Q.24 Indole acetic acid (IAA) is involved in ______.
(A) gravitropism (B) flowering (C) ripening (D) senescence
Indole-3-acetic acid (IAA), the primary natural auxin, primarily mediates tropic responses like gravitropism through asymmetric distribution that drives differential cell elongation. While IAA influences multiple growth processes, its core involvement in competitive exam contexts like CSIR NET centers on gravitropism, distinguishing it from other hormones.
Option Analysis
(A) Gravitropism
IAA, produced in shoot and root tips, redistributes laterally in response to gravity, causing roots to elongate more on the upper side (inhibiting lower-side growth) for positive gravitropism downward. This asymmetric auxin gradient is essential for root gravitropism, as confirmed in Arabidopsis studies where IAA transport mutants impair bending.
(B) Flowering
Flowering primarily involves florigen signals and gibberellins; IAA supports vegetative growth and apical dominance but does not directly trigger flowering.
(C) Ripening
Ethylene is the key hormone for fruit ripening via climacteric ethylene bursts; IAA levels decline during ripening and exogenous IAA does not hasten it.
(D) Senescence
Senescence features chlorophyll loss and is promoted by ethylene or ABA; IAA application can accelerate leaf senescence in some cases but generally maintains growth rather than driving aging.
Indole Acetic Acid (IAA), the most common natural auxin, plays a pivotal role in Indole Acetic Acid IAA gravitropism by creating gravity-responsive auxin gradients that direct root and shoot growth. This process ensures roots grow downward (positive gravitropism) and shoots upward, vital for plant anchorage and light access.
IAA Mechanism in Gravitropism
Gravity sensing via statoliths in root columella cells triggers IAA redistribution to the lower side. High IAA inhibits root elongation on the lower flank while promoting shoot cells, bending organs appropriately. PIN proteins facilitate this polar transport, as mutants show defective bending.
Comparison with Other Processes
| Process | Primary Hormone | IAA Role |
|---|---|---|
| Gravitropism | IAA (Auxin) | Directs via asymmetric gradients |
| Flowering | Gibberellins | Indirect via growth support |
| Ripening | Ethylene | Minimal; levels drop |
| Senescence | Ethylene/ABA | Can accelerate but not primary |
CSIR NET Relevance
In exams, IAA links directly to gravitropism over secondary roles, testing auxin specificity. Understanding IAA biosynthesis from tryptophan and transport reinforces its tropism primacy.
