Plant Hormones and Tropisms

Core Idea

Plants coordinate growth and respond to their surroundings with hormones, small signaling molecules that act at low concentration. The main classes are auxin, gibberellins, cytokinins, abscisic acid and ethylene, and each affects several processes. Tropisms are growth responses that bend a plant toward or away from a stimulus, such as light (phototropism) or gravity (gravitropism). The best-studied tropisms, phototropism and gravitropism, work by moving auxin to one side of an organ, so that the two sides grow at different rates.

How It's Best Learned

Grow seedlings in a box with light coming from one side and watch them bend; then repeat with the tips covered by foil. Lay a potted seedling on its side and watch the root turn down and the shoot turn up within a day or two. Build a table of the five hormone classes with one or two clear effects each, rather than memorizing long lists.

Common Misconceptions

• A shoot bends toward light because cells on the lit side grow faster. (Auxin moves to the shaded side, cells there elongate more, and the shoot curves toward the light.)
• Each plant hormone has a single job. (Each hormone affects many processes, and outcomes often depend on the balance between hormones, such as the ratio of auxin to cytokinin.)
• Plant hormones act only where they are made. (Many travel; auxin moves from cell to cell in a directed way, and ethylene is a gas that can pass between plants and fruits.)

Explainer

A plant cannot walk toward light or away from danger, but it can change how it grows. It does this with hormones that tell cells when to divide, how much to elongate and when to age or rest.

Five classes of hormones are best known. Auxin, chiefly indole-3-acetic acid (IAA), is made mostly in shoot tips and young leaves. It promotes cell elongation in stems and keeps side buds dormant, an effect called apical dominance. Pinch off a shoot tip and the side buds grow, making the plant bushier. Gibberellins promote stem elongation and help seeds break dormancy. They were first identified through a fungus that makes rice seedlings grow abnormally tall. Cytokinins promote cell division and slow the aging of leaves. In tissue culture, a high ratio of auxin to cytokinin favors root formation and a low ratio favors shoots. Abscisic acid (ABA) acts as a stress signal. It keeps seeds dormant and closes stomata during drought. Ethylene is a gas. It promotes fruit ripening and the dropping of leaves and fruit.

Tropisms show hormones at work. In phototropism, a shoot lit from one side bends toward the light. Charles Darwin and his son Francis showed in 1880 that the tip must see the light even though bending happens lower down. In the 1920s Frits Went collected the signal from shoot tips in small agar blocks. When he placed a block on one side of a cut stump, the stump bent away from that side. The signal was auxin. Light from one side causes auxin to move toward the shaded side. Cells there elongate more, and the shoot curves toward the light.

In gravitropism, roots grow down and shoots grow up. Specialized cells in the root cap contain statoliths, plastids filled with starch, that settle to the lower side of each cell. This shifts auxin toward the lower side of the root. In roots, high auxin slows elongation, so the upper side outgrows the lower side and the root turns down. In a shoot lying on its side, extra auxin on the lower side speeds elongation, so the shoot turns up.

Other responses include thigmotropism, growth in response to touch, which lets tendrils coil around supports. The general rule across these examples is that plants respond to the world by changing where and how fast their cells grow, and hormones carry the instructions.