In flowering plants, the pollen tube (PT) invades one synergid cell of the ovule, is enveloped by an invagination of the synergid plasma membrane termed the peritubular membrane (PRM), and ruptures to release two sperm cells for double fertilization. 3D live imaging in Arabidopsis established that PT entry, PRM formation, and rupture are genetically separable steps, but the mechanistic basis of this sequence is unknown: which enzymes locally degrade the synergid cell wall and which cell secretes them; what signal triggers rupture only after PT entry and how it is transduced; how PRM engulfment undermines PT wall integrity and kills the PT; whether the PT's transition cytosolic calcium spike drives entry or merely reflects mechanical acceleration; what the filiform apparatus membrane folds contribute; and how FERONIA controls the calcium spike amplitude and penetration speed. The mechanistic basis of this sequence remains unresolved in the literature to date.

Background

Double fertilization in flowering plants depends on a tightly choreographed invasion event at the micropylar end of the ovule. The pollen tube (PT), a tubular outgrowth of the pollen grain that carries the two immotile sperm cells, is guided to the ovule by female signals and arrives at the egg apparatus, which consists of the egg cell, the central cell, and two synergid cells. The synergids are haploid helper cells flanking the egg; their micropylar pole is elaborated into the filiform apparatus (FA), a dense array of plasma-membrane infoldings that enlarges the synergid surface and is the site where the PT is received. One synergid degenerates before or upon PT arrival and becomes the receptive synergid into which the PT grows. FERONIA (FER), a receptor-like kinase with an extracellular malectin-like (carbohydrate-binding) domain localized at the FA of the receptive synergid, was identified because in fer mutants the PT reaches the synergid but fails to rupture and instead continues to grow invasively through the female gametophyte (Huck et al. 2003), showing that rupture is controlled non-cell-autonomously from the synergid. The current signaling model holds that PT-derived RALF (rapid alkalinization factor) peptides bind, in an autocrine loop, to the ANXUR1/2 and BUPS1/2 receptor kinases at the PT tip to maintain PT integrity, and that female-side FER-containing complexes, together with LORELEI (LRE), a co-receptor anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) lipid tail whose anchoring was recently validated biochemically (Wang et al. 2025), participate in triggering rupture; the ovule-expressed peptide RALF34 can displace RALF4/19 from the PT receptor complex and burst PTs in vitro, yet ralf34 mutants are fully fertile, so the identity of the physiological rupture signal remains unsettled (Zakharova and Fedoreyeva 2025). Calcium is a second key messenger: synergid cells show characteristic cytosolic Ca2+ oscillations, fertilization is accompanied by Ca2+ spikes in the PT and Ca2+ flooding of the synergids (Hamamura et al. 2014), and the pollen-expressed Ca2+ pump AUTOINHIBITED Ca2+ ATPASE9 (ACA9), a plasma-membrane transporter that extrudes Ca2+ from the cytosol, is required for normal PT growth and fertilization (Schiøtt et al. 2004). What remained unknown until recently was the physical relationship between the invading PT and the synergid. Using 3D live imaging with PT-expressed Ca2+ sensors and membrane markers in Arabidopsis, Desnoyer et al. (2024, bioRxiv) showed that upon penetration the receptive synergid's plasma membrane invaginates and completely envelops the PT, forming a structure they named the peri-tubular membrane (PRM); that in fer mutants the PT still enters and forms a normal PRM but often fails to rupture, resulting in extensive distortions of the synergid membrane, decoupling entry from rupture; that a high-amplitude cytosolic Ca2+ 'transition spike' in the PT and Ca2+ flooding of both synergids — the receptive synergid and the persistent synergid (the second, non-receptive synergid, which survives until after fertilization) — accompany PRM formation; that in fer-1 the transition spike is still present in most penetrating PTs but its amplitude is significantly lower than in the wild type (a 2.8-fold change on average), and PTs interacting with fer-1 synergids show lower acceleration during the transition, growing faster during phase I but slower during the transition; and that ACA9-deficient PTs have reduced spike amplitude and frequently fail to penetrate, although those that do enter form a PRM and rupture normally. The work remained a preprint (version 1, July 2024) as of August 2026, but its findings and its discussion define the current frontier: the entry-and-rupture sequence is now anatomically described, yet every mechanistic question raised by these observations — enzymology of the wall breach, identity and transduction of the rupture signal, the death pathway of the engulfed PT, causal role of the Ca2+ spike, function of the FA folds, and the FER-dependent regulation of spike amplitude and penetration speed — is unanswered.

Problem Statement

Determine the molecular and cellular mechanism by which an Arabidopsis pollen tube penetrates the receptive synergid and subsequently ruptures inside it, as framed by the discovery of the peritubular membrane (PRM). A complete answer must resolve the following mechanistic questions on the PRM-mediated entry-rupture sequence: (1) how the PRM forms and how the synergid cell wall is locally degraded to permit PT entry — in particular, whether the wall-degrading enzymes are secreted by the PT or by the synergid, which enzymes they are, and how their activity is regulated; (2) what signal(s) trigger PT rupture only after the PT has entered the synergid and been enveloped by the PRM — in particular, whether the trigger is peptide-based (e.g., RALF peptides) or mechanical (a consequence of PRM envelopment), and how the signal is transduced to cause PT lysis; (3) how engulfment by the PRM compromises the integrity of the PT cell wall and leads to PT cell death; (4) whether the high-amplitude transition Ca2+ spike in the PT causally drives penetration or merely reflects mechanical acceleration of PT growth, and whether aca9 pollen tubes that arrest at the synergid do so from a signaling deficiency or from cytotoxic Ca2+ accumulation; (5) what functional contribution the pre-existing membrane folds of the filiform apparatus make to PT reception; and (6) which genetic and molecular pathway links FERONIA to the reduced transition-spike amplitude and slower penetration observed in fer mutants. These questions are facets of one integrated process, the PT entry-rupture sequence; the six facets are jointly required, and a resolution of only a subset is a partial answer rather than a complete one.

References

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