Abstract
SummaryPathogenic isolates of Fusarium oxysporum, distinguished as formae speciales (f. spp.) on the basis of their host specificity, cause crown rots, root rots and vascular wilts on many important crops worldwide. Fusarium oxysporum f. sp. cepae (FOC) is particularly problematic to onion growers worldwide and is increasing in prevalence in the UK. We characterized 31 F. oxysporum isolates collected from UK onions using pathogenicity tests, sequencing of housekeeping genes and identification of effectors. In onion seedling and bulb tests, 21 isolates were pathogenic and 10 were non‐pathogenic. The molecular characterization of these isolates, and 21 additional isolates comprising other f. spp. and different Fusarium species, was carried out by sequencing three housekeeping genes. A concatenated tree separated the F. oxysporum isolates into six clades, but did not distinguish between pathogenic and non‐pathogenic isolates. Ten putative effectors were identified within FOC, including seven Secreted In Xylem (SIX) genes first reported in F. oxysporum f. sp. lycopersici. Two highly homologous proteins with signal peptides and RxLR motifs (CRX1/CRX2) and a gene with no previously characterized domains (C5) were also identified. The presence/absence of nine of these genes was strongly related to pathogenicity against onion and all were shown to be expressed in planta. Different SIX gene complements were identified in other f. spp., but none were identified in three other Fusarium species from onion. Although the FOC SIX genes had a high level of homology with other f. spp., there were clear differences in sequences which were unique to FOC, whereas CRX1 and C5 genes appear to be largely FOC specific.
Highlights
Fusarium oxysporum is a major pathogen of many important crops worldwide, causing crown and root rots as well as vascular wilts (Leslie and Summerell, 2006)
SIX1, SIX3 (Avr2), SIX4 (Avr1) and SIX5 are recognized by resistance genes which have been introgressed into tomato (Houterman et al, 2008, 2009; Ma et al, 2015; Rep et al, 2004; Takken and Rep, 2010), whereas gene knock-outs have demonstrated that SIX1, SIX3, SIX5 and SIX6 contribute directly to virulence (Gawehns et al, 2014; Houterman et al, 2009; Ma et al, 2015; Rep, 2005; Takken and Rep, 2010)
In the onion seedling tests, significant differences were observed in the pathogenicity of the 32 F. oxysporum isolates (Table 1) for the two experiments using Napoleon and HZS onion cultivars (Fig. 1, P < 0.001)
Summary
Fusarium oxysporum is a major pathogen of many important crops worldwide, causing crown and root rots as well as vascular wilts (Leslie and Summerell, 2006). The soil-borne fungus affects a very wide range of crop hosts, including onion, leek, lettuce, tomato, brassicas, asparagus, cucurbits, peppers, coriander, spinach, basil, beans, peas, strawberry, watermelon and banana, and important non-food crops, such as carnation and narcissus (Leslie and Summerell, 2006; Michielse and Rep, 2009). Lycopersici (FOL), which infects tomato (Ma et al, 2010) This led to the discovery of lineage-specific mobile pathogenicity chromosomes which contain pathogenicity-related genes. These include Secreted In Xylem (SIX) genes, the products of which are small effector proteins secreted by FOL during the colonization of tomato plants (Ma et al, 2010). SIX genes have been found in other f. spp. of F. oxysporum (Fraser-Smith et al, 2014; Sasaki et al, 2015b), VC 2015 THE AUTHORS MOLECULAR PLANT PATHOLOGY PUBLISHED BY BRITISH SOCIETY FOR PLANT PATHOLOGY AND JOHN WILEY & SONS LTD
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