Differential apple transcriptomic responses to Penicillium expansum (pathogen) and Penicillium digitatum (non-host pathogen) infection

Penicillium expansum is the causal agent of blue mould of pome fruits and is responsible for important economical losses during postharvest handling in all producing countries. Although control of this pathogen can be achieved by using chemical fungicides, the appearance of resistant strains and increasing public concern about the use of chemicals in food products have motivated the study of host-pathogen interactions. To develop a better understanding of disease resistance mechanisms in apples, a comprehensive transcriptional analysis of apple gene expression in response to a compatible (P. expansum) and non-host (Penicillium digitatum) pathogen was conducted using an apple microarray of approximately 40,000 probes. The obtained data provide further evidence that apples inoculated with P. expansum exhibit significant upregulation of defense-related genes and genes involved in detoxification of reactive oxygen species. In contrast, apples inoculated with P. digitatum, a non-host pathogen, exhibited upregulation of genes involved in phenylpropanoid metabolism. To confirm the accuracy of the expression profiles obtained with the microarray, reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) was conducted for four genes involved in the phenylpropanoid pathway (PAL1, PAL2, COMT2 and POX64). Expression data were obtained for different time points after inoculation and fruit maturity stages. The highest expression level of the phenylpropanoid genes was detected 48 h after inoculation with P. expansum in both immature and mature apples. Collectively, the results of the present study support the hypothesis that apples exhibit a more complex and diverse defense response to the compatible pathogen than to the non-host pathogen. P. expansum, however, is able to overcome these defenses and successfully infect apples