Chapter 36

1. Fire produces ash, which enriches the soil with plant nutrients. A seed that germinates as a result of fire could have an advantage in such nutrient-rich soil. Such seeds might have a selective advantage in habitats such as forests, where periodic fires are natural events.

2. Both hormone receptors and photoreceptors are proteins that change when bound to a signal and set off a signal transduction response in a cell. Hormones are chemical in nature (such as small molecules, proteins, and steroids), whereas photoreceptors change when light binds to them.

3. Seeds would be treated with a mutagen and then planted. Those seeds that germinated immediately might have a mutation that affects dormancy. These mutant plants could be compared with wild-type plants to isolate the genes involved.

1. An inhibitor of mRNA translation would block the gibberellin-induced synthesis of hydrolases. A proteasome inhibitor would block the hydrolysis of the repressor that normally is broken down when gibberellin affects cells. So gibberellin signal transduction would be inhibited and there would be no increase in hydrolase synthesis. In the case of auxin and coleoptiles, without inhibitors, there would be cell expansion and coleoptile growth. These effects would not occur in the presence of mRNA translation and proteasome inhibitors.

2. Cells away from the lighted side receive more auxin. Cell elongation on the shaded side is accelerated with the input of auxin, which causes bending toward the light.

3. To test for the relationship between corn stunt spiroplasma infection and suspected inhibition of gibberellins, you could compare measurements of gibberellins in plants infected with the bacterium and in normal plants. A reduction of gibberellins in the spiroplasma-infected plants would support the hypothesis that the infection inhibits synthesis of gibberellins. Another approach would be to infect normal plants with the spiroplasma and then spray gibberellins on them. If this process reversed the stunt phenotype, it would be reasonable to conclude that the infection reduced synthesis of gibberellins.

1. Cytokinins stimulate axillary buds to grow into branches, which would make the plant more bushy. Auxins help maintain apical dominance, which prevents branching. Thus this plant most likely has a low ratio of auxin to cytokinin.

2. The charcoal in the bag absorbs ethylene gas, which is released by ripening fruits. The lack of ethylene prevents over-ripening and decay.

3. Both ethylene and brassinosteroids promote leaf senescence, so either or both of these hormones are most likely active. Cytokinins delay leaf senescence, so they are most likely inactive in this plant.

1. An action spectrum for the bending of coleoptiles toward light shows that blue light is most effective. Genetic screens for blue-light insensitivity have shown that there is a blue-light receptor present on normal plants that is not present in plants unable to respond to blue light.

2. Red light causes phytochrome to convert from the P_r to the P_fr form. The latter is active in promoting seed germination. Far-red light reduces P_fr by causing it to covert to P_r, which is inactive in promoting seed germination.

1. Light was shone from the right of the observer.

2. These data indicate that the tip is necessary for bending to light (if the tip was cut off, the plant did not bend). The fact that cutting the very apex (1.27 mm) of the tip allowed some bending indicates that there was not significant injury and the plant was still functional.

3. The data indicate that exposure of the tip to light is necessary for the bending response. The six coleoptiles that bent slightly even though they were covered possibly were ineffectually covered—that is, they might have had some light leakage, like that noted for the coleoptiles where the ink on the tubes was cracked.

1. 1. A high number means that more light energy at that wavelength is needed for the response (seed germination) than a low number.

   2. The highest efficiency was in the red part of the visible spectrum (600–680 nm).

      

2. 1. The photoreceptor has two forms that reversibly interconvert: red light stimulates the formation of the active receptor, and far-red light stimulates the conversion of the active receptor to its inactive form.

   2. A small amount of phytochrome was in the P_fr state in the dark.

1. Acting alone, phyC enhances leaf area, whereas phyA, phyB, and phyD do not.

2. Arabidopsis seeds would be treated with a mutagen and planted. Those plants with reduced leaf size would be isolated and grown to examine for phyC mutations.

3. The leaf area phenotypes produced by these three conditions are equal. All three groups have leaf areas about one-third smaller than that of the WT group. This suggests that (1) the phyA and phyC genes have the same effect on controlling leaf area (these results show the effect of the mutant, or nonfunctional gene, so the functional gene should increase leaf area by one-third); (2) either gene will work with equal effect in the absence of the other (shown by phyAD and phyCD); and (3) having both genes dysfunctional does not increase the effect (shown by phyACD).

4. A deficiency in phyB causes a very long leaf (a long petiole) and a very small leaf area (small leaf). This suggests that, if this gene were functional, it would control leaf growth by greatly inhibiting petiole length and greatly increasing leaf area. This would produce a leaf with a large surface area for photosynthesis, which could be highly adaptive. PhyA and phyC have the same effect on leaf area as phyB, but to a lesser extent. PhyA and phyC also inhibit petiole length, with phyA having the greater effect and phyC adding slightly to this effect. It is likely that, in plants in nature, the control of these two leaf features results from the ratio of the various phytochromes present.