Ascorbate oxidation stimulates rice root growth via effects on auxin and abscisic acid levels

Ascorbic acid (AA) and AA oxidation play a vital role in plant growth and development. In this study, we investigated the role of AA and AA oxidation in rice (Oryza sativa) root growth. Our results show, that rice AA biosynthesis mutant vitamin C 1 (vtc1) seedlings have a defect in radicle and early vegetative root growth. AA measurement displayed significantly lower levels of total AA, and mainly lower dehydroascorbic acid (DHA) in the roots of the vtc1 mutant. Phytohormone analysis shows that roots of the vtc1 mutant also contain lower levels of Indole-3-acetic acid (IAA) and abscisic acid (ABA). The vtc1 radicle root phenotype could be complemented by exogenous ABA or auxin (1- naphthalene acetic acid (NAA)) application, but not by AA application. Also, NAA and ABA treatments promoted radicle and early vegetative root growth similarly in WT as in the vtc1 mutant, implicating that they act downstream of AA biosynthesis. Both the radicle and the early vegetative root growth phenotype of vtc1 could be complemented by treatments with DHA or ascorbate oxidase (AO), the enzyme that oxidizes AA to DHA. Our data further demonstrate accumulation of IAA and ABA upon AO treatment in wildtype seedlings, implicating that AO-induced rice root growth is regulated via auxin and ABA levels. Taken together, these results imply that ascorbic acid and its oxidation stimulates rice root growth via positive effects on auxin and ABA levels.