Superoxide, hydrogen peroxide, and the respiratory burst of fungally infected plant cells.

The principal route of oxygen utilization in the respiratory burst of fungally infected plants was determined from stoichiometries of the uptake and electronic reduction of oxygen in cotton cells exposed to Aspergillus flavus walls. Using 2,2'-azino-di-(3-ethyl-benzothiazoline-6-sulfonic acid) and epinephrine as redox reagents to manipulate oxygen transitions, we found that oxygen consumption doubled when superoxide disproportionation was abolished and was abolished when disproportionation doubled. Of four possible pathways for oxygen consumption, only monovalent reduction of molecular oxygen to superoxide was consistent with this inversely proportional relationship. According to the observed rate of oxygen consumption in this pathway and in the absence of competition to disproportionation of superoxide, infected cells are capable of generating intracellular concentrations of 1 M hydrogen peroxide in 13 min.