CLOSE to the town of Ayr in Queensland, Australia, there is a field of unusual crops. The plants are a silvery shade of teal, with long fleshy leaves splaying out in all directions like thin, serrated knives. When Daniel Tan walks among them, the tallest stand two heads taller than him. There are thousands of these blue agaves here. Best known as the raw ingredient needed to make the fiery spirit tequila, they are more commonly found in Mexico than on Australia’s Pacific coast. Yet for Tan, a researcher at the University of Sydney, they are part of an impending global revolution.
We certainly need one. Plants provide us with food, fuel, building materials and natural beauty, all while locking away untold volumes of carbon dioxide that would otherwise crank up the planet’s thermostat. But as Earth’s population and temperature continue to rise, we will need more from our green allies. Our food requirements alone will be eye-watering. In 30 years, we may need to produce about 50 per cent more food to feed nearly 10 billion people – just as global warming is predicted to slash the yield of many major grain crops.
Researchers like Tan are looking to a radical solution, involving plants’ not-so-secret weapon: photosynthesis. We ultimately depend on this process, by which plants store energy from sunlight for everything that nourishes us. So it might seem odd to say it is scandalously inefficient. But it is – for most species. By understanding the secrets of plants such as agave with supercharged versions of photosynthesis, the hope is we can create a greener, cleaner, more secure future for us all.
Photosynthesis captures the power of sunlight to convert CO2 and water into sugars, which plants then use to fuel their growth. It is a wondrous thing. Yet despite the fact that evolution has had at least 2 billion years to perfect it, we have to content ourselves with the wonder that it is done at all, not that it is done well. The maximum conversion efficiency of solar energy to biomass in most plants is a disappointing 4.6 per cent.
This is true for the C3 version of photosynthesis, the metabolic process used by almost 90 per cent of plants, including wheat, rice and soya beans. The inefficiency comes down to an enzyme called rubisco. This piece of biochemical machinery picks up CO2 molecules and combines them with another compound to form a molecule containing three carbon atoms, as a first step in the production of sugar. The trouble is that 40 per cent of the time, rubisco slips and picks up oxygen instead, wasting energy. The problem gets worse when plants close their leaf pores, or stomata, to prevent water loss. Oxygen builds up inside the leaf and rubisco is even more likely to mistakenly grab it.
None of this mattered when rubisco evolved more than 3 billion years ago, when Earth’s atmosphere was rich in CO2 and almost free of oxygen. But as oxygen has become more abundant – ironically largely as a result of plants photosynthesising – it has become a roadblock to better photosynthesis.
Over the past 100 million years, some plants have found a workaround, evolving a process known as C4 photosynthesis. This splits the metabolic pathway involved in normal photosynthesis between two parts of their anatomy. First, they capture CO2 molecules in spongy cells called mesophylls beneath a leaf’s waxy protective layer, where they produce a four-carbon molecule. This molecule is then transported through special channels to cells clustered around leaf veins, where it is broken down to release CO2 again. Only here does rubisco come in, and with higher concentrations of CO2 present, it has fewer chances to grab oxygen. C4 plants also have enlarged chloroplasts, the parts of the cell where photosynthesis is conducted, which gives them an extra boost.
The benefits of these adaptations are stark. Although only about 4 per cent of plant species use C4 photosynthesis, they are responsible for about 23 per cent of the biomass produced on land. C4 crops include major sources of food such as maize and sugar cane, and pasture grasses that feed many of the animals we consume.
The warming planet is adding fuel to the idea that we could make more of these potent photosynthesis machines, for example by using genetic engineering to prod C3 plants into using the C4 pathway. Even if global warming is contained at 2°C this century, that could lower yields of C3 crops such as wheat, rice, maize and soya beans by between 6 and 15 per cent.
The C4 rice project is an international effort that kicked off in 2008 to transform the staple food of half the world’s population into a C4 crop. Rice lacks the special leaf structure of C4 plants, so its anatomy requires resculpting through the insertion of 20 or 30 new genes. “This is the biggest project in synthetic biology and genome engineering that’s around at the moment,” says Robert Furbank at the Australian National University in Canberra.
It initially took the team seven years to transplant six genes. But new techniques allowing multiple genes to be transferred at once moved the work along apace, and in 2017, the team announced it had created a proto-C4 rice species complete with those crucial intercellular channels and beefed-up chloroplasts.
Jane Langdale at the University of Oxford, coordinates the project. She expects C4 rice plants to be in field trials by 2030. “We may not get a perfect C4 rice, but we will get varieties that are better yielding,” she says. Meanwhile the International Rice Research Institute, which helped initiate the project, has grown rice plants under atmospheres with a higher than usual CO2 concentration in order to simulate what C4 rice would be like. Calculations based on these experiments suggest it would have a yield up to 50 per cent higher than the conventional crop.
But ambitious though the C4 rice project is, it won’t be enough. As the climate changes, we don’t just need crops that produce food more efficiently, we need them to do it under more taxing conditions. “Water is going to be the rate-limiting factor for agriculture in the context of our global climate crisis,” says John Cushman at the University of Nevada in the US. Drought is predicted to ravage many semi-arid regions over the coming century, with 45 per cent of land expected to have droughts that are more frequent, more intense and longer lasting. Turbocharged rice will be no use to anyone if it is simply too dry for it to grow.
There is, however, another trick up nature’s sleeve. About 7 per cent of plant species use a third kind of photosynthesis called crassulacean acid metabolism (CAM). Those silvery agave with the serrated leaves in Queensland are one; others include pineapple, aloe vera and vanilla.
Like C4 photosynthesis, CAM pre-concentrates CO2 to improve the performance of rubisco. But while C4 plants physically separate photosynthesis, CAM plants split it into time intervals. Unlike most vegetation, CAM plants open their stomata only in the cool of night to capture CO2. When the sun comes up, the stomata close to prevent water loss and the plants use stored CO2 to photosynthesise. Thanks to these adaptations, CAM plants only need about 20 per cent as much water as the least thirsty C3 and C4 crops.
Agave and its ilk have long been used for food (see “Sugar ‘n’ nice“). But they are increasingly being grown in new places and for unusual purposes. The point of Tan’s plantation is to test whether agave can be used to produce biofuel. Already used, for example, to supplement petrol in many parts of the world, biofuels are increasingly seen as a viable alternative to liquid fossil fuels, but are also controversial due to the land, water and other resources needed to grow them.
Tan and his colleagues recently published the first comprehensive life cycle assessment of agave bioethanol, examining greenhouse gas emissions, water consumption and environmental pollution. They found that it has a 60 per cent lower impact on global warming compared with ethanol derived from maize, and 30 per cent lower than that from sugar cane. It requires neither irrigation nor pesticides, because agave has no native pests in Australia.
Agave isn’t the only CAM crop with potential. Cushman leads a project growing the prickly pear cactus for food, animal feed, bioethanol and biogas. Native to the Americas, this cactus can thrive anywhere where the temperature remains mostly above freezing. This means a fifth of land that is unsuitable for other crops could be used to grow it. Field trials in Nevada have shown that a hectare of cactus produces as much as 44 tonnes of biomass each year, a similar productivity to maize and sugar cane.
Even if you don’t use the CAM plants for anything in particular, they are worth having around. Brazil and Tunisia have both planted prickly pears across areas equivalent to that of the Grand Canyon. Originally grown to feed cattle, scientists at the International Center for Agricultural Research in the Dry Areas in Tunisia observed that hedges comprising the cactus prevent erosion and boost the soil’s nitrogen content. In South Africa, which has seen extreme droughts over the past few years, some farmers are growing another CAM crop called spekboom to revive their parched land.
CAM plants are often thought to be slow to grow, but they don’t necessarily deserve this reputation. Annual crops like maize and soya beans grow fast, but only for one season, typically four to six months. Most CAM plants are perennials that grow continuously for years. “If you take seasonality out of the equation, some cultivated CAM species are just as productive,” says Cushman.
That isn’t the full story, however. Some CAM crops, including agave, flower and produce seeds only once towards the end of their lives. And their lives are long; one species of agave is known as the “century plant”, though in truth it lives about 30 years. To be commercially successful, agave must be propagated not by germinating seeds, but by cloning.
This creates several problems, including a world shortage of tequila in 2018 (see “Tequila sunset?“). A more serious issue is that the pollinators that feed on agave flower nectar – in Mexico this is largely bats – are threatened with extinction. The cloned crops, being so genetically similar, are also vulnerable to pests and disease. Prickly pears are at risk of infection by a stunting disease called “macho”. We don’t know its cause.
Cushman’s team is sequencing the DNA of prickly pear plants afflicted by macho to investigate the disease and prevent it from spreading. And several sustainable tequila projects have been established that allow a portion of the agave plantation to go untouched, so that the plants flower and can be pollinated naturally. It is estimated that if 5 per cent of the agave planted on a hectare of land is allowed to flower, that will provide enough food for about 90 bats each night.
Some are wondering if we can go further, with an effort akin to the C4 rice project that aims to combine the traits of C3 and CAM crops into the ultimate supercrop. Over the past five years, scientists have sequenced the genomes of several CAM plants. But there is a long road ahead. While we broadly understand how the CAM photosynthesis pathway operates, important details such as how regulatory enzymes fluctuate over time remain unclear.
For the moment, Cushman and his team are piecing together an understanding of CAM genetics with a view to developing a prototype CAM soya bean. He thinks we could have one in about five years, so it may be a while before we see them in the fields. In the meantime, more and more of Earth’s semi-arid land looks set to be planted with crops like agave. Its tall, teal leaves are going to become a lot more familiar.
newscientist.com, 15 July 2020
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