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Food for thought: Carbon

Written by Robert Griffin-Nolan, a 2017-2018 Sustainability Leadership Fellow and PhD Candidate in the Department of Biology and Graduate Degree Program in Ecology

Without photosynthesis, humans would not be here. Photosynthesis is arguably the most valuable biochemical process on Earth. Without it we would have no food. Plants “eat” carbon dioxide (CO2) from the atmosphere and use the Sun’s energy to convert it into sugars. When tending to your house plant, you probably do not consider CO2 concentrations. You focus on proper lighting or adequate watering, factors you can control. However, the amount of CO2 in the atmosphere can have huge implications for plant function. It likely drove the evolution of several plant adaptations we observe today. For plants, carbon is food – and some plants are better than others at using it. Improving the carbon use efficiency of plants has caught the attention of a global community of plant breeders interested in increasing crop yields to feed the expected 9.8 billion people that will share this planet in 2050.1

Why do plants differ in their ability to use carbon? With all the CO2 we’ve been pumping into the atmosphere, one would think plants should be saturated in a carbon feast! In fact, photosynthesis is riddled with inefficiencies which are revealed in light of evolution. Plant life has its origins in the ocean, an environment that provides structural stability and plentiful hydration yet has very little CO2 (i.e. plant food). One of the environmental drivers of plants moving onto land over 400 million years ago was the high amount of CO2 in the atmosphere, approximately 10x higher than today’s atmospheric concentrations – truly a plant feast!2 The enzymes, or proteins, plants use to reorganize carbon into sugars were saturated in CO2. Thus, there was no “enzymatic incentive” to use it efficiently. In this carbon rich world, terrestrial plants thrived and conquered much of Earth’s land surface. However, as photosynthetic rates soared, CO2 concentrations dropped, revealing a lethal flaw in the photosynthetic pathway.

Ribulose-1,5-bisphosphate carboxylase/oxygenase, more commonly referred to as RuBisCO, is the most abundant protein on Earth.3 It is responsible for catalyzing the carbon fixing step of photosynthesis. RuBisCO is not a loyal servant of carbon, however, and has an affinity for another molecule, oxygen. When oxygen interacts with RuBisCO, in a process called photorespiration, there is less RuBisCO available to grab onto CO2, and thus less sugar is produced.

In a CO2 rich world, this was not an issue for plants; however, around 30 million years ago, CO2 concentrations dropped to the point where photorespiration became problematic.4 There was not enough ‘plant food’ to go around! Around this time, however, a more efficient form of photosynthesis emerged in the paleo-botanical record: C4 photosynthesis. This new biochemical pathway compartmentalized RuBisCO in specific plant cells that were then pumped full of CO2 to essentially remove any chance of RuBisCO interacting with oxygen. Variations of this particular carbon concentrating mechanism evolved not once, but more than 45 times in different plant lineages. The process of natural selection was hard at work.

Scientists are still debating what truly drove the evolution of C4 photosynthesis.5 The emergence of C4 photosynthesis was geographically isolated to some of the warmest and periodically dry regions of the world – tropical grasslands. Why didn’t it happen in other parts of the world? This is likely because the carbon concentrating mechanism of C4 photosynthesis allows plants to use water more efficiently.

CO2 is not freely available to plants but must be purchased. The currency for buying CO2 is water. Most plants are extremely inefficient at using water. For every molecule of CO2 gained by a plant, up to 400 molecules of water are lost! By preventing CO2 and O2 from fighting over RuBisCO, C4 plants can close the microscopic pores in their leaves used for exchanging gases, prevent excessive water loss, and survive in arid climates. Whether it was changing CO2 concentrations, climate or other factors (e.g. increased fire frequency4) that drove the evolution of C4 photosynthesis, the success of this new photosynthetic pathway was clear – C4 plants, and largely C4 grasses, spread rapidly throughout the world creating our warm savannas and grasslands. While C4 plants comprise a mere 3% of vascular plants species, they make up 25% of global photosynthesis6 – and it is this that has caught the attention of plant breeders.

In 2008, the Bill and Melinda Gates foundation awarded the International Rice Research Institute a grant of $11.1 million to begin the C4 Rice Project (http://c4rice.irri.org/), a 15-year collaborative research effort including 12 institutions with the goal of genetically modifying rice to use C4 photosynthesis. Rice, a staple food source for much of the world, is a C3 plant meaning it uses the relatively ‘carbon inefficient’ photosynthetic pathway that evolved over 2.8 billion years ago (named after the three-carbon sugar initially produced, C4 photosynthesis makes a four-carbon sugar). Indeed, many of our crops are C3 plants. The C4 Rice Project aims to identify relevant genes that code for proteins involved in C4 photosynthesis and eventually re-engineer the entire cellular structure and biochemistry of rice to mimic the C4 pathway, allowing rice to use water, light, and nutrients more efficiently. The end result is potentially a 50% increase in crop yield!7

There are multiple benefits of having C4 crops (some researchers remain skeptical of this increase in yield). C4 plants use nitrogen, or the building block of proteins, much more efficiently than C3 plants. Given that RuBisCO is saturated in CO2 in C4 plants, there is less need for RuBisCO production, and thus less need for nitrogen (i.e. fertilizer). Conversion of crops to C4 plants would greatly reduce our need for synthetic fertilizer with their attendant greenhouse gas emissions and resulting widespread eutrophication of our water bodies. Additionally, C4 crops would require less irrigation, given the higher water use efficiency of the pathway. Droughts are expected to become more common with climate change, a trend that is already apparent in California and even here in Colorado (see map). There is a reason why C4 grasses rapidly dominated the most arid regions of the world – they are masters of environmental stress.

If you’re concerned about genetic modification of crops, think about one number – 9.8 billion. That’s how many people we’ll have to feed by 2050. C4 photosynthesis has already evolved multiple times over the course of evolutionary history, one of the most amazing examples of convergent evolution. Scientists are now trying to make it happen just one more time. 

References:

  1. United Nations, Department of Economic and Social Affairs, Population Division (2017). World Population Prospects: The 2017 Revision, Key Findings and Advance Tables. ESA/P/WP/248.
  2. Ehleringer, J. R., & Cerling, T. E. (1995). Atmospheric CO2 and the ratio of intercellular to ambient CO2 concentrations in plants. Tree Physiology, 15(2), 105-111.
  3. Ellis, R. J. (1979). The most abundant protein in the world. Trends in biochemical sciences, 4(11), 241-244.
  4. Sage, R. F. (2004). The evolution of C4 photosynthesis. New phytologist, 161(2), 341-370.
  5. Edwards, E. J., Osborne, C. P., Strömberg, C. A., Smith, S. A., & C4 Grasses Consortium. (2010). The origins of C4 grasslands: integrating evolutionary and ecosystem science. science, 328(5978), 587-591
  6. Still, C. J., Berry, J. A., Collatz, G. J., & DeFries, R. S. (2003). Global distribution of C3 and C4 vegetation: carbon cycle implications. Global Biogeochemical Cycles, 17(1).
  7. “C4 Rice Project”. http://c4rice.irri.org/

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