A partial pathogenicity chromosome in Fusarium oxysporum is sufficient to cause disease and can be horizontally transferred

Abstract

During host colonization, plant pathogenic fungi secrete proteins, called effectors, to facilitate infection. Collectively, effectors may defeat the plant immune system, but usually not all effectors are equally important for infecting a particular host plant. In Fusarium oxysporum f.sp. lycopersici, all known effector genes - also called SIX genes - are located on a single accessory chromosome which is required for pathogenicity and can also be horizontally transferred to another strain. To narrow down the minimal region required for virulence, we selected partial pathogenicity chromosome deletion strains by fluorescence-assisted cell sorting of a strain in which the two arms of the pathogenicity chromosome were labelled with GFP and RFP, respectively. By testing the virulence of these deletion mutants, we show that the complete long arm and part of the short arm of the pathogenicity chromosome are not required for virulence. In addition, we demonstrate that smaller versions of the pathogenicity chromosome can also be transferred to a non-pathogenic strain and they are sufficient to turn the non-pathogen into a pathogen. Surprisingly, originally non-pathogenic strains that had received a smaller version of the pathogenicity chromosome were much more aggressive than recipients with a complete pathogenicity chromosome. Whole genome sequencing analysis revealed that partial deletions of the pathogenicity chromosome occurred mainly close to repeats, and that spontaneous duplication of sequences in accessory regions is frequent both in chromosome deletion strains and in horizontal transfer (recipient) strains. Author SummaryFungal genomes can often be divided into a core genome, which is essential for growth, and an accessory genome which is dispensable. The accessory genome in fungi can be beneficial under some conditions. For example, in some plant-pathogenic fungi, virulence genes are present in the accessory genome, which enable these fungi to cause disease on certain hosts. In Fusarium oxysporum f.sp. lycopersici, which infects tomato, all host-specific virulence genes are located on a single accessory chromosome. This pathogenicity chromosome can be horizontally transferred between strains. Here, we found that many suspected virulence genes are in fact not required for virulence because strains without a large part of the pathogenicity chromosome, including these genes, showed no reduced virulence. In addition, we demonstrate that partial pathogenicity chromosomes can be horizontally transferred to a non-pathogen. Surprisingly, originally non-pathogenic strains that had received a partial pathogenicity chromosome were more virulent than strains that had received the complete pathogenicity chromosome.