Following statements are made regarding abscisic acid (ABA) signalling during water stress in plants.
A. ABA causes membrane depolarization by decreasing cytosolic calcium levels.
B. ABA causes alkalinisation of the cytosol which stimulates opening of K⁺ efflux channels.
C. ABA inhibits the activity of the plasma membrane H⁺-ATPase which results in membrane depolarization.
D. During stomatal closure, ABA induces reorganization of tubulin cytoskeleton mediated by Rho GTPases.
E. ABA-induced membrane depolarization occurs by release of calcium from endoplasmic reticulum and vacuoles.
Which one of the following options is a combination of all correct statements?

Correct option is D

Correct Answer:
(d) C, D and E
Explanation:
ABA signalling during drought stress increases cytosolic Ca²⁺ levels, not decreases them, leading to membrane depolarization; hence statement A is incorrect.
ABA inhibits the plasma membrane H⁺-ATPase, reducing proton extrusion and causing depolarization (C).
ABA also triggers cytoskeletal reorganization via Rho-type GTPases during stomatal closure (D).
The rise in cytosolic Ca²⁺ primarily results from release from internal stores such as ER and vacuoles, contributing to depolarization (E).
Information Booster :
· ABA is a key drought-stress hormone controlling stomatal movement.
· Elevated cytosolic Ca²⁺ acts as a second messenger in guard cells.
· Membrane depolarization favors K⁺ efflux, reducing guard cell turgor.
· Cytoskeletal dynamics are crucial for stomatal aperture regulation.
Additional Information (Incorrect Options):
Statement A: Incorrect because ABA increases, not decreases, cytosolic Ca²⁺.
Statement B: Cytosolic alkalinisation is not the primary driver of K⁺ efflux during ABA-induced stomatal closure.
Therefore, options including A or B are incorrect.