Differential roles, crosstalk and response to the Antifungal Protein AfpB in the three Mitogen-Activated Protein Kinases (MAPK) pathways of the citrus postharvest pathogen Penicillium digitatum.

Fungi have three mitogen-activated protein kinases (MAPKs): Kss1/Fus3 involved in the invasive growth and virulence of pathogens, Hog1 in response to osmotic stress, and Slt2/Mpk1 in response to cell wall (CW) stress. We conducted comparative analyses of these MAPKs in the phytopathogen Penicillium digitatum and studied their role in the mode of action of the novel self-antifungal protein AfpB. The sensitivity to different stresses of Δhog1 and the reduced growth of Δkss1 coincided with previous reports. However, Δslt2 showed a strong reduction of growth and conidiation, abnormal morphology, and sensitivity to CW stress and temperature. The complementation of Δslt2 validated this mutant. Immunodetection of P-Hog1 and P-Slt2 confirmed the loss and gain of MAPKs in the mutant and complemented strains. Mutants Δslt2 and Δkss1 showed a strong reduction in virulence, whereas Δhog1 was the least affected, and none sporulated during infection. We studied the MAPK signalling induction in response to different treatments. Our data revealed a complex crosstalk involving the three MAPKs, the differential responses of Hog1 and Slt2 to various stresses and their induction by AfpB or the fungicide fludioxonil (FD). Δhog1 resistance to FD confirmed that Hog1 mediates the activity of FD, whereas Δkss1 sensitivity is probably due to the basal activation of Hog1 in Δkss1. None of the three MAPK mutants showed increased sensitivity to AfpB, contrary to previous reports of other antifungal proteins, which indicates that the observed AfpB-mediated activation of Hog1 and Slt2 would not have a defensive role.