auxinen

Auxinen, or auxins, are a class of plant hormones that regulate growth and development. The most important natural auxin is indole-3-acetic acid (IAA); other naturally occurring auxins include indole-3-butyric acid (IBA) and phenylacetic acid. Synthetic auxins such as 2,4-D and NAA are widely used in horticulture and weed control.

Auxins are synthesized mainly in shoot tips and young leaves and are transported through the plant in a directional manner, a process known as polar auxin transport. This transport, which depends on PIN transport proteins and related carriers, creates gradients that guide growth responses, organ formation, and patterning, including root formation and tropisms.

At the cellular level, auxins modulate growth by influencing cell wall loosening and expansion, partly through activation of proton pumps that acidify the cell wall region. They also regulate gene expression via a receptor signaling pathway that includes TIR1/AFB proteins, AUX/IAA repressors, and ARF transcription factors. Binding of auxin leads to degradation of AUX/IAA proteins, freeing ARF factors to activate target genes involved in growth and development.

Physiological roles of auxins are diverse. They control apical dominance, promote root initiation, participate in vascular differentiation, influence fruit set and growth, and mediate responses to light and gravity (phototropism and gravitropism). Auxin signaling interacts with other hormones such as cytokinins, gibberellins, and ethylene to coordinate development and environmental responses.

Applications and implications include the rooting of cuttings using IBA, control of branching and fruit development, and the use of synthetic auxins as herbicides (for example, 2,4-D) that cause overstimulation in broadleaf weeds. Understanding auxins remains central to plant biology and agricultural practices.