OsMYB67 Knockout Promotes Rice Heading and Yield by Facilitating Copper Distribution in Panicles

Copper (Cu) is an essential micronutrient required for rice flowering and seed setting. Here, we identified that Cu-induced R2R3-MYB transcription factor, OsMYB67, acts as a negative regulator that controls rice heading and yield production by affecting Cu distribution in panicles. OsMYB67 was constitutively expressed, with the highest expression in the roots. OsMYB67 knockout did not affect plant growth, but significantly increased Cu concentrations in roots, shoots, and xylem sap at the seedling stage. At the reproductive stage, OsMYB67 mutants displayed an early heading phenotype, with significantly increased Cu distribution in panicles but decreased Cu distribution in leaves, whereas OsMYB67-overexpressing plants showed the opposite result. In addition, higher grain yield and Cu concentrations in seeds were observed in OsMYB67 mutants compared to the wild-type. The results of Y1H, transient co-expression, EMSA, in situ RT-PCR, and RT-qPCR showed that OsMYB67 directly binds to the promoter region of OsHMA9 and upregulates its expression. Significantly increased Cu concentrations were also observed in the roots, shoots, and seeds of oshma9 mutants, consistent with the results observed in OsMYB67 mutants. Interestingly, dramatically higher expression levels of OsATX1 and OsYSL16 were observed in the OsMYB67 mutants, which may contribute to the increased Cu distribution in the panicles.