11. During episodes of anoxia in plants, pyruvate produced in glycolysis is initially fermented to lactate. During later stage, there is an increase in the fermentation to ethanol and decrease in the fermentation to lactate, a phenomena which helps plants survive anoxia. Which of the following statements is correct about this change of fermentation flux from lactate towards ethanol?
(1) The cytosolic pH increases, thus activating both lactate dehydrogenase and pyruvate decarboxylase ativity
(2) cytosolic pH increases, thus inhibiting lactate The cytosolic pH increases, thus inhibiting lactate dehydrogenase activity and activating pyruvate decarboxylase activity.
(3) The cytosolic pH decreases, thus activating both lactate dehydrogenase and pyruvate decarboxylase activity.
(4) The cytosolic pH decreases, thus inhibiting lactate dehydrogenase and activating pyruvate decarboxylase activity

Introduction

During oxygen shortage (anoxia) in plants, pyruvate from glycolysis is first converted to lactate via lactate dehydrogenase. As anoxia progresses, plants shift fermentation towards ethanol production, a process tied to cytosolic pH changes that regulate enzyme activity and help manage cytoplasmic pH and energy status.

Explanation of Each Option

(1) Cytosolic pH increases, activating both lactate dehydrogenase and pyruvate decarboxylase

• Incorrect. pH generally decreases during anoxia; lactate dehydrogenase is inhibited, not activated.

(2) Cytosolic pH increases, inhibiting lactate dehydrogenase and activating pyruvate decarboxylase

• Incorrect. pH decrease, not increase, drives these enzyme activity changes.

(3) Cytosolic pH decreases, activating both lactate dehydrogenase and pyruvate decarboxylase

• Incorrect. Lactate dehydrogenase activity is inhibited at lowered pH to reduce lactate production.

(4) Cytosolic pH decreases, inhibiting lactate dehydrogenase and activating pyruvate decarboxylase

• Correct. Cytosolic acidification inhibits lactate dehydrogenase, reducing lactate buildup, and activates pyruvate decarboxylase, promoting ethanol fermentation, which helps maintain redox balance and cell survival.

Why Option (4) Is Correct

The shift in fermentation flux in plants during anoxia is a survival strategy. Lower cytosolic pH inhibits lactate dehydrogenase to prevent toxic lactate accumulation and stimulates pyruvate decarboxylase to enhance ethanol fermentation, which uses NADH and regenerates NAD⁺, supporting glycolysis continuation in low-oxygen conditions.

This knowledge is critical for understanding plant stress physiology and metabolic adaptation under hypoxic stress.