Twelve phenolic compounds with related structures were analyzed for their ability to act as chemoattractants for Agrobacterium tumefaciens C58C1 and as inducers of the Ti plasmid virulence operons. The results divided the phenolic compounds into three groups: compounds that act as strong vir inducers and are chemoattractants for A, tumefaciens C58C0 harboring the nopaline Ti plasmid pDUB1003A31, but not the isogenic cured strain; compounds that are at best weak vir inducers and are weak chemoattractants for Ti plasmid-harboring and cured A. tumefaciens C58C'; and compounds that are vir noninducers and are also nonattractants. A strong correlation between vir-inducing ability and Ti plasmid requirement for chemotaxis is thus established. In addition, chemical structure rules for vir induction and chemotaxis are outlined. Positive chemotaxis toward root and shoot homogenates from monocotyledonous and dicotyledonous plants was observed. At low extract concentrations, chemotaxis was enhanced by the presence of Ti plasmid. The chemoattractants do not derive from intact cell walls. Lack of attraction is not responsible for the apparent block to monocot transformation by A. tumefaciens.Plant wounding is a prerequisite for crown gall tumor induction by Ti-plasmid-harboring Agrobacterium tumefaciens. The Ti plasmid virulence operons, induced by wound exudates such as acetosyringone (24,32,33), mediate T-DNA transfer to the plant cell, where neoplastic overgrowth results from its expression (19,20,23).A. tumefaciens C58C1 possesses a highly sensitive chemotaxis system, which responds to a range of sugars and amino acids (19a). For example, the chemotactic peak for sucrose occurs at 10-6 M. Thus, release of these compounds from plant roots and chemotaxis of A. tumefaciens toward them can account for the prevalence of the bacterium in the rhizosphere (15,16).The majority of chemotactic responses in A. tumefaciens appear to be chromosomally encoded (3, 19a, 25). However, chemotaxis toward acetosyringone, one of the major plant phenolic inducers, requires the presence of a Ti plasmid and occurs with a threshold sensitivity of <108 M (3), some 1,000-fold below the maximal vir-inducing concentration (32). Agrobacterium strains harboring either octopine or nopaline Ti plasmids respond at this concentration (29), indicating that chemotaxis is important in guiding virulent A. tumefaciens toward a susceptible plant (30). The Ti plasmid genes involved in this specific chemotactic response are virA and virG (29). virA and virG are also involved in mediating vir induction in response to acetosyringone (18,21,34,37). This suggests a multifunctional role for virA and virG: at low concentrations of acetosyringone they trigger chemotaxis, whereas at high concentrations vir induction is effected.To confirm and extend our previous observations, we embarked upon a survey of the vir-inducing and chemotactic properties of a variety of related phenolic compounds found in plant exudates. We present evidence indicative of a strong correlation bet...