A Near-Isogenic Fusarium oxysporum f. sp. lycopersici Strain with a Novel Combination of Avirulence Characteristics

Phylogenetic and Biological Characterization of Fusarium oxysporum Isolates Associated with Onion in South Africa

Fusarium oxysporum f. sp. cepae causes Fusarium basal rot of onion, a disease of worldwide importance. Limited information is available on the phylogenetic diversity, vegetative compatibility groups (VCGs), mating type idiomorphs, and virulence of F. oxysporum isolates associated with onion. Therefore, these characteristics were investigated in 19 F. oxysporum f. sp. cepae isolates from Colorado, 27 F. oxysporum f. sp. cepae and 33 F. oxysporum isolates nonpathogenic to onion from South Africa. Six F. oxysporum f. sp. cepae VCGs (0421 to 0426) were identified, of which three were new. The dominant VCGs in Colorado and South Africa were VCG 0421 (47% of isolates) and VCG 0425 (74%), respectively. VCG 0423 was the only VCG that was shared between the two regions. Molecular phylogenies (intergenic spacer region of the rDNA, elongation factor 1α, and mitochondrial small-subunit) confirmed the polyphyletic nature of F. oxysporum f. sp. cepae and showed that some F. oxysporum f. sp. cepae and nonpathogenic F. oxysporum isolates were genetically related. Most F. oxysporum f. sp. cepae isolates clustered into two distinct, well-supported clades. The largest clade only contained highly virulent isolates, including the two main VCGs (0421 and 0425), whereas the basal clade mostly contained moderately virulent isolates. These groupings along with the VCG data provide an important basis for selection of isolates for use in breeding programs, and for the development of molecular makers to identify VCGs. Mating type genotyping revealed the distribution of both mating type (MAT1-1 and MAT1-2) idiomorphs across phylogenetic clades, and the fact that several isolates contained both idiomorphs.

Drifter, a novel, low copy hAT-like transposon in Fusarium oxysporum is activated during starvation

The facultative pathogenic fungus Fusarium oxysporum is known to harbour many different transposable and/or repetitive elements. We have identified Drifter, a novel DNA transposon of the hAT family in F. oxysporum. It was found adjoining SIX1-H, a truncated homolog of the SIX1 avirulence gene in F. oxysporum f. sp. lycopersici. Absence of a target site duplication as well as the 5' part of SIX1-H suggests that transposition of Drifter into the ancestor of SIX1-H was followed by loss of a chromosomal segment through recombination between Drifters. F. oxysporum isolates belonging to various formae speciales harbour between 0 and 5 full-length copies of Drifter and/or one or more copies with an internal deletion. Transcription of Drifter is activated during starvation for carbon or nitrogen.

A small, cysteine-rich protein secreted by Fusarium oxysporum during colonization of xylem vessels is required for I-3-mediated resistance in tomato

A 12 kDa cysteine-rich protein is secreted by Fusarium oxysporum f. sp. lycopersici during colonization of tomato xylem vessels. Peptide sequences obtained with mass spectrometry allowed identification of the coding sequence. The gene encodes a 32 kDa protein, designated Six1 for secreted in xylem 1. The central part of Six1 corresponds to the 12 kDa protein found in xylem sap of infected plants. A mutant that had gained virulence on a tomato line with the I-3 resistance gene was found to have lost the SIX1 gene along with neighbouring sequences. Transformation of this mutant with SIX1 restored avirulence on the I-3 line. Conversely, deletion of the SIX1 gene in a wild-type strain results in breaking of I-3-mediated resistance. These results suggest that I-3-mediated resistance is based on recognition of Six1 secreted in xylem vessels.