Says Winslow Briggs, the team's leader, "This discovery represents a remarkable evolutionary adaptation of a single basic function to serve widely divergent purposes."
The discovery, reported in this week's Science magazine, is the latest piece of the puzzle in understanding how blue light triggers plants to undergo a phototropic response, i.e., bend toward the light. For over a decade, Briggs and his colleagues at the Department of Plant Biology have been on the trail of the molecule or molecules that sense blue-light and initiate phototropism. A few years ago, they discovered a 120-kD plasma membrane protein that becomes heavily phosphorylated on irradiation with blue light. (Phosphorylation, or the attachment of phosphate groups, is a well-known way to regulate proteins.) Subsequent studies indicated that this protein was involved in an early step in the phototropism response pathway. They isolated several phototropic mutants at four distinct loci in Arabidopsis, and cloned the putative gene, NPH1, that encodes the protein.
Examining the protein more closely, they made the surprising discovery that it contains two related versions of a domain (designated LOV, for light, oxygen, or voltage) that shares significant similarity (36-56%) with domains from proteins in a large number of other organisms. Because the domain appears to be used for a variety of functions in the various proteins in which it is found, it was not possible to determine, by homology, what LOV does in the Arabidopsis gene. The authors suggest that LOV could be a flavin-binding site, and that once it is bound, it could act as a redox sensor. (A flavin is a member of a family of molecules, including riboflavin, that readily gains or loses electrons under reducing or oxidizing conditions, respectively. Flavins also absorb blue light.) Alternatively, the LOV domain could play a role in mediating protein-protein interactions. If so, photoactivation of the protein would likely alter its membrane properties on the shaded and illuminated sides of the stem. This would, in turn, lead to an alteration in growth hormone transport, diverting the hormone stream to the shaded side and causing that side to grow more rapidly than the illuminated side, and thus bend toward the light. Whatever it's function, the authors note that the LOV domain likely represents an important regulatory motif, not only for the phototropic response in plants but for a wide range of other functions in other organisms.
The work was funded by the National Science Foundation.
The Department of Plant Biology, located in Stanford, California, is one of five operating centers of the Carnegie Institution of Washington, a nonprofit research and educational organization founded in 1902 by Andrew Carnegie. The institution also conducts research in the earth sciences, developmental biology, and astronomy. The director of the Department of Plant Biology is Christopher Somerville. The institution's president is Maxine F. Singer.
*The scientists are Eva Huala, Paul Oeller, Emmanuel Liscum, In-Seob Han, and Elise Larsen all current or former postdoctoral fellows at the Department of Plant Biology and Winslow Briggs, staff member and former department director.