Pollen-pistil interactions: It takes two to tangle but a molecular cast of many to deliver

Sexual reproduction in land plants: an evolutionary perspective

KEY MESSAGE

We link key aspects of land plant reproductive evolution and detail how successive molecular changes leading to novel tissues and organs require co-evolution of communication systems between tissues. The transition of water-dependent reproduction of algae to mechanisms with very limited water dependence in many land plant lineages allowed plants to colonize diverse terrestrial environments, leading to the vast variety of extant plant species. The emergence of modified cell types, novel tissues, and organs enabled this transition; their origin is associated with the co-evolution of novel or adapted molecular communication systems and gene regulatory networks. In the light of an increasing number of genome sequences in combination with the establishment of novel genetic model organisms from diverse green plant lineages, our knowledge and understanding about the origin and evolution of individual traits that arose in a concerted way increases steadily. For example, novel members of gene families in signaling pathways emerged for communication between gametes and gametophytes with additional tissues surrounding the gametes. Here, we provide a comprehensive overview on the origin and evolution of reproductive novelties such as pollen grains, immobile sperms, ovules and seeds, carpels, gamete/gametophytic communication systems, double fertilization, and the molecular mechanisms that have arisen anew or have been co-opted during evolution, including but not limited to the incorporation of phytohormones, reactive oxygen species and redox signaling as well as small RNAs in regulatory modules that contributed to the evolution of land plant sexual reproduction.

Non-Target Metabolomics Reveals Changes in Metabolite Profiles in Distant Hybrid Incompatibility Between Paeonia sect. Moutan and P. lactiflora

Peonies are globally renowned ornamental plants, and distant hybridization is a key method for breeding new varieties, though it often faces cross-incompatibility challenges. The metabolic mechanisms underlying the crossing barrier between tree peony (Paeonia sect. Moutan) and herbaceous peony (P. lactiflora) remain unclear. To identify key metabolites involved in cross-incompatibility, we performed a cross between P. ostii 'Fengdanbai' (female parent) and P. lactiflora 'Red Sara' (male parent) and analyzed metabolites in the stigma 12 h after pollination using UPLC-MS. We identified 1242 differential metabolites, with 433 up-regulated and 809 down-regulated, including sugars, nucleotides, amino acids, lipids, organic acids, benzenoids, flavonoids, and alkaloids. Most differential metabolites were down-regulated in hybrid stigmas, potentially affecting pollen germination and pollen tube growth. Cross-pollinated stigma exhibited lower levels of high-energy nutrients (such as amino acids, nucleotides, and tricarboxylic acid cycle metabolites) compared to self-pollinated stigma, which suggests that energy deficiency is a contributing factor to the crossing barrier. Additionally, cross-pollination significantly impacted KEGG pathways such as nucleotide metabolism, purine metabolism, and vitamin B6 metabolism, with most metabolites in these pathways being down-regulated. These findings provide new insights into the metabolic basis of cross-incompatibility between tree and herbaceous peonies, offering a foundation for overcoming hybridization barriers in peony breeding.

Preparation technologies, structural characteristics and biological activities of polysaccharides from bee pollen: A review

Bee pollen, a natural honeybee product, is hailed as a treasure trove of human nutrition. Among the nourishing substances of bee pollen, the constituents with a low molecular weight (such as phenolic acids and flavonoid glycosides) have been extensively studied in the past decades, whereas the polysaccharides with a relatively high molecular weight have received much less attention. To deepen our understanding of bee pollen polysaccharides, this review summarizes the published findings related to their preparation technologies, structural characteristics and biological activities. Among the preparation technologies, ultrasonic-assisted extraction is currently the most effective technology for the recovery of polysaccharides from bee pollen, because ultrasound can crack the pollen exine into fragments and facilitate the release of polysaccharides present in the pollen intine. The preliminary structures, including the molecular weight and monosaccharide composition, of bee pollen polysaccharides have been widely reported, but their fine structures have not fully elucidated. Moreover, bee pollen polysaccharides have antioxidant, immunomodulatory, and antitumor activities, exhibiting potential application in functional foods. Furthermore, bee pollen polysaccharides can modulate the composition of gut microbiota and promote the production of short-chain fatty acids. It is expected that this review can provide inspiration for the development and utilization of bee pollen polysaccharides.

Advances in research on the control of pollen tube growth by calcium in higher plants

The function of the pollen tube in higher plants is to deliver two male gametes to the embryo sac to ensure successful double fertilization. During this process, many interactions occur among pollen tubes and pistil cells and tissues. Calcium ion (Ca2 +) dynamics mediate these interactions to ensure that the pollen tube grows with correct polarity and orientation to reach its target, the embryo sac. The pistil tissue, which contains abundant Ca2+, attracts the growing pollen tube via changes in Ca2+. Recent studies have shed light on the mechanisms of vacuole biogenesis and the Ca2+ action pathway in the growing pollen tube. This successfully explains the distribution characteristics of high Ca2+ at the tip of the pollen tube and the physiological mechanism of Ca2+ controlling pollen tube growth. In last stage of pollen tube growth, some studies indicated that the cessation of pollen tube growth and breaking of the tube in embryo sac maybe related to Ca2+ dynamic, which finishes its complex journey of pollen tube in vivo.

Multiple gatekeeping steps in pollination lock species specificity

In flowering plants, pollen grains must undergo a series of critical processes, including adhesion, hydration, and germination, which are dependent on the stigma, to develop a pollen tube. This pollen tube then penetrates the stigma to reach the internal tissues of pistil, facilitating the transport of non-motile sperm cells to the embryo sac for fertilization. However, a dry stigma, characterized by the absence of an exudate that typically envelops a wet stigma, functions as a multi-layered filter in adhesion, hydration, germination, and penetration that permits the acceptance of compatible pollen or tubes while rejecting incompatible ones, thereby protecting the embryo sac from ineffective fertilization and maintaining species specificity. Given the significance of these selective events, related research has consistently been at the forefront of reproductive studies, with notable advancements being made in recent times. In this review, we systematically synthesize the selective events and provide comprehensive, up-to-date summaries of occurrences on dry stigmas with a particular focus on the Brassicaceae family, following the chronological sequence of these events. Our objective is to update and elucidate the critical points within pollination, identify unresolved questions, and propose potential avenues for future research in other plant families.

Pollen-pistil interactions in divergent wide crosses lead to spatial and temporal pre-fertilization reproductive barrier in flax (Linum usitatissimum L.)

Linseed, has been a source of natural fiber for textile industries since its domestication. However, despite being the potential source of trait reservoir, the use of Linum wild genetic resources for the improvement of economic traits are not exploited widely. This is mainly due to the degree of genetic divergence that exists among the interspecific ecotypes causing crossability issues. Self-incompatibility due to the occurrence of heterostyly is very well reported in distantly related crop wild relatives of Linum and, the mechanism of self-incompatibility between different floral morphs is also studied. However, pollen germination and tube growth responses in the interspecific crosses are rarely studied. Thus, the present study was exclusively carried out to assess the major pre-zygotic barriers and their effect on pollen germination on foreign stigma using fluorescent microscopy of aniline blue stain-aided technology, to understand how the species barriers operate on pollen germination and pollen tube growth. The study revealed that the pollen-pistil interaction in the wide crosses among L. usitatissimum X L. grandiflorum was regulated by both temporal and spatial pre-fertilization barriers. Callose deposition within 2 h after pollination (HAP) at the stigma surface, was the major cause inhibiting pollen germination. Various kinds of aberrations started appearing during the 2-4 HAP. The complexity of interspecific hybridization was observed in terms of arrest of pollen tube (PT) growth in the ovary, ruptured, twisted and swollen pollen tube tip, tube growth in reverse direction, convoluted and terminated growth patterns. Inconsistent growth rates of pollen tubes to reach various stylar regions emphasizes the importance of studying these wild relatives for potential agricultural advancements. The results show that while distant hybridization with L. grandiflorum is less efficient, pollen tubes can still navigate the ovular tissues, albeit with some delay. This finding opens avenues for investigating factors that hinder viable seed formation, enhancing our understanding of reproductive success in distant hybridization with this species.

Mismatch between pollen and pistil size causes asymmetric mechanical reproductive isolation across Phlox species

Characterizing the mechanisms of reproductive isolation between lineages is key to determining how new species are formed and maintained. In flowering plants, interactions between the reproductive organs of the flower-the pollen and the pistil-serve as the last barrier to reproduction before fertilization. As such, these pollen-pistil interactions are both complex and important for determining a suitable mate. Here, we test whether differences in style length (a part of the pistil) generate a postmating prezygotic mechanical barrier between five species of perennial Phlox wildflowers with geographically overlapping distributions. We perform controlled pairwise reciprocal crosses between three species with long styles and two species with short styles to assess crossing success (seed set). We find that the heterospecific seed set is broadly reduced compared to conspecific cross success and reveal a striking asymmetry in heterospecific crosses between species with different style lengths. To determine the mechanism underlying this asymmetric reproductive isolating barrier, we assess pollen tube growth in vivo and in vitro. We demonstrate that pollen tubes of short-styled species do not grow long enough to reach the ovaries of long-styled species. We find that short-styled species also have smaller pollen and that both within- and between-species pollen diameter is highly correlated with pollen tube length. Our results support the hypothesis that the small pollen of short-styled species lacks resources to grow pollen tubes long enough to access the ovaries of the long-styled species, resulting in an asymmetrical, mechanical barrier to reproduction. Such reproductive isolating mechanisms, combined with additional pollen-pistil incompatibilities, may be particularly important for closely related species in geographic proximity that share pollinators.

The Role of Female and Male Genes in Regulating Pollen Tube Guidance in Flowering Plants

In flowering plants, fertilization is a complex process governed by precise communication between the male and female gametophytes. This review focuses on the roles of various female gametophyte cells-synergid, central, and egg cells-in facilitating pollen tube guidance and ensuring successful fertilization. Synergid cells play a crucial role in attracting the pollen tube, while the central cell influences the direction of pollen tube growth, and the egg cell is responsible for preventing polyspermy, ensuring correct fertilization. The review also examines the role of the pollen tube in this communication, highlighting the mechanisms involved in its growth regulation, including the importance of pollen tube receptors, signal transduction pathways, cell wall dynamics, and ion homeostasis. The Ca2+ concentration gradient is identified as a key factor in guiding pollen tube growth toward the ovule. Moreover, the review briefly compares these communication processes in angiosperms with those in non-flowering plants, such as mosses, ferns, and early gymnosperms, providing evolutionary insights into gametophytic signaling. Overall, this review synthesizes the current understanding of male-female gametophyte interactions and outlines future directions for research in plant reproductive biology.

Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation

How barriers to gene flow arise and are maintained are key questions in evolutionary biology. Speciation research has mainly focused on barriers that occur either before mating or after zygote formation. In comparison, postmating prezygotic (PMPZ) isolation-a barrier that acts after gamete release but before zygote formation-is less frequently investigated but may hold a unique role in generating biodiversity. Here we discuss the distinctive features of PMPZ isolation, including the primary drivers and molecular mechanisms underpinning PMPZ isolation. We then present the first comprehensive survey of PMPZ isolation research, revealing that it is a widespread form of prezygotic isolation across eukaryotes. The survey also exposes obstacles in studying PMPZ isolation, in part attributable to the challenges involved in directly measuring PMPZ isolation and uncovering its causal mechanisms. Finally, we identify outstanding knowledge gaps and provide recommendations for improving future research on PMPZ isolation. This will allow us to better understand the nature of this often-neglected reproductive barrier and its contribution to speciation.

FERONIA: A Receptor Kinase at the Core of a Global Signaling Network

Initially identified as a key regulator of female fertility in Arabidopsis, the FERONIA (FER) receptor kinase is now recognized as crucial for almost all aspects of plant growth and survival. FER partners with a glycosylphosphatidylinositol-anchored protein of the LLG family to act as coreceptors on the cell surface. The FER-LLG coreceptor interacts with different RAPID ALKALINIZATION FACTOR (RALF) peptide ligands to function in various growth and developmental processes and to respond to challenges from the environment. The RALF-FER-LLG signaling modules interact with molecules in the cell wall, cell membrane, cytoplasm, and nucleus and mediate an interwoven signaling network. Multiple FER-LLG modules, each anchored by FER or a FER-related receptor kinase, have been studied, illustrating the functional diversity and the mechanistic complexity of the FER family signaling modules. The challenges going forward are to distill from this complexity the unifying schemes where possible and attain precision and refinement in the knowledge of critical details upon which future investigations can be built. By focusing on the extensively characterized FER, this review provides foundational information to guide the next phase of research on FER in model as well as crop species and potential applications for improving plant growth and resilience.

Mismatch between pollen and pistil size causes asymmetric mechanical reproductive isolation across Phlox species

In flowering plants, pollen-pistil interactions can serve as an important barrier to reproduction between species. As the last barrier to reproduction before fertilization, interactions between these reproductive organs are both complex and important for determining a suitable mate. Here, we test whether differences in style length generate a post-mating prezygotic mechanical barrier between five species of perennial Phlox wildflowers with geographically overlapping distributions. We perform controlled pairwise reciprocal crosses between three species with long styles and two species with short styles to assess crossing success (seed set). We find that heterospecific seed set is broadly reduced compared to conspecific cross success and reveal a striking asymmetry in heterospecific crosses between species with different style lengths. To determine the mechanism underlying this asymmetric reproductive isolating barrier we assess pollen tube growth in vitro and in vivo. We demonstrate that pollen tubes of short-styled species do not grow long enough to reach the ovaries of long-styled species. We find that short-styled species also have smaller pollen and that both within and between species pollen diameter is highly correlated with pollen tube length. Our results support the hypothesis that the small pollen of short-styled species lacks resources to grow pollen tubes long enough to access the ovaries of the long-styled species, resulting in an asymmetrical, mechanical barrier to reproduction. Such mechanisms, combined with additional pollen-pistil incompatibilities, may be particularly important for closely related species in geographic proximity that share pollinators.

Signalling between the sexes during pollen tube reception

Plant reproduction is a complex, highly-coordinated process in which a single, male germ cell grows through the maternal reproductive tissues to reach and fertilise the egg cell. Focussing on Arabidopsis thaliana, we review signalling between male and female partners which is important throughout the pollen tube journey, especially during pollen tube reception at the ovule. Numerous receptor kinases and their coreceptors are implicated in signal perception in both the pollen tube and synergid cells at the ovule entrance, and several specific peptide and carbohydrate ligands for these receptors have recently been identified. Clarifying the interplay between these signals and the downstream responses they instigate presents a challenge for future research and may help to illuminate broader principles of plant cell-cell communication.

Characterization of pollen tube development in distant hybridization of Chinese cork oak (Quercus variabilis L.)

MAIN CONCLUSION

This work mainly found that the stigma and style of Q. variabilis did not completely lose the specific recognition towards heterologous pollen, a fact which is different from previous studies. Quercus is the foundation species in the Northern Hemisphere, with extreme prevalence for interspecific hybridization. It is not yet entirely understood whether or how the pollen tube-female tissue interaction contributes to the "extensive hybridization" in oaks. Pollen storage conditions correlate with distant hybridization. We conducted hybridization experiments with Q. variabilis as female and Q. variabilis and Q. mongolica as male parents. And the differences in pollen tube (PT) development between intra- and distant interspecific hybridization were studied by fluorescence microscopy and scanning electron microscopy (SEM). Our results showed that -20 °C allowed pollen of both species to maintain some viability. Both Q. variabilis and Q. mongolica pollen germinated profusely on the stigmas. SEM results indicated that in the intraspecific hybridization, Q. variabilis pollen started to germinate at 6 h after pollination (hap), PTs elongated significantly at 12 hap, and entered the stigma at 24 hap. By contrast, Q. mongolica pollen germinated at 15 hap, and the PTs entered the stigma at 27 hap. By fluorescence microscopical studies it was observed that some PTs of Q. variabilis gathered at the style-joining at 96 hap, unlike the Q. mongolica which reached the style junction at 144 hap. The above results indicate that the abundant germination of heterologous pollen (HP) on the stigma and the "Feeble specificity recognition" of the stigma and transmitting tract to HP may create opportunities for the "extensive hybridization" of oaks. This work provides a sexual developmental reference for clarifying the causes of Quercus "extensive hybridization".

Brassinosteroid recruits FERONIA to safeguard cell expansion in Arabidopsis

Plant cell expansion is driven by turgor pressure and regulated by hormones. How plant cells avoid cell wall rupture during hormone-induced cell expansion remains a mystery. Here we show that brassinosteroid (BR), while stimulating cell elongation, promotes the plasma membrane (PM) accumulation of the receptor kinase FERONIA (FER), which monitors cell wall damage and in turn attenuates BR-induced cell elongation to prevent cell rupture. The GSK3-like kinase BIN2 phosphorylates FER, resulting in reduced FER accumulation and translocation from endoplasmic reticulum to PM. By inactivating BIN2, BR signaling promotes dephosphorylation and increases PM accumulation of FER, thereby enhancing the surveillance of cell wall integrity. Our study reveals a vital signaling circuit that coordinates hormone signaling with mechanical sensing to prevent cell bursting during hormone-induced cell expansion.

Organogenic events during gynoecium and fruit development in Arabidopsis

Angiosperms are the most successful group of land plants. This success is mainly due to the gynoecium, the innermost whorl of the flower. In Arabidopsis, the gynoecium is a syncarpic structure formed by two congenitally fused carpels. At the fusion edges of the carpels, the carpel margin meristem forms. This quasi-meristem is important for medial-tissue development, including the ovules. After the double fertilization, both the seeds and fruit begin to develop. Due to the importance of seeds and fruits as major food sources worldwide, it has been an important task for the scientific community to study gynoecium development. In this review, we present the most recent advances in Arabidopsis gynoecium patterning, as well as some questions that remain unanswered.

Structural and molecular basis of pollen germination

Pollen germination is a prerequisite for double fertilization of flowering plants. A comprehensive understanding of the structural and molecular basis of pollen germination holds great potential for crop yield improvement. The pollen aperture serves as the foundation for most plant pollen germination and pollen aperture formation involves the establishment of cellular polarity, the formation of distinct membrane domains, and the precise deposition of extracellular substances. Successful pollen germination requires precise material exchange and signal transduction between the pollen grain and the stigma. Recent cytological and mutant analysis of pollen germination process in Arabidopsis and rice has expanded our understanding of this biological process. However, the overall changes in germination site structure and energy-related metabolites during pollen germination remain to be further explored. This review summarizes and compares the recent advances in the processes of pollen aperture formation, pollen adhesion, hydration, and germination between eudicot Arabidopsis and monocot rice, and provides insights into the structural basis and molecular mechanisms underlying pollen germination process.

Central-cell-produced attractants control fertilization recovery

Animal fertilization relies on hundreds of sperm racing toward the egg, whereas, in angiosperms, only two sperm cells are delivered by a pollen tube to the female gametes (egg cell and central cell) for double fertilization. However, unsuccessful fertilization under this one-pollen-tube design can be detrimental to seed production and plant survival. To mitigate this risk, unfertilized-gamete-controlled extra pollen tube entry has been evolved to bring more sperm cells and salvage fertilization. Despite its importance, the underlying molecular mechanism of this phenomenon remains unclear. In this study, we report that, in Arabidopsis, the central cell secretes peptides SALVAGER1 and SALVAGER2 in a directional manner to attract pollen tubes when the synergid-dependent attraction fails or is terminated by pollen tubes carrying infertile sperm cells. Moreover, loss of SALs impairs the fertilization recovery capacity of the ovules. Therefore, this research uncovers a female gamete-attraction system that salvages seed production for reproductive assurance.

Rapid alkalinization factor: function, regulation, and potential applications in agriculture

Rapid alkalinization factor (RALF) is widespread throughout the plant kingdom and controls many aspects of plant life. Current studies on the regulatory mechanism underlying RALF function mainly focus on Arabidopsis, but little is known about the role of RALF in crop plants. Here, we systematically and comprehensively analyzed the relation between RALF family genes from five important crops and those in the model plant Arabidopsis thaliana. Simultaneously, we summarized the functions of RALFs in controlling growth and developmental behavior using conservative motifs as cues and predicted the regulatory role of RALFs in cereal crops. In conclusion, RALF has considerable application potential in improving crop yields and increasing economic benefits. Using gene editing technology or taking advantage of RALF as a hormone additive are effective way to amplify the role of RALF in crop plants.

Pollen Coat Proteomes of Arabidopsis thaliana, Arabidopsis lyrata, and Brassica oleracea Reveal Remarkable Diversity of Small Cysteine-Rich Proteins at the Pollen-Stigma Interface

The pollen coat is the outermost domain of the pollen grain and is largely derived from the anther tapetum, which is a secretory tissue that degenerates late in pollen development. By being localised at the interface of the pollen-stigma interaction, the pollen coat plays a central role in mediating early pollination events, including molecular recognition. Amongst species of the Brassicaceae, a growing body of data has revealed that the pollen coat carries a range of proteins, with a number of small cysteine-rich proteins (CRPs) being identified as important regulators of the pollen-stigma interaction. By utilising a state-of-the-art liquid chromatography/tandem mass spectrometry (LC-MS/MS) approach, rich pollen coat proteomic profiles were obtained for Arabidopsis thaliana, Arabidopsis lyrata, and Brassica oleracea, which greatly extended previous datasets. All three proteomes revealed a strikingly large number of small CRPs that were not previously reported as pollen coat components. The profiling also uncovered a wide range of other protein families, many of which were enriched in the pollen coat proteomes and had functions associated with signal transduction, cell walls, lipid metabolism and defence. These proteomes provide an excellent source of molecular targets for future investigations into the pollen-stigma interaction and its potential evolutionary links to plant-pathogen interactions.