46. Following are certain statements regarding root growth and differentiation in plants:
A. Root hair, endodermis, xylem and phloem reach maturation in elongation zone of a developing root.
B. The root epidermal cells that are incapable of forming root hairs are called
atrichoblasts.
C. Quiescent center is present just above root cap.
D. In Arabidopsis, an auxin transporter (ABCB4) plays a role in root hair emergence by maintaining intracellular auxin concentration.
Which one of the following combination of statements is correct?
(1) A, B and C                                                (2) B, C and D
(3) A, C and D                                                (4) A, B and D

 

Explanation:

• (A) Root hair, endodermis, xylem and phloem reach maturation in elongation zone of a developing root — Incorrect. These tissues mature in the zone of maturation, not the elongation zone, which primarily features rapid cell elongation but not full differentiation.

• (B) The root epidermal cells incapable of forming root hairs are called atrichoblasts — Correct. Root epidermis differentiates into trichoblasts (root hair forming) and atrichoblasts (non-root hair cells).

• (C) The quiescent center is present just above the root cap — Correct. The quiescent center is a group of slowly dividing cells that maintain stem cells in the root apical meristem, located proximal to the root cap.

• (D) In Arabidopsis, an auxin transporter (ABCB4) plays a role in root hair emergence by maintaining intracellular auxin concentration — Correct. ABCB4 is involved in auxin efflux and influx regulation, critical for auxin accumulation in root hair cells promoting their emergence.

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Introduction

Root development involves coordination of cell differentiation and growth across distinct zones. The zone of maturation is where tissues like root hairs, endodermis, xylem, and phloem fully develop. Root epidermis differentiates into trichoblasts and atrichoblasts, while the quiescent center regulates stem cell maintenance near the root cap. Auxin transport, mediated by proteins such as ABCB4, orchestrates root hair formation.

Tissue Maturation and Zones in Root Development

• The zone of elongation mainly entails cell elongation without full tissue differentiation.

• Tissue maturation—including root hairs, vascular tissues (xylem, phloem), and endodermis—occurs later in the zone of maturation.

Epidermal Cell Types: Atrichoblasts and Trichoblasts

• Root epidermal cells specialized as atrichoblasts do not develop root hairs, contrasting with trichoblasts which do.

• This differentiation is crucial for root surface specialization and efficient water/nutrient absorption.

Quiescent Center’s Role and Position

• The quiescent center is a cluster of slowly dividing cells situated just above the root cap protecting and regulating surrounding stem cells.

• It maintains root growth and meristem function through signaling and cell cycle modulation.

ABCB4-Mediated Auxin Transport in Root Hair Emergence

• ABCB4, an ATP-binding cassette transporter in Arabidopsis, regulates directional auxin transport in roots.

• Auxin accumulates in trichoblast cells promoting root hair initiation and elongation; ABCB4 helps maintain this critical auxin gradient.

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Summary Table: Root Growth and Differentiation Components

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Conclusion

Statements B, C, and D accurately describe crucial aspects of root growth and differentiation related to epidermal cell types, stem cell niche, and auxin transport mechanisms. Statement A is incorrect regarding the zone of tissue maturation.