Department of Plant
Carnegie Institution for Science
260 Panama Street
Stanford, CA 94305
Phone: (650) 325-1521 x207
Fax: (650) 325-6857
The directional response of seedlings toward a light source (phototropism) is essential for the rapid orientation of recently germinated seedlings to maximize light capture for photosynthesis as early as possible following germination. Over a decade ago the Briggs lab discovered and first characterized the photoreceptor family that mediates this directional response and named the two members phototropin 1 and phototropin 2. The Briggs group also identified the light-absorbing chromophore as flavin mononucleotide (FMN). A year later they identified the two very similar protein domains that bind the FMN (110-amino-acid domains that they designated LOV domains as they were similar to domains in a wide range of otherwise entirely different signaling proteins responding to Light, Oxygen, or Voltage.) Another year later they first described the unique photochemistry that LOV domains undergo on photoexcitation: they form a carbon-sulfur covalent bond between a carbon of the flavins and the sulfur from a nearby cysteine. This causes a change in the protein confirmation leading to autophosphorylation and consequent activation of the photoreceptor. Over a matter of seconds or minutes in darkness, the protein returns to its inactive state, prepared for another photoexcitation. Since then the laboratory has done a great deal of biochemical and biophysical work characterizing the light reaction and from this work and that of others, we now know a great deal about it. We know only a little about its deactivation that occurs in darkness. However, we recently identified the phosphatase system that interacts with one of the phototropins to remove the phosphate, an important component in the deactivation.
It is now known that the phototropins mediate several other responses—leaf expansion, leaf orientation to light, chloroplast movement, stomatal opening, and rapid inhibition of hypocotyl grown in dark-grown seedlings. All of these responses relate to maximizing photosynthetic potential. Two other LOV-domain proteins, combining a single LOV domain with a downstream F-Box have been demonstrated to serve as photoreceptors involved in flowering responses to daylength and light perception in circadian rhythms.
The group is also interested in the interactions of phototropins with other plant photoreceptors, for example the phytochromes. The group recently showed that phytochrome A, excited by red light, could block blue-light-induced movement of phot1 away from the plasma membrane, an inhibition that likely sensitizes the plant to the blue light stimulus. This unique interaction is currently under investigation.
A variety of proteins harboring LOV domains are found in bacteria. Many of these are LOV domains upstream from histidine kinases, classic bacterial sensor proteins. In collaboration with others, the Briggs lab demonstrated that a LOV-histidine kinase in the animal pathogen Brucella was essential for virulence in an in vitro assay. However, virulence plummeted to 10 % of that expected in the light when the cultures were kept in the dark. These studies form the basis for an entirely new field of bacterial photophysiology involving determining the role of LOV-domain-based photosensors. The laboratory will be working with several of these (in non-pathogenic bacteria) in addition to working on the signal transduction pathways from the phortotropins.