Vegetative hyphal fusion is not essential for plant infection by Fusarium oxysporum

Departamento de Genética, Universidad de Córdoba, Campus de Rabanales Edif. C5, 14071 Córdoba, Spain

^* To whom correspondence should be addressed. Email: ge2dipia{at}uco.es.

	Abstract

Vegetative hyphal fusion (VHF) is a ubiquitous phenomenon in filamentous fungi whose biological role is poorly understood. In Neurospora crassa, the mitogen-activated protein kinase (MAPK) Mak-2 and the WW domain protein So are required for efficient VHF. A MAPK orthologous to Mak-2, Fmk1, was previously shown to be essential for root penetration and pathogenicity of the vascular wilt fungus Fusarium oxysporum. Here we took a genetic approach to test two hypotheses: 1) VHF and plant infection share common signalling mechanisms, and 2) VHF is required for efficient plant infection. F. oxysporum mutants lacking either Fmk1 or Fso1, an orthologue of N. crassa So, were impaired in fusion of vegetative hyphae and microconidial germ tubes. Double fmk1 fso1 mutants exhibited a more severe fusion phenotype than either single mutant, indicating that the two components function in distinct pathways. Both fso1 and fmk1 strains were impaired in the formation of hyphal networks on the root surface, a process associated with extensive VHF. The fso1 mutants exhibited slightly reduced virulence in tomato fruit infection assays, but in contrast to fmk1 strains, were still able to perform functions associated with invasive growth such as secretion of pectinolytic enzymes or penetration of cellophane sheets, and to infect tomato plants. Thus, although VHF per se is not essential for plant infection, both processes share some common signalling components suggesting an evolutionary relationship between the underlying cellular mechanisms.