36.1: Plant Hormones

Plant hormones—or phytohormones—are chemical molecules that modulate one or more physiological processes of a plant. In animals, hormones are often produced in specific glands and circulated via the circulatory system. However, plants lack hormone-producing glands.

Instead, plant hormones are often produced in regions of active growth, like the tips of roots and shoots. Additionally, even very low concentrations of plant hormones can have a profound effect on growth and development processes. For instance, auxins are produced predominantly in shoot tips and transported from cell to cell down the stem. Auxins mediate a plethora of plant responses, such as cell elongation, fruit development, and phototropism—a plant’s movement toward or away from light.

The classical plant hormones include auxins, gibberellins (GA), abscisic acid (ABA), cytokinins (CK), and ethylene (ET). More recently-discovered hormones include jasmonates (JA), brassinosteroids (BR), and peptides. These chemical compounds mediate crucial signaling cascades that ultimately lead to key processes associated with root and shoot development, flowering, fruit ripening, and plant morphogenesis.

For example, auxins and cytokinins are mediators of plant cell division, elongation, and differentiation. Ethylene, which is the only gaseous hormone in plants, mediates fruit ripening and the abscission—or detachment—of leaves and other parts of the plant. Many of these hormones are extensively used in standard agricultural practices and have become critical for crop propagation and harvesting. For example, to increase shelf life, fruits are often picked in a green, unripe state and later treated with ethylene to promote ripening.

Plant hormones are signaling molecules that are produced in small amounts in one part of the plant and are transported to other parts. They can trigger specific responses involved in plant growth and development.

Two major plant hormones, also called phytohormones, include the auxin and cytokinin family of chemicals. Auxins promote stem elongation and root initiation.

Auxins were famously shown by Darwin to be involved in phototropism—the movement of plants towards or away from light. For example, sunflowers respond to light by tracking the movement of the sun.

In response to light, auxins accumulate on the shaded side of a plant. This leads to cell elongation and bending of the plant towards the light.

Cytokinins are another group of phytohormones that promote plant cell division.

Cytokinins act along with auxins to mediate plant morphogenesis, which is the formation of different structures of the plant such as leaves, shoots, and roots. 

Another important phytohormone is ethylene, which is  a gaseous plant hormone.

Ethylene mediates abscission in plants—the detachment of leaves or fruits—as well as the ripening of fruits.

The effects of ethylene include the enzymatic breakdown of cell wall components resulting in softening of the fruit, an increase in sugar content, and a change in the color of the fruit. 

Many plant hormones are very useful for agricultural practices. For instance, auxin is used in the generation of roots in the vegetative propagation of plants. 

Fruits are often transported in an unripened state and exposed to ethylene to quickly induce ripening once they reach their destination far from where they were harvested.