Identification and characterization of the rice pre‐harvest sprouting mutants involved in molybdenum cofactor biosynthesis

Summary In cereal crops, ABA deficiency during seed maturation phase causes pre‐harvest sprouting ( PHS ), and molybdenum cofactor (MoCo) is required for ABA biosynthesis. Here, two rice PHS mutants F254 and F5‐1 were characterized. In addition to the PHS , these mutants showed pleiotropic phenotypes such as twisting and slender leaves, and then died when the seedling developed to four or five leaves. Map‐based cloning showed that Os CNX 6 and Os CNX 1 encoding homologs of MoaE and MoeA were responsible for F254 and F5‐1 mutants, respectively. Genetic complementation indicated that Os CNX 6 not only rescued the PHS and seedling lethal phenotype of the cnx6 mutant, but also recovered the MoCo‐dependent enzyme activities such as xanthine dehydrogenase ( XDH ), aldehyde oxidase ( AO ), nitrate reductase ( NR ) and sulfite oxidase ( SO ). Expression pattern showed that Os CNX 6 was richly expressed in seed during embryo maturation by quantitative reverse transcriptase PCR and RNA in situ hybridization. Furthermore, the Os CNX 6 overexpression plants can significantly enhance the MoCo‐dependent enzyme activities, and improved the osmotic and salt stress tolerance without unfavorable phenotypes. Collectively, these data indicated that Os CNX 6 participated in MoCo biosynthesis, and is essential for rice development, especially for seed dormancy and germination, and Os CNX 6 could be an effective target for improving abiotic stress tolerance in rice.