A role for oxidative stress in the elicitation of plant defense responses

Phytoalexins are low molecular weight phenylpropanoid compounds that accumulate in plant cells after exposure to incompatible microbes, often in association with the hypersensitive response. The mechanism of elicitation of these defense responses may involve the plant's production of an oxidative burst capable of peroxidizing the plant cell's plasmalemma. The oxidative burst is thought to consist of activated oxygen species such as H;Maximum chemiluminescence occurred between 10 and 20 minutes after addition of both live and autoclaved bacteria (Pseudomonas corrugata) to white clover (Trifolium repens) cell suspensions. Preincubation of plant cells in SOD (superoxide dismutase) consistently increased the chemiluminescence peak, though not to statistically significant levels. Preincubation of plant cells in catalase prevented the increase in chemiluminescence.;Significantly high levels of the phytoalexin medicarpin accumulated in white clover cell suspensions within 24 hours after addition of live bacteria. Autoclaved bacteria did not elicit medicarpin accumulation. Preincubation of plant cells in SOD and catalase at levels capable of quenching the oxidative burst did not decrease medicarpin accumulation after addition of live bacteria to plant cell suspensions. Similarly, live bacteria induced development of the hypersensitive response, but autoclaved bacteria did not. Preincubation of live bacteria in SOD and catalase did not interfere with development of the hypersensitive response in tobacco leaves.;These results suggest that an oxidative burst consisting partly of H;Ozone elicited medicarpin accumulation in white clover cells suspended in MES buffer, but not in cells suspended in nutrient medium. These results suggest that an oxidative stress can elicit specific defense responses if plant cells are denied access to metabolically useful energy.