Bacteria in soil makes flowers more attractive to pollinators, study finds

Bacteria that live in soil and help roots fix nitrogen may increase the reproductive capacity of some plants, according to a paper published in the American Journal of Botany describing a study of this mechanism in Chamaecrista latistipula, a legume belonging to the family Fabaceae, which includes beans and peas.

Bacteria in the soil enhance the attractiveness of the plant’s flowers to pollinators through a type of relationship known as mutualism, common among plants and animals. Microorganisms such as bacteria or fungi contribute to and benefit from mutualistic relationships with plants, whereby both parties obtain more nutrients or reproduce more vigorously, for example.

In the case of C. latistipula, a shrub whose range extends from Bolivia, Brazil and northeastern Argentina, the soil in which it inhabits tends to be poor in nutrients and its Reproduction depends on a specific type of pollinator.

“Its mutualistic relationship with nitrogen-fixing bacteria increases the supply of nutrients to its roots in exchange for the sugar they feed on,” said Anselmo Nogueira, professor at the Federal University of Natural and Human Sciences Center of the ABC (CCNH). -UFABC) in São Bernardo do Campo, State of São Paulo, Brazil.

“The plant also maintains a mutualistic relationship with a specific type of pollinator. Pollen stored in the anthers of its flowers is only released when they are vibrated, mainly when shaken by the females of certain species of bumblebees of the genus Bombus.”

A greenhouse experiment conducted at the Plant-Animal Interaction Laboratory, led by Nogueira, showed that these bacteria play an important role in making flowers attractive to bumblebees, especially for plants growing in nutrient-poor soil.

“We also observed a drastic effect that we did not expect. Since association with bacteria is very costly for the plant, we assumed that in nitrogen-rich soil, plants would simply take the nitrogen directly from the soil “But in our experiments, nutrient-rich soil does not produce healthy plants with attractive flowers,” said Caroline Souza, first author of the paper.

The experiment was part of the project “Synergistic effect of multiple mutualists on plants: how bacteria, ants and bees contribute to the evolution of a hyper-diverse lineage of legumes”, coordinated by Nogueira.

Bacteria, plants and insects

In the experiment, researchers monitored the growth of 60 C. latistipula plants from seed germination for 16 months. Half were grown in soil composed mainly of sand (90%) with a thin layer of organic topsoil (10%) and a low concentration of nutrients, notably nitrogen. The other half was grown in soil rich in organic matter and supplemented with potassium nitrate, which releases nitrogen into the soil. Soil acidity was monitored for six months in both cases to ensure that pH was neutral and did not influence root-bacteria interaction.

Before sowing, the seeds were sterilized with alcohol, sodium hypochlorite and hydrogen peroxide to eliminate bacteria that could influence the results, then rinsed with distilled water. The soil was sterilized at high temperature in an autoclave to eliminate microorganisms.

The two types of soil were then subjected to different treatments. A solution containing rhizobia (bacteria that fix nitrogen in plant roots) was added to half of the pots with sandy soil low in nitrogen and half of those with organic matter rich in nitrogen. The rest contained no bacteria. The rhizobia used in the experiment were isolated directly from root nodules found on C. latistipula in nature.

In the sandy soil low in nitrogen and without added bacteria, the plants grew very little and had persistent yellowish leaves due to the lack of nitrogen. Plants grown in sandy soil poor in nitrogen and supplemented with rhizobia developed satisfactorily.

“In sandy soil low in nitrogen and containing nitrogen-fixing bacteria, plants were almost twice as tall and three times larger than those grown in nitrogen-rich soil with organic matter and rhizobia. “, plants grown without rhizobia in sandy soils and soils rich in organic matter were shorter and smaller than those grown with rhizobia,” Nogueira said.

The researchers analyzed the flowers using a surface spectrophotometer, which measures how light is reflected. “Based on the flower reflectance measured in this way, we tested changes in color contrasts perceptible by bumblebees in the different soils with and without bacteria,” Souza said.

Significant differences were only detected in plants grown in sandy soil low in nitrogen and containing rhizobia: their anthers showed a pattern considered particularly attractive to bumblebees, which perceive the color spectrum differently from humans.

“The anthers contain the pollen and are only accessible to insects capable of making them vibrate, which exotic species like the European bee Apis mellifera, for example, cannot do,” Souza explained.

Pollen is an essential source of protein for the larvae of all bee species, including bumblebees and other native bees. Nutrients contained in pollen greatly influence the growth and development of larvae.

After taking the measurements, the researchers removed the plants from the pots to analyze their roots. The number of root nodules served as an indication of interaction with rhizobia.

Nodules are button-like structures that form on the roots of legumes following symbiotic infection by nitrogen-fixing bacteria. The mutualistic relationship with bacteria allows plants to make the amino acids they need.

Amino acids and their derivatives serve many functions in plants, contributing to protein synthesis, development, nutrition, and stress responses. In exchange, the plants provide the sugar bacteria with their energy and growth needs, allowing them to proliferate in the nodules.

In the experiment, plants grown in sandy soil low in nitrogen and inoculated with rhizobia had the most nodules.

“We now want to know whether this pollen, accessible only to female native bees, is enriched in proteins and amino acids due to the partnership between plants and bacteria. The increased attractiveness of flowers may be associated with greater amounts of high content of amino acids of quality resources, influenced by the high rate of nitrogen fixation of the roots,” said Nogueira.