During water stress, ABA increases dramatically in leaves causing stomatal closure. Given below are the various events involved in this process:

A. Opening of plasma membrane Ca²⁺ permeable ion channels and elevation of cytosolic Ca²⁺.
B. Activation of plasma membrane anion channels, efflux of anions and potassium ions.
C. Binding of ABA to cytosolic ABA receptor and inhibition of activity of Type 2C protein phosphatases (PP2Cs).
D. Phosphorylation and activation of NADPH oxidases (RBOH) and formation of apoplastic ROS.

Which one of the following options represents the correct sequence of events involved?

Correct option is D

Correct Answer: (4) C, D, A, B

Explanation – ABA Signaling and Stomatal Closure under Water Stress:

Abscisic acid (ABA) is a plant hormone that plays a key role in stomatal closure during water stress to reduce transpiration. The process follows a well-characterized signaling cascade.

Correct Sequence of Events:

C. Binding of ABA to cytosolic ABA receptor and inhibition of Type 2C protein phosphatases (PP2Cs)

First Step

• ABA is perceived by PYR/PYL/RCAR receptors in the cytosol.

• These receptors inhibit PP2Cs, which are negative regulators of ABA signaling.

D. Phosphorylation and activation of NADPH oxidases (RBOH) and formation of ROS

Second Step

• Inhibition of PP2Cs leads to activation of SnRK2 kinases.

• SnRK2 phosphorylates RBOH, leading to ROS generation, which acts as a secondary messenger.

A. Opening of Ca²⁺-permeable ion channels and elevation of cytosolic Ca²⁺

Third Step

• ROS promotes Ca²⁺ influx into guard cells.

• This elevates cytosolic Ca²⁺, a key trigger for further downstream responses.

B. Activation of anion channels, efflux of anions and K⁺, leading to loss of turgor and stomatal closure

Final Step

• Elevated Ca²⁺ levels activate anion and K⁺ efflux channels.

• Ion efflux → loss of guard cell turgor → stomatal closure