Plant Physiology (Bot 328), Final Exam NAME: KEY
Spring 1996
Please provide succinct answers in the space provided under each question. Unless otherwise noted in
the margin the value of each question is 2 points.
1. (a) In the equation growth rate = m(T - Y), define "m" and "Y".
Ans.: m=wall extensibility; Y= yield threshold.
(b) Name three proteins for which there are data to show that they affect "m".
Ans.: peroxidase; endo- and exo-glucanases; expansin
(c) For each protein named in (b) indicate how it alters "m".
Ans.: peroxidase: decreases m by cross-linking extensins; glucanases: increases m by
breaking wall glycosidic bonds.; expansin: increases m by breaking wall hydrogen bond
linkages.
4 pts. 2. (a) Typically when a wall protein is being considered as playing a major role in regulating
the growth rate of cells, the effect of pH on its activity is tested. In what way would
physiologists expect pH to alter the activity of a wall protein that promoted growth? Why?
Ans.: They would expect that the protein would have optimal activity at acid pH because
increased growth is usually accompanied by wall acidification.
(b) Cell X is an undifferentiated cell that is actively taking up salt. How would this activity
be expected to affect the growth of cell X? Explain your answer.
Ans.: Uptake of salt would lower the water potential of the cell and promote water uptake,
which would increase turgor and, all other things being equal, increased turgor would increase
growth.
4 pts. 3. (a) How could a yeast mutant help scientists understand sucrose uptake into plant better?
Your answer should specify what kind of yeast mutant would be helpful as well as how it could
be helpful.
Ans.: A yeast mutant defective in active sucrose uptake could be used to test whether a plant
gene that has been implicated as a sucrose transporter functions as a sucrose transporter by
complementing the defect in the mutant yeast.
(b) In Table 1 (p. 6) what is the expected basis for the inhibitory effects of 2,4 dinitrophenol
and N-ethylmaleimide?
Ans.: 2, 4 dinitrophenol is a protonophore. It would diffuse the proton gradient needed for
sucrose-proton co-transport, and thus inhibit sucrose uptake. Ethylmaleimide blocks certain
side chains on some transport proteins and thus blocks their transport activity.
3 pts. 4. (a) A cobweb better represents signalling pathways in plants and animals than a linear
array of sequential steps. Why?
Ans.: Because all signal transduction chains act in the context of other signalling pathways
that may use similar intermediates. Thus signal transduction chains for any one input signal
would be expected to intersect with and affect other signalling pathways, rather than function
in a linear, independent pathway.
3 pts. (b) To implicate step X in a signal transduction pathway initiated by sound, what three
general kinds of evidence would be helpful?
Ans.: (i) Find out if the level of X changes when the stimulus is given; (ii) Find out if blocking
or negating a change in X by the use of inhibitors also blocks the pathway initiated by the stimulus.
(iii) Find out if increasing X by the use of chemical agonists or antagonists can substitute for
the stimulus in inducing a typical physiological response.
3 pts. 5. (a) Nifedipine is effective in blocking the gravity response in Chara only if applied on
the ends of the Chara cell. Based on the inferences drawn from this result, why would you
predict that a phospholipase would disrupt a gravity response no matter whether it was
applied at the ends of the cell or in the middle?
Ans.: Nifedipine blocks calcium channels. Since this blocks the gravity responses if applied at
the cell ends, this implies that the gravity response requires regulated calcium transport at the
two ends of the cell. Lipases would destroy membrane integrity no matter where it was applied,
and without an integral membrane regulated transport of calcium at the two ends of the cell
would be impossible.
3 pts. (b) In the giant Chara cell, apparently no amyloplasts are needed to promote the gravity
response, but in normal higher plant cells, amyloplasts might significantly contribute to
how gravisensing works accommodate the relative participation of amyloplasts in different
sized cells?
Ans.: Wayne & Staves propose that the mass of the protoplasm serves as the gravity sensor.
The larger this mass the more sensitive the gravity response. Smaller cells may have too
small a mass to be very sensitive to gravity, but this sensitivity could be increased by adding
the "ballast" of dense amyloplasts.
3 pts 6. (a) In the germinating spore of the fern Ceratopteris, what experiment indicates that
gravity controls the direction of rhizoid emergence and growth?
Ans.: When the spores are rotated on a clinostat, both the gravity vector and the direction
of rhizoid emergence is randomized.
(b) In Ceratopteris the emerged rhizoid is itself not responsive to gravity. What prior
cellular event directed by gravity predicts the orientation of rhizoid emergence and growth?
Ans.: Nuclear migration.
7. (a) Comparing daylight to sunset to light under an ivy canopy, (i) which would have
the highest R:FR ratio & which would have the lowest? (ii) Which light would generate
the highest % of Pfr? (R=red light; FR=far-red lt.)
Ans.: (i) Daylight would have the highest R:FR ratio, ivy canopy light would have the lowest.
(ii) Daylight.
(b) Compare the three kinds of light noted in 7(a) for their effects on stem elongation in
shade-tolerant plants, assuming the quantity of each kind of light given was equivalent.
Ans.: None of them would have much of an effect on stem elongation in shade-tolerant plants.
(c) Compare the three kinds of light noted in 7(a) for their ability to initiate very low
fluence (VLF) phytochrome responses. Explain your answer.
Ans.: All three would initiate VLF responses, for even light enriched in FR sets up a
Pr-Pfr equilibrium that has enough Pfr to initiate these responses.
3 pts. 8. (a) Name three genes regulated by phytochrome and for each state its function.
Ans.: Genes for (1) Chl a/b protein: chloroplast protein that binds chl. (2) Small subunit
of RUBISCO: part of complex needed for CO2 fixation; (3) phytochrome: R photoreceptor
for photomorphogenesis.
3 pts. (b) If you wanted to put the expression of the gene for cytokinin synthase under
phytochrome control, what kind of genetic engineering transformation would you have
to do?
Ans.: Transform a plant with the orf for cytokinin synthase under the control of a light
regulatory element (promoter for phy gene).
9. (a) Regarding the data in Table 2 (p. 6), describe the most striking difference in the
appearance of a population with a high SD compared to a population with a low SD.
Ans.: Plants with a high SD would be growing in random directions; plants with a low SD
would be growing more uniformly upright.
(b) Do the results in Table 2 support the conclusion that PhyB alone controls hypocotyl
orientation in continuous R? Explain your answer.
Ans.: No, because plants that have inoperative PhyB gene still show light-regulated
hypocotyl orientation, as long as their PhyA gene is OK.
(c) Specify the strongest evidence in Table 2 that phytochrome controls the hypocotyl
orientation response in continuous R.
Ans.: When both PhyA and PhyB are inoperative the plants no longer show light-regulated
hypocotyl orientation.
10. (a) In the microinjection experiments of Neuhaus et al. hundreds of each kind of
microinjection were done, despite the tedium and difficulty of this technique. Were
these high number of repeats necessary? Explain.
Ans.: Most of the injections showed no response, because most of the microinjections
would be expected to damage the injected cell or to be delivered to a non-cytosolic site
(e.g., wall or vacuole), so to believe the low percentage that did show a response these
had to be repeated many times.
(b) The microinjection experiments implicated both G-proteins and Ca2+-activated
calmodulin as a mediator of at least some phy responses. Could the different actions
of G-proteins and Ca2+-activated calmodulin be causally linked in phy signalling? Explain
your answer.
Ans.: G-protein action could be linked to Ca-calmodulin action if G-protein led to increased
[Ca2+]cyt by way of activating a phospholipase C that would generate IP3 that would
bind to and open intracellular Ca2+ channels.
(c) What specific microinjection results suggested that Ca2+ was not a universal mediator
of all phy responses?
Ans.: Microinjection of Ca2+-activated calmodulin did not substitute for Pfr in inducing
anthocyanin synthesis.
11. (a) Specify the involvement of ATP and kinase action in the maintenance of the IP3
pathway of control of [Ca2+]cyt.
Ans.: The involvement of ATP and a kinase is necessary for the conversion of IP to PIP and
for the conversion of PIP to PIP2. Without PIP2, no IP3 could be made.
(b) What role does the plasma membrane play in the pathway leading to IP3 control of
Ca2+]cyt, and what is the molecular target of IP3 action in plant cells?
Ans.: The plasma membrane is where the PIP2 substrate is that is converted to IP3 by PLC.
The target of IP3 action in cells is a calcium channel on the ER, vacuolar, or nuclear
membrane that opens when it binds IP3.
(c) Fig. 1 shows that brief touch stimuli induce only a transient spike in the increase in
[Ca2+]cyt. Suggest two reasons why the increase in [Ca2+]cyt isn't sustained.
Ans.: (i) IP3 is very unstable and is quickly destroyed in cells; (2) Increase in [Ca2+]cyt
turns on CaM, which turns on pump that pumps Ca2+ out of the cell.
6 pts. 12. Fig. 2 shows the results of a tissue-printing experiment that assayed the distribution
of SAUR RNAs in relation to gravitropism. What is the most obvious inference drawn
from this result? Propose a plausible alternative hypothesis to explain the data and an
experiment that would help disprove one of the two alternatives.
Ans.: The gravitropic stimulus leads to an asymmetry of expression of IAA-regulated SAUR
genes, and thus probably also to an asymmetry of IAA.
Alternative hypothesis: Gravitropic stimulus leads to asymmetry of Ca2+ (or some other
regulatory factor) that (like IAA) also turns on the SAUR genes
Experiment: Assay whether Ca2+ (or other regulatory factor postulated) can turn on SAUR genes.
4 pts 13. (a) Recent in vitro experiments showed that GA induces an increase in [Ca2+]cyt in barley
aleurone cells, and that Ca2+ promotes secretion of "-amylase from aleurone cells. In vivo, what
would be the natural source of GA that would affect aleurone cells in barley and specify three
sequential downstream responses that would follow the secretion events initiated by GA action.
Ans.: Source = embryo; (1) "-amylase activity would increase in endosperm; (2) Starch in
endosperm would be converted to metabolizable sugars; (3) Increase in sugar nutrients would
promote embryo growth and seed germination.
3 pts. (b) Recent in vitro experiments show that auxin can also induce an increase in [Ca2+]cyt in
cells. Would you expect auxin also to induce "-amylase secretion in barley aleurone cells?
Explain.
Ans.: Not necessarily. There may not be IAA receptors in the aleurone cells whose activation is
linked to an increase in [Ca2+]cyt in cells.
3 pts. 14. (a) Agrobacterium tumefaciens induces tumors when it infects plant wounds. It is also used
as a vehicle for plant transformation, but in most of these uses it does not induce tumors.
Why not?
Ans.: Because tumor induction is due to genes in the Ti plasmid that turn on IAA and cytokinin
synthesis, and when the Ti plasmid is being used for most plant transformations, these genes are
engineered out of the plasmid.
3 pts. (b) Describe an experiment using A. tumefaciens in which the infection would result in rooty
tumors. Explain the relationship between your experimental design and the results obtained.
Ans.: Infect plant with A. tumefaciens carrying a Ti plasmid that is missing the genes for cytokinin
synthase, but still has the genes encoding enzymes in the biosynthetic pathway for auxin production.
This will result in a high ratio of IAA to cytokinin, which will induce a rooty tumor.
4 pts. 15. (a) When the insect Tyrannobugus rex attacks one of a large group of Hastalavista
babus plants in a field, the other plants begin making defense compounds that stunt T. rex's
growth. A scientist, S. Spielburg, uses two inhibitors, one of ACC synthase and one of
lipoxygenase, to test which of two alternative explanations of this defense response is more
likely to be correct. What are the two alternative hypotheses, and what is the rationale for
testing these particular two alternatives?
Ans.: Two alternatives: Is the defense response induced by the gas ethylene or by the gas
methyl jasmonate? Rationale: The defense response must be communicated by air, since the
plants are not physically connected to each other, and the two best known volatile hormones are
ethylene (whose synthesis requires the action of ACC synthase) and methyl jasmonate (whose
synthesis requires the action of a lipoxygenase).
(b) Which inhibitor is likely to block the defense response and why?
Ans.: Lipoxygenase inhibitor is likely to block the defense response, because such defense
responses have been shown to be induced by methyl jasmonate, but not by ethylene.
16. (a) SA is involved in SAR. What is SA and SAR and what is the experimental evidence
that SA is involved in SAR?
Ans.: SA = salicylic acid;SAR = systemic acquired resistance; SA is increased by the pathogen
infection; knocking out SA accumulation knocks out SAR response; treating leaf with SA can
induce resistance response.
4 pts. (b) In experiments designed to test the movement of the SAR signal in a competent plant,
two inoculations of viruses are used. Describe where each inoculation is given, roughly how
much time could elapse between inoculations, and the expected phenotypic response in each
case.
Ans.: Initial inoculation is given to one leaf on plant; a half-day or so later a second inoculation is
given to a another leaf on the same plant distant from the first leaf that was inoculated. In each
case both the first leaf inoculated and the second leaf inoculated show localized necrosis and
cell death near the inoculation sites, but the infection is contained typically within that leaf.
17. (a) Describe the phenotype of the det mutant in Arabidopsis when it is grown in darkness
for 10 days (i) on standard growth medium, and (ii) on standard growth medium supplemented
with brassinolide.
Ans.: (i) On standard growth medium, the det mutant has short hypocotyls, open first leaves and
a pale color; (ii) On a medium supplanted with brassinolide, the det mutant looks like a wild-type
etiolated seedling: tall and slender with unopened first leaves and a pale color.
4 pts. (b) In what gene is the det mutant found to be defective in most cases. How does this defect
relate to the answer given in 17 (b).
Ans.: The det mutant is defective in a reductase gene critical for the synthesis of brassinolide.
This fact, together with the fact that the det phenotype can be "corrected" by treatment with
brassinolide strongly suggests that brassinolide is a hormone that participates in the suppression
of genes needed to maintain the normal etiolated phenotype in wild-type plants, and that its
synthesis or function may be affected by the activation of phytochrome that converts the etiolated
phenotype to the de-etiolated phenotype.