Expression of fungal chitinolytic enzymes in transgenic apples confers high levels of resistance to scab.

Abstract

Apple (Malus pumila) cultivar Marshall McIntosh was transformed with gene ThEn-42 coding endochitinase from the biocontrol fungus Trichoderma harzianum by Agrobacterium-mediated transformation. Varying levels of expression of ThEn-42 were found in different lines. Transgenic MMc lines expressing ThEn-42 were more resistant to apple scab (Venturia inaequalis) than the parental lines as indicated with both grafted and own-rooted plants. There was a negative correlation between level of ThEn-42 expression and plant vigour. However, plants with intermediate levels of expression were substantially less susceptible than non-transgenics, and some plants with intermediate levels of resistance had good vigour. In another experiment, an exochitinase gene encoding N-acetyl-beta-D-glucosaminidase (ThNag-70) was cloned from T. harzianum, and obtained ThNag-70 transgenic-MMc lines were produced. A very high efficiency of transformation was obtained using the Agrobacterium strain EHA105-PCH32. Transgenic lines containing both ThEn-42 and ThNag-70 are currently being investigated. It is hypothesized that plants with "pyramided" genes, that express more than one synergistic protein, will be resistant to diseases even at relatively low levels of enzyme production and also have good vigour.