INDUCIBLE ENZYMES OF THE 9,1O-DIHYDRO-PHENANTHRENE PATHWAY - STERILE ORCHID PLANTS RESPONDING TO FUNGAL INFECTION

Bulbs, roots, and rhizomes of orchids may contain substantial amounts of 9,10-dihydrophenanthrenes being formed from bibenzyls in an oxidati ve coupling reaction. They are, therefore, products of the phenylpropa ne metabolism in combination with polyketide formation. In young steri le plants of Phalaenopsis species, however, these compounds were not d etectable. Infection by Botrytis cinerea and Rhizoctonia spp. caused a pronounced increase in the amount and enzyme activity of bibenzyl syn thase, the key enzyme of the pathway. Concomitantly, phenylalanine amm onia-lyase and an O-methyltransferase were induced. 3,5-Dihydroxy-3'-m ethoxybibenzyl and the phytoalexin hircinol (2,5-dihydroxy-4-methoxy-9 ,10-dihydrophenanthrene) were formed. Infection of leaves of young orc hid plants with Botrytis cinerea led to a more than 100-foId increase in bibenzyl synthase activity within one day. Using an antiserum raise d against a bibenzyl synthase from Bletilla striata we studied the cha nge of bibenzyl synthase protein during the time following elicitation . Roots infected with Rhizoctonia sp. showed a similar but less rapid response.