UW-Madison scientists recently obtained the detailed structure of a light-sensing protein, gaining a better understanding of the mechanics of how light governs plant growth and development. This discovery is the end result of almost 30 years of research in mapping phytochromes, plant proteins that can be manipulated to alter plant behavior to better suit agricultural needs.
'Once you have the blueprint, we know how to build the house. So, [graduate student] Jeremiah [Wagner] is now changing all of those amino acids to change the chemical properties of the photoreceptor,' said Richard Vierstra, genetics professor at UW-Madison. 'In doing so, we hope to then change how plants sense light, so we can change how they behave out in the field.'
Some beneficial plant adaptations include growing crops in closer proximity to one another, fruits that flower at different times during the season and slow-growing grass that requires less cutting. Any 'decision' that a plant makes based upon the light it receives can be manipulated, making this a valuable discovery for the agriculture industry.
Plants harness light for diverse processes. In photosynthesis, chloroplasts harness the energy from the light to assemble sugars as a food source. Phytochromes also absorb light, but treat it as a message about their surroundings or seasonal changes rather than a delivery of energy. Scattered sparsely throughout the cell, their interpretations of light durations and intensities cue a plant's reproduction and physical maturation.
'This molecule in plants senses red and far-red light and enables the plant a little bit of color vision,' Wagner said. 'And by determining the amount of light, they can grow taller, sense competition between other plants; they can tell them when to flower and what time of year it is.'
Scientists knew about the phytochrome's light-sensing role for decades, but had not fully realized the protein's structure. But when very similar phytochromes were found to exist in some bacteria, experimentation became easier.
Wagner isolated the phytochromes from bacteria cells and then created a bank of proteins to experiment upon. Using bacterial phytochromes for experimentation proved analogous to the plant version of the photoreceptor because, based on Wagner's research, each structure is built from the same raw materials.
'What he was able to show is that all the key amino acids that are found in the bacterial form are also found in the plant form. So it tells us that the structure we have for this bacterial version is probably the same structure the plant phytochromes use,' Viestra said.
For the next three years, Wagner attempted to crystallize the protein by mixing it with thousands of different chemical solutions. Once crystallized, the protein's molecular structure could be determined with X-ray diffraction. Light is shone into the crystallized phytochrome and the cloudy spots diffracting out of the prism structure represent electrons, which can be mapped on a computer.
'There have been labs all over the world trying to do this for almost 30 years...without success,' Vierstra said. 'And in fact, there were probably half a dozen labs across the world trying to do it at this moment.'
Wagner believes he succeeded with 'some diligence and probably a lot of luck.'
