OsCOP1 regulates embryo development and flavonoid biosynthesis in rice (Oryza sativa L.)

KEY MESSAGE: Novel mutations of OsCOP1 were identified to be responsible for yellowish pericarp and embryo lethal phenotype, which revealed that OsCOP1 plays a crucial role in flavonoid biosynthesis and embryogenesis in rice seed. Successful production of viable seeds is a major component of plant life cycles, and seed development is a complex, highly regulated process that affects characteristics such as seed viability and color. In this study, three yellowish-pericarp embryo lethal (yel) mutants, yel-hc, yel-sk, and yel-cc, were produced from three different japonica cultivars of rice (Oryza sativa L). Mutant seeds had yellowish pericarps and exhibited embryonic lethality, with significantly reduced grain size and weight. Morphological aberrations were apparent by 5 days after pollination, with abnormal embryo development and increased flavonoid accumulation observed in the yel mutants. Genetic analysis and mapping revealed that the phenotype of the three yel mutants was controlled by a single recessive gene, LOC_Os02g53140, an ortholog of Arabidopsis thaliana CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1). The yel-hc, yel-sk, and yel-cc mutants carried mutations in the RING finger, coiled-coil, and WD40 repeat domains, respectively, of OsCOP1. CRISPR/Cas9-targeted mutagenesis was used to knock out OsCOP1 by targeting its functional domains, and transgenic seed displayed the yel mutant phenotype. Overexpression of OsCOP1 in a homozygous yel-hc mutant background restored pericarp color, and the aberrant flavonoid accumulation observed in yel-hc mutant was significantly reduced in the embryo and endosperm. These results demonstrate that OsCOP1 is associated with embryo development and flavonoid biosynthesis in rice grains. This study will facilitate a better understanding of the functional roles of OsCOP1 involved in early embryogenesis and flavonoid biosynthesis in rice seeds.