Prehelminthosporol is the immediate precursor of helminthosporol, first isolated in 1965 from Helminthosporium sativum. Prehelminthosporol inhibits coleoptile growth at 1mM and 0.1mM, while significantly promoting growth at 0.01mM. Prehelminthosporol inhibits H+-ATPase in a dose-dependent manner, with complete inhibition at 500 μM, and inhibits ATP hydrolysis. Helminthosporol reduces Ca2+ uptake with maximal inhibition of 60% at 500 μM toxin. Prehelminthosporol stimulates 1,3-β-glucan synthase activity at concentrations below 100 μM, with higher concentrations being inhibitory.