Cell mechanics of pollen tube growth

An insight into the roles of ubiquitin-specific proteases in plants: development and growth, morphogenesis, and stress response

Ubiquitination is a highly conserved and dynamic post-translational modification in which protein substrates are modified by ubiquitin to influence their activity, localization, or stability. Deubiquitination enzymes (DUBs) counter ubiquitin signaling by removing ubiquitin from the substrates. Ubiquitin-specific proteases (UBPs), the largest subfamily of DUBs, are conserved in plants, serving diverse functions across various cellular processes, although members within the same group often exhibit functional redundancy. Here, we briefly review recent advances in understanding the biological roles of UBPs, particularly the molecular mechanism by which UBPs regulate plant development and growth, morphogenesis, and stress response, which sheds light on the mechanistic roles of deubiquitination in plants.

Forces on and in the cell walls of living plants

Environmental influences and differential growth subject plants to mechanical forces. Forces on the whole plant resolve into tensile forces on its primary cell walls and both tensile and compression forces on the secondary cell wall layers of woody tissues. Forces on cell walls are further resolved into forces on cellulose microfibrils and the noncellulosic polymers between them. Many external forces on plants oscillate, with time constants that vary from seconds to milliseconds. Sound waves are a high-frequency example. Forces on the cell wall lead to responses that direct the oriented deposition of cellulose microfibrils and the patterned expansion of the cell wall, leading to complex cell and tissue morphology. Recent experiments have established many of the details of which cell wall polymers associate with one another in both primary and secondary cell walls, but questions remain about which of the interconnections are load bearing, especially in primary cell walls. Direct cellulose-cellulose interactions appear to have a more important mechanical role than was previously thought, and some of the noncellulosic polymers may have a role in keeping microfibrils apart rather than cross-linking them as formerly envisaged.

A signaling peptide locks pollen tube walls

A protein-peptide complex generates and stabilizes a cell-wall carbohydrate lattice.

Actin cytoskeleton in the control of vesicle transport, cytoplasmic organization, and pollen tube tip growth

Pollen tubes extend rapidly via tip growth. This process depends on a dynamic actin cytoskeleton, which has been implicated in controlling organelle movements, cytoplasmic streaming, vesicle trafficking, and cytoplasm organization in pollen tubes. In this update review, we describe the progress in understanding the organization and regulation of the actin cytoskeleton and the function of the actin cytoskeleton in controlling vesicle traffic and cytoplasmic organization in pollen tubes. We also discuss the interplay between ion gradients and the actin cytoskeleton that regulates the spatial arrangement and dynamics of actin filaments and the organization of the cytoplasm in pollen tubes. Finally, we describe several signaling components that regulate actin dynamics in pollen tubes.

Optimization of In Vitro Germination, Viability Tests and Storage of Paeonia ostii Pollen

Paeonia ostii is an important woody oil crop mainly cross-pollinated. However, the low yield has become an important factor restricting the industrial development of P. ostii. Cross-pollination has become one of the important measures to increase the seed yield. Therefore, conservation of pollen with high vitality is crucial to ensure successful pollination of P. ostii. In this study, we found an effective methodological system to assess the viability, ability to germinate, and optimal storage conditions of P. ostii pollen grains. The optimal medium in vitro was 50 g/L sucrose, 100 mg/L boric acid, 50 g/L PEG6000, 100 mg/L potassium nitrate, 300 mg/L calcium nitrate, and 200 mg/L magnesium sulfate at pH 5.4. Optimal germination condition in vitro was achieved at 25 °C for 120 min, allowing easy observation of the germination percentage and length of the pollen tubes. In addition, the viability of pollen grains was assessed by comparing nine staining methods. Among them, MTT, TTC, benzidine-H₂O₂, and FDA were effective to distinguish between viable and non-viable pollen, and the results of the FDA staining method were similar to the pollen germination percentage in vitro. After evaluation of pollen storage, thawing and rehydration experiments showed that thawing at 4 °C for 30 min and rehydration at 25 °C for 30 min increased the germination percentage of pollen grains stored at low temperatures. The low-temperature storage experiments showed that 4 °C was suitable for short-term storage of P. ostii pollen grains, while -80 °C was suitable for long-term storage. This is the first report on the in vitro germination, viability tests, and storage of P. ostii pollen grains, which will provide useful information for P. ostii germplasm conservation and artificial pollination.

Autophagy and its mediated mitochondrial quality control maintain pollen tube growth and male fertility in Arabidopsis

Macroautophagy/autophagy, a major catabolic pathway in eukaryotes, participates in plant sexual reproduction including the processes of male gametogenesis and the self-incompatibility response. Rapid pollen tube growth is another essential reproductive process that is metabolically highly demanding to drive the vigorous cell growth for delivery of male gametes for fertilization in angiosperms. Whether and how autophagy operates to maintain the homeostasis of pollen tubes remains unknown. Here, we provide evidence that autophagy is elevated in growing pollen tubes and critically required during pollen tube growth and male fertility in Arabidopsis. We demonstrate that SH3P2, a critical non-ATG regulator of plant autophagy, colocalizes with representative ATG proteins during autophagosome biogenesis in growing pollen tubes. Downregulation of SH3P2 expression significantly disrupts Arabidopsis pollen germination and pollen tube growth. Further analysis of organelle dynamics reveals crosstalk between autophagosomes and prevacuolar compartments following the inhibition of phosphatidylinositol 3-kinase. In addition, time-lapse imaging and tracking of ATG8e-labeled autophagosomes and depolarized mitochondria demonstrate that they interact specifically via the ATG8-family interacting motif (AIM)-docking site to mediate mitophagy. Ultrastructural identification of mitophagosomes and two additional forms of autophagosomes imply that multiple types of autophagy are likely to function simultaneously within pollen tubes. Altogether, our results suggest that autophagy is functionally crucial for mediating mitochondrial quality control and canonical cytoplasm recycling during pollen tube growth.Abbreviations: AIM: ATG8-family interacting motif; ATG8: autophagy related 8; ATG5: autophagy related 5; ATG7: autophagy related 7; BTH: acibenzolar-S-methyl; DEX: dexamethasone; DNP: 2,4-dinitrophenol; GFP: green fluorescent protein; YFP: yellow fluorescent protein; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; PVC: prevacuolar compartment; SH3P2: SH3 domain-containing protein 2.

An actin mechanostat ensures hyphal tip sharpness in Phytophthora infestans to achieve host penetration

Filamentous plant pathogens apply mechanical forces to pierce their hosts surface and penetrate its tissues. Devastating Phytophthora pathogens harness a specialized form of invasive tip growth to slice through the plant surface, wielding their hypha as a microscopic knife. Slicing requires a sharp hyphal tip that is not blunted at the site of the mechanical interaction. How tip shape is controlled, however, is unknown. We uncover an actin-based mechanostat in Phytophthora infestans that controls tip sharpness during penetration. Mechanical stimulation of the hypha leads to the emergence of an aster-like actin configuration, which shows fast, local, and quantitative feedback to the local stress. We evidence that this functions as an adaptive mechanical scaffold that sharpens the invasive weapon and prevents it from blunting. The hyphal tip mechanostat enables the efficient conversion of turgor into localized invasive pressures that are required to achieve host penetration.

Interactions between a mechanosensitive channel and cell wall integrity signaling influence pollen germination in Arabidopsis thaliana

Cells employ multiple systems to maintain cellular integrity, including mechanosensitive ion channels and the cell wall integrity (CWI) pathway. Here, we use pollen as a model system to ask how these different mechanisms are interconnected at the cellular level. MscS-Like 8 (MSL8) is a mechanosensitive channel required to protect Arabidopsis thaliana pollen from osmotic challenges during in vitro rehydration, germination, and tube growth. New CRISPR/Cas9 and artificial miRNA-generated msl8 alleles produced unexpected pollen phenotypes, including the ability to germinate a tube after bursting, dramatic defects in cell wall structure, and disorganized callose deposition at the germination site. We document complex genetic interactions between MSL8 and two previously established components of the CWI pathway, MARIS and ANXUR1/2. Overexpression of MARISR240C-FP suppressed the bursting, germination, and callose deposition phenotypes of msl8 mutant pollen. Null msl8 alleles suppressed the internalized callose structures observed in MARISR240C-FP lines. Similarly, MSL8-YFP overexpression suppressed bursting in the anxur1/2 mutant background, while anxur1/2 alleles reduced the strong rings of callose around ungerminated pollen grains in MSL8-YFP overexpressors. These data show that mechanosensitive ion channels modulate callose deposition in pollen and provide evidence that cell wall and membrane surveillance systems coordinate in a complex manner to maintain cell integrity.

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.

Plant cell mechanobiology: Greater than the sum of its parts

The ability to sense and respond to physical forces is critical for the proper function of cells, tissues, and organisms across the evolutionary tree. Plants sense gravity, osmotic conditions, pathogen invasion, wind, and the presence of barriers in the soil, and dynamically integrate internal and external stimuli during every stage of growth and development. While the field of plant mechanobiology is growing, much is still poorly understood-including the interplay between mechanical and biochemical information at the single-cell level. In this review, we provide an overview of the mechanical properties of three main components of the plant cell and the mechanoperceptive pathways that link them, with an emphasis on areas of complexity and interaction. We discuss the concept of mechanical homeostasis, or "mechanostasis," and examine the ways in which cellular structures and pathways serve to maintain it. We argue that viewing mechanics and mechanotransduction as emergent properties of the plant cell can be a useful conceptual framework for synthesizing current knowledge and driving future research.

Microfluidics-Based Bioassays and Imaging of Plant Cells

Many plant processes occur in the context of and in interaction with a surrounding matrix such as soil (e.g. root growth and root-microbe interactions) or surrounding tissues (e.g. pollen tube growth through the pistil), making it difficult to study them with high-resolution optical microscopy. Over the past decade, microfabrication techniques have been developed to produce experimental systems that allow researchers to examine cell behavior in microstructured environments that mimic geometrical, physical and/or chemical aspects of the natural growth matrices and that cannot be generated using traditional agar plate assays. These microfabricated environments offer considerable design flexibility as well as the transparency required for high-resolution, light-based microscopy. In addition, microfluidic platforms have been used for various types of bioassays, including cellular force assays, chemoattraction assays and electrotropism assays. Here, we review the recent use of microfluidic devices to study plant cells and organs, including plant roots, root hairs, moss protonemata and pollen tubes. The increasing adoption of microfabrication techniques by the plant science community may transform our approaches to investigating how individual plant cells sense and respond to changes in the physical and chemical environment.

Apical pollen tube wall curvature correlates with growth and indicates localized changes in the yielding of the cell wall

The shape of the apical region of lily pollen tube changes rhythmically as the growth rate of the tube oscillates becoming alternately more prolate then back to oblate. We quantified shape change by calculating the curvature of the cross-sectional edge of the pollen tube tip and cross-correlating curvature changes with growth rate. The apical region takes the form of a partial elliptical spheroid, with variation in the length and location of the minor axis. During oscillation curvature profiles show a sharp increase in curvature at the "shoulders" of the apex when oblate, 4-7 μm from the flatter central zone. As the tip becomes more prolate, the "shoulders" decrease rapidly in curvature and move towards the growth axis as curvature at the tip increases. We understand curvature changes to represent differential changes in local wall expansion rates, driven by uniform turgor pressure and mediated by changes in wall polysaccharides. To become more oblate, the tip region must become less extensible than the "shoulder" region. And, as the tip becomes more prolate, the increased curvature must be due to increased local expansion. We found that changes in the growth velocity of the "shoulders" of the cell measured as the progress of the cell edge along the growth axis are cyclically out of phase with growth velocity at the tip such that the shoulder regions lag for part of the oscillation cycle, then "catch up" as the growth rate at the tip reaches a maximum and begins to decline. In this way the cell becomes oblate. Cell shape and growth rate oscillate in concert and are functionally related. Spatial change in edge growth rate points to important cellular locations for further investigation of vesicle movement and exocytosis, calcium gradients, and actin dynamics in lily pollen tubes.

KATANIN and cortical microtubule organization have a pivotal role in early pollen tube guidance

Following pollen deposition on the receptive surface of the stigma, pollen germinates a tube that carries male gametes toward the ovule where fertilization occurs. As soon as it emerges from the pollen grain, the pollen tube has to be properly guided through the pistil tissues so as to reach the ovule and ensure double fertilization. Chemical attractants, nutrients as well as receptor kinase-dependent signaling pathways have been implicated in this guidance. Recently, we showed in Arabidopsis that the microtubule severing enzyme KATANIN, by acting both on cortical microtubule (CMT) dynamics and cellulose microfibril (CMF) deposition, conferred particular mechanical properties to the papilla cell wall that act as active guidance factors. Here we confirm the importance of KATANIN and CMT orientation in pollen tube directionality by examining another katanin mutant.

Turgor regulation defect 1 proteins play a conserved role in pollen tube reproductive innovation of the angiosperms

Sexual reproduction in angiosperms is siphonogamous, and the interaction between pollen tube and pistil is critical for successful fertilization. Our previous study demonstrated that mutation of the Arabidopsis turgor regulation defect 1 (TOD1) gene leads to reduced male fertility, a result of retarded pollen tube growth in the pistil. TOD1 encodes a Golgi-localized alkaline ceramidase, a key enzyme for the production of sphingosine-1-phosphate (S1P), which is involved in the regulation of turgor pressure in plant cells. However, whether TOD1s play a conserved role in the innovation of siphonogamy is largely unknown. In this study, we provide evidence that OsTOD1, which is similar to AtTOD1, is also preferentially expressed in rice pollen grains and pollen tubes. OsTOD1 knockout results in reduced pollen tube growth potential in rice pistil. Both the OsTOD1 genomic sequence with its own promoter and the coding sequence under the AtTOD1 promoter can partially rescue the attod1 mutant phenotype. Furthermore, TOD1s from other angiosperm species can partially rescue the attod1 mutant phenotype, while TOD1s from gymnosperm species are not able to complement the attod1 mutant phenotype. Our data suggest that TOD1 acts conservatively in angiosperms, and this opens up an opportunity to dissect the role of sphingolipids in pollen tube growth in angiosperms.

KATANIN-dependent mechanical properties of the stigmatic cell wall mediate the pollen tube path in Arabidopsis

Successful fertilization in angiosperms depends on the proper trajectory of pollen tubes through the pistil tissues to reach the ovules. Pollen tubes first grow within the cell wall of the papilla cells, applying pressure to the cell. Mechanical forces are known to play a major role in plant cell shape by controlling the orientation of cortical microtubules (CMTs), which in turn mediate deposition of cellulose microfibrils (CMFs). Here, by combining imaging, genetic and chemical approaches, we show that isotropic reorientation of CMTs and CMFs in aged Col-0 and katanin1-5 (ktn1-5) papilla cells is accompanied by a tendency of pollen tubes to coil around the papillae. We show that this coiled phenotype is associated with specific mechanical properties of the cell walls that provide less resistance to pollen tube growth. Our results reveal an unexpected role for KTN1 in pollen tube guidance on the stigma by ensuring mechanical anisotropy of the papilla cell wall.

Flowering plants produce small particles known as pollen that – with the help of the wind, bees and other animals – carry male sex cells (sperm) to female sex cells (eggs) contained within flowers. When a grain of pollen lands on the female organ of a flower, called the pistil, it gives rise to a tube that grows through the pistil towards the egg cells at the base.

The surface of the pistil is covered in a layer of long cells named papillae. Like most plant cells, the papillae are surrounded by a rigid structure known as the cell wall, which is mainly composed of strands known as microfibrils. The pollen tube exerts pressure on a papilla to allow it to grow through the cell wall towards the base of the pistil. Previous studies have shown that the pistil produces signals that guide pollen tubes to the eggs. However, it remains unclear how pollen tubes orient themselves on the surface of papillae to grow in the right direction through the pistil.

Riglet et al. combined microscopy, genetic and chemical approaches to study how pollen tubes grow through the surface of the pistils of a small weed known as Arabidopsis thaliana. The experiments showed that an enzyme called KATANIN conferred mechanical properties to the cell walls of papillae that allowed pollen tubes to grow towards the egg cells, and also altered the orientation of the microfibrils in these cell walls. In A. thaliana plants that were genetically modified to lack KATANIN the pollen tubes coiled around the papillae and sometimes grew in the opposite direction to where the eggs were.

KATANIN is known to cut structural filaments inside the cells of plants, animals and most other living things. By revealing an additional role for KATANIN in regulating the mechanical properties of the papilla cell wall, these findings indicate this enzyme may also regulate the mechanical properties of cells involved in other biological processes.

Internally Controlled Methods to Quantify Pollen Tube Growth and Penetration Defects in Arabidopsis thaliana

Double-fertilization in angiosperms requires precise communication between the male gametophyte (pollen), the female tissues, and the associated female gametophyte (embryo sac) to facilitate efficient fertilization. Numerous small molecules, proteins, and peptides have been shown to impact double-fertilization through the disruption of pollen germination, pollen tube growth, pollen tube guidance, or pollen tube penetration of the female tissues. The genetic basis of signaling events that lead to successful double-fertilization has been greatly facilitated by studies in the model organism Arabidopsis thaliana, which possesses a relatively simple reproductive physiology and a widely available T-DNA mutant seed collection. In this chapter, we detail methods for determining the effects of single gene loss-of-function mutations on pollen behavior through the creation of an internally controlled fluorescent hemizygous complement line. By transforming a single copy of the disrupted gene back into the homozygous mutant background, a precise endogenous control is generated due to the fact that pollen containing equal ratios of mutant and complemented pollen can be collected from a single flower. Using this experimental design, we describe multiple assays that can be performed in series to assess mutant pollen defects in germination, pollen tube elongation rate, and pistil penetration, which can be easily quantified alongside a "near-wildtype" complemented counterpart.

Plasma membrane H+-ATPases sustain pollen tube growth and fertilization

Pollen tubes are highly polarized tip-growing cells that depend on cytosolic pH gradients for signaling and growth. Autoinhibited plasma membrane proton (H+) ATPases (AHAs) have been proposed to energize pollen tube growth and underlie cell polarity, however, mechanistic evidence for this is lacking. Here we report that the combined loss of AHA6, AHA8, and AHA9 in Arabidopsis thaliana delays pollen germination and causes pollen tube growth defects, leading to drastically reduced fertility. Pollen tubes of aha mutants had reduced extracellular proton (H+) and anion fluxes, reduced cytosolic pH, reduced tip-to-shank proton gradients, and defects in actin organization. Furthermore, mutant pollen tubes had less negative membrane potentials, substantiating a mechanistic role for AHAs in pollen tube growth through plasma membrane hyperpolarization. Our findings define AHAs as energy transducers that sustain the ionic circuit defining the spatial and temporal profiles of cytosolic pH, thereby controlling downstream pH-dependent mechanisms essential for pollen tube elongation, and thus plant fertility.

Boosting autophagy in sexual reproduction: a plant perspective

The key process of sexual reproduction is the successful fusion of the sperm and egg cell. Distinct from dynamic and flagellated animal sperm cells, higher flowering plant sperm cells are immotile. Therefore, plants have evolved a novel reproductive system to achieve fertilization and generate progenies. Plant sexual reproduction consists of multiple steps, mainly including gametophyte development, pollen-pistil recognition, pollen germination, double fertilization and postfertilization. During reproduction, active production, consumption and recycling of cellular components and energy are critically required to achieve fertilization. However, the underlying machinery of cellular degradation and turnover remains largely unexplored. Autophagy, the major catabolic pathway in eukaryotic cells, participates in regulating multiple aspects of plant activities, including abiotic and biotic stress resistance, pathogen response, senescence, nutrient remobilization and plant development. Nevertheless, a key unanswered question is how autophagy regulates plant fertilization and reproduction. Here, we focus on comparing and contrasting autophagy in several key reproductive processes of plant and animal systems to feature important distinctions and highlight future research directions of autophagy in angiosperm reproduction. We further discuss the potential crosstalk between autophagy and programmed cell death, which are often considered as two disconnected events in plant sexual reproduction.

The Plasma Membrane-An Integrating Compartment for Mechano-Signaling

Plants are able to sense their mechanical environment. This mechanical signal is used by the plant to determine its phenotypic features. This is true also at a smaller scale. Morphogenesis, both at the cell and tissue level, involves mechanical signals that influence specific patterns of gene expression and trigger signaling pathways. How a mechanical stress is perceived and how this signal is transduced into the cell remains a challenging question in the plant community. Among the structural components of plant cells, the plasma membrane has received very little attention. Yet, its position at the interface between the cell wall and the interior of the cell makes it a key factor at the nexus between biochemical and mechanical cues. So far, most of the key players that are described to perceive and maintain mechanical cell status and to respond to a mechanical stress are localized at or close to the plasma membrane. In this review, we will focus on the importance of the plasma membrane in mechano-sensing and try to illustrate how the composition of this dynamic compartment is involved in the regulatory processes of a cell to respond to mechanical stress.

Putrescine modifies the pollen tube growth of tea (Camellia sinensis) by affecting actin organization and cell wall structure

The aim of the current study was to examine the effect of different exogenous putrescine concentrations (200, 400, 600, and 800 μM) on the tea pollen performance. It was shown that putrescine has a dose-dependent effect on pollen performance. Results exhibited that pollen germination and tube elongation were induced by 200 and 400 μM putrescine treatment, especially, 400 μM putrescine-enhanced pollen performance. However, pollen performance was inhibited by higher concentrations of putrescine. Putrescine concentrations above 400 μM changed the actin filament distribution in pollen tubes by affecting the distribution of sucrose synthase enzyme. Alterations of the distribution on sucrose synthase enzyme also caused the alterations in the dispersion of cellulose and callose in the cell wall, and morphological alterations such as balloon-shaped and snake-shaped pollen tube tip accompanied them. Moreover, putrescine concentrations above 400 μM caused a decrease of ROS level in apex and led to chromatin condensation of the generative nucleus. In conclusion, exogenous putrescine application can be used as a pollen performance enhancer at low concentrations while the high concentrations cause adverse effects reducing fertilization success.

Exocytosis and Endocytosis: Yin-Yang Crosstalk for Sculpting a Dynamic Growing Pollen Tube Tip

The growing pollen tube has become one of the most fascinating model cell systems for investigations into cell polarity and polar cell growth in plants. Rapidly growing pollen tubes achieve tip-focused cell expansion by vigorous anterograde exocytosis, through which various newly synthesized macromolecules are directionally transported and deposited at the cell apex. Meanwhile, active retrograde endocytosis counter balances the exocytosis at the tip which is believed to recycle the excessive exocytic components for multiple rounds of secretion. Therefore, apical exocytosis and endocytosis are the frontline cellular processes which drive the polar growth of pollen tubes, although they represent opposite vesicular trafficking events with distinct underpinning mechanisms. Nevertheless, the molecular basis governing the spatiotemporal crosstalk and counterbalance of exocytosis and endocytosis during pollen tube polarization and growth remains elusive. Here we discuss recent insight into exocytosis and endocytosis in sculpturing high rates of polarized pollen tube growth. In addition, we especially introduce the novel integration of mathematical modeling in uncovering the mysteries of cell polarity and polar cell growth.

Mathematical principles and models of plant growth mechanics: from cell wall dynamics to tissue morphogenesis

Plant growth research produces a catalogue of complex open questions. We argue that plant growth is a highly mechanical process, and that mathematics gives an underlying framework with which to probe its fundamental unrevealed mechanisms. This review serves to illustrate the biological insights afforded by mathematical modelling and demonstrate the breadth of mathematically rich problems available within plant sciences, thereby promoting a mutual appreciation across the disciplines. On the one hand, we explain the general mathematical principles behind mechanical growth models; on the other, we describe how modelling addresses specific problems in microscale cell wall mechanics, tip growth, morphogenesis, and stress feedback. We conclude by identifying possible future directions for both biologists and mathematicians, including as yet unanswered questions within various topics, stressing that interdisciplinary collaboration is vital for tackling the challenge of understanding plant growth mechanics.

Insights into secrets along the pollen tube pathway in need to be discovered

The process of plant fertilization provides an outstanding example of refined control of gene expression. During this elegant process, subtle communication occurs between neighboring cells, based on chemical signals, that induces cellular mechanisms of patterning and growth. Having faced an immediate issue of self-incompatibility responses, the pathway to fertilization starts once the stigmatic cells recognize a compatible pollen grain, and it continues with numerous players interacting to affect pollen tube growth and the puzzling process of navigation along the transmitting tract. The pollen tube goes through a guidance process that begins with a preovular stage (i.e. prior to the influence of the target ovule), with interactions with factors from the transmitting tissue. In the subsequent ovular-guidance stage a specific relationship develops between the pollen tube and its target ovule. This stage is divided into the funicular and micropylar guidance steps, with numerous receptors working in signalling cascades. Finally, just after the pollen tube has passed beyond the synergids, fusion of the gametes occurs and the developing seed-the ultimate aim of the process-will start to mature. In this paper, we review the existing knowledge of the crucial biological processes involved in pollen-pistil interactions that give rise to the new seed.

Cellular basis of growth in plants: geometry matters

The growth of individual cells underlies the development of biological forms. In plants, cells are interconnected by rigid walls, fixing their position with respect to one another and generating mechanical feedbacks between cells. Current research is shedding new light on how plant growth is controlled by physical inputs at the level of individual cells and growing tissues. In this review, we discuss recent progress in our understanding of the cellular basis of growth from a biomechanical perspective. We describe the role of the cell wall and turgor pressure in growth and highlight the often-overlooked role of cell geometry in this process. It is becoming apparent that a combination of experimental and theoretical approaches is required to answer new emerging questions in the biomechanics of plant morphogenesis. We summarise how this multidisciplinary approach brings us closer to a unified understanding of the generation of biological forms in plants.

Evolution of Cell Wall Polymers in Tip-Growing Land Plant Gametophytes: Composition, Distribution, Functional Aspects and Their Remodeling

During evolution of land plants, the first colonizing species presented leafy-dominant gametophytes, found in non-vascular plants (bryophytes). Today, bryophytes include liverworts, mosses, and hornworts. In the first seedless vascular plants (lycophytes), the sporophytic stage of life started to be predominant. In the seed producing plants, gymnosperms and angiosperms , the gametophytic stage is restricted to reproduction. In mosses and ferns, the haploid spores germinate and form a protonema, which develops into a leafy gametophyte producing rhizoids for anchorage, water and nutrient uptakes. The basal gymnosperms (cycads and Ginkgo) reproduce by zooidogamy. Their pollen grains develop a multi-branched pollen tube that penetrates the nucellus and releases flagellated sperm cells that swim to the egg cell. The pollen grain of other gymnosperms (conifers and gnetophytes) as well as angiosperms germinates and produces a pollen tube that directly delivers the sperm cells to the ovule (siphonogamy). These different gametophytes, which are short or long-lived structures, share a common tip-growing mode of cell expansion. Tip-growth requires a massive cell wall deposition to promote cell elongation, but also a tight spatial and temporal control of the cell wall remodeling in order to modulate the mechanical properties of the cell wall. The growth rate of these cells is very variable depending on the structure and the species, ranging from very slow (protonemata, rhizoids, and some gymnosperm pollen tubes), to a slow to fast-growth in other gymnosperms and angiosperms. In addition, the structural diversity of the female counterparts in angiosperms (dry, semi-dry vs wet stigmas, short vs long, solid vs hollow styles) will impact the speed and efficiency of sperm delivery. As the evolution and diversity of the cell wall polysaccharides accompanied the diversification of cell wall structural proteins and remodeling enzymes, this review focuses on our current knowledge on the biochemistry, the distribution and remodeling of the main cell wall polymers (including cellulose, hemicelluloses, pectins, callose, arabinogalactan-proteins and extensins), during the tip-expansion of gametophytes from bryophytes, pteridophytes (lycophytes and monilophytes), gymnosperms and the monocot and eudicot angiosperms.