Fusarium oxysporum colonizes the stem of resistant tomato plants, the extent varying with the R-gene present

The plant immune system employs resistance (R) genes to detect the presence of pathogenic microbes by the avirulence (Avr) factors they produce. Whereas some R-genes confer extreme resistance, completely blocking pathogen proliferation, others act later during infection and allow initial microbial multiplication in the host. We hypothesized that transmembrane R-proteins – as opposed to intracellular R-proteins - may recognize and arrest pathogen entry earlier in the infection process. In tomato, two transmembrane R-proteins (I and I-3) and one intracellular R-protein (I-2) have been identified conferring resistance against the same pathogen: the soil borne fungus Fusarium oxysporum f.sp. lycopersici (Fol). We found that in all root inoculations with Fol on resistant tomato plants, the fungus was able to reach the vasculature of the stem. However, the extent of host vasculature colonization was less than that in susceptible plants. This indicates that a complete blockade of fungal ingress does not occur in any of the incompatible interactions. However, resistance mediated by the intracellular R-protein I-2 allowed more extensive fungal colonization than resistance mediated by the transmembrane R-proteins I or I-3. The first phases of invasion – penetration of the root epidermis, cortex colonization and early stage xylem colonization – are unaffected by R-gene mediated resistance of the host. We suggest that all 3 R-proteins only limit Fol colonization after the fungus has reached the xylem tissues, and that I and I-3 act prior to and/or are more effective than I-2 in reducing pathogen proliferation and spread.