Transcriptional Corepressor OsTPR1 Regulates Seedling Growth and Seed Size Development in Rice

Transcription factors play crucial roles in regulating gene expression, ensuring proper plant growth and development, and responding to biotic and abiotic signals. TOPLESS (TPL)/TOPLESS-related (TPR) corepressors are central regulators of gene expression that interact with various transcription factors, thereby regulating multiple signaling pathways or metabolic biosynthetic processes. In this study, we identified a rice TPR gene OsTPR1, which is induced on day 3 of germinating rice embryo. In gain- and loss-of-function studies performed with transgenic rice. The overexpressing (OsTPR1-Ox) line exhibited a longer phenotype, whereas the RNA interference (OsTPR1-Ri) line showed a semi-dwarf phenotype during rice seedlings development, and exogenous GA3 could rescue the semi-dwarf phenotype in OsTPR1-Ri seedlings. Interestingly, in germinating rice embryos, OsTPR1 was found to negatively regulate the expression of enzymes essential for GA biosynthesis genes GA20ox1 and GA3ox2 and the signaling repressor OsSLR1. In contrast, OsTPR1 positively regulates the expression of GA signaling responsive genes aAmy8 and OsEP3A. Furthermore, during seed development, OsTPR1 can positively regulate seed size development by increasing grain width and thickness, and the expression of OsTPR1::GUS is specifically expressed in the aleurone layer before the milk stage and in the endosperm at the dough stage. These results indicate that OsTPR1 plays a critical role in GA signaling in improving seedling growth through inhibiting the expression of OsSLR1 repressor, thereby de-repressing GA signaling responsive gene expression, whereas it also acts as a negative regulator that inhibits GA biosynthesis. These findings indicate that the OsTPR1 corepressor may interact with unknown proteins spatiotemporally to form functionally specific repressor complexes, regulate GA biosynthesis and signaling responses promptly, fine-tune seedling growth and ultimately regulate rice seed development. Results provide a new clue in regulating the GA signaling pathway and that the manipulation of OsTPR1 will have the potential to increase rice grain size and yield.