From Academic Kids

Scientific classification
Class:Alpha Proteobacteria

Agrobacterium tumefaciens
Agrobacterium rhizogenes

Agrobacterium is a genus of bacteria that cause tumors in plants. Agrobacterium tumefaciens is the most commonly studied species in this genus. Agrobacterium is well known for its ability to transfer DNA between itself and plants, and for this reason it has become an important tool for plant improvement by genetic engineering.

Taxonomic note: The Agrobacterium genus is quite heterogeneous, recent taxonomic studies have reclassified all of the Agrobacterium species in to new genera, such as Ruegeria, Pseudorhodobacter and Stappia. But most species have been reclassified as Rhizobium species.

Agrobacterium as a plant pathogen

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The large growths on these roots are galls are induced by Agrobacterium sp.

A. tumefaciens causes crown-gall disease in plants, the disease is characterised by a tumour like growth on the infected plant at the junction between the root and the shoot. Tumors are induced by the transfer of a transferred DNA (T-DNA) segment form the bacterial tumour-inducing (Ti) plasmid A. rhizogenes induces root tumors, its plasmid is call Ri (root-inducing).

The plasmid T-DNA is incorporated into the genome of the host cell by homologous recombination, and the virulence (vir) genes on the T-DNA are expressed, causing the gall to form. The T-DNA carries genes for the biosynthetic enzymes for the production of unusual amino acids octapine and nopaline. It also carries genes for the biosyntheis of plant hormones auxin and cytokinins. By altering the hormone balance in the plant cell, the division of those cells cannot be controlled by the plant, and tumors from.

Agrobacterium in biotechnology

The ability of Agrobacterium to transfer genes to plants has been exploited for genetic engineering for plant improvement. 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. A team of researchers led by Dr Mary-Dell Chilton were the first to demonstrate that the virulence genes could be removed without adversely affecting the ability of Agrobacterium to insert its own DNA into the plant genome.

The genes to be introduced into the plant are cloned into the plasmid, the plasmid will also contain a selectable marker, like an antibiotic resistance to enable selection for plants that have been successfully transformed. Plants are grown on media containing antibiotic following transformation, those that have not had the plasmid DNA integrated into their genome will die.

Transformation with Agrobacterium can be achieved in two ways. Protoplasts, or leaf-discs can be incubated with the Agrobacterium and whole plants regenerated using plant tissue culture. A common transformation protocol for Arabidopsis is the floral-dip method, the flowers are dipped in a culture of a Agrobacterium, the bacterium transforms the germline cells that make the female gametes. The seeds can then be screened for antibiotic resistance, plants that have not integrated the plasmid DNA will die.

Agrobacterium cannot infect all types of plant, there are many other techniques for transformation including the gene gun.

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