By Rowenna Hoskin | Science Editor
The tequila plant, agave, is part of a global agricultural revolution. It may hold the secrets to feeding the planet’s 3 billion people in the midst of a climate emergency.
Agave uses a highly efficient version of photosynthesis called Crassulacean acid metabolism (CAM) meaning it can grow in dry locations, a feature that will become increasingly important through climate change.
It is most commonly found in Mexico, but Australia’s Pacific coast has triggered an impending global revolution. Daniel Tan, a researcher at the University of Sydney views the agave plant as the key to fulfilling the massive food requirement of the human population. In just 30 years, we will have to produce 50% more food to feed 10 billion people. Unfortunately, climate change will mean that there is less viable land and more extreme weather conditions inhibiting the growth of crops.
Tan has a radical solution to tackle this impending crisis. Despite having two billion years to perfect the process, the majority of plants are very inefficient at harnessing the sun’s energy. Plants use sunlight to convert carbon dioxide and water into oxygen and sugars needed for growth. 90% of plants use C3 photosynthesis which has a maximum conversion efficiency of 4.6%. This disappointing figure is caused by the enzyme Rubisco which catalyses the carbon fixation stage in the Calvin cycle.
The reason for the inefficiency of 90% of plant species is that Rubisco not only catalyses this reaction, but it also catalyses a competing and wasteful reaction with oxygen. This initiates phosphorespiration which leads to a loss of fixed carbon and a waste of energy. This happens 40% of the time, and this percentage increases when plants close their pores to reduce water loss.
Rubisco’s inefficiency was not a problem when this process evolved billions of years ago because the earth’s atmosphere was rich in CO2 and almost free of oxygen. Due to billions of years of photosynthesis, the atmosphere has a higher abundance of oxygen meaning they are now competing with the carbon molecules.
C4 Photosynthesis as a solution:
Some plants have found a way around this inefficiency by developing C4 photosynthesis. This process splits the metabolic pathway between two parts of the plant’s anatomy. The first section contains the capture of carbon molecules in the spongy mesophyll layer beneath the leaf’s waxy protective layer. This is where the plant produces the 4 carbon molecules. These molecules are then transported through special channels to cells clustered around leaf veins where it is broken down to release the carbon molecules once again. Rubisco can then catalyse the reaction with less chance of accidently wasting energy on oxygen reactions as the concentration of carbon in this section of the plant is much higher.
Another adaptation of these C4 plants is that they can absorb more sunlight due to enlarged chloroplasts. Only 4% of plant species use this method of photosynthesis, despite being highly efficient.
Maize, sugar cane and pasture grasses are just a few C4 plants that we use in agriculture. These plants are responsible for 23% of biomass produced on land.
Scientists have been inspired by this natural adaptation and its potential; they are using genetic engineering to force C3 plants to use C4 pathways.
C3 plants species are predicted to be the worst affected by climate change. A 2°C change will decrease wheat and soya beans by 6-15% and rice will be increasingly vulnerable to droughts.
The Rice Project:
Scientists used this knowledge to kickstart their C4 rice project in 2008, with the aim of transforming the plant into a stable C4 crop. Rice lacks the special leaf structure of C4 plants meaning that it’s anatomy has had to be re-sculpted through the insertion of 20 to 30 genes.
It initially took the teams 7 years to transplant 6 genes within the rice plant, but new technology now allows multiple genes to be transferred at once. In 2017, the team announced that it has a proto-C4 rice species complete with the crucial intercellular channels and enlarged chloroplasts.
Jane Langdale, the coordinator of the project from the University of Oxford expects C4 rice plants to be in the field trials by 2030.
“We may not get a perfect C4 rice, but we will get varieties that are better yielding,” she says.
The International Rice Research Institute, which helped to start this project, has gone on to grow rice plants under atmospheres with a much higher CO2 concentration in order to stimulate what a C4 rice plant would be like. Through these calculations, data suggests that these plants would have a yield of up to 50% higher than the conventional rice crop.
As great as this sounds, C4 plants will not be enough to combat the mass nourishment needs of the 10 billion people by 2050. We don’t just need more efficiency; we need them to grow under much more extreme conditions.
“Water is going to be the rate-limiting factor for agriculture in the context of our global crisis,” says John Cushman at the University of Nevada in the US.
Global warming is set to increase the number of droughts we experience, with 45% of land predicted to experience more intense and long lasting droughts.
It is here that the tequila plant, otherwise known as Agave, comes into play. 7% of plant species use a different method of photosynthesis – these are CAM plants. Agave, pineapple, vanilla and aloe vera are just a few of these species.
CAM photosynthesis uses a similar method as C4 plants to bypass rubisco’s inefficiency. Pre-concentrating carbon dioxide to improve the performance of Rubisco, CAM plants split photosynthesis into time intervals. They open their stomata only at night when it is cool in order to capture carbon dioxide, when the sun rises, they close their stomata to reduce water loss. The plants then use the CO2 collected that night for photosynthesis throughout the day.
In recent years, agave has been increasingly planted for many more purposes than simply food. In Tan’s plantation, the agave is used to experiment with its potential to produce biofuel.
Biofuel is already widely used to supplement petrol in many countries, Tan’s project aims to create pure biofuel from agave plants. However, Biofuels are controversial due to the amount of water, land and other resources needed to grow them.
Tan and his team recently published their very first comprehensive life cycle assessment of agave bioethanol. The study examined the greenhouse gas emissions, the water consumption and the environmental pollution produced throughout the cycle.
The data revealed that agave bioethanol had a 60% lower impact on global warming compared with ethanol derived from maize and 30% lower than sugar cane. This is because it does not require irrigation or pesticides.
Agave is not the only CAM plant to hold immense potential in the race to harness the sun’s energy and feed billions of people. Cushman is leading a project that examines the prickly pear cacti potential for food, animal feed, bioethanol and biogas production. The species is native to the Americas and thrives anywhere with a temperature above freezing. This provides immense hope as it means that 1/5 of the planet which is unsuitable for other crops could be utilised to feed the population and fuel our lifestyles.
The prickly pear cacti is currently in field trials in Nevada. Research shows it has a similar productivity to maize and sugar cane.
Even if they are not planted for fuel or food, CAM plants have other benefits. In Brazil and Tunisia, prickly pear cacti were planted across an area equivalent to the grand canyon. Originally, they were planted for cattle feed but scientists at the International Centre for Agricultural Research in dry areas in Tunisia were pleasantly surprised to find that the CAM plants prevented erosion and boosted the soil’s nitrogen content.
This potential could be harnessed in areas such as South Africa which has seen an increase in extreme droughts over the past few years. Some farmers have begun to plant another CAM species called Spekboom in order to revive their scorched land.
CAM species have had some bad press in recent years as slow growers, but they don’t necessarily deserve this reputation. While compared on an annual level with crops like maize and soya beans, CAM plants take longer to flower and seed, this is because they continuously grow for many years, rather than only lasting one season like maize and soya.
The current issues:
Unfortunately, CAM species are currently faced with some problems. One of them is pollination – some CAM crops (including agave) flower and produce seeds at the end of their lifetime, a lifetime of which is relatively long.
This extended time period means that in order to be commercially effective, agave and other species must be propagated by cloning rather than waiting for them to naturally germinate seeds.
Another problem is the fact that it’s main pollinator, a species of bat, is threatened with extinction.
Cloning as a method of reproduction also creates two other issues. Cloning means they are genetically identical so have an increased vulnerability to pests and diseases due to their decreased variation.
Several sustainable tequila plant projects have been implemented in order to mitigate the impacts of cloned monoculture. They allow a small proportion of an agave plantation to remain natural and untouched, allowing it to grow through its natural lifespan without being cloned. This means that they can be pollinated naturally, and it is estimated that if 5% of agave planted on a hectare of land can flower and seed naturally it will provide enough food for 90 bats every night.
Could CAM plants be the future of agriculture?
There is potential for CAM species to follow in the footsteps of C4 plants and experiment with combining CAM with C3 plants to form a super crop.
In the past 5 years scientists have sequenced the genomes of several different CAM species but it is still a long way away from a CAM/C3 super crop.
Cushman and his team are concentrating on understanding CAM genetics with the aim of developing a CAM soya bean. This has the potential to reach field trials by 2025. In the meantime, more and more of the planet’s semi-arid land is set to be planted with crops like agave.
With scientists working hard to genetically modify our crops, we may be looking at a future of genetically spliced food. With a projected population of 10 billion by 2050, this may still not be enough. The race to harness the power of the sun is on.Science and Technology Rowenna Hoskin