Fusicoccin Activity and Binding in Arabidopsis thaliana

I have examined the activity and specific binding of the phytotoxin fusicoccin using Arabidopsis thaliana L. Col-O. Fusicoccin (10 micromolar) stimulates both proton extrusion and enlargement in isolated Arabidopsis leaf discs. Radiolabeled fusicoccin specifically binds to membranes (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 degrees C). The apparent dissociation constant for the specific binding at 20 degrees C is approximately 1.3 x 10(-8) molar. The results of this study are in general agreement with previous reports of fusicoccin binding and activity in other plant species.