Allelopathic bacteria and their impact on higher plants

The impact of allelopathic, nonpathogenic bacteria on plant growth in natur al and agricultural ecosystems is discussed. In some natural ecosystems, ev idence supports the view that in the vicinity of some allelopathically acti ve perennials (e.g., Adenostoma fasciculatum, California), in addition to a llelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annual s. In agricultural ecosystems allelopathic bacteria may evolve in areas whe re a single crop is grown successively, and the resulting yield decline can not be restored by application of minerals. Transfer of soils from areas wh ere crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibi lity of higher plants to deleterious rhizobacteria is often manifested in s andy or so-called skeletal soils. Evaluation of phytotoxic activity under c ontrolled conditions, as well as ways to apply allelopathic bacteria in the field, is approached. The allelopathic effect may occur directly through the release of alleloche micals by a bacterium that affects susceptible plant(s) or indirectly throu gh the suppression of an essential symbiont. The process is affected by nut ritional and other environmental conditions, some may control bacterial den sity and the rate of production of allelochemicals. Allelopathic nonpathogenic bacteria include a wide range of genera and secr ete a diverse group of plant growth-mediating allelochemicals. Although a l imited number of plant growth-promoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth-inhibiting allelochemicals have been isolated and characterized. Some species may prod uce more than one allelochemical; for example, three different phyotoxins, geldanamycin, nigericin, and hydanthocidin, were isolated from Streptomyces hygroscopicus. Efforts to introduce naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial h erbicides, phosphinothricin, a product of Streptomyces viridochromogenes, a nd bialaphos from S. hygroscopicus. Many species of allelopathic bacteria that affect growth of higher plants a re not plant specific, but some do exhibit specificity; for example, dicoty ledonous plants were more susceptible to Pseudomonas putida than were monoc otyledons. Differential susceptibility of higher plants to allelopathic bac teria was noted also in much lower taxonomical categories, at the subspecie s level, in different cultivars of wheat, or of lettuce. Therefore, when te st plants are employed to evaluate bacterial allelopathy, final evaluation must include those species that are assumed to be suppressed in nature. The release of allelochemicals from plant residues in plots of `continuous crop cultivation' or from allelopathic living plants may induce the develop ment of specific allelopathic bacteria. Both the rate by which a bacterium gains from its allelopathic activity through utilizing plant excretions, an d the reasons for the developing of allelopathic bacteria in such habitats, are important goals for further research.