Understanding cooperation and conflict in plant symbionts
Medicago truncatula plants in the greenhouse were infected with Sinorhizobium meliloti. Credit: Rebecca Batstone

Symbiosis is a long-term interaction between two organisms that benefits both of them. Researchers have debated if the interests of the symbionts always line up with their hosts or if genes that benefit symbionts might come at the expense of their hosts. This question is being investigated through a new study.

Crop health and our own health are governed by the organisms that we interact with. In agriculture, there's a big movement to try to engineer microbes to make plants happier. A natural population of symbionts that have been interacting with their host for a long time can be measured to see what genes in nature are doing.

Rebecca Batstone is a former graduate student at the Carl R. Woese Institute for Genomic Biology. The interests of the symbionts don't always align with that of the host they live in. We want to know how much alignment there is between hosts and symbionts.

Medicago truncatula, a native of the Mediterranean region, was examined by the researchers in their study. Nitrogen is supplied to the plant by the microbe in the root nodules. A competitive situation can occur in nature when a group pairs each strain with an individual plant.

Batstone said that they measured the plant's performance with the particular strain. In addition to measuring plant growth, we looked at the fitness of the microbes in the nodules and the size of them.

The researchers used a technique called genome-wide association to compare which genes are associated with plant growth.

"If there is an association, for example one gene of interest is strongly associated with plant growth, then it's indicative that the gene might be important for controlling that trait."

The researchers found that most of the genes they identified were associated with alignment.

There will be a separation between the interests of the host and symbionts in a competitive environment. The result shows that even though the symbionts aren't evolving in order to benefit their hosts, they can still be beneficial.

The group would like to look at more hosts to see if the trend continues with a larger sample of plant types. They want to test these interactions in different environments.

Plants get nitrogen from their symbionts, so we did everything under low nitrogen conditions. Batstone said that adding nitrogen to the system might cause more conflict.

More information: Rebecca T. Batstone et al, Phenotypic and genomic signatures of interspecies cooperation and conflict in naturally occurring isolates of a model plant symbiont, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.0477 Journal information: Proceedings of the Royal Society B