increases the number of vascular cells during secondary development. This complex developmental process is properly regulated by a network consisting of various hormonal signaling pathways. Genetic studies have revealed that mutants that are defective in BR biosynthesis or the BR signaling cascade exhibit a multifaceted vascular development phenotype. Furthermore, BR crosstalk with other plant hormones, including peptide hormones, coordinately regulates vascular development. Recently, the involvement of BR in vascular development, especially in xylem differentiation, has also been suggested in plant species other than the model plant Arabidopsis thaliana. In this review, we briefly summarize the recent findings on the roles of BR in primary and secondary vascular development in Arabidopsis and other species.