SMALL GRAIN 5 encodes a heat shock transcription factor controlling grain size and plant architecture in rice.

Grain size is a critical determinant of rice yield. Despite the identification of several genes involved in grain size regulation, the underlying molecular mechanisms remain incompletely understood. A comprehensive understanding of the genetic and molecular mechanisms that regulate seed size is crucial for improving crop yield. Here, we report the characterization of SMALL GRAIN 5 (SMG5), which encodes a heat shock transcription factor (HSF) and serves as a key regulator of both grain size and plant architecture in rice. The smg5 mutant exhibits pleiotropic phenotypes, including significantly reduced grain size, as well as decreased plant height, leaf width, tiller number, and grain number per panicle. SMG5 regulates grain size through both cell expansion and cell proliferation. It is expressed in developing panicles, and the GFP-SMG5 fusion protein is localized in the nucleus. Integrated RNA-seq and DAP-seq analyses reveal that SMG5 binds to the promoter region of DGS1 and directly modulates its expression. Notably, overexpression of SMG5 resulted in larger grains. These findings establish SMG5 as a key HSF that modulates grain size and plant architecture, providing insights into the regulatory network controlling grain development in rice.