Zinc regulates nitrogen fixation in legumes by controlling the activity of the transcription factor FUN. The study shows that zinc acts as an intracellular second messenger that links environmental changes to the regulation of nodule function. FUN is a bZIP-type transcription factor that senses zinc levels and controls the transition between an inactive filamentous megastructure and an active transcriptional regulator. Lower zinc concentrations in nodules, which occur in response to higher soil nitrate levels, dissociate the filament and activate FUN. FUN then targets multiple pathways to initiate nodule senescence. The zinc-dependent filamentation mechanism allows legumes to adapt nodule function to environmental nitrogen conditions. FUN regulates nodule senescence by controlling the expression of genes involved in nitrate signalling and nitrogen fixation, such as NRT2.1, HO1, and NAC094. The FUN sensor domain has homology to metal-binding proteins and is regulated by zinc, which changes its oligomeric state and activity. Zinc binds to the FUN sensor, altering its structure and activity, and this process is reversible. The study also shows that zinc levels in nodules decrease in response to nitrate, which releases active FUN and increases nodule senescence. The findings suggest that zinc acts as a second messenger that links nitrate availability to the regulation of nodule function. The results have implications for understanding the role of metal ions in integrating environmental signals with plant development and optimizing nitrogen fixation in legumes.Zinc regulates nitrogen fixation in legumes by controlling the activity of the transcription factor FUN. The study shows that zinc acts as an intracellular second messenger that links environmental changes to the regulation of nodule function. FUN is a bZIP-type transcription factor that senses zinc levels and controls the transition between an inactive filamentous megastructure and an active transcriptional regulator. Lower zinc concentrations in nodules, which occur in response to higher soil nitrate levels, dissociate the filament and activate FUN. FUN then targets multiple pathways to initiate nodule senescence. The zinc-dependent filamentation mechanism allows legumes to adapt nodule function to environmental nitrogen conditions. FUN regulates nodule senescence by controlling the expression of genes involved in nitrate signalling and nitrogen fixation, such as NRT2.1, HO1, and NAC094. The FUN sensor domain has homology to metal-binding proteins and is regulated by zinc, which changes its oligomeric state and activity. Zinc binds to the FUN sensor, altering its structure and activity, and this process is reversible. The study also shows that zinc levels in nodules decrease in response to nitrate, which releases active FUN and increases nodule senescence. The findings suggest that zinc acts as a second messenger that links nitrate availability to the regulation of nodule function. The results have implications for understanding the role of metal ions in integrating environmental signals with plant development and optimizing nitrogen fixation in legumes.