Soil bacteria makes flowers more attractive to pollinators

Bacteria that reside in soil and help plant roots fix nitrogen can significantly boost the reproductive capacity of certain plants by making the flowers more attractive, according to new research.

The study focuses on Chamaecrista latistipula, a legume in the Fabaceae family, which includes beans and peas.

Soil bacteria and plant flowers

Soil bacteria enhance the attractiveness of the plant’s flowers to pollinators through a type of mutualistic relationship. In mutualism, both organisms benefit – plants receive more nutrients, and microorganisms such as bacteria or fungi thrive.

Chamaecrista latistipula is a shrub native to regions of Bolivia, Brazil, and northeast Argentina, where it grows in nutrient-poor soils. It relies on a specific pollinator to reproduce successfully. 

“Its mutualistic relationship with nitrogen-fixing bacteria increases the supply of the nutrient to its roots in exchange for the sugar on which they feed,” explained Anselmo Nogueira, a professor at the Federal University of the ABC (UFABC), in São Paulo, Brazil.

Mutualistic relationship with pollinators

In addition to its relationship with bacteria, C. latistipula has a mutualistic relationship with specific pollinators. 

“The pollen stored in its flowers’ anthers is only released when they are made to vibrate, mainly by being shaken by females of some species of bumblebee in the genus Bombus,” Nogueira said.

Bacteria make the flowers appealing 

In a greenhouse experiment conducted at the Plant-Animal Interaction Laboratory, led by Nogueira, it was revealed that these bacteria play a significant role in making the flowers more appealing to bumblebees, especially in nutrient-poor soils. 

Unexpectedly, the researchers also found that nutrient-rich soil did not lead to healthy plants with attractive flowers. 

“Because the association with bacteria is very costly for the plant, we assumed that in nitrogen-rich soil the plants would simply take nitrogen directly from the soil, but in our experiments, nutrient-rich soil didn’t produce healthy plants with attractive flowers,” explained Caroline Souza, the first author of the article.

Focus of the research

The experiment was part of a broader project examining how bacteria, ants, and bees contribute to the evolution of legumes. 

The team monitored 60 C. latistipula plants over 16 months, from seed germination. Half were grown in nutrient-poor sandy soil, while the other half were in soil rich in organic matter, supplemented with potassium nitrate. 

Both soil types were sterilized to remove any existing microorganisms, and the seeds were sterilized before sowing to prevent external bacterial influence.

The role of nitrogen-fixing bacteria 

The experiment revealed that plants grown in sandy soil with added rhizobia (nitrogen-fixing bacteria) developed much better than those without bacteria or those grown in nutrient-rich soil. 

“In the nitrogen-poor sandy soil with nitrogen-fixing bacteria, the plants were almost twice as tall and three times larger than those grown in nitrogen-rich soil with organic matter and rhizobia,” Nogueira said. 

In contrast, plants grown without rhizobia, regardless of soil type, were smaller and less healthy.

Colors that attract bumblebees 

The researchers analyzed the flowers of these plants using a surface spectrophotometer to measure how light was reflected, testing for color contrasts perceptible to bumblebees. 

Significant differences were observed only in the plants grown in sandy soil with rhizobia, whose anthers displayed a pattern particularly attractive to bumblebees. 

Bumblebees perceive the color spectrum differently than humans, and the unique flower characteristics likely contributed to their attraction.

“The anthers contain the pollen and can be accessed only by insects capable of making them vibrate, which can’t be done by exotic species such as the European honeybee Apis mellifera,” Souza explained. 

Pollen is a vital protein source for bee larvae, influencing their growth and development.

Broader implications of the study 

After the flower analysis, the researchers examined the roots of the plants and found that the plants grown in nutrient-poor sandy soil with rhizobia had the most root nodules. 

These nodules are knob-like structures where nitrogen-fixing bacteria live in a symbiotic relationship with the plant. The bacteria help the plant produce amino acids needed for growth, while the plant supplies the bacteria with sugars for energy and reproduction.

The study suggests that the mutualistic relationship between plants and bacteria enhances the quality of the pollen, potentially increasing its protein and amino acid content, making the flowers more attractive to native bees. 

“The heightened attractiveness of the flowers may be associated with larger amounts of high-quality resources, influenced by the roots’ high nitrogen fixation rate,” Nogueira concluded.

The study is published in the American Journal of Botany. 

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