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The Virulence System of Agrobacterium Tumefaciens

1993; Springer Nature (Netherlands); Linguagem: Inglês

10.1007/978-94-017-0651-3_4

0924-1949

A. Beijersbergen, Paul J. J. Hooykaas,

Toxin Mechanisms and Immunotoxins

The gram-negative soil bacterium Agrobacterium tumefaciens causes the plant disease crown gall. This disease is characterized by the formation of tumors or crown galls at wound sites of infected dicotyledonous plants (for recent reviews see Kado, 1991; Winans, 1992; Zambryski, 1992; Hooykaas and Schilperoort, 1992). During tumor induction Agrobacterium attaches to the plant and transfers part of its tumor inducing (Ti) plasmid (Fig.l) to plant cells at wound sites. The transferred or T-DNA, which is surrounded by 24 basepair (bp) imperfect direct repeats or border repeats, becomes integrated in the plant cell nuclear DNA. Upon expression of the genes located on the T-DNA, proteins are produced involved in the production of the plant hormones auxin (IAA) and cytokinin (isopentenyl-AMP). These hormones cause the tumorous phenotype, characterized by the ability of the cells to proliferate unlimited and autonomously in the absence of added phytohormones. The T-DNA also encodes synthases catalyzing the formation of opines. These opines are mostly built from an amino acid and a sugar. Based on the kind of opines produced, Agrobacterium strains are classified as octopine, nopaline, succinamopine and leucinopine strains. The opines formed in the tumors are metabolized by the agrobacteria which induced tumor formation, but not by most other soil organisms. Thus, by genetic manipulation of plant cells Agrobacterium creates a favorable niche for itself, a process also called “genetic colonization”.