Ice-fighting Bacteria Could Help California Crops Survive Frost

But a scientist whose groundbreaking research linked frost damage to plant-dwelling bacteria thinks there’s a better way to fight frost. And it doesn’t require spending a fortune on equipment that consumes copious quantities of water and fossil fuels. 

 

Steven Lindow, a plant pathologist at the University of California, Berkeley, showed more than 40 years ago that frost-sensitive agricultural crops harbor bacteria called Pseudomonas syringae that cause ice to form at above-normal freezing temperatures. The bacteria are so adept at making ice that they’re routinely added to snow machines at ski resorts. 

 

Despite battles with anti-GMO activists who derailed his early work, Lindow has  continued to refine his approach ever since. A central focus involves exploiting the science of microbial ecology to enlist beneficial bacteria and other tools to displace ice-promoting strains. Creating conditions unfavorable to harmful strains could reduce dependence on fuel-hungry equipment and conserve water during dry years, which will become more common as the world warms.  

 

Plants are about 90 percent water, and sustain frost damage when ice crystals form between cells and pierce their membranes. In the absence of substances that make ice, they can “super cool,” or remain liquid at sub-freezing temperatures, Lindow said, contrary to the conventional wisdom that water always freezes at 32 degrees Fahrenheit. 

 

“And it turns out that bacteria are by far the most important determinants of how cold plants will get before they will start to freeze and get damaged,” he said.

 

Lindow’s early research showed that plants could avoid frost injury when they weren’t colonized by P. syringae bacteria, which cause frost damage in many valuable agricultural crops. 

 

After homing in on a protein responsible for the bacteria’s ice-making proclivity, Lindow figured out how to block it by deleting the gene that encodes the protein.

 

Lindow’s “ice minus” mutant bacteria—viewed by scientists as a landmark scientific achievement—triggered protests from consumers wary of genetically modified organisms and legal challenges that delayed testing for four years. He finally won approval to test his engineered bacteria in strawberries and potatoes in the late 1980s, after spending about $100,000 to meet the Environmental Protection Agency’s regulatory requirements. 

 

The mutant bacteria proved safe and effective. But vandals destroyed most of the plants.

 

Lindow’s ice-minus bacteria were never commercialized. No company wanted to assume the costs of developing and testing a product for regulatory scrutiny only to face public backlash.

 

“Nobody wants genetically engineered organisms,” Lindow said, with the resigned laugh of a battle-weary veteran of the GMO wars. 

 

But Lindow had discovered naturally occurring strains that also worked against the ice-promoting bacteria. 

 

Those naturally occurring, benign Pseudomonas strains left no room for harmful species to grow when applied to plants as sprays. One strain reached the market, but was sold not to prevent frost but for its ability to suppress fire blight, a devastating bacterial disease in apple and pear trees. That’s been disappointing, Lindow said, because the strain so effectively controls P. syringae, one of the most common, ice-producing bacteria. 

 

Such setbacks have not deterred Lindow’s research on combating frost by managing microbial communities. 

 

Instead of focusing on just the frost-vulnerable plant, though, Lindow has increasingly looked at the microbial ecology of the agricultural system as a whole. That means figuring out where ice-promoting bacteria come from, how they get to the plant and, crucially, how to disrupt the microbe’s colonization process.

 

He recently finished a multiyear trial to prevent frost by managing microbial communities on cover crops planted between vineyard rows to fertilize soil, manage weeds and curtail erosion, among other environmental benefits. His team worked with vintners in Mendocino and Lake counties, where about 700 growers farm more than 28,000 vineyard acres. 

 

“We’ve been modifying the local environment by removing cover crops in different ways,” Lindow said. “Depending on the situation, you can have a huge impact on the microbiology of the grapevine, especially early in the spring when frost is the biggest issue, by making these other sources of bacteria scarce.”­

 

 

Just a handful of bacterial species promote ice, and they all live on the leaves of plants. Plants are most vulnerable to frost when ice-forming bacteria settle on nascent growth. Although soil is teeming with bacteria, the plant-colonizing variety mostly emigrate from adjacent plants, Lindow said.

 

Concentrations of airborne bacteria are far higher when green plants are nearby than when they’re not. The number of bacteria coming from plants, he said, “is much more dramatic than I would have ever guessed.”

 

On farms, those bacteria are coming from cover crops. “People would really like to have cover crops for soil health and erosion control,” Lindow said. “But we recognize that they are major contributors to the freezing temperature of those plants.” 

 

Lindow wanted to make sure growers were aware of trade-offs. Cover crops, he found, support high populations of ice-promoting bacteria during the winter. He reasoned that removing the vegetation could thwart the harmful bacteria, and partnered with Glenn McGourty, emeritus viticulture and plant science advisor for Lake and Mendocino counties, to work with vintners. 

 

They experimented with trying to get the beneficial A506 bacteria to grow first on the cover crops and then spread into the vines, said McGourty. “It colonized some cover crops better than others.”

 

But it turns out that grapes don’t seem to support high populations of bacteria like other agricultural crops do, so it was unlikely that the good bacteria would get established anyway. Simply clearing out cover crops seemed to be the best way to prevent frost, said McGourty.