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As an increasing number of regions across the globe enter a state of drought, the need for an immediate solution is necessary. In California, water levels have reached all time lows, and a drought State of Emergency has been declared. However, drought is not just a California problem. According to the United States Drought Monitor, droughts are gradually intensifying in much of the west and southwest due to global warming. As of early April, nearly 37 percent of the United States was in at least a moderate drought. Although water usage restrictions have been put in place by the government of California, the drought crisis places a looming threat to America's large crop industry. 
Known as the "breadbasket of the world", the U.S. alone exports more corn, wheat, and soybeans than any other country, and global food prices have rapidly escalated because of the crop shortage in America. Approximately 80 percent of the country's corn crops and 11 percent of its soybean crops have been affected. Major droughts in the past three years alone have collectively caused more than ten billion dollars in losses of crops in the U.S. As it becomes increasingly clear that America's drought crisis is the world's problem, research is being conducted in the hopes of discovering a mechanism to increase drought tolerance in plants.
A team of researchers led by the Scripps Research Institute and the University of California, San Diego has "decoded" the structure of a critical molecule that allows plants to survive during harsh external conditions. Their research has monitored the detailed physical and biochemical mechanisms associated with drought resistance such as gas exchange characteristics, seed filling rate, and the role of growth and development enzymes. They have discovered that a molecular structure allows for drought resistant mechanisms in the plant to initiate. Their newly solved structure shows a representation of a key plant hormone known as abscisic acid or ABA attached to PYR1, its target protein. When plants detect dry environmental conditions, they release ABA, which allows them to conserve water from the roots to the leaves. Additionally, ABA causes plants to close microscopic pores (stomatas) to prevent water loss, slow their own growth, and keep their seeds dormant in the ground. A technique known as X-ray crystallography was used to determine the three-dimensional positions of the individual atoms of the PYR1 and ABA complex. This process revealed two copies of PYR1 closely fit together in plant cells. There, they were targeted by ABA. Each PYR1 molecule has an interior open space where a hormone molecule such as ABA can fit neatly into one of the spaces. This structural change causes the PYR1 molecule to close, and thus initiates plant processes for drought resistance. 
Although it is possible for crops to be sprayed with ABA to assist their survival during droughts, ABA is expensive to synthesize and inactive inside plant cells. However, the agrochemical mandipropamid, which has been widely used to control late blight of crops, seems to mimic the action of ABA in engineered crops. Researchers in UC-Riverside worked with Arabidopsis, a plant widely used in labs, and the tomato plant to synthetically develop a new version of these plants' ABA receptors engineered to respond to mandipropamid instead of ABA. When these drought-affected plants were sprayed with mandipropamid, they survived drought conditions by turning on the ABA pathway, which closed their stomatas to prevent further water loss.
This discovery proves that synthetic biological innovation allows for crop improvement and can support a growing world population. Ultimately, the mandipropamid agrochemical was repurposed for a new application by genetically engineering a plant receptor, and this is something that has not been done before. It is anticipated that this reprogramming strategy using synthetic biology will allow scientists to control other plant traits, such as growth rates or disease resistance. Although this seems to be very promising for crop shortages caused by drought conditions, it is still a while away before mandipropamid and genetically engineered plant receptors can be applicable for all plants affected by drought.
References:
The Atlantic. America's Drought is the World's Problem (2009).
Phys.org. Researchers examine how to minimize drought impact on important food crops (2015).
Phys.org. Scientists reveal secrets of drought resistance (2009).
Phys.org. Scientists reveal underpinnings of drought tolerance in plants (2015).
Phys.org. World's largest drought resistance experiment on chickpeas under way at UVA (2014).
Finkelstein, R. Abscibic Acid Synthesis and Response. Arabidopsis Book. 2013; 11
ScienceDaily. Scientists reprogram plants for drought tolerance (2015).
Zhu, J. K. Salt and Drought Stress Signal Transduction in Plants. Annu Rev Plant Biol. 2002; 53: 247-73
Image Credits:
1. The Plant Drought Image is by Wikipedia user CSIRO and is distributed via Wikimedia Commons under a CC-BY license.
2. The ABA and PYR1 Molecule Complex is from Unnatural agrochemical ligands for engineered abscibic acid receptors by Rodriguez and Lozano-Juste and is distributed by Cell.com.
