Enhanced disease resistance in transgenic carrot (Daucus carota L.) plants over-expressing a rice cationic peroxidase

Plant class III peroxidases are involved in numerous responses related to pathogen resistance including controlling hydrogen peroxide (H2O2) levels and lignin formation. Peroxidases catalyze the oxidation of organic compounds using H2O2 as an oxidant. We examined the mechanisms of disease resistance in a transgenic carrot line (P23) which constitutively over-expresses the rice cationic peroxidase OsPrx114 (previously known as PO-C1) and which exhibits enhanced resistance to necrotrophic foliar pathogens. OsPrx114 over-expression led to a slight enhancement of constitutive transcript levels of pathogenesis-related (PR) genes. These transcript levels were dramatically increased in line P23 compared to controls [GUS construct under the control of 35S promoter (35S::GUS)] when tissues were treated with cell wall fragments of the fungal pathogen Sclerotinia sclerotiorum (SS-walls), and to a lesser extent with 2,6-dichloroisonicotinic acid. There was no basal increase in basal H2O2 levels in tissues of the line P23. However, during an oxidative burst response elicited by SS-walls, H2O2 accumulation was reduced in line P23 despite, typical media alkalinization associated with oxidative burst responses was observed, suggesting that OsPrx114 was involved in rapid H2O2 consumption during the oxidative burst response. Tap roots of line P23 had increased lignin formation in the outer periderm tissues, which was further increased during challenge inoculation with Alternaria radicina. Plant susceptibility to a biotrophic pathogen, Erysiphe heraclei, was not affected. Disease resistance to necrotrophic pathogens in carrot as a result of OsPrx114 over-expression is manifested through increased PR transcript accumulation, rapid removal of H2O2 during oxidative burst response and enhanced lignin formation.