Light Sensing Helps Bacteria Survive Daily Environmental Shifts

Scientists know that many bacteria have proteins that give them the ability to sense light. This ability helps some bacteria use light for energy. Many other bacteria cannot use light as energy but can still sense light. How these non-photosynthetic bacteria benefit from using light cues has not been well understood.

Iowa State University researchers are illuminating the role of light sensing in the lives of these simple organisms. The scientists discovered that several types of bacteria that grow on plants and in soil can use light to anticipate an imminent, potentially deadly loss of water from their environment.

Light as Cue

At the very onset of light – even before warming and evapotranspiration begin – the bacteria express genes that allow them to accumulate compounds to protect themselves from water loss in preparation for daily warming and drying conditions.

We tend to think of bacteria as a family of simple organisms that just react to their environments. This work demonstrates a level of sophistication not usually attributed to bacteria. We documented that the bacteria we studied used light as a cue, anticipating something that was going to happen, something which they have learned happens almost every day."

Gwyn A. Beattie, the Robert Earle Buchanan Distinguished Professor of Bacteriology for Research and Nomenclature at Iowa State

Beattie led the research team that reported these findings recently in the Proceedings of the National Academy of Science. The team included Bridget Hatfield, a doctoral student in genetics at the time who is now with Kemin Industries, and Breah LaSarre, a research scientist in plant pathology, entomology and microbiology. Other collaborators on the project included Haili Dong and Meiling Liu, former postdocs at Iowa State University, and Dan Nettleton, chair of the Department of Statistics and Distinguished Professor and Laurence H. Baker Chair in Biological Statistics.

They were studying Pseudomonas syringae, a bacterial species that is a common resident of plant leaves. It is often swept up into the air where it moves long distances, having been found in clouds and even glacial ice. In all of these environments, the bacteria are exposed to daily changes in light and moisture.

Moments of Discovery

The study was originally inspired by pure curiosity, according to Beattie, whose foundational research often focuses on trying to understand how bacteria interact with their environment. She was looking at genes in the bacteria, trying to identify how and why they express genes differently in different environments.

As she and her graduate students kept digging deeper into Pseudomonas syringae's genome, they had some surprising moments of discovery. One was finding light-sensing proteins. The second was that these proteins influence the expression of genes that help bacteria survive periods of drying .

"We first imagined that bacteria living in dew on plants leaves would simultaneously experience sunlight and evaporation of morning dew, making it unclear how they were benefitting from sensing light," Beattie said. "But then, in an 'aha' moment, we realized that the bacteria may activate changes as light hits the leaves, before the leaves heat up and evaporation occurs."

"To confirm this finding, we set up experiments to check and double-check our results, to convince ourselves these things weren't all happening simultaneously."

Their experiments included climbing to the rooftop of the agronomy building at 4:30 a.m., on a day predicted to be without clouds. They hauled equipment for measuring light and evaporation from simulated leaves to a clear spot where morning rays would not be interrupted by shade.

"It was early, and it was freezing. But it was worth it," LaSarre said. "We proved to ourselves that the bacteria's responses are, in fact, based on discrete events – first exposure to light, then exposure to drying."

In fact, they eventually learned that light affected almost one-third of the microbe's genes, indicating that the response is significant in its evolutionary history. They hypothesize that this ability to respond to light preemptively before a drying event gives the bacteria a competitive edge for survival.

"That's one thing that's exciting about this finding," Breah said. "Many have suggested that bacteria are evolving anticipatory strategies to improve their fitness, but there are few cases where this has been clearly demonstrated with experimental data. This helps build the case."

The project has been supported by competitive grants from the USDA's National Institute of Food and Agriculture through its Agricultural Food Research Initiative and a joint program with the National Science Foundation.

The discovery has broad implications for applied research in the future, suggested Beattie. For example, she wants to learn more about how broadly this ability to use light as a cue is shared by other bacteria.

"It would be interesting to learn how other microbes could have evolved to use light to signal other oncoming stresses – or opportunities. Or maybe they use different signals altogether," she said. "This could inform research on ways to use these types of microbial responses to develop smarter, more sustainable crop protection products, such as microbial products that use a flash of light to activate protective functions before they are applied on a field."