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Factors Affecting the Elicitation of Sesquiterpenoid Phytoalexin Accumulation by Eicosapentaenoic and Arachidonic Acids in Potato ¹

Richard M. Bostock², Roger A. Laine³ and Joseph A. Ku

Department of Plant Pathology, University of Kentucky, Lexington, Kentucky 40546, Department of Biochemistry, University of Kentucky, Lexington, Kentucky 40536

Eicosapentaenoic and arachidonic acids in extracts of Phytophthora infestans mycelium were identified as the most active elicitors of sesquiterpenoid phytoalexin accumulation in potato tuber slices. These fatty acids were found free or esterified in all fractions with elicitor activity including cell wall preparations. Yeast lipase released a major portion of eicosapentaenoic and arachidonic acids from lyophilized mycelium. Concentration response curves comparing the elicitor activity of the polyunsaturated fatty acids to a cell-free sonicate of P. infestans mycelium indicated that the elicitor activity of the sonicated mycelium exceeded that which would be obtained by the amount of eicosapentaenoic and arachidonic acids (free and esterified) present in the mycelium. Upon acid hydrolysis of lyophilized mycelium, elicitor activity was obtained only from the fatty acid fraction. However, the fatty acids accounted for only 21% of the activity of the unhydrolyzed mycelium and the residue did not enhance their activity. Centrifugation of the hydrolysate, obtained from lyophilized mycelium treated with 2N NaOH, 1 molarity NaBH₄ at 100°C, yielded a supernatant fraction with little or no elicitor activity. Addition of this material to the fatty acids restored the activity to that which was present in the unhydrolyzed mycelium. The results indicate that the elicitor activity of the unsaturated fatty acids is enhanced by heat and base-stable factors in the mycelium.

³ Recipient of Grants AM25101 and GM23902 from the National Institutes of Health, and Equipment Grants from the Fleischmann Foundation and from the Graduate School of the University of Kentucky.

¹ Supported in part by Grant 78-59-2211-0-1-063-1 of the United States Department of Agriculture, Science and Education Administration, and a grant from the Rockefeller Foundation. Journal Paper No. 81-11-267 of the Kentucky Agricultural Experiment Station, Lexington, KY 40446.

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