CSIR NET Life Sciences System Physiology - Plant Flashcards

24 sample cards from the System Physiology - Plant deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Outline the ionic mechanism of light-induced stomatal opening.

   Blue light activates the guard-cell H+-ATPase, hyperpolarizing the membrane; this drives K+ uptake through inward channels, accumulation of Cl- and malate, lowering water potential so water enters, increasing turgor and opening the pore.

2. Define short-day and long-day plants in terms of the critical night length.

   Short-day (long-night) plants flower when the night exceeds a critical length; long-day (short-night) plants flower when the night is shorter than a critical length. The dark period length is the decisive factor.

3. What is florigen, and which gene encodes it?

   Florigen is the mobile flowering signal produced in leaves and transported through the phloem to the shoot apex; it is the protein product of the FT (FLOWERING LOCUS T) gene.

4. What molecular components make up the plant circadian clock's core feedback loop?

   A transcription-translation feedback loop: morning genes CCA1 and LHY repress the evening gene TOC1, while TOC1 (and the PRR proteins) feed back to regulate CCA1/LHY, producing ~24 h oscillations.

5. Distinguish apoplastic, symplastic, and transmembrane pathways of water/solute movement across roots.

   Apoplastic: through cell walls and intercellular spaces (blocked at the Casparian strip). Symplastic: cell-to-cell through plasmodesmata. Transmembrane: repeatedly crossing plasma membranes. Water must enter symplast at the endodermis.

6. State the cohesion-tension theory of water transport (transpiration pull).

   Transpiration from leaves lowers water potential, pulling water up the xylem; continuous water columns are held together by cohesion (H-bonding) and adhesion to xylem walls, creating tension that draws water from roots to leaves.

7. What is the proton-coupled mechanism of phloem loading (apoplastic loading)?

   Companion cells use a plasma membrane H+-ATPase to pump protons out; sucrose is then co-transported into the sieve element–companion cell complex against its gradient via sucrose-H+ symporters (SUC/SUT), driving pressure-flow.

8. Explain Munch's pressure-flow hypothesis of phloem transport, including unloading.

   At sources, sugar loading raises osmotic pressure drawing in water (high turgor); at sinks, unloading lowers turgor; the resulting pressure gradient drives bulk flow of phloem sap from source to sink, where assimilates are unloaded (symplastically or apoplastically).

9. From which precursor are all terpenes built, and what are the two biosynthetic routes in plants?

   All terpenes derive from the C5 isoprene units IPP and DMAPP. Plants make these via the cytosolic mevalonate (MVA) pathway and the plastidic MEP (non-mevalonate/DOXP) pathway.

10. Classify terpenes by isoprene (C5) unit number with one example each.

    Monoterpenes (C10, e.g., menthol), sesquiterpenes (C15, e.g., farnesol), diterpenes (C20, e.g., gibberellins), triterpenes (C30, e.g., sterols), tetraterpenes (C40, e.g., carotenoids).

11. What is the general biosynthetic origin of plant phenolic compounds?

    Most phenolics arise from the phenylpropanoid pathway, beginning with phenylalanine deaminated by phenylalanine ammonia-lyase (PAL) to cinnamic acid; this leads to flavonoids, lignin, tannins, and coumarins.

12. Give the major classes of nitrogen-containing secondary metabolites with examples.

    Alkaloids (e.g., nicotine, morphine, caffeine), cyanogenic glycosides (release HCN), glucosinolates (mustard oils), and non-protein amino acids; many derive from amino acids and serve in defense.

13. List three ecological roles of plant secondary metabolites.

    Defense against herbivores and pathogens (toxins, deterrents), protection from UV/abiotic stress (flavonoids), and attraction of pollinators/seed dispersers (pigments and scents); some are allelopathic.

14. What is systemic acquired resistance (SAR) and its key signaling molecule?

    SAR is a broad, long-lasting whole-plant resistance induced after a local infection; salicylic acid is the key signal, leading to expression of PR (pathogenesis-related) proteins.

15. Differentiate PTI and ETI in plant immunity.

    PTI (PAMP-triggered immunity) is the first layer, where surface receptors (PRRs) recognize conserved microbial patterns (PAMPs). ETI (effector-triggered immunity) is a stronger response where intracellular R proteins recognize pathogen effectors, often causing a hypersensitive response.

16. What is the hypersensitive response (HR) in plant defense?

    HR is rapid, localized programmed cell death at the infection site that restricts pathogen spread (especially biotrophs), accompanied by ROS bursts and reinforcement of cell walls.

17. Which hormone signaling pathways defend against chewing insects versus biotrophic pathogens?

    Jasmonic acid (JA) signaling defends mainly against chewing insects and necrotrophic pathogens; salicylic acid (SA) signaling defends against biotrophic pathogens. The two pathways are often mutually antagonistic.

18. What are compatible solutes (osmolytes) and give three examples used in osmotic/salt stress.

    Compatible solutes are non-toxic organic molecules accumulated to lower osmotic potential and protect macromolecules without disrupting metabolism. Examples: proline, glycine betaine, and sugars/sugar alcohols (e.g., trehalose, mannitol).

19. What is the SOS pathway role in salt tolerance?

    The Salt Overly Sensitive (SOS) pathway: salt raises cytosolic Ca2+; SOS3 senses Ca2+ and activates the SOS2 kinase, which activates the SOS1 plasma membrane Na+/H+ antiporter to extrude Na+, maintaining Na+/K+ homeostasis.

20. How do plants protect against high-temperature (heat) stress at the molecular level?

    Heat stress triggers heat shock factors (HSFs) that induce heat shock proteins (HSPs/chaperones) which prevent protein aggregation and refold denatured proteins; plants also adjust membrane lipid saturation.

21. What are LEA proteins and when are they expressed?

    Late Embryogenesis Abundant (LEA) proteins are hydrophilic protective proteins induced by ABA during seed desiccation and under water/cold/salt stress; they stabilize membranes and proteins during dehydration.

22. What protective mechanisms do plants use against cold/freezing stress?

    Cold acclimation via the CBF/DREB transcription factors induces COR (cold-regulated) genes, accumulation of compatible solutes and antifreeze proteins, increased membrane lipid unsaturation, and dehydrins to limit ice damage.

23. Compare PEP carboxylase and Rubisco as CO2-fixing enzymes.

    PEP carboxylase fixes HCO3- (no oxygenase activity, high affinity, no photorespiration) onto PEP. Rubisco fixes CO2 onto RuBP but also reacts with O2 (oxygenase activity causing photorespiration) and has lower CO2 affinity.

24. What are the three biochemical subtypes of C4 plants based on the decarboxylating enzyme?

    NADP-malic enzyme (NADP-ME) type, NAD-malic enzyme (NAD-ME) type, and PEP carboxykinase (PEPCK) type, differing in the enzyme that releases CO2 in the bundle sheath.

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