Evidence against the existence of genes for resistance to Fusarium oxysporum f. sp. lycopersici races 1 and 2 on Solanum pennellii chromosome 7 additional to I-3

Abstract

The I-3 gene for resistance to Fusarium oxysporum f. sp. lycopersici races 2 and 3 was introgressed into tomato (Solanum lycopersicum) from its wild relative Solanum pennellii. Additional genes for resistance to races 1 and 2 are suggested to exist alongside I-3 on S. pennellii chromosome 7. To examine the existence of a separate gene for race 2 resistance, we tested recombinants generated during the isolation of I-3 that lacked I-3 but carried the region suggested to confer race 2 resistance. Disease assays showed these lines had no resistance to race 2. To confirm this result, we tested the introgression lines IL7-2 and IL7-4, which together cover the entirety of S. pennellii chromosome 7 (including I-3), for resistance to race 2 carrying a knockout of Avr3. Loss of Avr3 resulted in a loss of race 2 resistance, indicating that I-3 is the only gene conferring race 2 resistance in these two lines. To examine the existence of a gene for race 1 resistance (designated I-1 in the literature), we tested the tomato line E779, which carries I-3 but lacks the I gene conferring race 1 resistance. Disease assays showed E779 had intermediate resistance to race 1, but no resistance to the Avr3-knockout derivative, indicating that I-3 alone is able to confer race 1 resistance. Finally, we examined the race-1-resistant tomato line 90E402F, which is supposed to carry I-1, for the presence of S. pennellii markers on chromosome 7. We found no S. pennellii markers in the region supposed to carry I-1, but instead found that this line carries the I gene. Thus, we found no evidence to support the existence of additional genes for race 1 and 2 resistance on S. pennellii chromosome 7, but instead found that I-3 alone is sufficient to explain the observed resistance to our isolates of races 1, 2 and 3.