Agrobacterium in plant disease, biological disease control and plant genetic engineering

Plant pathogenic strains of Agrobacterium cause crown gall and hairy-root diseases. The abnormal cell proliferation of diseased tissues results from elevated plant hormonal levels within them. These levels are consequent upon the transfer of DNA portions (T-DNA) from Agrobacterium plasmids to host nuclei where they are integrated and where genes for hormone synthesis, borne on the T-DNA, are expressed in transformed host cells. This process has been exploited in plant genetic engineering by producing Agrobacterium mutants with deleterious genes removed from T-DNA, and with selected beneficial genes inserted in their place. Such mutants are able to infect and transform plants so that the beneficial genes are transferred to, and expressed in, the host plants, endowing them with desirable characteristics, including insect and disease resistance and herbicide tolerance. Biological control of Agrobacterium disease has been achieved using non-pathogenic Agrobacterium strains which produce an antibiotic specifically inhibitory to Agrobacterium pathogens. Use of one of these biocontrol strains is the first instance of a genetically engineered micro-organism being released commercially.