STANFORD, Calif. – New research finds that certain soil microbes influence how plants and bees interact.

When Aidee Guzman, assistant professor of biology at Stanford University, began studying for her doctoral degree, she was interested in exploring bee health and conservation. One question that drove her curiosity was understanding how to make agricultural systems more nutritious for bees.

In her pursuit to study bee health and conservation, Guzman came across the mutualistic relationship between mycorrhizal fungi and plants. Mycorrhizal fungi are a group of fungi that form symbiotic relationships with the roots of plants, greatly increasing the absorptive area of a plant and enhancing nutrient and water uptake. In turn, the fungi receive access to sugars produced from photosynthesis.

Guzman wanted to know if a plant that has more positive associations with mycorrhizal fungi below ground led to more positive effects for the plant above ground. Newly published research conducted by Guzman and colleagues investigated how arbuscular mycorrhizal fungi (AMF), a type of mycorrhiza in which the fungus penetrates the roots of a vascular plant forming arbuscules, influenced floral traits.

The research focused on summer squash (Cucurbita pepo L. var. cylindrica). The research team found that in a laboratory setting, squash plants inoculated with AMF demonstrated enhanced flower size and number of flowers as well as nectar volume and pollen protein. Bee visitation to plants was highest for plants inoculated with a diverse composition of AMF species.

Interestingly, a trade-off was observed between higher AMF concentrations and flower size. Especially high AMF root colonization led to smaller flowers, adding to the idea that not all AMF are the same and have varying effects on flower size, nectar volume, and other measures studied.

“The way we are thinking (about the trade-off) – and obviously this would mean more research – is that we think when plants associate with mycorrhizal fungi, they produce more hyphae, so they are able to explore the soil more, and then that leads to more positive benefits,” Guzman said. “But potentially when they associate with mycorrhizal fungi that just establish themselves in the root, they might be associating with cheater AMF that might not actually provide many nutrients but take a lot of carbon from the plant instead.”

Guzman added that many people ask about mycorrhizal fungi inoculants to improve plant health. She said many inoculants have low species diversity, and it’s not possible to guarantee that the inoculant captures the diversity of functions that the fungi could provide to plants.

Additionally, Guzman was surprised to learn that the main driver of bee visitation in the study was bigger flower sizes. Prior to the study, she thought that bee visitation would be driven by the nutritional value of the nectar and pollen. Still, the results of the study showed that AMF can lead to increased protein content in pollen, increasing nutritional value for bees.

“There’s a potential through these associations and by fostering beneficial organisms in the soil, that we could improve or at least contribute to the health of bee communities,” she said. “For people focused on pollinator conservation, we may have been overlooking these other factors (in soil) that might help bees and bee health in their communities.”

In the future, Guzman is interested in exploring if management practices lead to certain effects on AMF diversity and if those effects led to certain outcomes in bee communities and studying plant-pollinator interactions on active farms. Her research group is also looking into the interactions between the “three sisters” (squash, corn and bean) crops to understand the cascading effects of multiple plants in close proximity.

For farmers interested in supporting bee health and bee communities, Guzman said that plant diversity is important, and that published research indicates hedgerows attract a lot of pollinators. Previously, she studied monoculture squash farms versus farms with squash and other crops, curious to see if fields plentiful with squash flowers would support squash bees, specialized pollinators that only eat squash pollen. Interestingly, she found that squash bees preferred to pollinate squash plants at polyculture farms because they offered a more diverse source of nectar.

She also added that bees need to eat beyond just the months that crops are flowering, advocating for floral diversity and floral strips that bloom at different points in the season.