Oxygen metabolism in plant/bacteria interactions: characterization of the oxygen uptake response of plant suspension cells

In recent years the accumulation of reactive oxygen species (ROS) has been studied in plant cell suspension systems treated with bacterial pathogens. However, the associated utilization of molecular oxygen has not been well characterized. Using a multi-electrode oxygen analyser, the rates of oxygen consumption by tobacco cells during bacterial interactions were monitored. Heat-killed (HK) bacteria, which initiate an immediate ROS response in plant cells, were used as an elicitor to avoid complications of oxygen consumption by viable bacteria. An increase in oxygen uptake by the tobacco cells occurred within 4 min after addition of HK-bacteria and lasted for about 10 min, returning to a steady state at approximately twice the initial basal rate. The initial burst in oxygen uptake coincided with production of H2O2. Calculation of the total oxygen consumption by the plant cells indicated that less than 5 % of the increased oxygen uptake was utilized in ROS production. Use of respiratory inhibitors indicated that respiration, especially the cytochrome pathway, played a significant role in this response. Results from the use of K-252, a protein kinase inhibitor, and DPI, an inhibitor of membrane bound NADPH oxidases, indicated that triggering of the oxygen uptake response may involve protein phosphorylation and is at least partially activated by the membrane bound NADPH oxidase activity. The involvement of mitochondrial respiration in the oxygen uptake response described here indicates that early events in plant recognition of pathogens involves more of the cellular machinery than previously hypothesized.