Identification of Dipterocarpol as Isolation Factor for the Induction of Primary Isolation of Frankia from Root Nodules of Alnus glutinosa (L.) Gaertner. Anton Quispel. Departments of Plant Molecular Biology, Botanical Laboratory, University of Leiden, 2311 VJ Leiden. Anders Baerheim Svendsen(2), Jan Schripsema(2), Wim J. Baas(4), Cees Erkelens(3), and Johan Lugtenburg(3). Departments of (1)Plant Molecular Biology, Botanical Laboratory, University of Leiden, 2311 VJ Leiden, (2)Pharmacognosy and (3)Organic Chemistry, Gorlaeus Laboratories, University of Leiden, 2333 AL Leiden, and (4)General Botany, Botanical Laboratory, University of Utrecht, 3512 PN, Utrecht, The Netherlands. . MPMI 2:107-112. Accepted 20 January 1989. Copyright 1989 The American Phytopathological Society.

In different types of actinorhizal root nodules, primary isolation of Frankia from the endophytic state within short periods of incubation is only possible after the addition of root-lipid extracts. Because substances like Tween 80 or lecithin, which stimulate growth during subsequent cultivation, cannot substitute for the root lipids for this effect, it was concluded that a more specific isolation factor was involved. An active compound was isolated from root-lipid extracts by thin-layer chromatography and identified as dipterocarpol (hydroxydammarenone II) by mass spectrometry and by ¹H and ¹³C nuclear magnetic resonance spectroscopy. The identification of dipterocarpol as the active isolation factor was confirmed by biological tests with an authentic sample of dipterocarpol. The isomer hydroxydammarenone I was active as well. Because this is the first indication that triterpenes of this type are involved in symbiotic plant-microbe interactions, some possible explanations are discussed. A hypothesis is based on adaptive differences between the cell membranes of Frankia under endophytic and free-living conditions.