VirA, the plant-signal receptor, is responsible for the Ti plasmid-specific transfer of DNA to maize by Agrobacterium.

Agrobacteria exhibit marked Ti (tumor-inducing)/Ri (root-inducing) plasmid specificity in their interaction with the Gramineae. In this study, we have used the technique of "agroinfection," in which Agrobacterium-mediated delivery of viral genomes into plants is detected by the development of viral disease symptoms, to identify the region of the Ti plasmid which is responsible for the major differences seen in the ability of nopaline- vs. octopine-type Ti plasmids to transfer maize streak virus (MSV) DNA to maize. Introduction of fragments of the C58 (nopaline-type) Ti plasmid into strains containing an octopine-type Ti plasmid showed that a fragment containing the nopaline-type virA locus was able to complement these normally non-agroinfectious strains to high levels of MSV DNA transfer. Octopine-type virA mutant strains that express vir genes at high levels in the absence of the plant inducing compound acetosyringone also efficiently transferred MSV DNA. These findings imply a functional difference between the virA gene products encoded by octopine- and nopaline-type Ti plasmids which has a profound effect on their ability to mediate DNA transfer to maize.