Explain the role of auxin in controlling shoot growth, limited to: (a) auxin is made in the shoot tip (b) auxin spreads through the plant from the shoot tip (c) auxin is unequally distributed in response to light and gravity (d) auxin stimulates cell elongation

Sourav PanNovember 3, 2024

Answered

Auxin is a key plant hormone that plays a crucial role in controlling shoot growth and directing plant responses to environmental stimuli such as light and gravity. Here’s an explanation of the role of auxin in this process, focusing on the specified points:

(a) Auxin is Made in the Shoot Tip

• Production Location: Auxin is primarily synthesized in the apical meristem located at the shoot tip (also known as the terminal bud). This region contains actively dividing cells that contribute to the growth of new shoots and leaves.
• Significance: The concentration of auxin is highest at the shoot tip, where it initiates growth and development processes. This localized production is essential for directing growth towards favorable conditions.

(b) Auxin Spreads Through the Plant from the Shoot Tip

• Transport Mechanism: Once produced in the shoot tip, auxin is transported downwards through the plant via a process called polar transport. This movement occurs primarily through parenchyma cells and involves both active transport and diffusion.
• Role in Growth Regulation: As auxin spreads from the shoot tip, it influences various growth processes along the stem, including cell division and elongation, thereby regulating overall shoot development.

(c) Auxin is Unequally Distributed in Response to Light and Gravity

• Phototropism: When light shines on one side of a plant, auxin redistributes towards the shaded side. This uneven distribution causes cells on the shaded side to elongate more than those on the illuminated side, resulting in bending towards the light source (positive phototropism).
• Gravitropism: Similarly, when a plant is tilted or experiences gravitational pull, auxin accumulates on the lower side of the stem. This causes differential growth where cells on the lower side elongate more than those on the upper side, leading to upward bending of the stem (negative gravitropism).

(d) Auxin Stimulates Cell Elongation

• Mechanism of Action: Auxin promotes cell elongation by stimulating several physiological processes:
  • It activates proton pumps in cell membranes, leading to an increase in hydrogen ion concentration in the cell wall. This acidification activates enzymes that loosen cell wall structures, allowing cells to expand more easily.
  • Auxin also promotes the synthesis of proteins involved in cell wall loosening and expansion.
• Effect on Growth: By stimulating cell elongation, auxin directly influences how quickly and effectively a plant can grow towards light or adjust its orientation in response to gravity. This growth response is crucial for optimizing resource acquisition (light for photosynthesis and nutrients from soil).