Growth Kinetics and Histopathology ofXanthomonas campestsrispv.glycinesin Leaves of Resistant and Susceptible Soybeans

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Abstract
In soybeans, a very high level of resistance to the bacterial pustule disease is conferred by a recessive gene designated rxp. Studies were undertaken to characterize this resistance mechanism. The first trifoliolate leaves of resistant and susceptible soybean plants were inoculated with Xanthomonas campestris pv. glycines by using an airbrush. Inoculum concentrations, were varied to produce initial bacterial populations ranging from 5 to 1,600 colony-forming units (cfu)/cm2, and pathogen populations were assessed over a 9-day period. At all inoculum concentrations, pathogen populations increased at similar rates in both resistant and susceptible leaves. The mean generation time was 10.49 hr. For both susceptible and resistant cultivars, the number of pustules formed 10 days after inoculation was proportional to the inoculum concentration. The number of pathogen cells required to initiate a single pustule was at least six times greater in the resistant cultivars than in the susceptible cultivars. Pustules, which resulted from hypertrophy of mesophyll cells, were smaller on resistant cultivars. Anatomical responses to infection were similar in resistant and susceptible cultivars, although the responses were of somewhat reduced intensity in the resistant plants. These results indicate that the rxp gene confers resistance, at least in part, by increasing the number of bacterial cells necessary for infection rather than by restricting pathogen growth within host tissues.