Polyphenolic and Anthocyanin Responses to Postharvest Fungal Pathogen Infection in Purple and Green near Isogenic Pepper (Capsicum annuum) Lines

Postharvest fungal pathogens, such as Botrytis cinerea, Alternaria alternata and Fusarium culmorum, pose major challenges for pepper (Capsicum annuum) storage and shelf-life. To explore the basis of induced resistance, near isogenic lines (NILs) differing in pigmentation (green vs. purple fruits and their red ripe counterparts) were artificially inoculated and evaluated for disease severity by phenotyping and by qPCR, and metabolite composition by spectroscopy and by HPLC. Infection severity was strongly dependent on whether purple or green NILs were infected and on ripening stage: economically ripe fruits were most susceptible to B. cinerea, whereas biologically ripe fruits displayed higher infection rates with A. alternata. In the case of B. cinerea infection, detailed HPLC analysis revealed that chlorogenic acid and p-coumaric acid were identified as infection-responsive metabolites after analyzing the metabolite changes upon infection. Total anthocyanin content and delphinidin derivatives measured decreased upon infection; however, this effect was not significant in correlation with the infection severity, indicating that B. cinerea infection in low, moderate or severe amounts will lead to the degradation of these compounds. Overall, our findings indicate that anthocyanin accumulation alone did not confer resistance to B. cinerea in pepper, whereas specific hydroxycinnamic acids emerged as infection-responsive markers.