The soil phytopathogen Agrobacterium tumefaciens causes crown galldisease in a wide range of plant species. The neoplastic growth at theinfection sites is caused by transferring, integrating, and expressingtransfer DNA (T-DNA) from A. tumefaciens into plant cells. A transzeatinsynthesizing (tzs) gene is located in the nopaline-type tumor-inducingplasmid and causes trans-zeatin production in A. tumefaciens.Similar to known virulence (Vir) proteins that are induced by the vir geneinducer acetosyringone (AS) at acidic pH 5.5, Tzs protein is highlyinduced by AS under this growth condition but also constitutively expressedand moderately upregulated by AS at neutral pH 7.0. We foundthat the promoter activities and protein levels of several AS-induced virgenes increased in the tzs deletion mutant, a mutant with decreasedtumorigenesis and transient transformation efficiencies, in Arabidopsisroots. During AS induction and infection of Arabidopsis roots, the tzsdeletion mutant conferred impaired growth, which could be rescued bygenetic complementation and supplementing exogenous cytokinin. Exogenouscytokinin also repressed vir promoter activities and Vir proteinaccumulation in both the wild-type and tzs mutant bacteria with ASinduction. Thus, the tzs gene or its product, cytokinin, may be involved inregulating AS-induced vir gene expression and, therefore, affect bacterialgrowth and virulence during A. tumefaciens infection
