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