Plotting a course: multiple signals guide pollen tubes to their targets

Morphological Study on the Differentiation of Flower Buds and the Embryological Stages of Male and Female Floral Organs in Lespedeza davurica (Laxm.) Schindl. cv. JinNong (Fabaceae)

Lespedeza davurica (Laxm.) is a leguminous plant with significant ecological benefits, but its embryonic development mechanism remains unclear. We investigated the flower bud differentiation, megaspore and microspore formation, gametophyte development, and embryo and endosperm development in L. davurica. Our aim was to elucidate the relationship between the external morphology and internal development processes of male and female floral organs during growth, as well as the reproductive factors influencing fruiting. The results indicated that although the pistil develops later than the stamen during flower bud differentiation, both organs mature synchronously before flowering. L. davurica pollen exhibits three germination grooves, a reticulate outer wall, and papillary structures on the anther surface. In vivo pollination experiments revealed abnormal spiral growth of L. davurica pollen tubes within the style and the occurrence of callus plugs, which may reduce the seed setting rate. The anther wall development follows the dicotyledonous type, with tetrads formed through microspore meiosis exhibiting both left-right symmetry and tetrahedral arrangements. L. davurica has a single ovule, and the embryo sac develops in the monosporic polygonum type. After dormancy, the zygote undergoes multiple divisions, progressing through spherical, heart-shaped, and torpedo-shaped embryo stages, culminating in a mature embryo. A mature seed comprises cotyledons, hypocotyl, embryo, radicle, and seed coat. Phylogenetic tree analysis reveals a close genetic relationship between L. davurica and other leguminous plants from the genera Lespedeza and Medicago. This study provides valuable insights into the regulation of flowering and hybrid breeding in leguminous plants and offers a new perspective on the development of floral organs and seed setting rates.

Distribution of exchangeable Ca2+ during the process of Larix decidua Mill. pollination and germination

The involvement of Ca2+ ions in angiosperms sexual processes is well established, while in gymnosperms, such knowledge remains limited and is still a topic of discussion. In this study, we focused on Larix decidua, using Alizarin-red S staining and the pyroantimonate method to examine the tissue and subcellular distribution of free and loosely bound Ca2+ ions at different stages of the male gametophyte's development and its interaction with the ovule. Our findings show that in larch, both the germination of pollen grains and the growth of pollen tubes occur in an environment rich in Ca2+. These ions play a crucial role in the adhesion of the pollen grain to the stigmatic tip and its subsequent movement to the micropylar canal. There is a significant presence of free and loosely bound Ca2+ ions in both the fluid of the micropylar canal and the extracellular matrix of the nucellus. As the pollen tube extends through the nucellus, we observed a notable accumulation of Ca2+ ions just above the entry to the mature archegonium, a region likely crucial for the male gametophyte's directional growth. Meanwhile, the localized presence of free and loosely bound Ca2+ ions within the egg cell cytoplasm may inhibit the pollen tubes growth and rupture, playing an important role in fertilization.

Arabidopsis pollen-specific glycerophosphodiester phosphodiesterase-like genes are essential for pollen tube tip growth

In angiosperms, pollen tube growth is critical for double fertilization and seed formation. Many of the factors involved in pollen tube tip growth are unknown. Here, we report the roles of pollen-specific GLYCEROPHOSPHODIESTER PHOSPHODIESTERASE-LIKE (GDPD-LIKE) genes in pollen tube tip growth. Arabidopsis thaliana GDPD-LIKE6 (AtGDPDL6) and AtGDPDL7 were specifically expressed in mature pollen grains and pollen tubes and green fluorescent protein (GFP)-AtGDPDL6 and GFP-AtGDPDL7 fusion proteins were enriched at the plasma membrane at the apex of forming pollen tubes. Atgdpdl6 Atgdpdl7 double mutants displayed severe sterility that was rescued by genetic complementation with AtGDPDL6 or AtGDPDL7. This sterility was associated with defective male gametophytic transmission. Atgdpdl6 Atgdpdl7 pollen tubes burst immediately after initiation of pollen germination in vitro and in vivo, consistent with the thin and fragile walls in their tips. Cellulose deposition was greatly reduced along the mutant pollen tube tip walls, and the localization of pollen-specific CELLULOSE SYNTHASE-LIKE D1 (CSLD1) and CSLD4 was impaired to the apex of mutant pollen tubes. A rice pollen-specific GDPD-LIKE protein also contributed to pollen tube tip growth, suggesting that members of this family have conserved functions in angiosperms. Thus, pollen-specific GDPD-LIKEs mediate pollen tube tip growth, possibly by modulating cellulose deposition in pollen tube walls.

LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Increasing evidence suggests that posttranscriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this messenger RNA (mRNA)-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, the loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely posttranscriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Increasing evidence suggests that posttranscriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this messenger RNA (mRNA)-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, the loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely posttranscriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

Arabinogalactan glycoprotein dynamics during the progamic phase in the tomato pistil

Pistil AGPs display dynamic localization patterns in response to fertilization in tomato. SlyFLA9 (Solyc07g065540.1) is a chimeric Fasciclin-like AGP with enriched expression in the ovary, suggesting a potential function during pollen-pistil interaction. During fertilization, the male gametes are delivered by pollen tubes to receptive ovules, deeply embedded in the sporophytic tissues of the pistil. Arabinogalactan glycoproteins (AGPs) are a diverse family of highly glycosylated, secreted proteins which have been widely implicated in plant reproduction, particularly within the pistil. Though tomato (Solanum lycopersicum) is an important crop requiring successful fertilization for production, the molecular basis of this event remains understudied. Here we explore the spatiotemporal localization of AGPs in the mature tomato pistil before and after fertilization. Using histological techniques to detect AGP sugar moieties, we found that accumulation of AGPs correlated with the maturation of the stigma and we identified an AGP subpopulation restricted to the micropyle that was no longer visible upon fertilization. To identify candidate pistil AGP genes, we used an RNA-sequencing approach to catalog gene expression in functionally distinct subsections of the mature tomato pistil (the stigma, apical and basal style and ovary) as well as pollen and pollen tubes. Of 161 predicted AGP and AGP-like proteins encoded in the tomato genome, we identified four genes with specifically enriched expression in reproductive tissues. We further validated expression of two of these, a Fasciclin-like AGP (SlyFLA9, Solyc07g065540.1) and a novel hybrid AGP (SlyHAE, Solyc09g075580.1). Using in situ hybridization, we also found SlyFLA9 was expressed in the integuments of the ovule and the pericarp. Additionally, differential expression analyses of the pistil transcriptome revealed previously unreported genes with enriched expression in each subsection of the mature pistil, setting the foundation for future functional studies.

Pollen biology and hormesis: Pollen germination and pollen tube elongation

This paper evaluated the occurrence of hormetic dose responses in pollen reported over the past eight decades. Hormetic doses responses were induced by a wide range of chemical and physical agents in 34 plant species for pollen germination and pollen tube growth/elongation. Agents inducing such hormetic dose/concentration responses in pollen included nutrients, growth-promoting agents, plant and animal hormones, toxic substances, including heavy metals such as cadmium, gaseous pollutants such as ozone, as well as ionizing and non-ionizing radiation. This paper provides further evidence for the broad generality of the hormesis dose response, supporting substantial prior findings that the hormetic response is independent of biological model, inducing agent, and endpoints measured. Given the widespread potential of inducing hormetic dose responses in pollen, these findings indicate the need to explore their emerging biological, ecological, agricultural, economic and public health implications.

Paving the Way for Fertilization: The Role of the Transmitting Tract

Angiosperm reproduction relies on the precise growth of the pollen tube through different pistil tissues carrying two sperm cells into the ovules’ embryo sac, where they fuse with the egg and the central cell to accomplish double fertilization and ultimately initiate seed development. A network of intrinsic and tightly regulated communication and signaling cascades, which mediate continuous interactions between the pollen tube and the sporophytic and gametophytic female tissues, ensures the fast and meticulous growth of pollen tubes along the pistil, until it reaches the ovule embryo sac. Most of the pollen tube growth occurs in a specialized tissue—the transmitting tract—connecting the stigma, the style, and the ovary. This tissue is composed of highly secretory cells responsible for producing an extensive extracellular matrix. This multifaceted matrix is proposed to support and provide nutrition and adhesion for pollen tube growth and guidance. Insights pertaining to the mechanisms that underlie these processes remain sparse due to the difficulty of accessing and manipulating the female sporophytic tissues enclosed in the pistil. Here, we summarize the current knowledge on this key step of reproduction in flowering plants with special emphasis on the female transmitting tract tissue.

Flow Chamber Assay to Image the Response of FRET-Based Nanosensors in Pollen Tubes to Changes in Medium Composition

Pollen tubes growing in the transmitting tract are presented with an extracellular matrix rich in a variety of substances. The expression of a multitude of genes for transport proteins in the pollen tube indicates that pollen tubes take up at least some of the components provided by the transmitting tract, for example nutrients, ions, or signaling molecules. FRET (Förster resonance energy transfer)-based nanosensors are perfectly suited to study the uptake of these molecules into pollen tubes. They are genetically encoded and can easily be expressed in Arabidopsis pollen tubes. Furthermore, the method is noninvasive and nanosensors for a wide range of substances are available. This chapter will describe the design of plasmids required to generate stable Arabidopsis lines with a pollen tube-specific expression of nanosensor constructs. We also present a method to germinate Arabidopsis pollen tubes in a flow chamber slide that allows the perfusion of the pollen tubes with liquid medium supplemented with the substrate of the nanosensor. Simultaneous evaluation of the FRET efficiency of the nanosensor by confocal microscopy reveals whether the substance is taken up by the pollen tubes. Together with the great number of available nanosensors this method can generate a detailed picture of the substances that are taken up during pollen tubes growth.

Deficiency of rice hexokinase HXK5 impairs synthesis and utilization of starch in pollen grains and causes male sterility

There is little known about the function of rice hexokinases (HXKs) in planta. We characterized hxk5-1, a Tos17 mutant of OsHXK5 that is up-regulated in maturing pollen, a stage when starch accumulates. Progeny analysis of self-pollinated heterozygotes of hxk5-1 and reciprocal crosses between the wild-type and heterozygotes revealed that loss of HXK5 causes male sterility. Homozygous hxk5-1, produced via anther culture, and additional homozygous hxk5-2, hxk5-3 and hxk5-4 lines created by CRISPR/Cas9 confirmed the male-sterile phenotype. In vitro pollen germination ability and in vivo pollen tube growth rate were significantly reduced in the hxk5 mutant pollen. Biochemical analysis of anthers with the mutant pollen revealed significantly reduced hexokinase activity and starch content, although they were sufficient to produce some viable seed. However, the mutant pollen was unable to compete successfully against wild-type pollen. Expression of the catalytically inactive OsHXK5-G113D did not rescue the hxk5 male-sterile phenotype, indicating that its catalytic function was responsible for pollen fertility, rather than its role in sugar sensing and signaling. Our results demonstrate that OsHXK5 contributes to a large portion of the hexokinase activity necessary for the starch utilization pathway during pollen germination and tube growth, as well as for starch biosynthesis during pollen maturation.

MADS-Box and bHLH Transcription Factors Coordinate Transmitting Tract Development in Arabidopsis thaliana

The MADS-domain transcription factor SEEDSTICK (STK) controls several aspects of plant reproduction. STK is co-expressed with CESTA (CES), a basic Helix-Loop-Helix (bHLH) transcription factor-encoding gene. CES was reported to control redundantly with the brassinosteroid positive signaling factors BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1) and BEE3 the development of the transmitting tract. Combining the stk ces-4 mutants led to a reduction in ovule fertilization due to a defect in carpel fusion which, caused the formation of holes at the center of the septum where the transmitting tract differentiates. Combining the stk mutant with the bee1 bee3 ces-4 triple mutant showed an increased number of unfertilized ovules and septum defects. The transcriptome profile of this quadruple mutant revealed a small subset of differentially expressed genes which are mainly involved in cell death, extracellular matrix and cell wall development. Our data evidence a regulatory gene network controlling transmitting tract development regulated directly or indirectly by a STK-CES containing complex and reveal new insights in the regulation of transmitting tract development by bHLH and MADS-domain transcription factors.

Arabinogalactan proteins mediate intercellular crosstalk in the ovule of apple flowers

AGP-rich glycoproteins mediate pollen-ovule interactions and cell patterning in the embryo sac of apple before and after fertilization. Glycoproteins are significant players in the dialog that takes place between growing pollen tubes and the stigma and style in the angiosperms. Yet, information is scarce on their possible involvement in the ovule, a sporophytic organ that hosts the female gametophyte. Apple flowers have a prolonged lapse of time between pollination and fertilization, offering a great system to study the developmental basis of glycoprotein secretion and their putative role during the last stages of the progamic phase and early seed initiation. For this purpose, the sequential pollen tube elongation within the ovary was examined in relation to changes in arabinogalactan proteins (AGPs) in the tissues of the ovule before and after fertilization. To evaluate what of these changes are developmentally regulated, unpollinated and pollinated flowers were compared. AGPs paved the pollen tube pathway in the ovules along the micropylar canal, and the nucellus entrance toward the synergids, which also developmentally accumulated AGPs at the filiform apparatus. Glycoproteins vanished from all these tissues following pollen tube passage, strongly suggesting a role in pollen-ovule interaction. In addition, AGPs marked the primary cell walls of the haploid cells of the female gametophyte, and they further built up in the cell walls of the embryo sac and developing embryo, layering the interactive walls of the three generations hosted in the ovule, the maternal sporophytic tissues, the female gametophyte, and the developing embryo.

Transcriptome Analysis Provides Insight into the Molecular Mechanisms Underlying gametophyte factor 2-Mediated Cross-Incompatibility in Maize

In maize (Zea mays L.), unilateral cross-incompatibility (UCI) is controlled by Gametophyte factors (Ga), including Ga1, Ga2, and Tcb1; however, the molecular mechanisms underpinning this process remain unexplored. Here, we report the pollination phenotype of an inbred line, 511L, which carries a near-dominant Ga2-S allele. We performed a high-throughput RNA sequencing (RNA-Seq) analysis of the compatible and incompatible crosses between 511L and B73, to identify the transcriptomic differences associated with Ga2-mediated UCI. An in vivo kinetics analysis revealed that the growth of non-self pollen tubes was blocked at the early stages after pollination in 511L, maintaining the UCI barrier in Ga2. In total, 25,759 genes were expressed, of which, 2063 differentially expressed genes (DEGs) were induced by pollination (G_GG, G_GB, B_BB, B_BG). A gene ontology (GO) enrichment analysis revealed that these genes were specifically enriched in functions involved in cell wall strength and pectic product modification. Moreover, 1839, 4382, and 5041 genes were detected to differentially express under same pollination treatments, including B_G, BG_GG, and BB_GB, respectively. A total of 1467 DEGs were constitutively expressed between the two inbred lines following pollination treatments, which were enriched in metabolic processes, flavonoid biosynthesis, cysteine biosynthesis, and vacuole functions. Furthermore, we confirmed 14 DEGs related to cell wall modification and stress by qRT-PCR, which might be involved in Ga2-S-mediated UCI. Our results provide a comprehensive foundation for the molecular mechanisms involved in silks of UCI mediated by Ga2-S.

AP1G mediates vacuolar acidification during synergid-controlled pollen tube reception

Double fertilization in angiosperms requires the delivery of immotile sperm through pollen tubes, which enter embryo sacs to initiate synergid degeneration and to discharge. This fascinating process, called pollen tube reception, involves extensive communications between pollen tubes and synergids, within which few intracellular regulators involved have been revealed. Here, we report that vacuolar acidification in synergids mediated by AP1G and V-ATPases might be critical for pollen tube reception. Functional loss of AP1G or VHA-A, encoding the γ subunit of adaptor protein 1 or the shared component of two endomembrane V-ATPases, respectively, impaired synergid-controlled pollen tube reception and caused partial female sterility. AP1G works in parallel to the plasma membrane-associated receptor FERONIA in synergids, suggesting that synergid-mediated pollen tube reception requires proper sorting of vacuolar cargos by AP1G. Although AP1G did not mediate the targeting of V-ATPases, AP1G loss of function or the expression of AP1G-RNAi compromised vacuolar acidification mediated by V-ATPases, implying their genetic interaction. We propose that vacuolar acidification might represent a distinct cell-death mechanism specifically adopted by the plant phylum, which is critical for synergid degeneration during pollen tube reception.

LlFH1-mediated interaction between actin fringe and exocytic vesicles is involved in pollen tube tip growth

Pollen tube tip growth is an extreme form of polarized cell growth, which requires polarized exocytosis based on a dynamic actin cytoskeleton. However, the molecular basis for the connection between actin filaments and exocytic vesicles is unclear. Here, we identified a Lilium longiflorum pollen-specific formin (LlFH1) and found that it localized at the apical vesicles and plasma membrane (PM). Overexpression of LlFH1 induced excessive actin cables in the tube tip region, and downregulation of LlFH1 eliminated the actin fringe. Fluorescence recovery after photobleaching (FRAP) analysis revealed that LlFH1-labeled exocytic vesicles exhibited an initial accumulation at the shoulder of the apex and coincided with the leading edge of the actin fringe. Time-lapse analysis suggested that nascent actin filaments followed the emergence of the apical vesicles, implying that LlFH1 at apical vesicles could initiate actin polymerization. Biochemical assays showed that LlFH1 FH1FH2 could nucleate actin polymerization, but then capped the actin filament at the barbed end and inhibited its elongation. However, in the presence of lily profilins, LlFH1 FH1FH2 could accelerate barbed-end actin elongation. In addition, LlFH1 FH1FH2 was able to bundle actin filaments. Thus, we propose that LlFH1 and profilin coordinate the interaction between the actin fringe and exocytic vesicle trafficking during pollen tube growth of lily.

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

Pollen-pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels). In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope. Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination. Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.

Arabinogalactan Proteins as Interactors along the Crosstalk between the Pollen Tube and the Female Tissues

Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants. Pollen tube growth along the pistil tissues requires a multiplicity of signaling pathways to be activated and turned off precisely, at crucial timepoints, to guarantee successful fertilization and seed production. In the recent years, an outstanding effort has been made by the plant reproduction scientific community in order to better understand this process. This resulted in the discovery of a fairly substantial number of new players essential for reproduction, as well as their modes of action and interactions. Besides all the indications of AGPs involvement in reproduction, there were no convincing evidences about it. Recently, several studies came out to prove what had long been suggested about this complex family of glycoproteins. AGPs consist of a large family of hydroxyproline-rich proteins, predicted to be anchored to the plasma membrane and extremely rich in sugars. These two last characteristics always made them perfect candidates to be involved in signaling mechanisms, in several plant developmental processes. New findings finally relate AGPs to concrete functions in plant reproduction. In this review, it is intended not only to describe how different molecules and signaling pathways are functioning to achieve fertilization, but also to integrate the recent discoveries about AGPs along this process.

AtTMEM18 plays important roles in pollen tube and vegetative growth in Arabidopsis

In flowering plants, pollen tube growth is essential for delivery of male gametes into the female gametophyte or embryo sac for double fertilization. Although many genes have been identified as being involved in the process, the molecular mechanisms of pollen tube growth remains poorly understood. In this study, we identified that the Arabidopsis Transmembrane Protein 18 (AtTMEM18) gene played important roles in pollen tube growth. The AtTMEM18 shares a high similarity with the Transmembrane 18 proteins (TMEM18s) that are conserved in most eukaryotes and may play important roles in obesity in humans. Mutation in the AtTMEM18 by a Ds insertion caused abnormal callose deposition in the pollen grains and had a significant impact on pollen germination and pollen tube growth. AtTMEM18 is expressed in pollen grains, pollen tubes, root tips and other vegetative tissues. The pollen-rescued assays showed that the mutation in AtTMEM18 also caused defects in roots, stems, leaves and transmitting tracts. AtTMEM18-GFP was located around the nuclei. Genetic assays demonstrated that the localization of AtTMEM18 around the nuclei in the generative cells of pollen grains was essential for the male fertility. Furthermore, expression of the rice TMEM18-homologous protein (OsTMEM18) driven by LAT52 promoter could recover the fertility of the Arabidopsis attmem18 mutant. These results suggested that the TMEM18 is important for plant growth in Arabidopsis.

Pollen from Arabidopsis thaliana and other Brassicaceae are functionally omniaperturate

PREMISE OF THE STUDY

Most pollen walls are interrupted by apertures, thin areas providing access to stigmatic fluids and exit points for pollen tubes. Unexpectedly, pollen tubes of Arabidopsis thaliana are not obligated to pass through apertures and can instead take the shortest route into the stigma, passing directly through a nonaperturate wall.

METHODS

We used stains and confocal microscopy to follow early pollen tube formation in A. thaliana and 200+ other species. We germinated pollen in vitro and in situ (at control and high humidities) and also used atomic force microscopy to assay material properties of nonaperture and aperture walls.

KEY RESULTS

Pollen tubes of A. thaliana breached nonaperture walls despite these being an order of magnitude stiffer than aperture walls. Breakout was associated with localized swelling of the pectin-rich (alcian blue positive) intine. The precision of pollen tube exit at the pollen-stigma interface was lost at high humidity. Pollen from ∼4% of the species surveyed exhibited breakout germination behavior; all nine breakout species identified so far are in the Brassicaceae family (∼25% of the Brassicaceae sampled) and are scattered across seven tribes.

CONCLUSIONS

The polarity of pollen germination in A. thaliana is externally induced, not linked to aperture location. The biomechanical force for breaking nonaperture walls is found in localized swelling of intine pectins. As such, the pollen from A. thaliana, and likely many Brassicaceae family members, are functionally omniaperturate. This new mechanism for germination between extant apertures raises questions about exine porosity and the diversity of mechanisms across taxa.

Retarded Embryo Development 1 (RED1) regulates embryo development, seed maturation and plant growth in Arabidopsis

Plant seeds accumulate large amounts of protein and carbohydrate as storage reserves during maturation. Thus, understanding the genetic control of embryo and seed development may provide bioengineering tools for yield improvement. In this study, we report the identification of Retarded Embryo Development 1 (RED1) gene in Arabidopsis, whose two independent T-DNA insertion mutant lines, SALK_085642 (red1-1) and SALK_022583 (red1-2), show a retarded embryo development phenotype. The embryogenesis process ceases at the late heart stage in red1-1 and at the bent-cotyledon stage in red1-2, respectively, resulting in seed abortion in both lines. The retarded embryo development and seed abortion phenotypes reverted to normal when RED1 complementation constructs were introduced into mutant plants. Small red1-2 homozygous plants can be successfully rescued by culturing immature seeds, indicating that seed abortion likely results from compromised tolerance to the desiccation process associated with seed maturation. Consistent with this observation, red1-2 seeds accumulate less protein, and the expression of two late embryo development reporter transgenes, LEA::GUS and β-conglycinin::GUS, was significantly weak and started relatively late in the red1-2 mutant lines compared to the wild type. The RED1 gene encodes a plant specific novel protein that is localized in the nucleus. These results indicate that RED1 plays important roles in embryo development, seed maturation and plant growth.

Interactions between pollen tube and pistil control pollen tube identity and sperm release in the Arabidopsis female gametophyte

Flowering plants have immotile sperm that develop within the pollen cytoplasm and are delivered to female gametes by a pollen tube, a highly polarized extension of the pollen cell. In many flowering plant species, including seed crop plants, hundreds of pollen tubes grow towards a limited number of ovules. This system should ensure maximal fertilization of ovules and seed production; however, we know very little about how signalling between the critical cells is integrated to orchestrate delivery of two functional sperm to each ovule. Recent studies suggest that the pollen tube changes its gene-expression programme in response to growth through pistil tissue and that this differentiation process is critical for pollen tube attraction by the female gametophyte and for release of sperm. Interestingly, these two signalling systems, called pollen tube guidance and pollen tube reception, are also species-preferential. The present review focuses on Arabidopsis pollen tube differentiation within the pistil and addresses the idea that pollen tube differentiation defines pollen tube identity and recognition by female cells. We review recent identification of genes that may control pollen tube-female gametophyte recognition and discuss how these may be involved in blocking interspecific hybridization.

Phytosulfokine peptide signaling controls pollen tube growth and funicular pollen tube guidance in Arabidopsis thaliana

Phytosulfokine (PSK) is a peptide growth factor that requires tyrosine sulfation carried out by tyrosylprotein sulfotransferase (TPST) for its activity. PSK is processed from precursor proteins encoded by five genes in Arabidopsis thaliana and perceived by receptor kinases encoded by two genes in Arabidopsis. pskr1-3 pskr2-1 and tpst-1 knockout mutants displayed reduced seed production, indicative of a requirement for PSK peptide signaling in sexual plant reproduction. Expression analysis revealed PSK precursor and PSK receptor gene activity in reproductive organs with strong expression of PSK2 in pollen. In support of a role for PSK signaling in pollen, in vitro pollen tube (PT) growth was enhanced by exogenously added PSK while PTs of pskr1-3 pskr2-1 and of tpst-1 were shorter. In planta, growth of wild-type pollen in pskr1-3 pskr2-1 and tpst-1 flowers appeared slower than growth in wild-type flowers. But PTs did eventually reach the base of the style, suggesting that PT elongation rate may not be responsible for the reduced fertility. Detailed analysis of anthers, style and ovules did not reveal obvious developmental defects. By contrast, a high percentage of unfertilized ovules in pskr1-3 pskr2-1 and in tpst-1 siliques displayed loss of funicular PT guidance, suggesting that PSK signaling is required to guide the PT from the transmitting tract to the embryo sac. Cross-pollination experiments with wild-type, pskr1-3 pskr2-1 and tpst-1 male and female parents revealed that both the PT and the female sporophytic tissue and/or female gametophyte contribute to successful PT guidance via PSK signaling and to fertilization success.

Arabinogalactan proteins: rising attention from plant biologists

Key message: AGP update: plant reproduction. Arabinogalactan proteins (AGPs) are a large family of hydroxyproline-rich proteins, heavily glycosylated, ubiquitous in land plants, including basal angiosperms and also in many algae. They have been shown to serve as important molecules in several steps of the reproductive process in plants. Due to their special characteristics, such as high sugar content and their means of association with the membrane, they are often perceived as likely candidates for many different aspects of the reproductive process such as signalling molecules, cell identity determinants, morphogens, nutrient sources and support for pollen tube growth, among others. Nevertheless, the study of these proteins pose many difficulties when it comes to studying them individually. Most of the work done involved the use of the β-glucosyl Yariv reagent and antibodies that recognize the carbohydrate epitopes only. Recently, new approaches have been used to study AGPs largely based in the remarkable growing volume of microarray data made available. Either using older techniques or the most recent ones, a clearer picture is emerging for the functions and mode of action of these molecules in the plant reproductive processes. Here, we present an overview about the most important studies made in this area, focusing on the latest advances and the possibilities for future studies in the field.

Real-time automated characterization of 3D morphology and mechanics of developing plant cells

In this article, we introduce the real-time cellular force microscope (RT-CFM), a high-throughput microrobotic platform for mechanical stimulation and characterization of single cells. We developed computer vision algorithms that fully automate the positioning of target cells and localization of the sensor tip. The control and acquisition architecture dramatically increases the accuracy, speed, and reliability of force measurements. Pollen tubes provide an ideal model system for the study of plant mechanics at the single-cell level. To quantitatively obtain the physical properties of the plant cell wall, we generated topography and stiffness measurements from 3D scans of living, growing pollen tubes. We report techniques for real-time monitoring and analysis of intracellular calcium fluxes during mechanical intervention. Our platform is compatible with various imaging systems and enables a powerful screening technology to facilitate biomechanical and morphological characterization of developing cells.

Differential expression patterns of arabinogalactan proteins in Arabidopsis thaliana reproductive tissues

Arabinogalactan proteins (AGPs) are heavily glycosylated proteins existing in all members of the plant kingdom and are differentially distributed through distinctive developmental stages. Here, we showed the individual distributions of specific Arabidopsis AGPs: AGP1, AGP9, AGP12, AGP15, and AGP23, throughout reproductive tissues and indicated their possible roles in several reproductive processes. AGP genes specifically expressed in female tissues were identified using available microarray data. This selection was confirmed by promoter analysis using multiple green fluorescent protein fusions to a nuclear localization signal, β-glucuronidase fusions, and in situ hybridization as approaches to confirm the expression patterns of the AGPs. Promoter analysis allowed the detection of a specific and differential presence of these proteins along the pathway followed by the pollen tube during its journey to reach the egg and the central cell inside the embryo sac. AGP1 was expressed in the stigma, style, transmitting tract, and the chalazal and funiculus tissues of the ovules. AGP9 was present along the vasculature of the reproductive tissues and AGP12 was expressed in the stigmatic cells, chalazal and funiculus cells of the ovules, and in the septum. AGP15 was expressed in all pistil tissues, except in the transmitting tract, while AGP23 was specific to the pollen grain and pollen tube. The expression pattern of these AGPs provides new evidence for the detection of a subset of specific AGPs involved in plant reproductive processes, being of significance for this field of study. AGPs are prominent candidates for male-female communication during reproduction.

Brassinosteroids promote Arabidopsis pollen germination and growth

Pollen tubes are among the fastest tip-growing plant cells and represent an excellent experimental system for studying the dynamics and spatiotemporal control of polarized cell growth. However, investigating pollen tube tip growth in the model plant Arabidopsis remains difficult because in vitro pollen germination and pollen tube growth rates are highly variable and largely different from those observed in pistils, most likely due to growth-promoting properties of the female reproductive tract. We found that in vitro grown Arabidopsis pollen respond to brassinosteroid (BR) in a dose-dependent manner. Pollen germination and pollen tube growth increased nine- and fivefold, respectively, when media were supplemented with 10 µM epibrassinolide (epiBL), resulting in growth kinetics more similar to growth in vivo. Expression analyses show that the promoter of one of the key enzymes in BR biosynthesis, CYP90A1/CPD, is highly active in the cells of the reproductive tract that form the pathway for pollen tubes from the stigma to the ovules. Pollen tubes grew significantly shorter through the reproductive tract of a cyp90a1 mutant compared to the wild type, or to a BR perception mutant. Our results show that epiBL promotes pollen germination and tube growth in vitro and suggest that the cells of the reproductive tract provide BR compounds to stimulate pollen tube growth.

Efficient plant male fertility depends on vegetative nuclear movement mediated by two families of plant outer nuclear membrane proteins

Increasing evidence suggests that nuclear migration is important for eukaryotic development. Although nuclear migration is conserved in plants, its importance for plant development has not yet been established. The most extraordinary plant nuclear migration events involve plant fertilization, which is starkly different from that of animals. Instead of evolving self-propelled sperm cells (SCs), plants use pollen tubes to deliver SCs, in which the pollen vegetative nucleus (VN) and the SCs migrate as a unit toward the ovules, a fundamental but barely understood process. Here, we report that WPP domain-interacting proteins (WIPs) and their binding partners the WPP domain-interacting tail-anchored proteins (WITs) are essential for pollen nuclear migration. Loss-of-function mutations in WIT and/or WIP gene families resulted in impaired VN movement, inefficient SC delivery, and defects in pollen tube reception. WIPs are Klarsicht/ANC-1/Syne-1 Homology (KASH) analogs in plants. KASH proteins are key players in animal nuclear migration. Thus, this study not only reveals an important nuclear migration mechanism in plant fertilization but also, suggests that similar nuclear migration machinery is conserved between plants and animals.

Genetic and cellular analysis of cross-incompatibility in Zea mays

Three genetic systems conferring cross-incompatibility have been described in Zea mays: Teosinte crossing barrier1-strong (Tcb1-s) found in teosinte, and Gametophyte factor1-strong (Ga1-s) and Ga2-s found in maize and teosinte. The reproductive barrier between maize and some weedy teosintes is controlled by the Tcb1-s locus. Multi-generation inheritance experiments on two independent Tcb1-s lineages show that the Tcb1-s barrier is unstable in some maize lines. Reciprocal crosses between Tcb1-s tester plants and three recombinants in the Tcb1-s mapping region demonstrate that the Tcb1-s haplotype contains separable male and female components. In vivo assays of the dynamics of pollen tube growth and pollen tube morphology during rejection of incompatible pollen in silks carrying the Tcb1-s, Ga1-s, or Ga2-s barriers showed that, in all three, pollen tube growth is slower than in compatible crosses at early stages and had ceased by 24 h after pollination. In all three crossing barrier systems, incompatible pollen tubes have clustered callose plugs in contrast to pollen tubes of compatible crosses. Incompatible pollen tubes growing in the Tcb1-s, Ga1-s, and Ga2-s silks have different morphologies: straight, curved, and kinked, respectively. The distinct morphologies suggest that these crossing barriers block incompatible pollen through different mechanisms. This study lays the foundation for cloning the Tcb1 genes and provides clues about the cellular mechanisms involved in pollen rejection in the Tcb1-s, Ga1-s, and Ga2-s crossing barriers.

Transcript profiles of maize embryo sacs and preliminary identification of genes involved in the embryo sac-pollen tube interaction

The embryo sac, the female gametophyte of flowering plants, plays important roles in the pollination and fertilization process. Maize (Zea mays L.) is a model monocot, but little is known about the interactions between its embryo sac and the pollen tube. In this study, we compared the transcript profiles of mature embryo sacs, mature embryo sacs 14-16 h after pollination, and mature nucelli. Comparing the transcript profiles of the embryo sacs before and after the entry of the pollen tube, we identified 3467 differentially expressed transcripts (3382 differentially expressed genes; DEGs). The DEGs were grouped into 22 functional categories. Among the DEGs, 221 genes were induced upon the entry of the pollen tube, and many of them encoded proteins involved in RNA binding, processing, and transcription, signaling, miscellaneous enzyme family processes, and lipid metabolism processes. Genes in the DEG dataset were grouped into 17 classes in a gene ontology enrichment analysis. The DEGs included many genes encoding proteins involved in protein amino acid phosphorylation and protein ubiquitination, implying that these processes might play important roles in the embryo sac-pollen tube interaction. Additionally, our analyses indicate that the expression of 112 genes encoding cysteine-rich proteins (CRPs) is induced during pollination and fertilization. The CRPs likely regulate pollen tube guidance and embryo sac development. These results provide important information on the genes involved in the embryo sac-pollen tube interaction in maize.

The juxtamembrane and carboxy-terminal domains of Arabidopsis PRK2 are critical for ROP-induced growth in pollen tubes

Polarized growth of pollen tubes is a critical step for successful reproduction in angiosperms and is controlled by ROP GTPases. Spatiotemporal activation of ROP (Rho GTPases of plants) necessitates a complex and sophisticated regulatory system, in which guanine nucleotide exchange factors (RopGEFs) are key components. It was previously shown that a leucine-rich repeat receptor-like kinase, Arabidopsis pollen receptor kinase 2 (AtPRK2), interacted with RopGEF12 for its membrane recruitment. However, the mechanisms underlying AtPRK2-mediated ROP activation in vivo are yet to be defined. It is reported here that over-expression of AtPRK2 induced tube bulging that was accompanied by the ectopic localization of ROP-GTP and the ectopic distribution of actin microfilaments. Tube depolarization was also induced by a potentially kinase-dead mutant, AtPRK2K366R, suggesting that the over-expression effect of AtPRK2 did not require its kinase activity. By contrast, deletions of non-catalytic domains in AtPRK2, i.e. the juxtamembrane (JM) and carboxy-terminal (CT) domains, abolished its ability to affect tube polarization. Notably, AtPRK2K366R retained the ability to interact with RopGEF12, whereas AtPRK2 truncations of these non-catalytic domains did not. Lastly, it has been shown that the JM and CT domains of AtPRK2 were not only critical for its interaction with RopGEF12 but also critical for its distribution at the plasma membrane. These results thus provide further insight into pollen receptor kinase-mediated ROP activation during pollen tube growth.

Speed dating, rejection, and finding the perfect mate: advice from flowering plants

Angiosperm pollen tubes extend through pistil tissue to deliver a pair of immotile sperm cells to female gametes for double fertilization. The extended journey of the pollen tube requires extensive cell:cell interactions that guide the pollen tube to its destination and instruct it to stop growing and burst. Once sperm cells are released the molecular exchanges between male and female continue, resulting in sperm activation and gamete fusion. Finally, there is evidence that gamete fusion can feed back on the pollen tube attraction mechanism so that additional pollen tubes can be attracted only if the first sperm cells fail to fertilize. We review progress toward defining the molecules mediating each of these exchanges and describe how small cysteine-rich peptides are a major mode of cellular communication.

OsAP65, a rice aspartic protease, is essential for male fertility and plays a role in pollen germination and pollen tube growth

Aspartic proteases (APs) comprise a large proteolytic enzyme family widely distributed in animals, microbes, viruses, and plants. The rice genome encodes 96 APs, of which only a few have been functionally characterized. Here, the identification and characterization of a novel AP gene, OsAP65, which plays an indispensable role in pollen tube growth in rice, is reported. The T-DNA insertion line of OsAP65 caused severe segregation distortion. In the progeny derived from an individual heterozygous for the T-DNA insertion, the wild type and T-DNA-carrying heterozygote segregated at a ratio close to 1:1, while homozygotes of disrupted OsAP65 (OsAP65-/-) were not recovered. Reciprocal crosses between heterozygotes and wild-type plants demonstrated that the mutant alleles could not be transmitted through the male gamete. Examination of the anthers from heterozygous plants revealed that the mutant pollen matured normally, but did not germinate or elongate. OsAP65 was expressed in various tissues and the transcript level in heterozygous plants was about half of the amount measured in the wild-type plants. The subcellular localization showed that OsAP65 is a pre-vacuolar compartment (PVC) protein. These results indicated that OsAP65 was essential for rice pollen germination and tube growth.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Arabidopsis COBRA-LIKE 10, a GPI-anchored protein, mediates directional growth of pollen tubes

Successful reproduction of flowering plants requires constant communication between female tissues and growing pollen tubes. Female cells secrete molecules and peptides as nutrients or guidance cues for fast and directional tube growth, which is executed by dynamic changes of intracellular activities within pollen tubes. Compared with the extensive interest in female cues and intracellular activities of pollen tubes, how female cues are sensed and interpreted intracellularly in pollen is poorly understood. We show here that COBL10, a glycosylphosphatidylinositol (GPI)-anchored protein, is one component of this pollen tube internal machinery. Mutations in COBL10 caused gametophytic male sterility due to reduced pollen tube growth and compromised directional sensing in the female transmitting tract. Deposition of the apical pectin cap and cellulose microfibrils was disrupted in cobl10 pollen tubes. Pollen tube localization of COBL10 at the apical plasma membrane is critical for its function and relies on proper GPI processing and its C-terminal hydrophobic residues. GPI-anchored proteins are widespread cell sensors in mammals, especially during egg-sperm communication. Our results that COBL10 is critical for directional growth of pollen tubes suggest that they play critical roles in cell-cell communications in plants.

Extra-gynoecial pollen-tube growth in apocarpous angiosperms is phylogenetically widespread and probably adaptive

• Fusion of floral carpels (syncarpy) in angiosperms is thought to have allowed for significant improvements in offspring quantity and quality in syncarpous species over gymnosperms and apocarpous (free-carpelled) angiosperms. Given the disadvantages of apocarpy, it remains an evolutionary puzzle why many angiosperm lineages with free carpels (apocarpy) have been so successful and why some lineages show reversals to apocarpy. • To investigate whether some advantages of syncarpy may accrue in other ways to apocarpous species, we reviewed previous studies of pollen-tube growth in apocarpous species and also documented pollen-tube growth in nine additional apocarpous species in six families. • Anatomical studies of a scattering of apocarpous paleodicots, monocots, and eudicots show that, after transiting the style, 'extra' pollen tubes exit fully fertilized carpels and grow to other carpels with unfertilized ovules. In many species this occurs via openings in the simple carpels, as we report here for Sagittaria potamogetifolia, Sagittaria pygmaea, Sedum lineare, and Schisandra sphenanthera. • The finding that extra-gynoecial pollen-tube growth is widespread in apocarpous species eliminates the possibility of a major fitness cost of apocarpy relative to syncarpy and may help to explain the persistence of, and multiple reversals to, apocarpy in the evolutionary history of angiosperms.

Perspectives on Systematic Analyses of Gene Function in Arabidopsis thaliana: New Tools, Topics and Trends

Since the sequencing of the nuclear genome of Arabidopsis thaliana ten years ago, various large-scale analyses of gene function have been performed in this model species. In particular, the availability of collections of lines harbouring random T-DNA or transposon insertions, which include mutants for almost all of the ~27,000 A. thaliana genes, has been crucial for the success of forward and reverse genetic approaches. In the foreseeable future, genome-wide phenotypic data from mutant analyses will become available for Arabidopsis, and will stimulate a flood of novel in-depth gene-function analyses. In this review, we consider the present status of resources and concepts for systematic studies of gene function in A. thaliana. Current perspectives on the utility of loss-of-function and gain-of-function mutants will be discussed in light of the genetic and functional redundancy of many A. thaliana genes.

Arabidopsis CSLD1 and CSLD4 are required for cellulose deposition and normal growth of pollen tubes

The cell wall is important for pollen tube growth, but little is known about the molecular mechanism that controls cell wall deposition in pollen tubes. Here, the functional characterization of the pollen-expressed Arabidopsis cellulose synthase-like D genes CSLD1 and CSLD4 that are required for pollen tube growth is reported. Both CSLD1 and CSLD4 are highly expressed in mature pollen grains and pollen tubes. The CSLD1 and CSLD4 proteins are located in the Golgi apparatus and transported to the plasma membrane of the tip region of growing pollen tubes, where cellulose is actively synthesized. Mutations in CSLD1 and CSLD4 caused a significant reduction in cellulose deposition in the pollen tube wall and a remarkable disorganization of the pollen tube wall layers, which disrupted the genetic transmission of the male gametophyte. In csld1 and csld4 single mutants and in the csld1 csld4 double mutant, all the mutant pollen tubes exhibited similar phenotypes: the pollen tubes grew extremely abnormally both in vitro and in vivo, which indicates that CSLD1 and CSLD4 are not functionally redundant. Taken together, these results suggest that CSLD1 and CSLD4 play important roles in pollen tube growth, probably through participation in cellulose synthesis of the pollen tube wall.

Arabidopsis AtVPS15 is essential for pollen development and germination through modulating phosphatidylinositol 3-phosphate formation

Arabidopsis thaliana phosphatidylinositol 3-kinase (AtVPS34) functions in the development and germination of pollen by catalyzing the biosynthesis of phosphatidylinositol 3-phosphate (PI3P). In yeast, Vps15p is required for the membrane targeting and activity of Vps34. The expression of Arabidopsis thaliana VPS15 (AtVPS15), an ortholog of yeast Vps15, is mainly detected in pollen grains and pollen tubes. To determine its role in pollen development and pollen tube growth, we attempted to isolate the T-DNA insertion mutants of AtVPS15; however, homozygous lines of atvps15 were not obtained from the progeny of atvps15/+ heterozygotes. Genetic analysis revealed that the abnormal segregation is due to the failure of transmission of the atvps15 allele through pollen. Most pollen grains from the atvps15/+ genotype are viable, with normal exine structure and nuclei, but some mature pollen grains are characterized with unusual large vacuoles that are not observed in pollen grains from the wild AtVPS15 genotype. The germination ratio of pollen from the atvps15/+ genotype is about half when compared to that from the wild AtVPS15 genotype. When supplied with PI3P, in vitro pollen germination of the atvps15/+ genotype is greatly improved. Presumably, AtVPS15 functions in pollen development and germination by regulating PI3P biosynthesis in Arabidopsis.

Using maize as a model to study pollen tube growth and guidance, cross-incompatibility and sperm delivery in grasses

BACKGROUND

In contrast to animals and lower plants such as mosses and ferns, sperm cells of flowering plants (angiosperms) are immobile and require transportation to the female gametes via the vegetative pollen tube cell to achieve double fertilization. The path of the pollen tube towards the female gametophyte (embryo sac) has been intensively studied in many intra- and interspecific crossing experiments with the aim of increasing the gene pool of crop plants for greater yield, improved biotic and abiotic stress resistance, and for introducing new agronomic traits. Many attempts to hybridize different species or genotypes failed due to the difficulty for the pollen tubes in reaching the female gametophyte. Detailed studies showed that these processes are controlled by various self-incompatible (intraspecific) and cross-incompatible (interspecific) hybridization mechanisms.

SCOPE

Understanding the molecular mechanisms of crossing barriers is therefore of great interest in plant reproduction, evolution and breeding research. In particular, pre-zygotic hybridization barriers related to pollen tube germination, growth, guidance and sperm delivery, which are considered the major hybridization controls in nature and thus also contribute to species isolation and speciation, have been intensively investigated. Despite this general interest, surprisingly little is known about these processes in the most important agronomic plant family, the Gramineae, Poaceae or grasses. Small polymorphic proteins and their receptors, degradation of sterility locus proteins and general compounds such as calcium, γ-aminobutyric acid or nitric oxide have been shown to be involved in progamic pollen germination, adhesion, tube growth and guidance, as well as sperm release. Most advances have been made in the Brassicaceae, Papaveraceae, Linderniaceae and Solanaceae families including their well-understood self-incompatibility (SI) systems. Grass species evolved similar mechanisms to control the penetration and growth of self-pollen to promote intraspecific outcrossing and to prevent fertilization by alien sperm cells. However, in the Poaceae, the underlying molecular mechanisms are still largely unknown.

CONCLUSIONS

We propose to develop maize (Zea mays) as a model to investigate the above-described processes to understand the associated intra- and interspecific crossing barriers in grasses. Many genetic, cellular and biotechnological tools including the completion of a reference genome (inbred line B73) have been established in the last decade and many more maize inbred genomes are expected to be available soon. Moreover, a cellular marker line database as well as large transposon insertion collections and improved Agrobacterium transformation protocols are now available. Additionally, the processes described above are well studied at the morphological level and a number of mutants have been described already, awaiting disclosure of the relevant genes. The identification of the first key players in pollen tube growth, guidance and burst show maize to be an excellent grass model to investigate these processes in more detail. Here we provide an overview of our current understanding of these processes in Poaceae with a focus on maize, and also include relevant discoveries in eudicot model species.

Identification and characterization of TcCRP1, a pollen tube attractant from Torenia concolor

BACKGROUND AND AIMS

During sexual reproduction in higher angiosperms, the pollen tubes are directed to the ovules in the pistil to deliver sperm cells. This pollen tube attraction is highly species specific, and a group of small secreted proteins, TfCRPs, are necessary for this process in Torenia fournieri.

METHODS

A candidate pollen tube attractant protein in Torenia concolor, a related species of T. fournieri, was isolated and the attractant abilities between them were compared.

KEY RESULTS

TcCRP1, an orthologous gene of TfCRP1 from T. concolor, is expressed predominantly in the synergid cell. The gene product attracted pollen tubes in a concentration-dependent manner, but attracted fewer pollen tubes from the other species.

CONCLUSIONS

The results indicated that this class of CRP proteins is a common pollen tube attractant in Torenia species. The sequence diversity of these proteins is important for species-specific pollen tube attraction.

Rapid tip growth: insights from pollen tubes

Pollen tubes extend rapidly in an oscillatory manner by the extreme form of polarized growth, tip growth, and provide an exciting system for studying the spatiotemporal control of polarized cell growth. The Rho-family ROP GTPase is a key signaling molecule in this growth control and is periodically activated at the apical plasma membrane to spatially define the apical growth region and temporally precede the burst of growth. The spatiotemporal dynamics of ROP GTPase is interconnected with actin dynamics and polar exocytosis that is required for tip-targeted membrane and wall expansion. Recent advances in the study of the mechanistic interlinks between ROP-centered signaling and spatiotemporal dynamics of cell membrane and wall remodeling will be discussed.

Sporophytic control of pollen tube growth and guidance in grasses

Pollen tube growth and guidance in the female tissues of flowering plants is a long-studied and anatomically well-described process. A large number of gene products and chemical compounds involved have been identified in the last 20 years, and some underlying molecular mechanisms including self-incompatibility in the Brassicaceae, Solanaceae and Papaveraceae are now well understood. However, the largest part of the pollen tube pathway inside the transmitting tract towards the ovule harbouring the female gametophyte still requires intensive investigations. Especially in the economically most import plant family, the Poaceae or grasses, progamic pollen tube development is barely understood. Using maize as a model, we propose to divide pollen tube germination, growth and guidance towards the female gametophyte into five distinct phases. The model is adapted from Arabidopsis thaliana, taking anatomical differences and novel genetic and cellular studies into consideration. With the exception of Phase V, all phases seem to be under sporophytic control in grasses.

The pollen tube journey in the pistil and imaging the in vivo process by two-photon microscopy

The process of pollen germination and tube growth in the pistil involves a series of cell-cell interactions, some facilitating fertilization while others prohibiting pollen tube access to the female gametophyte, either because of incompatibility or as a result of mechanisms to avert polyspermy and to ensure reproductive success. Understanding pollen tube growth and guidance to the female gametophyte has long been a pursuit among plant biologists, and observations indicate that diverse strategies may be adopted by different plant species. Recent studies in Arabidopsis, maize, and Torenia fournieri suggest that low molecular weight secretory molecules probably play major roles in the short-range attraction of pollen tubes to the female gametophyte. The process of pollen tube growth in the pistil occurs beneath several cell layers so much of the information that conveys the intimate partnership between penetrating pollen tubes and the female tissues has come from fixed samples and observations of in vitro pollen tube growth responses to female factors. A unique glimpse of the in vivo pollen germination and tube growth process is provided here by intra-vital two-photon excitation (TPE) microscopy of pollinated Arabidopsis pistils that remained on intact plants. Further discoveries of critical factors of male or female origins and how they control the pollen tube growth and fertilization process will broaden our understanding of the common themes and diverse strategies that plants have evolved to ensure reproductive success. The advancement of imaging technology to monitor pollination and fertilization and the development of probes to monitor various aspects of the pollen tube growth process, including pollen intracellular dynamics, will allow us to superimpose details obtained from studying pollen tube growth in culture conditions to interpret and understand the in vivo events.

Interdependence of endomembrane trafficking and actin dynamics during polarized growth of Arabidopsis pollen tubes

During polarized growth of pollen tubes, endomembrane trafficking and actin polymerization are two critical processes that establish membrane/wall homeostasis and maintain growth polarity. Fine-tuned interactions between these two processes are therefore necessary but poorly understood. To better understand such cross talk in the model plant Arabidopsis (Arabidopsis thaliana), we first established optimized concentrations of drugs that interfere with either endomembrane trafficking or the actin cytoskeleton, then examined pollen tube growth using fluorescent protein markers that label transport vesicles, endosomes, or the actin cytoskeleton. Both brefeldin A (BFA) and wortmannin disturbed the motility and structural integrity of ARA7- but not ARA6-labeled endosomes, suggesting heterogeneity of the endosomal populations. Disrupting endomembrane trafficking by BFA or wortmannin perturbed actin polymerization at the apical region but not in the longitudinal actin cables in the shank. The interference of BFA/wortmannin with actin polymerization was progressive rather than rapid, suggesting an indirect effect, possibly due to perturbed endomembrane trafficking of certain membrane-localized signaling proteins. Both the actin depolymerization drug latrunculin B and the actin stabilization drug jasplakinolide rapidly disrupted transport of secretory vesicles, but each drug caused distinct responses on different endosomal populations labeled by ARA6 or ARA7, indicating that a dynamic actin cytoskeleton was critical for some steps in endomembrane trafficking. Our results provide evidence of cross talk between endomembrane trafficking and the actin cytoskeleton in pollen tubes.

Sporophytic control of pollen tube growth and guidance in maize

Pollen tube germination, growth, and guidance (progamic phase) culminating in sperm discharge is a multi-stage process including complex interactions between the male gametophyte as well as sporophytic tissues and the female gametophyte (embryo sac), respectively. Inter- and intra-specific crossing barriers in maize and Tripsacum have been studied and a precise description of progamic pollen tube development in maize is reported here. It was found that pollen germination and initial tube growth are rather unspecific, but an early, first crossing barrier was detected before arrival at the transmitting tract. Pollination of maize silks with Tripsacum pollen and incompatible pollination of Ga1s/Ga1s-maize silks with ga1-maize pollen revealed another two incompatibility barriers, namely transmitting tract mistargeting and insufficient growth support. Attraction and growth support by the transmitting tract seem to play key roles for progamic pollen tube growth. After leaving transmitting tracts, pollen tubes have to navigate across the ovule in the ovular cavity. Pollination of an embryo sac-less maize RNAi-line allowed the role of the female gametophyte for pollen tube guidance to be determined in maize. It was found that female gametophyte controlled guidance is restricted to a small region around the micropyle, approximately 50-100 microm in diameter. This area is comparable to the area of influence of previously described ZmEA1-based short-range female gametophyte signalling. In conclusion, the progamic phase is almost completely under sporophytic control in maize.

Mechanistic insights from a quantitative analysis of pollen tube guidance

BACKGROUND

Plant biologists have long speculated about the mechanisms that guide pollen tubes to ovules. Although there is now evidence that ovules emit a diffusible attractant, little is known about how this attractant mediates interactions between the pollen tube and the ovules.

RESULTS

We employ a semi-in vitro assay, in which ovules dissected from Arabidopsis thaliana are arranged around a cut style on artificial medium, to elucidate how ovules release the attractant and how pollen tubes respond to it. Analysis of microscopy images of the semi-in vitro system shows that pollen tubes are more attracted to ovules that are incubated on the medium for longer times before pollen tubes emerge from the cut style. The responses of tubes are consistent with their sensing a gradient of an attractant at 100-150 mum, farther than previously reported. Our microscopy images also show that pollen tubes slow their growth near the micropyles of functional ovules with a spatial range that depends on ovule incubation time.

CONCLUSIONS

We propose a stochastic model that captures these dynamics. In the model, a pollen tube senses a difference in the fraction of receptors bound to an attractant and changes its direction of growth in response; the attractant is continuously released from ovules and spreads isotropically on the medium. The model suggests that the observed slowing greatly enhances the ability of pollen tubes to successfully target ovules. The relation of the results to guidance in vivo is discussed.

A gain-of-function mutation of Arabidopsis lipid transfer protein 5 disturbs pollen tube tip growth and fertilization

During compatible pollination of the angiosperms, pollen tubes grow in the pistil transmitting tract (TT) and are guided to the ovule for fertilization. Lily (Lilium longiflorum) stigma/style Cys-rich adhesin (SCA), a plant lipid transfer protein (LTP), is a small, secreted peptide involved in pollen tube adhesion-mediated guidance. Here, we used a reverse genetic approach to study biological roles of Arabidopsis thaliana LTP5, a SCA-like LTP. The T-DNA insertional gain-of-function mutant plant for LTP5 (ltp5-1) exhibited ballooned pollen tubes, delayed pollen tube growth, and decreased numbers of fertilized eggs. Our reciprocal cross-pollination study revealed that ltp5-1 results in both male and female partial sterility. RT-PCR and beta-glucuronidase analyses showed that LTP5 is present in pollen and the pistil TT in low levels. Pollen-targeted overexpression of either ltp5-1 or wild-type LTP5 resulted in defects in polar tip growth of pollen tubes and thereby decreased seed set, suggesting that mutant ltp5-1 acts as a dominant-active form of wild-type LTP5 in pollen tube growth. The ltp5-1 protein has additional hydrophobic C-terminal sequences, compared with LTP5. In our structural homology/molecular dynamics modeling, Tyr-91 in ltp5-1, replacing Val-91 in LTP5, was predicted to interact with Arg-45 and Tyr-81, which are known to interact with a lipid ligand in maize (Zea mays) LTP. Thus, Arabidopsis LTP5 plays a significant role in reproduction.