Ri plasmid

The root inducing (Ri) -plasmid of Rhizobium rhizogenes (formerly Agrobacterium rhizogenes) is a plasmid capable of undergoing horizontal gene transfer of its transfer DNA (T-DNA), upon contact with a plant host. The T-DNA of the Ri-plasmid affects the plant host in such a way, that gene expression is altered, especially in regard to phytohormonal balances, metabolism and certain phenotypical characteristics.

The Ri-plasmid is generally classified based on the type of opines produced, and four have been described so far: the agropine, cucumopine, mannopine, and mikinopine types. While all types of Ri-plasmid contain T-DNA, the agropine plasmid contains both a TL and a TR-domain (left and right, resp.). The TL-DNA is reminiscent of the T-DNA of the other strain types, containing a virulence region with a set of virulence (vir) genes, opine synthesis genes, root oncogenic loci (rol) genes (rolA, rolB, rolC and rolD) and a number of other genes with unidentified functions (open reading frames (ORFs)). The TR-DNA resembles the T-DNA of the Ti-plasmid (pTi) found in Agrobacterium tumefaciens, and carries two codes for auxin biosynthesis genes (aux1 and aux2), homologous to the pTi tms1 and tms2 regions.

The Ri-phenotype
Upon infection with R. rhizogenes and subsequent integration of the Ri-plasmid, the host plant displays phenotypical characteristics aptly named the hairy root disease with the so-called Ri-phenotype. The phenotypical changes include but are not limited to increased, agravitropic root growth and root hair growth, shortened internodes, wrinkled leaves and reduced apical dominance, dwarfism and early flowering. Several of these physical traits are of interest in the commercial breeding of horticultural and agricultural plants. The different rol genes have been shown to have different effects on their host plant, both when incorporated into the plant genome separately and in combination with one another; for example, rolA has been shown to exhibit inhibitory effects on the rolB and rolC domains, and rolC is by now essentially known to induce dwarfism in its plant host.