What is the role of auxin in phototropism and gravitropism, and how does it affect plant growth rates?

SouravNovember 12, 2024

Answered step-by-step

Auxin is a crucial plant hormone that plays a significant role in coordinating growth responses to environmental factors such as light (phototropism) and gravity (gravitropism). Here’s an overview of how auxin functions in these processes and its impact on plant growth rates.

Role of Auxin in Phototropism

1. Mechanism of Action:
   • Light Perception: When a plant is exposed to directional light, auxin redistributes unevenly within the plant. It accumulates on the shaded side of the stem.
   • Cell Elongation: The increased concentration of auxin on the shaded side stimulates cell elongation in that region. This occurs through the activation of proton pumps, which lower the pH in the cell wall, allowing wall-loosening proteins (expansins) to facilitate cell expansion. As a result, the cells on the shaded side elongate more than those on the illuminated side, causing the stem to bend towards the light source.
2. Impact on Growth Rates:
   • By promoting elongation on one side of the stem, auxin enhances overall growth rates in response to light. This adaptation allows plants to maximize photosynthesis by orienting their leaves towards sunlight, thereby improving energy capture and growth efficiency.

Role of Auxin in Gravitropism

1. Mechanism of Action:
   • Gravity Perception: In response to gravity, auxin also redistributes within the plant. When a plant is tilted or placed horizontally, auxin accumulates on the lower side of the root or shoot.
   • Differential Growth: For roots (positive gravitropism), higher auxin concentrations inhibit cell elongation, resulting in slower growth on the lower side and allowing roots to grow downwards. Conversely, for shoots (negative gravitropism), auxin promotes cell elongation on the lower side, encouraging upward growth against gravity.
2. Impact on Growth Rates:
   • The differential distribution of auxin allows plants to maintain proper orientation and stability, which is essential for optimal nutrient uptake and structural integrity. This coordinated growth response ensures that roots grow deeper into the soil while shoots reach upward toward light sources, promoting overall plant health and growth rates