Agriculture would be transformed if crops could produce fertiliser as legumes do. New research suggests it might be easier than we thought
Editorial: “We need another green revolution“
THE 20th century brought us a wealth of life-changing innovations, from the mass-produced automobile to the television and the silicon chip. Arguably, though, the came from Fritz Haber in 1909. Haber was the first to pull nitrogen from the air to produce ammonia – a key component of the artificial fertilisers that allow food production to keep pace with an escalating world population.
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But evolution beat Haber to the prize of “free” fertiliser by some 60 million years. That was when legumes entered into a symbiotic relationship with nitrogen-fixing soil bacteria known as rhizobia and began generating their own supply of fertiliser from the air.
On 20 April, a group of 20 researchers gathered in Seattle for a meeting hosted by the Bill and Melinda Gates Foundation to discuss what’s been considered an impossible dream: to somehow teach the legume’s trick to all other major crops. If they are successful, we might need a lot less of the artificial fertilisers that Haber gave to the world – a legacy tainted by the ecological damage the fertilisers do to ecosystems (see “The ecological cost of feeding the world”).
The challenge seems relatively straightforward: recreate the symbiotic relationship between legumes and rhizobia in cereals (see diagram). Twenty years ago researchers predicted that they would soon have the job licked after discovering the group of bacterial genes that makes the fertiliser, including those for producing the nitrogen-fixing enzyme nitrogenase. Just transfer these into cereals and problem solved, or so they thought.
Preliminary attempts to modify the genetic make-up of crops suggested the researchers were wildly optimistic. Transferring one or a few functioning genes into a plant was just about possible at the time, but the wholesale transfer of the 50-odd nitrogen-fixing bacterial genes was far too ambitious.
“Twenty years ago researchers discovered the group of bacterial genes that makes the fertiliser”
“I don’t think anyone said ‘let’s give up’, but everyone realised it was a lot harder than they thought,” says of the University of Alberta in Edmonton, Canada, a key organiser of the Seattle meeting.
Now it seems that this level of genetic modification may not even be necessary. In January Fabienne Maillet at France’s Laboratory of Plant-Microorganism Interactions in Castanet-Tolosan demonstrated that the signalling network legumes use to communicate with rhizobia is unexpectedly similar to the one cereals already use to talk to the fungi in soil that deliver other vital nutrients to plant roots. Crucially, the “let me in” chemical signals the soil fungi produce to gain access to the roots of plants are broadly the same as the signals that rhizobia use to signal their presence to legumes (Nature, ).
“We thought we had to reinvent the whole process in cereals, and that became an argument against it,” says of the John Innes Centre in Norwich, UK. “Now, we’ve discovered that the machinery to do it all is already there in cereals, so it’s a question of tweaking it, not reinventing everything.”
Oldroyd says that the links between land plants and fungi stretch back some 400 million years, dwarfing rhizobia’s 60-million-year-old pas-de-deux with legumes. The implication is that rhizobia “stole” the ancient fungal signal to communicate with plants, using it to trick their way into legumes and form their nitrogen-fixing partnership. This suggests that all other plants have the machinery to evolve the relationship too. Earlier this year biologists proved the principle in the only non-legume, as far as they know, that has naturally evolved a symbiotic relationship with rhizobia. The small tropical tree Parasponia formed the attachment by modifying the signalling pathways used to communicate with symbiotic fungi (Science, ).
“All plants have the machinery to evolve a nitrogen-fixing partnership themselves”
Exploiting this machinery will provide an easier route to fertiliser-free cereals, says Oldroyd. The priority will be to tweak cereals so that they recognise and admit rhizobia. “Down-regulating the plant defences and allowing colonisation of the root is the key step,” he says. With luck, he adds, other important components – such as forming the nodules that provide the rhizobia with a suitable habitat – might then fall into place automatically.
Oldroyd discussed his plans in Seattle, as did , also at the John Innes Centre. Dixon is still convinced that rhizobia’s nitrogen-fixing toolkit can be inserted successfully into crops, giving existing organelles such as chloroplasts a new role as nitrogen fixers. Twenty years after the idea first arose it is finally beginning to bear fruit: recently, in a study too commercially sensitive to be published, Dixon and colleagues loaded nitrogen-fixing genes into the chloroplasts of algae. “It’s only a beginning,” Dixon says.
of the University of Bremen, Germany, was also in Seattle to present a “third way” towards nitrogen fixation: simply find plant-dwelling bacteria which fix nitrogen, but don’t need to form nodules to pass it to their plant hosts. “We discovered the first one in 1986,” she says. Since then, she’s been studying how the BH72 strain of the bacterial genus Azoarcus fixes nitrogen in kallar grass, native to Pakistan. Almost all the nitrogen-fixing – or “diazotrophic” – bacteria she’s identified so far are found in grasses, the most productive in varieties of Brazilian sugar cane. They make so much nitrogen themselves, she says, that they require only about a quarter of the artificial fertiliser usually applied.
Good says it is difficult to predict which option will gain favour and that all are worth pursuing to help small farms increase their productivity while minimising environmental damage. But he acknowledges that optimism on its own is not enough. “We need to be realistic because we’ve over-promised and under-delivered in this field before,” he says. “We’ll make lots of mistakes, but we’re at a stage where we can do it a lot faster and more efficiently than before.”
The ecological cost of feeding the world
Although artificial fertiliser helps to feed half the world’s population by boosting crop yields, we pay a heavy price in the damage done to human health and the environment. These words of warning come from 200 researchers who last month launched the , the first continent-wide assessment of the impact of nitrogen pollution on health and the environment.
It estimated that nitrogen pollution in Europe costs each European citizen around $1000 per year, 40 per cent of it to repair the damage to agriculture. Because farmers apply fertiliser so liberally, 40 to 70 per cent of the nitrate it contains drains off into waterways and the sea. Here, it nourishes algae which multiply and suffocate other marine and aquatic life.
Following on from the report, 350 scientists met in Edinburgh, UK, to discuss the environmental impact of nitrogen. They launched the to call for global action to curb pollution, and for farmers to use nitrates more efficiently.
of the University of Alberta in Edmonton, Canada, who organised last month’s summit on “self-fertilising cereals”, says that the escalating damage from fertiliser pollution, coupled with the threat of climate change, make it imperative to pump resources into the agricultural equivalent of the 1960s “moon shot” – although it could take 10 to 25 years to succeed.
