Phytohormones in the Penicillium digitatum-citrus fruit interaction

Early- and long-term variations in indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acid (JA)-related metabolites occurring during Penicillium. digitatum infection in the flavedo (outer peel part) and albedo (inner peel part) of Navelate orange (Citrus sinensis (L.) Osbeck), and in the flavedo of its mutant Pinalate, which is deficient in abscisic acid (ABA), were compared. The levels of these metabolites were also examined in the fungal spores either obtained from in vitro grown cultures or isolated from infected fruit. The hormones were determined by ultra-high performance chromatography (UHPLC) connected to a Qtrap triple quadrupole mass spectrometer. The results showed that IAA barely changed either in the albedo of Navelate or in the flavedo of the mutant, but sharply and transiently increased in the flavedo of the parental. This highlight the relevance of IAA in very early infection perception. Moreover, results showed that IAA levels were higher in zones clearly showing mycelium or spores than in the healthy or macerated zones. They also indicate that SA participates in the initial signalling of both the flavedo and the albedo upon fungal attack and demonstrated the relevance of the interaction between this hormone and ABA in the citrus fruit-P. digitatum interaction. The concentration of cis(+)- 12-oxo-phytodienoic acid (OPDA) dropped during the initiation of maceration and at early infection stages. The fungus redirected jasmonate metabolism towards the synthesis of methyl jasmonate (MeJA) and jasmonoylisoleucine (JAIle) from OPDA in a later infection stage. Finally, the results demonstrated for the first time that P. digitatum is able of producing SA, IAA, OPDA, JA, MeJA and JAIle, and that the fungus' ability to produce OPDA was reduced, and that of IAA and JA stimulated, because of the fruit influence.