FIG 4.

R. solanacearum does not require nitrate assimilation for wild-type growth, virulence, or competitive fitness in tomato xylem. (A) Growth of wild-type strain UW551 (■) or nasA mutant (□) in either filter-sterilized xylem sap or water inoculated with 10⁴ CFU of bacteria. Growth was assessed 72 h later by dilution plating. The data shown are means of nine replicates; P > 0.05 (Mann-Whitney t test). (B) Disease progress of tomato plants inoculated directly into xylem tissue via a cut leaf petiole with 2,000 CFU of either R. solanacearum wild-type UW551 (black) or nasA mutant (white). Plants were rated daily on a 0 to 4 disease index scale; n = 10; P > 0.05 (repeated-measures ANOVA). (C) Competitive indices of marked R. solanacearum strains following cut-petiole inoculation of tomato plants with a 1:1 mixture of wild-type and nasA mutant bacteria (2,000 cells total). Portions (100 mg) of plant tissue were ground and dilution plated to quantify each strain at either the first sign of disease (disease index 1; n = 29) or at 48 h postinoculation (n = 10). The relative competitive index was calculated as the ratio of CFU of nas mutant recovered/g of stem to the CFU of UW551 wild-type recovered/g of stem, adjusted to inoculum levels. Horizontal lines represent the medians. The nasA mutant significantly outcompeted wild-type bacteria (P = 0.0026 and 0.0059 [Wilcoxon signed-rank test] for disease index 1 and 48 h postinfection [HPI], respectively).