The present disclosure relates to compositions and methods useful for the production of heterologous proteins with reduced O-mannosylation in filamentous fungal cells, such as Trichoderma cells. More specifically, the invention provides a PMT-deficient filamentous fungal cell comprising a) at least a first mutation that reduces an endogenous protease activity compared to a parental filamentous fungal cell which does not have said first mutation, and, b) at least a second mutation in a PMT gene that reduces endogenous O-mannosyltransferase activity compared to a parental filamentous fungal cell which does not have said second mutation, wherein said filamentous fungal cell is selected from the group consisting of Trichoderma, Neurospora, Myceliophthora or Chrysosporium cell.