Agrobacterium
Scientific classification
Domain:	Bacteria
Kingdom:	Pseudomonadati
Phylum:	Pseudomonadota
Class:	Alphaproteobacteria
Order:	Hyphomicrobiales
Family:	Rhizobiaceae
Genus:	Agrobacterium Conn 1942 (Approved Lists 1980)
Type species
Agrobacterium radiobacter (Smith and Townsend 1907) Conn 1942 (Approved Lists 1980)
Species
"Agrobacterium albertimagni"Salmassiet al. 2002 Agrobacterium arsenijeviciiKuzmanovićet al. 2019 "Agrobacterium bohemicum"Zahradniket al. 2018 Agrobacterium cavaraeFlores-Félixet al. 2020 "Agrobacterium deltaense"Yanet al. 2017 Agrobacterium fabacearumDelamutaet al. 2020 "Agrobacterium fabrum"Lassalleet al. 2011 Agrobacterium larrymooreiBouzar and Jones 2001 Agrobacterium nepotum(Puławskaet al. 2012) Mousaviet al. 2016 Agrobacterium pusense(Pandayet al. 2011) Mousaviet al. 2016 Agrobacterium radiobacter(Beijerinck and van Delden 1902) Conn 1942 (Approved Lists 1980) Agrobacterium rosaeKuzmanovićet al. 2019 Agrobacterium rubi(Hildebrand 1940) Starr and Weiss 1943 Agrobacterium salinitoleransYanet al. 2017 Agrobacterium skierniewicense(Puławskaet al. 2012) Mousaviet al. 2016 Agrobacterium tumefaciens
Synonyms[1]
PolymonasLieske 1928

Agrobacterium is agenus ofGram-negativebacteria established byH. J. Conn that useshorizontal gene transfer to causetumors in plants.Agrobacterium tumefaciens is the most commonly studiedspecies in this genus.Agrobacterium is well known for its ability to transferDNA between itself and plants, and for this reason it has become an important tool forgenetic engineering.

Leading up to the 1990s, the genusAgrobacterium was used as awastebasket taxon. With the advent of16S sequencing, manyAgrobacterium species (especially the marine species) were reassigned to genera such asAhrensia,Pseudorhodobacter,Ruegeria, andStappia.^[2][3] The remainingAgrobacterium species were assigned to three biovars: biovar 1 (Agrobacterium tumefaciens), biovar 2 (Agrobacterium rhizogenes), and biovar 3 (Agrobacterium vitis). In the early 2000s,Agrobacterium was synonymized with the genusRhizobium.^[4] This move proved to be controversial.^[5][6] The debate was finally resolved when the genusAgrobacterium was reinstated^[7] after it was demonstrated that it wasphylogenetically distinct fromRhizobium^[8][9] and thatAgrobacterium species were unified by a uniquesynapomorphy: the presence of the protelomerase gene,telA, which causes all members of the genus to have a linearchromid.^[10] By this time, however, the threeAgrobacterium biovars had become defunct; biovar 1 remained withAgrobacterium, biovar 2 was renamedRhizobium rhizogenes, and biovar 3 was renamedAllorhizobium vitis.

Agrobacterium tumefaciens causes crown-gall disease in plants. The disease is characterised by atumour-like growth orgall on the infected plant, often at the junction between the root and the shoot. Tumors are incited by theconjugative transfer of a DNA segment (T-DNA) from the bacterial tumour-inducing (Ti)plasmid. The closely related species,Agrobacterium rhizogenes, induces root tumors, and carries the distinct Ri (root-inducing) plasmid. Although the taxonomy ofAgrobacterium is currently under revision it can be generalised that 3 biovars exist within the genus,Agrobacterium tumefaciens,Agrobacterium rhizogenes, andAgrobacterium vitis. Strains withinAgrobacterium tumefaciens andAgrobacterium rhizogenes are known to be able to harbour either a Ti or Ri-plasmid, whilst strains ofAgrobacterium vitis, generally restricted to grapevines, can harbour a Ti-plasmid. Non-Agrobacterium strains have been isolated from environmental samples which harbour a Ri-plasmid whilst laboratory studies have shown that non-Agrobacterium strains can also harbour a Ti-plasmid. Some environmental strains ofAgrobacterium possess neither a Ti nor Ri-plasmid. These strains are avirulent.^[11]

The plasmid T-DNA is integrated semi-randomly into thegenome of the host cell,^[12] and the tumor morphology genes on the T-DNA are expressed, causing the formation of a gall. The T-DNA carries genes for the biosynthetic enzymes for the production of unusualamino acids, typicallyoctopine ornopaline. It also carries genes for the biosynthesis of theplant hormones,auxin andcytokinins, and for the biosynthesis ofopines, providing a carbon and nitrogen source for the bacteria that most other micro-organisms can't use, givingAgrobacterium aselective advantage.^[13] By altering the hormone balance in the plant cell, the division of those cells cannot be controlled by the plant, and tumors form. The ratio of auxin to cytokinin produced by the tumor genes determines the morphology of the tumor (root-like, disorganized or shoot-like).

Although generally seen as an infection in plants,Agrobacterium can be responsible foropportunistic infections in humans with weakenedimmune systems,^[14][15] but has not been shown to be a primary pathogen in otherwise healthy individuals. One of the earliest associations of human disease caused byAgrobacterium radiobacter was reported by Dr. J. R. Cain in Scotland (1988).^[16] A later study suggested thatAgrobacterium attaches to and genetically transforms several types of human cells by integrating its T-DNA into the human cell genome. The study was conducted using cultured human tissue and did not draw any conclusions regarding related biological activity in nature.^[17]

The ability ofAgrobacterium to transfergenes toplants and fungi is used inbiotechnology, in particular,genetic engineering forplant improvement. Genomes of plants and fungi can be engineered by use ofAgrobacterium for the delivery of sequences hosted inT-DNA binary vectors. A modified Ti or Ri plasmid can be used. The plasmid is 'disarmed' by deletion of the tumor inducing genes; the only essential parts of the T-DNA are its two small (25 base pair) border repeats, at least one of which is needed for plant transformation.^[18][19] The genes to be introduced into the plant are cloned into a plant binary vector that contains the T-DNA region of the disarmedplasmid, together with a selectable marker (such asantibiotic resistance) to enable selection for plants that have been successfully transformed. Plants are grown on media containing antibiotic following transformation, and those that do not have the T-DNA integrated into their genome will die. An alternative method isagroinfiltration.^[20][21]

Transformation withAgrobacterium can be achieved in multiple ways.Protoplasts or alternatively leaf-discs can be incubated with theAgrobacterium and whole plants regenerated usingplant tissue culture. Inagroinfiltration theAgrobacterium may be injected directly into the leaf tissue of a plant. This method transforms only cells in immediate contact with the bacteria, and results in transient expression of plasmid DNA.^[22]

Agroinfiltration is commonly used to transform tobacco (Nicotiana). A common transformation protocol forArabidopsis is the floral dip method:^[23] Aninflorescence is dipped in a suspension ofAgrobacterium, and the bacterium transforms thegermline cells that make the femalegametes. Theseeds can then be screened for antibiotic resistance (or another marker of interest). Plants that have not integrated the plasmid DNA will die when exposed to the antibiotic.^[20]

Agrobacterium is listed as being the vector of genetic material that was transferred to these USA GMOs:^[24]

• Soybean
• Cotton
• Maize
• Sugar Beet
• Alfalfa
• Wheat
• Rapeseed Oil (Canola)
• Creeping bentgrass (for animal feed)
• Rice (Golden Rice)

Thetransformation of fungi usingAgrobacterium is used primarily for research purposes,^[25][26] and follows similar approaches as for plant transformation. TheTi plasmid system is modified to include DNA elements to select for transformed fungal strains, after co-incubation ofAgrobacterium strains carrying these plasmids with fungal species.

TheAgrobacterium genome consists of three parts: a circularchromosome, a linear chromosome/chromid, and (in some species) aTi plasmid.^[27]

The sequencing of thegenomes of several species ofAgrobacterium has permitted the study of the evolutionary history of these organisms and has provided information on thegenes and systems involved in pathogenesis, biological control andsymbiosis. One important finding is the possibility thatchromosomes are evolving fromplasmids in many of these bacteria. Another discovery is that the diverse chromosomal structures in this group appear to be capable of supporting both symbiotic and pathogenic lifestyles. The availability of the genome sequences ofAgrobacterium species will continue to increase, resulting in substantial insights into the function and evolutionary history of this group of plant-associated microbes.^[28]

Marc Van Montagu andJozef Schell at theUniversity of Ghent (Belgium) discovered the gene transfer mechanism betweenAgrobacterium and plants, which resulted in the development of methods to alterAgrobacterium into an efficient delivery system for gene engineering in plants.^[18][19] A team of researchers led byMary-Dell Chilton were the first to demonstrate that the virulence genes could be removed without adversely affecting the ability ofAgrobacterium to insert its own DNA into the plant genome (1983).^[29]

• Agroinfiltration
• Marc Van Montagu
• Rhizobium rhizogenes (formerlyAgrobacterium rhizogenes)

• Current taxonomy ofAgrobacterium species, and newRhizobium names
• Agrobacteria is used as gene ferry - Plant transformation withAgrobacterium]

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