Abstract
A mutant (M3 strain) of Agrobacterium tumefaciens was isolated by transposon (Tn5) mutagenesis. The M3 strain was avirulent on all host plants tested, Daucus carota, Solanum tuberosum and Kalanchoe pinnata. The avirulent phenotype of M3 strain was due to an insertion of Tn5 into the chvA chromosomal virulence gene. Using this M3 strain (chvA:: Tn5) and two other strains, B119 strain (acvB: Tn5) and A208 strain (parent, virulent), the functions of the chvA and acvB chromosomal virulence genes of A. tumefaciens were analyzed. When M3, B119 and A208 strains were co-cultured with tobacco BY2 suspension-culture cells in the presence and absence of acetosyringone (As), efficient T-DNA transfer from an Agrobacterium cell to the nucleus of a tobacco cell occurred only with A208 strain in the presence of As. However, the T-strand (an intermediate form of T-DNA for transfer) was produced normally in all three strains in the presence of As. Analyses with tricine-SDS-polyacrylamide gel electrophoresis and electron microscopy demonstrated that pili, which are assumed to mediate T-DNA transfer into the host cell, were not produced on the cell surface of the M3 strain in either media with or without As. On the other hand, perfect pili were produced on the cell surface of the A208 strain in the presence of As. On the cell surface of the B119 strain, “imperfect” pili which were thinner than those on the A208 strain were produced in the presence of As. Taken together, these results indicate that chvA and acvB likely play roles in virulence by influencing formation of the pili through which the T-strands are thought to be transfer into host cells.
