Identification of differentially expressed genes in a resistant versus a susceptible blueberry cultivar after infection by Colletotrichum acutatum.

Anthracnose fruit rot, caused by the fungus Colletotrichum acutatum, is an important disease of blueberry worldwide. The cultivar Elliott is resistant, severely restricting fungal growth and sporulation relative to the susceptible cultivar Jersey. The objectives of this research were as follows: (i) to analyse pre-penetration events in 'Elliott' and 'Jersey' at different stages of fruit development; (ii) to identify putative defence genes in 'Elliott' fruit; and (iii) to monitor the timing of the oxidative burst in both cultivars. Light microscopy revealed no differences in the pre-penetration activities of C. acutatum on the immature fruit of both cultivars. However, at fruit ripening, conidia germinated and formed appressoria more rapidly on 'Jersey' than on 'Elliott' fruit. Using suppression subtractive hybridization, 37 differentially expressed sequence tags (ESTs) were detected in 'Elliott' versus 'Jersey' upon infection. Several of the ESTs had homology to known plant defence genes, such as a class II chitinase, pathogenesis-related protein 10 (PR10) and β-1,3-glucanase. Two putative genes involved in oxidative stress were identified: a metallothionein-like protein and monodehydroascorbate reductase. ESTs of fungal origin were also detected. Many ESTs had no homology to known genes. Using semi-quantitative and quantitative reverse transcription-polymerase chain reaction (RT-PCR), the expression of most of the candidate genes was detected earlier in 'Elliott' than in 'Jersey', some within 24 h post inoculation (hpi). Monitoring of the oxidative burst showed that the overall H(2) O(2) concentration was two to three times higher in 'Elliott' than in 'Jersey' at 24 hpi. The elucidation of the basis of resistance to C. acutatum in blueberry will facilitate the development of new anthracnose fruit rot-resistant cultivars.