Characterization of genes associated with induced resistance against Penicillium expansum in apple fruit treated with quercetin

Penicillium expansum causes blue mould, a serious postharvest disease of apples, and is the main producerof the mycotoxin patulin. Since control by synthetic fungicides is less accepted by consumers, the demandfor alternative means is pressing. In a recent study, the flavonoid quercetin, although scarcely effectivein in vitro assays against P. expansum growth, significantly reduced blue mould rots on ‘Golden Delicious’apples, suggesting an enhancement of host disease resistance. To confirm or reject this hypothesis, genesdifferentially expressed in quercetin-treated ‘Golden Delicious’ apples were identified by suppressionsubtractive hybridization (SSH). A pool of 88 unique gene transcripts was obtained. Several sequencesrevealed high similarities with different classes of pathogenesis-related proteins (RNase-like PR10 andPR8), or with proteins expressed under stress conditions. Other transcripts had high similarity to genesof unknown function or genes coding for proteins having a role in pathogen recognition and in signallingpathways. SSH data were validated by analysing the expression of 14 genes by quantitative real-timePCR (qPCR). Eleven genes proved to be up-regulated at a medium-high level in freshly harvested apples.Among these, 5 genes selected for temporal expression profiling revealed the existence of a combinedeffect, particularly at 24 or 48 h, between wounding and phenolic treatment. These results provide evidencethat quercetin induces resistance to P. expansum in apples, by acting on the transcription level ofgenes involved in several distinct metabolic processes.