Agrobacterium Tumefaciens

Agrobacterium Tumefaciens - Agrobacterium tumefaciens is a soil borne bacterium. Like Rhizobium, the nitrogen fixing bacterial genus, Agrobacterium has developed a way of living in and deriving nourishment from plant tissues. However, unlike Rhizobium, Agrobacterium is a parasite and provides no benefit to the plant that it colonizes. Instead, it causes crown gall disease.

Agrobacterium can infect many dicot species. In the normal disease process the bacterium enters the plant at the site of a wound. The infection often occurs at the crown (the junction of the root and shoot) but may involve an upper part of the stem, the petiole, or the leaf.

The bacterium attachs to the wall of a cell and "transforms" it. The word "transform" was used before the involvement of DNA transfer was known and referred simply to the observation that infected cells acquired new properties. Infected cells proliferate at the site of infection, forming a tumor or "gall".

The tumor can be fatal to young plants if. it grows large enough to crush the vascular system; more often the tumor is merely debilitating and unsightly. Transformed cells also have the ability to grow in culture on a medium that lacks the hormones auxin and cytokinin.

Finally, transformed cells produce opines, such as octopine, and nopaline, chemicals formed from two amino acids. Arginine and alanine form octopine while arginine and glutamine produce nopaline. Octopine and nopaline are not found in normal plant cells.

There are two more or less distinct types of crown galls. The first type grows as a relatively unorganized callus on artificial medium and on host plants. Cells of this type produce octopine. The second type grows as a callus containing green islands. The islands show a variable amount of organization, inducing production of multiple shoots. Cells of this type produce nopaline.

The two types are associated with different strains of A. tumefaciens. There is also a related disease ("hairy root disease") in which infected tissues proliferate in root tissue and produce an opine. This disease is associated with the bacterium A. rhizogenes.

Formation of opines explains the ecological significance of tumor formation. Each strain of Agrobacterium synthesizes enzymes (permease, dehydrogenase) that allow it to metabolize the specific type of opine formed by the tumor it induces. Thus by stimulating the plant to form opines, the bacteria insure themselves a supply of nutrients specifically designed for them.

Growth of the infected tissue is important because it increases the amount of opine forming tissue. It is possible, too, that the physical characteristics of the tumors parenchymal cells with extensive intercellular spaces provide a good habitat for the bacteria.

Early studies of the mechanism of "transformation" demonstrated that tumorous tissue remained transformed even in the absence of infecting bacteria. (The bacteria could be removed with antibiotics such as penicillin.).