Bot 305; Test #2 Name: KEY

Summer 2000

Provide succinct answers in the space provided after each question. Unless otherwise noted each question is worth 3 points.

1. (a) On the whole -plant level, how is gravitropic curvature achieved?

Ans.: By differential growth of the two sides of the curving organ.

(b) Give one line of evidence that asymmetry of auxin may be involved in root gravitropism.

Ans.: Application of auxin to the bottom side of a root that cannot respond to gravity because it is missing its root cap, can induce that root to curve downward.

(c) In the gravitropic response of corn coleoptiles, calcium accumulates asymmetrically in the wall of cells preferentially on the upper side of this organ. How could this event influence gravitropism?

Ans.: Increased calcium in the wall will decrease the extensibility of that wall, thus the gravity-induced calcium asymmetry in coleoptiles could help mediate the gravity-induced growth asymmetry, which is characterized by decreased growth on the upper side.

2. (a) What are amyloplasts and what cells in the root cap have the most amyloplasts?

Ans.: Amyloplasts are plastids (membrane-bound organelles) containing starch. Columella cells in the root cap have the most amyloplasts.

(b) What experiment indicates that heavy particles play a role in the ability of crayfish to swim "right side up" with respect to gravity?

Ans.: If iron filings are used by crayfish to replace the heavy particles in their inner ear when they molt, then magnets can move filings and determine orientation of crayfish.

3. (a) What is the gravity response in Chara, and how is this response orientation dependent?

Ans.: When Chara cells are placed in a vertical orientation the rate of cytoplasmic flow downward is 10% faster than the rate of flow upward. When Chara cells are in a horizontal orientation the rate of cytoplasmic flow to the right and to the left is equal.

(b) Proteases inhibit the gravity response in Chara. State where exactly the proteases need to be applied to do this, and name one protein target of protease action that would be important for the gravity response mechanism.

Ans.: To be effective the proteases need to be applied in the upper and lower wall of a vertically oriented Chara cell. One protein target would be integrins, another would be the large wall proteins to which integrins bind.

(c) What is the molecular event that RGDS is inhibiting when it blocks the gravity response in Chara, and what component of the gravity-sensing mechanism would contain the RGD sequence?

Ans.: The molecular event is the binding of integrins to large proteins anchored in the wall matrix. The proteins to which integrins bind contain the RGD sequence.

4. (a) If you turn a Ceratopteris spore upside down 24 h after its germination was initiated by light, in which direction would it rhizoid emerge? What conclusion follows from this observation?

Ans.: Its rhizoid would emerge pointing upward. This observation indicates that the time during which gravity fixes the polarity of rhizoid emergence and growth must occur during the first 24 h after light initiates germination.

(b) What is the most rapid cellular response to a gravity stimulus so far detected in Ceratopteris?

Ans.: The most rapid response detected so far is the orientation of a calcium current which enters the bottom and exits the top of the cell.

(c) What is the experimental evidence that the gravity response noted in 4(b) is important for determining the subsequent developmental polarity regulated by gravity in Ceratopteris?

Ans.: Blocking the current with the calcium channel blocker nifedipine blocks the ability of gravity to direct the polarity of development in Ceratopteris.

5. (a) What kind of substance in a cell initiates all light responses in plants and animals?

Ans.: A pigment.

(b) An action spectrum for a response allows a scientist to predict what critical information about the substance named in 3(a)?

Ans.: The color or absorption spectrum of the pigment.

(c) What color of light would allow you to see objects most effectively? Explain why that color of light is so effective for vision.

Ans.: Green, because the pigment that initiates vision (rhodopsin) absorbs green light most effectively.

6. (a) Would red light effectively promote phototropism? Explain your answer.

Ans.: No, because phototropism is promoted by a pigment that absorbs blue light best, and phytochrome absorbs red light best.

(b) Two different genes that encode photoreceptor proteins have been shown to be involved in responses that are initiated by protein-flavin complexes. What are these genes and what growth response is initiated by the photoreceptor each one encodes?

Ans.: The genes are nph1, which encodes the photoreceptor protein for phototropism, and cry1, which encodes the photoreceptor for the blue light induced inhibition of hypocotyl elongation.

(c) In their search for the two different genes noted in 6(b), scientists were aided by a process called a mutant screen. How would scientists generate a population of mutants for such a screen? Illustrate what a scientist would be looking for in a mutant screen for one of the two genes noted in 6(b).

Ans.: One way scientists would generate a population of mutant seeds for such a screen would be to irradiate the seeds with gamma rays. In a mutant screen for the photoreceptor for phototropism, scientists would look for plants in a mutant population that would not curve toward unilateral blue light. In a mutant screen for the photoreceptor for blue-light inhibition of hypopcotyl growth, scientists would look for plants that grew the same length as dark grown plants when they were grown under blue light.

7. (a) What is meant by the statement that phytochrome responses are photoreversible?

Ans.: Red light will convert one form of phy (Pr) to another (Pfr), and far-red light will convert Pfr back to Pr, thus the two forms of phy are interconvertible by light irradiations.

(b) Far-red light can inhibit the germination of certain seeds. How does this response help insure the survival of the plants that arise from these seeds?

Ans.: Far-red light is the main part of sunlight that is transmitted through green leaves. Seeds getting FR are probably in a place crowded by other plants, so probably their survival chances would be low if they germinated in such a crowded environment.

(c) Why does the response noted above have to be photoreversible in order to have survival value in nature?

Ans.: In nature wind would occasionally blow aside a leaf above a seed, thus exposing the seed to unfiltered sunlight, which would initiate the germination sequence. If the return of shaded light (namely, FR light) when the wind subsided could not reverse this process, the seed would be irreversibly induced to germinate even though it would still be at a competitive disadvantage.

8. (a) Some seeds do not germinate well if they are under the canopy of other plants. What experiment shows that this response is due to light quality rather than light quantity under the canopy?

Ans.: Place a red filter over the leaf-shaded seeds. Although the filter reduces the amount of light reaching the seeds it increases the percentage of the light that is red, and the seeds germinate.

(b) For so-called "sun plants" changing the quality of light they receive changes the form of their growth. Explain what this photomorphogenesis response is and what is its survival value to the plant.

Ans.: The response is that in shaded light (i.e., when it is under the canpopy of other plant)s the plant allocates more of its resources to growing taller and less of its resources to putting out more leaves and branches. This helps the survival of the plant by helping it grow above the canopy of surrounding plants and thus compete more favorably for the light.

9 (a) Illustrate what is meant by a circadian rhythm by giving an example of one in humans.

Ans.: The blood pressure of humans varies rhythmically every day (24 h), peaking at the same hour each day and having a low level at the same hour each day.

(b) Illustrate what is meant by entrainment of a rhythm by light by contrasting a rhythm that is entrained by light with one that is not entrained.

Ans.: A typical circadian rhythm that is not entrained by light does not have a periodicity of exactly 24 h; usually it is a few h less than or more than 24 h. Those that are entrained have a periodicity closer to 24 h.

(c) Scientists recently concluded what was the identity of the principal gene that encodes the photoreceptor protein for the entrainment response. What is the gene, and what is the evidence for this conclusion?

Ans.: The gene is cry-1, and the evidence is that plants or animals that have a defective version of this gene do not have their circadian rhythms entrained by light.