Molecular Plant Pathology: Identification of pathogenicity-related genes in Fusarium oxysporum f. sp. cepae
Pathogenic isolates of Fusarium oxysporum, distinguished as formae speciales (f. spp.) based on their host specificity, cause crown rots, root rots and vascular wilts on many important crops worldwide. F. oxysporum f. sp. cepae (FOC) is particularly problematic to onion growers worldwide and is increasing in prevalence in the UK. We characterised 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 while 10 were non-pathogenic. Molecular characterisation 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 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 characterised 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 sequence which were unique to FOC while CRX1 and C5 genes appear to be largely FOC specific.
Plant Pathology: Identification of differential resistance to six Fusarium oxysporum f. sp. cepae isolates in commercial onion cultivars through the development of a rapid seedling assay
Fusarium oxysporum isolates collected from onions in the UK and other countries were characterized using sequences of the transfer elongation factor 1-α (TEF) gene and compared with published sequence data for 10 other isolates. Isolates associated with diseased onion bulbs in the UK formed two clades. Isolates from both clades were selected for pathogenicity testing and to develop a rapid seedling assay to screen commercial onion cultivars for resistance to F. oxysporum f. sp. cepae (FOC), the cause of basal rot. Differences in the levels of aggressiveness between isolates were observed and isolates from both clades were pathogenic. Differences in resistance/susceptibility were also observed amongst 10 commercial onion cultivars, with cvs Ailsa Craig Prizewinner and White Lisbon showing the highest levels of resistance. The results from the seedling assay were supported by those from a subsequent onion bulb rot assay. Thus, this study reports the development of a rapid, simple and repeatable seedling assay that can be used to screen large numbers of onion cultivars for resistance to FOC and which is indicative of resistance at the bulb stage.