How do bacteria affect plant growth?

The ability of helpful microorganisms to compete with one another for host infection and the capacity of hosts to select between them increases evolutionary conflict that is projected to destabilize mutualism.

Plants create relationships with bacteria in their growing soil. For example, legumes benefit from a symbiotic relationship with microorganisms that live in nodules in their roots and “fix” nitrogen in the atmosphere to make it available for the legumes’ growth. But are microorganisms always good for plants? Or does competition for plant access across strains diminish the function that bacteria ultimately provide?

A team of scientists from the University of California, Riverside, conducted studies to answer these concerns and better understand the competition process. Acmispon strigosus, a native California plant with nodules, and eight nitrogen-fixing bacterial strains were utilized.

The researchers infected plants with each of the eight strains to directly test their potential to infect and benefit the plants. They next infected other plants with pairs of bacterial strains to evaluate each strain’s competitive capacity and influence on plant performance. The researchers discovered that competition among helpful soil bacteria strains reduces the service the bacteria deliver to their hosts. 

Joel Sachs, a professor of evolution, ecology, and organismal biology, who led the research team, said, “More specifically, we found an interstrain competition that occurs in the soil before the bacteria infect the plant causes fewer of the bacteria to colonize the plant, resulting in the plant gaining smaller benefits in the end, To understand symbiosis, we often use sterile conditions where one strain of bacteria is ‘inoculated’ or introduced into an otherwise sterile host. Our experiments show that making that system slightly more complex simply by using two bacterial strains at a time fundamentally shifts the balance of benefits that the hosts receive, reshaping our understanding of how symbiosis works.”

Sachs explained that exploiting the benefits that microorganisms may bring to crops by sustainably boosting growth without the environmental costs of chemical fertilizers is a key challenge in agriculture. His group examines rhizobia, which are microorganisms that help plants develop. Rhizobial competition has long been a source of concern for sustainable agriculture. Rhizobia bacteria generate root nodules on legumes, which fix nitrogen for the plant in return for carbon from photosynthesis. Growers have long sought to use rhizobia to sustainably fertilize staple legume crops such as soybeans, peanuts, peas, and green beans. 

Sachs, who chairs the Department of Evolution, Ecology, and Organismal Biology, said, “One might think using rhizobia as inoculants should allow growers to minimize the use of chemical nitrogen, which is environmentally damaging,” “But such rhizobial inoculation is rarely successful. When growers inoculate their crops with high-quality rhizobia strains that fix much nitrogen, these ‘elite’ strains get outcompeted by indigenous rhizobia already in the soil, providing little or no benefit to hosts.”

This interstrain rivalry that happens in the soil before the bacteria infect the plant causes fewer bacteria to colonize the plant, resulting in the plant obtaining less benefits. The researchers discovered that competition between types of helpful bacteria in the soil reduces the bacteria’ service to their hosts.

A significant difficulty in agriculture is utilizing the benefits that microorganisms may provide to crops by sustainably boosting growth without the environmental costs of chemical fertilizers. Their research focuses on rhizobia, which are microorganisms that encourage plant growth. Rhizobial competition has long been a source of concern for sustainable agriculture. Growers have long sought to use rhizobia to sustainably fertilize staple legume crops such as soybeans, peanuts, peas, and green beans.

Sachs and his colleagues used previously sequenced bacterial genomes in their investigations. They also characterized the strains, which ranged from highly advantageous to inefficient at nitrogen fixation, to determine their exact benefit to the target plant species. The researchers sequenced the contents of almost 1,100 nodules from plants infected with one of 28 distinct strain combinations.

Based on previous research, the researchers created mathematical models to anticipate how much advantage co-inoculated plants will acquire. “clonally infected” (infected with one strain). This allowed the researchers to calculate the growth deficit caused by interstrain competition.

According to researchers, to uncover and cultivate a bacterial strain that is highly favorable to plants, scientists must conduct tests under extremely clean conditions. However, the plant is covered in microorganisms in the field, confounding the picture. In their trials, they progressed from employing one strain to a pair of strains to observe how this affected plant growth. Surprisingly, with only two strains, many of our forecasts failed.

While experiments are required to determine how helpful a bacterial strain is, experiments that assess the strain’s competitiveness against a panel of other bacterial strains are also needed, according to Rahman. His research discovered that some of the most significant strains can benefit plant growth. However, that effect is considerably decreased when combined with any other strain. Furthermore, it is critical to understand when the interstrain competition occurs: before or after the bacteria interact with the plant. Our findings support the former and serve as a good guide for future research to develop superior strains for crop delivery.

According to Sachs and Rahman, sustainable growth practices must be vital to new agriculture to feed a growing population with a finite resource base. This will necessitate abandoning polluting practices such as applying massive amounts of chemical nitrogen to the soil. Understanding how to transfer beneficial bacteria to a specific host in an effective manner is a critical challenge in medicine, agriculture, and cattle research. This work has opened up new research paths to improve sustainable agriculture practices by finding that interstrain dynamics can limit the benefits of symbiosis.

He said, “It’s important to remember that bacteria are shaped by natural selection. Some of them may be highly competitive in entering the nodule to infect the plant but not be very beneficial to the plant, and that could be a trait that wins out in nature. Suppose we leverage microbial communities for the services they can provide to plants and animals. In that case, we need to understand interstrain dynamics in these communities.”

Journal Reference:

1. Arafat Rahman,Joel L. Sachs, etal. Competitive interference among rhizobia reduces benefits to hosts. Current Biology. DOI: 10.1016/j.cub.2023.06.081