Agrobacterium rhizogenes inserts T-DNA into the genomes of the host plant root cells

Abstract

Agrobacterium rhizogenes, which induces hairy root disease of dicotyledonous plants1, is closely related to Agrobacterium tumefaciens, the causative agent of crown gall disease1–3. Virulence in both species is conferred by large plasmids4–7. Infected plant tissue synthesizes novel metabolites, opines8–11, that are not found in normal plant tissues. The pattern of opines synthesized is determined by the type of virulence plasmid in the bacterium, and in general virulence plasmids confer on the host bacterium the ability to catabolize the same opines (refs 8,12, 13 and A.P. et al., in preparation). Opine synthesis persists when the affected plant tissue is cultivated in vitro in the absence of the pathogenic bacterium9–11,14,15, which in the case of crown gall tumours is a consequence of gene transfer from the pathogen to the plant cells. A small specific part of the tumour-inducing (Ti) plasmid of A. tumefaciens, termed T-DNA (transferred DNA), is incorporated into host plant nuclear DNA16–21 and transcribed into mRNA18,22–25. A specific region of T-DNA confers the ability to synthesize the characteristic opine26,27. Synthesis of opines is thus a natural example of genetic engineering in which the agent is the Ti plasmid of,A. tumefaciens. The discovery of opines in roots induced by A. rhizogenes (ref. 11 and A.P. et al., in preparation) suggested that they too might contain T-DNA derived from the virulence plasmid of the pathogen. We report here DNA hybridization studies that confirm this hypothesis.