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The virulence (vir) genes are required in the early stages of plant tumor formation and are located together on the tumor-inducing (Ti) plasmid in Agrobacterium tumefaciens. Five of the vir genes are expressed inducibly in response to the following monocyclic phenolic compounds: acetosyringone, catechol, gallate, P-resorcylate, protocatechuate, p-hydroxybenzoate, and vanillin. Of these compQunds, only the latter five served as chemoattractants and only the latter three served as growth substrates for A. tumefaciens A348. Strain A136, isogenic except for lack of the Ti plasmid, demonstrated chemotactic behavior and nutritional capabilities similar to those of strain A348. The chemotactic response to the vir gene inducers was expressed constitutively.

Acetosyringone, a phenolic inducer of the virulence (vir) genes of Agrobacterium tumefaciens, inhibited the growth of the nopaline-type strains T37 and C58 incubated under acidic conditions. In the course of a 6-day incubation with acetosyringone, avirulent clones were produced in different proportions by strains T37 and C58 and also by a spontaneous variant of strain C58, denominated C58F. The proportion of avirulent clones in acetosyringone-treated cultures often exceeded 50%o for strains T37 and C58F and was of the order of 1% for strain C58. Control cultures not exposed to acetosyringone did not yield avirulent clones. Two other vir inducers, sinapinic acid and syringaldehyde, also inhibited growth and promoted accumulation of avirulent clones in cultures of strains C58F and T37. On the other hand, various acetosyringone analogs reported not to induce the vir genes did not act as growth inhibitors. All of the T37 and most of the C58F avirulent clones examined still carried a Ti plasmid. In all instances examined, avirulent clones still carrying a Ti plasmid were mutated in this plasmid. Mutants of strain C58F lacked the capacity to induce a virB::lacZ fusion in the presence of acetosyringone.The causal agent of crown gall, Agrobacterium tumefaciens, infects plant wounds where it detects the presence of specific compounds of plant origin. The virulence (vir) genes present on the Ti plasmids ofA. tumefaciens are then induced. Expression of the vir genes in the bacterium leads to the transfer to host plant cells of a particular region of the Ti plasmids, called the transferred (T-) DNA. This DNA of bacterial origin becomes stably integrated into the nuclear genome of the plant cells.

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Twelve strains of Agrobacterium radiobacter isolated from naturally occurring crown galls or soil were found to be avirulent on sunflower, tomato, Kalanchoe, and carrot. Eleven strains contained plasmids of molecular weights 77 x 106 to 182 x 106 as determined by electron microscopy. One strain contained only a smaller plasmid (50 x 106 daltons). Several strains had both large and small (ca. 11 x 106 daltons) plasmids; one strain contained two large plasmids (112 x 106 and 136 x 106 daltons). Hybridization reactions of virulence plasmids from Agrobacterium tumefaciens strains C58 and A6 with plasmids from each of the A. radiobacter strains revealed that some A. radiobacter plasmids had less than 10% homology to either the C58 or A6 plasmids. Plasmids from some strains had approximately 50% homology with the C58 plasmid, but only one A. radiobacter plasmid contained more than 10%o homology to the A6 plasmid. The