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.