Inoculating Soybean Plants at Different Growth Stages: Selecting Bradyrhizobium, Sinorhizobium, or Both

Soybean plants rely on nitrogen for growth, however atmospheric nitrogen (N2) is not a usable form and needs to be converted into different forms that are available to the plant. Nitrogen fixing bacteria such as rhizobia form symbiotic relationships with Soybeans in exchange for shelter and excess plant sugars. As a result, the plant can get the nitrogen it needs. There are four main genera of rhizobia that can form root nodules on Soybean including Bradyrhizobium, Sinorhizobium, Mesorhizobium, and Rhizobium. At this point, the effect that age of the host plant has on the selection of rhizobia within root nodules is unknown. It is also unknown whether the cell density of two different compatible rhizobia in the rhizosphere influences their selection. This study is focused on the Bradyrhizobium and Sinorhizobium genera and whether soybean plant age plays a role in the selection of rhizobial endophytes. The other focus looks at what effect rhizobial abundance in the rhizosphere has on endophyte selection. Soybeans were grown for fifty-six days and were inoculated with Bradyrhizobium diazoefficiens (previously known as Bradyrhizobium japonicum) USDA 110 (B. diazoefficiens) and Sinorhizobium fredii USDA 191 (S. fredii) at different cell density ratios (1:1, 1:100, 100:1) at different plant growth stages (T0, T2, T4). I assessed the relative bacterial abundance within root nodules across all treatments using Illumina paired-end DNA sequencing of the bacterial 16S rRNA gene. I observed a preferential selection of S. fredii over B. diazoefficiens as plant age increased and across all cell density ratios. In summary, the age of the host plant could be a driving factor for endophyte selection in root nodules. Cell density does not seem to influence selection as the host plant matures.