Characterization of three Agrobacterium tumefaciens avirulent mutants with chromosomal mutations that affect induction of vir genes

Abstract

Three Agrobacterium tumefaciens mutants with chromosomal mutations that affect bacterial virulence were isolated by transposon mutagenesis. Two of the mutants were avirulent on all hosts tested. The third mutant, Ivr-211, was a host range mutant which was avirulent on BryophyUum diagremontiana, Nicotiana tabacum, N. Infection of susceptible dicotyledonous plants by Agrobacterium tumefaciens results in crown gall tumor formation. The mechanism by which the bacterium infects the plant is not fully understood. An early step in tumor formation involves the attachment of the bacterium to host plant cells (12). Once attached to the plant, the bacterium begins a process that results in the movement of tumor-inducing (Ti) plasmid DNA sequences from the bacterium into the plant host genome (2, 28). Depicker et al. have identified the DNA sequences transferred and termed them T-DNA (5). Without successful transfer of the T-DNA from the bacterium to the plant, tumor formation cannot occur. This paper examines the properties of three avirulent TnS insertion mutants of A. tumefaciens,. These TnS insertions are contained within the chromosome of the bacterium; thus, the insertions have no direct affect on the vir region of the Ti plasmid which is responsible for T-DNA transfer. The only previously identified chromosomal genes required for virulence are chvA and chvB (6), att (16), pscA (27) (also called exoC [1]), chvD (32), and chvE (9). Mutations in each of the first four genes listed result in the inability of the bacteria to attach to host cells and the consequent loss of bacterial virulence. Mutations in chvD result in reduced virulence on Bryophyllum diagremontiana (visible tumors were still formed) and reduced induction of virG by acetosyringone (32). Transposon insertions in chvE result in a * Corresponding author.

Mutation of the miaA gene of Agrobacterium tumefaciens results in reduced vir gene expression

“…The four-to fivefold reduction in the number of tumors per potato disk induced by the miaA mutant strain appears very minor compared with the effect of mutations in the chvE (30) or ivr loci (41). We did not observe a decreased virulence in the mutant strain with any of the other virulence assays, although these assays are generally perceived not to be as sensitive as the potato disk assay and thus may not allow the detection of minor virulence determinants.…”

The chromosomal response regulatory gene chvI of Agrobacterium tumefaciens complements an Escherichia coli phoB mutation and is required for virulence

“…Metts et al isolated three chromosomal loci that are necessary for virulence and vir gene expression (40). Insertions in any one of these loci disrupt the cellular lipopolysaccharide composition (40). The present study suggests that at least one additional chromosomal locus may be involved in vir gene regulation and virulence.…”

“…Disruption of this gene attenuated virG induction by acidic pH and phosphate starvation, although these effects could be indirect (61). Metts et al isolated three chromosomal loci that are necessary for virulence and vir gene expression (40). Insertions in any one of these loci disrupt the cellular lipopolysaccharide composition (40).…”

A chromosomally encoded two-component sensory transduction system is required for virulence of Agrobacterium tumefaciens

“…Mutations that result in reduced vir gene induction by acetosyringone have recently been described. Those in the ivr loci drastically reduce tumor-forming ability (52), while those in miaA affect virulence only slightly (24). Significantly, chromosomal virulence genes specifically involved in T-DNA processing and transport have not been identified.…”