Inducible Enzymes of the 9,10-Dihydro-phenanthrene Pathway. Sterile Orchid Plants Responding to Fungal Infection. Thorsten Reinecke. Department of Biochemistry, Philipps-University, Marburg, Germany Received 15 February 1994. Helmut Kindl Department of Biochemistry. Philipps-University, Marburg, Germany Received 15 February 1994. MPMI 7: 449-454. Accepted 6 May 1994. Copyright The American Phytopathological Society.

Bulbs, roots, and rhizomes of orchids may contain substantial amounts of 9, 10-dihydrophenanthrenes being formed from bibenzyls in an oxidative coupling reaction. They are, therefore, products of the phenylpropane metabolism in combination with polyketide formation. In young sterile plants of Phalaenopsis species, however, these compounds were not detectable. Infection by Botrytis cinerea and Rhizoctonia spp. caused a pronounced increase in the amount and enzyme activity of bibenzyl synthase, the key enzyme of the pathway. Concomitantly, phenylalanine ammonia-lyase and an O-methyltransferase were induced. 3, 5-Dihydroxy-3' -methoxybibenzyl and the phytoalexin hircinol (2,5-dihydroxy-4-methoxy-9, 10-dihydrophenanthrene) were formed. Infection of leaves of young orchid plants with Botrytis cinerea led to a more than 100-fold increase in bibenzyl synthase activity within one day. Using an antiserum raised against a bibenzyl synthase from Bletilla striata we studied the change of bibenzyl synthase protein during the time following elicitation. Roots infected with Rhizoctonia sp. showed a similar but less rapid response.