Agave may be most associated with tequila, but this plant has a less familiar use — it’s teaching scientists about how to craft more drought-resistant plants.
The hardy succulent, along with species like prickly pear (an edible cactus), pineapple and vanilla orchids, has evolved over millions of years to perform a different kind of photosynthesis that allows the plants to survive in semiarid environments where water isn’t always readily available.
The process is called crassulacean acid metabolism, or CAM, and a small group of scientists have been studying it for several decades because the plants that have it use less water. However, it has only been in the last couple of years that a growing number of researchers have been attempting to fully identify and transfer this photosynthetic pathway to other plant species.
Re-creating an entire metabolic pathway in a plant is far from a simple task. Once scientists figure out all the genes associated with its basic function, as well as its regulation, they then have to find a way to add that genetic material into the target plant, or make existing genes and proteins within the plant work the way they want them to. Altogether, that could involve somewhere around 100 genes, the researchers said, though they don’t know the exact number yet.
Xiaohan Yang, a staff scientist in the Biosciences Division at Oak Ridge National Laboratory, is one of the researchers working to figure out how to get CAM to work in other types of plants. He said interest in CAM has increased rapidly in the last few years alone, as concern about the effects of climate change on drought has gone up and more funding from the federal government has come in.
What makes photosynthesis in agave and cactus so different? Unlike most plants that take up carbon dioxide through stomata in their leaves during the day (known as C3 and C4 plants), CAM plants absorb most of their CO2 at night. This timing shift means less water evaporates off of the leaves through transpiration. In fact, CAM plants require between a fifth and a third of the water that C3 and C4 plants need, respectively.
However, CAM plants also need a way of storing carbon overnight, because just like other plants, they cannot use it to build energy reserves like sugars and starches without sunlight. They do this by temporarily fixing carbon in a transient pool of mostly malic acid. When the sun rises, the plants break down the organic acids, releasing the CO2. At this point, the plant is able to perform photosynthesis like a C3 plant, except the stomata don’t have to stay open because the carbon is already available in the leaf.
The challenge for researchers like Yang is to find a way to get other plants to create this nocturnal carbon storage. Since the genomes of a number of different CAM plants have been sequenced in the past two years, researchers are beginning to develop a better understanding of how the pathway works.
"We have a very good idea of what genes are important for CAM species," Yang said. "Right now, we are working on how those genes come together, and then we test their efficiency."
Tougher plants for a tough global food problem
If they are successful and CAM plants are productive enough, the research could potentially have a significant impact on global food security. As human-caused climate change drives temperatures higher over the course of the century, the Intergovernmental Panel on Climate Change (IPCC) predicts droughts will likely become longer and more severe. At the same time, the global population is expected to exceed 9 billion, putting added pressure on available water resources. Under these conditions, having more plants that can produce food for humans and animals, as well as fiber and fuel with less water, will become more important, according to the researchers.
The federal government is interested on its applications to biofuels. Yang is part of a five-year research project funded by the Department of Energy, which is focused on engineering the CAM photosynthetic pathway into bioenergy crops. His lab is focusing specifically on poplar, a fast-growing tree used for its woody biomass. The $14.3 million grant is divided among five institutions in the United States and the United Kingdom, with $6.3 million going to Oak Ridge National Laboratory; the University of Tennessee, Knoxville; and Newcastle University. Another sub-team that included the University of Nevada, Reno, and the University of Liverpool received $7.6 million in funding.
Last week, 51 researchers from nine different countries co-authored an article published in the journal New Phytologist‘s debut Viewpoints section, broadly outlining what needs to happen in the field in order to further CAM research and engineering. Yang was the article’s lead author. He said the "road map" article was meant not just for scientists studying CAM, but also for a more general audience to introduce them to the research. The authors also discussed how existing CAM plants could be developed into major crops.
Research on CAM plants is part of a broader international effort to identify and eventually engineer genetic traits that would enable plants to become more drought-resistant. In a study published in the journal Plant Cell last month, researchers at Seoul National University in South Korea found an important transcription factor (NAC016) in the model plant Arabidopsis thaliana that promotes drought stress response. According to the study’s lead author, Nam-Chon Paek, the findings could provide insight into how to develop drought-tolerant plants through either conventional breeding or biotechnology.
Earlier this week, researchers at Kansas State University published a study that showed identifying specific genomic signatures associated with adaptation could predict how different varieties of sorghum would respond to environmental stresses like drought.
"This is an approach that allows us to look at varieties in a crop gene bank and say, ‘Hey, there is something useful in this variety,’" said Geoffrey Morris, an assistant professor of agronomy at KSU and lead author of the study.
The study doesn’t connect plant genetics to specific traits; rather, the method is based on an association between geographic location and certain genomic markers, he said.
"What we’re looking at is near-term use. Most crop genetic laboratories can take advantage of this," Morris said, adding that research into CAM engineering is much more specialized and could take many years to complete.
Though CAM researchers can’t say for sure how long it will take them to develop a CAM pathway in a C3 or C4 plant, Yang said that conceptually, at least, their approach was the best.
Evolution played its part
"Evolution already gave us the answer: CAM evolved to be the most [water] efficient," he said.
Unlike other plants that grow deeper root systems or die back during dry periods, CAM species have a way to conserve water "like a camel," he said.
Yang and his colleagues aren’t trying to make a C3 that could fix carbon like a CAM plant all the time. Instead, they are planning to create a C3 hybrid that will be able to switch to a more water-saving metabolism if exposed to drought or high-salinity conditions. Or, as report co-author John Cushman referred to it, they want to make a plant that would have "CAM on demand."
The researchers are focusing on creating a hybrid with C3 plants because CAM plants evolved from them somewhere between 10 million and 30 million years ago, according to Cushman, who is a professor in the Department of Biochemistry and Molecular Biology at the University of Nevada, Reno, and part of the five-year research project. Before that can happen, there is still a great deal of research that needs to be done, he said.
"The pathways are very complex; you don’t want to re-engineer something until we have a good sense of the blueprint," he said. "The way these pathways have evolved, there is a concerted set of genes involved … we don’t think a few changes around the edges will be enough."
So far, it’s unclear just how many genes are involved in CAM. While adding a new metabolic pathway to plant species will be a complicated process, CAM plants do share a lot of molecular components and genes with C3 and C4 plants, which may help to facilitate the process, Yang said.
One reason for optimism is that these dually capable plants already exist in nature. Clusia pratensis is what is known as a facultative CAM. With normal rainfall, the Panamanian plant will take in CO2 during the day as it acts like a C3 plant, but during dry periods, it begins to take in CO2 at night.
"This is the perfect example in nature that [C3 and CAM] can co-exist in a single plant," Yang said. "That species is kind of magic."
Clusia pratensis isn’t the only facultative CAM plant. Cushman studies Mesembryanthemum crystallinum, also known as the common ice plant. Research has helped to show what genes are recruited in CAM, though the approach may not be uniform from species to species.
Ideally, the best C3 plant to use in CAM engineering would be one that already has a fully sequenced genome, can be easily transformed, would have a large impact on food or biofuel production, and isn’t currently able to thrive in dryland areas.
Over the remaining two years of the DOE grant, researchers will work to transfer CAM into the model plant Arabidopsis thaliana and the poplar.
"We’ll start small to work with the core metabolism, and then we’ll work outward," Cushman said.
Eventually, if they are successful with transferring CAM into C3 plants, the researchers may add CAM to C4 plants like corn and sorghum to increase their water efficiency, as well, the report’s authors wrote.