BES1 hinders ABSCISIC ACID INSENSITIVE5 and promotes seed germination in Arabidopsis

Summary Proper regulation of seed germination is essential for the successful propagation of a plant. The transcription factor ABSCISIC ACID INSENSITIVE 5 ( ABI 5) of the abscisic acid (ABA) signaling pathway plays a central role in the inhibition of seed germination. ABI 5 is precisely regulated by the core ABA signaling components and multiple other factors. However, the complex regulatory network of ABI 5 remains largely unknown. In this study, we determined the biochemical interaction between ABI 5 and the BRINSENSITIVE 1 ( BRI 1)‐ EMS ‐ SUPPRESSOR 1 ( BES 1) transcription factor of the brassinosteroid ( BR ) signaling pathway, as well as the function of BES 1 regulating ABI 5 during seed germination in Arabidopsis . BES 1 directly interacts with ABI 5 both in vitro and in vivo . The bZIP domain of ABI 5, which is responsible for DNA binding, is critical for ABI 5 binding to BES 1. The interaction of BES 1 with ABI 5 significantly suppressed the binding of ABI 5 to the promoter regions of downstream genes, which resulted in their reduced expression and consequently facilitated seed germination. This study shed new light on the coordination of multiple signaling pathways during seed germination. In particular, BES 1 directly binds to ABI 5, which interferes with its transcriptional activity and suppresses ABA signaling output.