Agrobacterium: A disease-causing bacterium

The common use of Agrobacterium as a gene vector for plants has somewhat obscured the fact that this bacterium remains an important plant pathogen. Pathogenic strains of the genus Agrobacterium cause unorganized tissue growth called crown gall or profuse abnormal root development called hairy root. Agrobacterium tumefaciens induces galls on roots and crowns of several fruit and forest trees and ornamental plants. A. vitis is responsible for the crown gall disease of grapevine, while A. rhizogenes induces abnormal rooting on its hosts. Plants tissues that become diseased undergo physiological changes resulting in weak growth, low yields or even death of the entire plant. Tumors originate from dividing plant cells, e. g. from cambium. Thus the cambial region becomes unable to differentiate into efficient phloem and xylem elements leading to deficient nutrient transport. Symptoms may appear on roots, crowns and aerial parts of attacked plants (Figure 1–1). Tumors are usually comprised of unorganized tissue, but sometimes they differentiate into roots or shoots. This depends on the host plant, the position on the infected plant or the inducing bacterium (Figure 1–2). As indicated by several reviews, crown gall has been a worldwide problem in agriculture for over hundred years (Moore and Cooksey, 1981; Burr et al., 1998; Burr and Otten, 1999; Escobar and Dandekar, 2003).