Fusicoccin-induced catalase inhibitor is produced independently of H+-ATPase activation and behaves as an organic acid.

The phytotoxin fusicoccin (FC) was found to induce an increase in apoplastic H₂O₂ content in Arabidopsis thaliana cells, apparently linked to the presence of an as yet unidentified catalase inhibitor detectable even in the external medium of FC-treated cells. This study, aimed to further characterize the inhibitor's features, shows that (1) FC-induced H₂O₂ accumulation increases as a function of FC concentration and correlates to the amount of inhibitor released at apoplastic level. The pattern of H+ efflux, conversely, does not fit with that of these two parameters, suggesting that neither the production nor the release of the catalase inhibitor is linked to the main role of FC in activating the plasma membrane (PM) H+-ATPase; (2) treatment with 10 µM erythrosine B (EB) early and totally inhibits net H+ and K+ fluxes across the PM, indicative of the H+ pump activity; nevertheless, also in these conditions a huge FC-induced H₂O₂ accumulation occurs, confirming that this effect is not related to the FC-induced PM H+-ATPase activation; (3) the inhibitor's release increases with time in all conditions tested and is markedly affected by extracellular pH (a higher pH value being associated to a larger efflux), in agreement with a weak acid release; and (4) the inhibitor can be almost completely recovered in a CH₂C₂-soluble fraction extracted from the incubation medium by sequential acid-base partitioning which contains nearly all of the organic acids released. These final results strongly suggest that the metabolite responsible for the FC-induced catalase inhibition belongs to the organic acid class.