A Single Laccase Acts as a Key Component of Environmental Sensing in a Broad Host Range Fungal Pathogen

Abstract

Secreted laccases are important enzymes on an ecological scale for their role in mediating plant-fungal interactions, but their function in fungal pathogenesis has yet to be elucidated. Ascomycete laccases have been primarily associated with cell wall melanin deposition, and laccase mutants in ascomycete species often demonstrate reduced pigmentation. In this study, a putatively secreted laccase, Sslac2, was characterized from the broad host-range plant pathogen Sclerotinia sclerotiorum, which is largely unpigmented and is not dependent on melanogenesis for plant infection. Of the seven putative laccases in the S. sclerotiorum genome, Sslac2 was the only one found to be highly upregulated during pathogenesis of soybeans and was additionally found to be induced during growth on solid surfaces. Gene knockouts of Sslac2 demonstrate wide ranging developmental defects, including abolished sclerotial formation, and are functionally non-pathogenic on unwounded tissue. While these mutants demonstrated a clear radial growth defect, enhanced growth was observed in liquid culture, likely due to altered hydrophobicity and thigmotropic responsiveness. Sslac2 mutants were also unable to respond to a host environment, and accordingly unable to differentiate penetration structures, respond appropriately to chemical stress, or produce the key virulence factor oxalic acid. Transmission and scanning electron microscopy of WT and mutant strains show apparent differences in extracellular matrix structure that may explain the inability of the mutant to respond to environmental cues. Targeting Sslac2 using host-induced gene silencing significantly improved resistance to S. sclerotiorum, suggesting that fungal laccases could be a valuable target of disease control. Collectively, we identified a laccase critical to the development and virulence of the broad host-range pathogen S. sclerotiorum and propose a potentially novel role for fungal laccases in modulating environmental sensing.