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."

Meet the Overcompensators, Plants That Get Tougher and Meaner When Attacked

If plants could be stars in a cowboy film, the scarlet gilia would be one of the meanest wildflowers west of the Mississippi.

You can find it standing tall among the sagebrush on mountainsides, its red flowers blazing. Drought can't always stop it. Shade won't faze it. And when mule deer and elk start grazing on it early in the season, it comes back bigger and stronger, with more defenses and a posse of new plants.

Biologists call outlaw plants like this the overcompensators.

“It's a little counterintuitive,” said Miles Mesa, a graduate student at The University of Illinois, Urbana-Champaign who led a new study into these types of plants. “After some animal comes by and eats it, the plant actually does better.”

In the study published this month in the journal Ecology, scientists showed for the first time that in an experiment, damaging some plants set off a molecular chain of events that caused them to grow back bigger, and produce more seeds and chemical defenses simultaneously. At the genetic level, the two tactics for plant survival worked hand in hand — at least in Arabidopsis thaliana, a kind of mustard plant often used for research.

Ken Paige, an evolutionary ecologist also at he University of Illinois and principal investigator of the study, first observed overcompensation in the scarlet gilia in 1987. He described plants that would make more flowers, stems and seeds when their main stems were cut off or eaten.

At the time, being eaten was believed to be bad for plants — always. It took a decade's worth of seeing the contrary for other biologists to believe it.

Dr. Paige started looking for a molecular mechanism behind overcompensation in some versions of Arabidopsis. As he damaged their main stems, he started seeing indications that not only did they get bushier and produce more seeds, but they also ramped up their chemical defenses.

At one point in time, theory pitted regrowth, also known as tolerance, against defense: with limited energy, a plant had to pick one or the other.

But in the past decade, more researchers can't find a trade-off, said Anurag Agrawal, an ecologist at Cornell University who studies plant-herbivore interactions and was not involved in the study.

Dr. Paige thinks a special process at the molecular level helps plants that overcompensate employ both strategies.

Most plants respond to damage with a process called endoreduplication, in which a cell can copy its DNA over and over without splitting into two cells. This gives the plant bigger cells with multiple energy factories to accomplish a variety of tasks. Many damaged plants only show minimal levels of endoreduplication. But the overcompensators go into overdrive with the process.

In the case of the study's mustard plants, they were able to grow bigger and also produce glucosinolate, the sulfurish, bitter chemical compound in mustard, kale, cabbage and horseradish.

And the new research finds that when it comes to building up tolerance or defenses, for at least some plants, you can't have one without the other.

“What this paper shows is that, in practice, defense and regrowth actually go hand-in-hand because the genetics of defense and regrowth are similar,” Josh Banta a biologist at The University of Texas at Tyler, who was not involved in the study. “Like it or not, theory be darned.”

But even the baddest cowboys are not immortal, the researchers found. If they cut the main stem and 75 percent of its leaves, even overcompensators can't rebound.

This tough guy tactic may be a special case, said Dr. Agrawal, but Mr. Mesa and Dr. Paige think it could be generalized to many other plants. Depending on much that turns out to be true, future research could one day help farmers grow super crops that made more food without having to use as many pesticides. But as the story often goes for basic genetic research on crops, results that could be applied are a ways off.

Can Plants Hear?

Pseudoscientific claims that music helps plants grow have been made for decades, despite evidence that is shaky at best. Yet new research suggests some flora may be capable of sensing sounds, such as the gurgle of water through a pipe or the buzzing of insects.

In a recent study, Monica Gagliano, an evolutionary biologist at the University of Western Australia, and her colleagues placed pea seedlings in pots shaped like an upside-down Y. One arm of each pot was placed in either a tray of water or a coiled plastic tube through which water flowed; the other arm had only soil. The roots grew toward the arm of the pipe with the fluid, regardless of whether it was easily accessible or hidden inside the tubing. "They just knew the water was there, even if the only thing to detect was the sound of it flowing inside the pipe," Gagliano says. Yet when the seedlings were given a choice between the water tube and some moistened soil, their roots favored the latter. Gagliano hypothesizes that these plants use sound waves to detect water at a distance but follow moisture gradients to home in on their target when it is closer.

The research, reported earlier this year in Oecologia, is not the first to suggest flora can detect and interpret such information. A 2014 study showed the rock cress Arabidopsis, a relative of cabbage, can distinguish between caterpillar chewing sounds and wind vibrations—the plant produced more chemical toxins after "hearing" a recording of feeding insects. "We tend to underestimate plants because their responses are usually less visible to us. But leaves turn out to be extremely sensitive vibration detectors," says lead study author Heidi Appel, an environmental scientist now at the University of Toledo.

Another hint that plants can hear comes from the phenomenon of "buzz pollination," in which a bee buzzing at a particular frequency has been shown to stimulate pollen release. Other experiments have found that sounds can lead to hormonal changes in plants, influence their oxygen uptake and change their growth rates. A study published earlier this year revealed that sound waves can even influence gene expression in Arabidopsis.

Michael Schöner, a biologist at University of Greifswald in Germany, who was not involved in the new research, believes that plants may have organs that can perceive noises. "Sound vibrations could trigger a response of the plant via mechanoreceptors—these could be very fine, hairy structures, anything that could work like a membrane," he says.

This research raises questions about whether acoustic pollution affects plants as well as animals, Gagliano observes: "Noise could block information channels between plants, for example, when they need to warn each other of insects." So next time you turn on a noisy leaf blower or a hedge trimmer in your garden, consider the lilies.