D-amino acid aminotransferase1 regulates grain chalkiness in rice by modulating endoplasmic reticulum stress response

D-amino acids are key components of the bacterial cell wall and play important roles in neural communication and inflammatory responses in animals. However, knowledge about D-amino acid metabolism and physiological functions in plants is limited. Here, we isolated and characterized a rice D-amino acid aminotransferase1, OsDAAT1, which maternally regulates rice grain chalkiness through map-based cloning and a subsequent complementation test. We found that OsDAAT1 is highly expressed in the vascular tissue of rice nodes and is capable of interconverting different D-amino acids in vitro. Mutation of OsDAAT1 results in elevated D-alanine levels in stems, nodes, and developing grains. The disruption of D-amino acid metabolism subsequently leads to significantly altered peptide/protein isomerization, including some key enzymes involved in starch and protein biosynthesis. These changes trigger severe endoplasmic reticulum stress and ultimately leads to chalky grains. Furthermore, we identified OsDAAT1Hap1as a low-chalkiness haplotype, and historical frequency analysis suggests that OsDAAT1 may have undergone selection during rice domestication. Overall, our findings uncover a previously unrecognized role in D-amino acid metabolism in plants and facilitate the practical use of OsDAAT1 in grain appearance quality improvement in rice.