The OsMAPK5–OsWRKY72 module negatively regulates grain length and grain weight in rice.

ABSTRACT Grain size and grain weight are important determinants for grain yield. In this study, we identify a novel OsMAPK5–OsWRKY72 module that negatively regulates grain length and grain weight in rice. We found that loss‐of‐function of <italic>OsMAPK5</italic> leads to larger cell size of the rice spikelet hulls and a significant increase in both grain length and grain weight in an <italic>indica</italic> variety Minghui 86 (MH86). OsMAPK5 interacts with OsMAPKK3/4/5 and OsWRKY72 and phosphorylates OsWRKY72 at T86 and S88. Similar to the <italic>osmapk5</italic> MH86 mutants, the <italic>oswrky72</italic> knockout MH86 mutants exhibited larger size of spikelet hull cells and increased grain length and grain weight, whereas the <italic>OsWRKY72</italic>‐overexpression MH86 plants showed opposite phenotypes. OsWRKY72 targets the W‐box motifs in the promoter of <italic>OsARF6</italic>, an auxin response factor involved in auxin signaling. Dual‐luciferase reporter assays demonstrated that OsWRKY72 activates <italic>OsARF6</italic> expression. The activation effect of the phosphorylation‐mimicking OsWRKY72T86D/S88D on <italic>OsARF6</italic> expression was significantly enhanced, whereas the effects of the OsWRKY72 phosphorylation‐null mutants were significantly reduced. In addition, auxin levels in young panicles of the <italic>osmapk5</italic> and <italic>oswrky72</italic> mutants were significantly higher than that in the wild‐type MH86. Collectively, our study uncovered novel connections of the OsMAPKK3/4/5‐OsMAPK5‐mediated MAPK signaling, OsWRKY72‐mediated transcription regulation, and OsARF6‐mediated auxin signaling pathways in regulating grain length and grain weight in an <italic>indica</italic>‐type rice, providing promising targets for molecular breeding of rice varieties with high yield and quality. [ABSTRACT FROM AUTHOR]