Bacterial phytotoxins, syringomycin, syringostatin and syringotoxin, exert their effect on the plasma membrane H + ‐ATPase partly by a detergent‐like action and partly by inhibition of the enzyme

Syringostatin is a newly discovered phytotoxin produced by a phytopathogenic bacterium Pseudomonas syringae pv. syringae lilac isolate. The effects of syringostatin and the similar phytotoxins, syringomycin and syringotoxin, on H − ‐ATPase activity were investigated using cultured mung bean ( Vigna radiata L. cv. Ryokuto) cells or plasma membrane vesicles isolated from mung bean hypocotyls. 31 P‐NMR analysis of cultured cells treated with syringostatin revealed that the cytoplasmic pH was decreased. When plasma membrane was prepared by a two‐step method (Dextran gradient followed by a sucrose gradient). syringostatin, syringomycin and syringotoxin inhibited the H + ‐ATPase activity in a dose‐dependent manner. In contrast, these toxins stimulated H + ‐ATPase activity when plasma membrane was prepared by a one‐step method (sucrose gradient). While these toxins inhibited the H + ‐ATPase activity of inside‐out plasma membrane vesicles, the H + ‐ATPase activity of right‐side‐out vesicles was stimulated. The detergent. Triton X‐100, abolished this stimulatory effect of the toxins on the H + ‐ATPase of right‐side‐out vesicles and of one‐step purified plasma membrane. The toxins also inhibited the activity of the plasma membrane H + ‐ATPase solubilized with deoxycholate and Zwittergent 3–14. Taken together, these results indicate that these toxins exert their effects partly by a detergent‐like action on the plasma membrane and partly by inhibition of the enzyme.