Contribution of the putative clathrin adaptor AP-1 subunit β to cell wall integrity and virulence factor secretion in the plant pathogen Botrytis cinerea.

In filamentous fungi, the secretory pathway plays a key role in polarized growth by delivering vesicles carrying lipids and enzymes required for cell wall synthesis and nutrition. In pathogenic species, it also supports the delivery of virulence factors. In metazoa and yeast, secretory vesicle formation occurs at the Golgi apparatus and endosomes through a complex and still partially characterized machinery involving clathrin and adaptor proteins such as AP-1. In filamentous fungi, the molecular mechanisms controlling these processes remain poorly understood. Using a mutant underexpressing the β-subunit of the heterotetrameric adaptor AP-1, we investigated the role of this adaptor in the phytopathogenic fungus Botrytis cinerea. Reduced expression of AP-1β resulted in pleiotropic developmental defects, altered cell wall integrity, and loss of polarized trafficking of a class III chitin synthase. Moreover, the AP-1β mutant is affected in the secretion of specific classes of hydrolytic enzymes and displays a severe defect in pathogenicity. Together, these results highlight the importance of AP-1-dependent trafficking pathways in the biology and virulence of B. cinerea.