GRE Subject Test: Biology : Understanding Plant Hormones

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Plant Hormones

Indole-3-acetic acid (IAA) is the most abundant hormone in plants. IAA falls under which classification of plant hormone?

Possible Answers:

Gibberellin

Abscisic acid

Auxin

Cytokinins

Ethylene

Correct answer:

Auxin

Explanation:

The correct answer is auxin. IAA is the most abundant auxin naturally found in plants, and plays key roles in plant development and growth. 

Example Question #2 : Understanding Plant Hormones

Perishable fruit is often picked before it is ripe. This hard, unripe fruit is easier to transport as it is less likely to bruise or spoil while in transit. Once fruit reaches its destination, it can be sprayed with a plant hormone that will promote its rapid ripening. What hormone is most likely used for this function?

Possible Answers:

Gibberellins

Ethylene

Abscisic acid (ABA)

Cytokinins

Correct answer:

Ethylene

Explanation:

Ethylene gas promotes fruit ripening; in a classic example of a positive feedback loop, ripe fruit causes the production of more ethylene gas, which promotes more fruit ripening (this is the origin of the phrase "one bad apple spoils the bunch!")

Cytokinins stimulate cell division and cytokinesis, and they can delay senescence (deterioration due to age, as in the case of cells). Gibberellins promote cell growth, and also promote stem and leaf elongation. Abscisic acid (ABA) promotes seed dormancy, inhibits plant growth, and closes plant stomata during water shortages. 

Example Question #3 : Understanding Plant Hormones

Which of the following is not true of auxins, a class of plant hormones?

Possible Answers:

They enhance apical dominance, which promotes plants' upward growth, as opposed to lateral growth

Indoleacetic acid (IAA) is a natural auxin

They promote fruit ripening, which triggers the production of more auxins

They are responsible for phototropism—plants' tendency to grow toward light sources

Correct answer:

They promote fruit ripening, which triggers the production of more auxins

Explanation:

Auxins play a number of crucial roles in plant growth, behavior, and development. They were the first major class of plant hormone to be discovered and are present in all parts of a plant in varying concentrations. Some of their more notable functions include causing phototropism, enhancing apical dominance, and softening the cell wall to stimulate cell elongation and growth. Indoleacetic acid (IAA) is a naturally occurring auxin, though some synthetic auxins are used as weed killers.

Ethylene gas is a plant hormone that promotes fruit ripening in a positive feedback loop. Ripe fruit produces more ethylene gas, which promotes more fruit ripening, and so on. It is not a kind of auxin.

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