FIG 5.

nasA contributes to tomato plant bacterial wilt following a period of soil survival. (A) Virulence assay comparison. In a between-host virulence assay (see panel B), unwounded plants of moderately wilt-resistant tomato H7996 were inoculated with 10⁸ CFU of R. solanacearum/g soil. After 20 days, the plants were removed from each pot, and 10 days later a susceptible Bonny Best plant was transplanted into each pot. This is in contrast to the standard soil soak virulence assay (see the results presented in Fig. 2), where 10⁸ CFU of R. solanacearum were added to pots containing a 3-week-old Bonny Best plant. (B) Between-host virulence assay. The percentages of Bonny Best tomato plants showing any wilt symptoms each day after transplanting into pots previously containing a H7996 plant inoculated with either wild-type UW551 (■) or nasA mutant (□) strains are shown. This assay was repeated twice, with 20 plants/treatment in total (P = 0.0009 [repeated-measures ANOVA]). (C) Soil survival assessments. Pots were inoculated via soil soak as described above with 10⁸ CFU of either nasA mutant or wild-type R. solanacearum/g of soil. At 0 h, 6 h, 5 days, and 15 days postinoculation, 1-g soil samples were dilution plated to determine the population sizes of R. solanacearum in each pot. (D) Soil survival competitive index assessments. Pots were inoculated via soil soak with a 1:1 ratio of nasA mutant and wild-type R. solanacearum cells. Population sizes of each strain were determined by dilution plating on selective media at 6 h and 5 days postinoculation. The relative competitive index was calculated as the ratio of CFU nasA mutant to CFU wild-type recovered/g of stem, adjusted to inoculum levels. Horizontal bars represent the medians.