INTRODUCTION: The NIGT1/HHO subfamily of transcription factors in plants plays a crucial role in perceiving and integrating nutrient signals, regulating developmental processes, and mediating stress responses. However, the functions of NIGT1/HHO proteins in hormone signaling and their involvement in biotic and abiotic stress responses remain incompletely characterized. OBJECTIVES: We aimed to elucidate how OsHHO3 integrates into brassinosteroid (BR) and abscisic acid (ABA) signaling pathways, thereby modulating disease resistance and drought tolerance in rice. METHODS: We employed protein interaction assays (e.g., yeast two-hybrid, co-immunoprecipitation assays and pull-down assays), phosphorylation modification assays (e.g., in vivo and in vitro kinase assays), transcription factor binding assays (e.g., dual-luciferase reporter assay, electrophoretic mobility shift assay and chromatin immunoprecipitation followed by qPCR), and phenotypic and biochemical assays of transgenic rice lines (overexpression and CRISPR-Cas9 knockouts). RESULTS: The gain-of-function mutant hho3-D and OsHHO3-overexpressing lines show BR deficiency and BR insensitivity, indicating that OsHHO3 participates in BR signaling. The GSK3-like kinase GSK2 interacts with and phosphorylates OsHHO3 at Ser283 and Ser312, contributing to its stabilization. Acting as a transcriptional repressor, OsHHO3 directly binds the promoter of OsBZR1 and suppresses its expression. OsHHO3 also physically interacts with OsBZR1, and promotes its degradation through the 26S proteasome pathway. Furthermore, OsHHO3 negatively regulates disease resistance by directly repressing defense-related genes. Knockout of OsHHO3 leads to enhanced resistance both to fungal blast and bacteria blight, without compromising rice growth. In addition, OsHHO3 and GSK2 positively regulate drought tolerance, likely via the ABA signaling pathway. Accordingly, OsHHO3 directly represses ABA-catabolic genes as well as other negative regulators of drought tolerance. Notably, co-expression with GSK2 significantly enhances the repressive activity of OsHHO3 on its target genes. CONCLUSION: Our findings underscore the importance of the GSK2-OsHHO3 module in the coordinated regulation of disease resistance and drought tolerance via interplaying BR and ABA signaling in rice.
Fang et al. (Mon,) studied this question.