A system of defence in depth provided by vascular parenchyma cells of tomato in response to vascular infection with Fusarium oxysporum f. sp. lycopersici, race 1

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The penetration and colonization of xylem parenchyma cells surrounding xylem vessels of tomato infected with Fusarium oxysporum f. sp. lycopersici were examined. The primary xylem tissues of both resistant and susceptible near-isolines were found to be highly resistant to infection with only 5% and 3%, respectively, of the first tier of cells and none of the second layer of cells being invaded. The secondary xylem tissues were considerably more susceptible to infection, and here the difference between resistant and susceptible cultivars was pronounced. Secondary tissues were selected in which at least three layers of xylem parenchyma cells abutted the infected vessels. The cells in these layers were designated contact cells (those in immediate contact with the vessels) and first and second adjacent cells. The progress of infection and apparent defence responses were recorded within these three layers of cells at 3 and 6 days after inoculation. These data were later combined when it became apparent that by 3 days after inoculation a successful defence had already been established within the three cell layers, or the infection front had moved beyond them. The percentages of cells infected within these three cell layers were 52, 15 and 3, respectively, in the resistant cultivar, with a mean of 23%. In the susceptible cultivar these percentages were 90, 83, and 70, respectively, with a mean of 81%. Although similar responses, callose deposition and an apparent hypersensitive reaction were observed in both cultivars, the responses, especially that of callose deposition, appeared to be much stronger and to provide a more effective defence in the resistant cultivar than in the susceptible cultivar.