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Q24 - Nitric Oxide and Guard Cell K+ Currents

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Stomatal Physiology & Gas Exchange

Electrophysiological measurements were carried out on bean guard cells to measure outward K+ currents and how they are modified by various factors. Assume the K+ currents shown are proportional to net changes in intracellular K+ concentration. Currents (I) across the plasma membrane at different voltages (V) were determined after each step of four different experiments:

Experiment 1 - Step 1: incubated in washing buffer. Step 2: exposed to exogenous nitric oxide (NO). Step 3: washed in washing buffer.

Experiment 2 - Step 1: injected with buffer to set cytoplasmic pH to 7.4. Step 2: exposed to exogenous NO. Step 3: washed in washing buffer.

Experiment 3 - Step 1: incubated in washing buffer. Step 2: exposed to exogenous NO. Step 3: exposed to BAL, a reducing agent known to reduce disulphide bridges in proteins. Step 4: exposed to exogenous NO again.

Experiment 4 - Step 1: incubated in washing buffer. Step 2: exposed to PAO, an oxidising agent that localises to membranes within the cell. Step 3: washed in washing buffer containing 0.3mM BAL.

Four current-voltage (I-V) curve panels, one per experiment, x-axis V (mV, -100 to 50), y-axis I (nA). Experiment 1: Step 1 and Step 3 curves both rise steeply above V=0 (Step 3 highest, up to ~1.4 nA); Step 2 (NO-exposed) stays nearly flat near zero throughout. Experiment 2: same pattern - Step 1/Step 3 rise steeply, Step 2 (NO, with pH clamped to 7.4) stays flat. Experiment 3: Step 1 rises steeply; Step 2 (NO only) stays flat/suppressed; Steps 3 and 4 (after BAL, even with repeated NO exposure) rise again nearly as high as Step 1 - the suppression is reversed. Experiment 4: Step 1 rises steeply; Step 3 (PAO then BAL) also rises to a similar high level; Step 2 (PAO alone) stays flat/suppressed. Fig.1.

Based on the information above, indicate with an X if each of the following statements is true (T) or false (F).

Q24.1. In Experiment 1 (washing buffer → NO exposure → wash), guard cells have higher turgor and a more open stoma after NO treatment compared to before.
Q24.2. Intracellular pH significantly influences the NO-mediated reduction of K+ efflux.
Q24.3. BAL (a reducing agent that breaks disulphide bridges) is a hydrophilic molecule.
Q24.4. According to these data alone, infection of a bean leaf with a pathogen results in stomatal closure.
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key