Fungal plant pathogens secrete effector molecules to establish disease on their hosts, and plants in turn use immune receptors to try to intercept these effectors. The tomato immune receptor Ve1 governs resistance to race 1 strains of the soil-borne vascular wilt fungi Verticillium dahliae and Verticillium albo-atrum , but the corresponding Verticillium effector remained unknown thus far. By high-throughput population genome sequencing, a single 50-Kb sequence stretch was identified that only occurs in race 1 strains, and subsequent transcriptome sequencing of Verticillium -infected Nicotiana benthamiana plants revealed only a single highly expressed ORF in this region, designated Ave1 (for A virulence on Ve1 tomato ). Functional analyses confirmed that Ave1 activates Ve1-mediated resistance and demonstrated that Ave1 markedly contributes to fungal virulence, not only on tomato but also on Arabidopsis . Interestingly, Ave1 is homologous to a widespread family of plant natriuretic peptides. Besides plants, homologous proteins were only found in the bacterial plant pathogen Xanthomonas axonopodis and the plant pathogenic fungi Colletotrichum higginsianum , Cercospora beticola , and Fusarium oxysporum f. sp. lycopersici . The distribution of Ave1 homologs, coincident with the presence of Ave1 within a flexible genomic region, strongly suggests that Verticillium acquired Ave1 from plants through horizontal gene transfer. Remarkably, by transient expression we show that also the Ave1 homologs from F. oxysporum and C. beticola can activate Ve1 -mediated resistance. In line with this observation, Ve1 was found to mediate resistance toward F. oxysporum in tomato, showing that this immune receptor is involved in resistance against multiple fungal pathogens.

Knowledge of pathogen biology and genetic diversity is a cornerstone of effective disease management, and accurate identification of the pathogen is a foundation of pathogen biology. Species names provide an ideal framework for storage and retrieval of relevant information, a system that is contingent on a clear understanding of species boundaries and consistent species identification. Verticillium, a genus of ascomycete fungi, contains important plant pathogens whose species boundaries have been ill defined. Using phylogenetic analyses, morphological investigations and comparisons to herbarium material and the literature, we established a taxonomic framework for Verticillium comprising ten species, five of which are new to science. We used a collection of 74 isolates representing much of the diversity of Verticillium, and phylogenetic analyses based on the ribosomal internal transcribed spacer region (ITS), partial sequences of the protein coding genes actin (ACT), elongation factor 1-alpha (EF), glyceraldehyde-3-phosphate dehydrogenase (GPD) and tryptophan synthase (TS). Combined analyses of the ACT, EF, GPD and TS datasets recognized two major groups within Verticillium, Clade Flavexudans and Clade Flavnonexudans, reflecting the respective production and absence of yellow hyphal pigments. Clade Flavexudans comprised V. albo-atrum and V. tricorpus as well as the new species V. zaregamsianum, V. isaacii and V. klebahnii, of which the latter two were morphologically indistinguishable from V. tricorpus but may differ in pathogenicity. Clade Flavnonexudans comprised V. nubilum, V. dahliae and V. longisporum, as well as the two new species V. alfalfae and V. nonalfalfae, which resembled the distantly related V. albo-atrum in morphology. Apart from the diploid hybrid V. longisporum, each of the ten species corresponded to a single clade in the phylogenetic tree comprising just one ex-type strain, thereby establishing a direct link to a name tied to a herbarium specimen. A morphology-based key is provided for identification to species or species groups.

Verticillium dahliae infecting tomato can be differentiated into races 1 and 2 based on differential pathogenicity on tomato cultivars carrying resistance gene Ve1. Although no commercial cultivars resistant to race 2 are available, race 2‐resistant rootstock cultivars Aibou and Ganbarune‐Karis have been bred in Japan. Nevertheless, the resistance of these rootstocks appears to be unstable in commercial tomato fields. Pathogenicity assays conducted under controlled conditions revealed that these rootstock cultivars are resistant to some isolates of race 2; this resistance is controlled by a single dominant locus, denoted by V2, based on segregation of resistance in F2 populations from selfed rootstock cultivars. However, some other isolates of race 2 can overcome this resistance. Therefore it is proposed that the current race 2 of V. dahliae should be divided into two races, i.e. ‘race 2’ (nonpathogenic on Aibou) and ‘race 3’ (pathogenic on Aibou). The distribution of these races was surveyed in 70 commercial tomato fields in Hida, Gifu Prefecture, Japan. Race 3 was found in 45 fields, indicating that race 3 had already spread throughout the region. On the other hand, 25 fields had only race 2, and thus race 2‐resistant rootstocks would be effective for disease management in these fields. Races 2 and 3 could not be identified by genomic Southern hybridization probed with a telomere sequence, nor with previously reported race‐specific PCR assays. Elucidation of race‐determining mechanisms and development of methods for quick race identification should be made in future studies.

Plant pathogens secrete effector molecules during host invasion to promote colonization. However, some of these effectors become recognized by host receptors to mount a defence response and establish immunity. Recently, a novel resistance was identified in wild tomato, mediated by the single dominant V2 locus, to control strains of the soil-borne vascular wilt fungus Verticillium dahliae that belong to race 2. With comparative genomics of race 2 strains and resistancebreaking race 3 strains, we identified the avirulence effector that activates V2 resistance, termed Av2. We identified 277 kb of race 2-specific sequence comprising only two genes encoding predicted secreted proteins that are expressed during tomato colonization. Subsequent functional analysis based on genetic complementation into race 3 isolates and targeted deletion from the race 1 isolate JR2 and race 2 isolate TO22 confirmed that one of the two candidates encodes the avirulence effector Av2 that is recognized in V2 tomato plants. Two Av2 allelic variants were identified that encode Av2 variants that differ by a single acid. Thus far, a role in virulence could not be demonstrated for either of the two variants.

Verticillium dahliae, a soilborne plant pathogen, causes wilt disease in many important crops. We reported previously that the mating type gene MAT1-2-1 is spread to isolates of this asexual fungus. However, we did not determine whether V. dahliae is homothallic or heterothallic because the opposite mating type gene, MAT1-1-1, had not been identified. In the present study, we identified the MAT1-1-1 gene from an isolate lacking MAT1-2-1 and the mating type idiomorphs of V. dahliae. Each isolate we tested contained either the MAT1-1 or MAT1-2 idiomorph, indicating that the asexual fungus V. dahliae is potentially heterothallic.