EFFECT OF THE FUNGAL PATHOGEN BIPOLARIS-SOROKINIANA TOXIN PREHELMINTHOSPOROL ON BARLEY ROOT PLASMA-MEMBRANE VESICLES

The effects of the phytotoxin prehelminthosporol from the fungal patho gen Bipolaris sorokiniana on the activities of the plasma membrane H+- ATPase, Ca2+- ATPase, and 1,3-beta-glucan synthase in barley roots wer e investigated in vitro. Plasma membranes were isolated by aqueous pol ymer two-phase partitioning. Proton pumping by the H+-ATPase was drast ically reduced in a dose-dependent manner, with complete inhibition at 500 mu M prehelminthosporol, while inhibition of ATP hydrolysis was l ess drastic, with a 35% inhibition at 500 mu M toxin. Ca2+ uptake was also reduced, although to a lesser extent than proton pumping, with a maximal inhibition of 60% at 500 mu M toxin. The 1,3-beta-glucan synth ase activity was weakly stimulated at toxin concentrations below 100 m u M, with higher concentrations being inhibitory. Taken together, our results indicate that prehelminthosporol exerts its effect in at least two ways. First, it appears to disrupt the membrane barrier and thus seriously interfere with establishment of the proton gradient that dri ves ion and nutrient uptake and affect the efflux of Ca2+ to maintain the low cytoplasmic concentration of Ca2+ essential for the function o f Ca2+ as a messenger in signal transduction. Secondly, it appears to inhibit the enzyme activities. The 1,3-beta-glucan synthase is activat ed by Ca2+ and can operate in the presence of low concentrations of pr ehelminthosporol (which may induce Ca2+ leakage into the cytoplasm and hence activate the enzyme), to produce callose to seal leaky membrane s.