Fusicoccin Activity and Binding in Arabidopsis thaliana

Abstract

ABSTRACr I have examined the activity and specific binding of the phytotoxin fusicoccin using Arabidopsis thaliana L. Col-O. Fusicoccin (10 micro-molar) stimulates both proton extrusion and enlargement in isolated Arabidopsis leaf discs. Radiolabeled fusicoccin specifically binds to mem-branes (13,000 to 100,000g subcellular fraction) from cultured cells of Arabidopsis. The specific binding of this phytotoxin to putative receptor sites in Arabidopsis membranes is both pH-sensitive (pH optimum = 5.5 to 6.0) and heat-labile (10 min at 70°C). The apparent dissociation constant for the specific binding at 20°C is approximately 1.3 x 108 molar. The results of this study are in general agreement with previous reports of fusicoccin binding and activity in other plant species. Fusicoccin, a fungal phytotoxin produced by Fusicoccum amygdali, affects a wide variety of physiological processes in higher plants (7). Evidence from both in vivo and in vitro exper-iments supports the hypothesis that FC2 primarily acts by stim-ulating the activity of ATPase proton pumps in the plasma membrane of plant cells (7, 9, 10). The putative receptor site for FC is not, however, the ATPase, but another protein located in the plant plasma membrane (13, 14). Recent evidence suggests that FC may also cause a blockage of a K+ efflux channel in the plasma membrane in addition to stimulating the proton pump (3). Thus, it appears that the binding of this toxin to its receptor may affect at least two transport mechanisms in the plasma membrane of higher plants. This supports the idea that the target