ACTIVE OXYGEN SPECIES PRODUCTION IN TOBACCO CELLS ELICITED BY CRYPTOGEIN

We analyse the relationship between active oxygen species (AOS) produc tion and pH changes induced in tobacco cells by cryptogein, a fungal p roteinaceous elicitor of defence mechanisms in plants, When tobacco ce lls were treated with cryptogein, an intracellular acidification, an a lkalinization of the extracellular medium and a transient burst of AOS (H2O2) were observed, Treatment of elicited cells with either dipheny leneiodonium (DPI), an inhibitor of the neutrophil NADPH oxidase, or T iron, which scavenges O-2(.-), abolished AOS production, These data su ggest the involvement of a NADPH oxidase-like enzyme leading to H2O2 p roduction through O-2(.-) dismutation, Although H2O2 production could be, per se, the origin of the pH changes observed, we showed that it w as not the main cause, since DPI and Tiron did not inhibit extracellul ar alkalinization, On the other hand, cryptogein-induced changes in pH could be abolished using fusicoccin (FC), which is known to stimulate the plasmalemma H+-ATPase, Consequently, the observed changes in pH i nduced by cryptogein could be mainly due to the inhibition of the plas malemma H+-ATPase activity. Furthermore, changes in extracellular pH w ere shown to modulate the intensity of AOS production by elicited cell s, The possible regulation of the NAD(P)H oxidase activity of plant ce lls by changes in pH is further discussed.