Transgenic N. glauca plants expressing bacterial virulence gene virF are converted into hosts for nopaline strains of A. tumefaciens

Abstract

Tumours are induced by Agrobacterium tumefaciens on a variety of plants. The virulence determinants of A. tumefaciens reside on a large tumour-inducing (Ti) plasmid. This plasmid carries two regions essential for tumour induction, namely the T region and the Vir region. During infection the T region is transferred to the plant cell, where it becomes stably integrated in one of the host chromosomes as T-DNA. Expression of T-DNA leads to the production of the plant hormones auxin and cytokinin, as well as to the synthesis of specific amino-acid derivatives termed opines. Agrobacterium strains are classified according to the types of opines produced by the tumours they induce. The Vir region contains genes that are expressed in the bacterium and are required for T-DNA transfer to plant cells, and several other genes that affect the efficiency of transfer and the host range. Vir regions from different Ti plasmids may vary slightly in the genes they contain: for instance, the virF gene, which is present in the Vir-region of octopine Ti plasmids, is absent from nopaline Ti plasmids. Mutation of the virF gene leads to a weakened virulence of octopine strains on tomato and Nicotiana glauca (shrub tobacco). Nopaline strains are strongly attenuated in N. glauca compared with octopine strains because of the absence of the virF virulence gene from the Ti plasmid in nopaline strains. The virF gene product may be transferred to and be active in plant cells. Here we isolate transgenic N. glauca plants in which the virF coding sequence is expressed using the cauliflower mosaic virus 35S promoter. The presence of the VirF protein converts the non-host N. glauca into a host for tumour formation by A. tumefaciens nopaline strains and octopine virF mutants. Our results indicate that certain virulence gene products such as the VirF protein may be transferred to plant cells during tumour induction, where they function as mediators of T-DNA transfer.