Agrobacterium rhizogenes:: recent developments and promising applications

Agrobacterium rhizogenes is the etiological agent for hairy-root disease (also known as root-mat disease). This bacterium induces the neoplastic growth of plant cells that differentiate to form '' hairy roots.'' Morphologically, A. rhizogenes-induced hairy roots are very similar in structure to wild-type roots with a few notable exceptions: Root hairs are longer, more numerous, and root systems are more branched and exhibit an agravitropic phenotype. Hairy roots are induced by the incorporation of a bacterial-derived segment of DNA transferred (T-DNA) into the chromosome of the plant cell. The expression of genes encoded within the T-DNA promotes the development and production of roots at the site of infection on most dicotyledonous plants. A key characteristic of hairy roots is their ability to grow quickly in the absence of exogenous plant growth regulators. As a result, hairy roots are widely used as a transgenic tool for the production of metabolites and for the study of gene function in plants. Researchers have utilized this tool to study root development and root-biotic interactions, to overexpress proteins and secondary metabolites, to detoxify environmental pollutants, and to increase drought tolerance. In this review, we provide an up-to-date overview of the current knowledge of how A. rhizogenes induces root formation, on the new uses for A. rhizogenes in tissue culture and composite plant production (wild-type shoots with transgenic roots), and the recent development of a disarmed version of A. rhizogenes for stable transgenic plant production.