Research Brief: Grassland plants react unexpectedly to high levels of carbon dioxide

Plants are responding in unexpected ways to increased carbon dioxide in the air, according to a twenty-year study conducted by researchers at the University of Minnesota and published in the journal Science. For the first 12 years, researchers found what they expected regarding how different types of grasses reacted to carbon dioxide. However, researchers' findings took an unanticipated turn during the last eight years of the study.

Researchers planted 88 plots with two different types of grasses, warm-season C4 grasses and cool-season C3 grasses, and exposed them to different levels of carbon dioxide, current carbon dioxide levels and the elevated levels the Earth might experience later this century due to human activity.

"Because carbon dioxide is needed by plants to grow, we expected grasses that have the C3 photosynthetic pathway to grow more under elevated CO2, because these plants are known to be able to increase their CO2 capture as CO2 levels rise. We also expected that growth of grasses with the C4 photosynthetic pathway would not be affected by higher CO2 levels, because these plants are generally less able to capture extra CO2 as CO2 levels rise," said University of Minnesota Professor Peter Reich. "While that held true for the first dozen years, that pattern changed."

Researchers found that during the last eight years of the study, C4 plant species grew more in an elevated CO2 environment than C3 plants. While it's uncertain why this shift happened, these findings could have significant implications.

"If mature grasslands worldwide behave like our experiment did, this could have long lasting impacts on how we think about the conservation and restoration of grasslands around the world," Reich said. "Grasslands cover between 30 and 40 percent of land and play a key role in soaking up carbon dioxide released by burning fossil fuels."

Along with impacts on conservation and restoration planning, these data could be used to help computer models better predict how plants will respond to changing CO2 concentrations in the atmosphere.

"Our results suggest that the predictions made by these models might not be quite right and that we should not be overly confident about our assumptions regarding where, and by how much, land ecosystems will keep absorbing extra CO2 out of the air," Reich said.

Reich, a professor with the College of Food, Agricultural and Natural Resource Sciences' (CFANS) Department of Forest Resources and Institute on the Environment (IonE) fellow, was the lead researcher on the study. Other study investigators included Professor Sarah Hobbie and graduate student Melissa Pastore, with the Department of Ecology, Evolution and Behavior in the College of Biological Sciences, and Professor Tali Lee from the University of Wisconsin, Eau Claire.

About University of Minnesota College of Food, Agricultural and Natural Resource Sciences

The University of Minnesota College of Food, Agricultural and Natural Resource Sciences (CFANS) brings science-driven innovators together to discover hands-on solutions to global challenges. With 10 research and outreach centers across Minnesota, the Minnesota Landscape Arboretum, and the Bell Museum of Natural History, CFANS offer unparalleled experiential learning opportunities for students and the community. CFANS students enter career fields with some of the best job outlooks in the country, including 13 undergraduate majors and more than 25 minors ranging from agricultural education and marketing communications to conservation biology and forest and natural resource management, health and nutrition, to the future of food and agriculture management with a focus on business and technology.

Plants really do feed their friends

"For more than a century, it's been known that plants influence the makeup of their soil microbiome, in part through the release of metabolites into the soil surrounding their roots," said Berkeley Lab postdoctoral researcher Kateryna Zhalnina, the study's lead author. "Until now, however, it was not understood whether the contents of this cocktail released by plants was matched by the feeding preferences of soil microbes in a way that would allow plants to guide the development of their external microbiome."

The study, "Dynamic root exudate chemistry and microbial substrate preferences drive patterns in rhizosphere microbial community assembly," has just been published in the journal Nature Microbiology. The corresponding authors were Berkeley Lab scientists Trent Northen and Eoin Brodie.

Microbes within soil improve the ability of plants to absorb nutrients and resist drought, disease, and pests. They mediate soil carbon conversion, affecting the amount of carbon stored in soil or released into the atmosphere as carbon dioxide. The relevance of these functions to agriculture and climate are being observed like never before.

Just one gram of soil contains tens of thousands of microbial species. Scientists have long known that plants impact the composition of the soil microbiome in the area surrounding their roots by sending out chemicals (metabolites). Prior work by Mary Firestone, Berkeley Lab faculty scientist and a professor of microbiology at UC Berkeley, had shown that plants were consistently selecting or suppressing the same types of microbes over time in the root zone, suggesting some form of synchronization between plant and microbiome development.

Yet, little research had gone into the relationship between specific metabolites that plants release and the microbes consuming them. The new study brought together experts in soil science, microbial and plant genomics, and metabolomics to explore these potential metabolic connections. Their study took a close look at the rhizosphere of an annual grass (Avena barbata) common in California and other Mediterranean ecosystems.

The Berkeley Lab team felt the time was ripe for doing so. As pressure mounts for farmers to grow enough healthy crops to meet a burgeoning population's needs, and for new land management strategies that improve soil carbon storage to reduce atmospheric CO2 and produce healthy soils, the soil microbiome is the subject of more in-depth scientific research than ever before.

The researchers set out to determine the relationship between microbes that consistently bloomed near the grass roots and the metabolites released by the plant. Their first step was to collect soil from the University of California's Hopland Research and Extension Center in northern California. Brodie, deputy director of Berkeley Lab's Climate and Ecosystem Sciences Division, and his group used what they knew about the lifestyles of these soil bacteria to develop specialized microbial growth media to cultivate hundreds of different bacterial species. They then selected a subset that either flourished or declined as roots grew through the soil.

This collection of microbes was then sent to the Joint Genome Institute (JGI), a DOE Office of Science User Facility, where their genomes were sequenced to provide clues as to why their responses to roots differed. This analysis suggested that the key to success for microbes that thrived in the rhizosphere was their diet.

Northen, senior scientist in Berkeley Lab's Environmental Genomics and System Biology Division, is fascinated by the chemistry of microbiomes, and his group has developed advanced mass spectrometry-based exometabolomic approaches to elucidate metabolic interactions between organisms. Zhalnina and Northen combined their expertise to identify what the more successful microbes surrounding the roots of the Avena grasses preferred to eat.

Using a hydroponic setup at the JGI, they immersed plants at different developmental stages in water to stimulate them to exude their metabolites, then measured the metabolites being released by the plants using mass spectrometry. Subsequently, the cultivated soil microbes were fed a cocktail of root metabolites, and the researchers used mass spectrometry to determine which microbes preferred which metabolites.

They found that the microbes that flourished in the area around plant roots preferred a diet more rich in organic acids than the less successful microbes in the community.

"Early in its growth cycle, the plant is putting out a lot of sugars, 'candy', which we find many of the microbes like," Northen said. "As the plant matures, it releases a more diverse mixture of metabolites, including phenolic acids. What we discovered is that the microbes that become more abundant in the rhizosphere are those that can use these aromatic metabolites."

Brodie describes these phenolic acids as very specific compounds released by plants throughout their development. Phenolic acids are often associated with plant defenses or plant-microbe communication. This indicates to Brodie that as they establish the microbial community within the rhizosphere, plants could be exuding metabolites like phenolic acids to help them control the types of microbes thriving around their roots.

"We've thought for a long time that plants are establishing the rhizosphere best suited to their growth and development," said Brodie. "Because there are so many different types of microbes in soil, if the plants release just any chemical it could be detrimental to their health.

"By controlling the types of microbes that thrive around their roots, plants could be trying to protect themselves from less friendly pathogens while promoting other microbes that stimulate nutrient supply."

Zhalnina, Firestone, Northen, and Brodie believe their findings have great potential to influence additional scientific and applied research. Zhalnina points out that a lot of research and development is currently underway by government and industry to harness the power of microbes that improve plant yield and quality of soil to help meet society's growing demands for a sustainable food supply.

She said, "It's exciting that we can potentially use the plant's own chemistry to help nourish beneficial microbes within soil. Population growth, especially, has created a demand for identifying more reliable ways to manipulate the soil microbiome for beneficial outcome."

How plants 'muscle up' against bacteria in the cold

Michigan State University scientists have furthered our understanding on how a plant protein, called CAMTA, helps plants strengthen themselves as they anticipate long periods of cold, such as three to four months of winter in the American midwest or northern Europe.

The long-term goal behind the research is to breed or create plants with higher tolerance to wild swings in temperature. The study is published in the journal The Plant Cell.

CAMTA proteins are universally found across plants, and they help turn on genes that communicate freezing tolerance to these plants. In the study, CAMTA proteins were observed to also control how plants defend against harmful bacteria under long-term cold conditions.

In the cold, plants generally build up high levels of salicylic acid, or SA, a compound that protects them against bacteria.

"At warm temperatures CAMTA proteins, specifically the N-terminus (the start of the proteins), block the system that produces SA," said Yong Sig Kim, a post-doctoral student in the lab of University Distinguished Professor and MSU Foundation Professor Michael Thomashow.

When it gets cold for a long enough period, an unknown signal is generated that modifies CAMTA to allow SA production to turn on. In that case, the C-terminus, or the bottom of an amino acid chain that is stopped by a free carboxyl group, detects the signal -- possibly a rise in cellular calcium levels -- that enables SA biosynthesis.

This observation reverses current accepted models, which proposed instead that the C-terminus blocked SA production.

Why does tolerance to the cold instigate bacterial defenses?

"SA doesn't protect the plant from the cold, per se. Instead, we think the plants enhance their immune systems in the cold as a general preemptive strategy," Kim said.

Although plants take measures to survive the cold, they still get injured, and their structures are destabilized, which makes them more vulnerable to bacterial infection.

So, weakened plants keep their guard up as a precaution. It is similar to how humans take preventative measures to stay healthy – eat well, sleep eight hours, hydrate, etc.

This knowledge has long-term potential impact on agricultural production. For example, according to the EPA, in 2010 and 2012, high nighttime temperatures affected corn yields across the U.S. Corn Belt, and premature budding due to a warm winter caused $220 million in losses of Michigan cherries in 2012.

 "The field of plant defense is gradually revealing how protection mechanisms against the elements and against other living beings are interrelated," Kim said.

Will winter's bitter cold damage plants?

The new year is arriving in a blast of bitter cold. People can snuggle indoors where it's warm, but what about plants? Will the harsh weather do them harm?

"For plants that are fully dormant, there shouldn't be any problem," said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. Dormancy is the resting state that plants enter to protect themselves against winter cold and drought.

The aboveground parts of trees and shrubs have bark to protect them. Buds at the tips of branches are shielded by bud scales. Perennials have allowed their stalks and leaves to die back already. Plants' roots are still alive, but they are safe in the ground. "As long as the roots are underground, they are protected," Yiesla said.

The main risk she sees is for evergreens, which may not have fully entered their dormant state because of warm weather in the fall. Evergreen needles retain some water all winter and could be dried out by bitter cold and wind.

"Depending on how long the cold goes on and how low the temperature falls, we may see a little more winter damage on evergreens than we sometimes do," she said. "But it won't be apparent until spring, and there's nothing a homeowner can do about it now."

The greatest danger isn't from cold, but from warm weather that can follow. Chicago often swings between cold and warm spells, and plants can be harmed by the quick shifts. When soil warms, dormant plants may start to wake up and then be hit hard by the next cold snap.

Plants with shallow roots, such as hydrangeas and hellebores, are especially vulnerable, along with species that are not quite hardy here, such as oakleaf hydrangea and butterfly bush.

"The best way to prevent damage from freeze-thaw cycles is to make sure there's a layer of mulch over the roots," Yiesla said. The mulch will insulate the soil, so it stays evenly cold.

You can use wood chips or fallen leaves, but another source of mulch is handy at this time of year: evergreen boughs. "Cut up your Christmas tree and lay the branches on the soil around vulnerable plants," she said. "They will help insulate the soil."

Be good to your house plants, they’ll be good to you

 Indoor plants are wonderful things. They are green and beautiful and even shiny, if you bother to dust the leaves or give them a bath now and then. House plants take in carbon dioxide and release oxygen, helping to clear the air and make the inside of your home a healthier place for you and your family. Indoor plants are especially nice to have around in the autumn and through the winter when the garden outside is often less attractive and accessible.

But at the moment I feel a touch guilty talking about house plants. A few weeks ago, I finally had to toss a rubber plant that had died, slowly and painfully, fallen brown leaf by fallen brown leaf. It was beautiful when I bought it and plunked it down on the raised hearth, on the wall farthest from the living room's largest window. It was big and gorgeous for several weeks, sitting in a large green pot, but then the leaves began to brown, curl and fall. Watching this lovely plant die was heartbreaking, but did I do any research to find out what was wrong? Unfortunately, no, until it was too late. That was when I discovered the poor thing wasn't getting enough light and was possibly getting too much water. I moved it into the dining room next to a window, but by then the die was cast.

Tossing that rubber plant was a rather expensive lesson in doing research sooner rather than later. I've had great luck with other house plants — mother-in-law's tongue, pothos, and various palms — and thought I knew what I was doing. But no. Googling “rubber plant” after I bought it would have saved me much grief, and my rubber plant might still be with us.

If you do a bit of research first, raising house plants and keeping them healthy isn't difficult. I've had some house plants for many years. I moved nine months ago, and it took a while to find the optimal spot for all of them. But here's the thing. With these plants, I paid attention to their ups and downs and responded accordingly. For some crazy reason, I had too much faith in that poor rubber plant to pay proper attention.

House plants do need different care from outdoor plants. They are highly sensitive to light levels, and less water is usually preferable to too much. But it is easy to find many indoor plants that thrive in many different light conditions and that conform to your personal plant care philosophy, whether it be to do everything on a carefully set schedule or on a more catch-as-catch-can basis.

For the catch-as-catch-can indoor gardener, the mother-in-law's tongue and pothos in my living room are the perfect solution. Mother-in-law's tongue, also known as the snake plant, does well in low light and is almost impossible to kill. It can be watered as seldom as once a month. The pothos vine likes a bit more light (I combined the two plants just for the photo), but it will signal when it needs water by starting to look limp and wilted. A good soaking in the kitchen sink will refresh it.

Children will enjoy the pothos vine because it is simple to create more plants. When a vine gets too long, just cut it off and put the cut end in a jar filled with water on a window sill. It will grow roots and can then be planted in a new pot. Where you had one plant, you now have two.

There are dozens of other indoor possibilities. The air plant is extremely easy to grow, and no soil is involved. Just soak each plant for two or three hours every ten days or so, and they will thrive on a saucer or dish on a counter or bookcase in indirect light.

Aloe likes indirect light and a good soak about once a week. Aloe also has excellent healing properties. If someone gets a burn in the kitchen, break a leaf open and smear the aloe sap on the burn.

Diffenbachia likes filtered light, perhaps by a curtained window. Spider plants like lots of light and a weekly watering. Peace lilies prefer indirect light. They are another plant that lets you know when it needs water. African violets are lovely in a kitchen window.

Palm trees are often easy to grow indoors and may remind you of sunnier climes in the depths of winter. The parlor palm prefers low light and may suffer if exposed to direct sunlight. It grows very slowly and may take years to reach its full height of three to four feet. I have had great luck with the Kentia palm. In its native habitat, it can reach 60 feet. But indoors it tops out at 6 to 12 feet. This plant benefits from periodic soaking in a bucket or deep sink to thoroughly wet the entire root ball.

Orchids are another winter delight. I avoid them in the summer when I'm running ceiling fans because the plants dry out.

Of course nothing beats a Christmas cactus at this time of year. Give it indirect light and occasional water, and its festive color will brighten your holidays into the new year.