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Wounded plant cell produces acetosyringone.

Acetosyringone activates vir genes.

Agrobacterium cell

Acetosyringone activates vir genes.

Copy of T DNA is transferred and integrated into a plant chromosome.

Agrobacterium cell

Plant cell synthesizes auxins, cytokinins, opines; tumor forms.

Plant cell nucleus

Plant cell nucleus

FIGURE 9-26 Transfer of DNA to plant cells by a bacterial parasite.

(a) The Ti (tumor-inducing) plasmid of Agrobacterium tumefaciens.

(b) Wounded plant cells produce and release the phenolic compound acetosyringone. When Agrobacterium detects this compound, the virulence (vir) genes on the Ti plasmid are expressed. The vir genes encode enzymes needed to introduce the T DNA segment of the Ti plasmid into the genome of nearby plant cells. A single-stranded copy of the T DNA is synthesized and transferred to the plant cell, where it is converted to duplex DNA and integrated into a plant cell chromosome. The T DNA encodes enzymes that synthesize both plant growth hormones and opines (see Fig. 9-2 7); the latter compounds are metabolized (as a nutrient source) only by Agrobacterium. Expression of the T DNA genes by transformed plant cells thus leads to both aberrant plant cell growth (tumor formation) and the diversion of plant cell nutrients to the invading bacteria.

the bacterium (Fig. 9-27). The first group consists of plant growth hormones (auxins and cytokinins) that stimulate growth of the transformed plant cells to form the crown gall tumor. The second constitutes a series of unusual amino acids called opines, which serve as a food source for the bacterium. The opines are produced in high concentrations in the tumor cells and secreted to the surroundings, where they can be metabolized only by Agrobacterium, using enzymes encoded elsewhere on the Ti plasmid. The bacterium thereby diverts plant resources by converting them to a form that benefits only itself.

Opines

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