Trihelix Transcription Factor OsTGS1 Regulates Grain Size and Weight in Rice

Grain size is one of the major factors determining rice grain yield. Nevertheless, our knowledge of the molecular mechanisms underlying the control rice grain size remains limited. Trihelix proteins are plant-specific transcription factors that regulate plant growth and development. However, their roles in modulating grain size in cereal crops are largely unknown. Here, we report the rice trihelix family gene Oryza sativa trihelix transcription factor related to grain size 1 (OsTGS1) as a novel regulator of grain size and weight. Mutation of OsTGS1 leads to large and heavy grains, whereas overexpression of OsTGS1 results in small and light grains. OsTGS1 regulates grain size by influencing cell division and cell expansion in spikelet hulls. OsTGS1 is expressed in various tissues, and its expression level increases during panicle development. The OsTGS1 protein is localized to the nucleus and exhibits transcriptional repressor activity. The screening of interacting proteins via a yeast two-hybrid assay revealed that OsTGS1 interacted with GSK3/SHAGGY-LIKE KINASE2 (GSK2), an important regulator of various agronomic traits, including grain size, in rice. Moreover, ostgs1 mutants are hypersensitive to exogenous brassinosteroid treatment, indicating that OsTGS1 may be involved in brassinosteroid signaling. Our study reveals the role of OsTGS1 in controlling grain size and provides a new gene resource for improving grain weight in rice.