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  • Reengineering Photosynthesis

    The effectiveness and efficiency of photosynthesis determine the amount of food and oxygen available on this planet. That, in turn, determines the population of humans and other species that can survive and thrive here. Soaring populations and rising affluence, require a greater abundance and variety of food than ever before. How can we enhance photosynthetic mechanisms to ensure these needs are met? What barriers and constraints could get in the way? And what business opportunities and threats are involved? We’ll show you.

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    Nothing is more important to human health and well-being than an adequate supply of nutritious food. Over the last 50 years, malnutrition has invariably been the result of failures to make food accessible, not its global production. In fact, over this period, we have seen large surpluses of all the major crops, and that’s why shortages have remained a distant concern for most of the global population. The most important primary foodstuffs, in terms of millions of metric tons, are corn, rice, wheat, and soybeans. These four crops account for about two-thirds of the calories consumed worldwide. Moreover, the average global yield per unit area of land, for each of these crops, has more than doubled since 1960.

    So, why are we discussing “food security” in 2020? One reason is that these global surpluses in staple crops have led to a steady decline in spending on plant science research and crop improvement, evident at the global level.

    But that shift in priorities may prove short-sighted. The global population is expected to increase from just over 7.3 billion today to 9.5 billion by 2050, a 30% increase. Furthermore, an increasing proportion of the population will be urban and middle class, resulting in diets shifting increasingly from staples to processed foods, upgraded with more meat and dairy products; these require large amounts of primary foodstuffs to produce. For example, 10 lbs of cattle feed are required to produce each pound of live cattle. Thus, an increase in urban population will result in an increased demand for high-quality animal products, requiring an increase in crop production that is substantially faster than that estimated based solely on the projected population growth. This trend is expected to continue, and experts predict that the world will need 80% more primary foodstuffs in 2050 than in 2020.

    So, is our current rate of increase in crop yields sufficient to meet this rising demand? That doesn’t appear to be the case. If current rates of crop yield improvement per hectare are simply maintained into the future, supply will fall seriously below demand by 2050. And the resulting rise in global food prices is expected to have the largest impact in the poorest tropical countries, which also have the highest population growth. A compounding factor is that improvement in subsistence crops in these tropical countries has tended to lag global average increases in the four leading crops. For example, the global average increase in yield per hectare of cassava, a major staple for sub-Saharan Africa, between 1960 and 2010 was 63%. This is less than half of the 171% increase for wheat over the same period. The problem is further compounded by the fact that the rate of improvement in yield even for our major crops is stagnating or even moving into reverse, in some areas of the globe. For instance, China, India, and Indonesia are the world’s largest producers of rice; yields per hectare in these countries increased by an average of 36% between 1970 and 1980. However, they only increased by 7% between 2000 and 2010.
     
    This begs the question: “Why are yield improvements stagnating?”

    The gains of the Green Revolution were achieved largely through improved genetics coupled with enhanced agronomy and crop protection that allowed for the realization of higher “genetic yield potential.”We can begin to understand these gains by defining them in mathematical terms.Yield potential (or Yp) is the mass of harvested material per hectare of land that a genotype of a crop can produce in a given environment in the absence of biotic and abiotic stresses. Improved Yield potential was achieved during the Green Revolution, in particular, by selecting genotypes that partitioned more of their biomass into the harvested product. For example, the selection of dwarfed genotypes of wheat resulted in more biomass in the grain and less in the stem. The proportion of a plant’s biomass that is invested into the harvested product, e.g., the grain of rice, is termed the harvest index. The yield potential of a given genotype is the product of the solar radiation received by a unit area of land over the growing season and the efficiencies with which the crop intercepts that radiation, converts the intercepted radiation into biomass energy, and then partitions the biomass into the harvested part of the plant.

    With reference to this equation, the Green Revolution increased both solar efficiency and harvest index. In fact, over the past 50 years, the harvest index has almost doubled in the major grain crops and now stands at 60% for modern rice, wheat, and soybean varieties. However, a living plant can’t be made only of seeds, it needs structural components like stem roots, and pod casings to support the seed at harvest. So, there is little room remaining for genetic improvement in the harvest index. Similarly, the proportion of visible sunlight that is intercepted by the crop over the growing season has reached 80-to-90% for modern crops; that indicates that this determinant of yield potential is also very close to its biological limits.

    So, what can be done? The one area in which there has been little or no improvement in conversion efficiency of visible solar energy; remains at about 2%, which is roughly one-fifth of the theoretical efficiency of 10% for C3 crops such as wheat and rice and 13% for C4 crops such as corn and sorghum. Therefore, this component of the equation appears to be a very promising focus for further enhancement of yield potential.

    Conversion efficiency depends on the chemical efficiency of the process of photosynthesis, net of respiratory losses by the crop. Concern over global climate change motivated many studies of the effects of elevated CO2 on crop production and photosynthesis.CO2 is a limiting factor for photosynthesis in C3 crops, so we know that the primary effect of increased CO2 is to artificially boost the photosynthetic rate. Invariably, this results in increased yield, demonstrating that there would be a clear benefit to yield if total crop photosynthesis could be increased genetically in crops.

    Yet, this also raises the question: “If photosynthesis has such a strong influence on crop yield, why have traditional breeding and selection for higher yield delivered so little improvement in photosynthetic efficiency?” There are several reasons for this effect.

    Within a crop species and its relatives, there is huge variation in solar efficiency and in factors affecting the harvest index, such as the proportion of biomass invested in leaves during vegetative growth, rates of leaf growth, size of leaves, and leaf longevity. This has provided breeders with many variations in selecting for improved solar efficiency and harvest index. By contrast, the process of photosynthesis is highly conserved, not only within a crop species but across a wide range of plants. Further, directed breeding efforts have screened for germplasm with high light-saturated photosynthetic rates at the leaf level and selection here has often been at the expense of other traits. For example, selection for higher light-saturated rates of leaf photosynthesis alone has often indirectly selected for lower total leaf area, offsetting any advantage at the crop level. This approach also ignores the fact that about half of crop “carbon gain” occurs under light-limited conditions.

    So, how can we increase photosynthetic efficiency, and why might this be an appealing strategy for a second Green Revolution when it was not for the first one?

    Three factors make improving overall crop photosynthetic efficiency a real possibility, today.

    The first one is based on our understanding of the photosynthetic process. In the 50 years since the start of the first Green Revolution, knowledge of the photosynthetic process has exploded. From light capture by pigment molecules to the production of storage carbohydrates; this fundamental process for all life on Earth is now understood in great detail. For higher plants, some algal species, and photosynthetic bacteria, not only is every step known, but the structures of the key proteins have been unraveled at high resolution revealing the mechanism of their action. Meanwhile, the gene coding for the key components has been characterized.

    The second factor lies in the emergence of high-performance computing. The rapid growth of computational power and new software tools has allowed the simulation of photosynthetic kinetic models of the complete process as well as the application of optimization routines. Not only can the metabolic pathways and their cellular organization be represented “in silico,” there is now the opportunity to integrate these into realistic representations of the whole canopy of a crop. This facilitates predictions of the optimal distribution of resources at the sub-cellular, cellular, leaf, and whole-crop level. High-performance computing allows the “in silico” investigation of thousands of permutations of up-and-down regulation of genes and proteins involved in photosynthesis, as well as the impacts of the potential addition of foreign genes and pathways to plants. This lets researchers identify the best targets for practical manipulation.

    Finally, the third factor is the advance of genetic engineering technology. Genome editing and synthetic biology, is now becoming increasingly routine for a wide range of crops.

    Combined, these three factors are allowing scientists to take an informed and directed approach to engineering improved photosynthetic efficiency. And the results promise to be nothing short of revolutionary.

    Given this trend, we offer the following forecasts for your consideration.

    First, by 2050, commercial crops engineered to simplify and reduce photorespiration will increase food production from the big four crops by 40 percent under real-world agronomic conditions.

    Experts estimate that today the agricultural production lost due to photorespiration each year just in the Midwest United States could feed 200 million people. Plant respiration is normal and it’s supposed to happen when it’s dark. So-called photorespiration happens when the sun is shining and it wastes resources that should be devoted to photosynthesis. Researchers have found ways to simplify and reroute photorespiration saving enough resources to boost plant growth dramatically. Already, researchers at the University of Illinois are translating their initial findings in order to boost the yield of soybeans, cowpeas, rice, potatoes, tomatoes, and eggplants.

    Second, over the next decade, the productivity of engineered food crops will jump further because of the enhanced production of key natural enzymes.

    Consider the role of RuBisCO, the world’s most common enzyme. Increased RuBisCO assists the biological machinery used by plants such as corn to convert atmospheric carbon dioxide into sugar during photosynthesis. Researchers eliminated a key bottleneck in corn photosynthesis by getting the plant to overexpress a key chaperone enzyme called RuBisCO Assembly Factor 1, helping the plant to make more RuBisCO.With the chaperone enzyme, the scientists in effect lowered a chemical speed bump that limits the rate at which RuBisCO can attain the right biological architecture leading the plants to accumulate more of it. As a result, photosynthesis is more efficient and produces more food from a given amount of sunlight.

    Third, other more subtle bio-engineering changes will make photosynthesis even more productive and reduce the amount of water that crops need.

    Put simply, plants are factories that manufacture food from light and carbon dioxide via a process called photosynthesis. As we’ve highlighted, parts of this complex process are hindered by a lack of raw materials and machinery. To optimize food production, scientists from the University of Essex recently resolved two major photosynthetic bottlenecks boosting plant productivity by 27 percent under real-world field conditions. As described in the journal Nature Plants, this photosynthetic hack also conserves water.

    Fourth, even without manmade improvements to crops, photosynthesis will rise sharply as atmospheric CO2 increases.

    In spite of falling CO2 output in OECD countries, both manmade and natural emissions are rising atmospheric CO2 levels. According to research published in the journal Nature, once C02 levels double, worldwide photosynthesis will rise by one-third. Obviously, this would increase the food supply for all species and make the planet far greener. Importantly, the resulting rise in biomass would also act to slow the rate of increase in C02, preventing runaway warming. And,

    Fifth, in the medium term, what scientists have learned about photosynthesis will impact far more than the food supply.

    Chemists at the University of Illinois are using artificial photosynthesis to produce liquid fuels from water, carbon dioxide, and visible sunlight. By converting carbon dioxide into more complex molecules like propane, solar energy become much more useful. For instance, it can be used when the sun is not shining, stored until times of peak demand, and stored in a form that is dramatically more energy-dense than batteries.

    [References]
    1. Cell.2015.Stephen P. Long, Amy Marshall-Colon, & Xin-Guang Zhu. Meeting the Global Food Demand of the Future by Engineering Crop Photosynthesis and Yield Potential.


    2. Nature.2016.Sabrina Wenzel, Peter M. Cox, Veronika Eyring & Pierre Friedlingstein. Projected land photosynthesis constrained by changes in the seasonal cycle of atmospheric CO2.

    3. Nature Plants.2018.Coralie E. Salesse-Smith, Robert E. Sharwood, Florian A. Busch & Johannes Kromdijk, Viktoriya Bardal, David B. Stern. Overexpression of Rubisco subunits with RAF1 increases Rubisco content in maize.

    4. Molecular Plant.2019.Bo-Ran Shen, Li-Min Wang, Xiu-Ling Lin, Zhen Yao, Hua-Wei Xu, Cheng-Hua Zhu, Hai-Yan Teng, Li-Li Cui, E.-E. Liu, Jian-Jun Zhang, Zheng-Hui He & Xin-Xiang Peng. Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice.

    5. Matthias M. May, Kira Rehfeld. ESD Ideas: Photoelectrochemical carbon removal as negative emission technology. Earth System Dynamics, 2019; 10 (1): 1.

    6. Paul F. South, et al. Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field. Science, Jan 4th, 2019.