In the 1980s, scientists studied the model plant Arabidopsis thaliana to learn which genes are expressed in response to different hormones. The experimental treatment involved spraying plants with different solutions containing different hormones; control plants were sprayed with water. The hormone treatments were successful in that several genes were strongly up-regulated. However, the same genes were also turned on in the plants sprayed with water. Further experiments showed that what the plants were responding to was neither the hormones nor the water, but the fact that their stems were bent back and forth when the plants were sprayed. This led to the discovery of touch-sensitive genes in plants, which are activated by mechanical perturbation.

This discovery confirmed what foresters and horticulturists had long known. Plants exposed to wind produce stems that are shorter and wider than ones grown in more protected sites. In fact, commercial greenhouses often install fans, in part so that their plants will produce stems robust enough that the plants can thrive when moved outdoors. Flexing a stem back and forth triggers an increase in the synthesis of ethylene. As noted in section 31.4, cells treated with ethylene expand more in diameter and less in length, resulting in shorter and thicker stems.