Auxins
ஆக்ஸின்கள்
Stop confusing Auxins with mere growth hormones. They are the primary architects of plant polarity and apical dominance. If you miss the distinction between IAA and synthetic analogs like 2,4-D, you are losing marks on a guaranteed question.
The idea
Auxins are growth-promoting Plant Growth Regulators (PGRs), primarily Indole-3-acetic acid (IAA). They are synthesized in the growing apices of stems and roots before migrating to the site of action. They facilitate cell division and cell enlargement, directly driving elongation. Their most critical physiological role is maintaining apical dominance, where the terminal bud inhibits the growth of lateral buds.
Why it matters
They exist to ensure the plant prioritizes vertical growth and root development, allowing the plant to compete for light and resources effectively.
NEET lens
NEET focuses on the distinction between natural (IAA, IBA) and synthetic (NAA, 2,4-D) auxins. Memorize the specific functions: apical dominance, parthenocarpy in tomatoes, and weed control. Expect a match-the-following question on PGR functions. Never confuse Auxins with Gibberellins; Gibberellins promote internode elongation, while Auxins drive cell enlargement.
Clinical significance
While Auxins are plant-specific, their mechanism of action via auxin-response factors (ARFs) mirrors the signaling pathways of human steroid hormones. Disruption in plant growth regulation leads to ecosystem collapse, as seen in the misuse of synthetic auxins as herbicides. Understanding these pathways is fundamental to grasping how chemical signals regulate complex biological development.
Introduction
Auxins (primarily indole-3-acetic acid, IAA) are plant hormones produced mainly in shoot apical meristems that promote cell elongation, control phototropism and gravitropism, and regulate apical dominance.
Core concept
Produced in: shoot apex, developing leaves, seeds. Moved by polar auxin transport (PAT) — strictly downward (basipetal in shoot, acropetal in root) via PIN efflux carriers. Effects: low concentrations promote cell elongation (in shoots). High concentrations inhibit (in roots — roots are more sensitive). Phototropism: light causes auxin redistribution from lit side to shaded side → shaded side cells elongate more → bending toward light. Gravitropism: auxin accumulates on lower side → shoots grow up, roots grow down (due to differential sensitivity).
Go deeper
Acid growth hypothesis: auxin promotes H⁺ secretion into cell wall → wall loosening (expansins activated) → water uptake → cell expansion. This explains why cell elongation follows auxin application within minutes.
From Class 10
Plants bend toward light. A hormone causes cells on the shaded side to elongate more than cells on the lit side. That hormone is auxin (IAA).