Nectar microbes: undercover manipulators of flower scent and pollinator behavior

 

Evobites Carpenter Bee
Pollinators are attracted to flowers for a multitide of reasons, not just their smells and colors. (Photo Credit: Michelle Fearon, PhD Candidate in Ecology and Evolutionary Biology at the University of Michigan)

 

We’ve all been there. Completely stuffed, can’t stand the sight of more food, and in desperate need of a nap. A few weeks ago, I reached this state after attending a decadent brunch buffet. Imagine a seemingly endless number of tables covered with poached eggs, crisp slices of bacon, and waffles oozing with syrup. Each item showcases a unique color palette and tantalizing aroma—altogether, it can be nearly overwhelming. Bees, on the other hand, are masters of the brunch buffet. They experience situations like this nearly every day during the spring. But instead of our typical breakfast fare, they feast on flowers.

The accepted story is that plants put on showy displays and offer tasty treats to insect visitors in exchange for pollen transportation to other flowers. Similar to how we wander around a restaurant scoping out options for our breakfast plates, bees buzz around patches of flowers in search of sugary nectar. To navigate their buffet options, they rely on a variety of floral cues, such as color and scent, to find nutrient rich resources. By using these floral cues to select only the tastiest options, they avoid getting to that dreaded overstuffed state. The idea that floral displays and rewards evolved to attract pollinators formed the foundation for decades of research on plant-pollinator mutualisms, but a recent study revealed other players in these interactions that are changing the way pollinators select their buffet treats.

These hidden players are nectar microbes—yeasts and bacteria that live in flower nectar. Although previous studies have found that nectar microbes modify the chemical composition of nectar, few studies have evaluated their effects on floral scent. “It hasn’t really been demonstrated that microbes can alter how a plant smells, especially how a flower smells,” says senior author Rachel Vannette from the University of California, Davis. “Evidence is accumulating that floral scents are really dynamic and part of it is due to the microbial community living in the flowers.” Because bees, and other pollinators, use scents to determine whether they’ll visit a flower, changes in floral scents due to microbes may alter the way plants and pollinators interact. For instance, if microbes produce scents that are unattractive to pollinators, then pollinators may stay away from stinky flowers, ultimately reducing the amount of resources they consume and pollen transferred to other plants.  

To determine how microbes influence floral scent and plant-pollinator interactions, Vannette and her colleagues first characterized the smells of two fungal species and two bacterial species frequently found in nectar. By growing these microbes in the lab and identifying the chemical compounds they produced, the scientists discovered that each microbial species emitted a unique blend of compounds, resulting in a distinct smell. The researchers then wanted to determine whether these smells had any impact on pollinator behavior, and if so, whether these effects differed among microbial species because of their unique aromas.

To do this, they made five nectar solutions in the lab; four were treated with one microbial species each (either a fungus or a bacterium), whereas the fifth nectar solution did not contain microbes. Then they introduced honey bee pollinators to each solution and measured how often they fed on the nectar. Some nectar solutions were exceptionally enticing to the honey bees, including the solution containing one of the fungal species and one without microbes. But honey bees found the other solutions to be quite revolting and avoided feeding on them. If the bees were at a nectar buffet, they would fill their plates with the microbe-free and tasty fungal solution, leaving the other microbe filled nectars behind.

Although some microbes may not deter pollinators from visiting flowers, others reduce a pollinator’s likelihood of feeding on nectar. Pollinators that avoid feeding on nectar probably consume fewer nutrients and transfer less pollen to other plants. Over time, these changes can negatively affect plant health and impact the evolution of plant-pollinator interactions. To ensure flowers remain attractive to pollinators, some plants produce antimicrobial compounds to prevent microbes from growing in nectar. Other plants, however, host entire microbial communities in nectar, which have the potential to either enhance or destabilize plant relationships with pollinators.

Although scientists have known about the presence of microbes in flower nectar for some time, this study uncovered mechanisms for how microbes alter the way plants, pollinators, and microbes interact. “This is one of the first papers providing actual data suggesting that the volatiles, in addition to the chemistry of the nectar itself, is being affected by microbes,” says Tadashi Fukami, a plant ecologist at Stanford University. “It’s an additional piece of the puzzle in terms of what microbes in the nectar are doing to plant-pollinator mutualisms.”

This study also stimulates broader questions regarding the evolution of plant-pollinator relationships. Although we’re only beginning to understand the role microbes play in altering floral cues, such as scent and nectar chemistry, it is clear that they can be important manipulators of plants and pollinators. By understanding how microbes shape the way plants and pollinators interact, we can start to explore how these players might evolve in response to one another. Maybe one day, these tiny microbes will give rise to a new suite of sweets for bees.

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