Resistance of apple leaves to infection by Colletotrichum fructicola acts independently of hypersensitive reaction and PR-1 and PR-10 gene expression

Colletotrichum fructicola is currently one of the most important fungal pathogens affecting apple production in the subtropics. This work was aimed at studying the resistance mechanisms of apple leaves during early stages of pathogen infection. Seedlings from ‘Fuji’ × ‘Gala’ crosses, resulting in resistant and susceptible plants (1:1) were used in an assay model. Disease severity reached 19% on susceptible plants at 7 days after inoculation with C. fructicola. Enzymatic activity of ascorbate peroxidase, guaiacol peroxidase, and superoxide dismutase were similar between resistant and susceptible leaves, whereas glutathione reductase activity was enhanced in inoculated resistant plants. Hydrogen peroxide (H2O2) leaf content showed a decrease only in inoculated resistant leaves at 12 h after inoculation (hai). H2O2 accumulation associated with the hypersensitive reaction (HR) was observed at a low frequency (ca. one reaction in 3000 penetration attempts) in resistant leaves. Pathogenesis-related 1 (PR-1) gene expression was not affected by inoculation with C. fructicola up to 48 hai. Whereas PR-10 gene expression was significantly increased in leaves of both genotypes at 24 and 48 hai. Results suggest that the monogenic recessive resistance of apple to GLS acts independently of HR and both PR-1 and PR-10 genes expression.