'Gala' apple transformed for scab resistance with cloned Vf gene region construct

The transformation of apple for resistance to scab Venturia inaequalis is an objective of several research projects today. While many apple cultivars have been bred over the last 30 years throughout the world for scab resistance, none has had any significant commercial success. The attempts to transform the most valuable apple cultivars for resistance to the pathogen, which is perhaps the major threat to this fruit species, began with the positive results from the mapping of the Vf gene. This allowed the construction of a BAC library and subsequently a contig of about 350 Kb spanning the Vf region. These steps enabled the working group formed by the University of Bologna's DCA and the Swiss Federal Institute of Technology in Zurich to identify four open reading frames (ORFs) whose sequences are homologous to receptor-like genes like those conferring resistance to Cladosporium fulvum in tomato. Hence the acronym HcrVf (homologous to Cladosporium fulvum resistance genes of the Vf region). These sequences, found as a cluster, are likely to be involved in Vf resistance and form the basis for the design of plasmid constructs for the introduction of these ORFs into scab-susceptible apple cultivars. The HcrVf2 sequence, cloned into binary vector pCAMBIA2301, was introgressed into 'Gala' apple using Agrobacterium tumefaciens, both leaf and internode explants were used. PCR assays of regenerated plantlets showed that almost 90% had HcrVf2 and the nptII genes. The acquired resistance was corroborated by repeated in vitro infection with V. inaequalis conidia. The definitive demonstration of acquired resistance is to be conducted in vivo using V. inaequalis inoculum in greenhouse.