Like automatic street lamps switched on and off by photocells, plants have light receptors that control seed germination, reproduction, and stem and leaf growth. Nearly 100 years of studies have led biologists to believe that one major light receptor, called phytochrome, steps up responses to sunlight, but new research at UC Davis indicates otherwise. The findings could lead to improved agricultural productivity. When plant biochemist J. Clark Lagarias and his graduate students used sophisticated genetic analyses to study phytochrome's biochemical activity in blue-green algae, "the results were unexpected," Lagarias says. Rather than being activated by sunlight, phytochrome is inactivated, and apparently puts the brakes on the light response when sunlight is in short supply. The researchers described their work in the Sept. 5 issue of the journal Science. Lagarias suggests that in higher plants, "Phytochrome tells the buried seed, 'Until there's light here, you don't want to germinate,' or tells the shaded sprout, 'There's not enough light here; grow stems faster. Don't spend your time on photosynthesis; don't expand your leaves; concentrate your energy on getting us out of this darkness.' " "If we can understand these molecules better," Lagarias adds, "we can design ways to enhance them." A crop plant might be re-engineered to ignore shade cast by a neighbor, allowing greater planting density and higher yields. Or two plants' light responses might be synchronized to make them flower at the same time, allowing crosses between a hearty fruit producer and a disease-resistant relative.