Partial deletion of the small chromosome produces loss of pathogenicity in Fusarium oxysporum

Virginia Casado-del Castillo 1 Riccardo Baroncelli 2 Michael R. Thon 1 Ernesto P. Benito 1 Jose M. Diaz-Minguez 1
1CIALE (Instituto Hispano-Luso de Investigaciones Agrarias), Departamento de Microbiología y Genética, Universidad de Salamanca, Villamayor, Salamanca, Spain
2Laboratoire Universitaire de Biodiversité et Écologie Microbienne, ESIAB, Université de Brest, Plouzané, France

Chromosome 14 is the smallest chromosome of the standard genome of Fusarium oxysporum (F. oxysporum f. sp. lycopersici strain 4287) and has been described as a ‘pathogenicity chromosome’. It contains loci that encode virulence/pathogenicity factors and confers pathogenicity to non-pathogenic strains after its transfer from a pathogenic strain. Also, it has been recently shown that complete loss of this chromosome results in the loss of pathogenicity, although partial deletions that affect only supercontig 22 do not reduce virulence (Vlaardingerbroek et al., 2016). This chromosome is likely equivalent to the smallest chromosome of F. oxysporum f. sp. phaseoli (FOP).

The FTF gene family is composed of two pathogenicity factors: FTF1, with multiple paralogues all located in the small chromosome of highly virulent strains of FOP, and FTF2, a single copy factor located in the core genome. Both factors are involved in virulence/pathogenicity (Niño-Sánchez et al., 2016). We describe here the isolation and characterization of some strains carrying a partial deletion of the small chromosome (FOP-SP1sChr-pΔ). The region deleted includes all the paralogues of FTF1. FOP-SP1sChr-pΔ mutants show a complete loss of pathogenicity on common bean plants, suggesting that the deleted fragment harbours the relevant set of genes required to produce disease in this forma specialis. Although no Fusarium wilt symptoms were observed in common bean plants inoculated with FOP-SP1sChr-pΔ mutants, confocal laser microscopic analysis revealed the ability of these strains to colonize the host, indicating a behaviour similar to that shown by endophytic strains

Niño-Sánchez et al., 2016. Mol. Plant Pathol. 17, 1124–1139. Vlaardingerbroek et al., 2016. Mol. Plant Pathol. 17, 1455–1466.