BPH1 negatively regulates ABA signaling via AtSAP9 degradation

Previous studies have shown that BPH1 represses the abscisic acid (ABA)-mediated cellular responses. To further understand the mechanism of action of BPH1 in ABA signaling, the putative binding partners of BPH1 were investigated. Arabidopsis stress associated protein 9 (AtSAP9), which acts as a positive regulator of ABA signaling, has been identified as a BPH1-binding protein. Both BPH1 and AtSAP9 proteins were localized in the nucleus and cytosol, and a direct interaction between BPH1 and AtSAP9 was confirmed using yeast two-hybrid and bimolecular fluorescence complementation assays. The cell-free degradation assay indicated that MBP-AtSAP9 protein was degraded more slowly when incubated with the bph1 extracts than with Col-0 extracts, and that its degradation was dependent on the presence of the proteasome inhibitor MG132. Negative regulation of AtSAP9 protein stability by BPH1 was also confirmed in planta. Despite the E3 ubiquitin ligase activity of AtSAP9, the protein level of BPH1 was unaffected by AtSAP9. Collectively, these results indicate that BPH1, a CRL3 substrate receptor, functions as a repressor of ABA signaling, potentially through ubiquitin-proteasome system-dependent degradation of AtSAP9.