1.10.13
CHARACTERIZATION OF E-PHYTOL AS AN INITIATION FACTOR FOR OOSPORE FORMATION OF PHYTOPHTHORA CACTORUM AND P. PARASITICA

HJ JEE1, CS TANG2 and WH KO3

1Division of Plant Pathology, National Institute of Agricultural Science and Technology, RDA, Suwon 441-707, Korea; 2Department of Environmental Biochemistry and 3Department of Plant Pathology, University of Hawaii, Honolulu, HI 96822, USA

Background and objectives
Although a 3-beta-hydroxysterol has been widely recognized as necessary for the sexual reproduction of a number of pythiaceous fungi, conflicting data exist in the literature [1]. In this report, we provide evidence that E-phytol, a widespread diterpene alcohol, initiates oospore formation in Phytophthora cactorum and P. parasitica in the absence of sterols.

Results and conclusion
Phytophthora cactorum and P. parasitica did not form oospores when cultured on basal medium consisting of high-purity SeaKem agarose, distilled water, mineral nutrients, FeEDTA, L-asparagine, L-serine and glucose. Oospore formation was greatly stimulated, however, when the medium was supplemented with plant products such as V8 juice and seed oils. In corn oil, the oospore stimulatory activities were found in the non-saponifiable fraction, and the active compounds were not 3-beta-hydroxysterols as suggested by digitonin precipitation. Consecutive-column, thin-layer and HPLC chromatography were used for the pre-purification of the active principles. Among 13 HPLC fractions collected, three stimulated P. cactorum and P. parasitica, and the other four only stimulated P. cactorum. The most active compound in the fractions was identified as E-phytol based on the comparison of GC-MS data with those of the standard compound isolated from the commercial phytol, which was a mixture of the E- and Z-isomers. While E-phytol was highly active in the initiation and proliferation of oospores of the two Phytophthora spp. [2] at 0.005 p.p.m., cholesterol stimulated P. cactorum only at over 0.05 p.p.m. However, the oospores induced by E-phytol alone were either morphologically incomplete or distorted. Therefore, E-phytol served only as an oospore initiation factor. Results indicated that a number of chemicals stimulated sexual reproduction of Phytophthora, and the dependency of these initiation and normalization factors appeared to be different in P. cactorum and P. parasitica. Other factor(s) in the corn oil are required to fulfil requirements for the formation of normal oospores. Isolation and identification of factors leading to the formation of normal oospores are in progress.

References
1. Erwin DC, Ribeiro OK, 1996. Phytophthora Diseases Worldwide. APS Press, St Paul, Minnesota, pp. 46-48.
2. Jee HJ, Tang CS, Ko WH, 1997. Microbiology 143, 1631-1638.