HOS15 impacts DIL9 protein stability during drought stress in Arabidopsis.

HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 15 (HOS15) acts as a substrate receptor of E3 ligase complex, which plays a negative role in drought stress tolerance. However, whether and how HOS15 participates in controlling important transcriptional regulators remains largely unknown. Here, we report that HOS15 physically interacts with and tightly regulates DROUGHT-INDUCED LIKE 19 (DIL9) protein stability. Moreover, application of exogenous abscisic acid (ABA) stabilizes the interaction between DIL9 and HOS15, leading to ABA-induced proteasomal degradation of DIL9 by HOS15. Genetic analysis revealed that DIL9 functions downstream to HOS15 and that the drought tolerance of hos15-2 plants was impaired in dil9/hos15 double mutants. Notably, DIL9 is directly associated with the promoter regions of ABF transcription factors and facilitates their expression, which is pivotal in enhancing ABA-dependent drought tolerance. Collectively, these findings demonstrate that HOS15 consistently degrades DIL9 under normal condition, while stress (drought/ABA) promotes the DIL9 activity for binding to the promoter regions of ABFs and positively regulates their expression in response to dehydration.