The endosperm begins its life in an unusual way. Rather than immediately forming cells separated by membranes and walls, it undergoes coenocytic growth, a phase in which nuclei divide repeatedly without accompanying cytokinesis, producing a single multinucleate cytoplasm that expands to fill the growing seed. During this period, in Arabidopsis, the expansion of the maternal seed coat together with the proliferation of the endosperm largely determines the final size of the seed. This continues until the embryo reaches the early heart stage, a morphological landmark at which the first obvious organogenesis of the plant body becomes visible. At that point, the endosperm changes course dramatically and begins to cellularize, partitioning its many nuclei into individual cells that will later serve as storage and transport tissue.

What makes this transition remarkable is that it is not uniform across the seed. Cellularization starts at the micropylar pole, the region of the seed where the embryo resides, and it stops before reaching the opposite, chalazal pole. The result is a polarized tissue: cellularized endosperm near the embryo and a persistent non-cellularized, multinucleate zone at the chalazal end that remains specialized for nutrient uptake from maternal tissues. This spatial asymmetry is not a curiosity; it is functionally central to how the seed feeds itself. Yet despite decades of work on endosperm development, the molecular mechanism that imposed this polarity remained unclear. Two genes were known to be essential players: HAIKU2, abbreviated IKU2, which encodes a leucine-rich repeat receptor kinase, and MINI3, which encodes a transcription factor. Loss-of-function mutations in either gene produce small seeds because endosperm cellularization is disrupted, but how these components generated a polarized pattern was unknown.

The new study resolves this puzzle by showing that the transcription factor MINI3 is expressed in two distinct phases. Early in seed development, MINI3 is active in the central cell before fertilization and in the proliferating endosperm afterward. Later, its expression becomes restricted to the micropylar endosperm, precisely the region where cellularization begins. This biphasic pattern immediately suggested a regulatory logic: an early, broadly distributed phase followed by a later, spatially restricted phase that must depend on some positional cue. The question was where that cue comes from, and the answer turned out to lie in a dialogue between the embryo and its surrounding endosperm.

Genetic evidence pointed the way. In iku2 mutants, the late micropylar expression of MINI3 is reduced, indicating that IKU2 is required for the polar phase of MINI3 transcription. Even more striking, the polar expression of MINI3 is also diminished in seeds in which only the central cell has been fertilized and no zygote or embryo is present. This observation implies that a signal derived from the zygote or the young embryo is needed to establish the micropylar pattern of MINI3 expression. Consistent with this interpretation, IKU2 protein is localized to the membrane of the central cell in the immediate vicinity of the egg cell at the micropylar pole, placing the receptor exactly where an embryo-derived signal would be expected to arrive.

Receptor kinases of the leucine-rich repeat family typically detect small extracellular peptides, and IKU2 belongs to subclade XI of this receptor family, a group for which members of the PIPL peptide family have previously been reported as ligands. This gave the researchers a defined search space. They screened for PIPL-family peptides on the basis of their expression patterns, looking specifically for genes active in zygotes or early embryos, the tissues positioned to send a signal toward the neighboring endosperm. This strategy yielded PROPIPL7 as the leading candidate, a gene whose expression profile matched the predicted source of the polarizing signal.

Functional tests then confirmed the hypothesis. When PIPL7 was applied to a chimeric receptor consisting of the extracellular domain of IKU2 fused to the intracellular signaling domain of FLAGELLIN SENSING 2, a well-characterized immune receptor commonly used as a reporter for ligand-receptor interactions, the construct was activated in Nicotiana benthamiana. This demonstrated biochemically that PIPL7 can be recognized by IKU2. The genetic evidence was equally compelling: propipl7 mutants recapitulate the phenotype of iku2 mutants, and double mutants between the two show no additive effect, the classic signature of two genes acting in the same pathway. Together, these results indicate that PIPL7 is the in vivo ligand of IKU2, completing a signaling axis that runs from the embryo, through a peptide ligand, to an endosperm-expressed receptor kinase, and finally to the transcription factor MINI3 that drives cellularization at the micropylar pole.

One additional finding adds an intriguing layer of redundancy and regulation to the system. PIP2, a peptide from the same family whose expression is induced by pathogens in other tissues, can rescue the propipl7 mutant phenotype when supplied in the relevant context. This suggests that PIPL-family peptides are functionally interchangeable to a considerable degree, and that the specificity of the developmental pathway is achieved not by unique ligand-receptor chemistry but by tissue-specific transcriptional regulation. In other words, many peptides could in principle activate the receptor, but only the embryo-expressed PROPIPL7 gene is transcribed at the right time and place to do so during normal seed development. This is an elegant example of how plants can build developmental specificity on top of a promiscuous signaling system, reserving a general-purpose signaling module for a highly specialized patterning role.

The broader significance of this work extends beyond Arabidopsis seed biology. Seed size is an agronomically critical trait, and the IKU2-MINI3 pathway has long been recognized as a key regulator of the maternal and filial determinants that set final seed dimensions. By identifying the embryo-derived peptide signal that polarizes endosperm cellularization, the study closes a mechanistic gap in our understanding of how the two products of double fertilization coordinate their development. It establishes, at the level of a defined ligand-receptor pair, that the embryo actively instructs the endosperm rather than merely responding to it, reframing the seed as a system of intercompartmental communication. The micropylar-to-chalazal polarity of the endosperm, which underpins the seed’s nutrient uptake architecture, can now be traced to a peptide signal originating in the zygote or early embryo and read out by a receptor positioned at the central cell membrane adjacent to the egg. For researchers interested in plant polarity, peptide signaling, and seed development, the finding provides both a satisfying answer to a long-standing question and a template for dissecting similar intertissue dialogues in other plant organs, where short-range peptide signals are increasingly recognized as the lingua franca of coordinated development.

Subject of Research: Embryo-to-endosperm peptide signaling that polarizes endosperm cellularization in Arabidopsis seeds

Article Title: Embryo talks to endosperm

Article References: Trösch, R. (2026). Embryo talks to endosperm. Nature Plants. https://doi.org/10.1038/s41477-026-02443-0

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02443-0

Keywords: Arabidopsis thaliana, double fertilization, endosperm, seed development, IKU2, MINI3, PIPL7, receptor kinase, peptide signaling, cellularization, plant polarity, seed size

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