The pathogen-induced defence compounds of apple (Malus × domestica) and other Malinae are biphenyls and dibenzofurans. Their biosynthesis was studied using cell cultures of Sorbus aucuparia (Malinae), which produce biphenyls and dibenzofurans upon treatment with elicitors derived from the scab-causing fungus (Venturia inaequalis) and the fire blight-causing bacterium (Erwinia amylovora). Upon feeding of the cell cultures with radioactively labelled tracers, 3,5-dihydroxybiphenyl proved to be the precursor of both biphenyls and dibenzofurans. 3,5-Dihydroxybiphenyl is formed by biphenyl synthase (BIS), a type III polyketide synthase. Conversion to aucuparin involves two non-sequential O-methylation reactions, which flank a 4-hydroxylation step. Genes encoding the soluble O-methyltransferases and the membrane-bound cytochrome P450 enzyme were cloned and functionally characterized. The conversion of biphenyls to dibenzofurans is still under study. In fire blight-infected shoots of apple cultivar 'Holsteiner Cox', biphenyls and dibenzofurans were detected in the transition zone of stems. The flanking stem segments, i.e., the necrotic and healthy zones, lacked detectable quantities of phytoalexins. This also applied to leaves. BIS of apple is encoded by a gene family, whose members fall into four subfamilies that are differentially regulated after the inoculation of shoots with E. amylovora. Stems contained only BIS3 transcripts. The maximum expression level in the transition zone was approx. 4,000 times that in the respective stem segment of mock-inoculated control shoots. Leaves expressed the BIS2 gene; however, no enzyme protein was immunodetectable. The BIS3 protein was immunochemically localized to the parenchyma of the bark. Dot-shaped immunofluorescence was restricted to the junctions between neighbouring cortical parenchyma cells, which may point to an association of the enzyme with plasmodesmata in primary pit fields. Studies of a potential metabolon organization are in progress. Our results pave the way for the exploitation of the phytoalexin biosynthetic pathway to improve the resistance potential of valuable cultivars.