Agrobacterium Ti plasmid indoleacetic acid gene is required for crown gall oncogenesis

A gene (iaaP) necessary for virulence and indoleacetic acid (IAA) production has been located on a nopaline Ti plasmid of Agrobacterium tumefaciens C58. iaaP function was established by using transformation to insert nopaline or octopine Ti plasmids into an avirulent, Ti plasmid-free mutant 1D1293-3 that was defective in IAA synthesis (iaaC(-)). The resulting transformants produced increased levels of IAA and virulence was restored. When these transformants were cured of their Ti plasmid, virulence and high IAA production levels were concomitantly lost. A Tn5 mutagenized TiC58 plasmid, deficient in the ability to direct increased synthesis of IAA, was inserted by transformation into mutant 1D1293-3. The resulting transformants 1D1293-3 (TiC58::Tn5) remained avirulent and iaaP(-). Restriction analysis of the TiC58::Tn5 plasmid DNA identified the iaaP gene at 20.9 kilobases to the left of the T-DNA. A major aromatic-amino-acid aminotransferase is coded by the iaaC gene, but not by the iaaP gene. The possible reasons for the iaaP locus to be situated outside the T-DNA region are discussed.