Expression of an Agrobacterium Ti plasmid gene involved in cytokinin biosynthesis is regulated by virulence loci and induced by plant phenolic compounds.

The nopaline-type Ti plasmid T37 of Agrobacterium tumefaciens carries two distinct genes that encode enzymes involved in cytokinin biosynthesis. In this report, we show that the level of expression of one of these genes was increased dramatically by culture conditions that increased the expression of Ti plasmid virulence genes, including coculture with plant cells or treatment with acetosyringone, a plant phenolic compound. When this nopaline-type Ti plasmid gene was introduced into Agrobacterium strains containing an octopine-type Ti plasmid, similar induction of expression by culture conditions was observed, and analysis of virulence region mutants demonstrated that this induction was under the control of the virA and virG regulatory loci. We further show that induction was strongly pH dependent in octopine strains but, under the conditions examined, pH independent in nopaline strains.