Allelopathic Bacteria

The impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems has been discussed. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostomafasciculatum Hook. & Am. in California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems where a crop is grown successively, the resulting yield decline cannot always be restored by application of minerals, but yield can often be improved by soil disinfestation. Transfer of soils from the areas where crop suppression had been recorded into an unaffected area, induced crop retardation without readily apparent symptoms of plant disease. The allelopathic effect may occur directly, through release of allelochemicals by a bacterium that affects susceptible plant(s) or indirectly, through suppression of an essential symbiont. The process is affected by nutritional and other environmental conditions that control bacterial density and the rate of production of allelochemicals. Among these, water stress was suggested to govern the susceptibility of downy brome (Bromus tectorum L.) to deleterious rhizobacteria. Allelopathic nonpathogenic bacteria are found in a wide range of genera and secrete a diverse group of plant growth-mediating allelochemicals. Although a limited number of plant growthpromoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth inhibitors have been isolated and characterized. Efforts to use naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial herbicides, phosphinothricin, a product of Streptomyces viridochromogenes, and bialaphos from S. hygroscopicus. Although many species of allelopathic bacteria are not plant specific, some do exhibit specificity. For example, dicotyledonous plants were more susceptible to Pseudomonas putida than were monocotyledons. Differential susceptibility of higher plants was noted also in much lower taxonomical categories, at the sub-species level, such as in different cultivars of wheat or of lettuce. Therefore, when test plants are used to evaluate allelopathy, final evaluation must include those species that are suppressed in nature. Release of allelochemicals from plant residues in plots of continuous crop cultivation or from aromatic shrubs may induce development of specific allelopathic bacteria. Both the rate by which a bacterium gains from its allelopathic activity through utilizing plant excretions, and the reasons for developing allelopathic bacteria where the same crop is grown repeatedly on the same soil, are important goals for further research.