Abstract: The microbiome of legumes such as soybean are a key mediator of the plants growth traits. For example, Bradyrhizobia that reside within the nodules of the soybean root have been shown to provide the plant with significant levels of nitrogen through fixation of atmospheric N2. It was once widely thought that Bradyrhizobia were essentially the only bacteria residing within nodules, but now it is known that nodules can have many different bacterial phyla. The role that this diversity plays in soybean growth is not known. Evidence suggests that soybean benefits from phosphorous-solubilizing and siderophore producing bacteria. Here, we describe the microbiome of different soybean microbial habitats with a focus on understanding their compositional similarities, whether these habitats supply bacterial inoculum for each other.
Soil, rhizosphere, root nodules, and seeds were sampled to determine bacterial composition by describing the 16s rRNA genes from three different soybean cultivars. 16s rRNA genes between three cultivars were averaged to determine the dominant families that reside within each habitat. Dominant families are defined as composing at least 1% of the microbial community in its respective habitat.
The rhizosphere contained about 1/5 of the number of families found in the soil. Half of the dominant families in the rhizosphere were not dominant in the soil. All of the dominant families in the nodule are also dominant in the rhizosphere.
As expected, though habitat composition was variable, there were also clear trends between habitats regardless of cultivar. The results support the idea that there may be selective filtering of bacterial taxa moving from the soil, to the rhizosphere, and into the nodule. For example, Paenibacillaceae were dominant in the rhizosphere and nodule, but not in the soil. Although this family does occur in the soil under 1%, one or many mechanisms associated with the roots and nodules promote the growth of this family over others.
The composition of dominant microbes in nodule is a subset of the dominant microbes in the rhizosphere. This indicates that the rhizosphere provides the inoculum for the nodule.
Bradyrhizobiaceae are mutualists with soybean, but what are the functions of other soybean microbiome taxa: associates or mutualists or antagonists? Eventually, we will determine the importance of these bacterial taxa to soybean health, growth, and occurrence of disease.
