Question

Rotenone is a chemical often used to kill insect pests on crop plants and fishes in lakes. Rotenone acts by inhibiting electron transport from the NADH dehydrogenase enzyme in Complex I to ubiquinone in the mitochondrial electron transport chain. Which one of the following explains why plants can tolerate rotenone application?

• A. The Complex I in plants is resistant to rotenone.
• B. Plants inactivate rotenone by enzymatic degradation.
• C. Plants have specific channels that efflux rotenone out of the cell.
• D. Plants have additional NAD(P)H dehydrogenases that are resistant to rotenone.

Hint:

Remember the plant mitochondrial “{bypass toolkit}”: Type II NAD(P)H dehydrogenases (external & internal) feed electrons to ubiquinone without proton pumping, and AOX can further bypass Complex III–IV. These features let plants keep respiration going when Complex I is inhibited.

Answer 1

The Correct Option is D

Step 1: What rotenone does
Rotenone binds to Complex I (NADH:ubiquinone oxidoreductase) at the ubiquinone-binding site. This blocks electron transfer from FMN/Fe–S centers to ubiquinone (Q). Consequences:
Electron flow from NADH is halted at Complex I.
Complex I cannot pump protons from matrix to intermembrane space.
Mitochondrial $\Delta p$ (proton motive force) and ATP synthesis fall sharply.
Step 2: Why plants cope better than animals
Plant mitochondria possess a suite of rotenone-insensitive Type II NAD(P)H dehydrogenases embedded in the inner mitochondrial membrane:
External-facing enzymes (e.g., NDB family) oxidize cytosolic NAD(P)H and pass electrons directly to ubiquinone.
Internal-facing enzymes (e.g., NDA/NDC) oxidize matrix NAD(P)H and also reduce ubiquinone.
These enzymes bypass Complex I (they do not pump protons), maintaining electron flow to Q and onward to Complex III/IV (often assisted by alternative oxidase, AOX, to relieve over-reduction and limit ROS). Energy yield is lower (no pumping), but respiration and redox balance are sustained, conferring tolerance to rotenone.
Step 3: Eliminate incorrect options
(A) Incorrect. Plant Complex I is not intrinsically rotenone-resistant; it is inhibited similarly to animal Complex I.
(B) Unlikely/Incorrect as primary explanation. While plants can metabolize some xenobiotics, rotenone tolerance in vivo is explained chiefly by ETC bypass, not specific detoxification.
(C) Incorrect. No specific rotenone efflux channels are known to account for whole-plant tolerance.
(D) Correct. Presence of additional rotenone-insensitive NAD(P)H dehydrogenases that feed electrons to Q provides a physiological bypass of Complex I.
Therefore, the correct explanation is (D).