Dev. Cell 41, 47–58 (2017)

Credit: ELSEVIER

Brassinosteroids (BRs) are plant steroids that control development via a signaling cascade that includes a plasma membrane receptor kinase, BRASSINOSTEROID INSENSITIVE 1 (BRI1), and the transcription factors BRI1-EMS-SUPPRESSOR 1 (BES1) and BRASSINAZOLE-RESISTANT 1 (BZR1). A process called photomorphogenesis incorporates light signals to regulate plant development, but the mechanisms of the light–brassinosteroid interaction are not well understood. The stability and phosphorylation states of BES1 and BZR1 are known to be important during BR signaling. To better understand the role of BES1 and BZR1 stability, Yang et al. identified and explored the function of BES1- and BZR1-binding proteins. A two-hybrid screen identified several Arapbidopsis thaliana RING finger E3 ubiquitin ligases, SINAT1–SINAT5, that bound more specifically to dephosphorylated BES1 than to the phosphorylated form. In vitro ubiquitination assays, RNAi, overexpression, and degradation assays confirmed that BES1 interacts with SINATs, causing BES1 degradation and diminished BR signaling, ultimately resulting in effects on plant growth and morphology. Further RNAi experiments showed that BES1 is genetically epistatic to SINATs, consistent with SINAT regulation of BES1. Finally, the authors showed that SINAT levels are increased by light through a mechanism dependent on their own E3 ligase activity, thereby explaining the light-induced destabilization of dephosphorylated BES1. MB