Regulation of grain protein content by the amino acid permease gene OsAAP4 in rice

Rice grain protein content is crucial for its quality and nutritional value, but the mechanisms of nitrogen transport to the grain remain poorly understood. This study demonstrates that the amino acid transporter OsAAP4 regulates protein accumulation in rice grains. OsAAP4 is expressed during grain filling, increasing at the mid-filling stage. Analysis of aap4 mutants in Zhonghua11 and Nipponbare backgrounds revealed significantly reduced grain yield and polished rice protein content compared to wild-type. Furthermore, mutant grains showed markedly lower nitrogen content at both filling and maturity stages. Analysis of amino acid concentrations revealed that the contents of most amino acids in the grains of the aap4 mutant were significantly lower than those in the wild type. Nitrogen content analysis indicated that the aap4 mutation had a minimal impact on plant nitrogen content, suggesting that the reduced grain protein content in the aap4 mutant was due to its loss of function within the grain. Yeast heterologous complementation experiments suggest that OsAAP4 may mediate the transmembrane transport of amino acids. Further investigation revealed that OsAAP4 can encode a plasma membrane localized protein and is expressed in the vascular bundles of grains during filling. Interestingly, overexpression of OsAAP4 simultaneously increased both rice yield and grain protein content. Transcriptomic analysis revealed that OsAAP4 may influence plant hormone signaling, protein formation on the ER, and various amino acid metabolic processes in grains. Collectively, these findings indicate that OsAAP4 might plays a crucial role in the translocation of amino acids into grains, thereby regulating grain protein content.