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 pr…
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.
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Q1: What are plant hormones and how do they differ from animal hormones?
Plant hormones, or phytohormones, are signaling molecules produced in small amounts in one part of a plant and transported to other parts to trigger specific growth and development responses. Unlike animal hormones produced in specialized glands, plant hormones are often produced in regions of active growth, such as shoot and root tips. Even very low concentrations of plant hormones can profoundly affect plant physiology.
Q2: How do auxins cause plants to bend toward light?
Auxins are plant hormones that promote stem elongation and root initiation. In response to light, auxins accumulate on the shaded side of a plant stem, causing cells on that side to elongate more than cells on the illuminated side. This differential growth bends the plant toward the light, a process studied through photoreceptors and plant responses to light.
Q3: What role do cytokinins play in plant development?
Cytokinins are phytohormones that promote plant cell division and work together with auxins to mediate plant morphogenesis—the formation of different plant structures such as leaves, shoots, and roots. This coordinated hormone action is essential for proper plant growth and the development of distinct tissues and organs throughout the plant body.
Q4: Why is ethylene considered unique among plant hormones?
Ethylene is the only gaseous plant hormone, distinguishing it from other phytohormones that are typically transported cell-to-cell. It mediates abscission—the detachment of leaves and fruits—and fruit ripening. Ethylene triggers enzymatic breakdown of cell wall components, resulting in fruit softening, increased sugar content, and color changes that signal ripeness.
Q5: How are plant hormones used in agricultural practices?
Plant hormones have become critical for crop propagation and harvesting. Auxins are used to generate roots during vegetative propagation of plants. Fruits are often picked in an unripe state and transported to distant markets, then exposed to ethylene gas to quickly induce ripening. This practice extends shelf life and ensures fruits reach consumers in optimal condition.
Q6: What are the classical plant hormones and their primary functions?
The classical plant hormones include auxins, gibberellins, abscisic acid, cytokinins, and ethylene. These phytohormones mediate crucial signaling cascades leading to root and shoot development, flowering, and fruit ripening. More recently discovered hormones include jasmonates, brassinosteroids, and peptides, which further expand the complexity of plant hormone regulation.
Q7: Where are plant hormones produced and how are they transported?
Plant hormones are often produced in regions of active growth, particularly shoot tips and root tips. Auxins, for example, are produced predominantly in shoot tips and transported from cell to cell down the stem. This cell-to-cell transport mechanism allows hormones to reach target tissues throughout the plant and coordinate growth responses across distant plant organs.