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 T_L and a T_R-domain (left and right, resp.). The T_L-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 T_R-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.