Daily RC Article 27

Phytopathogens


Paragraph 1

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.

Paragraph 2

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.

Paragraph 2

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.

Topic and Scope:

Agriculture; specifically, the effects of using genetically altered bacteria for specific agricultural purposes.

Purpose and Main Idea:

The author’s purpose is to describe a debate over the agricultural benefits of using genetically altered bacteria on crops. The main idea, not fully evident until the second and third para, is that some are encouraged by the research in this field, while others still remain sceptical.

Paragraph Structure:

Paragraph 1 essentially establishes the lingo for the passage. It begins with some brief background information (the impact of phytopathogens, crop rotation, and “suppressive soil” on agricultural yields), and then moves on to the topic at hand: Phytopathogens, or “bad” bacteria— “bad” because they’re harmful to crops—may be suppressed by putting “good” bacteria such as Pseudomonas fluorescens into the soil. The rest of the first paragraph simply provides some evidence in support of this point by citing increased crop yields.

In Paragraph 2 we learn that increased crop yields suggest that putting genetically altered “good” bacteria into the soil might be beneficial to agriculture in general. A few examples are given, and we find out that there are both proponents and opponents of such a course of action.

Paragraph 3 simply takes the debate a step further by giving us additional  information about the work being done by the proponents—those who want to use genetically altered bacteria on the soil.

The Big Picture:

  • Although some test-takers get flustered by natural science passages, they’re really not much different from social science or humanities passages. Sure, they often contain some technical words or phrases, but these will be defined and explained in the passage. As with all passages, nail down the author’s purpose, structure, and main idea, and you’ll be in a position to answer most if not all of the questions.
  • Never try to memorize details—many of them will not be tested. Instead, keep in mind the big idea, in this case that there is a debate over whether releasing genetically altered bacteria into the soil is a good idea. That such a debate is taking place is the message the author wants to convey.
  • Strive to understand the author’s place in the argument. This author’s tone, despite the seemingly rosy ending, can only be described as neutral and objective. He cites examples as well as the proponents’ case and the opponents’ reservations about bacterial use in agriculture. And in the end, it is “many”—not necessarily him, the author—who are encouraged by the prospects for the procedure. Detached, objective, informative; not at all unusual for a natural science passage.

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