Auxins

Auxins

Auxins are a class of plant hormones that play a crucial role in the coordination of many growth and behavioral processes in the plant's life cycle. They are essential for plant body development and are involved in processes such as cell elongation, root formation, and response to light and gravity.

History

The study of auxins began in the late 19th and early 20th centuries. The term "auxin" is derived from the Greek word "auxein," which means "to grow." The first auxin to be discovered was indole-3-acetic acid (IAA), which remains the most well-known and studied auxin.

Chemical Structure

Auxins are generally indole-based compounds, with IAA being the most common naturally occurring auxin. The structure of IAA consists of an indole ring and an acetic acid side chain. Other synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are used in agriculture as herbicides.

Biosynthesis

Auxins are primarily synthesized in the shoot apical meristem and young leaves. The biosynthesis of IAA involves several pathways, including the tryptophan-dependent pathway, where tryptophan is converted into IAA through a series of enzymatic reactions.

Transport

Auxins are transported throughout the plant via two main mechanisms: polar transport and non-polar transport. Polar auxin transport is a directional movement of auxin molecules from cell to cell, primarily from the shoot towards the root. This transport is facilitated by specific auxin transport proteins, such as PIN proteins and AUX1/LAX family transporters.

Functions

Auxins have a wide range of functions in plant growth and development:

• Cell Elongation: Auxins promote cell elongation by loosening the cell wall, allowing cells to expand.
• Apical Dominance: Auxins produced in the apical bud suppress the growth of lateral buds, maintaining apical dominance.
• Root Initiation: Auxins stimulate the formation of lateral and adventitious roots.
• Phototropism and Gravitropism: Auxins mediate the plant's response to light (phototropism) and gravity (gravitropism) by redistributing auxin concentration in response to environmental stimuli.

Applications

Auxins have several practical applications in agriculture and horticulture:

• Rooting Hormones: Synthetic auxins are used to promote root formation in plant cuttings.
• Herbicides: Compounds like 2,4-D are used as selective herbicides to control broadleaf weeds.
• Fruit Development: Auxins can be applied to promote fruit set and prevent premature fruit drop.

See Also

• Plant hormone
• Gibberellin
• Cytokinin

References

• Taiz, L., & Zeiger, E. (2010). Plant Physiology. Sinauer Associates.
• Davies, P. J. (2010). Plant Hormones: Biosynthesis, Signal Transduction, Action! Springer.