Fusicoccin Counteracts the n-Ethylmaleimide and Silver-Induced Stimulation of Oxygen Uptake in Egeria densa Leaves

It was previously shown that a number of sulfhydryl [SH] group reagents (N-ethylmaleimide [NEM], iodoacetate, Ag(+), HgCl(2), etc.) can induce a marked, transitory stimulation of O(2) uptake (QO(2)) in Egeria densa leaves, insensitive to CN(−) and salicylhydroxamic acid and inhibited by diphenylene iodonium and quinacrine. The phytotoxin fusicoccin (FC) also induces a marked increase in O(2) consumption in E. densa leaves, apparently independent of the recognized stimulating action on the H(+)-ATPase. In this investigation we compared the FC-induced increase in O(2) consumption with those induced by NEM and Ag(+), and we tested for a possible interaction between FC and the two SH blockers in the activation of QO(2). The results show (a) the different nature of the FC- and NEM- or Ag(+)-induced increases of QO(2); (b) that FC counteracts the NEM- (and Ag(+))-induced respiratory burst; and (c) that FC strongly reduces the damaging effects on plasma membrane permeability observed in E. densa leaves treated with the two SH reagents. Two alternative models of interpretation of the action of FC, in activating a CN(−)-sensitive respiratory pathway and in suppressing the SH blocker-induced respiratory burst, are proposed.