Improving Photosynthesis

Photosynthesis is the basis of plant growth, and improving photosynthesis can contribute toward greater food security in the coming decades as world population increases. Multiple targets have been identified that could be manipulated to increase crop photosynthesis. The most important target is Rubisco because it catalyses both carboxylation and oxygenation reactions and the majority of responses of photosynthesis to light, CO₂, and temperature are reflected in its kinetic properties. Oxygenase activity can be reduced either by concentrating CO₂ around Rubisco or by modifying the kinetic properties of Rubisco. The C₄ photosynthetic pathway is a CO₂-concentrating mechanism that generally enables C₄ plants to achieve greater efficiency in their use of light, nitrogen, and water than C₃ plants. To capitalize on these advantages, attempts have been made to engineer the C₄ pathway into C₃ rice (Oryza sativa). A simpler approach is to transfer bicarbonate transporters from cyanobacteria into chloroplasts and prevent CO₂ leakage. Recent technological breakthroughs now allow higher plant Rubisco to be engineered and assembled successfully in planta. Novel amino acid sequences can be introduced that have been impossible to reach via normal evolution, potentially enlarging the range of kinetic properties and breaking free from the constraints associated with covariation that have been observed between certain kinetic parameters. Capturing the promise of improved photosynthesis in greater yield potential will require continued efforts to improve carbon allocation within the plant as well as to maintain grain quality and resistance to disease and lodging.Photosynthesis is the process plants use to capture energy from sunlight and convert it into biochemical energy, which is subsequently used to support nearly all life on Earth. Plant growth depends on photosynthesis, but it is simplistic to think that growth rate directly reflects photosynthetic rate. Continued growth requires the acquisition of water and nutrients in addition to light and CO₂ and, in many cases, involves competition with neighboring plants. Biomass must be invested by the plant to acquire these resources, and respiration is necessary to maintain all the living cells in a plant. Photosynthetic rate is typically measured by enclosing part of a leaf in a chamber, but to understand growth, one needs to consider the daily integral of photosynthetic uptake by the whole plant or community and how it is allocated. Almost inevitably, changing photosynthesis in some way requires more resources. Consequently, in order to improve photosynthesis, one needs to consider the tradeoffs elsewhere in the system. The title, “Improving Photosynthesis,” could be interpreted in many ways. For this review, I am restricting the scope to focus on crop species growing under favorable conditions.To support the forecast growth in human population, large increases in crop yields will be required (Reynolds et al., 2011; Ziska et al., 2012). Dramatic increases in yield were achieved by the Green Revolution through the introduction of dwarfing genes into the most important C₃ cereal crops rice (Oryza sativa) and wheat (Triticum aestivum). This allowed greater use of fertilizer, particularly nitrogen, without the risk of lodging, where the canopy collapses under the weight of the grain, causing significant yield losses (Stapper and Fischer, 1990). It also meant that biomass allocation within the plant could be altered to increase grain mass at the expense of stem mass now that the plants were shorter. Retrospective comparisons of cultivars released over time, but grown concurrently under favorable conditions with weed, pest, and disease control and physical support to prevent lodging, reveal that while modern cultivars yield more grain, they have similar total aboveground biomass (Austin et al., 1980, 1989).It is interesting to revisit the review by Gifford and Evans (1981): “over the course of evolution from the wild plant to modern cultivar, carbon partitioning was improved. Thus, as remaining scope for further improvement in carbon allocation must be small, it would be better to aim at increasing photosynthetic and growth rates. Alternatively, as partitioning is where flexibility has been manipulated in the past, it is better to aim for further increases in harvest index.” Just over 30 years have passed since this was published, and yield gains made by plant breeders have continued to come largely from increasing carbon allocation into grain (Fischer and Edmeades, 2010) and selecting for increased early vigor (Richards et al., 2010). By contrast, selection based on improving photosynthesis has yet to be achieved. Plants need leaves and roots to capture light, water, and nutrients for growth and stems to form the leaf canopy and support the flowers and grain, so further increases in harvest index may lead to a decrease in yield. Therefore, in order to increase yield potential further, it is necessary to increase total biomass. If light interception through the growing season is already fully exploited, then increasing biomass requires that photosynthesis be increased. It is the realization that further significant increases in yield potential will not be possible by continuing the current strategy that has turned attention toward improving photosynthesis. Recent technological developments now provide us with the means to engineer changes to photosynthesis that would not have been possible previously.