For example, genes responsible for production of insecticidal compounds have been transposed from other bacteria into pseudomonads that colonize corn roots.
Some proponents have gone further and suggest that genetic alteration techniques could create organisms with totally new combinations of desirable traits not found in nature. Proponents, on the other hand, argue that this particular strain is altered only by the removal of the gene responsible for the strain’s propensity to cause frost damage, thereby rendering it safer than the phytopathogen from which it was derived. Opponents of such research have objected that the deliberate and large-scale release of genetically altered bacteria might have deleterious results. Some experiments suggest that deliberately releasing altered nonpathogenic Pseudomonas syringae could crowd out the nonaltered variety that causes frost damage. For example, a form of phytopathogen altered to remove its harmful properties could be released into the environment in quantities favorable to its competing with and eventually excluding the harmful normal strain.
These improvements in crop yields through the application of Pseudomonas fl uorescens suggest that agriculture could benefi t from the use of bacteria genetically altered for specifi c purposes.
Similar treatment of sugar beets, cotton, and potatoes has had similar results. Wheat yield increases of 27 percent have been obtained in fi eld trials by treatment of wheat seeds with fl uorescent pseudomonads. There is now considerable experimental support for this view. This suggests that the presence of such bacteria suppresses phytopathogens.
While there may be many reasons for this phenomenon, it is clear that levels of certain bacteria, such as Pseudomonas fl uorescens, a bacterium antagonistic to a number of harmful phytopathogens, are greater in suppressive than in nonsuppressive soil. However, even if crops are not rotated, the severity of diseases brought on by such phytopathogens often decreases after a number or years as the microbial population of the soil changes and the soil becomes “suppressive” to those diseases. The problem can be cured by crop rotation, denying the pathogens a suitable host for period of time. One reason for this is that harmful bacterial phytopathogens, organisms parasitic on plant hosts, increase in the soil surrounding plant roots. Cultivation of a single crop on a given tract of land leads eventually to decreased yields.
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