Relief for parched plants

Blazing heat and drought across Russia have withered much of the country's wheat crop, triggering a dramatic rise in food prices worldwide. But what if plants could survive long periods without water?

Image: Domesticated barley (Hordeum vulgare)
Wikimedia commons,
3268zauber

Groups of scientists around the world are working on engineering crops that can do just that. And with temperatures and drought frequency expected to continue to climb, and an increasing demand on dwindling fresh water supplies, the linkurl:need for drought-resistant plants;http://www.the-scientist.com/2009/09/1/30/1/ is more pressing than ever. "The number one limiting factor on [crop] yield in the world is available water," said Mark Lawson, an agricultural scientist at linkurl:Monsanto,;http://www.monsanto.com/Pages/default.aspx in St. Louis, Mo. And in many countries around the world, "drought is essentially an annual occurrence." But scientists are looking for ways to tweak plant physiology to enable crops to feed the world using significantly less water. Many research groups have focused on improving root systems, which linkurl:function like miniature straws to slurp water.;http://www.the-scientist.com/blog/display/57677/ Healthier, more abundant roots mean a plant can take up water from the soil more efficiently. Other scientists have begun to identify the subtle genetic differences between domesticated crops and their more drought-resistant wild relatives in search of water-saving tricks. Plant biologist linkurl:Sean Cutler;http://cutlerlab.blogspot.com/ of the University of California, Riverside, for example, has focused on the plant hormone abscisic acid (ABA). When a plant's roots sense a shortage of water in the surrounding soil, they synthesize ABA, which tells the plant to close microscopic pores on their leaves through which moisture can escape. Although closing these microscopic pores, known as stomata, prevents water loss, it also blocks the entry of carbon dioxide, a critical component of photosynthesis. Without sufficient CO2, plants can't photosynthesize and crop yield drops. By studying how ABA triggers stomata closing, Cutler hopes to harness the plant's ability to conserve water without negatively affecting photosynthesis. His goal is to create a molecule that selectively activates certain ABA pathways under drought conditions to maximize water conservation, as well as photosynthesis. Crop plants don't respond well to a lack of water, he said. "What we want to do is tinker with the physiology a bit so that we can get better yields when conditions aren't ideal." Paleobotanist linkurl:Robin Allaby,;http://www2.warwick.ac.uk/fac/sci/lifesci/people/rallaby/ at the University of Warwick in the UK, thinks that looking at the wild ancestors of current crops may be one of the keys to creating more drought-resistant plants. Domesticated barley (Hordeum vulgare) produces six smaller grains on each stalk, compared to two slightly larger grains on the wild variety. The greater number of grains and their higher protein content give them tremendous advantage as a food source, but previous research has shown that six-grain barley plants need more water than two-grain domestic barley.In Qasr Ibrim, an archaeological site in southern Egypt, Allaby found evidence that the domestic barley grown there reverted to the wild-type two grain, and he suspects the barley may have evolved rapidly in response to drought conditions. "When plants need to change," he said, "they can change very, very quickly. They're very evolvable." This swift evolutionary response to drought may give scientists genetic clues about how crops can better survive water stress. One such drought-resistant technique comes in the form of an enzyme known as proton pyrophosphatase in Arabidopsis. Proton pyrophosphatases reside in plant organelle membranes and maintain the proper electrical charge in the organelle by moving protons across the membrane. In Arabidopsis, this enzyme helps the plant survive stress from drought and salty soil by sequestering toxic sodium ions accumulated from the water the plants drink and storing them in the vacuole, a plant cell's storage tank, where they cannot harm the cell. Furthermore, linkurl:Roberto Gaxiola;http://sols.asu.edu/people/faculty/rgaxiola.php of Arizona State University recently found that the enzyme also enlarges the root system, which aids in water absorption under drought conditions. Gaxiola suspects the enzyme may increase the plant's sensitivity to growth hormones known as auxins, leading to greater root growth. "[Arabidopsis] can get water more efficiently because they have an enhanced root system." These plants still can experience drought, he added -- they just experience it later.Genetically modified corn that carries a single bacterial gene has also shown increased drought resistance. The agricultural giant Monsanto has focused on a type of transgenic corn carrying the cold shock protein B (CSPB) gene from Bacillus subtilis, a common soil bacterium. Plants under stress often have misfolded RNA and proteins. Lawson and his colleagues believe that the CSPB gene may mitigate some of that misfolding and help the plant cell run more efficiently, just as it does in Bacillus. This improves RNA translation and photosynthesis capability, which ultimately increases yield.The impact of the CSPB transgenic corn, or any other drought-resistant crop, is hard to calculate, Lawson said. Still, "being able to mitigate the effects that drought has on crops would be an extremely important characteristic to get into farmers' hands." Although their approaches may differ, these scientists believe that more than one pathway exists towards the final goal of improving drought tolerance in crops. Even seemingly small improvements may have large affects."If it's a severe drought, like what they have in Russia right now, and we could boost yields by 20 percent in [those] bad years," Cutler said, "that would be huge."
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[February 2008]