Dissection of the Fusarium I2 Gene Cluster in Tomato Reveals Six Homologs and One Active Gene Copy

Abstract

The I2 locus in tomato confers resistance to race 2 of the soil-borne fungus Fusarium oxysporum f sp lycopersici. The selective restriction fragment amplification (AFLP) positional cloning strategy was used to identify I2 in the tomato genome. A yeast artificial chromosome (YAC) clone covering approximately 750 kb encompassing the I2 locus was isolated, and the AFLP technique was used to derive tightly linked AFLP markers from this YAC clone. Genetic complementation analysis in transgenic R1 plants using a set of overlapping cosmids covering the I2 locus revealed three cosmids giving full resistance to F. o. lycopersici race 2. These cosmids shared a 7-kb DNA fragment containing an open reading frame encoding a protein with similarity to the nucleotide binding site leucine-rich repeat family of resistance genes. At the I2 locus, we identified six additional homologs that included the recently identified I2C-1 and I2C-2 genes. However, cosmids containing the I2C-1 or I2C-2 gene could not confer resistance to plants, indicating that these members are not the functional resistance genes. Alignments between the various members of the I2 gene family revealed two significant variable regions within the leucine-rich repeat region. They consisted of deletions or duplications of one or more leucine-rich repeats. We propose that one or both of these leucine-rich repeats are involved in Fusarium wilt resistance with I2 specificity.

Potato chromosomes IX and XI carry genes for resistance to potato virus M

“…The co-linear region of tomato chromosome 11 harbors the I2 locus conferring resistance to race 2 of F. oxysporum f. sp. lycopersici (Ori et al 1997;Simons et al 1998;Huang et al 2005). The Rm locus reported here was at a close genetic distances of 0.8 cM from the marker GP250.…”

“…The PVM resistance gene Rm is therefore a new member of this resistance gene cluster located on potato chromosome XI and the first one conferring resistance to a virus. Sequence analysis of the I2 locus in tomato (Ori et al 1997;Simons et al 1998) and the R3a locus in potato (Huang et al 2005) revealed a complex structure of clustered resistance gene homologs. Based on its position, the gene Rm could be another member of these gene families.…”

“…The marker UBC822 1079 linked to Rm showed significant DNA sequence similarity to the I2C-1 resistance gene against F. oxysporum f. sp. lycopersici located on tomato chromosome 11 (Simons et al 1998). The marker UBC878 962 tightly linked to Gm shared high identity with sequences upstream of both the Rx1 gene for resistance to PVX and the Gpa2 gene for resistance to the potato cyst nematode Globodera pallida (van der Vossen et al 2000).…”

“…UBC822 1079 shared 89% sequence identity with tomato I2C-1 resistance gene (AF004878.1) to Fusarium oxysporum f. sp. lycopersici located on tomato chromosome 11 (Simons et al 1998). …”

Identification and evidence of positive selection upon resistance gene analogs in cotton (Gossypium hirsutum L.)

“…These two degenerate primers were particularly designed based on the above conserved domains of the I2 gene of tomato, which confers resistance to race 2 of Fusarium oxysporum f. sp. lycopersici (Simons et al 1998) and of the Arabidopsis RPS2 gene, which encodes a protein that specifically recognizes Pseudomonas syringae pv. tomato strains (Axtell et al 2001).…”