PLANT BIOLOGY 112 Spring 2019
Answers to Homework Questions #3 Page 1
Answers to Homework Questions #3
1. State whether the following genes are positive or negative regulators of
photomorphogenesis and explain why. Predicts the phenotype of plants that
overexpress each of these genes when they are grown in the light and in the dark. PHY,
COP1, HY5
a. Positive and negative regulators.
i. Negative regulator - COP1 because recessive mutations in this gene
cause photomorphogenesis in the dark. Therefore, the wild type gene
must repress photomorphogenesis.
ii. Positive regulators - PHY and HY5 because recessive mutations in these
genes cause loss of photomorphogenesis. Therefore, the wild type
genes must activate photomorphogenesis.
b. Overexpression of PHY
i. Light: Because there is an overabundance of phytochrome, the
transgenic plant is likely to be excessively photomorphogenic. Therefore,
it is likely to be shorter and darker green than nontransgenic plants.
ii. Dark: Phytochrome requires light to be activated. Therefore, in the dark,
a seedling overexpressing PHY is likely to look like a nontransgenic
seedling - skotomorphogenic.
c. Overexpression of COP1
i. Light: Two answers are possible. 1) COP1 is localized to the cytoplasm
in the light and inaccessible to HY5. Therefore, the plant is likely to look
photomorphogenic in the light. 2) Overexpression of COP1 causes the
transgenic plant to be partially skotomorphogenic in the light (this is an
actual experimental result). Presumably, some COP1 enters the nucleus
and causes partial degradation of HY5.
ii. Dark: COP1 should localize to the nucleus in the dark and cause the
degradation of HY5. The transgenic plant is likely to look
skotomorphogenic in the dark.
d. Overexpression of HY5
i. Light: Transgenic plants are likely to be excessively photomorphogenic as
with PHY overexpressing plants or they may display only limited aspects
of excessive photomorphogenesis. This is because other transcription
factors are likely to activate light regulated genes in the light signal
transduction pathway.
ii. Dark: Two answers are possible. 1) COP1 normally causes the
degradation of HY5 in the dark. Therefore, plants overexpressing HY5
may look like wild type plants in the dark. That is, they may be
skotomorphogenic in the dark. 2) If HY5 levels are so high that COP1 is
unable to process all of the protein, the transgenic plant may look partially
or completely photomorphogenic in the dark.
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, PLANT BIOLOGY 112 Spring 2019
Answers to Homework Questions #3 Page 2
2. You obtain two types of mutant seeds from another researcher. One mutant has
reduced gibberellic acid (GA) levels because it has a defective GA biosynthetic gene.
The other mutant has a defect in the GA signaling pathway and is insensitive to GA.
The other researcher is jealous of your success, so he did not label the two types of
mutant seeds. How can you distinguish the two type of mutants after they have
germinated and are growing on soil.
One way to distinguish between the two mutant plants is to apply GA to the
plants and observe the extent of stem elongation that occurs. The GA
biosynthesis mutant can still response to GA, so it should elongate in response to
GA treatment. The GA signaling mutant will be unable to respond to GA
treatment and stem elongation will not occur.
3. Strigolactone is a plant hormone that inhibits
branching. You have identified several four Plant Genotype Number of Branches
recessive mutations that appear to affect Wild Type 6
branching and/or strigolactone signaling. Which
of the mutations are positive regulators of bch1 1
branching? Which of the mutations are negative
bch2 15
regulators of branching? Which of the mutations
are positive regulators of strigolactone signaling? bch3 10
Which of the mutations are negative regulators of
strigolactone signaling? Describe the order in bch4 3
which the genes act in the pathway. bch2 bch3 10
bch2 bch1 1
a. Positive regulators of branching: bch1,
bch2 bch4 9
bch4. Negative regulators of branching:
bch2, bch3
b. Positive regulators of strigolactone signaling: bch2, bch3. Negative regulators of
strigolactone signaling: bch1, bch4
c. Gene order:
i. BCH3 BCH2 BCH1
(1) BCH3 is epistatic to BCH2, same signaling state, BCH3 is
upstream of BCH2
(2) BCH1 is epistatic to BCH2, opposite signaling state, BCH1 is
downstream of BCH2
ii. BCH4 does not display epistasis with BCH2 and must operate in a
different pathway
4. You are interested in studying the Abscisic Acid (ABA) signaling pathway. You know
that drought induces production of ABA which leads to closed stomata.
Drought º ABA º Closed stomata
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Answers to Homework Questions #3 Page 1
Answers to Homework Questions #3
1. State whether the following genes are positive or negative regulators of
photomorphogenesis and explain why. Predicts the phenotype of plants that
overexpress each of these genes when they are grown in the light and in the dark. PHY,
COP1, HY5
a. Positive and negative regulators.
i. Negative regulator - COP1 because recessive mutations in this gene
cause photomorphogenesis in the dark. Therefore, the wild type gene
must repress photomorphogenesis.
ii. Positive regulators - PHY and HY5 because recessive mutations in these
genes cause loss of photomorphogenesis. Therefore, the wild type
genes must activate photomorphogenesis.
b. Overexpression of PHY
i. Light: Because there is an overabundance of phytochrome, the
transgenic plant is likely to be excessively photomorphogenic. Therefore,
it is likely to be shorter and darker green than nontransgenic plants.
ii. Dark: Phytochrome requires light to be activated. Therefore, in the dark,
a seedling overexpressing PHY is likely to look like a nontransgenic
seedling - skotomorphogenic.
c. Overexpression of COP1
i. Light: Two answers are possible. 1) COP1 is localized to the cytoplasm
in the light and inaccessible to HY5. Therefore, the plant is likely to look
photomorphogenic in the light. 2) Overexpression of COP1 causes the
transgenic plant to be partially skotomorphogenic in the light (this is an
actual experimental result). Presumably, some COP1 enters the nucleus
and causes partial degradation of HY5.
ii. Dark: COP1 should localize to the nucleus in the dark and cause the
degradation of HY5. The transgenic plant is likely to look
skotomorphogenic in the dark.
d. Overexpression of HY5
i. Light: Transgenic plants are likely to be excessively photomorphogenic as
with PHY overexpressing plants or they may display only limited aspects
of excessive photomorphogenesis. This is because other transcription
factors are likely to activate light regulated genes in the light signal
transduction pathway.
ii. Dark: Two answers are possible. 1) COP1 normally causes the
degradation of HY5 in the dark. Therefore, plants overexpressing HY5
may look like wild type plants in the dark. That is, they may be
skotomorphogenic in the dark. 2) If HY5 levels are so high that COP1 is
unable to process all of the protein, the transgenic plant may look partially
or completely photomorphogenic in the dark.
This study source was downloaded by 100000870954916 from CourseHero.com on 02-20-2024 22:26:37 GMT -06:00
https://www.coursehero.com/file/42011438/9-Homework-3-Answers-Updated-052019pdf/
, PLANT BIOLOGY 112 Spring 2019
Answers to Homework Questions #3 Page 2
2. You obtain two types of mutant seeds from another researcher. One mutant has
reduced gibberellic acid (GA) levels because it has a defective GA biosynthetic gene.
The other mutant has a defect in the GA signaling pathway and is insensitive to GA.
The other researcher is jealous of your success, so he did not label the two types of
mutant seeds. How can you distinguish the two type of mutants after they have
germinated and are growing on soil.
One way to distinguish between the two mutant plants is to apply GA to the
plants and observe the extent of stem elongation that occurs. The GA
biosynthesis mutant can still response to GA, so it should elongate in response to
GA treatment. The GA signaling mutant will be unable to respond to GA
treatment and stem elongation will not occur.
3. Strigolactone is a plant hormone that inhibits
branching. You have identified several four Plant Genotype Number of Branches
recessive mutations that appear to affect Wild Type 6
branching and/or strigolactone signaling. Which
of the mutations are positive regulators of bch1 1
branching? Which of the mutations are negative
bch2 15
regulators of branching? Which of the mutations
are positive regulators of strigolactone signaling? bch3 10
Which of the mutations are negative regulators of
strigolactone signaling? Describe the order in bch4 3
which the genes act in the pathway. bch2 bch3 10
bch2 bch1 1
a. Positive regulators of branching: bch1,
bch2 bch4 9
bch4. Negative regulators of branching:
bch2, bch3
b. Positive regulators of strigolactone signaling: bch2, bch3. Negative regulators of
strigolactone signaling: bch1, bch4
c. Gene order:
i. BCH3 BCH2 BCH1
(1) BCH3 is epistatic to BCH2, same signaling state, BCH3 is
upstream of BCH2
(2) BCH1 is epistatic to BCH2, opposite signaling state, BCH1 is
downstream of BCH2
ii. BCH4 does not display epistasis with BCH2 and must operate in a
different pathway
4. You are interested in studying the Abscisic Acid (ABA) signaling pathway. You know
that drought induces production of ABA which leads to closed stomata.
Drought º ABA º Closed stomata
This study source was downloaded by 100000870954916 from CourseHero.com on 02-20-2024 22:26:37 GMT -06:00
https://www.coursehero.com/file/42011438/9-Homework-3-Answers-Updated-052019pdf/
