Selective Inhibition of K⁺, Na⁺, Cl⁺, and $\text{PO}_{4}{}^{3-}$ Uptake in Zea mays L. By Bipolaris (Helminthosporium) maydis Race T Pathotoxin: Evidence for a Plasmalemma Target Site?

Pathotoxin preparations were obtained from either axenic culture filtrate of race T of Bipolaris maydis (Nisikado) Shoemaker (new culture media and toxin purification procedures are described) or extracts of maize leaves infected with the fungus. The toxins (10-6 to 10-8 M) caused inhibition of $[{}^{86}\text{Rb}]\text{K}^{+}$ uptake in leaf discs and apical root segments of Zea mays L. cv W64A Texas (Tcms) and normal (N) cytoplasms. Significant inhibition was measurable as early as 5 min after adding toxin. In Tcms per cent inhibition was increased by increasing toxin concentration and time in toxin, by using solution at pH 5 rather than pH 7, by decreasing external KCl concentration over the range 50 to 0.1 mM (in the presence of 0.5 mM CaSO4), or by exposing leaf discs to light rather than dark during the uptake period in toxin. Root uptake of 22Na+ and 36Cl- was inhibited to a lesser extent than K+. Inhibition of ^{32}\text{PO}{}_{4}{}^{3-}$ uptake occurred after 40 min when cyclosis had ceased. When combined with data in the literature, our data indicate that the plasmalemma is the probable primary site of toxin action in N and Tcms maize. Comparison of the effects of toxin on K+ uptake in N and Tcms maize suggests the existence of more than one mode of toxin action: a weak disruptive effect in N and Tcms, and in addition, specific membrane sites in Tcms involved in monovalent ion uptake. Six genotypes in N or Tcms cytoplasm which exhibited different degrees of disease susceptibility in the field showed a corresponding gradation of susceptibility to the toxin when a K+ uptake bioassay was used. This correlation is strong evidence that the sites of toxin action affecting K+ transport have characteristics closely related to cellular factors regulating susceptibility to fungal attack.