The inducible 9, 10-dihydrophenanthrene pathway: characterization and expression of bibenzyl synthase and S-adenosylhomocysteine hydrolase.

Tricyclic 9,10-dihydrophenanthrenes originate from phenylpropane derivatives by chain elongation and cyclization according to the polyacetate rule. Bibenzyls are bicyclic intermediates, and O-methylation is a prerequisite for their conversion into dihydrophenanthrenes. cDNA clones encoding bibenzyl synthases and S-adenosylhomocysteine hydrolase of the orchid Phalaenopsis sp. were isolated from a cDNA library representing the stage of elicitor-induced plants. The deduced amino acid sequences of two clones, pBibSy811 and pBibSy212, indicated that we obtained two full-length sequences of bibenzyl synthases characterized by their homology to stilbene synthases previously investigated. That indeed bibenzyl synthase cDNAs rather than a homologous stilbene synthase cDNA or chalcone synthase cDNA have been isolated was demonstrated by expression of two enzymatically active bibenzyl synthase proteins in Escherichia coli. These proteins showed virtually the same selectivity towards m-hydroxyphenylpropionyl-CoA as substrate as the enzyme isolated from orchid plants. In young sterile Phalaenopsis plants, the formation of both bibenzyl synthase mRNAs and S-adenosylhomocysteine hydrolase mRNAs was increased upon elicitation more than 100-fold. The time courses of gene expression exhibited transient profiles, reaching maximum mRNA levels 20 h after onset of fungal infection followed by a rapid decline to 40 h.