Chitinases of Trichoderma atroviride Induce Scab Resistance and Some Metabolic Changes in Two Cultivars of Apple

This study reports the combination of a transgene-mediated defense mechanism with a conventionally bred resistance in order to improve apple scab resistance. Two cultivars of apple (Galaxy, scab-susceptible, and Ariane, carrying the Vf resistance gene) were transformed with endochitinase and exochitinase genes derived from the biocontrol fungus Trichoderma atroviride. The obtained transgenic lines were analyzed for the expression of both genes and resistance to two races of the pathogen Venturia inaequalis: the common race 1 and race 6 which overcomes the resistance conferred by the Vf gene. A negative correlation between growth of transgenic lines and endochitinase activity was observed. Reduced growth appeared to be associated with high lignin content and high peroxidase as well as glucanase activity, suggesting that endochitinase activity may disturb the metabolism of the plant. Scab inoculation with races 1 and 6 performed in a growth chamber on 14 lines of normal vigor identified 6 lines with significantly enhanced resistance. Ten lines with reduced vigor were tested in vitro with a bioassay on rooted shoots. All lines expressing high endochitinase activity exhibited a significant reduction of scab symptoms.