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Modifying crop genetics to resist drought, conserve water

Agriculture monopolises 90% of global freshwater, availability of clean water resources shrinking

Scientists have improved how a crop uses water by 25% without compromising yield by altering the expression of one gene that is found in all plants, as reported in a study published on Tuesday in the journal Nature Communications. Agriculture monopolises 90% of global freshwater. However, its production still needs to dramatically increase to feed and fuel this century’s growing population.

Crop production is the world’s largest consumer of freshwater. The availability of clean water resources is shrinking because of several factors that include urbanisation, human population growth, and climate change, which presents a challenge to optimal growing environments.

The study is part of the international research project Realising Increased Photosynthetic Efficiency (RIPE) that is supported by the Bill & Melinda Gates Foundation, the Foundation for Food and Agriculture Research, and the UK Department for International Development.

RIPE Director Stephen Long, Ikenberry endowed chair of plant biology and crop science, described the findings of the research as “a major breakthrough,” according to a RIPE press release.

He added, “crop yields have steadily improved over the past 60 years, but the amount of water required to produce one tonne of grain remains unchanged—which led most to assume that this factor could not change. Proving that our theory works in practice should open the door to much more research and development to achieve this all-important goal for the future.”

According to the paper, the international team who conducted the research increased the levels of a photosynthetic protein (PsbS) to conserve water by tricking plants into partially closing their stomata, the microscopic pores in the leaf that allow water to escape. Stomata are the gatekeepers to plants: when open, carbon dioxide enters the plant to fuel photosynthesis, but water is allowed to escape through the process of transpiration. Co-lead author Katarzyna Glowacka, a postdoctoral researcher who led this research at the Carl R Woese Institute for Genomic Biology (IGB), explained, “these plants had more water than they needed, but that will not always be the case,” adding, “when water is limited, these modified plants will grow faster and yield more—they will pay less of a penalty than their non-modified counterparts.”

PsbS is a key part of a signalling pathway in the plant that relays information about the quantity of light. By increasing PsbS, the signal says there is not enough light energy for the plant to photosynthesise, which triggers the stomata to close since carbon dioxide is not needed to fuel photosynthesis.

The researchers improved the plant’s water use efficiency—the ratio of carbon dioxide entering the plant to that of escaping water—by 25% without significantly sacrificing photosynthesis or yield in real-world field trials. The carbon dioxide concentration in our atmosphere has increased by 25% in just the past 70 years, allowing the plant to amass enough carbon dioxide without fully opening its stomata. “Evolution has not kept pace with this rapid change, so scientists have given it a helping hand,” said Long, who is also a professor of crop sciences at Lancaster University, according to the statement.

To reach their findings, the researchers tested their hypothesis using tobacco, a model crop that is easier to modify and faster to test than other crops. Now they will apply their discoveries to improve the water-use-efficiency of food crops and test their efficacy in water-limited conditions.

“Making crop plants more water-use efficient is arguably the greatest challenge for current and future plant scientists,” said co-lead author Johannes Kromdijk, a postdoctoral researcher at the IGB. “Our results show that increased PsbS expression allows crop plants to be more conservative with water use, which we think will help to better distribute available water resources over the duration of the growing season and keep the crop more productive during dry spells,” he added.

The research reports that there are four factors that can trigger stomata to open and close: humidity, carbon dioxide levels in the plant, the quality of light, and the quantity of light. This study is the first report of hacking stomatal responses to the quantity of light.

A previous study showed that increasing PsbS and two other proteins can improve photosynthesis and increase productivity by as much as 20%. Now, the researchers of the new study plan to combine the gains from these two studies to improve production and water-use by balancing the expression of these three proteins.

This research complements previous work, also published in Nature Communications in 2017. In that study, scientists at the US Department of Energy’s Oak Ridge National Laboratory have identified a common set of genes that enable different drought-resistant plants to survive in semi-arid conditions, which could play a significant role in bioengineering and creating energy crops that are tolerant to water deficits. Semi-arid conditions include desert environments such as North Africa, including Egypt.

Depending on a form of photosynthesis, known as crassulacean acid metabolism (CAM), plants thrive in drylands by keeping their stomata, or pores, shut during the day to conserve water and open at night to collect carbon dioxide. This form of photosynthesis has evolved over millions of years, building water-saving characteristics in plants such as the Kalanchoë, orchid, and pineapple.

“CAM is a proven mechanism for increasing water-use efficiency in plants,” ORNL co-author Xiaohan Yang said in the statement of the study. “As we reveal the building blocks that make up CAM photosynthesis, we will be able to bioengineer the metabolic processes of water-heavy crops such as rice, wheat, soybeans, and poplar to accelerate their adaptation to water-limited environments.”

Scientists are studying a variety of drought-resistant plants to unlock the mystery of CAM photosynthesis. For this work, the ORNL-led team sequenced the genome of Kalanchoë fedtschenkoi, an emerging model for CAM genomics research because of its relatively small genome and amenability to genetic modification. They have investigated and compared the genomes of K fedtschenkoi, Phalaenopsis equestris (orchid) and Ananas comosus (pineapple) using ORNL’s Titan supercomputer.

“It is widely accepted that some unrelated plants exhibit similar characteristics under similar environmental conditions, a process known as convergent evolution,” said the co-author.

As part of the research, the team of scientists have identified 60 genes that exhibited convergent evolution in CAM species, including convergent daytime and night-time gene expression changes in 54 genes, as well as protein sequence convergence in six genes.

The researchers discovered a novel variant of phosphoenolpyruvate carboxylase, or PEPC.

PEPC is an important worker enzyme which is responsible for the night-time fixation of carbon dioxide into malic acid. And then, malic acid is converted back to carbon dioxide for photosynthesis during the day.

“These convergent changes in gene expression and protein sequences could be introduced into plants that rely on traditional photosynthesis, accelerating their evolution to become more water-use efficient,” said Xiaohan.

To address concerns over freshwater, engineering CAM photosynthesis into food and energy crops could reduce agricultural water use and boost crops’ resilience when the water supply is less than desirable.

Co-author and chief executive officer of the Center for Bioenergy Innovation, Jerry Tuskan, explained, “studying the genome of water-efficient plants may also provide insights into a plant’s ability to use slightly saline water and maintain growth under higher temperature and lower clean water availability.” He further added, “if we can identify the mechanisms for water-use efficiency, we could move this trait into agronomic plants, supply non-potable water as irrigation to those plants and save the clean water for drinking.”

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