Biological Control of Crown Gall

Agrobacterium radiobacter strain K84 is a naturally occurring bacterium that controls crown gall, a plant tumor caused by the related soil bacterium A. tumefaciens. Crop losses caused by crown gall occur worldwide and can be extensive, particularly in nurseries growing rosaceous plants, grapevines, and stone fruit trees. A. tumefaciens enters the plants through wounds and causes tumors, which can weaken, reduce the aesthetic quality, and eventually kill the host plant. There are no effective chemical controls currently available for crown gall.

The biological control agent A. radiobacter strain K84 has been used commercially for more than a decade in many regions of the world, including Australia, Greece, Israel, Italy, Japan, New Zealand, South Africa, Spain, and the United States. K84 can be applied to wounds on cuttings, bare-rooted seedlings, grafts, and on field-grown plants. K84 protects wounds from infection by A. tumefaciens in part because of the production of agrocin 84, an antibiotic with specific toxicity against sensitive strains of A. tumefaciens.

Genes determining the production of agrocin 84 and immunity of the host bacterium to agrocin 84 are present on pAgK84, an indigenous, conjugative plasmid of strain K84. If plasmid pAgK84 is transferred to A. tumefaciens through the natural process of bacterial conjugation, the pathogen becomes immune to agrocin 84 and less sensitive to biological control by strain K84. In response to concerns that the predominance of A. tumefaciens harboring pAgK84 may reduce the efficacy of biological control, a derivative strain of K84, lacking a region (tra) required for conjugal transfer of pAgK84, has been constructed (Jones et al., 1988). This strain cannot transfer pAgK84 to other bacteria and its use is expected to minimize the risk that biological control will break down due to the presence of agrocin 84-resistant strains of A. tumefaciens in nursery soils. The strain containing the tra deletion is used commercially in Australia.

Gene flow has apparently occurred from cultivated rye (Secale cereale spp.) to wild relatives in California, where a weedy rye probably derived from a cross between S. cereale and S. montanum (wild relative) has become increasingly crop-like. This introgression has proceeded to such an extent that farmers are said to have abandoned efforts to grow cultivated rye for human consumption; instead they deliberately sow hybrids for forage (Jain, 1977; Suneson et al., 1969).

Although hybridization between a crop and its wild relative may not be preventable, there is little likelihood that desirable domesticated traits will be retained in the wild relative. Much of the emphasis in plant breeding has been on traits that would reduce adaptation to the wild. Important commercial traits, such as pest resistance, that have the potential to alter the ecology of wild relatives have not been a problem, with the possible exception of gene transfer from cultivated sorghum to johnsongrass (National Research Council, 1989a). In gen-



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