Characterizing Symbiotic Interactions Between Paraburkholderia Phymatum and Its Legume Hosts

Abstract

The nitrogen-fixing symbiosis between legumes and rhizobia plays a vital role in agriculture and minimizes the negative impacts of N-fertilizer. Existing knowledge of mechanisms necessary for symbiosis is based on studies carried out in axenic laboratory conditions using papilionoid crop legumes and associated α-rhizobia. In contrast, very little is known about the mechanisms of symbiosis between β-rhizobia symbionts and their Caesalpinioid legume hosts. Therefore, it is crucial to extend research beyond crop legumes and their symbionts to obtain an enhanced understanding of legume-rhizobia symbiosis. Towards this long-term goal, this study is focused on determining the mechanisms of symbiosis between the Mimosoid legume Mimosa pudica and the β-rhizobia Paraburkholderia phymatum. Our results confirmed that bacterial lipochitooligosaccharides (LCO) signaling molecules (Nod factors) are essential for the nodulation of M. pudica by P. phymatum. We also show that a putative glycosyltransferase gene involved in lipopolysaccharide (LPS) biosynthesis is required for nodule formation and development. However, further studies are required to confirm the exact role of LPS in establishing symbiosis. Since M. pudica is not native to the Midwestern USA, we have established that native rhizobia do not nodulate M. pudica in soil. The lack of native bacteria gives an opportunity to examine the interactions among the inoculants, indigenous bacteria, and the host plant. Using our model system, we show that P. phymatum inoculation significantly affects the root-associated microbial community of M. pudica, and P. phymatum can successfully colonize and form nodules on M. pudica roots in soil. Our results provide new information that will likely contribute to the development and optimize legume-rhizobia symbiosis for successful use in sustainable agriculture.