Effect of the fungal pathogen Bipolaris sorokiniana toxin prehelminthosporol on barley root plasma membrane vesicles

The effects of the phytotoxin prehelminthosporol from the fungal pathogen Bipolaris sorokiniana on the activities of the plasma membrane H⁺-ATPase, Ca²⁺- ATPase, and 1, 3-β-glucan synthase in barley roots were investigated in vitro. Plasma membranes were isolated by aqueous polymer two-phase partitioning. Proton pumping by the H⁺-ATPase was drastically reduced in a dose-dependent manner, with complete inhibition at 500 μM prehelminthosporol, while inhibition of ATP hydrolysis was less drastic, with a 35% inhibition at 500 μM toxin. Ca²⁺ uptake was also reduced, although to a lesser extent than proton pumping, with a maximal inhibition of 60% at 500 μM toxin. The 1, 3-β-glucan synthase activity was weakly stimulated at toxin concentrations below 100 μM, 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 drives ion and nutrient uptake and affect the efflux of Ca²⁺ to maintain the low cytoplasmic concentration of Ca²⁺ essential for the function of Ca²⁺ as a messenger in signal transduction. Secondly, it appears to inhibit the enzyme activities. The 1, 3-β-glucan synthase is activated by Ca²⁺ and can operate in the presence of low concentrations of prehelminthosporol (which may induce Ca²⁺ leakage into the cytoplasm and hence activate the enzyme), to produce callose to seal leaky membranes.