How a Single Peptide-Receptor Duo Governs Plant Immunity and Growth

Deep inside every plant cell, a molecular conversation is constantly underway, one that decides whether the organism invests its energy in growing taller or in fighting off an invading fungus. At the heart of this decision-making sits a ligand-receptor pair that has quietly become one of the most studied signaling modules in plant biology: the Rapid Alkalinization Factor peptides, known as RALFs, and their receptor kinase FERONIA, named after the Etruscan goddess of fertility. A comprehensive review published in the journal Crop Health by Xing-Yan Chen, Jia Chen, Fan Xu, and Xin-Zhong Cai of Zhejiang University and Hunan University now pulls together the exploding body of work on this pair, tracing its evolutionary origins, its many roles in reproduction and stress tolerance, and its surprising exploitation by fungal and nematode pathogens that have learned to mimic the plant’s own peptides to disarm its defenses.

The RALF story began in tobacco, where the peptide was first identified as a growth inhibitor. RALFs are small, cysteine-rich peptides of roughly five kilodaltons that earned their name from their ability to rapidly raise the pH of the space outside plant cells, the apoplast. Mature RALF peptides are cut from larger precursor proteins at a specific recognition site for a protease called SITE-1 PROTEASE, and they carry two signature features: a YISY motif essential for binding to receptors, and four conserved cysteines that form disulfide bonds holding the peptide in its active three-dimensional shape. Once released, RALF1 binds its receptor and triggers phosphorylation of a proton pump called AHA2, shutting down the flow of protons into the cell wall. The resulting alkalinization blocks cell expansion, which is how the peptide restrains root growth. The RALF family has expanded dramatically across the plant kingdom, with 37 members in Arabidopsis, 43 in rice, 18 in soybean, and 34 in maize, and comparative analysis of 795 RALFs from 51 species places them into four major clades that diversified rapidly after the split of eudicots and monocots roughly 145 million years ago.

FERONIA itself belongs to the CrRLK1L family of receptor-like kinases, a group of 17 proteins in Arabidopsis distinguished by two malectin-like domains in their extracellular region. These domains share homology with carbohydrate-binding modules, hinting at the family’s role in monitoring the cell wall. To understand where FER came from, the review’s authors searched 26 plant genomes for FER-like proteins and found 50 homologs, but none in green algae. Their phylogenetic analysis, built from 67 proteins across mosses, lycophytes, ferns, gymnosperms, and flowering plants, revealed a striking pattern: the FER-like proteins of early-diverging lineages cluster with non-FER CrRLK1L members rather than with true FER, suggesting that FER as we know it originated early in the differentiation of flowering plants. The domain architecture proved highly conserved, with nearly all homologs carrying the characteristic malectin-like domains, a transmembrane segment, and an intracellular kinase domain, though the kinases split between serine/threonine and tyrosine types with no obvious lineage pattern.

What makes FERONIA extraordinary is its pleiotropy. In reproduction, the receptor performs a dual role: it ensures sperm delivery and blocks polyspermy. When a pollen tube arrives at the female gametophyte, FER, together with the GPI-anchored protein LORELEI and the calcium channel NORTIA, senses pollen-derived RALF peptides and triggers a calcium-dependent surge of reactive oxygen species that ruptures the pollen tube and releases the sperm cells. After fertilization, FER, ANJEA, and HERCULES RECEPTOR KINASE 1 detect pollen tube-specific RALFs secreted into the septum, establishing a polytubey block that prevents additional pollen tubes from entering the same ovule. On the stigma, a lock-and-key system operates: stigmatic RALF1/22/23/33 peptides bound to FER complexes form a barrier against unwanted pollen tubes, while compatible pollen carries its own RALF peptides that outcompete the stigmatic ligands and unlock the gate. In roots, FER regulates root hair growth through the RAC/ROP signaling pathway and NADPH oxidase-dependent ROS production, and it controls vacuolar expansion through interactions with cell wall proteins of the LRX family.

The same module also underpins abiotic stress tolerance. Under high salinity, the protease S1P cleaves RALF22, and the mature peptide, together with RALF23, drives FER internalization, part of the LRX3/4/5-RALF22/23-FER module that modulates jasmonic acid, salicylic acid, abscisic acid, and ROS levels. FER additionally phosphorylates the photoreceptor phytochrome B, linking salt stress to growth suppression through nuclear phyB accumulation. Loss-of-function mutants are hypersensitive to both cold and heat, and in apple, the FER-like kinase MdMRLK2 enhances cold resistance by promoting the accumulation of water-insoluble pectin, lignin, cellulose, and hemicellulose, while also improving drought tolerance by modulating energy metabolism and free amino acid production. Cadmium stress represses FER expression in roots, and fer mutants accumulate roughly half the cadmium of wild-type seedlings, implicating the receptor in metal ion homeostasis as well.

The review’s central focus, however, is immunity, and here the picture becomes genuinely dramatic. FER acts as a scaffold receptor that facilitates the formation of immune complexes between the pattern recognition receptors EFR and FLS2 and their coreceptor BAK1, the assembly that initiates pattern-triggered immunity upon detection of bacterial flagellin or elongation factor peptides. But the same receptor can be turned against the plant. Fungi and nematodes encode their own RALF-like peptides, and at least some of these molecular mimics are functionally active. The fungus Fusarium oxysporum secretes a RALF that Arabidopsis FER perceives, raising the pH around the root and promoting fungal infection. Root-knot nematodes of the species Meloidogyne incognita express two RALF-like genes, MiRALF1 and MiRALF3, in their esophageal glands during the parasitic stages; both peptides bind directly to the extracellular domain of FER, and both the peptides and the receptor are required for successful nematode parasitism in Arabidopsis and rice. Whether fungal RALFs arose through horizontal gene transfer or convergent evolution remains unresolved.

The experimental evidence across crops reveals a receptor with context-dependent, sometimes contradictory effects. In Arabidopsis, fer mutations increase resistance to Fusarium oxysporum, the powdery mildew Golovinomyces orontii, and Botrytis cinerea, yet heighten susceptibility to the oomycete Hyaloperonospora arabidopsidis, the anthracnose fungus Colletotrichum higginsianum, and the necrotroph Sclerotinia sclerotiorum. Synthetic RALF22 peptide induces resistance to Sclerotinia in wild-type plants, an effect abolished in fer mutants. In rice, mutations in the FERONIA-like gene FLR2 confer broad-spectrum resistance to the blast fungus Magnaporthe oryzae, cutting fungal biomass to 10 to 20 percent of wild-type levels, and remarkably, this resistance comes with no growth penalty, a rare and valuable combination for breeders. In tomato, the FER homolog SlFERL positively regulates immunity to Botrytis, whereas in apple, MdMRLK2 negatively regulates resistance to Valsa canker. In soybean, the malectin-like receptor kinase GmLMM1 suppresses resistance to Phytophthora sojae and to bacterial pathogens. The direction of FER’s effect evidently depends on the pathogen’s lifestyle, the host species, and the tissue involved.

Mechanistically, the review catalogues at least nine ways FER shapes immune outcomes. Beyond scaffolding PRR complexes, FER organizes the nanoscale distribution of FLS2 and BAK1 within the plasma membrane, keeping these immune receptors properly positioned for signaling. It senses cell wall integrity through interactions with pectin and LRX proteins, and cooperates with CELLULOSE SYNTHASE-INTERACTIVE PROTEIN 1 to monitor wall status. It generates ROS through the RAC/ROP and RIPK-RBOHD pathways, activates MAP kinase cascades, and undergoes regulated endocytosis and recycling through clathrin-dependent trafficking. Perhaps most striking is a recent discovery: rhizobacteria trigger accumulation of the metalloproteinase At2-MMP, which cleaves FER within its intracellular juxtamembrane region, and the truncated receptor then translocates into the nucleus to activate antimicrobial responses in specific root zones. FER also phosphorylates and destabilizes MYC2, the master transcription factor of jasmonic acid signaling, thereby bolstering salicylic acid-mediated immunity against the bacterium Pseudomonas syringae, while RALF23 reverses this effect and favors infection.

The translational implications are considerable, even if no RALF-FER-based crop protection strategy has yet reached the field. The rice FLR2 result demonstrates that manipulating this pathway can deliver durable, broad-spectrum disease resistance without sacrificing yield, addressing the central trade-off that has long plagued resistance breeding. The authors identify several open questions that will determine how quickly the pair can be exploited: the binding specificity of individual RALFs for FER, the dose-dependent effects that distinguish protective peptides from immunosuppressive ones, the possibility of FER-independent RALF signaling suggested by recent structural work on RALF22 in the cell wall, and the interplay between FER-mediated cell wall integrity sensing and the newly discovered SCOOP18-MIK2 immune module. As pathogens continue to deploy peptide mimics against plant surveillance systems, understanding this master ligand-receptor pair may prove essential for building the next generation of disease-resistant crops.

Subject of Research: The RALF-FERONIA ligand-receptor signaling pair in plant immunity, development, and pathogen interactions

Article Title: RALF-FER, a master ligand‒receptor pair in plant health

Article References: Chen, X.-Y., Chen, J., Xu, F., & Cai, X.-Z. (2025). RALF-FER, a master ligand‒receptor pair in plant health. Crop Health, 3(1), Article 4. https://doi.org/10.1007/s44297-024-00040-1

Image Credits: AI Generated

DOI: 10.1007/s44297-024-00040-1

Keywords: FERONIA, RALF peptides, plant immunity, receptor-like kinases, pattern-triggered immunity, fungal pathogens, nematode parasitism, crop resistance, cell wall integrity, reactive oxygen species, peptide signaling, plant-pathogen interactions