Effect of Helminthosporium sacchari-toxin on cell membrane potential of susceptible sugarcane.
The effect of the host-specific toxin produced by H. [Drechslera] sacchari (HS-toxin) on cell membrane potential of susceptible sugarcane was investigated. Membrane depolarization was detected at concn as low as 50 nsmallcap˜M HS-toxin. The onset of membrane depolarization was observed after a lag phase of 4-10 min depending on the toxin concn. The energy-dependent component of membrane potential (Em-p) was inhibited by the toxin. However, membrane repolarized, i.e. Em-p, was re-established after removing the toxin from the bathing solution. At low toxin concn Em-p was lost in the dark but was partially recovered in the light and more fully in the light with the addition of fusicoccin. The HS-toxin-induced membrane depolarization was prevented by temp. >30°C but subsequent lowering to 26° resulted in membrane depolarization. Treatments with a nontoxic lower homologue of HS-toxin (lacking one galactose unit in the molecule) protected cells against HS-toxin-induced membrane depolarization. Pretreatments of tissues with galactose or raffinose did not prevent HS-toxin-induced depolarization. It is postulated that HS-toxin causes a loss of H+ gradient across the plasmalemma. However, it is unlikely that the H+ pump is the site of the toxin action since in the presence of the toxin the pump can be activated by light and fusicoccin.