Bacterial Growth, Toxin Production, and Levels of Ornithine Carbamoyltransferase in Resistant and Susceptible Cultivars of Bean Inoculated with Pseudomonas phaseolicola
Time-course studies of growth and toxin production in culture were made with 4 isolates of P. phaseolicola: G-50 and HB-36 (race 2) and HB-46 and HB-20 (race 1). In culture, all isolates increased approximately to the same population level (108-109 cells/ml) and all isolates except HB-20 produced toxin. Time-course experiments to measure growth, toxin production and changes in the level of the enzyme, ornithine carbamoyltransferase (OCT), in bean leaf tissue, were made by inoculating 2 resistant and 2 susceptible bean cultivars with isolates HB-36 and HB-20 of the pathogen in separate experiments. In leaves of the susceptible bean cultivars ''Resistant Cherokee Wax'' and ''Red Kidney'', both isolates increased to 109 cells/1.2 cm2 of leaf area from an initial level of 103 cells, but only HB-36 produced toxin. Also, in leaves of susceptible cultivars ''Resistant Cherokee Wax'' and ''Red Kidney'', inoculated with HB-36, OCT activity averaged 53% and 71%, respectively, of that found in noninocualted leaves. In the resistant cultivars ''P.I. 150514'' and ''GN #27'', both isolates had much slower rates of multiplication than in susceptible cultivars; the highest number of bacterial cells observed was 105 cells/1.2 cm2 leaf area. In spite of this substantial increase in number of cells, no toxin was detected in resistant leaf tissues, including those that were inoculated with the toxin-producing isolate (HB-36) and there was no significant inhibition of OCT until 3 days after inoculation. Inhibition coincided with the appearance of the tissue browning and cell collapse characteristic of a hypersensitive reaction.