Biphenyls and dibenzofurans are the phytoalexins of the Malinae, a subtribe of the Rosaceae (formerly subfamily Maloideae). The Malinae include a number of economically important fruit trees, such as apple and pear, and the medicinally used whitethorn. The accumulation of biphenyls and dibenzofurans is an active pathogen defense strategy, which is restricted to the Malinae. The contribution of phytoalexin formation to protection against fire blight and scab and to overcoming replant disease (soil sickness) is under study.
The key enzyme of the biosynthesis is biphenyl synthase (BIS), which was first detected in elicitor-treated cell cultures of Sorbus aucuparia. In the apple genome sequence, the BIS gene family consists of at least nine members, which are differentially regulated. BIS3 was immunochemically localized to the junctions between neighboring cortical parenchyma cells of the transition zone in infected stems. This zone between the necrotic and healthy tissues accumulates the phytoalexins.
When incubated with salicoyl-CoA rather than benzoyl-CoA as a starter substrate, BIS catalyzes a single condensation step with malonyl-CoA to yield 4-hydroxycoumarin. This is a new PKS-catalyzed biosynthetic pathway. 4-Hydroxycoumarin is the precursor of dicoumarol, which is the forerunner of the warfarin group of anticoagulants.
The physiological product of BIS is 3,5-dihydroxybiphenyl, which is formed from benzoyl-CoA and three molecules of malonyl-CoA. The conversion to aucuparin involves two non-sequential O-methylation reactions, which flank a 4-hydroxylation step. The genes encoding the soluble O-methyltransferases and the membrane-bound cytochrome P450 enzyme were cloned and functionally characterized. The enzymatic conversion of biphenyls to dibenzofurans is still open.
