Plants’ surprising guard against freezing: acid

Biochemistry


Posted August 4, 2016 | View original publication

A built-in litmus test may help some plants sense when it’s time to protect cells from freezing over, according to new research from the University of Nebraska-Lincoln and Michigan State University.

The researchers found that the acidity of cells in the flowering plant Arabidopsis thaliana increased dramatically when exposed to freezing temperatures. This rise in acidity appears to trigger a protein named Sensitive to Freezing 2, or SFR2, which previous research has shown helps buffer against the cellular breakdown that otherwise accompanies freezing.

“Literally no information was known about how freezing-specific responses like those of SFR2 were happening,” said co-author Rebecca Roston, UNL assistant professor of biochemistry. “Now we have maybe the first clue.

“One of the first things you have to understand before you can engineer tolerance (to freezing) is what are the signals that allow tolerance to occur. The idea is that we’re studying the signaling mechanism … with the goal of understanding how to promote that response.”

To test that acidification plays a role in activating SFR2, Roston and her colleagues floated the Arabidopsis plants on acetic acid while keeping temperatures well above freezing. The team found that the acid alone produced essentially the same changes as the ice-cold temperatures.

Those changes include a series of chemical reactions among the lipids that act as building blocks of cell membranes, said co-author and UNL doctoral student Allison Barnes. Remodeling the membranes can make them more elastic, Barnes said, allowing them to better withstand the rigors of freezing.

The cells’ organelles and gel-like cytosol also begin accumulating sugars and amino acids that lower the temperature at which ice will crystallize, said Roston.

“This is fairly similar to making popsicles,” Roston said. “If you make popsicles, you know that they freeze so much slower than ice cube trays. The kids are like, ‘I want them now! I just put them in the freezer. Why aren’t they done?’ It just lengthens the time at which it stays (above freezing).”

The team, which included Michigan State’s Christoph Benning, also showed that introducing magnesium to the cells can accelerate SFR2 activation. This makes sense, Roston said, because photosynthesis-driving chloroplasts can spill magnesium into the surrounding cytosol when their own membranes are damaged via freezing.

Yet Roston emphasized that much remains to be learned and confirmed before fully understanding the freezing response, which does not operate the same in every plant. One such mystery: How do other stressors, such as drought and oxygen deficiency, seem to raise acidity without activating SFR2?

“There are still a lot of unknowns,” Roston said. “We don’t know whether the acidity is a primary signal. We don’t know how direct the connection (between acidity and SFR2) is.

“Even if you have a thorough knowledge of everything that responds (to freezing), and you put all of those enzymes into a non-freezing tolerant plant, if it doesn’t receive the correct signals, it’s not going to respond. These are the things that we are now trying to investigate.”

The authors published their findings in the July edition of the journal Plant Physiology. They received support from the U.S. Department of Energy, which funds the Benning-directed Plant Research Laboratory at Michigan State, and the National Science Foundation, which funds Roston through Nebraska’s Experimental Program to Stimulate Competitive Research.