Ripening-induced defence signalling in Botrytis cinerea-infected tomato fruits involves activation of ERF.F4 by a MYC2-NOR/RIN protein complex.

Fleshy fruits are commercially and nutritionally important but become more susceptible to pathogens as they ripen. Active defence mechanisms continue to function in ripe fruits but they are influenced by ripening regulators and ripening outcomes. Botrytis cinerea is one of the most important fruit fungal pathogens of tomato and jasmonic acid (JA) is the major defence phytohormone involved in response to B. cinerea infection. Pathogens induce the accumulation of bioactive jasmonoyl-isoleucine (JA-Ile), which transmits defence signals through a hub transcription factor (TF) MYC2 that regulates defence responses. This has been clarified in depth in leaves, but how ripening, and in particular the regulators NOR and RIN influence JA-Ile-mediated defence responses remains unclear. In this article, we demonstrate that knocking-out NOR or RIN either singly (CR-NOR, CR-RIN) or together (CR-NOR/RIN) weakens JA-Ile accumulation in response to B. cinerea infection in tomato fruit. Both NOR and RIN physically interact with MYC2, and 110 of 569 differentially expressed genes (DEGs) in B. cinerea-infected wild-type (WT) fruits were identified as direct targets of MYC2. These included ethylene response factors (ERFs) such as ERF.A2, B12, C3, F4, G2/PTI6, whose transcripts accumulated in WT but were greatly reduced in either CR-NOR or CR-RIN infected fruits. Only MYC2 binding to ERF.F4 was severely impaired in CR-NOR and CR-RIN fruits. ERF.F4 transactivates PR-STH2 and activates defence signalling in B. cinerea-infected fruit. The increase in susceptibility to infection caused by promotion of ripening by RIN and NOR is thus partially compensated by their positive action in defence signalling.