Bacteria that Associate with Plants

Members of two related genera use very different means to obtain the nutrients needed for growth from plants. Agrobacterium species are plant pathogens that cause tumorlike growths, whereas Rhizobium species form a mutually beneficial relationship with certain types of plants.

The Genus Agrobacterium

Agrobacterium species have an unusual mechanism of gaining a competitive advantage in soil. They cause plant tumors, a manifestation of their ability to genetically alter a plant for their own benefit (figure 11.20). These Gram-negative rod-shaped bacteria enter the plant via a wound, and they then transfer to the

Tumor Caused Insect
Figure 11.20 Plant Tumor Caused by Agrobacterium tumefaciens

5 mm

Figure 11.21 Symbiotic Relationship Between Rhizobia and Certain Plants

(a) Root nodules. (b) Scanning electron micrograph of bacterial cells within a nodule.

5 mm

Figure 11.21 Symbiotic Relationship Between Rhizobia and Certain Plants

(a) Root nodules. (b) Scanning electron micrograph of bacterial cells within a nodule.

286 Chapter 11 The Diversity of Prokaryotic Organisms which binds and controls the levels of O2. Within the resulting microaerobic confines of the nodule, the bacteria are able to fix nitrogen. Rhizobia residing within plant cells are examples of endosymbionts, organisms that provide a benefit to the cells they reside within. Rhizobia are Gram-negative. ■ symbiotic nitrogen fixers, p. 779

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    What are the causative agent of disease of vegetation?
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