Ascorbic acid (AA) is the major antioxidant buffer produced in the shoot tissue of plants. Previous studies on root-knot nematode (RKN; Meloidogyne graminicola)-infected rice (Oryza sativa) plants showed differential expression of AA-recycling genes, although their functional role was unknown. Our results confirmed increased dehydroascorbate (DHA) levels in nematode-induced root galls, while AA mutants were significantly more susceptible to nematode infection. External applications of ascorbate oxidase (AO), DHA, or reduced AA, revealed systemic effects of ascorbate oxidation on rice defence versus RKN, associated with a primed accumulation of H₂O₂ upon nematode infection. To confirm and further investigate these systemic effects, a transcriptome analysis was done on roots of foliar AO-treated plants, revealing activation of the ethylene (ET) response and jasmonic acid (JA) biosynthesis pathways in roots, which was confirmed by hormone measurements. Activation of these pathways by methyl-JA, or ethephon treatment can complement the susceptibility phenotype of the rice Vitamin C (vtc1) mutant. Experiments on the jasmonate signalling (jar1) mutant or using chemical JA/ET inhibitors confirm that the effects of ascorbate oxidation are dependent on both the JA and ET pathways. Collectively, our data reveal a novel pathway in which ascorbate oxidation induces systemic defence against RKNs.

Ascorbic acid (AA) and AA oxidation play a vital role in plant growth and development. In this research we investigated their role in rice (Oryza sativa) root growth. Monitoring of germinating and early vegetative seedlings of the AA biosynthesis mutant vtc1 and its corresponding wild type (WT, ‘Nipponbare’) showed vtc1 mutant with 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 show roots of the vtc1 mutant with lower levels of Indole-3-acetic acid (IAA) and abscisic acid (ABA). The vtc1 radicle and very early vegetative root phenotype could be complemented by exogenous ABA or auxin (1- naphthalene acetic acid (NAA)) application, but not by AA application. However, at the later vegetative root growth phase only NAA could complement this growth phenotype. Our data revealed that, vtc1 germinating seedling and early vegetative root growth can be complemented by DHA and ascorbate oxidase (AO), the enzyme that oxidizes AA to DHA treatments. Also, these treatments promoted radicle and early vegetative root growth similarly in WT as in the vtc1-mutant, implicating that these hormones act downstream of AA biosynthesis. Our data show significantly higher concentration of IAA and ABA upon AO treatment in WT seedlings, implicating that AO-induced rice root growth is regulated via auxin and ABA levels. Taken together, these results imply that ascorbate oxidation stimulates rice root growth via positive effects on auxin and ABA levels.