Oxygen metabolism in plant cell culture bacteria interactions:: role of bacterial concentration and H2O2-scavenging in survival under biological and artificial oxidative stress

Preliminary studies with plant cell suspensions indicated that high concentrations of bacterial inoculum can significantly reduce the H2O2 levels detected immediately following inoculation. Further investigation suggested that bacterial concentration is an important factor in H2O2-scavenging, and is often overlooked in both plant/pathogen interactions as well as in vitro survival studies. In vitro studies with Pseudomonas spp. characterized the relationship between H2O2-scavenging and culturability of bacteria. Because H2O2 permeates membranes rapidly, the intracellular scavenging activity of the bacteria could be detected as a decrease in the external H2O2 concentration. Therefore, the effective scavenging activity of a given suspension of bacteria was proportional to the bacterial concentration. The culturability of populations of bacteria exposed to H2O2 concentrations which are intolerable to individual cells (greater than or equal to 100 mu M) was also dependent on bacterial concentration. Studies with a catalase-deficient mutant of P. putida indicated that catalase, due to its high K-m, had little effect on scavenging H2O2 at the lower concentrations of H2O2 (< 100 mu M) that are likely to be encountered in suspension cells and other plant tissues. Using high concentrations of bacteria (1 x 10(9) cfu ml(-1)), there was little difference between the catalase-deficient mutant and various isolates of Pseudomonas spp. in the reduction of H2O2 levels in suspension cells. The studies indicated that an additional scavenging system other than catalase exists in bacteria and this may play an important role in scavenging H2O2 during plant/bacteria interactions. (C) 1997 Academic Press Limited.