Suppression of Crown Gall Disease by Rhizosphere Bacteria and Agrobacterium-Specific Bacteriophages

Strains of Agrobacterium tumefaciens induce crown gall tumors on many economically important dicotyledonous crops, while Agrobacterium vitis strains cause crown gall disease mainly in grape. The limited biocontrol of these pathogens, particularly A. vitis, emphasizes the need for alternative biocontrol agents. Here, we show three novel approaches to protect tomato (Solanum lycopersicum) plants against crown gall disease under greenhouse conditions. A broad-range antagonist, Serratia plymuthica IC1270, significantly suppressed the development of tumors on tomato seedlings when applied before injection of Agrobacterium strains into the wounded stems. The antagonist persisted in the stems of tomato seedlings for at least a month after application via root-soaking or wound-zone inoculation. Strain IC1270 produced volatile organic compounds that strongly inhibited the Agrobacterium strains' growth in vitro. The alkyl sulfide dimethyl disulfide, the major headspace volatile produced by strain IC1270, was found to be emitted from stem tissues of tomato plants treated with this bacterium and suppressed the growth of Agrobacterium in plate assays, suggesting involvement of this volatile compound in the biocontrol activity of strain IC1270 toward crown gall disease. A similar biocontrol effect was achieved with plant growth-promoting bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase which degrades ACC, the immediate precursor of ethylene in plants, and thereby lowers plant ethylene levels. Additionally, several lines of Agrobacterium phages showed an ability to protect tomato plants against crown gall tumors. Application of these phages is apparently not risky for beneficial rhizobial microflora.