<scp>OsPT4</scp> Facilitates Selenomethionine Transport and Biosynthesis to Enhance Seed Accumulation in Rice: Molecular Mechanisms and Biotechnological Potential

Selenium (Se) is a vital micronutrient for humans, with important functions for health and anti-cancer properties. Organic Se shows higher antioxidant activity and much lower toxicity compared to inorganic Se, making it safer for use. Selenomethionine (SeMet) is one of the primary forms of organic Se. OsPT4, the high-affinity phosphate (Pi) transporter (PHT) of rice, has been investigated for its role in the transport of the different forms of Se, and its effects on the accumulation of SeMet in this study. The OsPT4 mutant and overexpression lines were used as research materials. Phenotypic analyses revealed that OsPT4 confers improved Se tolerance in shoots upon selenite exposure. Heterologous expression assays in Xenopus laevis oocytes and yeast systems and translocation assays in different transgenic lines of rice confirmed OsPT4-mediated selenite and SeMet transport activity, establishing its responsibility for root-to-shoot Se translocation. Transcriptomic profiling, amino acid quantification and qRT-PCR analyses further indicated that OsPT4 up-regulates methionine (Met) biosynthesis, the direct precursor of SeMet. Notably, OsPT4 significantly increased SeMet accumulation and promoted the formation of Se-rich micron-sized spherical particles in seeds under Se supplementation. These findings provide mechanistic insights into OsPT4-mediated SeMet trafficking and metabolism, advancing strategies for developing Se-biofortified rice cultivars with enhanced nutritional and therapeutic value.