The story begins with a family of small peptides called C-TERMINALLY ENCODED PEPTIDES, or CEPs. When a root senses that the soil around it is deficient in nitrogen, it ramps up production of these peptide hormones, which travel through the xylem from the root to the shoot. High in the leaf phloem, these mobile peptides are detected by a receptor called CEP RECEPTOR 1, or CEPR1. This detection event is the pivot of the entire signalling cascade: it triggers the production of shoot-to-root mobile signals, including transcription factors known as CEPDs, that descend back into the roots and activate high-affinity nitrate transporters such as NRT2.1 in the roots that still dwell in nitrogen-rich patches of soil. The result is a beautifully compensatory system in which nitrogen-starved roots effectively recruit their better-positioned siblings to forage harder on behalf of the whole plant.

But there was a puzzle at the heart of this pathway. In other peptide hormone systems in plants, a single receptor kinase rarely acts alone. The classic example is the brassinosteroid pathway, where the receptor BRI1 requires its co-receptor BAK1 to achieve full signalling output. Co-receptors confer specificity, amplify signals, and often determine which downstream responses are engaged. Researchers therefore suspected that CEPR1, too, might depend on an as-yet-unidentified partner to translate CEP binding into the specific output of nitrogen-demand signalling. Yet years of genetic screening and biochemical analysis had failed to turn up such a partner, leaving the mechanism of CEP-dependent CEPR1 activation unknown.

The Nagoya University team, led by Yoshikatsu Matsubayashi with Mari Ogawa-Ohnishi and Taizo Nomura as co-first authors, took a biochemical approach to the problem. Using co-immunoprecipitation experiments in Arabidopsis thaliana, they searched for proteins that physically associate with CEPR1, but only in the presence of CEP peptides. This ligand-dependent interaction screen revealed a leucine-rich repeat receptor kinase that binds CEPR1 in a strikingly CEP-dependent manner. The protein, encoded by the gene At5g63710, belonged to a clade within leucine-rich repeat receptor kinase subgroup II that the researchers describe as the last functionally uncharacterized branch of that subgroup in Arabidopsis. They christened it CEP RECEPTOR INTERACTOR, CERI, reflecting both its discovery method and its molecular role.

Genetics quickly confirmed what biochemistry had suggested. Mutant plants in which the CERI gene was knocked out using CRISPR-Cas9 lost the ability to mount a systemic nitrogen-demand response. When the roots of these ceri mutants were starved of nitrogen, the leaves failed to properly induce the CEPD genes that normally relay the demand signal back down to the roots, and the compensatory boost in nitrate uptake in nitrogen-rich roots was impaired. Crucially, however, the loss of CERI did not disrupt every CEPR1-dependent function. The researchers showed that CEPR1-mediated regulation of root system architecture, a separate developmental response in which CEP signalling influences lateral root growth, remained intact in the ceri mutants. This dissociation was the key insight: CEPR1 can still signal without CERI, but only the systemic nitrogen-demand output requires the co-receptor.

That specificity is what makes CERI so interesting from a mechanistic standpoint. Rather than being a generic amplifier of all CEPR1 signalling, CERI appears to act as a specificity factor, selectively enabling one branch of the receptor’s output while leaving others untouched. This resembles the modular logic seen in other receptor kinase systems, where different co-receptors or phosphorylation patterns channel a single receptor into distinct cellular responses. The finding suggests that the architecture of plant peptide signalling is more layered than a simple ligand-receptor pair, and that the identity of the co-receptor can determine which physiological programme a receptor complex engages.

At the molecular level, the team dissected how CEP binding brings CEPR1 and CERI together and what happens next. Using mass spectrometry, they mapped phosphorylation sites on both proteins and found that CEP treatment triggers phosphorylation of the activation loops of both CEPR1 and CERI, the regulatory segments that must be phosphorylated for a kinase domain to become catalytically active. This pattern of CEP-dependent transphosphorylation is a hallmark of receptor kinase complex activation, mirroring the sequential phosphorylation events described for the BRI1-BAK1 brassinosteroid complex. Mutations that eliminate these activation-loop phosphorylation sites abolish signalling, confirming that the kinase activities of both proteins are essential. In other words, CEP binding assembles an active receptor kinase duo, and the coordinated phosphorylation of both partners is what generates the intracellular signal that ultimately commands the roots to take up more nitrate.

The evolutionary dimension of the work adds further resonance. CEP peptides and their signalling pathways are specific to seed plants, and the genomes of nonflowering land plants have revealed how peptide signalling expanded over the course of plant evolution. Identifying CERI as the functional co-receptor for nitrogen-demand signalling not only completes the circuit in Arabidopsis but also provides a reference point for tracing how this sophisticated long-distance communication system was assembled over hundreds of millions of years. Because the CEP-CEPR1 pathway also intersects with plant immunity, with recent work showing that CEP signalling coordinates immune responses with nitrogen status, understanding the receptor complex at atomic and genetic resolution has implications well beyond nutrient physiology.

The practical stakes are considerable. Nitrogen fertilizer underpins modern agriculture, but applied nitrogen is notoriously inefficient: a large fraction is lost to leaching, volatilization, and denitrification, polluting waterways and generating the potent greenhouse gas nitrous oxide. A crop plant with a sharpened nitrogen-demand signalling system could, in principle, fine-tune its uptake more precisely to what it actually needs, absorbing nitrates more efficiently when and where they are available. Because CERI specifically gates the systemic demand response without interfering with other developmental functions of CEPR1, it offers an appealing target for breeding or genome editing: enhancing CERI-dependent signalling could boost nitrogen use efficiency while leaving root architecture and other CEP-dependent traits unaltered. The Nagoya team’s work, rooted in fundamental curiosity about how plants talk to themselves, thus delivers both a completed signalling circuit and a candidate molecular lever for one of agriculture’s most pressing challenges. The raw mass spectrometry data underlying the study have been deposited in the ProteomeXchange consortium via the jPOST repository, and the Arabidopsis lines generated in the study are available from the corresponding author, ensuring that other laboratories can immediately build on this newly completed map of the plant nitrogen-demand circuit.

Subject of Research: Systemic nitrogen-demand signalling in plants via the CEP-CEPR1-CERI receptor kinase pathway

Article Title: An LRR co-receptor kinase essential for systemic nitrogen-demand signalling

Article References: Ogawa-Ohnishi, M., Nomura, T., Hayashi, Y., Yamashita, Y., Noda, S., Suda, R., Ohkubo, Y., & Matsubayashi, Y. (2026). An LRR co-receptor kinase essential for systemic nitrogen-demand signalling. Nature Plants. https://doi.org/10.1038/s41477-026-02391-9

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02391-9

Keywords: CERI, CEPR1, CEP peptides, nitrogen-demand signalling, leucine-rich repeat receptor kinase, co-receptor, Arabidopsis thaliana, nitrate uptake, plant signalling, root architecture, nitrogen use efficiency, Nature Plants