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Eukaryotic Cell, January 2008, p. 162-171, Vol. 7, No. 1
1535-9778/08/$08.00+0     doi:10.1128/EC.00258-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Vegetative Hyphal Fusion Is Not Essential for Plant Infection by Fusarium oxysporum ^,

Departamento de Genética, Universidad de Córdoba, 14071 Córdoba, Spain

Received 19 July 2007/ Accepted 9 November 2007

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, that (i) VHF and plant infection have signaling mechanisms in common and (ii) VHF is required for efficient plant infection. F. oxysporum mutants lacking either Fmk1 or Fso1, an orthologue of N. crassa So, were impaired in the fusion of vegetative hyphae and microconidial germ tubes. fmk1 fso1 double 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 have some signaling components in common, suggesting an evolutionary relationship between the underlying cellular mechanisms.

Published ahead of print on 26 November 2007.

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