Multiple Copies of virG Allow Induction of Agrobacterium tumefaciens vir Genes and T-DNA Processing at Alkaline pH. Chang-Nong Liu. Department of Biological Sciences, Purdue University, West Lafayette, IN 47907 U.S.A. Todd R. Steck, Lillian L. Habeck, Jill A. Meyer, and Stanton B. Gelvin. Department of Biological Sciences, Purdue University, West Lafayette, IN 47907 U.S.A. MPMI 6:144-156. Accepted 19 October 1992. Copyright 1993 The American Phytopathological Society.

Previous work indicated that the virulence (vir) genes of the octopine-type Ti (tumor-inducing) plasmid pTiA6 of Agrobacterium tumefaciens are induced by phenolic plant signal molecules only in acidic medium (pH <6.5). Upon induction of the vir genes, the T-DNA (transferred DNA) is processed from the Ti plasmid and is transferred to plant cells. We report here that several vir genes of pTiA6 can be induced to high levels by acetosyringone in both minimal and rich media at a pH greater than 7.5 when multiple (5-10) copies of virG are present in A. tumefaciens. In AB minimal medium, the extent of induction (measured by the expression of vir gene::lacZ fusions) at alkaline pH (>7.5) can be as high as 41% of that at acidic pH (5.5). In LB rich medium with a pH greater than 8.0, vir gene induction could be up to fourfold that of the level in the corresponding acidic medium. The induction of octopine-type vir genes at alkaline pH depended on the source of virG gene present in multiple copies within the bacterial cell: In some instances, multiple copies of virG from the nopaline-type Ti plasmid pTiC58 did not affect induction at alkaline pH, whereas multiple copies of virG from the octopine-type Ti plasmid pTiA6 or the agropine-type Ti plasmid pTiBo542 did. After 12 hr of induction, virD and virG induction by acetosyringone at both acidic and alkaline pH correlated well with the production of processed T-DNA intermediates. The correlation was poor after induction for 24 hr. The induction of virE did not correlate with T-DNA processing at either early or late times. These data show that the presence of multiple copies of virG in A. tumefaciens can alter the pH-sensitivity profile of vir gene induction, suggesting that virG, as well as virA, may play a role in the pH response to plant phenolic signal molecules.