Natural variation in theOsbZIP18promoter contributes to branched-chain amino acid levels in rice

Branched-chain amino acids (BCAAs) are essential amino acids that must be obtained from the diet for humans and animals, and they play important roles in various aspects of plant growth and development. Although BCAA biosynthetic pathways in higher plants have been uncovered, knowledge of their genetic control is still limited, and no positive regulators have been identified to date. Here, we showed that variation in BCAA levels in rice is attributable to differential transcription ofOsbZIP18, a basic leucine zipper (bZIP) transcription factor, due to polymorphisms in its promoter. Functional analysis revealed thatOsbZIP18positively regulates BCAA synthesis by binding directly to the ACE and C-boxcis-elements in the promoters of the biosynthetic genesbranched-chain aminotransferase1(OsBCAT1) andOsBCAT2. We further demonstrated thatOsbZIP18is strongly induced by nitrogen (N) deficiency and that N starvation results in enhanced BCAA levels in an OsbZIP18-dependent manner. Overall, we identified OsbZIP18, a positive regulator of BCAA biosynthesis, which contributed to natural variation in BCAA levels and mediated BCAA accumulation throughde novosynthesis by directly modulating the key biosynthetic genesOsBCAT1andOsBCAT2.