UDP-glucose epimerase 1, moonlighting as a transcriptional activator, is essential for tapetum degradation and male fertility in rice

Multiple enzymes perform moonlighting functions distinct from their main roles. UDP-glucose epimerases (UGEs), a subclass of isomerases, catalyze the interconversion of UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal). We identified a rice male-sterile mutant, osuge1, with delayed tapetum degradation and abortive pollen. The mutant osuge1 protein lacked UDP-glucose epimerase activity, resulting in higher UDP-Gal content and lower UDP-Glc levels in the osuge1 mutant compared with the wild type. Interestingly, we discovered that OsUGE1 participates in the TIP2/bHLH142–TDR–EAT1/DTD transcriptional regulatory cascade involved in tapetum degradation, in which TIP2 and TDR regulate the expression of OsUGE1 while OsUGE1 regulates the expression of EAT1. In addition, we found that OsUGE1 regulates the expression of its own gene by directly binding to an E-box element in the OsUGE1 promoter. Collectively, our results indicate that OsUGE1 not only functions as a UDP-glucose epimerase but also moonlights as a transcriptional activator to promote tapetum degradation, revealing a novel regulatory mechanism of rice reproductive development. This study reveals that OsUGE1 not only functions as a UDP-glucose epimerase but also moonlights as a transcriptional activator to promote tapetum degradation. OsUGE1 catalyzes the interconversion of UDP-Glc and UDP-Gal and also participates in the TIP2/bHLH142-TDR-EAT1/DTD pathway to show moonlighting transcriptional activation activity by directly regulating the expresson of EAT1 during tapetum development. .