Question

Which one of the following statements on Casparian strips is correct?

• A. Casparian strips are specific to vascular plants found in epidermal cells.
• B. Casparian strips are modifications mostly found in shoot tissue.
• C. Casparian strips act as a cellular barrier to allow selective nutrient uptake and exclusion of pathogens.
• D. Casparian strips are common in root endodermal cells of non-vascular plants.

Hint:

Think “{Root endodermis = apoplast checkpoint}.” Casparian strips force symplastic entry through transport proteins, ensuring selectivity and protecting the stele from unregulated solutes and pathogens.

Answer 1

The Correct Option is C

Step 1: What is a Casparian strip?
Casparian strips are narrow, belt-like bands impregnated with suberin (and often lignin) deposited in the radial and transverse walls of endodermal cells of roots in vascular plants (angiosperms and gymnosperms). These hydrophobic bands are formed early during endodermal differentiation (the “Casparian band” stage).
Step 2: Functional consequence (apoplast block ⇒ forced membrane crossing)
Water and solutes moving through the apoplast (cell wall continuum) cannot cross the Casparian strip because suberin is impermeable to ions and water. Therefore, solutions must enter the symplast through an endodermal plasma membrane, where selective transporters and channels regulate uptake into the stele. This:
Ensures selective nutrient uptake (e.g., K$^+$, NO$_3^-$, PO$_4^{3-}$) and prevents loss/backflow.
Acts as a defense barrier, restricting pathogens/toxins traveling apoplastically.
Creates a root pressure seal for xylem loading.
Mature endodermis may later develop suberin lamellae and even tertiary (U-shaped) lignified thickenings; passage cells remain less suberized opposite protoxylem poles for controlled entry.
Step 3: Evaluate each option carefully
(A) Incorrect. Right taxonomic scope (vascular plants) but wrong cell type: Casparian strips are not epidermal; they are in the root endodermis.
(B) Incorrect. Location is root, not shoot. (An exodermis with similar bands may occur in some roots, but not in shoots.)
(C) Correct. By blocking the apoplast and forcing membrane transit, Casparian strips provide a selective barrier, enabling nutrient selectivity and helping exclude pathogens/toxins.
(D) Incorrect. Non-vascular plants (e.g., bryophytes) lack true roots/endodermis; hence Casparian strips are not “common” there.
Therefore, the correct statement is (C).