cDNA-AFLP Analysis of Differentially Expressed Transcripts in Partially Resistant Apple Cultivar Leaves Infected by Venturia inaequalis

Apple scab is one of the most important diseases of apple. Although resistance genes governing qualitative resistance have been isolated and characterized, the biological roles of genes governing quantitative resistance are still unknown. This study investigated the molecular and biochemical mechanisms involved in the partial resistance against Venturia inaequalis of an old Belgian apple cultivar 'President Roulin'. Gene expression in both resistant and susceptible apple cultivars after scab inoculation was studied using the cDNA-AFLP method. From about 10,250 cDNA fragments detected, 252 bands were up- or down-regulated in the resistant cultivar 48 hours after scab inoculation. From these amplicons, 135 appeared to be regulated only in the resistant cultivar, but not in the susceptible cultivar 'Gala'. Some 224 fragments were constitutively expressed by the resistant cultivar only, and were not affected by the fungal infection. All the pathogen-responsive fragments and some of the fragments showing constitutive expression were cloned, sequenced and compared with GENBANK accessions using blastx. Homologies to known genes revealed several proteins belonging to the NBS-LRR and LRR-RLK classes of plant R genes. Other defense-related transcripts were also identified and could play a role in a partial scab defense response. Expression of these genes was verified by real-time quantitative PCR.