Bacteria that live in the soil and help roots fix nitrogen can enhance the ability of certain plants to reproduce, according to an article published in American Journal of Botany describing a study of this mechanism in Chamaecrista latistipula, a legume belonging to the Fabaceae family, which includes beans and peas.
Soil bacteria enhance the attractiveness of plant flowers to pollinators through a type of relationship known as mutualism, which is widespread in 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 with a native range that includes Bolivia, Brazil and northeastern Argentina, the soil it inhabits tends to be poor in nutrients and depends on a specific type of pollinator to reproduce.
“Its mutual 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 ABC’s Center for Natural and Human Sciences (CCNH). -UFABC) in São Bernardo do Campo, São Paulo State, Brazil.
“The plant also has a mutualistic relationship with a certain type of pollinator. The pollen stored in the anthers of its flowers is only released when they are vibrated, mainly by being shaken by the females of some species of bumblebee in the genus Bombus.”
A greenhouse experiment at the Plant-Animal Interaction Laboratory, which Nogueira directs, showed that these bacteria play a significant role in making flowers attractive to bumblebees, especially plants growing in nutrient-poor soil.
“We also noticed a drastic effect that we didn’t expect. Because association with bacteria is very costly to the plant, we assumed that in nitrogen-rich soil the plants would simply take nitrogen directly from the soil, but in our nutrient-rich experiments, the nutrient-rich soil did not produce healthy plants with attractive flowers,” he said. said Caroline Souza, first author of the article.
The experiment was part of the project “The synergistic effect of multiple mutualists on plants: how bacteria, ants and bees contribute to the evolution of a hyperdiverse legume lineage”, which is coordinated by Nogueira.
Bacteria, plants and insects
In the experiment, the researchers monitored the growth of 60 C. latistipula plants from germinating seeds for 16 months. Half were grown in soil consisting mostly of sand (90%) with a thin layer of organic soil (10%) and a low concentration of nutrients, especially 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 the pH was neutral and did not influence the root-bacteria interaction.
Before sowing, the seeds were sterilized with alcohol, sodium hypochlorite and hydrogen peroxide to remove bacteria that could influence the results, then they were rinsed with distilled water. The soil was sterilized at a high temperature in an autoclave to eliminate microorganisms.
The two soil types were then subjected to different treatments. A solution containing rhizobia (nitrogen-fixing bacteria in plant roots) was added to half of the pots with nitrogen-poor sandy soil and half of those with nitrogen-rich organic matter. The rest had no bacteria. The rhizobia used in the experiment were isolated directly from root nodules found on C. latistipula in the wild.
In the nitrogen-poor sandy soil with no added bacteria, the plants grew very little and had persistently yellow leaves from the lack of nitrogen. Plants grown in nitrogen-poor sandy soil with added rhizobia developed satisfactorily.
“In nitrogen-poor sandy soil with 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. On the other hand, plants grown without rhizobia in both sandy soil and soil rich in organic matter were shorter and smaller than those grown with rhizobia,” said Nogueira.
The researchers analyzed the flowers using a surface spectrophotometer, which measures how light is reflected. “Based on the flower’s reflectance measured in this way, we tested changes in color contrasts perceived by bumblebees in different soils with and without bacteria,” said Souza.
Significant differences were detected only in plants grown in nitrogen-poor sandy soil with rhizobia: their anthers displayed a pattern considered particularly attractive to bumblebees, which perceive the color spectrum differently than humans.
“The anthers contain pollen and can only be accessed by insects capable of making them vibrate, which cannot be done by exotic species like the European honey bee Apis mellifera, for example,” explained Souza.
Pollen is an essential source of protein for the larvae of all bee species, including bumblebees and other native bees. Pollen nutrients greatly influence larval growth and development.
After taking the measurements, the researchers removed the plants from their pots to analyze their roots. The number of root nodules served as an indication of the interaction with rhizobia.
Nodules are button-like structures that form on the roots of leguminous plants as a result of symbiotic infection with nitrogen-fixing bacteria. The mutual relationship with bacteria allows plants to produce the amino acids they need.
Amino acids and their derivatives perform many functions in plants, contributing to protein synthesis, development, nutrition, and stress responses. In return, the plants provide the sugar bacteria need for energy and growth, allowing them to proliferate in the nodules.
In the experiment, plants grown in nitrogen-poor sandy soil and inoculated with rhizobia had the most nodules.
“Now we want to know if this pollen, which is accessible only to native female bees, is enriched with proteins and amino acids due to the partnership between plants and bacteria. The increased attractiveness of the flowers may be associated with higher amounts of quality resources, influenced by the high rate of nitrogen fixation of the roots,” said Nogueira.
More information:
Caroline Souza et al., Nitrogen-fixing bacteria enhance floral attractiveness in a tropical legume species during nutrient limitation, American Journal of Botany (2024). DOI: 10.1002/ajb2.16363