21. Indole acetic acid (IAA) is involved in _____.

(A) gravitropism

(B) flowering

(D) senescence

Indole Acetic Acid (IAA) Function Explained | Role of Auxin in Gravitropism

Introduction

Indole-3-acetic acid (IAA) is the principal naturally occurring auxin in higher plants and is one of the most important plant growth regulators. Auxins regulate numerous physiological and developmental processes including cell elongation, apical dominance, vascular differentiation, root initiation, fruit development, and various tropic responses. The discovery of auxin was one of the major milestones in plant physiology because it explained how plants respond to environmental stimuli such as gravity and light.

Among the many functions of IAA, its role in gravitropism is particularly significant. Gravitropism is the directional growth response of plant organs to gravity. Auxin redistributes within plant tissues in response to gravitational stimulation, producing unequal rates of cell elongation on opposite sides of roots and shoots. This differential growth enables roots to grow downward into the soil while shoots grow upward toward the sky.

Correct Answer

Correct Option: (A) Gravitropism

Detailed Explanation

Indole-3-acetic acid (IAA) is the major natural auxin synthesized primarily in the shoot apical meristem, young leaves, and developing seeds. One of its most important physiological functions is regulating gravitropism, the directional growth of plants in response to gravity.

When a plant organ changes its orientation, specialized gravity-sensing cells called statocytes detect the change. Dense starch-containing organelles known as statoliths settle under the influence of gravity, triggering the redistribution of auxin to the lower side of the organ. In shoots, the higher concentration of auxin stimulates greater cell elongation on the lower side, causing the shoot to bend upward. In roots, however, higher auxin concentrations inhibit cell elongation, causing the root to bend downward. Thus, auxin controls both positive and negative gravitropism through differential growth.

Although auxin influences many developmental processes, it is not the principal hormone responsible for flowering, fruit ripening, or senescence. Flowering is mainly regulated by photoperiodic pathways involving florigen and gibberellins in many species. Fruit ripening is primarily controlled by ethylene, while senescence is promoted mainly by ethylene and abscisic acid. Therefore, the correct answer is gravitropism.

Explanation of Each Option

Option (A): Gravitropism

This option is correct. Auxin redistribution produces unequal cell elongation on opposite sides of roots and shoots, generating the characteristic bending response to gravity.

Option (B): Flowering

This option is incorrect. Although auxin may indirectly influence floral development, flowering is primarily regulated by photoperiod, florigen, and gibberellins rather than IAA.

Option (C): Ripening

This option is incorrect. Fruit ripening is mainly regulated by ethylene, particularly in climacteric fruits such as tomato, banana, apple, and mango.

Option (D): Senescence

This option is incorrect. Leaf and flower senescence are primarily promoted by ethylene and abscisic acid, whereas auxin generally delays the formation of the abscission layer.

Why Option (A) is Correct

IAA regulates the redistribution of growth between opposite sides of plant organs after gravity perception. This differential growth enables shoots to exhibit negative gravitropism and roots to exhibit positive gravitropism, making auxin the principal hormone involved in gravitropic responses.

Why the Other Options are Incorrect

Why Option (B) is Incorrect

Flowering depends mainly on environmental signals such as photoperiod and involves floral induction pathways rather than auxin as the primary regulator.

Why Option (C) is Incorrect

Ethylene is the principal hormone responsible for fruit ripening, not indole acetic acid.

Why Option (D) is Incorrect

Senescence is largely controlled by ethylene and abscisic acid. Auxin generally helps maintain tissue integrity and delays organ abscission.

Comparison of All Options

Option	Physiological Process	Major Hormone	Correct or Incorrect
A	Gravitropism	Auxin (IAA)	Correct
B	Flowering	Florigen and Gibberellins	Incorrect
C	Fruit Ripening	Ethylene	Incorrect
D	Senescence	Ethylene and ABA	Incorrect

Major Functions of Indole Acetic Acid (IAA)

Function	Role of IAA
Cell Elongation	Stimulates elongation of shoot cells
Gravitropism	Controls directional growth in response to gravity
Phototropism	Regulates bending toward light
Apical Dominance	Suppresses growth of lateral buds
Root Initiation	Promotes adventitious root formation
Vascular Differentiation	Stimulates xylem and phloem development
Fruit Development	Promotes fruit growth and parthenocarpy

Major Plant Hormones and Their Primary Functions

Plant Hormone	Primary Function
Auxin (IAA)	Cell elongation, tropic responses
Cytokinin	Cell division and shoot initiation
Gibberellin	Stem elongation and seed germination
Ethylene	Fruit ripening and senescence
Abscisic Acid (ABA)	Stress response and stomatal closure

Auxin Distribution During Gravitropism

Plant Organ	Effect of Higher Auxin Concentration
Shoot	Stimulates cell elongation, causing upward bending
Root	Inhibits cell elongation, causing downward bending

Biological Significance

Auxin-mediated gravitropism enables plants to orient themselves correctly within their environment. Shoots grow upward to maximize light capture for photosynthesis, while roots grow downward to absorb water and minerals and provide mechanical support. This adaptive growth response ensures optimal resource acquisition, enhances survival, and contributes significantly to plant fitness under changing environmental conditions.

Final Answer