The research, led by Xiaobin Wei, Mengyi Yao and Pingping Zhao of Henan University together with colleagues at the Shanghai Institute of Materia Medica and Guangzhou Medical University, focused on the signalling module formed by the SCREW/CTNIP peptide family, their receptor NUT/HSL3 and the co-receptor BAK1/SERK3. Earlier work had established that SCREW-type peptides, sometimes described as phytocytokines, promote the reopening of stomata that have closed in response to the stress hormone abscisic acid or to microbe-associated molecular patterns, thereby restoring photosynthesis and gas exchange while lowering apoplast water saturation to inhibit pathogen proliferation. What remained unknown was precisely how the receptor recognizes these peptides and assembles the active signalling complex on the cell surface.

To answer that question, the team turned to cryo-electron microscopy. They determined the structure of the extracellular domain of the Arabidopsis receptor AtNUT on its own and, more importantly, the structure of the ternary complex in which AtNUT binds the peptide SCREW2 and recruits the co-receptor AtBAK1. The resulting maps, deposited in the Electron Microscopy Data Bank under accession codes EMD-65054 and EMD-65053, with atomic coordinates in the Protein Data Bank as entries 9VH3 and 9VH2, provide the first three-dimensional portrait of this immune-modulating receptor assembly.

The most striking discovery concerns the shape of the peptide itself. Most plant peptides that bind leucine-rich repeat receptor kinases adopt one of two well-characterized conformations: an omega-like, hairpin-shaped fold seen in complexes such as HAESA–IDA–SERK1, HSL1–CLE9–SERK1 and PXY–CLE41–SERK2, or a fully extended configuration seen in the immune complexes MIK2–SCOOP12–BAK1 and FLS2–flg22–BAK1. SCREW2 does neither. Instead, it adopts what the authors call a cross-ribbon conformation, an inverted arrangement in which the peptide’s termini cross over in a way that has not been observed for any other LRR receptor-binding peptide. This distinctive fold is stabilized by intramolecular interactions, including conserved disulfide bonds and hydrogen-bonding networks that tie the N- and C-terminal regions together and lock the central loop into place.

The cross-ribbon architecture is not merely a structural curiosity; it is the molecular basis of specificity. Because the peptide presents its recognition surfaces in an arrangement unique to the SCREW family, only AtNUT possesses a binding surface complementary to it. The structure shows how conserved residues of SCREW2, including the completely conserved serine and asparagine residues near the N-terminus and the highly conserved CTNIP motif within the central loop, dock into a matching pocket on AtNUT. Once the peptide is seated in its receptor, its spatial configuration creates the platform needed to recruit AtBAK1, the promiscuous SERK-family co-receptor that participates in a wide range of plant receptor kinase signalling pathways, from brassinosteroid perception to innate immunity.

One of the most unexpected findings involves a sugar. The structure revealed an N-linked glycan attached to asparagine 449 of AtNUT, positioned directly within the peptide-binding interface. The glycan forms a hydrogen bond with the arginine 63 residue of SCREW2, effectively acting as a molecular handhold that helps grip the peptide. Biochemical assays using surface plasmon resonance confirmed that mutating the N449 glycosylation site significantly weakens SCREW2 binding. Sequence alignments across Brassicaceae species show that the N449 site and the surrounding N-X-S/T glycosylation motifs are conserved, suggesting that this sugar-assisted recognition mechanism is a general feature of NUT-family receptors rather than an idiosyncrasy of Arabidopsis.

The structural insights translated directly into functional predictions. When the researchers mutated the SCREW2 recognition sites on AtNUT, the consequences rippled through the entire downstream signalling cascade. Mitogen-activated protein kinase activation, a hallmark early response of pattern-triggered immunity, was significantly attenuated, as was the expression of PTI marker genes measured by RT-qPCR. These experiments demonstrate that the precise peptide-receptor contacts visualized in the cryo-EM maps are not incidental; they are load-bearing elements of the signalling pathway that counteracts abscisic acid- and MAMP-triggered stomatal closure.

The work also clarifies how the SCREW/NUT module fits into the broader logic of stomatal immunity. Pathogen attack triggers closure through well-mapped pathways involving abscisic acid receptors of the PYR/PYL/RCAR family, the SLAC1 anion channel and calcium-permeable channels such as OSCA1.3. Reopening, by contrast, requires actively counteracting these closure signals. The new structures suggest a model in which SCREW peptide binding induces heterodimerization of AtNUT and AtBAK1, initiating a transphosphorylation cascade that pushes guard cells back toward the open state. In this view, the plant does not simply reverse closure passively but deploys a dedicated peptide-receptor system whose activation is gated by the unique cross-ribbon geometry of its ligand.

From an applied perspective, the findings open concrete avenues for crop improvement. Stomatal dynamics sit at the heart of the trade-off between drought tolerance, photosynthetic productivity and disease resistance, and engineering this balance has long been hampered by incomplete knowledge of the underlying receptors. Because the SCREW recognition surface, the N449 glycan dependence and the co-receptor recruitment interface are now defined at the residue level, breeders and synthetic biologists have a structural template for tuning stomatal reopening without broadly disabling immune signalling. Sequence comparisons across angiosperms, from wheat and rice to grapevine and tomato, indicate that SCREW homologs and their conserved cysteine scaffolds are widespread, raising the prospect that the mechanism characterized here in Arabidopsis can be translated to staple crops.

The study also adds a new chapter to the structural biology of plant receptor kinases. For years, the field has recognized that leucine-rich repeat receptors rely on a common architectural principle: a peptide ligand bridges the receptor and a SERK co-receptor, with the peptide’s C-terminus typically engaging the co-receptor’s recruitment loop. The SCREW2 structure breaks that pattern, demonstrating that peptide geometry itself can be the determinant of receptor identity and that glycans can participate directly in ligand recognition rather than merely assisting protein folding and quality control. As cryo-EM continues to expose the extracellular signalling complexes of plant membranes, cases like SCREW/NUT are likely to multiply, revealing a molecular vocabulary far richer than the canonical models anticipated. For now, the cross-ribbon peptide stands as a vivid reminder that in biology, shape is often the message.

Subject of Research: Cryo-EM structural mechanism of SCREW/CTNIP peptide recognition by the NUT/HSL3–BAK1 receptor complex in plant stomatal immunity

Article Title: Cross-ribbon conformation defines peptide recognition in SCREW/CTNIP–NUT/HSL3 signalling

Article References: Wei, X., Yao, M., Zhao, P., Zhang, X., Zhang, K., Hu, W., Shi, J., Guo, S., Li, R., Wang, W., Xu, Y., Guo, S., Xu, H. E., & Song, C.-P. (2026). Cross-ribbon conformation defines peptide recognition in SCREW/CTNIP–NUT/HSL3 signalling. Nature Plants, 12(9), 1769-1781. https://doi.org/10.1038/s41477-026-02376-8

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02376-8

Keywords: SCREW/CTNIP peptides, NUT/HSL3 receptor, BAK1/SERK3 co-receptor, cryo-EM structure, cross-ribbon conformation, stomatal immunity, N-glycosylation, pattern-triggered immunity, plant receptor kinases, stomatal reopening, Arabidopsis, Nature Plants

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