Nectar-living microbes release scents or volatile compounds, too, and can influence a pollinator's foraging preference, according to newly published research led by UC Davis community ecologist Rachel Vannette.
The groundbreaking research, published in the current edition of New Phytologist journal, shows that nectar-inhabiting species of bacteria and fungi “can influence pollinator preference through differential volatile production,” said Vannette, an assistant professor in the UC Davis Department of Entomology and Nematology.
“This extends our understanding of how microbial species can differentially influence plant phenotype and species interactions through a previously overlooked mechanism,” Vannette said. “It's a novel mechanism by which the presence and species composition of the microbiome can influence pollination.”
“Broadly, our results imply that the microbiome can contribute to plant volatile phenotype,” she said. “This has implications for many plant-insect interactions.”
Their paper, titled “Nectar-inhabiting Microorganisms Influence Nectar Volatile Composition and Attractiveness to a Generalist Pollinator,” may explain in part the previous documented extreme variation floral volatiles that Robert Junker of University of Salzburg, Austria, and his team found; New Phytologist published their work in March 2017.
Although microbes commonly inhabit floral nectar, microbial species differ in volatile profiles, they found. “Honey bees detected most of the microbial volatiles or scents that we tested,” Vannette said, “and they distinguished the solutions of yeasts or bacteria based on volatiles only.” This suggests that pollinators could choose among flowers based on the microbes that inhabit those flowers.
The yeast Metschnikowia reukaufii produced the most distinctive compounds (some shared with the fruity flavors in wine) and was the most attractive of all microbes compared. This yeast is commonly found in flower nectar and is thought to hitch a ride on pollinators to travel from one flower to the next. Its scent production may help it attract pollinators, which then help the yeast disperse among flowers.
The Harry H. Laidlaw Jr. Honey Bee Research Facility, UC Davis, provided the honey bees. More than 20 species of flowers--mostly natives--were used in the survey, including canyon delphinium or canyon larkspur (Delphinium nudicaule), sticky monkey flower (Mimulus aurantiacus), salvia (Lepechinia calycina) and purple Chinese houses (Collinsia heterophylla). The samplings were done in the spring and early summer, when the natives are at their peak.
Co-authors of the paper are Caitlin Rering, postdoctoral fellow at USDA-ARS, Gainesville, Fla.; John Beck researcher at USDA-ARS; Griffin Hall, junior specialist in the Vannette lab; and Mitch McCartney in UC Davis Department of Mechanical and Aerospace Engineering.
The USDA and USDA-ARS funded the research.
- Evolution of floral signals and flower morphology
- Pollinator-drive speciation
- Evolution of floral mimicry
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In newly published research in the journal Ecology, Vannette noted that floral nectar is produced by many plants to reward pollinators, but this sugary secretion often contains chemical compounds that are bitter tasting or toxic, and can deter pollinators. Plants including citrus (Citrus), tobacco (Nicotiana), milkweed (Asclepias), turtlehead (Chelone), Catalpa, and others produce nectar containing bioactive compounds, including deterrent or toxic compounds.
“This poses a paradox of toxic nectar: why are deterrent or harmful compounds present in a resource intended to attract pollinators?” she asked. “One hypothesis is that these compounds reduce microbial growth, which could otherwise spoil the nectar resource.”
Vannette, an assistant professor in the UC Davis Department of Entomology and Nematology, and her colleague Tadashi Fukami, associate professor at Stanford University, tested this hypothesis by growing yeasts and bacteria in sugar solutions spiked with a chemical compounds that are found in nectar.
“We examined effects on the growth of microbes isolated from nectar and non-nectar sources. Contrary to expectations, chemical compounds only weakly inhibited microbial growth in most cases. Interestingly, some microorganisms even grew better in the presence of plant compounds, like nicotine. But most surprising, we found that microbial growth in nectar reduced nectar toxicity, decreasing the concentration of chemical compounds in some nectar solutions.”
Microbial effects on nectar, in turn, increased consumption of nectar containing chemical compounds by honey bee pollinators, she said. “We found that microorganisms in nectar can both reduce the concentration of some plant compounds in nectar and increase consumption of nectar that does contain these compounds. This indicates that although ‘toxic nectar' does not strongly inhibit microbial growth in nectar, microbes modify the palatability of nectar to pollinators, which can change foraging behaviors and may reduce selection on this trait in nectar.”
The paper, exploring the effects of nectar-inhabiting microbes on chemical compounds found in nectar and nectar consumption by pollinators, “demonstrates that the compounds in nectar—such as on citrus blossoms--do not inhibit microbial growth, Vannette said. “However, yeasts and bacteria that grow in nectar can modify the effects of plant chemical compounds on pollinator foraging and nectar consumption..”
In her abstract, Vannette wrote “Secondary metabolites that are present in floral nectar have been hypothesized to enhance specificity in plant-pollinator mutualism by reducing larceny by non-pollinators, including microorganisms that colonize nectar. However, few studies have tested this hypothesis. Using synthetic nectar, we conducted laboratory and field experiments to examine the effects of five chemical compounds found in nectar on the growth and metabolism of nectar-colonizing yeasts and bacteria, and the interactive effects of these compounds and nectar microbes on the consumption of nectar by pollinators.”
“In most cases, focal compounds inhibited microbial growth, but the extent of these effects depended on compound identity, concentration, and microbial species. Moreover, most compounds did not substantially decrease sugar metabolism by microbes, and microbes reduced the concentration of some compounds in nectar. Using artificial flowers in the field, we also found that the common nectar yeast Metschnikowia reukaufii altered nectar consumption by small floral visitors, but only in nectar containing catalpol. This effect was likely mediated by a mechanism independent of catalpol metabolism. Despite strong compound-specific effects on microbial growth, our results suggest that the secondary metabolites tested here are unlikely to be an effective general defense mechanism for preserving nectar sugars for pollinators. Instead, our results indicate that microbial colonization of nectar could reduce the concentration of secondary compounds in nectar and, in some cases, reduce deterrence to pollinators.”
The research, “Nectar Microbes Can Reduce Secondary Metabolites in Nectar and Alter Effects on Nectar Consumption by Pollinators,” appears on the Ecology website, http://onlinelibrary.wiley.com/doi/10.1890/15-0858.1/full
The research was funded by the Gordon and Betty Moore Foundation, the National Science Foundation, and Stanford University.
Future work will examine how microbial modification of nectar traits influences floral attractiveness, how microbial growth may modify the specificity of plant-pollinator interactions, and if microbial effects vary among plant species.
Vannette, a former postdoctoral fellow at Stanford University, joined the UC Davis Department of Entomology and Nematology in September 2015. “I am interested in understanding and predicting how microbial communities influence interactions between plants and insects,” she said. “In the Vannette lab (in Briggs Hall), we use tools and concepts from microbial ecology, chemical ecology, and community ecology to better understand the ecology and evolution of interactions among plants, microbes and insects."
Ecology journal research paper