Synergistic activity of endochitinase and exochitinase from Trichoderma atroviride (T-harzianum) against the pathogenic fungus (Venturia inaequalis) in transgenic apple plants

Genes from the biocontrol fungus Trichoderma atroviride encoding the antifu ngal proteins endochitinase or exochitinase (N-acetyl-beta -D-hexosaminidas e) were inserted into 'Marshall McIntosh' apple singly and in combination. The genes were driven by a modified CaMV35S promoter. The resulting plants were screened for resistance to Venturia inaequalis, the causal agent of ap ple scab, and for effects of enzyme expression on growth. Disease resistanc e was correlated with the level of expression of either enzyme when express ed alone but exochitinase was less effective than endochitinase. The level of expression of endochitinase was negatively correlated with plant growth while exochitinase had no consistent effect on this character. Plants expre ssing both enzymes simultaneously were more resistant than plants expressin g either single enzyme at the same level; analyses indicated that the two e nzymes acted synergistically to reduce disease. Selected lines, especially one expressing low levels of endochitinase activity and moderate levels of exochitinase activity, were highly resistant in growth chamber trials and h ad negligible reduction in vigor relative to control plants. We believe tha t this is the first report of resistance in plants induced by expression of an N-acetylhexosaminidase and is the first report of in planta synergy bet ween an exochitinase and an endochitinase.