COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: PLANT PHYSIOLOGY – Sensory Photobiology of Plants.
Syllabus Outline
- Structure, function and mechanisms of action of phytochromes.
- Cryptochromes, Cryptochromes and Phototropins.
- Stomatal movement.
- Photoperiodism and biological clocks.
This quiz contains 25 concept-based, most frequently asked MCQs on “PLANT PHYSIOLOGY – Sensory Photobiology of Plants”. Each question has a single correct/most appropriate answer.
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1. Which structure in Arabidopsis roots acts as the main site of gravity perception (gravitropism)?
A) Root hairs
B) Epidermis
C) Elongation zone
D) Root cap columella cells
2. How does active phytochrome (Pfr) suppress etiolation in the nucleus?
A) Pfr binds directly to HY5, preventing its degradation by COP1, while simultaneously inducing PIF genes.
B) Pfr represses COP1 E3 ligase activity and induces the phosphorylation and degradation of PIF factors.
C) The 26S proteasome degrades Pfr, and degradation products inhibit COP1 nuclear import.
D) Pfr promotes PHYA conversion to inactive Pr, reducing dark-signalling repressors.
3. Which plant hormone primarily mediates the bending response in phototropism?
A) Cytokinin
B) Gibberellin
C) Auxin
D) Abscisic acid
4. How does increasing light intensity affect the Pr ↔ Pfr interconversion rate and the Pfr/Ptotal ratio at equilibrium under continuous red light?
A) Intensity increases the Pfr/Ptotal ratio but slows the interconversion rate.
B) Intensity accelerates the interconversion rate but does not substantially alter the Pfr/Ptotal ratio at equilibrium.
C) High intensity red light drives Pfr levels to near 100%.
D) The Pr ↔ Pfr interconversion rate and the Pfr/Ptotal ratio are determined by temperature.
5. The skippy mutant shows de-etiolation (short hypocotyls) even in the dark, but has normal phytochromes and cryptochromes. The most likely defective protein is:
A) HY5, a positive regulator of photomorphogenesis.
B) AHA1, the plasma membrane H+-ATPase.
C) LHY, a component of the circadian clock.
D) COP1, an E3 ubiquitin ligase.
6. Thigmomorphogenesis refers to:
A) Growth in response to touch
B) Movement of leaves in response to touch
C) Root bending due to gravity
D) Circadian leaf movement
7. In Mimosa pudica, the electrical signalling involved in rapid leaf movements is mainly due to:
A) Phytochrome activation
B) Water loss from pulvinar cells due to ion fluxes
C) Increased turgor in guard cells
D) Potential difference between plants and the touched object
8. During gravitropism, which signalling molecule acts downstream of statolith sedimentation to trigger asymmetric auxin distribution in the root tip?
A) Cytosolic calcium ions
B) Reactive oxygen species
C) Phytochrome-interacting factors
D) Gibberellin oxidases
9. Which of the following statements correctly describes the mechanism of UV-B perception by the UVR8 receptor?
A) UVR8 binds the flavin chromophore to absorb UV-B light.
B) UVR8 undergoes conformational change upon dissociation of tryptophan dimers.
C) UVR8 phosphorylates COP1 directly under UV-B exposure.
D) UVR8 binds cryptochrome to enhance blue-light signalling.
10. Canopy shade decreases the Red: Far Red ratio, favouring inactive Pr. Which molecular event leads directly to stem elongation?
A) Increased degradation of PIF factors.
B) Increased nuclear COP1 activity.
C) Increased stability of PIF factors.
D) Increased sequestration of Pr in the cytosol.
11. What specific biochemical change occurs upon blue light absorption by Phototropins to initiate signalling?
A) FAD oxidation and COP1 binding.
B) Autophosphorylation of Ser residues in the photoreceptor’s kinase activation loop.
C) Chromophore (FAD) dissociation.
D) Conversion of Pr to Pfr in the LOV domain.
12. Which pair of photoreceptors work antagonistically to regulate seed germination and flowering time?
A) Cryptochrome and phototropin
B) Phytochrome and phototropin
C) Phytochrome and UVR8
D) Phytochrome and cryptochrome
13. Which of the following processes involves plant perception of sound or vibration?
A) Thigmonasty
B) Sonotropism
C) Mechanosensing
D) Gravitropism
14. Which response is mediated by both PHOT1 and PHOT2, but shows higher light sensitivity via PHOT1?
A) Chloroplast avoidance response.
B) Suppression of LHY/CCA1 transcription.
C) Stomatal opening.
D) Inhibition of hypocotyl elongation.
15. How does photoactivated Cryptochrome 1 (CRY1) promote de-etiolation?
A) CRY1 is ubiquitinated by COP1, which activates downstream targets.
B) CRY1 is degraded, stabilising positive regulators like HY5.
C) CRY1 inhibit COP1 E3 ligase activity and prevents HY5 degradation.
D) CRY1 induces phosphorylation and nuclear export of COP1.
16. Cryptochromes share similarity with DNA photolyases. Which characteristic primarily distinguishes cryptochromes in Arabidopsis?
A) Photolyases require blue light; cryptochromes use UV-A light.
B) Cryptochromes regulate development; photolyases mediate DNA repair.
C) Cryptochromes contain FAD; photolyases use a pterin chromophore.
D) Cryptochromes are photo-stable.
17. Blue light initiates stomatal opening, but red light is also needed for optimal opening. What is the primary role of red light?
A) Convert Pr to Pfr, which directly phosphorylates PHOT1.
B) Provide ATP via guard cell photosynthesis to fuel the H+−ATPase.
C) Inhibit SLAC1 anion channel activity.
D) Directly activate PHOT2.
18. What is the immediate osmotic effect of activated H+−ATPase activity in guard cells?
A) Depolarisation, driving K+
B) Breakdown of triacylglycerols.
C) Increased efflux of anions.
D) Hyperpolarisation, driving K+
19. Abscisic Acid triggers stomatal closure. What is the primary downstream target responsible for Abscisic Acid-induced rapid depolarisation of the guard cell plasma membrane?
A) The H+−ATPase
B) Inward−rectifying K+ channels
C) S-type anion channels
D) PHOT1 receptors.
20. What is the correct sequence of molecular events leading from Abscisic Acid perception to guard cell closure?
A) Abscisic Acid perception → K+ influx → Hyperpolarisation → Turgor increase.
B) Abscisic Acid perception → SLAC1 activation → Anion efflux → Depolarization → K+
C) Abscisic Acid perception → AHA1 inhibition → K+ efflux →
D) Abscisic Acid perception → PHOT1 activation → 14-3-3 binding → Turgor loss.
21. Which statements about stomatal function are correct?
I – Blue light activates the PM H+−ATPase via PHOT1/PHOT2.
II – ABA inactivates S−type anion channels (SLAC1) via depolarisation
III – The net movement of K+ and malate regulates turgor.
IV – Inward-rectifying K+ channels are active when the membrane is depolarised.
A) I and III only
B) I, II and III only
C) I, III and IV only
D) I and III
22. Which statements regarding Cryptochromes (CRYs) and Phototropins (PHOTs) are generally true?
I – CRYs are mainly involved in rapid motor responses.
II – PHOTs redundantly regulate stomatal opening and leaf expansion.
III – CRY photoactivation often leads to photooligomerization, crucial for signalling.
IV – Both CRYs and PHOTs are FAD-binding protein kinases.
A) I, II and IV only
B) II and III only
C) I, III and IV only
D) I, II, III and IV
23. Which components are necessary for maintaining etiolation (skotomorphogenesis) in dark-grown seedlings?
I – Stable and active COP1 E3 ubiquitin ligase.
II – Stable, high-abundance PIF transcription factors.
III – High concentration of Pfr phytochrome.
IV – High activity of HY5 transcription factor.
A) I and IV only
B) I and II only
C) II and III only
D) I, II, and IV
24. Assertion (A): Amyloplasts in root cap columella cells act as statoliths for gravity perception.
Reason (R): Sedimentation of amyloplasts triggers redistribution of auxin transporters through Ca²-dependent signalling.
A) Both A and R are true, and R is the correct explanation of A.
B) Both A and R are true, but R is not the correct explanation of A.
C) A is true, but R is false.
D) Both A and R are false.
25. Assertion (A): Light-activated photomorphogenesis involves Pfr promoting PIF degradation and repressing COP1 E3 ligase activity.
Reason (R): Both PIF and COP1 repress photomorphogenesis; Pfr must neutralise both for efficient de-etiolation.
A) Both (A) and (R) are true, and (R) is the correct explanation for (A).
B) Both (A) and (R) are true, but (R) is NOT the correct explanation for (A).
C) (A) is true, but (R) is false.
D) (A) is false, but (R) is true.
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References
- Hopkins, W. G., & Hüner, N. P. A. (2008). Introduction to Plant Physiology, John Wiley & Sons. 4th Edition.
- Pandey, S. N., & Sinha, B. K. (2015). Plant Physiology, Vikas Publishing House. 4th Edition.
- Jain, V. K. (2018). Fundamentals of Plant Physiology, S. Chand Publishing. 20th Edition.
- Verma, Mohit & Verma, S. K. (2022). A Textbook of Plant Physiology, Biochemistry and Biotechnology, S. Chand Publishing. 13th Edition.
- Lincoln Taiz, Ian Max Møller, Angus Murphy, and Eduardo Zeiger (2022). Plant Physiology and Development, Oxford University Press, 7th Edition.
- Nelson, David L. & Cox, Michael M. (2021). Lehninger Principles of Biochemistry, W. H. Freeman, 8th Edition.
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