Cultivation of a single crop on a given tract of land leads eventually to decreased yields. One reason for this is that harmful bacterial phytopathogens, organisms parasitic on plant hosts, increase in the soil surrounding plant roots. The problem can be cured by crop rotation, denying the pathogens a suitable host for a period of time. However, even if crops are not rotated, the severity of diseases brought on by such phytopathogens often decreases after a number of years as the microbial population of the soil changes and the soil becomes “suppressive” to those diseases. While there may be many reasons for this phenomenon, it is clear that levels of certain bacteria, such as Pseudomonas fluorescens, a bacterium antagonistic to a number of harmful phytopathogens, are greater in suppressive than in nonsuppressive soil. This suggests that the presence of such bacteria suppresses phytopathogens. There is now considerable experimental support for this view. Wheat yield increases of 27 percent have been obtained in field trials by treatment of wheat seeds with fluorescent pseudomonads. Similar treatment of sugar beets, cotton, and potatoes has had similar results.
These improvements in crop yields through the application of Pseudomonas fluorescens suggest that agriculture could benefit from the use of bacteria genetically altered for specific purposes. 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. Some experiments suggest that deliberately releasing altered nonpathogenic Pseudomonas syringae could crowd out the nonaltered variety that causes frost damage. Opponents of such research have objected that the deliberate and large-scale release of genetically altered bacteria might have deleterious results. 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.
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. For example, genes responsible for production of insecticidal compounds have been transposed from other bacteria into pseudomonads that colonize corn roots. Experiments of this kind are difficult and require great care: such bacteria are developed in highly artificial environments and may not compete well with natural soil bacteria. Nevertheless, proponents contend that the prospects for improved agriculture through such methods seem excellent. These prospects lead many to hope that current efforts to assess the risks of deliberate release of altered microorganisms will successfully answer the concerns of opponents and create a climate in which such research can go forward without undue impediment.
15. Which one of the following best summarizes the main idea of the passage?
(A) Recent field experiments with genetically altered Pseudomonas bacteria have shown that releasing genetically altered bacteria into the environment would not involve any significant danger.
(B) Encouraged by current research, advocates of agricultural use of genetically altered bacteria are optimistic that such use will eventually result in improved agriculture, though opponents remain wary.
(C) Current research indicates that adding genetically altered Pseudomonas syringae bacteria to the soil surrounding crop plant roots will have many beneficial effects, such as the prevention of frost damage in certain crops.
(D) Genetic alteration of a number of harmful phytopathogens has been advocated by many researchers who contend that these techniques will eventually replace such outdated methods as crop rotation.
(E) Genetic alteration of bacteria has been successful in highly artificial laboratory conditions, but opponents of such research have argued that these techniques are unlikely to produce organisms that are able to survive in natural environments.
16. The author discusses naturally occurring Pseudomonas fluorescens bacteria in the first paragraph primarily in order to do which one of the following?
(A) prove that increases in the level of such bacteria in the soil are the sole cause of soil suppressivity
(B) explain why yields increased after wheat fields were sprayed with altered Pseudomonas fluorescens bacteria
(C) detail the chemical processes that such bacteria use to suppress organisms parasitic to crop plants, such as wheat, sugar beets, and potatoes
(D) provide background information to support the argument that research into the agricultural use of genetically altered bacteria would be fruitful
(E) argue that crop rotation is unnecessary, since diseases brought on by phytopathogens diminish in severity and eventually disappear on their own
17. It can be inferred from the author’s discussion of Pseudomonas fluorescens bacteria that which one of the following would be true of crops impervious to parasitical organisms?
(A) Pseudomonas fluorescens bacteria would be absent from the soil surrounding their roots.
(B) They would crowd out and eventually exclude other crop plants if their growth were not carefully regulated.
(C) Their yield would not be likely to be improved by adding Pseudomonas fluorescens bacteria to the soil.
(D) They would mature more quickly than crop plants that were susceptible to parasitical organisms.
(E) Levels of phytopathogenic bacteria in the soil surrounding their roots would be higher compared with other crop plants.
18. It can be inferred from the passage that crop rotation can increase yields in part because
(A) moving crop plants around makes them hardier and more resistant to disease
(B) the number of Pseudomonas fluorescens bacteria in the soil usually increases when crops are rotated
(C) the roots of many crop plants produce compounds that are antagonistic to phytopathogens harmful to other crop plants
(D) the presence of phytopathogenic bacteria is responsible for the majority of plant diseases
(E) phytopathogens typically attack some plant species but find other species to be unsuitable hosts
19. According to the passage, proponents of the use of genetically altered bacteria in agriculture argue that which one of the following is true of the altered bacteria used in the frost-damage experiments?
(A) The altered bacteria had a genetic constitution differing from that of the normal strain only in that the altered variety had one less gene.
(B) Although the altered bacteria competed effectively with the nonaltered strain in the laboratory, they were not as viable in natural environments.
(C) The altered bacteria were much safer and more effective than the naturally occurring Pseudomonas fluorescens bacteria used in earlier experiments.
(D) The altered bacteria were antagonistic to several types of naturally occurring phytopathogens in the soil surrounding the roots of frost-damaged crops.
(E) The altered bacteria were released into the environment in numbers sufficient to guarantee the validity of experimental results.
20. Which one of the following, if true, would most seriously weaken the proponents’ argument regarding the safety of using altered Pseudomonas syringae bacteria to control frost damage?
(A) Pseudomonas syringae bacteria are primitive and have a simple genetic constitution.
(B) The altered bacteria are derived from a strain that is parasitic to plants and can cause damage to crops.
(C) Current genetic-engineering techniques permit the large-scale commercial production of such bacteria.
(D) Often genes whose presence is responsible for one harmful characteristic must be present in order to prevent other harmful characteristics.
(E) The frost-damage experiments with Pseudomonas syringae bacteria indicate that the altered variety would only replace the normal strain if released in sufficient numbers.