TWISTED DWARF 1 Associates with BRASSINOSTEROID-INSENSITIVE 1 to Regulate Early Events of the Brassinosteroid Signaling Pathway

A genome-wide screen for mutants showing altered brassinosteroid (BR) sensitivity or bri1-like phenotypes resulted in the identification of two new mutant alleles of TWISTED DWARF 1 (TWD1), twd1-4, and twd1-5. Previous studies indicated that TWD1, also named as ULTRACURVATA 2 or FKBP42, associates with auxin efflux transporters and is essential for their biological functions. Although earlier reports showed that BR signaling is downregulated in twd1, how TWD1 is integrated in BR signaling has not been elucidated. Here, we provide genetic and biochemical evidence demonstrating that TWD1 interacts with the BR receptor BRI1 in vivo in a BR-independent manner. Further analyses indicated that TWD1 modulates the BR signal transduction not by altering ER quality control or protein abundance of BRI1; instead, TWD1 appears to be critical in BR-induced interaction of BRI1 and its co-receptor BAK1, as well as BR-induced phosphorylation of these two proteins. These results provide better understanding of the early events of the BR signaling pathway.