Rhizobium rhizogenes

Rhizobium rhizogenes (formerly Agrobacterium rhizogenes) is a Gram-negative soil bacterium that produces hairy root disease in dicotyledonous plants. R. rhizogenes induces the formation of proliferative multiple-branched adventitious roots at the site of infection, so-called 'hairy roots'.^[3] It also induces galls.^[4]: 39

Rhizobium rhizogenes
Scientific classification
Domain:	Bacteria
Kingdom:	Pseudomonadati
Phylum:	Pseudomonadota
Class:	Alphaproteobacteria
Order:	Hyphomicrobiales
Family:	Rhizobiaceae
Genus:	Rhizobium
Species:	R. rhizogenes
Binomial name
Rhizobium rhizogenes (Riker et al. 1930) Young et al. 2001[1][a]
Synonyms
Agrobacterium rhizogenes (Riker et al. 1930) Conn 1942 Agrobacterium rhizogenes (Riker et al. 1930) Conn 1942 (Approved Lists 1980) emend. Sawada et al. 1993

In the rhizosphere, plants may suffer from wounds by soil pathogens or other sources. This leads to the secretion of phenolic compounds like acetosyringone which have chemotactic effects that attract the bacteria. Under such conditions, certain bacterial genes are turned on leading to the transfer of its T-DNA from its root-inducing plasmid (Ri plasmid) into the plant through the wound. After integration and expression, in vitro or under natural conditions, the hairy root phenotype is observed, which typically includes overdevelopment of a root system that is not completely geotropic, and altered (wrinkled) leaf morphology, if leaves are present.^[5] R. rhizogenes also propagates as a seed-borne pathogen.^[4]: 39

Bacterial genes may be retained within the plant,^[6] and sweet potato for instance has retained and actively expresses A. rhizogenes genes which are not present in other members of its genus.^[7]

The hairy roots are grown in vitro in bioreactors to study their soil interaction with other pathogens like fungi and nematodes. This technique has also led to the commercial production of certain metabolic compounds that the plant is known to secrete, especially in regard to the medicinal plants that are difficult to cultivate in sufficient quantities by other means.^[8] The root cultures are also used for genetic engineering.^[9][10]

Applications

Strain K84 produces agrocin 84, a type of bacteriocin, that inhibits the growth of Agrobacterium tumefaciens (possibly a misclassified A. radiobacter) which causes the disease.^[11] Strain K1026, which is descended from K84 by the removal of potentially-pathogenic DNA sequences, has been used as a biopesticide on stone fruit (such as cherries and plums), nut trees, and ornamentals to protect them from crown gall disease.^[12] Strain K84 and its descendant K1016 are now properly classified as belonging to Rhizobium rhizogenes. They were formerly considered A. radiobacter.^[13][14]