Improving photosynthesis: the final frontier?
EAST LANSING, MI. – The famous Star Trek opening phrase, “to boldly go where no man has gone before” is apt to describe photosynthesis scientists toiling away in their labs today. Their gaze, however, is not turned toward the stars, but fixed on the microscopic. And their mission: to feed a global population that could reach 9 billion people by 2050. To accomplish such a feat, humanity needs a new agricultural revolution to continue boosting crop production.
Since the Green Revolution of the 1960s, humans have adopted technologies such as fertilizers, pesticides, and high-yield crops towards that end. They have yet to fully comprehend the heart of the matter, photosynthesis, which is how plants and some microorganisms make their own food and nourish the rest of the Earth’s food chain.
At its most basic, photosynthesis uses three ingredients – sunlight, water, and carbon dioxide (CO2) – to make sugars that fuel living beings. Here is the rub. In crop plants, at most 5% of light energy captured from the sun ends up as suitable biomass for human consumption. The remaining 95% of that sunlight is reflected or lost. In theory, that 5% number could be much higher. But improving the efficiency of photosynthesis is complicated because the process involves hundreds of parts that work together in a tightly choreographed fashion. No single scientific discipline can crack this system on its own.
At the MSU-Department of Energy Plant Research Laboratory (PRL), an interdisciplinary team of 12 laboratories, scientists are devising new tools and methods in order to tackle this problem and to better understand how photosynthesis works in the ‘real world.’ What follows is a brief snapshot of their latest research.
Building a holistic picture of photosynthesis
Decades of research have given us a basic understanding of the main workings of photosynthesis, but scientists have not mapped out how the entire system runs. Photosynthesis includes components operating on different time and physical scales, ranging from sub-millisecond chemical reactions in plant cells to leaves falling and regenerating over the seasons.
To build a complete picture of photosynthesis, PRL scientists are examining how plant and bacterial components – such as ones that capture sunlight, others that make energy-rich products, or regulatory ‘valves’ that balance the system – talk to teach other at various levels.
At the cellular level: New research is revealing how the chloroplast, the compartment that captures sunlight, constantly morphs its shape and membrane composition to adapt to dynamic changes in the environment. Another line of research shows how photosynthesis relies on backup systems to keep from breaking down. These systems sense temperature and light availability in an organism’s surroundings and use that information to accordingly speed up or slow down photosynthetic production.
At the organism level: Scientists are demonstrating how organisms balance photosynthetic performance against other metabolic needs. There are times where photosynthesis takes the back seat. For example, when a plant is warding off danger – say a caterpillar starts chewing on a leaf – photosynthesis and plant growth slow down. Scientists suspect that, when in crisis mode, plants have sensors that funnel resources towards producing defense chemicals. This shift in metabolic priorities keeps plants protected, but it takes resources away from plant growth.
In a related study, researchers showed that tomato plants do not deal well with simultaneous heat and caterpillar threats. The plants defend themselves against the caterpillars, but the effort prevents them from dealing with the harmful effects of heat. The result is overheated, weakened crops with reduced photosynthesis.
At the ecological level: PRL researchers are helping climate scientists refine their predictive models. Current models assume that plants’ ability to absorb CO2 during photosynthesis will help offset our carbon emissions. Recent PRL research suggests that scientists might be overestimating how much CO2 plants can suck from the atmosphere, as plants cannot produce sugars fast enough when they get too much CO2. By 2100, planet Earth could end up with gigatons more carbon in the atmosphere than previously predicted, because of this limitation.
In order to examine photosynthesis at all these levels, MSU’s PRL scientists are creating new mathematical models and tools to quantify photosynthetic processes and to identify bottlenecks in these processes that reduce their productivity. The end goal is to identify better photosynthetic components that could someday be engineered or bred into crop plants. But, to test if any changes to plants actually improve photosynthesis, scientists need to get out of the lab.
Plants have it rough in the wild
Here is a common anecdote from lab scientists: a researcher discovers a single genetic mutation that affects one component of photosynthesis. Lo and behold, yield doubles – the mutant plant in the lab grows larger than its wild cousin! Then, the scientist takes that plant for a ‘test drive’ and grows it in an outdoor field. Within days, the plant has withered and died.
Studying plants in the controlled conditions of the lab is perfect for making new discoveries, but that approach does not tell the whole story. Plants are coddled in the lab. Light quality is constant. Temperatures are comfortable. There are no caterpillars to eat them or other plants competing for nutrients.
In nature, however, conditions are ever-changing. Drought. Frost. Light flickering through leaves or clouds moving across the sky. Photosynthesis rates must adapt accordingly – speed up or slow down – so the plant or bacterial components work in harmony. Mess up the balance, and a plant will build up harmful chemicals that kill it. For example, on sunny days, plants get a lot of sunlight. If they let that intense torrent of energy course through their leaves without any controls, the light capturing systems overload and crash. (It is like speeding in a car with faulty brakes.) In response, plants release some of that light energy as heat to slow down photosynthesis and ease the pressure.
This is one example of how plants are nimble. It also illustrates how changing one gene in the lab could disrupt a plant’s internal monitoring systems and threaten its survival in the real world. PRL scientists are developing new tools that probe these checks and balances so they can someday improve photosynthesis intelligently.
One such tool takes the sophisticated techniques of the lab to the field. A hand-held device, called MultispeQ, clips on to a leaf and spits out a large set of photosynthesis and plant health data, even down to identifying which genes might be more active in that leaf. Collected data is uploaded to an open-source cloud service, where scientists, breeders, and farmers all around the world can analyze it.
Another tool brings nature to the lab. Growth chambers allow scientists to recreate weather patterns – say a cloudy, hot day – and conduct high-throughput tests on plants, in the lab. The chamber is equipped with cameras that produce heat maps of photosynthetic activity in real time. Scientists get to watch how their plants react, down to the level of individual leaves, to surrounding conditions.
These new tools are producing big data sets that are yielding new insights. PRL scientists can now predict how much biomass some young plants will yield, come harvest season. They are also uncovering minute details about how the electrochemical balance inside certain photosynthetic parts can slow down or speed up the entire photosynthetic process. And remember that 5% biomass number? The new tools are showing us that it will be difficult to increase it. Nature has exquisitely tuned photosynthesis to balance productivity with safety. We are still endeavoring to understand this balancing act.
Cyanobacteria and futuristic technologies powered by photosynthesis
We need to go back billions of years to trace the origin of photosynthesis to ancient microorganisms called cyanobacteria. Cyanobacteria are tiny, each individual 25 times smaller than a human hair. But in groups, these microorganisms dominate many of the planet’s ecosystems – from clinging to icebergs in the Arctic to living on the edges of hot springs in Yellowstone National Park. Cyanobacteria get a bad reputation because they cause the occasional toxic bloom in a sea or lake. But this is a minor flip side of being one of the most productive photosynthetic organisms – even more so than plants.
The main reason cyanobacteria are photosynthetic powerhouses is that, unlike plants, they include miniature compartments – or nanofactories – that hoard some of the photosynthetic resources and processing machinery. Keeping materials in that tight space removes inefficiencies and boosts productivity. PRL scientists were the first to produce a detailed snapshot – at atomic-level resolution – of an intact nanofactory and its outer shell. The ability to see nanofactories has opened the research floodgates, with scientists currently investigating how a nanofactory’s outer shell is assembled.
Inspired by nanofactories’ productive power, PRL scientists aim to build artificial nanofactories and retrofit them with custom machines that enhance photosynthesis or even power new applications. Imagine: an artificial nanofactory that produces renewable biofuels for our planes and cars; another that produces industrial materials, like rubber (which currently comes from trees or petroleum); or one that produces cancer drugs that can be targeted, with laser-like precision, to afflicted cells.
These are dreams, but researchers are using a new approach, called synthetic biology, to transform dreams into reality. Synthetic biologists, in the vein of engineers, study individual biological parts and use that information to create new components or entire living cells or to redesign systems that are already found in nature. Currently, scientists have gained the capacity to produce fully fledged nanofactory shells in test tubes – all synthetic, made in the lab. Researchers are also developing methods to insert raw materials or processing machinery of choice into nanofactories. As proof of concept, they have managed to incorporate inorganic (gold particles) and organic (fluorescent proteins) materials inside artificial nanofactories.
It seems fitting that cyanobacteria, which evolved photosynthesis and ushered in complex life on Earth billions of years ago, could help us address today’s energy and climate problems.
The upcoming photosynthesis revolution
From invisible nanofactories to the plants surrounding us, there is an abundance of photosynthetic energy to tap into. It will take time for science to harness that potential. The effort requires collaborations between scientific fields and labs worldwide. Keep in mind, this endeavor will not benefit only humans. Emerging challenges in agriculture – such as insect invasions or extreme weather events caused by climate change – are existential threats to photosynthetic organisms. Unlike us, these organisms cannot run from adverse situations. They may need our help to survive.