Optimizing Nitrogen Fixation in Legumes–A New Horizon

“Thank goodness I still live in a world of telephones, car batteries, handguns and many things made of ZINC. ~Bart Simpson

Recent research uncovers zinc’s role in nitrogen fixation in legumes, alongside new insights into the transcriptional regulator FIXATION UNDER NITRATE (FUN) (Lin et al., 2024). These findings hold the potential to revolutionize legume agriculture by optimizing crop performance and decreasing dependence on synthetic fertilizers. Understanding how zinc and FUN influence nitrogen fixation can enhance nitrogen availability, boost crop yields, and promote more sustainable farming practices.

Nitrogen fixation is a biological process in which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds that plants can utilize. This conversion is carried out by microorganisms that possess the enzyme nitrogenase, including those in the root nodules of legumes and free-living bacteria in the soil. The fixed nitrogen is then used by plants to produce proteins, nucleic acids, and other essential molecules, supporting their growth and development. By fixing nitrogen, plants can thrive in environments with low soil nitrogen. Nitrogen fixation enriches soil fertility and reduces the need for synthetic fertilizers, supporting sustainable agriculture (Giller, 2001; Herridge et al., 2008). In legumes, maintaining nitrogen balance is essential and depends on managing nitrogen acquired from the soil alongside the nitrogen fixation in the root nodules. Zinc, an essential micronutrient, plays a significant role in plant physiological functions, including enzyme activation, protein synthesis, and gene expression regulation. It influences plant growth and development, particularly through its involvement in chlorophyll and auxin biosynthesis (Cakmak and Kürşat, 2008; Marschner, 2012).

A research team from Aarhus University, in collaboration with the Polytechnic University of Madrid and the European Synchrotron Radiation Facility, identified zinc as a secondary signal that integrates environmental factors and regulates nitrogen fixation efficiency in legumes. Their study, published in Nature, reveals that a transcription factor FIXATION UNDER NITRATE (FUN), acts as a zinc sensor, interpreting signals within nodules to control nitrogen fixation (Lin et al., 2024). The identification of zinc as an intracellular second messenger or signaling molecule was a significant discovery, achieved through extensive plant genetic screenings. Zinc is essential for controlling FUN by toggling it between an inactive filamentous mega-structure and an active transcriptional regulator state. In nodules, reduced zinc levels, caused by increased soil nitrate concentrations, lead to the disassembly of the filamentous structure and activate FUN. Activated FUN then functions as a transcriptional regulator, initiating nodule senescence or breakdown by modulating the expression of genes involved in nitrate signaling and nitrogen fixation (Fig. 1). This zinc-dependent mechanism allows nodules to adjust their function based on the environmental nitrogen levels.

Figure 1. Schematic showing how Zinc and Fixation Under Nitrate (FUN) regulate nodule function

Variations in zinc levels impact the activity of the FUN protein, which affects nodule function in plants. This mechanism connects soil nitrate availability with the transcriptional regulation of nodule metabolism. The post-translational regulation of FUN enables plants to adapt to fluctuating nitrate levels by gradually decreasing zinc concentrations, thereby enhancing the availability of active FUN and optimizing nodule function according to environmental conditions. Overall, these findings highlight how metal ions like zinc play a role in integrating environmental signals into plant development. Ongoing research aims to investigate how zinc signals are generated and interpreted by FUN, with the hope of applying these findings to other legume crops such as faba beans, soybeans, and cowpeas.  

In legumes, the symbiotic relationship with nitrogen-fixing bacteria in root nodules is affected by environmental factors such as temperature fluctuations, drought, flooding, soil salinity, and high soil nitrogen levels (Batstone et al., 2020). Enhancing nitrogen fixation could benefit both legumes and subsequent crops by improving soil nitrogen utilization, which leads to more efficient farming practices and a reduced need for synthetic fertilizers (Nishida and Suzaki, 2018; Lin et al., 2021). This research has significant implications for increasing crop yields and minimizing environmental impact, addressing both ecological and economic concerns (Yu et al., 2023).

Micronutrient and biostimulant products often incorporate zinc to improve physiological processes like enzyme activation and nutrient uptake by providing crops with a readily available source of zinc (González-Mendoza et al., 2020; Imran et al., 2021). Zinc’s role in increasing crop resilience against extreme weather conditions further underscores its importance, potentially leading to more consistent yields, reduced reliance on artificial fertilizers, and the possibility of cultivating legumes in previously unsuitable areas. The number of products containing zinc in their nutritional guarantee can vary widely based on market trends and new product introductions.

• MYR ZINC by Hello Nature
• Epivio-Zn by Syngenta
• Zn-LSA by Syngenta Biologicals
• CELL POWER® Protein Plus® Zinc 1-1-0 Stimulate by The Catalyst Product Group Micro-400 by AgroLiquid
• ZINCMAX® by Lachlan
• Zinc Complex by Sinclair
• Chlorodrive Foliar by Valent BioSciences
• Microblend- Foliar-Zn-Mn-B
• Ruffin Tuff Zn10% by ATP Ag
• Bio Zinc Rich by Maruti Agro Biotech
• Dora Che Zn by Dora Agritech

*This is not an exhaustive list but just a sample of what is available; for all of the available Zn products please refer to specific market research reports or industry databases.

Zinc mediates control of nitrogen fixation via transcription factor filamentation was published on June 27, 2024, in Nature and can be accessed at DOI: 10.1038/s41586-024-07607-6  . The paper is authored by Jieshun Lin1#*, Peter K. Bjørk1#, Marie V. Kolte1, Emil Poulsen1, Emil Dedic1, Taner Drace13, Stig U Andersen1, Marcin Nadzieja1, Huijun Liu1, Hiram Castillo-Michel2, Viviana Escudero34, Manuel González-Guerrero34, Thomas Boesen1, Jan Skov Pedersen5, Jens Stougaard1, Kasper R. Andersen1*, and Dugald Reid6,7*.

*corresponding author

• Department of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark, [email protected]
• ID21 Beamline, European Synchrotron Radiation Facility, Grenoble, France.
• Centro de Biotecnología y Genómica de Plantas (UPM-INIA/CSIC), Universidad Politécnica de Madrid, Pozuelo de Alarcón, Spain.
• Escuela Técnica Superior de Ingeniería Agronómica, Alimentaria y de Biosistemas. Universidad Politécnica de Madrid, Madrid, Spain.
• Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus, Denmark.
• La Trobe Institute for Sustainable Agriculture and Food (LISAF), La Trobe University, Melbourne, Victoria, Australia, [email protected]
• Department of Animal, Plant and Soil Sciences, School of Agriculture Bioscience and Environment, La Trobe University, Melbourne, Victoria, Australia, [email protected]

References

Lin J, Roswanjaya YP, Kohlen W, Stougaard J, Reid D (2021) Nitrate restricts nodule organogenesis through inhibition of cytokinin biosynthesis in Lotus japonicus. Nat Commun 12: 6544. doi: 10.1038/s41467-021

Batstone RT, Peters MAE, Simonsen AK, Stinchcombe JR, Frederickson ME (2020) Environmental variation impacts trait expression and selection in the legume–rhizobium symbiosis. Am J Bot 107: 195–208. doi: 10.1002/ajb2.1432>/p>

Nishida H, Suzaki T (2018) Nitrate-mediated control of root nodule symbiosis. Curr Opin Plant Biol 44: 129–136. doi: 10.1016/j.pbi.2018.04.006

Lin J, Li X, Luo Z, Mysore KS, Wen J, Xie F (2018) NIN interacts with NLPs to mediate nitrate inhibition of nodulation in Medicago truncatula. Nat Plants 4: 942–952. doi: 10.1038/s41477-018-0261-3

Yu H, Xiao A, Wu J, et al. (2023) GmNAC039 and GmNAC018 activate the expression of cysteine protease genes to promote soybean nodule senescence. Plant Cell 35: 2929–2951. doi: 10.1093/plcell/koad129

Lin J, Bjørk PK, Kolte MV, et al. (2024) Zinc mediates control of nitrogen fixation via transcription factor filamentation. Nature 631: 164–169. doi: 10.1038/s41586-024-07607-6

Giller KE (2001) Nitrogen Fixation in Tropical Cropping Systems. CABI Publishing, Wallingford, UK

Herridge DF, Peoples MB, Boddey RM (2008) Global inputs of biological nitrogen fixation in agricultural systems. Plant Soil 311: 1–18

Marschner H (2012) Marschner’s Mineral Nutrition of Higher Plants, 3rd Ed. Academic Press, London

Cakmak I, Kürşat Y (2008) Zinc deficiency in plants: A review. Sci Res Essays 3: 176–186

González-Mendoza D, Montalvo JG, del Río MÁ (2020) Zinc in plant physiology: Recent advances and future prospects. J Plant Nutr Soil Sci 183: 309–320

Imran M, Ali Shah SZ, Khan MN, Ullah N (2021) Zinc-based biostimulants for plant growth enhancement: An overview. J Soil Sci Plant Nutr 21: 100–114