Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called 'crown gall', via the transfer and integration of transferred
DNA (
T-DNA) from the bacterium into the plant genome. We characterized an
acetosyringone (AS)-induced tumour-inducing (Ti) plasmid gene, tzs (trans-
zeatin synthesizing), that is responsible for the synthesis of the
plant hormone cytokinin in
nopaline-type A. tumefaciens strains. The loss of Tzs
protein expression and trans-
zeatin secretions by the tzs frameshift (tzs-fs) mutant is associated with reduced
tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs-fs mutant with a wild-type tzs gene restored wild-type levels of trans-
zeatin secretions and transformation efficiencies. Exogenous application of
cytokinin during
infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium-produced
cytokinin. Interestingly, although the tzs-fs mutant displayed reduced
tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced
tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans-
zeatin at early stage(s) of the
infection process, modulates plant transformation efficiency by A. tumefaciens.