The soil phytopathogen Agrobacterium tumefaciens causes crown gall disease in a wide range of plant species. The neoplastic growth at the
infection sites is caused by transferring, integrating, and expressing transfer
DNA (
T-DNA) from A. tumefaciens into plant cells. A trans-
zeatin synthesizing (tzs) gene is located in the
nopaline-type
tumor-inducing plasmid and causes trans-
zeatin production in A. tumefaciens. Similar to known virulence (Vir)
proteins that are induced by the vir gene inducer
acetosyringone (AS) at acidic pH 5.5, Tzs
protein is highly induced by AS under this growth condition but also constitutively expressed and moderately upregulated by AS at neutral pH 7.0. We found that the promoter activities and
protein levels of several AS-induced vir genes increased in the tzs deletion mutant, a mutant with decreased
tumorigenesis and transient transformation efficiencies, in Arabidopsis roots. During AS induction and
infection of Arabidopsis roots, the tzs deletion mutant conferred impaired growth, which could be rescued by genetic complementation and supplementing exogenous
cytokinin. Exogenous
cytokinin also repressed vir promoter activities and Vir
protein accumulation in both the wild-type and tzs mutant bacteria with AS induction. Thus, the tzs gene or its product,
cytokinin, may be involved in regulating AS-induced vir gene expression and, therefore, affect bacterial growth and virulence during A. tumefaciens
infection.