Biphenyls are unique phytoalexins produced by plants belonging to Pyrinae, a subtribe of the economically important Rosaceae family. The formation of aucuparin, a well-known biphenyl, is induced by yeast extract (YE) in cell cultures of Sorbus aucuparia. However, the molecular mechanism underlying YE-induced activation of biphenyl biosynthesis remains unknown. Here we demonstrate that the addition of YE to the cell cultures results in a burst of reactive oxygen species (ROS; H2O2 and O-2(-)), followed by transcriptional activation of the biphenyl synthase 1 gene (BIS1) encoding the key enzyme of the biphenyl biosynthetic pathway and aucuparin accumulation. Pretreatment of the cell cultures with ROS scavenger dihydrolipoic acid and NADPH oxidase-specific inhibitor diphenylene iodonium abolished all of the above YE-induced biological events. However, when the cell cultures was pretreated with superoxide dismutase specific inhibitor N,N-diethyldithiocarbamic acid, although O-2(-) continued to be generated, the H2O2 accumulation, BIS1 expression and aucuparin production were blocked. Interestingly, exogenous supply of H2O2 in the range of 0.05-10 mM failed to induce aucuparin accumulation. These results indicate that endogenous generation of H2O2 rather than that of O-2(-) is a key factor in YE-induced accumulation of biphenyl phytoalexins in cell cultures of S.aucuparia.