BAF1 and Brassinosteroid related mutant in Arabidopsis

Brassinosteroids (BRs) regulate plant growth, development and stress responses by activating the core transcription factor BRI1-EMS-SUPPRESSOR1 (BES1). The E3 Ubiquitin ligase(s) that modify BES1 for autophagy-mediated degradation remain to be fully defined. In this study, we identified an F-box family E3 ubiquitin ligase termed BES1-ASSOCIATED F-BOX1 (BAF1). BAF1 interacts with and ubiquitinates BES1. Accordingly, BES1 stability and protein levels were reduced in BAF1 overexpression plants but increased in a baf1 loss-of-function mutant and in BAF1-DF (BAF1 with F-box deleted, dominant-negative form) overexpression lines. Selective autophagy of BES1, but not bulk autophagy, was significantly compromised in the baf1 mutant under sucrose starvation. BES1 degradation mediated by BAF1 could be blocked through autophagy but not proteasome inhibitors, suggesting that BAF1 mediates BES1 degradation largely through autophagy. baf1 and BAF1-DF overexpression plants had increased BR-regulated growth but were sensitive to long-term sucrose starvation, while BAF1 overexpression plants had decreased BR-regulated growth but were highly tolerant of sucrose starvation. Our results not only established BAF1 as a novel E3 ubiquitin ligase that targets BES1 for degradation through selective autophagy pathway, but also revealed a mechanism for plants to reduce growth during sucrose starvation by targeting this central growth regulator. 3 or 4 biological replicates samples of 250nM brassicanazole treated or untreated wild-type, bes1-D and baf1 seedlings were prepared for transcriptome profiling. Wild-type plants are control plants, and untreated plants are the reference condition.