Inhibition of catalase activity as an early response of Arabidopsis thaliana cultured cells to the phytotoxin fusiccoccin

In Arabidopsis thaliana cells, fusicoccin (FC) treatment induced an early and marked increase in the extracellular H2O2 level. It also increased the huge hypo-osmotic stress-induced oxidative wave and, in addition, prevented the H2O2 peak drop. These effects were apparently not linked to changes in either cytoplasmic pH or cytoplasmic free calcium concentration, since they occurred independently of the activity state of the plasma membrane (PM) H+-ATPase and neither influx nor efflux of 45 Ca2+ was modified by FC. In the presence of diphenylene iodonium (DPI), inhibiting the PM NADPH oxidase presumably responsible for reactive oxygen species (ROS) production, no apoplastic H2O2 development was detected either with or without FC. However, no increase in DPI-sensitive ferricyanide reduction, but rather a gradual decrease, occurred with FC. These results suggested that the H2O2 increase observed with FC was not due to a overproduction of RIDS but, more probably, to a reduced capability of FC-treated cells to degrade the H2O2 formed. This view, at first supported by the finding that FC-treated cells failed to break down exogenously supplied H2O2, was clearly confirmed by a series of measurements on exogenous catalase activity, tested in cell-free media of FC-treated samples. This assay, in fact, allowed ascertainment and partial characterization of an as yet unidentified factor increasingly accumulating in the incubation medium of FC-treated cells, behaving as a non-competitive catalase inhibitor and able to reduce markedly the cell's capability for H2O2 scavenging.