CHARACTERIZATION OF BIBENZYL SYNTHASE CATALYZING THE BIOSYNTHESIS OF PHYTOALEXINS OF ORCHIDS

Fungal infection of orchid rhizomes leading to enhanced formation of b ibenzyls and 9,10-dihydrophenanthrenes caused a substantial increase i n the activity of bibenzyl synthase. From rhizomes of Bletilla striata induced by slicing and subsequent incubation for 24 hr, bibenzyl synt hase was extracted and purified 237-fold to apparent homogeneity. The enzyme belonging to the class of polyketide synthases forming an aroma tic ring system uses dihydro-m-coumaroyl-CoA and three molecules of ma lonyl-CoA as substrates. The bibenzyl synthase had a K-m value of 25 m u M for malonyl-CoA and a K-m value of 1 mu M for dihydro-m-coumaroyl- CoA. Bibenzyl synthase is a homodimer with a subunit M, of 46 000. Pol yclonal antibodies were raised in rabbits and used to analyse the amou nt of bibenzyl synthase in various orchid tissues before and after eli citation.