Nov. 30, 1998 John Christie and Winslow Briggs, scientists at the Carnegie Institution of Washington's Department of Plant Biology, and others report in the November 26 issue of Science that they have isolated the protein that is the UV-A/blue light photoreceptor for phototropism the process whereby plants grow in the direction of light.
While research into red light photoreceptors has been progressing for decades, that for phototropism has long been a stumbling block. "This problem has been around for many years," says Briggs, "and has generated far more heat than light." Identifying the several photoreceptors that receive the many light signals that regulate plant development is crucial for a thorough understanding of how plants grow and how that growth is regulated.
Photoreceptors that are activated by light provide important signals to the developing plant, allowing it to make the most of its environment. All of these photoreceptors are proteins with some colored compound, known as a chromophore, bound to them. It is the chromophore that absorbs visible light. In phototropism, growing toward light has the obvious consequence of improving the plant's chance to generate its own food through photosynthetic processes.
For the past six decades, scientists have been searching for the photoreceptor for phototropism, and have simultaneously debated what the chromophore might be a carotenoid or a flavin (flavins are related to the B-vitamin riboflavin.) In their paper, Christie, Briggs et al. show that the protein NPH1, acting with so-called LOV domains, binds a flavin, and is the long-sought photoreceptor.
The LOV domains, characterized earlier this year by the Briggs laboratory, are found in all sorts of proteins that detect changes in redox status as affected by light, oxygen, or voltage (hence LOV). In the plant, NPH1 becomes phosphorylated (phosphate groups are added to the protein) on exposure to blue light; this reaction is one of the earliest biochemical steps in phototropism. In this study, the researchers expressed the NPH1 gene in insect cells and subsequently found that the protein produced had exactly the same light sensitivity as the protein from the plant. A flavin was bound to the protein as its chromophore. Hence, they concluded that NPH1 was the photoreceptor for the phosphorylation reaction, as no other plant proteins were present to play the role. Since the researchers had previously shown that NPH1 was essential to phototropism, it followed that NPH1 was the photoreceptor for phototropism.
By understanding biochemical reactions elicited by light in higher plants, scientists in the future will gain a much greater understanding of plant development. With this increased comprehension, progress in agricultural research could follow. The experiments performed at the Department of Plant Biology represent one step towards this future goal.
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