Optimizing plant-microbe interactions for sustainable supply of Nitrogen for bioenergy crops

Nitrogen (N) often is limited in plants, affecting plant growth and productivity. Large amounts of N-fertilizer is added to compensate, which in turn has consequences of heavy energy demand, greenhouse gas emission and eutrophication from agricultural field run-offs. It is well known that plants are strongly affected by their associated microbes. Diazotrophs are known to fix atmospheric N to ammonia in diverse plants. Our research focuses on investigating interaction between plants and diazotrophic bacteria to explore sustainable availability of adequate amounts of fixed N to the host-plant. As part of this project, several phylogenetically diverse endophytic and root-associated bacterial isolates were obtained from roots, leaves, rhizoplane and rhizosphere of Tobacco (Nicotiana tabacum) and Switchgrass (Panicum virgatum). As demonstrated by PCR amplification of nifH and by measuring nitrogenase activity, several of them are potential N₂-fixing bacteria. To validate N₂-fixing activity of isolated endophytic diazotrophs in-planta, fluorescent-tagged strains were used to re-infect aseptic Tobacco and Switchgrass seedlings and examined by different imaging techniques. In addition, we are investigating their ability to solubilize Phosphorus, another key element for plant life. We examined the utilization and transformation of tobacco root exudates by one of the N₂-fixing isolate closely related to Azospirillum lipoferum. Preliminary results reveal utilization of organic acids malate, glycolate, itaconate, citraconate from the exudate cocktail. An understanding of key metabolites exchanged and key pathways stimulated in both the plants and the N₂-fixing strains will allow us to design robust strategies to engineer diverse crops with decreased dependency on fertilizers.