Biological nitrogen fixation maintains carbon/nitrogen balance and photosynthesis at elevated CO2

Authors: Brooks, Matthew; SZETO, RONNIA - University Of Illinois

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2024
Publication Date: 3/13/2024
Citation: Brooks, M.D., Szeto, R.C. 2024. Biological nitrogen fixation maintains carbon/nitrogen balance and photosynthesis at elevated CO2. Plant Cell and Environment. 47(6):2178-2191. https://doi.org/10.1111/pce.14873.
DOI: https://doi.org/10.1111/pce.14873

Interpretive Summary: While increasing atmospheric CO2 concentrations has the potential to stimulate photosynthesis and crop yields, the extent of this benefit is often negated by nitrogen limitation. Understanding how plants respond to elevated CO2 will enable the development of strategies that will maximize crop yields in future climates and support increasing agricultural needs. Legumes, which are capable of acquiring biologically fixed nitrogen through association with bacteria, are predicted to respond better to elevated CO2. We tested the importance of biological nitrogen fixation by growing alfalfa lines with and without mutations that make them incapable of acquiring fixed nitrogen at different nitrogen doses and at ambient or elevated CO2. We show that biological nitrogen fixation is important for alfalfa plants to maintain carbon/nitrogen balance and photosynthesis and identify regulatory genes involved in the underlying gene expression networks. By directly addressing the role of biological nitrogen fixation on responses to elevated CO2 within a species, this work improves our understanding of the limitations to plant growth in future climates.

Technical Abstract: Understanding how crops respond to elevated CO2 is necessary to ensure increasing demands on agriculture are met. Crops often do not achieve potential yields at high CO2 due photosynthetic down-regulation, often associated with nitrogen limitation. Legumes have been proposed to have an advantage at elevated CO2 due to their ability to exchange carbon for nitrogen. Here, the effects of biological nitrogen fixation on the physiological and gene expression responses to elevated CO2 were examined across nitrogen treatments by comparing alfalfa lines with mutations that makes their nodules incapable of nitrogen fixation to wild-type. While plants capable of biological nitrogen fixation only had increased biomass under conditions of elevated CO2 and the lowest nitrogen dose, elemental analysis revealed broader changes in carbon/nitrogen balance at elevated CO2. Lower photosynthetic rates at elevated CO2 were correlated to the imbalance in carbon/nitrogen. Genome-wide transcriptional responses were used to build a gene regulatory network and identify transcription factors important to carbon/nitrogen signaling. This work supports the hypothesis that maintenance of carbon/nitrogen homeostasis at elevated CO2 can be achieved in plants capable of biological nitrogen fixation. Important regulators of carbon-nitrogen signaling networks were revealed and include the alfalfa GLK ortholog.