Biphenyl and dibenzofuran formation in fire blight-infected malus domestica cultivars

In response to pathogen attack, the Pyrinae (formerly Maloideae) produce biphenyls and/or structurally related dibenzofurans as phytoalexins. The carbon backbone of these inducible defence compounds is formed by biphenyl synthase (BIS). The enzyme catalyzes the sequential condensation of benzoyl-CoA with three molecules of malonyl-CoA to yield 3,5-dihydroxybiphenyl, precursor of the biphenyl and dibenzofuran phytoalexins. BIS belongs to the superfamily of type III polyketide synthases (PKSs), which generate a diverse array of plant secondary metabolites. Formation of biphenyls and dibenzofurans in response to fire blight (Erwinia amylovora) infection was studied in Malus species and Malus domestica cultivars. In response to elicitation, cell cultures of mountain ash (Sorbus aucuparia) accumulated three biphenyls and two dibenzofurans. These compounds were isolated and their structures were determined. Similar phytoalexins were also detected in fire blightinfected M. domestica shoots. Interestingly, co-occurrence of both classes of phytoalexins is rare. Therefore, S. aucuparia cell cultures could serve as a model system for studying the biosynthesis of biphenyl and dibenzofuran phytoalexins.