Palmer pigweed (Amaranthus palmeri) haunts the fields of many farmers around the nation. It grows more than six centimeters a day, can reach a stunning height of 2.5 meters, and can produce up to 600,000 seeds in its lifetime. Not to mention its notoriously tough stem that makes it nearly impossible for farming equipment to uproot.

Prior to the age of selective herbicides and GMOs (Genetically Modified Organisms), farmers fought weeds with tillage and hand weeding. However, tillage of large fields was both time-consuming and detrimental for the environment because it left valuable topsoil exposed to wind and water erosion. A more viable solution was imperative, and this came with the advent of Monsanto’s glyphosate active herbicide patent in the 1970s. Large-industrialized crop farmers around the world were finally able to control their unwanted weed populations. Sure, there were other herbicides before glyphosate, such as triazine, but those herbicides easily contaminated groundwater and harmed wildlife species through runoff waste. Roundup was a miracle chemical for farmers. It was able to kill a broad spectrum of weeds, and it was able to break down quickly to reduce its environmental impact.

Initially, glyphosate, or better known for its commercial name Roundup, was introduced as a nonselective herbicide (one that can kill all plants) before it became a narrow-spectrum or selective herbicide (one that kills a targeted weed). Once Roundup became a selective herbicide in 1990, sales took off as Monsanto created its Roundup Ready crops that were genetically modified to tolerate the chemical. Glyphosate herbicides kill plants by blocking the EPSPS enzyme, an enzyme involved in the synthesis of plant amino acids and vitamins. There are several ways in which a plant can be genetically modified to be tolerant to glyphosate. A widely used strategy is to incorporate a soil bacterium gene that produces a glyphosate-tolerant form of EPSPS. The incorporation of glyphosate-tolerant crops allowed farmers to spray their fields to kill weeds while leaving their crops unharmed. Today, Roundup Ready crops account for nearly 90 percent of soybeans and 70 percent of corn and cotton grown in the United States. However, Monsanto’s genetically modified crops have been notoriously nicknamed as terminator seeds because they are sterile. Each year, farmers must purchase the most recent strain of seed from Monsanto to continue using glyphosate herbicides.

But farmers sprayed so much Roundup that the very weeds which were targeted quickly evolved to survive it. Just as the heavy use of antibiotics led to the rise of drug-resistant supergerms, American farmer’s widespread usage of Roundup has led to the growth of new strains of “superweeds”. The first resistant species to pose an actual threat to agriculture production was spotted in a Delaware soybean farm in 2000. Since then, the problem has spread to almost uncontrollable levels with superweeds infesting more than seven million of the 170 million acres planted with corn, soybean, and cotton in the U.S. According to the U.S. Department of Agriculture (USDA), there are 383 known weed varieties that have the genetic defenses to tolerate one or more herbicides.

In the words of Mike Owen, a weed scientist at Iowa State University, what we’re seeing is Darwinian evolution in fast-forward. Herbicide resistance is widely recognized as the result of adaptive evolution of weed populations to the selection pressure exerted by glyphosate herbicides. The least herbicide-sensitive plants have a selective advantage in weed populations with heavy doses of herbicides. Thus, they are able to increase in frequency and establish a population shift towards a population of herbicide-resistant plants.

Even though the race for new herbicides to replace glyphosate is on, weed scientists argue that the golden solution of glyphosate herbicides is too valuable to be thrown away. It is estimated that the total amount of United States farmland afflicted with Round-up resistant weeds is relatively small. Most scientists agree that farming in a post-Roundup age will be costly and inefficient. Farmers must revert to crop rotations, cultivations, tillage, and limited herbicide usage. However, if farmers across the nation do their part by rotating crops constantly and using various herbicides, this will slow the development of glyphosate-resistant weeds over time.

References:

Delye, C. et al. Deciphering the evolution of herbicide resistance in weeds. Cell 29, 649-658 (2013).

ISAAA. Herbicide Tolerance Technology: Glyphosate and Glufosinate (2014).

MIT.edu. About Roundup Ready Crops (2009).

MIT.edu. The Roundup Ready Controversy (2009).

Nature. A growing problem (2014).

Nature. War on weeds loses ground (2012).

Neuman, W. & Pollack, A. "Farmers Cope with Roundup-Resistant Weeds." The New York Times. May 30, 2010.

Phys.org. US 'superweeds' epidemic shines spotlight on GMOs (2014).

ScienceDaily. Farmers Relying on Herbicide Roundup Lose Some of its Benefit (2009).

Image Credits:

1. The Amaranthus palmeri Image is by Wikipedia user Pompilid and is distributed via Wikimedia Commons under a CC-BY license.

2. The Consecutive steps of herbicide action figure is from Deciphering the evolution of herbicide resistance in weeds by Christophe Delye, Marie Jasieniuk, and Valerie Le Corre and is distributed by Cell.com.

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.