THE US is in the grip of the worst drought in over 50 years. Across the nation, crops that should be at their greenest in July are instead small and withered, and are expected to produce 35 per cent less food than normal. During such droughts, plants that have been genetically modified to need less water become more attractive. But the expense and time needed to get GM plants to market has many looking for faster solutions.
One shortcut might lie in the plant microbiome - the consortium of fungi, bacteria and viruses that live in the root systems of every plant. Plants that live in extreme environments, such as the slopes of Mount Everest or the deserts of Utah, use the microbiome to survive stressful conditions. “Plants can’t do it on their own,” says Russell Rodriguez of the University of Washington in Seattle.
In exchange for nutrition, the symbiotic microorganisms help the plants take up nitrogen from the soil and protect them from heat, drought and disease-causing organisms.
In 2002, Rodriguez and colleagues were studying a grass - Dichanthelium lanuginosum - that grows at 70 °C at the geothermal hotsprings of Yellowstone. When the team sterilised the grass seeds to remove the fungi that grow inside the plant, also known as endophytes, the grass could no longer grow at high temperatures (Science, DOI: 10.1126/science.1078055).
That gave them an idea: perhaps transferring the microbiome of a drought-tolerant plant to a normal plant would help it use less water.
To test the idea, Rodriguez and his colleagues isolated spores from D. lanuginosum’s endophytes and sprayed them onto wheat seeds, which normally grow at temperatures up to 38 °C. With the spores, the wheat could grow at 70 °C and needed up to 50 per cent less water than normal (The ISME Journal, DOI: 10.1038/ismej.2007.106).
Different microbiomes can confer a range of superpowers to a number of crops. Rodriguez’s group have also isolated endophytes from a salt-loving dunegrass (Leymus mollis), and a strawberry plant (Fragaria vesca) that grows at high altitude at temperatures as low as 5 °C. Rice plants that had been sprayed with the fungi became able to tolerate salt and cold, respectively. They also grew five times larger and needed half the water of normal plants (PLoS One, DOI: 10.1371/journal.pone.0014823).
The results were immediate: within 24 hours of being sprayed, the seeds began sprouting a greater number of longer roots than untreated seeds, and the team found that they expressed genes involved in stress-resistance and drought-tolerance. That suggests endophytes could help crops cope with droughts like the one afflicting the US.
Rodriguez thinks the fungi are jump-starting the plants’ metabolism, although the exact mechanism is still unclear. “The plant has the ability to do all this, it just can’t get its act together without the fungi,” he says.
While attempts to genetically engineer plants to become drought-tolerant involve switching on metabolic pathways one at a time - a costly, drawn-out process - the fungi appear to activate them all in one go. “Nature’s figured it out, we haven’t,” says Jerry Barrow, now retired from New Mexico State University in Las Cruces.
Regina Redman, Rodriguez’s collaborator and partner, has developed the spores as a powder that can coat any crop seed. The pair have started a company,Symbiogenics, which is carrying out field trials on rice sprayed with the fungusTrichoderma, isolated from dunegrass. The fungus allows the rice to grow at cold temperatures in salty environments; rising sea-levels due to climate change makes salt-tolerance a sought-after trait.
Initial results show treated plants can yield 35 per cent more grain than untreated ones. A second field trial in corn is underway in Michigan, in the heart of the drought. Based on lab results, Rodriguez says they expect that the endophytes will lessen the amount of water the plant needs.
What’s more, lab tests suggest endophytes do not harm the plant in wet conditions, in contrast to drought-tolerant GM plants, which tend to grow poorly when the weather turns.
As a result, endophytes have a definite advantage over GM crops: farmers could decide whether to spray their seeds with them at the beginning of the planting season rather than gambling on a drought-tolerant variety, Rodriguez says. He is now trying to isolate endophytic fungi on different continents. If each continent has its own library, we could avoid introducing fungi from the US into a crop system in Africa, for instance.
Barrow and Mary Lucero, also at New Mexico State University, have transferred endophytic fungi and bacteria from the drought-tolerant desert plants Atriplex canescens and Bouteloua eriopoda into tomatoes, chillies and grasses that would serve as feedstock for cattle. They found that yields increased in all three crops (USDA Forest Service Proceedings, 2008, p 83).
Rather than isolating individual species of fungi, Lucero believes it might be more effective to harness the whole microbial community by mulching up drought-tolerant plants’ roots and growing crops in them. “We don’t really know how many microbes are in there; we’re looking at one little snapshot,” she says. The crosstalk between the different species of microbe might be as important as that between the microbes and the plants, she adds.
Either way, transferring plant microbiomes might be a fast way to meet the UN’s Food and Agriculture Organization’s goal to double global food production by 2050. With droughts such as the one affecting the US expected to become more frequent over coming decades, plant biologists aren’t hopeful that they can meet this goal through genetic engineering.
“Biotechs can’t work fast enough to meet the pressures of 7 billion people and climate change,” says Lucero. “To meet food demands, we need to adapt quickly. Microbial communities have always adapted quickly.”
- From issue 2875 of New Scientist magazine, page 8-9.
