Mechanical design of apertures and the infolding of pollen grain

A Simple and User-Friendly Method for High-Quality Preparation of Pollen Grains for Scanning Electron Microscopy (SEM)

Pollen is becoming an increasingly important subject for molecular researchers in genetic engineering, plant breeding, and environmental monitoring. To broaden the scope of these studies, it is essential to develop accessible methods for scientists who are not specialized in palynology. The article presents a simplified technical procedure for preparing pollen grains for scanning electron microscopy (SEM). The protocol is convenient for any molecular laboratory due to its small set of reagents, ease of execution, low cost, does not require special equipment, and takes only one hour to complete. The high penetrating ability of formaldehyde and the final delicate dehydration using hexamethyldisilazane (HMDS) instead of critical point drying allow for sufficient preservation of the architecture of the aperture, which is considered a gateway for the passage of biomolecules. The method was successfully applied to pollen grains of representatives of dicotyledons (beetroot, petunia, radish, tomato and tobacco) and monocotyledons (lily, onion, corn, rye and wheat). Species studied included insect-pollinated (entomophilous) and wind-pollinated (anemophilous) species. A comparative analysis of the sizes of fresh living pollen grains under a light microscope and those prepared for SEM showed some shrinkage. Quantitative analysis of the degree of pollen grain shrinkage showed that this process depends on the initial shape of dry pollen grains, and the number and structure of apertures. The results support the theoretical model of the folding/unfolding pathways of pollen grains.

OsSRF8 interacts with OsINP1 and OsDAF1 to regulate pollen aperture formation in rice

In higher plants, mature male gametophytes have distinct apertures. After pollination, pollen grains germinate, and a pollen tube grows from the aperture to deliver sperm cells to the embryo sac, completing fertilization. In rice, the pollen aperture has a single-pore structure with a collar-like annulus and a plug-like operculum. A crucial step in aperture development is the formation of aperture plasma membrane protrusion (APMP) at the distal polar region of the microspore during the late tetrad stage. Previous studies identified OsINP1 and OsDAF1 as essential regulators of APMP and pollen aperture formation in rice, but their precise molecular mechanisms remain unclear. We demonstrate that the Poaceae-specific OsSRF8 gene, encoding a STRUBBELIG-receptor family 8 protein, is essential for pollen aperture formation in Oryza sativa. Mutants lacking functional OsSRF8 exhibit defects in APMP and pollen aperture formation, like loss-of-function OsINP1 mutants. OsSRF8 is specifically expressed during early anther development and initially diffusely distributed in the microsporocytes. At the tetrad stage, OsSRF8 is recruited by OsINP1 to the pre-aperture region through direct protein-protein interaction, promoting APMP formation. The OsSRF8-OsINP1 complex then recruits OsDAF1 to the APMP site to co-regulate annulus formation. Our findings provide insights into the mechanisms controlling pollen aperture formation in cereal species.

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.

Ontogeny of the pollinium in Hoya carnosa provides new insights into microsporogenesis

The presence of a pollinium is a distinct character in Apocynaceae which is important for phylogenetic analysis. The pollinium of Hoya has an outer sporopollenin wall and a pellucid margin which are adaptive features. However, their ontogeny and related evolutionary implications are not entirely understood. Therefore, a representative species Hoya carnosa was selected to investigate the pollinium development using light and electron microscopy and cytochemical tests. In contrast to the microsporogenesis in most angiosperms, which is associated with callose, the non-callosic intersporal walls in Hoya carnosa, together with the successive cytokinesis and linear form of the tetrad, represent an alternative pattern of microsporogenesis. This pattern has specific implication for the early stages of pollen morphogenesis. The absence of exine and apertures in the pollen grains in the pollinium could result from a combination of factors including the absence of callose in the early stages and the modifications in later developmental pathways, e.g., the sporopollenin accumulation pathway. The pollinium wall is an exine without stratification, its surface lacks sculptures, and it provides structural support and protection. The pollen tubes germinate through the pellucid margin and germinating ridge which are specialized features. The pellucid margin originates from aborted microspores. The germinating ridge that lies on the outer side of the pellucid margin develops in the same way as a classic pollen exine. The pollen grains are aggregated by intine fusion which is favorable for tube germination and growth. Comparing Asclepiadoideae with the other two subfamilies of Apocynaceae that develop a pollinium, the pollinium of Asclepiadoideae has reduced deposition of sporopollenin in the inner walls but an increase in the outer pollinium wall, thus making the inner walls more reduced and simplified, and the outer walls more solid. The adaptive characters of the pollen wall structure and the cohesion mechanism suggest that the pollinium of Hoya carnosa is a derived form of pollen aggregation.

Identification of Candidate Genes Involved in the Determinism of Pollen Grain Aperture Morphology by Comparative Transcriptome Analysis in Papaveraceae

In the last decade, certain genes involved in pollen aperture formation have been discovered. However, those involved in pollen aperture shape remain largely unknown. In Arabidopsis, the interaction during the tetrad development stage of one member of the ELMOD protein family, ELMOD_E, with two others, MCR/ELMOD_B and ELMOD_A, can change the morphology of apertures from colpus (elongated) to pore (round). Here, comparative transcriptome analysis is used to identify candidate genes involved in the determination of pollen aperture morphology in Papaveraceae (order Ranunculales). Furthermore, the role of ELMOD genes in the genetic determinism of aperture shape was tested by comparative analysis of their expression levels using RNA-seq data and RT-qPCR. Two pairs of species belonging to two different subfamilies were used. Within each pair, one species has colpate pollen and the other porate (Fumarioideae-Dactylicapnos torulosa, 6-colpate, and Fumaria bracteosa, pantoporate; Papaveroideae-Eschsholzia californica, 5-7 colpate, and Roemeria refracta, 6-porate). The transcriptomes were obtained at the tetrad stage of pollen development. A total of 531 DEGs were found between the colpate and porate pollen species groups. The results from RNA-seq and RT-qPCR indicate that pollen aperture shape is not determined by the relative expression levels of ELMOD family genes in Papaveraceae. However, genes related to callose wall formation or cytoskeleton organisation were found, these processes being involved in pollen aperture formation. In addition, transcriptomes from anthers with pollen during the tetrad stage of three species (D. torulosa, R. refracta, and F. bracteosa) were obtained for the first time. These data will be available for further studies in the field of floral evolution and development.

Extraordinary microcarriers derived from spores and pollens

Spores and pollens refer to the reproductive cells of seed plants and asexually reproducing sporophytes, exhibiting a natural core-shell structure and exquisite surface morphology. They possess extraordinary dimensional homogeneity, porosity, amphiphilicity and adhesion. Their sporopollenin exine layer endows them with chemically stable, UV resistant, and biocompatible properties, which can also be facilely functionalized due to sufficient groups on the surface. The unique characteristics of spores and pollens have facilitated a wide range of applications in drug carriers, biological imaging, food science, microrobotics, environmental purification, flexible electronics, cell scaffolds, 3D printing materials and biological detection. This review showcases the common structural composition and physicochemical properties of spores and pollens, describes the extraction and processing methods, and summarizes the recent research on their applications in various fields. Following these sections, this review analyzes the existing challenges in spores and pollen research and provides a future outlook.

Sporogenesis in Physcomitrium patens: Intergenerational collaboration and the development of the spore wall and aperture

Although the evolution of spores was critical to the diversification of plants on land, sporogenesis is incompletely characterized for model plants such as Physcomitrium patens. In this study, the complete process of P. patens sporogenesis is detailed from capsule expansion to mature spore formation, with emphasis on the construction of the complex spore wall and proximal aperture. Both diploid (sporophytic) and haploid (spores) cells contribute to the development and maturation of spores. During capsule expansion, the diploid cells of the capsule, including spore mother cells (SMCs), inner capsule wall layer (spore sac), and columella, contribute a locular fibrillar matrix that contains the machinery and nutrients for spore ontogeny. Nascent spores are enclosed in a second matrix that is surrounded by a thin SMC wall and suspended in the locular material. As they expand and separate, a band of exine is produced external to a thin foundation layer of tripartite lamellae. Dense globules assemble evenly throughout the locule, and these are incorporated progressively onto the spore surface to form the perine external to the exine. On the distal spore surface, the intine forms internally, while the spiny perine ornamentation is assembled. The exine is at least partially extrasporal in origin, while the perine is derived exclusively from outside the spore. Across the proximal surface of the polar spores, an aperture begins formation at the onset of spore development and consists of an expanded intine, an annulus, and a central pad with radiating fibers. This complex aperture is elastic and enables the proximal spore surface to cycle between being compressed (concave) and expanded (rounded). In addition to providing a site for water intake and germination, the elastic aperture is likely involved in desiccation tolerance. Based on the current phylogenies, the ancestral plant spore contained an aperture, exine, intine, and perine. The reductive evolution of liverwort and hornwort spores entailed the loss of perine in both groups and the aperture in liverworts. This research serves as the foundation for comparisons with other plant groups and for future studies of the developmental genetics and evolution of spores across plants.

Curvature-driven instabilities in thin active shells

Spontaneous material shape changes, such as swelling, growth or thermal expansion, can be used to trigger dramatic elastic instabilities in thin shells. These instabilities originate in geometric incompatibility between the preferred extrinsic and intrinsic curvature of the shell, which may be modified by active deformations through the thickness and in plane, respectively. Here, we solve the simplest possible model of such instabilities, which assumes the shells are shallow, thin enough to bend but not stretch, and subject to homogeneous preferred curvatures. We consider separately the cases of zero, positive and negative Gauss curvature. We identify two types of supercritical symmetry-breaking instability, in which the shell's principal curvature spontaneously breaks discrete up/down symmetry and continuous planar isotropy. These are then augmented by inversion instabilities, in which the shell jumps subcritically between up/down broken symmetry states and rotation instabilities, in which the curvatures rotate by 90° between states of broken isotropy without release of energy. Each instability has a thickness-independent threshold value for the preferred extrinsic curvature proportional to the square root of Gauss curvature. Finally, we show that the threshold for the isotropy-breaking instability is the same for deep spherical caps, in good agreement with recently published data.

In vitro experiments and kinetic models of Arabidopsis pollen hydration mechanics show that MSL8 is not a simple tension-gated osmoregulator

Pollen, a neighbor-less cell containing the male gametes, undergoes mechanical challenges during plant sexual reproduction, including desiccation and rehydration. It was previously shown that the pollen-specific mechanosensitive ion channel MscS-like (MSL)8 is essential for pollen survival during hydration and proposed that it functions as a tension-gated osmoregulator. Here, we test this hypothesis with a combination of mathematical modeling and laboratory experiments. Time-lapse imaging revealed that wild-type pollen grains swell, and then they stabilize in volume rapidly during hydration. msl8 mutant pollen grains, however, continue to expand and eventually burst. We found that a mathematical model, wherein MSL8 acts as a simple-tension-gated osmoregulator, does not replicate this behavior. A better fit was obtained from variations of the model, wherein MSL8 inactivates independent of its membrane tension gating threshold or MSL8 strengthens the cell wall without osmotic regulation. Experimental and computational testing of several perturbations, including hydration in an osmolyte-rich solution, hyper-desiccation of the grains, and MSL8-YFP overexpression, indicated that the cell wall strengthening model best simulated experimental responses. Finally, the expression of a nonconducting MSL8 variant did not complement the msl8 overexpansion phenotype. These data indicate that contrary to our hypothesis and to the current understanding of MS ion channel function in bacteria, MSL8 does not act as a simple membrane tension-gated osmoregulator. Instead, they support a model wherein ion flux through MSL8 is required to alter pollen cell wall properties. These results demonstrate the utility of pollen as a cellular scale model system and illustrate how mathematical models can correct intuitive hypotheses.

Generating images of hydrated pollen grains using deep learning

Pollen grains dehydrate during their development and following their departure from the host stigma. Since the size and shape of a pollen grain can be dependent on environmental conditions, being able to predict both of these factors for hydrated pollen grains from their dehydrated state could be beneficial in the fields of climate science, agriculture, and palynology. Here, we use deep learning to transform images of dehydrated Ranunculus pollen grains into images of hydrated Ranunculus pollen grains. We also then use a deep learning neural network that was trained on experimental images of different genera of pollen grains to identify the hydrated pollen grains from the generated transformed images, to test the accuracy of the image generation neural network. This pilot work demonstrates the first steps needed towards creating a general deep learning-based rehydration model that could be useful in understanding and predicting pollen morphology.

Role of Sporopollenin Shell Interfacial Properties in Protein Adsorption

Sporopollenin shells isolated from natural pollen grains have received attention in translational and applied research in diverse fields of drug delivery, vaccine delivery, and wastewater remediation. However, little is known about the sporopollenin shell's potential as an adsorbent. Herein, we have isolated sporopollenin shells from four structurally diverse pollen species, black walnut, marsh elder, mugwort, and silver birch, to study protein adsorption onto sporopollenin shells. We investigated three major interfacial properties, surface area, surface functional groups, and surface charge, to elucidate the mechanism of protein adsorption onto sporopollenin shells. We showed that sporopollenin shells have a moderate specific surface area (<12 m²/g). Phosphoric acid and potassium hydroxide treatments that were used to isolate sporopollenin shells from natural pollen grains also result in the functionalization of sporopollenin shell surfaces with ionizable groups of carboxylic acid and carboxylate salt. As a result, sporopollenin shells exhibit a negative ζ potential in the range of -75 to -82 mV at pH 10 when dispersed in water. The ζ potentials of sporopollenin shells remain negative in the pH range of 2.5-11, with the absolute value of ζ potential showing a decrease with the decrease in pH. The negative surface charge promotes the adsorption of protein onto the sporopollenin shell via electrostatic interaction. Despite having a moderate surface area, sporopollenin shells adsorb a significant amount of lysozyme (145-340 μg lysozyme per mg of sporopollenin shells). Lysozyme adsorption onto sporopollenin shells alters the surface, and the surface charge becomes positive at acidic pH. Overall, this study demonstrates the potential of sporopollenin shells to adsorb proteins, highlights the critical role of sporopollenin shell's interfacial properties in protein adsorption, and identifies the mechanism of protein adsorption on sporopollenin shells.

Mechanics of inactive swelling and bursting of porate pollen grains

The structure of pollen grains, which is typically characterized by soft apertures in an otherwise stiff exine shell, guides their response to changes in the humidity of the environment. These changes can lead to desiccation of the grain and its infolding but also to excessive swelling of the grain and even its bursting. Here we use an elastic model to explore the mechanics of pollen grain swelling and the role of soft, circular apertures (pores) in this process. Small, circular apertures typically occur in airborne and allergenic pollen grains so that the bursting of such grains is important in the context of human health. We identify and quantify a mechanical weakness of the pores, which are prone to rapid inflation when the grain swells to a critical extent. The inflation occurs as a sudden transition and may induce bursting of the grain and release of its content. This process crucially depends on the size of the pores and their softness. Our results provide insight into the inactive part of the mechanical response of pollen grains to hydration when they land on a stigma as well as bursting of airborne pollen grains during changes in air humidity.

Hierarchical magnetic nanoparticles for highly effective capture of small extracellular vesicles

Small extracellular vesicles (EVs) have various functions through the transfer of specific biomolecules. However, it is still challenging to capture small EVs with high sensitivity and specificity. Herein, inspired by the unique burry structure and the strong adhesion ability of pollen grains, we presented a novel Fe₃O₄@MgSiO₃ hierarchical magnetic nanoparticles (HNPs) as nanocarriers for the capture of small EVs. The NPs were generated through the solvothermal method and further modified with branching dendrimers to exhibit a hierarchical morphology. The enlarged surface area facilitated high-efficient capture of small EVs through specific recognition of aptamer probes and the small EVs surface markers. Besides, the magnetic core of the NPs allowed them to be isolated under the action of an external magnetic field, and thus the captured small EVs could be easily separated from plasma. These results indicated that the HNPs could serve as excellent nanocarriers for small EVs capture and related biomedical applications.

Consequences of whole genome duplication for 2n pollen performance

The vegetative cell of the angiosperm male gametophyte (pollen) functions as a free-living, single-celled organism that both produces and transports sperm to egg. Whole-genome duplication (WGD) should have strong effects on pollen because of the haploid to diploid transition and because of both genetic and epigenetic effects on cell-level phenotypes. To disentangle historical effects of WGD on pollen performance, studies can compare 1n pollen from diploids to neo-2n pollen from diploids and synthetic autotetraploids to older 2n pollen from established neo-autotetraploids. WGD doubles both gene number and bulk nuclear DNA mass, and a substantial proportion of diploid and autotetraploid heterozygosity can be transmitted to 2n pollen. Relative to 1n pollen, 2n pollen can exhibit heterosis due to higher gene dosage, higher heterozygosity and new allelic interactions. Doubled genome size also has consequences for gene regulation and expression as well as epigenetic effects on cell architecture. Pollen volume doubling is a universal effect of WGD, whereas an increase in aperture number is common among taxa with simultaneous microsporogenesis and pored apertures, mostly eudicots. WGD instantly affects numerous evolved compromises among mature pollen functional traits and these are rapidly shaped by highly diverse tissue interactions and pollen competitive environments in the early post-WGD generations. 2n pollen phenotypes generally incur higher performance costs, and the degree to which these are met or evolve by scaling up provisioning and metabolic vigor needs further study.