Plant Hormones and Signaling

A handful of small signaling molecules — auxin, cytokinin, gibberellin, abscisic acid, ethylene, and others — coordinate nearly every aspect of plant growth and the plant's response to its environment.

Definition

Plant hormones (phytohormones) are naturally occurring signaling molecules, active at low concentrations, that regulate growth and development, and signaling is the cellular machinery by which their presence is perceived and translated into responses.

Scope

This topic covers the major classes of plant hormones, their biosynthesis and transport, their receptors and signal-transduction pathways, and how they interact to control cell division, elongation, differentiation, dormancy, senescence, and environmental responses.

Core questions

• What are the major classes of plant hormones and their characteristic effects?
• How are hormone signals perceived and transduced inside plant cells?
• How do hormones interact and cross-talk to control development and stress responses?

Key theories

Hormonal control of tropic growth

Differential distribution of auxin across an organ drives differential cell elongation, producing the bending responses to light and gravity that orient plant growth.

Receptor-mediated signal transduction

Each hormone is perceived by specific receptors that trigger signaling cascades, often acting through regulated protein degradation to change gene expression and physiology.

Mechanisms

Auxin is transported directionally by influx and efflux carriers, establishing concentration gradients that pattern growth; its perception by TIR1/AFB receptors promotes degradation of Aux/IAA repressors, releasing transcription factors. Other hormones act through distinct receptors and cascades — for example, abscisic acid binding to PYR/PYL receptors inhibits protein phosphatases to trigger stress responses, and ethylene perception by membrane receptors regulates ripening and senescence. Reverse-genetic resources such as genome-wide insertion mutant collections have been central to dissecting these pathways.

Clinical relevance

Synthetic plant growth regulators and hormone-based treatments are widely used in agriculture and horticulture for rooting cuttings, controlling fruit set and ripening, managing weeds, and synchronizing crop development.

History

Darwin's experiments on phototropism pointed to a transmissible growth influence; Went isolated this substance as auxin in 1926, and the subsequent century identified the full complement of hormones and, with molecular genetics in model plants, their receptors and signaling pathways.

Key figures

• Frits Went
• Charles Darwin
• Kenneth Thimann

Seminal works

• taiz2015
• buchanan2015

Frequently asked questions

What does auxin do in plants?

Auxin promotes cell elongation and is distributed unevenly to drive tropic bending toward light and with gravity; it also influences root initiation, vascular patterning, and apical dominance.

Why is ethylene important for fruit?

Ethylene is a gaseous hormone that triggers and accelerates ripening in many fruits, which is why it is managed in storage and transport to control when produce ripens.