When rains fail somewhere, it becomes a problem everywhere. Drought’s economic effects are almost without parallel: Neither disease nor insect pests cost farmers more. Its human effects are profound: When fields dry and crops wither, food prices go up and people go hungry. When people go hungry, they become desperate, sometimes violent. A drought-related spike in global food prices three years ago, for example, sparked riots throughout the developing world.
For now, at least, no one can change the weather. But MU scientists believe they can do the next best thing — changing the plants that depend on it.
Specifically, MU scientists are asking fundamental questions about certain cellular adaptations that help some crops deal with drought better than others. What chemical adjustments do these plants’ cells make in order to tolerate long stretches without rain? What triggers the beneficial cellular changes? Can these biochemical mechanisms be transferred from one crop variety to another?
Since the earliest days of agriculture, farmers in dry places have been at the mercy of the weather. MU researchers working at the University’s Bradford Farm Research and Extension Center near Columbia are turning that approach upside down: They’re creating droughts.
It’s possible thanks to a barn-sized scientific instrument they’ve constructed at the Bradford Farm site. That instrument, called a drought simulator, or rain shelter, is designed to create a precisely controlled mini-drought on test plots.
The shelter consists of two 50-foot by 100-foot buildings. Its roofs and walls are built of translucent polycarbonate plastic panels. The walls’ bases are mounted on wheels that fit into what look like small railroad tracks. When it rains, remote sensors measure the rainfall. When a predetermined moisture level is reached, the shelters rumble down the tracks to shield the test plot from more rain.
This setup, built with part of a $1.5 million grant from the Missouri Life Sciences Research Board, allows scientists to exactly calibrate the severity of the drought they create. Two more drought simulators are planned for other regions of the state.
“This is not a new idea, by any means,” says MU’s Felix Fritschi, an assistant professor of plant sciences and one of four co-principal investigators on the project. For decades, he says, scientists have operated less sophisticated versions of drought simulators. Today’s technology gives his simulators a big performance boost. “Now we can deploy them using the rain sensors, “ he says. “We don’t have to go out there every time it looks like rain.”
This new facility allows researchers to apply the same scientific controls they would build into laboratory experiments to study actual conditions in the field. That’s important because every drought is different — in length, severity and the stage of development at which a crop plant is stressed.
With the drought simulator, “we now have control over the time the drought hits, how intense the drought is and how long it lasts,” Fritschi says. “This facility will allow us to begin to address the more complex questions of what’s happening in the field.”
few of these questions are more fraught with difficulty than those involving the molecular processes by which plant roots adapt their growth processes to dry conditions.
Scientists know that drought can stimulate root growth, which can help plants search for moisture deeper in the soil. That’s the reason, for example, that homeowners should not over-water newly planted lawns, so grass roots have a chance to get established and survive dry spells later in the season.
“Roots are a difficult part of a plant to study because they’re underground,” says Robert Sharp, an MU plant science professor whose research team has developed model systems to study root growth in the laboratory. “We know much less about root biology than we know about shoot biology,” he says. “It’s an understudied but vitally important area, one that desperately needs more research interest.”
It’s also an area in which MU has a core of research expertise that’s probably unique in the United States, Sharp says.
Sharp’s lab is studying how the roots of corn and soybean plants adapt and continue to grow as soil dries. He’s also identifying the plant genes, proteins and metabolites that trigger those adaptive changes.
Sharp and his team study seedlings grown in a “humid room” — a temperature-controlled dark room maintained at near-saturation relative humidity — which simulates the underground environment roots experience. They also examine the root growth of more mature plants. These are grown without watering in 5-foot-tall Plexiglas tubes housed in a “controlled-environment” chamber. Both systems allow scientists to observe root growth underground and then harvest the roots to study their cell biology.
Scientists like Sharp have found that a lack of water induces some plants to produce certain chemical compounds, among them hormones that are of interest to drought researchers. “Plants, like animals, have hormones, and different hormones have different functions,” he says. “The one hormone that has been studied the most in plants under drought conditions is abscisic acid or ABA.”
The substance is present at low levels in well-watered plants, but it accumulates to very high levels in plants stressed by drought. Sharp’s team discovered that the accumulation of ABA in corn roots is required for their successful adaptation and growth under dry conditions. The field drought simulators will help the MU team begin to examine the importance of this and other phenomena in roots experiencing drought under natural conditions.
MU plant scientist Gary Stacey is taking a “systems” approach in developing a model that will use single plant cells. For years, Stacey has been studying how Rhizobium, a genus of bacteria found in soil, colonizes soybean roots. He is expanding that research to also study how, during droughts, factors such as water availability and soil temperature influence root growth. Stacey is particularly interested in soybean root hairs, single-celled structures that grow on root surfaces and look like fuzz to the naked eye.
Although soybean plants have many different types of tissues, root hairs are particularly important, says Stacey, an endowed professor of soybean biotechnology: “Their primary function is to increase root surface area and to maximize nutrient and water uptake.”
Investigating root hairs involves raising different genetic strains of soybeans in growth chambers, then subjecting the plants to varying levels and combinations of heat and dehydration stress. Next, the researchers freeze tissue samples in liquid nitrogen, fixing a biochemical snapshot of a specific growth phase. They then carefully swirl the samples to shear off the root hairs for further study.
The aim, Stacey says, is to use advanced genomics to study drought-related changes in proteins, metabolites and other compounds inside cells. “Doing drought research is not easy. You would think you could just take water away, but it’s not that simple.”
There are, he continues, many other variables to consider: sunlight levels, water infiltration of soils, and plant-atmosphere gas exchanges among them. “Look at the most difficult scientific research problems, such as cancer. The problem with cancer is it’s a single word, but it’s many different diseases.”
This complexity is why researchers take a systemic approach to drought. “In the 21st century,” says Stacey, “we’re getting a lot more interested in understanding how molecules interact with each other.”
That interest has led Stacey to team up with Henry Nguyen, another MU geneticist and plant sciences endowed professor, to track those interactions. The goal, the scientists say, is to combine their expertise in plant physiology, molecular genetics and computational biology to create a comprehensive database of all proteins and metabolites — substances involved in or produced by a plant’s metabolism — found in soybean seeds, leaves and roots.
“Plants turn certain proteins and metabolites on or off or up or down in response to environmental cues,” says Nguyen, who also serves as director of MU’s National Center for Soybean Biotechnology. Charting these, he says, will help the researchers track molecular-level changes in soybeans grown under drought and other stresses.
Eventually, the scientists hope, the information will allow plant breeders to develop more drought-tolerant plants. It will also help future investigators create computer models for predicting the effects of environmental conditions.
Drought weakens plants’ natural defenses, making them more susceptible to attacks from insects and disease. Bruce Hibbard is studying the interaction between drought and an insect pest called the Western corn rootworm. Either factor can affect crop yields. Taken together, they can be a devastating one-two punch on corn crops.
“If you have both rootworm and drought, it’s not two plus two equals four; it’s more like two plus two equals 10 in terms of yield damage,” says Hibbard, a researcher with the USDA’s Agricultural Research Service and an MU adjunct plant science professor.
“Now that there are transgenic genes that work pretty well for controlling rootworm, and genes coming to the market that are supposed to have drought tolerance, we decided to look at this interaction and try to understand it a little better,” he says.
The working theory might be fairly basic, Hibbard says. “Rootworms eat the corn roots, and therefore you don’t have the ability to bring water and minerals to the plant.”
There’s another impact as well. An attack by rootworms can stimulate re-growth in the corn plant’s root system. “The interaction of root re-growth and drought is also not understood, and this is another aspect we will evaluate,” says Hibbard.
mel oliver, another ARS scientist and an adjunct plant sciences professor at MU, is exploring how plants process and retain the water these roots manage to extract. His findings thus far represent yet another promising approach to mitigating drought’s impact on agriculture. “Most crop plants avoid drought. They put mechanisms in place to retain water and keep water from being lost. It’s not really drought tolerance; it’s drought avoidance,” he says.
Maize, for instance, responds to dehydration by producing compounds called osmolytes within its cells. Osmolytes help the plant retain water and, along with antioxidants, protect cells from damage. “You also get new proteins being synthesized that help to protect plant cells,” he says. “We don’t know much about this in maize, and that’s one of the things we’re interested in finding out.”
Oliver also studies how some plants are able to survive severe dehydration — what are called “resurrection plants.” Some mosses, for example, can dry to a brittle brown and survive for years in a dried state. When water is added to them they turn green and come back to life within minutes. “The hope is that we can look at novel ways in which plants tolerate drying and use those in either a breeding program or through biotechnology,” he says.
At the moment, his lab is considering two closely related South African grasses belonging to the Sporobolus group. One species is drought tolerant. The other isn’t. Sporobolus’ response to drought is one of the very few differences between them — what is called a “phylogenetic sister group” contrast.
Oliver and his colleagues examined what effect dehydration had on the stew of metabolic by-products in each variety of Sporobolus. They found that the drought-tolerant variety was producing and storing amino acid compounds that help prevent water loss and store nitrogen. The sensitive plants don’t store these compounds, but use them instead for growth.
“The tolerant one has evolved a priming mechanism, because it knows it’s going to dry,” says Oliver. “It really is sitting there ready to dry. The sensitive one isn’t ready. It doesn’t have time to respond to drying, so it dies. We want to look for plants that are a little better primed. By knowing what’s involved in tolerance, we can start to engineer for it or look for natural variation in plants.”
Drought has always been problematic for agriculture, he adds. And it’s only going to get worse. “Water is getting more and more limited,” says Oliver. “You can look at drought in so many ways: from a social point of view, an economic point of view, an agricultural point of view, and from a food security and national security point of view. Drought has a huge impact and huge consequences across all the doings of mankind.”