Targeted Mutagenesis of POLYAMINE OXIDASE 5 that Negatively Regulates Mesocotyl Elongation Enables the Generation of Direct-seeding Rice with Improved Grain Yield.

Abstract Under conditions of labor or resource scarcity, direct seeding, rather than transplantation, is the preferred mode of rice (Oryza sativa) cultivation. This approach requires varieties that exhibit uniform seedling emergence. Mesocotyl elongation (ME), the main driver of rapid emergence of rice seedlings from soil, is enhanced by darkness and inhibited by light. Plant polyamine oxidases (PAOs) oxidize polyamines (PAs) and release H2O2. Here, we established that OsPAO5 expression in rice seedlings is increased in the presence of light and inhibited by darkness. To determine its role in ME, we created OsPAO5 mutants using CRISPR/Cas9. Compared with the wild type, pao5 mutants had longer mesocotyls, released less H2O2, and synthesized more ethylene. The mutant seedlings emerged at a higher and more uniform rate, indicating their potential for use in direct seeding. Nucleotide polymorphism analysis revealed that an SNP (PAO5−578G/A) located 578 bp upstream of the OsPAO5 start codon alters its expression, and was selected during rice mesocotyl domestication. The PAO5−578G genotype conferring a long mesocotyl mainly exists in wild rice, most Aus accessions, and some Geng (Japonica) accessions. Intriguingly, knocking out OsPAO5 can remarkably increase the grain weight, grain number, and yield potential. In summary, we developed a novel strategy to obtain elite rice with higher emergence vigor and yield potential, which can be conveniently and widely used to breed varieties of direct-seeding rice.