Natural variations of OsAUX5, a target gene of OsWRKY78, control the neutral essential amino acid content in rice grains

Grain essential amino acid (EAA) levels contribute to rice nutritional quality. However, the molecular mech-anisms underlying EAA accumulation and natural variation in rice grains remain unclear. Here we report the identification of a previously unrecognized auxin influx carrier subfamily gene, OsAUX5, which encodes an amino acid transporter that functions in uptake of multiple amino acids. We identified an elite haplotype of Pro::OsAUX5Hap2 that enhances grain EAA accumulation without an apparent negative effect on agronomic traits. Natural variations of OsAUX5 occur in the cis elements of its promoter, which are differentially acti-vated because of the different binding affinity between OsWRKY78 and the W-box, contributing to grain EAA variation among rice varieties. The two distinct haplotypes were shown to have originated from different Oryza rufipogon progenitors, which contributed to the divergence between japonica and indica. Introduction of the indica-type Pro::OsAUX5Hap2 genotype into japonica could significantly increase EAA levels, indicating that indica-type Pro::OsAUX5Hap2 can be utilized to increase grain EAAs of japonica vari-eties. Collectively, our study uncovers an WRKY78-OsAUX5-based regulatory mechanism controlling grain EAA accumulation and provides a potential target for breeding EAA-rich rice.

Automated summary

In this study, a previously unrecognized auxin influx carrier subfamily gene, OsAUX5, was identified to enhance the accumulation of essential amino acids (EAAs) in rice grains. Natural variations of OsAUX5 were found to contribute to the variation of grain EAAs among rice varieties. The introduction of OsAUX5 genotype from indica rice into japonica rice significantly increased EAA levels, suggesting a potential strategy for breeding EAA-rich rice.