Do seasons impact the root microbiome of wild Poa annua populations?

Low temperatures, one of the main characteristics of winters in temperate climates, are restrictive for agriculture, because they negatively affect plant growth and development. To alleviate cold stress, plants are able to establish a symbiosis with soil bacteria. Annual bluegrass ( Poa annua ) is adapted to low temperatures, has a global distribution, and colonizes a wide range of environments, including polar regions. Here, Poa annua plants were harvested randomly within a one square meter plot at five sites in Flanders, Belgium, during September (summer samples) and March (winter samples) over 2019 and 2020. For each time point, location and season, ten samples of bulk soil, rhizosphere soil and root endosphere were obtained, making a total of 570 samples. As specialized bacteria in the winter root microbiome may reduce the negative impact of cold, we wanted to unravel how winter temperatures affect the root microbiome. Amplicon sequencing of 16S rRNA genes revealed that location was the factor with the largest influence on the bacterial community composition, followed by compartment (root endosphere, rhizosphere, and bulk soil), season, and year. While season was not the main determining factor of the root endosphere, comparison of the summer and winter samples allowed the identification of cold-enriched root-colonizing bacterial families. Interestingly, one ASV corresponding to Massilia was enriched in all sites during the winter season and, thus, may represent an interesting taxonomic group for the development of biostimulants to help plants cope with cold conditions.