Effect of Helminthosporium sacchari-toxin on cell membrane potential of susceptible sugarcane

The effect of the host-specific toxin produced by Helminthosporium sacchari (HS-toxin) on cell membrane potential of susceptible sugarcane was investigated. Membrane depolarization was detected at concentrations as low as 50 nm HS-toxin. The onset of membrane depolarization was observed after a lag phase of 4–10 min depending on the toxin concentration.

The energy-dependent component of membrane potential (E_m–p) was inhibited by the toxin. However, membrane repolarized, i.e. E_m–p, was re-established after removing the toxin from the bathing solution. At low toxin concentrations E_m–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 temperatures above 30°C but subsequent lowering of temperature to 26°C 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 is causing 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.