An insight on the ethylene biosynthetic pathway of two major fruit postharvest pathogens with different host specificity: Penicillium digitatum and Penicillium expansum

Although several studies have reported the stress-induced ethylene burst occurring in fruit in response to pathogen attack, relatively little is known about the role of this hormone in the fungal metabolism of many postharvest pathogens. To gain further knowledge, we investigated the ethylene production capacity and which ethylene biosynthetic pathways (including the 1-aminocyclopropane-1-carboxylic acid (ACC), the 2-oxoglutarate (OXO) and the 2-keto-4-methylthiobutyric acid (KMBA) pathways) were used under different in vitro conditions by Penicillium digitatum and Penicillium expansum. Both pathogens were capable of producing ethylene in vitro using different pathways but were dependant on the growing conditions. For instance, both the KMBA and the OXO pathways seemed to be responsible for ethylene biosynthesis in P. digitatum when grown in more stressful conditions (limited surface and nutrient availability as in PDA tubes), while only the KMBA pathway seemed to be activated under other culture conditions (i.e. on PDA plates) or when the fungal mycelium was placed within aqueous solutions containing ethylene precursors. In contrast, the KMBA appeared to be the exclusive pathway responsible for ethylene production by P. expansum yet only when the fungal mycelium was in contact with aqueous solutions containing ethylene precursors. In both pathogens, ethylene production seemed to be synchronized with certain stages of fungal growth rather than conidiation, suggesting a putative role of this hormone in fungal metabolism.