Issue Date: September 28, 2009
Getting By On Little Water
According to the witty accusation often attributed to Mark Twain, everyone talks about the weather, but no one does anything about it. Scientists still may not be able to do anything about the weather, but they are working to make plants that thrive despite it.
By probing the biological and chemical bases of some plants’ innate ability to flourish in regions and periods of limited precipitation, researchers in industry, academia, and government labs are learning to confer drought tolerance to major food, feed, and fiber crops including rice, corn, and cotton. These scientific advances, which have been mounting in recent months, come as the combined effects of surging world population and climate change may be poised to deliver a potent one-two punch to global food supplies in the coming years.
According to estimates by the United Nations’ Food & Agriculture Organization (FAO), by 2030, the world’s farmers will need to grow 30% more grain than they do currently to feed a projected global population of 8.3 billion people. Coupled with predictions by FAO and the National Oceanic & Atmospheric Administration of more frequent and more intense dry spells in the years ahead, increasing agricultural output to meet global needs is going to be a tough challenge.
Farming more land isn’t an option. “There really isn’t any additional arable land being brought into production nowadays, and there are few places in the world where you could even look to find such land,” says David A. Fischhoff, vice president for technology strategy and development at agriculture technology company Monsanto. “We need to live with the footprint we have and at the same time produce enough food to feed the world’s growing population,” he adds.
Robert Berendes, head of business development at Switzerland-based Syngenta, points out that economic growth further increases the challenge. As per capita income increases, as is happening in China, for example, demand for food—and in particular meat, which depends on animal feed—increases faster than population. “So we need to produce more food using the same amount of land but with reduced amounts of water,” Berendes says. For that reason, researchers in various organizations are focusing on developing plants that produce ample yields even when water is scarce.
Monsanto, for example, is in the final stage of commercializing drought-tolerant corn. Earlier this summer, in collaboration with Germany-based BASF, Monsanto issued a statement disclosing the identity of the gene that will lie at the heart of what the companies are calling the world’s first biotechnology-derived (meaning genetically engineered) drought-tolerant crop. The companies say the product will enhance yields relative to common varieties of corn by some 10% in areas prone to the kind of moderate drought faced by western Nebraska. That region tends to get less rainfall than eastern parts of the Corn Belt. The biotech product is targeted for release in about two years.
Other companies, including Syngenta and DuPont, are also gearing up to sell drought-tolerant varieties of corn. Several other crops including rice, wheat, soy, canola, sunflower, and cotton are also being studied by various agribusiness companies that aim to improve yields of those crops during dry spells.
To agronomists, drought refers to a water shortage that causes stress in a given crop. A severe drought will destroy any crop. But even a moderate lack of water can stress a plant. Corn, for example, may show effects of moderate water shortage through wilting, loss of leaves, a decrease in the size and number of kernels per ear, or a decrease in the number of ears raised per acre.
Many generations of breeders have slowly improved crop varieties, by crossing (mating) pairs of plants that exhibit certain characteristics, such as high yield under ideal conditions and strong stalks under dry conditions; waiting for the “offspring” to mature; and then selecting the ones with the best traits for the next round of mating. That traditional approach to breeding, which is slow and labor intensive, is giving way to much faster breeding techniques supported by molecular biology, Berendes says. By examining plant DNA in search of telltale signs of traits before breeding, researchers can now predict which crosses are most likely to be successful, thereby dramatically saving time.
Even so, thousands of experimental lines of plants are bred and tested under controlled conditions in Monsanto’s drought-tolerance program, according to Robert S. Reiter, who serves as the company’s vice president of breeding technology. “It’s like looking at an enormous number of combinations of naturally occurring corn genes and searching for ones that have the best ability to tolerate drought,” he explains. But even with all the cross-breeding, some genetic variations that could further enhance corn’s ability to stand up to dry growing conditions still won’t be found in corn’s gene pool. That’s where genetic engineering comes in.
Monsanto and BASF found that a gene first identified in bacteria that were stressed by subjecting them to cold temperatures can help plants tolerate stress induced by drought. That gene is cspB, from Bacillus subtilis. It codes for a cold-shock protein that acts as an RNA chaperone. Such chaperones bind to and assist RNA in carrying out its basic biological functions by correcting misfolded forms of RNA. That bacterial gene has been inserted into the companies’ drought-tolerant corn.
Normally when a plant undergoes some kind of stress, it starts to “hunker down, shut off fundamental processes, and conserve resources,” Reiter says. In contrast, corn enhanced with the drought-tolerance gene remains in a more normal metabolic state when water is scarce. Reiter notes that years of controlled field tests have shown a yield enhancement relative to corn lacking the extra gene of about 6–15%, depending on the year and drought intensity. Importantly, however, that stress tolerance does not lead to a decrease in crop productivity when the plants are well watered, he adds.
Unlike corn, sorghum tolerates drought well naturally. That cereal species feeds millions of people mainly in developing countries and is used to make biofuel in the U.S. It originates from Africa and is well adapted to growing in arid climates. Details of sorghum’s knack for thriving in hot, dry weather aren’t fully understood. But earlier this year, a large team of researchers led by plant geneticist Andrew H. Paterson of the University of Georgia, Athens, took a key step toward uncovering the biochemical basis of sorghum’s inherent drought tolerance by determining and analyzing sorghum’s complete genome sequence and comparing it with other plants’ (Nature 2009, 457, 551).
According to the team, sorghum’s extra copies of certain genes coupled with its unusual photosynthetic pathway may play key roles in the plant’s overall drought response. Specifically, compared with the relatively small rice genome, sorghum has four additional copies of a certain regulatory gene that activates other genes when stressed by drought. Sorghum also contains a surplus of hydrogen-bond-breaking enzymes known as expansins that are believed to be linked to plant durability.
Furthermore, photosynthesis in sorghum proceeds by way of the C4 pathway, so called because the process initially combines CO2 with a three-carbon compound to make a C4 molecule (oxaloacetic acid). Corn, which is a distant relative of sorghum, is also a C4 plant. In contrast, rice, wheat, and most other plants depend on C3 photosynthesis, which incorporates CO2 to make a three-carbon compound in the first step. The C4 process involves a fast-acting enzyme that enables plants to take up atmospheric CO2 through pores known as stomata faster than C3 varieties. When the pores open to allow CO2 to diffuse in and O2 to diffuse out, water also escapes from the plant. The upshot is that C4 plants keep their stomata closed more of the time than C3 plants do and thereby efficiently minimize water loss—an important survival trick for plants growing in arid conditions.
Direct chemical methods for mitigating the effects of drought are also in the works. One example, based on the properties of 1-methylcyclopropene, is being developed jointly by Syngenta and AgroFresh, a Dow subsidiary. Ordinarily, crops exposed to drought and other types of stress produce ethylene, which triggers a number of negative responses including wilting, reduced photosynthesis, and poor pollination. The companies say that treating stressed corn, cereals, and other crops with Invinsa, a soon-to-be-released product based on 1-methylcyclopropene, protects the plants from the effects of moderate drought. They explain that the compound binds to the plants’ ethylene receptors, thereby blocking ethylene from transmitting the stress signals.
Abscisic acid is another plant hormone that may lie at the center of future drought-protection treatments for crops. For years, researchers have known that when plants dry up or are exposed to cold, levels of abscisic acid in leaves soar. The rising concentration of abscisic acid inhibits plant growth, conserves water, and generally signals plants to slow down, which helps them cope with adverse growing conditions. Applying abscisic acid directly to farm crops isn’t an option because the compound is expensive and sensitive to light. So researchers have tried to tease out the molecular basis of abscisic acid’s action to find suitable chemical treatments that can protect crop yields in the face of drought.
Earlier this year, a team of researchers led by University of California, Riverside, plant biologist Sean R. Cutler reported in Science the discovery of such a molecule (2009, 324, 1068). The group found that pyrabactin, a stable synthetic sulfonamide, turns on the abscisic acid signaling pathway in the plant Arabidopsis thaliana, a widely studied model organism. Cutler explains that abscisic acid and its mimic, pyrabactin, trigger the signaling process by binding to a protein called PYR1, which in turn binds to and inhibits the action of a phosphatase enzyme.
A half century ago, advances in breeding led to enormous boosts in grain yields, bringing self-sufficiency to developing countries. Labeled the green revolution, those agricultural practices enabled food production to keep pace with worldwide population growth. Since then, crop yields have increased dramatically, but so has population—and it continues to grow.
To meet skyrocketing food-production needs, scientists such as Monsanto’s Fischhoff are working to double yields in the U.S. of key crops such as corn by 2030 relative to yields in 2000. The strategy, he says, is to continue improving plant traits such as drought tolerance through further advances in breeding, biotechnology, and agronomic practices. “It’s a tall order,” Fischhoff admits, “but I think it’s an achievable goal.”
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