Reversible Histone H2B Monoubiquitination Fine-tunes Abscisic Acid Signaling and Drought Response in Rice.

Histone H2B monoubiquitination (H2Bub1) has been implicated in several important physiological and developmental processes, but its role in the regulation of plant stress responses remains elusive. Here, we report that H2Bub1 is crucially involved in abscisic acid (ABA) signaling and drought response in rice. We found the rice HISTONE MONOUBIQUITINATION2 (OsHUB2), an E3 ligase for H2Bub1, interacted with OsbZIP46, a key transcription factor regulating ABA signaling and drought response in rice. Genetic analyses suggest that OsHUB2, up-regulated by drought and ABA, positively modulates ABA sensitivity and drought resistance. The H2Bub1 levels were increased in the OsbZIP46 target genes under the drought stress and ABA treatments, and the increased H2Bub1 levels were positively correlated to the increased expression levels of the target genes. Interestingly, MODD, a reported suppressor of ABA signaling and drought resistance through mediation of OsbZIP46 deactivation and degradation, can repress the H2Bub1 level in the OsbZIP46 target genes by recruiting a putative deubiquitinase OsOTLD1. Suppression of OsOTLD1 in vivo resulted in increased H2Bub1 levels and expression of OsbZIP46 target genes. These findings established an elaborate mechanism of histone monoubiquitination in the fine-turning of ABA signaling and drought response via balancing H2Bub1 deposition and removal.