THE EFFECT OF STEROLS ON PHYTOALEXIN, STEROID GLYCOALKALOID, AND STEROL ACCUMULATION IN POTATO-TUBER DISCS INOCULATED WITH PHYTOPHTHORA-INFESTANS OR TREATED WITH ARACHIDONIC-ACID

Potato tuber discs contain low Levels of steroid glycoalkaloids (SGA) and free sterols at 0 h after slicing. Aging the discs increased SGA a nd free sterol levels in the outer 3 mm of disc surface. Inoculating t he discs with incompatible races of Phytophthora infestans or treatmen t with arachidonic acid (AA) caused a hypersensitive reaction (HR), wh ich included the accumulation of sesquiterpenoid phytoalexins and a ma rked reduction in the accumulation of steroid glycoalkaloids (SGA) and free sterols, largely beta-sitosterol and stigmasterol. Inoculation w ith a compatible race elicited little and delayed phytoalexin accumula tion, modest reductions in SGA and little or no reduction of free ster ol accumulation. Treatment of tuber discs with 1 or 2 mu g/disc stigma sterol or beta-sirosterol before inoculation with an incompatible race allowed colonization by the fungus, markedly decreased phytoalexin ac cumulation and decreased the suppression of SGA and free sterol accumu lation. Treatment with AA did not prevent colonization by either compa tible or incompatible races of P, infestans in the sterol-treated disc s. Neither sterol elicited phytoalexin accumulation, nor significantly affected SGA and free sterol accumulation in the absence of the fungu s. The sterols also did not affect AA-elicited phytoalexin accumulatio n the suppression of SGA nor free sterol accumulation in the absence o f the fungus. (C) 1995 Academic Press Limited.