Q&A: Can plants help reverse climate change?

UNIVERSITY PARK, Pa. — Heatwaves are arriving sooner and becoming hotter, with the United Kingdom recording May 25 as its hottest day in May since tracking began more than a century ago, only for the record to break again the next day. While humans can turn to artificial means of cooling, such as air conditioning or swimming pools, plants are left to cope with heat and frequently co-occurring droughts on their own. Sarah M. Assmann, Waller Professor of Biology at Penn State, is working to better understand how plants respond to environmental signals — and is applying that understanding to develop crops more resilient to environmental stress.

In this Q&A, Assmann discussed how plants cope with environmental stress, as well as how more resilient crops could not only increase agricultural food security but also help cool the warming climate.

Q: How does plant physiology — the physical and chemical processes that keep plants alive — influence a crop’s ability to survive drought and heat stress?

Assmann: Just like people have physiology that allows them to sense and respond to the environment, so do plants. Both drought and heat are environmental conditions that lead to a water deficit in a plant. In a drought, the plant can’t get as much water from the soil, and heat drives evaporation of water from the plant. The plant loses water to hotter or drier air through microscopic pores on the leaves called stomata.

When plants lose water through their stomata, it is like how humans sweat — water escapes our pores and eventually evaporates from our skin. That’s how we cool off, and so do plants.

If there is both heat and drought together, as there often are, the plant has a conundrum: How can it stay cool yet retain enough water for its other needs? It comes down to their stomata. Plants can regulate the size of their stomata by changing the volume of pairs of cells that surround each pore. These cells, called guard cells, have highly precise sensing mechanisms that detect heat and drought via hormonal signaling.

With drier soil, the plant produces more of a hormonal signal called abscisic acid that travels from the roots to the leaves. The guard cells perceive this and other signals and narrow the stomatal pore. That reduces water loss via evaporative cooling, but it also heats the plant. This also leads to a further complication, because it’s through the stomata that plants take up carbon dioxide from the atmosphere for photosynthesis — the process through which plants convert carbon dioxide into sugars and other carbohydrates.

Plants exhibit intricate physiological responses that change on a scale of seconds to minutes to control stomatal aperture sizes and balance these opposing priorities of optimizing carbon dioxide uptake and regulating water loss.

Q: With the changing climate bringing increasingly frequent and severe heatwaves and droughts, how are plants coping?

Assmann: Some can manage through their physiological balancing act, but many of them aren’t coping so well. We see the impacts on crop yields, with wheat and rice as examples of major crops that are predicted to decline in global yields over time. That’s why developing crops that can better resist this more extreme weather is a major priority for those of us who work in agriculture and food security.

We’re specifically looking at varieties of rice, a staple food crop for half of the world’s population. If you go to the grocery story, you’ll see a few types of rice — basmati or jasmine, for example — but there are thousands of varieties. Are some of those better adapted to survive heat and drought? If so, maybe we can introduce those adaptive mechanisms into farmer-preferred varieties to improve crop performance in future climates.

Q: Why is water management central to climate-resilient agriculture?

Assmann: Consider rice. It’s a hugely water-intensive crop, with almost double the water requirements of most other crops. When water is plentiful, rice fields are flooded. The plants get plenty of water; it isn’t a problem. But for farmers who can’t afford or can’t access that type of irrigation, the crop lives or dies by the rain.

So, the question is if it is all or nothing — can you get enough rice yield with less water? That’s what we’re working on. We’re seeing how little water you can supply and still have a good yield. In particular, we’re looking at which rice varieties might perform well with limited water. If the crop needs less irrigation, farmers who irrigate can save money on water, and those who can’t afford irrigation can still have a bountiful harvest when rainfall is inadequate.

Also, irrigation machinery uses fuel, which contributes to greenhouse gas emissions and global warming. Flooding fields creates an anerobic condition, where less oxygen gets to the soil, and this provides a setting that favors the proliferation of microorganisms that produce methane — another emission that contributes to global warming. So, producing rice that need less water, or shorter durations of water access, is another way that global warming can be ameliorated.

Q: How could your research contribute to regenerative agriculture practices?

Assmann: An analogy that may resonate is personalized medicine. We, as humans, know more and more about our own genetics and what genes or gene sequences might predispose us to develop certain diseases or to be less or more able to tolerate environmental stresses. The same thing is true for crops and for plants in general.

Each variety of rice, for example, has its own, personalized genome. The more we learn about those genomes, how they differ and how they confer various traits to the plant that influence their ability to respond to the environment, the more we can incorporate that information into developing plants that will perform well in future environments.

Q: How does understanding plant signaling shape the future of farming?

Assmann: This work is an excellent example of the importance of basic research. Basic research is the foundation of all advances in medicine and in agriculture. My primary focus is on employing molecular and physiological techniques to understand the messenger pathways by which signals such as heat and drought are converted into cellular responses — like abscisic acid hormonal signals influencing how guard cells swell or shrink to control stomatal pore size. That understanding is contributing to developing crops that can resist current environmental threats, ultimately improving global food security.

And, of course, this work is extremely collaborative. My collaborators on rice drought resilience are based in the Philippines, England, France and Germany, as well as in the U.S. — and that’s just for one project. Science really transcends country boundaries.

Research on rice environmental responses in the Assmann laboratory is supported by grants from the U.S. National Science Foundation and the U.S. Department of Agriculture’s National Institute of Food and Agriculture.