by | Dec 23, 2024
To make crops more nutritious and attractive for essential pollinators like bees, researchers are looking below the soil to an enigmatic group of fungi which may enhance the floral traits that attract and feed pollinators.
Arbuscular mycorrhizal fungi are a type of fungus that form symbiotic relationships with nearly all plants on the planet. “To really understand mycorrhizal fungi, you have to go back 450 million years,” said Aidee Guzman, assistant professor of biology at Stanford University.
In prehistoric times, land plants had limited root systems that struggled to obtain certain nutrients, like phosphorus and nitrogen, from the soil. Enter fungi that first colonized root systems, growing networks of filaments called hyphae, which they use to scavenge the soil for the valuable nutrients. These fungi which live within the roots can then directly give these nutrients to plants in exchange for carbon which the plants have fixed into of sugary substances during photosynthesis.
During her Ph.D., which she completed at the University of California Berkely, Guzman wanted to find ways to make crops more nutritious for bees. In doing so, bee populations would be healthier and the crops would benefit from increased visits and pollination by the insects.
“I started thinking about plants and where they get the nutrients,” she said, which brought her to soil and the mycorrhizal fungi. Given that the fungi provide nutrients that are essential for producing the pollen, nectar, and flowers that bees need to survive, exploring the relationship between nutrients, fungi, and plant traits seemed like an promising avenue of inquiry.
The only problem: the complexity of the plant-fungi-environment interaction.
In nature, there isn’t just one type of arbuscular mycorrhizal fungi. “There’s hundreds, if not thousands of mycorrhizal fungal species,” said Guzman. Furthermore, each species can behave differently depending on the plant it is growing with and the environmental conditions in the soil, making this a complex web of variables to sift through.
Many previous studies on these fungi-plant relationships limited this complexity for practical reasons by using a single fungi species in experiments or simply looked at plant growth in the presence or absence of fungi.
“When we set the experiment up, we decided to use different communities of mycorrhizal fungi,” said Guzman.
The communities they chose were based on informal classifications made by other researchers in the field. Broadly speaking, they could classify communities of fungi as competitors, stress tolerators, or ruderal fungi. The competitors grow fast and outcompete others when establishing colonies in the roots.
Stress-tolerant communities withstand events like drought and heat better than others, and the ruderal communities grow fast, have a high turnover and perform well in disturbed environments.
To make things more difficult, Guzman wanted to test more than just plant growth — she wanted to pinpoint relationships between these fungi communities and specific plant traits important for pollinators, like flower size and the nutritional value of pollen and nectar.
To do this, she needed to track multiple plant and fungal traits in high and low nutrient soil conditions, when plants were grown together with each fungi community on their own, or with a mixture of all three. Then, she needed to see how attractive the plants were to bees.
Unsurprisingly, plants grew bigger with any community or mix or fungi than without. “But when we started looking at the floral traits, that’s where we started seeing differences between the communities,” said Guzman.
“For example, associations with some mycorrhiza fungi resulted in more protein rich pollen, whereas some associations resulted in higher nectar volume,” she said.
That no one community of fungi or mixture of communities clearly showed a benefit across all plant traits and pollinator attractiveness highlights the complexity of these interactions. However, the main trait which attracted the pollinators turned out to be flower size. When digging deeper into the data, Guzman saw that plants whose fungal communities grew more hyphae, the fungal filaments which grab nutrients from the soil, grew bigger flowers.
“If the plants end up associating with mycorrhiza fungi that are really good at colonizing their roots, but they’re not growing enough hyphae, then they might actually give carbon with nothing in return,” she explained.
She also saw that in general the mixed communities of fungi produced the most hyphae. This is likely due to a mixed community being able to compensate for each other’s weaknesses. “You have [arbuscular mycorrhizal fungi] that are able to do things that the other ones can’t,” she said, “and they complement each other and maybe that’s why we saw more hyphae.”
One of the important takeaways of this work for Guzman is how much there is still to be uncovered about the fungi–plant relationship, and researchers need to increase the complexity and scope of their experiments.
“Farmers, when they come onto a farm, they’re not just managing for biomass or plant growth, they’re managing the soil, they’re managing for pollination, they’re managing all the pieces together,” said Guzman. “So we need more studies that integrate all those components.”
Reference: Aidee Guzman, et al. Arbuscular mycorrhizal interactions and nutrient supply mediate floral trait variation and pollinator visitation, New Phytologist (2024). DOI: 10.1111/nph.20219
Feature image credit: Reba Spike on Unsplash
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