The virulence genes of Agrobacterium are required for this organism to genetically transform plant cells. We show that vir gene expression is specifically induced by a small (<1000 Da) diffusible plant cell metabolite present in limiting quantities in the exudates of a variety of plant cell cultures. Active plant cell metabolism is required for the synthesis of the vir -inducing factor, and the presence of bacteria does not stimulate this production. vir -inducing factor is ( i ) heat and cold stable; ( ii ) pH stable, although vir induction with the factor is sensitive above pH 6.0; and ( iii ) partially hydrophobic. Induction of vir gene expression was assayed by monitoring β-galactosidase activity in Agrobacterium strains that carry gene fusions between each of the vir loci and the lac Z gene of Escherichia coli. vir -inducing factor (partially purified on a C-18 column) induces both the expression in Agrobacterium of six distinct loci and the production of T-DNA circular molecules, which are thought to be involved in the transformation process. vir -inducing factor potentially represents the signal that Agrobacterium recognizes in nature as a plant cell susceptible to transformation.

Stachel, S. E., Nester, E. W., & Zambryski, P. C. (1986). A plant cell factor induces Agrobacterium tumefaciens vir gene expression. Proceedings of the National Academy of Sciences, 83(2), 379–383.