Gibberellins For years, farmers in Japan knew of a disease that weakened rice plants by causing them to grow unusually tall. The plants would flop over and fail to produce a high yield of rice grain. Farmers called the disease the “foolish seedling” disease. In 1926, Japanese biologist Eiichi Kurosawa discovered that a fungus, Gibberella fujikuroi, caused this extraordinary growth. His experiments showed that the fungus produced a growth-promoting substance.

In fact, the chemical produced by the fungus mimicked hormones produced naturally by plants. These hormones, called gibberellins, stimulate growth and may cause dramatic increases in size, particularly in stems and fruits.

Abscisic Acid Gibberellins also interact with another hormone, abscisic acid, to control seed dormancy. Abscisic acid inhibits cell division, thereby halting growth.

Recall that seed dormancy allows the embryo to rest until conditions are good for growth. When seed development is complete, abscisic acid stops the seed's growth and shifts the embryo into a dormant state. The embryo rests until environmental events shift the balance of hormones. Such events may include a strong spring rain that washes abscisic acid away. (Gibberellins do not wash away as easily.) Without the opposing effect of abscisic acid, the gibberellins can signal germination.

Abscisic acid and gibberellins have opposite effects, much like the auxins and cytokinins. The opposing effects of plant hormones contribute to the balance necessary for homeostasis.

Ethylene One of the most interesting plant hormones, ethylene, is actually a gas. Fruit tissues release small amounts of the hormone ethylene, stimulating fruits to ripen. Ethylene also plays a role in causing plants to seal off and drop organs that are no longer needed. For example, petals drop after flowers have been pollinated, leaves drop in autumn, and fruits drop after they ripen. In each case, ethylene signals cells at the base of the structure to seal off from the rest of the plant by depositing waterproof materials in their walls.

FIGURE 24–16 A Summary of Plant Hormones This table lists some of the effects of the major plant hormones and where the hormones can be found in the plant body. Interpret Tables Name two pairs of hormones that work in opposition to each other.

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