Research Interests: The Lagarias Laboratory
Plants and other oxygenic photosynthetic organisms harvest light energy from the sun to fix carbon diode into the food and fiber required to sustain the biosphere. Since plants cannot move, they must cope with regular day-night cycles, seasonal cycles and neighboring vegetation, all of which influence the intensity, direction and spectral quality of light impinging on their leaves. To optimize light harvesting under low light conditions and to minimize photodamage when light is too bright, plants must continuously adjust the sensitivity of light harvesting by regulating the components of the photosynthetic apparatus or by altering the architecture of their leaves and stems. The ability to sense the ambient light level is conferred by protein light sensors that perceive different colors of visible light that are most effective in photosynthesis. For plants, red light is the most effective wavelength to drive photosynthesis therefore the most important of these photosensors, the phytochromes, are most sensitive to this region of the light spectrum.
Our laboratory studies phytochromes, which are protein-based sensors that strongly absorb red light due to a blue-colored (bilin) pigment that is physically attached. Red light absorption by the Pr form of phytochromes drives conversion to the turquoise-colored Pfr form that absorbs strongly in the near infrared (far-red) region. Unlike rhodopsins that primarily sense visible light, this red light-activated species of phytochrome can be switched off by far-red light that lies beyond the range of human vision. This red/far-red reversibility is the basis for phytochrome’s major role in plants, that of a shade detector. Through the action of phytochromes, plants seek to avoid shade that is depleted in the energy-rich red region of the spectrum. This growth response reflects the most obvious of the ‘shade avoidance’ responses that include altered leaf architecture and early flowering (see associated figure). Since these responses contribute to significant crop yield reduction in high-density field plantings, an understanding of the molecular basis of phytochrome signaling is expected to lead to new approaches to sustain crop yield at increased plant density.
Project 1 (link to abstract) focuses on understanding the molecular mechanism of light sensing and signal transduction by phytochromes. These studies seek to define how bilin and light signals are perceived by, and propagated within, the phytochrome molecule that effect transduction to downstream targets. Ubiquitous to aerobic organisms, bilins also play signaling roles in metazoans - their levels being linked to anoxia, xenobiotic-induced oxidative stress and vascular damage. The mechanism of this ancient signaling pathway is therefore of fundamental importance for understanding and regulating analogous signaling systems in other eukaryotes, including humans. In addition to leading to new approaches to regulate light responsiveness and productivity of plants, the primary food and energy source for life on earth, another long term objective of these studies is development of phytochrome-based approaches for new light-based therapies. Key collaborators on this project include Delmar S. Larson, Richard A. Mathies, Thomas R. Huser, Peter A. Jacobi and Julie A. Leary.
For more information about phytochromes and their distribution, click here.
For more information about phytochrome photochemistry, click here.
Project 2 (link to abstract) focuses on the biosynthesis and biological function of phytobilins. Phytobilins are linear tetrapyrrole pigments, produced exclusively by oxygenic photosynthetic organisms, whose optical and photochemical properties are exquisitely tuned by the proteins with which they are associated. When bound to (apo)phycobiliproteins, phytobilins harvest light throughout the visible spectrum and efficiently transfer this energy to photosynthetic reaction centers, enabling cyanobacteria, red algae and cryptophytes to colonize light-limiting environments unsuitable for other photosynthetic organisms. Phytobilins also play a key role as light harvesting pigments of plant and cyanobacterial phytochromes. This project focuses on members of the ferredoxin-dependent bilin reductase (FDBR) superfamily of enzymes that mediate the ferredoxin-dependent reduction of biliverdin IXα (BV) via protein-bound radical intermediates. Primarily using mechanistic biochemical, optical spectroscopy and molecular genetic approaches, our studies integrate x-ray crystallography, EPR spectroscopy and molecular modeling/protein design in collaboration with the groups of Andrew J. Fisher, R. David Britt, Michael Toney, and Takayuki Kohchi.
For more information about ferredoxin-dependent bilin reductases, click here.
Project 3 (link to abstract) focuses on re-engineering phytochromes to function as genetically encoded fluorescent and photoswitchable protein probes. Funded by the NSF Center for Biophotonics Science and Technology (CBST), this project exploits the ability to reconstitute photoactive and fluorescent phytochromes in living cells. Through directed evolution of phytochrome and bilin biosynthetic enzyme genes, bioorganic chemistry and in vivo functional screens, we seek to develop novel phytochrome-based fluorescent and photoswitchable biomolecules tailored for use as molecular tags and/or as regulators of gene expression in living plant, microbial and animal cells. Key collaborators on this project include Delmar S. Larson, Thomas R. Huser and Susan Spiller.
For more information about directed evolution of phytochromes, click here.