Plant growth regulators are organic compounds synthesized in one area of the plant and moved to another. These can have physiological effects at very low concentrations. Coleoptile tips responded to light when the tip was exposed. When the tip was removed and placed on gelatin, which was then placed on another coleoptile, the coleoptile curved away from the gelatin. In other words, the photoreceptor was in the tip, but the message to bend was transported from the tip to a lower region of the tissue.
Light drives the interconversion of two forms of phytochroms.
P fr has significant biological effects, such as stimulation of seed germination. Phytochrome may activate G proteins, which stimulate a series of cellular events.