The Importance of Bacterial Resource-Hoarding for the Soybean-Rhizobia Symbiosis.

The nitrogen-fixing bacteria known as rhizobia can dramatically improve yield and nitrogen-use efficiency of soybean. However, agronomic benefits from the soybean-rhizobia symbiosis have been limited, in part, due to poor persistence of introduced inoculum strains. Inside soybean nodules, rhizobia can allocate plant photosynthate either to respiration that drives nitrogen-fixation, or to energy storage in the lipid polyhydroxybutyrate (PHB), which can comprise upwards of 50% of cell biomass. We hypothesize that storing more energy as PHB improves long-term persistence in the soil (after nodules senesce) at the expense of nitrogen fixation efficiency and plant benefit. To investigate this hypothesis, we are comparing PHB-hoarding phenotypes and plant benefit among rhizobia strains isolated from a soybean field under conventional management. So far, we have identified several strains that differ in PHB-accumulation, but do not differ in several measures of plant benefit. These include nitrogen fixation efficiency measured as respiration cost per unit of nitrogen fixation activity and as g shoot mass per g nodule mass. We are working to expand our sample to assess relationships among PHB-accumulation, plant benefit, and other rhizobial traits that could contribute to plant benefit or soil persistence. This research addresses fundamental questions about the legume-rhizobium symbiosis that could help inform strategies for improving agronomic benefits. For example, is there an inherent tradeoff between plant benefit and long-term persistence in the soil? If not, it may be possible to develop persistent inoculum strains that can compete against resident rhizobia. If a tradeoff exists, it may be more valuable to focus on strategies to increase current-year benefits from rhizobia, such as breeding plants to allocate more resources to better-performing nodules.