The E3 ligase OsPUB33 controls rice grain size and weight by regulating the OsNAC120-BG1 module.

Grain size and weight are important determinants of crop yield. Although the ubiquitin pathway has been implicated in the grain development in rice (Oryza sativa), the underlying genetic and molecular mechanisms remain largely unknown. Here, we report that the plant U-box E3 ubiquitin ligase OsPUB33 interferes with the OsNAC120-BG1 module to control rice grain development. Functional loss of OsPUB33 triggers elevated photosynthetic rates and greater sugar translocation, leading to enhanced cell proliferation and accelerated grain filling. These changes cause enlarged spikelet hulls, thereby increasing final grain size and weight. OsPUB33 interacts with transcription factor OsNAC120, resulting in its ubiquitination and degradation. Unlike OsPUB33, OsNAC120 promotes grain size and weight: OsNAC120-overexpression plants harbor large and heavy grains, whereas osnac120 loss-of-function mutants produce small grains. Genetic interaction analysis supports that OsPUB33 and OsNAC120 function at least partially in a common pathway to control grain development, but have opposite functions. Additionally, OsNAC120 transcriptionally activates BIG GRAIN1 (BG1), a prominent modulator of grain size, whereas OsPUB33 impairs the OsNAC120-mediated regulation of BG1. Collectively, our findings uncover an important molecular framework for the control of grain size and weight by the OsPUB33-OsNAC120-BG1 regulatory module and provide promising targets for improving crop yield.