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Summary: Drought stress has detrimental effects on plants. Although the abscisic acid (ABA)‐mediated drought response is well established, defensive mechanisms to cope with dehydration‐induced proteotoxicity have been rarely studied. DRR1 was identified as an Arabidopsis drought‐induced gene encoding an ER‐localized RING‐type E3 Ub ligase. Suppression of DRR1 markedly reduced tolerance to drought and proteotoxic stress without altering ABA‐mediated germination and stomatal movement. Proteotoxicity‐ and dehydration‐induced insoluble ubiquitinated protein accumulation was more obvious in DRR1 loss‐of‐function plants than in wild‐type plants. These results suggest that DRR1 is involved in an ABA‐independent drought stress response possibly through the mitigation of dehydration‐induced proteotoxic stress. [ABSTRACT FROM AUTHOR]

Summary: Oryza sativa BRASSINAZOLE RESISTANT 1 (OsBZR1) is the closest rice homolog of the Arabidopsis BZR1 and bri1‐EMS‐SUPPRESSOR 1 (BES1)/BZR2 transcription factors. OsBZR1 plays a central role in the rice brassinosteroid signaling pathway. Despite its functional importance, the control mechanism by which the cellular stability of OsBZR1 is regulated has not yet been fully elucidated. Here, we report that a rice U‐box E3 ubiquitin (Ub) ligase OsPUB24 acts as a negative regulator in the BR signaling pathway via the 26S proteasome‐dependent degradation of OsBZR1. The ospub24 T‐DNA knock‐out mutant and Ubi:RNAi‐OsPUB24 knock‐down rice plants displayed enhanced seedling growth, increased lamina joint bending, and hypersensitivity to brassinolide (BL). The expressions of the BR biosynthetic genes suppressed by BR in a negative feedback loop were lower in the mutant progeny than in the wild‐type rice plants, which indicated increased BR responses in the mutant line. OsPUB24 ubiquitinated OsBZR1, resulting in the proteasomal degradation of OsBZR1. In addition, the stability of OsPUB24 was downregulated by BL and bikinin, an inhibitor of Oryza sativa Shaggy/GSK3‐like kinase 22 (OsSK22). OsSK22, the homolog of Arabidopsis BRASSINOSTEROID INSENSITIVE 2 (BIN2) protein kinase, phosphorylated OsPUB24 and elevated the cellular stability of OsPUB24. Our findings suggest that OsPUB24 participates in OsBZR1 turnover, and that the regulatory networks of OsPUB24, OsSK22 and OsBZR1 are crucial for fine‐tuning the BR response in rice. Significance Statement: OsBZR1 transcription factor is a key component of the rice brassinosteroid signaling pathway. Here, we show that OsBZR1 is subject to control by two negative factors, OsSK22 protein kinase and OsPUB24 E3 ubiquitin ligase, in the rice BR signaling pathway. This study illustrates that the signaling process of BR is cooperatively regulated by two distinct types of post‐translational modifications, phosphorylation and ubiquitination. [ABSTRACT FROM AUTHOR]