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.