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Kaitlin Palla, University of Tennessee Bredesen Center for Interdisciplinary Research and Graduate Education student, studies agave plants in Oak Ridge National Laboratory's greenhouse as part of the lab's research into a water efficient mode of photosynthesis called crassulacean acid metabolism, or CAM. Credit: ORNL

Hot regions will become hotter, and dry areas will become drier: this is a prediction of global warming. What that means for the future of important bioenergy and food crops is yet to be seen.  However, scientists from around the globe are not wasting any time. 

Researchers, including John Cushman, Ph.D., from the University of Nevada, and Xiaohan Yang, Ph.D., from the Department of Energy’s Oak Ridge National Laboratory are delving into the mechanisms behind how certain crops can tolerate severe drought, with the hopes of translating the new understandings into a way to mitigate the potential effects of climate change on important food and bioenergy sources.

“It’s simple really,” Cushman, a biochemist and molecular biologist, said. “We release carbon dioxide and other greenhouse gases into the atmosphere and average temperatures around the world increase. More heat leads to greater soil drying and more water loss from plants so that they can stay cool, both of which lead to greater probability of drought stress. So, one of the predictions of global warming is that with all of this heating, we are going to need to develop more drought-tolerant plants in the very near future.”  

CAM to the rescue

A specific type of photosynthesis known as crassulacean acid metabolism, or CAM, allows certain plants to conserve water and flourish in semi-arid conditions and is a major focus of both Cushman and Yang’s research.

CAM is present in more than 6 percent of all vascular plant species across 36 different plant families, making it a fairly widespread ecological adaptation, Cushman explained.

Many economically valuable crops use a more common type of photosynthesis, known as C3 photosynthesis.

“In C3 photosynthesis plants take up and fix CO2 during the day when stomata are open, resulting in substantial water loss,” Cushman told LabOutlook. “In contrast, CAM plants take up CO2 during the night and keep their stomata closed during all or most of the day, thereby substantially reducing their water loss due to evapotranspiration.”

“CAM is a biological innovation that allows plants to thrive in water-limited environments such as arid deserts and areas with a pronounced seasonal drought,” Yang told LabOutlook.

Due to this capability, CAM plants are between 5- and 20-fold more water-use efficient than C3 photosynthesis plants. 

Understanding this process at a basic level could lead to real-world applications in efforts to develop drought-resistant food and bioenergy crops, Yang said.

The unassuming ice plant could become an ingenious weapon in the fight against a warming climate that threatens to limit regions suitable for growing biofuel crops.
Photo by John Cushman, University of Nevada, Reno

Cushman’s work will focus on the ice plant, which is unique because it is the first reported species that can switch from the C3 pathway to CAM when stressed by factors such as drought and salinity. This makes it a good model for studying CAM because the entire mode of photosynthesis is inducible, Cushman explained. The ice plant is what is referred to as a facultative CAM plant, and it allows researchers to readily identify the genetic components involved in CAM.

Yang’s team recently published work investigating the agave plant, which was chosen because it’s an economically important CAM crop species, and has large potential for production of biofuel, fiber, food and animal feed in water-limited areas.

All about genes

One of the main aspects of study in drought resistant crops is understanding the genetic underpinnings that control the CAM process.

Cushman said his team will be creating a gene atlas of the ice plant, which is a comprehensive collection of expressed genes within an organism.

“The importance of creating a gene atlas is that scientists can know precisely when and where each gene is expressed throughout the plant,” he noted.

This type of information is crucial because it provides possible clues to the function of a particular gene, Cushman said. The function of about 40 percent of all genes is still largely unknown for most organisms, he added.

“By providing more information about gene function, this can help scientists understand the role of genes that are potentially involved in conferring drought resistance.”

Understanding how environmental stress and the circadian clock control the expression of CAM are major research objectives for Cushman’s lab. The team will perform integrated transcriptome, proteome, and metabolome analyses using the ice plant.

In a paper published in Nature Plants, Yang reported the metabolic and genetic underpinnings that allow the agave plant to flourish in semi-arid climates.

To do so the team compared molecular characteristics of agave to a control plant, Arabidopsis, using mass spectrometry, and evaluating genetic behavior over a 24-hour period.

Biochemist and molecular biologist John Cushman at the University of Nevada, Reno, pictured here with orchid plants he is studying. Photo: Courtesy of the University of Nevada, Reno

They found that the timing of nighttime compared to daytime stomatal activity varied notably between the two species.

“This study revealed rescheduled temporal expression of genes associated with signal transduction mechanisms that regulate stomatal opening/closing and provided a comprehensive resource (including metabolomics, transcriptomics and proteomics) to inform efforts to engineer more water-use efficient CAM pathway traits into economically valuable C3 crops,” Yang said.

Cushman’s research into the ice plant, which originates in the Namibian desert of Africa, and Yang’s work with agave and Kalanchoë, were both selected by the DOE’s Joint Genome institute as part of 37 projects in their Community Science Program.  The program was created to provide the scientific community with access to high-throughput sequencing and other resources at the DOE for projects relevant to DOE missions, and aims to advance genome science-based research.

Another international consortium, led by Henk Hilhorst, associate professor at Wageningen University in the Netherlands, is also using genome sequencing to understand how the ‘resurrection plant’ Xerophyta viscosa tolerates a severe dry environment for long periods of time.

That team investigated fluctuations in gene expression patterns during dehydration so they could identify genes that allow the plant to survive desiccation, or the process of extreme drying.  Their findings were published April 4 in Nature Plants.

Synthetic biology: A path to drought-resistant crops

The overall goal of both Cushman and Yang’s work is to move the CAM pathway into a C3 photosynthesis plant using synthetic biology approaches to improve the overall water-use efficiency of food and bioenergy crops.

“We will catalog patterns of gene expression to know exactly which genes are important for doing CAM, and that’s why the ice plant is such an important model, and that’s why the DO is interested in it,” Cushman said. “So now, we can take those genes and reengineer them back to C3 photosynthesis plant like wheat or rice, or a woody bioenergy feedstock like poplar, and we hope to make those more water-use efficient.”

Yang suggested techniques such as rational design of genetic circuit, high-throughput assembly of DNA parts, genome editing and gene stacking plants could be used to achieve this.

Engineering crops to use less water isn’t only important because of potential dry periods, according to Cushman.

“Water will become an increasingly precious commodity in the future due to global warming, and more and more of the human population is predicted to experience water insecurity,” Cushman said. “Thus, improving the water-use efficiency of crops will also become more important because agriculture consumes about 70% of blue (surface and ground) water resources on the planet.”

While the projects are ongoing, Cushman said it will take several years to completely understand how these plants tolerate drought and heat.  

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