Gravitropism

Gravitropism is a directional growth movement which occurs in plants and fungi, allowing them to grow in response to gravity. This phenomenon is also known as geotropism. The primary role of gravitropism is to guide roots to grow downwards into the soil (positive gravitropism) and shoots to grow upwards, away from the earth (negative gravitropism), thus ensuring that the plant can both support itself and obtain water and nutrients from the soil.

Mechanism

The mechanism of gravitropism is complex and involves the perception of gravity by specialized cells, followed by a signal transduction pathway that leads to the differential growth of plant organs. In plants, Amyloplasts (starch-containing organelles) within the Statocytes in root cap and shoot tissues act as gravity sensors. These amyloplasts sediment in the direction of gravity, exerting pressure on the endoplasmic reticulum, which is thought to trigger a biochemical response leading to the growth modulation.

Auxin Distribution

The plant hormone Auxin plays a crucial role in gravitropism. Auxin distribution within the plant tissue changes in response to gravity, leading to differential growth. In roots, auxin accumulates on the lower side, inhibiting cell elongation and causing the root to bend downwards. Conversely, in shoots, auxin promotes cell elongation on the lower side, causing the shoot to bend upwards.

Types of Gravitropism

Gravitropism can be classified into two main types based on the direction of growth in relation to gravity:

• Positive Gravitropism: Growth towards the direction of gravity. This is observed in roots.
• Negative Gravitropism: Growth away from the direction of gravity. This is observed in shoots.

Function

Gravitropism allows plants to orient themselves correctly in their environment, ensuring that roots grow into the soil to absorb water and nutrients, while shoots grow upwards to access sunlight for photosynthesis. This directional growth is crucial for plant stability, nutrient uptake, and overall survival.

Research and Applications

Research into gravitropism has implications for agriculture, as understanding how plants sense and respond to gravity can help in developing crops that are more resilient to environmental stresses. Additionally, with the advent of space exploration, studying plant growth in microgravity environments has become important for developing life support systems in space habitats.

See Also

• Phototropism
• Thigmotropism
• Hydrotropism
• Plant hormone
• Amyloplast
• Statocyte