Transcriptomic Insight into Early Response of Apple Infected with Penicillium expansum Exposed to Blue LED Light

The aim of the study was to investigate the early response of apple fruit infected with Penicillium expansum (P. expansum) to blue light-emitting diode (LED) light (BLL) irradiation. To focus our study on the interaction between apple fruit, the pathogen, and BLL, the effect of BLL was also studied on apples without P. expansum and P. expansum grown on malt extract agar (MEA). Transcriptome analysis revealed that the most pronounced responses among biological processes were observed in inoculated apples under BLL. The upregulated processes included water transport, response to heat, and response to high light intensity. The defence response of apples was enhanced by the upregulation of thaumatin-like proteins and caffeic acid 3-O-methyltransferase, while the cellular response to phosphate deficiency and the regulation of multicellular organism development were downregulated. In P. expansum grown on apples under BLL, transcriptome analysis revealed downregulation of genes related to signalling, response to organic compounds, and regulation of metabolic and biosynthetic processes, while genes involved in the biosynthesis of secondary metabolites were upregulated. In addition, the expression of patulin cluster genes was predominantly downregulated in P. expansum. The significant upregulation of genes related to cryptochrome inhibition, defence response, and caffeic acid metabolism in apples under BLL, together with the reduced virulence of P. expansum, contributes to the inhibition of fungal growth.