Meng et al. (2020) showed that the seven-transmembrane MILDEW RESISTANCE LOCUS O proteins MLO5, MLO9 and MLO15 are required for Arabidopsis pollen tubes to respond to ovule-derived guidance cues, and proposed that MLO5/MLO9 selectively recruit Ca2+-channel CNGC18-containing vesicles to the pollen-tube plasma membrane through the R-SNAREs VAMP721/VAMP722 acting in trans. What remains unresolved, and what this problem targets, is the coupling mechanism itself: which ovule-derived signals and pollen-tube receptors activate or spatially restrict the MLO5/MLO9 module at the apex, what makes this engagement species-specific, whether additional plasma-membrane proteins or channels are selectively recruited, whether the MLO-SNARE recruitment mechanism operates in other cellular contexts, and what molecular machinery (tethers, regulators) underlies the transient trans interaction between MLO-bearing membranes and CNGC18-bearing vesicles. Later work established that MLO-family proteins can themselves act as Ca2+ channels activated by RALF peptide signaling in pollen-tube integrity, pollen-tube reception, root-hair growth and root development, sharpening an unresolved question central to this problem: during guidance, does the ovule-cue-triggered Ca2+ influx flow through MLO proteins as channels, through CNGC18 recruited by MLO5/MLO9, or both.

Background

During flowering-plant reproduction, the pollen tube is a tubular cell that grows from the pollen grain through the pistil toward the ovule to deliver the immotile sperm cells; its directional growth toward the ovule is called pollen tube guidance. Ovules emit diffusible attractant signals, notably the LURE peptides (secreted cysteine-rich defensin-like peptides produced by the synergid cells of the female gametophyte), whose perception by pollen-tube receptor kinases such as PRK6 biases growth toward the micropyle. A tip-focused cytosolic Ca2+ gradient is essential for pollen-tube elongation and guidance, and the cyclic nucleotide-gated channel CNGC18, a plasma-membrane Ca2+-permeable ion channel activated by cyclic nucleotides, is required for this gradient and for guidance to ovules (Frietsch et al. 2007; Gao et al. 2016). How the extracellular attractant cues are converted into the spatially patterned Ca2+ influx that steers the tube was unknown. Meng et al. (2020) provided a molecular link: in Arabidopsis thaliana, loss of the pollen-expressed MILDEW RESISTANCE LOCUS O (MLO) proteins MLO5 and MLO9 (members of a conserved family of seven-transmembrane proteins first identified as powdery mildew susceptibility factors) causes pollen tubes to twist and pile up after sensing ovular cues, a phenotype distinct from the ovule-bypass phenotype of attractant-signal mutants. Mechanistically, MLO5 and MLO9 were shown to interact with CNGC18 and with the R-SNARE proteins VAMP721/VAMP722 (membrane-fusion components of vesicle trafficking) and to recruit CNGC18-containing vesicles to the plasma membrane in trans mode, i.e. via an interaction between an MLO-bearing membrane and a CNGC18-bearing vesicle membrane before fusion. Meng et al. thus proposed MLOs as tethering factors that couple perception of ovular signals to selective exocytosis of a Ca2+ channel. Subsequent work transformed the surrounding landscape. Gao et al. (2022) showed that in synergid cells the MLO-family protein NORTIA (NTA/MLO7) is itself a calmodulin-gated Ca2+ channel activated when the FERONIA-LORELEI receptor complex perceives pollen-tube-derived RALF peptides (a family of secreted peptide hormones) during pollen-tube reception. Gao et al. (2023) showed that in the pollen tube MLO1, MLO5, MLO9 and MLO15 are Ca2+-permeable channels activated downstream of autocrine RALF4/19 peptides (secreted by the pollen tube itself), the ANX1/2-BUPS1/2-LLG2/3 receptor complex, and the MARIS kinase, maintaining tube integrity, while noting that CNGC18 was not activated by the activated MARIS variant and leaving CNGC18's role in that process open. Ogawa et al. (2025) extended FER/MLO-dependent Ca2+ influx to root-hair tip growth, Binci et al. (2026) to RALF34-triggered Ca2+ signaling in Lotus japonicus roots, and Huebbers et al. (2026) provided computational structural support for MLO oligomers forming membrane-spanning pores. These results establish that peptide-receptor signaling can gate MLO Ca2+ influx in several cell types, yet none of them addresses the guidance step: the identity of the ovule-derived signal(s) and pollen-tube receptor(s) that engage MLO5/MLO9 and CNGC18 during attraction, the determinants of the species-specificity Meng et al. observed (the twisting response was reduced with ovules of spl/ino mutants defective in ovule and female-gametophyte development, or with Arabidopsis lyrata ovules), any additional plasma-membrane cargoes selectively recruited by MLO5/MLO9, the generality of the MLO-SNARE channel-recruitment mechanism in other contexts such as pathogen response or pollen-tube rupture, and the molecular details (tethering factors, stoichiometry, regulators) of the transient trans interaction. Reviews through 2025-2026 (Li and Xiao 2025; Zhong, Lan and Qu 2025; Zheng et al. 2024) treat this perception-to-Ca2+-coupling gap as an open question in the field.

Problem Statement

The problem is to determine the molecular mechanism that couples the perception of ovule-derived attractant signals to the MLO5/MLO9-dependent recruitment and activation of the Ca2+ channel CNGC18 at the pollen-tube plasma membrane during guidance in Arabidopsis thaliana - that is, to close the gap between female-derived cue perception and the polarized Ca2+ influx that steers tip growth. A complete answer must jointly establish: (i) the ovule-derived diffusible signal(s) and the pollen-tube surface receptor(s) and upstream regulators through which they activate or spatially restrict MLO5/MLO9 (and/or CNGC18) at the apex, distinguishing this ovule-cue route from the autocrine pollen-tube RALF4/19 pathway that activates MLO channels for tube integrity; (ii) the molecular determinants of the species-specificity of this engagement, given that the twisting response of mlo5 mlo9 pollen tubes is reduced with spl/ino ovules defective in female-gametophyte development and with ovules of the related species Arabidopsis lyrata; (iii) whether additional plasma-membrane proteins or other ion channels are selectively regulated by MLO5/MLO9 and contribute to guidance; (iv) whether the MLO-mediated selective recruitment of channels via the R-SNAREs VAMP721/VAMP722 operates as a general mechanism in other cellular contexts (for example pathogen response or pollen-tube rupture) and what additional components are required for full coupling of perception to exocytosis; and (v) the mechanistic details of the proposed transient trans interaction between MLO-bearing membranes and CNGC18-bearing vesicles, including the identity of any additional tethering factors and regulators. The answer must also resolve whether the guidance-relevant Ca2+ influx is carried by CNGC18 recruited by MLO5/MLO9, by MLO proteins themselves acting as Ca2+ channels, or by both, since these two mechanisms coexist unresolved in the current literature. Parts (i)-(iii) and (v) form the mandatory core of a complete answer: a solution that establishes the guidance-specific coupling mechanism is complete even if part (iv) - the operation of this mechanism in other cellular contexts - is left open; each part (i)-(v) is independently answerable and may be reported separately.

References

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