The root inducing (Ri) -plasmid of ''

Rhizobium rhizogenes ''Rhizobium rhizogenes'' (formerly ''Agrobacterium rhizogenes'') is a Gram-negative soil bacterium that produces hairy root disease in Dicotyledon, dicotyledonous plants. ''R. rhizogenes'' induces the formation of proliferative multiple-branched ...

'' (formerly ''

Agrobacterium ''Agrobacterium'' is a genus of Gram-negative bacteria established by Harold J. Conn, H. J. Conn that uses horizontal gene transfer to cause tumors in plants. ''Agrobacterium tumefaciens'' is the most commonly studied species in this genus. ''Agr ...

rhizogenes'') is a

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...

capable of undergoing

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

of its

transfer DNA The transfer DNA (abbreviated T-DNA) is the transferred DNA of the tumor-inducing (Ti) plasmid of some species of bacteria such as ''Agrobacterium tumefaciens'' and '' Agrobacterium rhizogenes (actually an Ri plasmid)''. The T-DNA is transferred ...

(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 Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

is altered, especially in regard to phytohormonal balances, metabolism and certain

phenotypical In genetics, the phenotype () is the set of observable characteristics or phenotypic trait, traits of an organism. The term covers the organism's morphology (biology), morphology (physical form and structure), its Developmental biology, develo ...

characteristics. The Ri-plasmid is generally classified based on the type of

opines Opines are low molecular weight compounds found in plant crown gall tumors or hairy root tumors produced by pathogenic bacteria of the genus ''Agrobacterium'' and ''Rhizobium''. Opine biosynthesis is catalyzed by specific enzymes encoded by gen ...

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 Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

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 In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames wil ...

(ORFs)). The T_R-DNA resembles the T-DNA of the Ti-plasmid (pTi) found in ''

Agrobacterium tumefaciens ''Agrobacterium tumefaciens'' (also known as ''Rhizobium radiobacter'') is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram-negative soil bacterium. Symptoms are cause ...

'', and carries two codes for

auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...

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 In botany, apical dominance is the phenomenon whereby the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems; on a branch the main stem of the branch is further dominant over its own side twigs. P ...

, 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.

References

{{reflist Plasmids