Antagonism-driven upregulation of the pulcherriminic acid biosynthetic pathway in Metschnikowia pulcherrima and Leptosphaeria maculans interaction.

Metschnikowia pulcherrima is a promising biological control agent (BCA), particularly intended for postharvest fruit protection. The research presents pulcherriminic acid (PA) producing isolates obtained from organic fruit surfaces that are antagonistic towards Leptosphaeria maculans, the causal agent of phoma stem canker. A highly antagonistic strain identified as M. pulcherrima was selected for studying the interactions with L. maculans in various in vitro culture techniques. In solid co-cultures, increased iron bioavailability restricted PA diffusion in the yeast colony microenvironment, significantly reducing antagonism. Double-plate assays indicated that volatile organic compounds (VOCs) contribute to the total inhibitory effect. When evaluating liquid co-cultures, complete pathogen inhibition was achieved within the tested yeast concentrations. The employed culture system simultaneously promoted yeast competitive advantage and allowed for the monitoring of expression changes within the PUL gene cluster. Pathogen presence induced a significant sequential expression pattern within the PUL gene cluster: an early strong upregulation of PUL2 encoding PA biosynthesis enzyme and PUL3 encoding the PA and pulcherrimin transporter, followed by sustained increased expression of PUL3, and finally very strong induction of PUL4. No changes were observed in SNF2 expression in co-culture. This model elucidates the molecular mechanisms of yeast antagonism induction, which is essential for developing effective, sustainable plant formulations for preharvest and postharvest protection.