Hacking Photosynthesis

If we could change photosynthesis, we could dramatically improve crop yields. And the breakthroughs have begun to come.

The most important rate limiting primary biological phenomenon on this planet is photosynthesis, which is responsible for the primary productivity of phototrophs (light harvesting or photosynthesizing green plants). This is the process by which plants convert solar energy into biomass, the true measure of plant productivity.

The natural rate of photosynthesis has evolved into a rather slow process. One of the dreams of plant biologists has been to boost the photosynthetic rate. This was not possible until the dawn of modern biotechnology and genetic engineering. That is, not until the advent of gene-splicing techniques in the mid-1980s. Since the 1990s, the rapid advances in recombinant DNA technology has progressed in astounding ways, which is why the last 70 years have been dubbed the “golden age” of biology. Crop yields are not increasing to keep pace with food demand from a burgeoning population. Boosting of photosynthesis in rice and wheat comes at a time when the yields have been dangerously leveling off for the past four decades. It is the fatigue of the Green Revolution, according to Dr M.S. Swaminathan.

The supercharged photosynthesis is called C4 photosynthesis, an international project at the International Rice Research Institute (IRRI) in Manila, the Philippines, funded by the Bill and Melinda Gates foundation to a global consortium of 12 rice researchers. When the C4 rice plant is planted, it will tower over the conventional rice crop within weeks. Researchers say that C4 photosynthesis will increase the yield of rice by over 50 per cent.

The first exciting evidence of the effect of C4 photosynthesis in rice was seen by Paul Quick et al at IRRI late last year. For this to succeed, not only the C4 photosynthesis genes, but the anatomy of leaf cells known as Kranz anatomy needs to be engineered, which is a daunting task, as scientists do not know how those cells are put together. Thomas Brutnell of the Danforth Plant Science Centre in St Louis, says that with the help of gene editing techniques like CRISPR/Cas9, there is a possibility for altering dozens of genes that might be controlling the formation of Kranz anatomy cells.

In September 2014, Maureen Hanson and her colleagues published a landmark paper in Nature where they showed that by introducing a functional Rubisco (a key enzyme in photosynthesis) from synecchococcus elongatus, a cyanobacterium, into tobacco, they knocked out its native gene coding for the large subunit of Rubisco. These transcriptomic tobacco lines showed higher photosynthetic rates than the non-transgenic tobacco controls. This is a fine example of highly refined genetic engineering of chloroplasts, a sub-cellular organelle, and not the nucleus.

The world needs an evergreen revolution and that is possible only through genetic engineering that is much reviled by the environmental activists. Gains from conventional crop improvement are far less than the rate of population growth. If a dwarfing gene called Norin was responsible for the first revolution, Rubisco in photosynthesis could be the next gene that could unleash the evergreen (gene) revolution to increase plant productivity.

There is now a whole array of redesigns of photosynthesis to increase crop productivity. Some of the designs are really futuristic, to be enabled by developments in synthetic biology, another frontier biotechnology area that is emerging fast. If agricultural production has to be double by 2050, it behooves to double the food production per hectare of existing cropland at a time when the world’s major food crops’ yields have been stagnating.

Water has become a critical limiting factor. Although there are some modest options to develop drought-tolerant crop varieties, there are few options to dramatically reduce water consumption in agriculture. Crops’ abilities to fully harness midday light intensity is still low, and options to improve the photosynthetic efficiency look highly promising. Many targets of opportunity exist to tinker with photosynthesis to increase its efficiency.

A large part of this inefficiency is ascribed to the ability of leaves to absorb the quantum of sunlight. What happens is that even if lots of light is absorbed, it is not converted into biomass but is dissipated. Lowering light absorption seems to be one important opportunity for crop synthetic genomics. An efficient arrangement to convert absorbed light into biomass can possibly be achieved by replacing the photosystem I—a cluster of protein complexes that work with other such complexes to take care of the basic photochemistry of photosynthesis—by replacing chlorophyll with chlorophyll b.

Secondly, one can improve carbon uptake by improving and rejigging the carbon uptake machinery. The third method of improvement is to improve carbon conversion. A major productivity limitation is photorespiration, in which there is a net loss of fixed carbon. Fixing carbon is energy-expensive, but one that can be afforded due to the conversion of this energy into biomass. The C4 photosynthesis research programme led by IRRI is already showing success in this approach.

Tools and technologies that are relevant to engineering photosynthesis are cyanobacterial transformation, nuclear transformation, plastid transformation, mitochondrial transformation, multi-gene engineering, protein design, and synthetic genomics.

Genetic engineering combined with synthetic biology is the only way to enhance the photosynthetic machinery. Conventional plant breeding simply cannot do it. Because engineering photosynthesis involves dozens of genes, synthetic chromosomes have to be synthesized, requiring an unprecedented scale of genetic engineering. Many of these approaches are currently limited by our abilities to introduce, position, and regulate the inserted gene(s).

The outlook for tinkering with photosynthesis looks promising, although we have miles to go. The field has been excited by the new opportunities afforded by advances in genetic engineering and synthetic biology. One critical and challenging question for which getting answers will not be easy for the moment is how alterations made to the photosynthetic process at the level of the chloroplast will scale to a whole canopy, whose complex seasonal development ultimately determines biomass production and yield.

A continuous span from cellular metabolism to the agricultural field will be essential for successfully redesigning photosynthesis to sustainably meet global food and bioenergy demand according to Donald Ort and his colleagues at the University of Illinois who have just published a futuristic scenario of redesigning photosynthesis.

Improving biological photosynthesis and biological nitrogen fixation by endowing this property to non-legumes still remains a huge challenge. But, thanks to the Gates foundation, funds are available for a variety of collaborative research groups to tackle the problems in a coordinated way.

Some of the giants in India’s private sector should take a cue from the likes of the Gates foundation to loosen their purse strings to fund such nationally important research projects. Such funding can come from any industrial sector in the name of corporate social responsibility and true national spirit to tackle problems of food security in the not so distant a future.