Phosphoenolpyruvate reallocation links nitrogen fixation rates to root nodule energy state

Legume-rhizobium symbiosis in root nodules fixes nitrogen to satisfy the plant’s nitrogen demands. The nodules’ demand for energy is thought to determine nitrogen fixation rates. How this energy state is sensed to modulate nitrogen fixation is unknown. Here, we identified two soybean (Glycine max) cystathionine β-synthase domain–containing proteins, nodule AMP sensor 1 (GmNAS1) and NAS1-associated protein 1 (GmNAP1). In the high–nodule energy state, GmNAS1 and GmNAP1 form homodimers that interact with the nuclear factor-Y C (NF-YC) subunit (GmNFYC10a) on mitochondria and reduce its nuclear accumulation. Less nuclear GmNFYC10a leads to lower expression of glycolytic genes involved in pyruvate production, which modulates phosphoenolpyruvate allocation to favor nitrogen fixation. Insight into these pathways may help in the design of leguminous crops that have improved carbon use, nitrogen fixation, and growth. Description Nitrogen fixation kept in balance Legumes such as soybeans can assimilate atmospheric nitrogen with the assistance of symbiotic bacteria that reside in root nodules. Nitrogen fixation is energy intensive, and the cellular energy status in the nodule determines the rate of nitrogen fixation. Ke et al. investigated that linkage using molecular sensors in the root nodule that respond to AMP concentrations. The sensors regulate access of a nuclear regulatory factor to certain glycolytic genes, thus shifting allocation of phosphoenolpyruvate between competing pathways to drive up nitrogen fixation when the nodule has enough energy. —PJH Cellular energy status regulates the physiology of nitrogen-fixing nodules in soybeans.