Source leaves and CRF2 coordinate systemic nitrate responses in Arabidopsis

<title>Abstract</title> <p>Plants adjust nitrate acquisition to spatially heterogeneous soils, but how shoots decode root-derived systemic signals remains largely unclear. Using Arabidopsis split-root assays, we show that source leaves are necessary and sufficient for the early root transcriptional response to systemic nitrogen demand. Tissue-resolved analyses then identify the cytokinin-responsive transcription factor CRF2 as a source-leaf-regulated integrator of root nitrate availability and trans-zeatin signaling. CRF2 is dispensable for the first induction of nitrate transporter genes, but required to sustain N-demand signaling regulating i) gene expression, ii) high-affinity nitrate influx, iii) root biomass allocation and integrated nitrate acquisition under heterogeneous supply. Mutant transcriptomic analyses further revealed CRF2’s role as an integrator of systemic nitrate status to source-leaf physiology. This includes shoot i) thermal regulation and ii) Fe/Zn/Mg homeostasis. These findings reveal functional specialization within the rosette and position source leaves as regulatory compartments coupling systemic nitrate information to root acquisition and shoot metabolic state.</p>