Holly Vuong, from CSIRO in Canberra, Australia, recently had a paper published in Journal of Ecology entitled “Host species and environmental variation can influence rhizobial community composition”. Below, Holly presents the context of her study on plant-microbes interactions in nutrient and water limited Australian soils and summarizes the findings of her experiment.

Out of about 1,350 Acacia species in the world, approximately 1000 are present in Australia making it a major hotspot for these leguminous (seed pod) plants. The key to Acacia survival in this nutrient and often water deficient environment is their association with important microbes in the soil. But this association is not a one-way street, rather, the microbes can affect how well the plants grow, and the plants can select which microbes it interacts with. This symbiotic relationship can be complex and multiple factors can affect these interactions.

Rhizobia are a special group of bacteria in the soil that aid in the survival of leguminous plant species. Rhizobia take nitrogen from the air, which the plants cannot use in this gaseous state (N₂), and convert it into a usable organic form that they offer to their plant host. In return, rhizobia get plant carbohydrates (i.e. sugars) and protection by living within nodules on the plant roots.

Given this symbiotic relationship, we might expect that both the host plant species and the bacteria have co-evolved to be more suited to each other. This would lead to a lower (genetic) diversity of rhizobial strains associating with the plant host. However, soils from the field often show that these microbes are even more diverse than the host plants, leading us to wonder what factors affect rhizobial composition and diversity.

Holly’s paper was published in Volume 105, Issue 2: http://onlinelibrary.wiley.com/doi/10.1111/1365-2745.12687/full

Acacia stenophylla and A. salicina are two common species found throughout the eastern temperate environments of Australia. Our lab has studied these species for the past two decades in order to understand their host-rhizobial interactions. We know that some rhizobial strains are specialists for some host species, while other strains are more generalists, meaning they will associate with multiple host plant species with positive effects for the host plants. Furthermore, some rhizobial strains can be poor nitrogen fixers but still form associations with the host plant.

To investigate the role of the host species and environmental variability (combinations of no or high nitrogen, low or high phosphorous, and low or high water levels), we germinated both host species from sterilised seeds and planted individual seedlings in steam sterilised potting mix in 10L containers within a glasshouse. The next day, we inoculated our experimental plants with 1ml of our rhizobial mix, which included two specialist strains (one for each host species), two generalist strains, and two generalist strains with poor efficiency in nitrogen fixing. We applied the environmental treatment and harvested half the plants at 19 weeks, and the rest at 30 weeks. We used genetic sequencing to determine the rhizobial community composition of those six strains within the root nodules.

Our results indicated that host species was the biggest factor affecting the relative abundance of these microbes, and interactions with nutrients and water also influenced the microbial community composition. The generalist strains were as abundant or more abundant than the specialist strains within their host species.

To further our understanding of the ecological and evolutionary factors that promote and drive rhizobial genetic variability, we need to examine host-rhizobial interactions in natural settings. This could include using soils from the field to inoculate potted glasshouse plants and examining how rhizobia associate with their host plants across a larger biogeographical space.

Holly Vuong, CSIRO Agriculture and Food, Canberra, Australia

Read the full paper here: http://onlinelibrary.wiley.com/doi/10.1111/1365-2745.12687/full