Pollen Tube Growth Rates and the Diversification of Flowering Plant Reproductive Cycles

Flower opening dynamics, pollen-ovule ratio, stigma receptivity and stigmatic pollen germination (in-vivo) in Chaenomeles speciosa (Sweet) Nakai

Although Chaenomeles is widely used in horticulture, traditional Chinese medicine and landscape greening, insufficient research has hindered its breeding and seed selection. This study investigated the floral phenology, floral organ characteristics, palynology, and breeding systems of Chaenomeles speciosa (Sweet) Nakai. The floral characteristics of C. speciosa were observed both visually and stereoscopically. The microstructures of the flower organs were observed using scanning electron microscopy. Pollen stainability was determined using triphenyl tetrazolium chloride staining. Stigma receptivity was determined using the benzidine-H2O2 method and the post-artificial pollination pollen germination method. The breeding system was assessed based on the outcrossing index and pollen-ovule ratio. The flowers of C. speciosa were bisexual with a flowering period from March to April. The flowering periods of single flowers ranged from 8 to 19 d, and those of single plants lasted 18-20 d. The anthers were cylindrical, with the base attached to the filament, and were split longitudinally to release pollen. The flower had five styles, with a connate base. The ovaries had five carpels and five compartments. The inverted ovules were arranged in two rows on the placental axis. The stigma of C. speciosa was dry and had many papillary protrusions. In the early flowering stage (1-2 d of flowering), the pollen exhibited high stainability (up to 84.24%), but all stainability was lost at 7 d of flowering. Storage at - 20 °C effectively delayed pollen inactivation. The stigma receptivity of C. speciosa lasted for approximately 7 days, and the breeding system was classified as outcrossing with partial self-compatibility.

Evolutionary analysis of the LORELEI gene family in plants reveals regulatory subfunctionalization

A signaling complex comprising members of the LORELEI (LRE)-LIKE GPI-anchored protein (LLG) and Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE (CrRLK1L) families perceive RAPID ALKALINIZATION FACTOR (RALF) peptides and regulate growth, reproduction, immunity, and stress responses in Arabidopsis (Arabidopsis thaliana). Genes encoding these proteins are members of multigene families in most angiosperms and could generate thousands of signaling complex variants. However, the links between expansion of these gene families and the functional diversification of this critical signaling complex as well as the evolutionary factors underlying the maintenance of gene duplicates remain unknown. Here, we investigated LLG gene family evolution by sampling land plant genomes and explored the function and expression of angiosperm LLGs. We found that LLG diversity within major land plant lineages is primarily due to lineage-specific duplication events, and that these duplications occurred both early in the history of these lineages and more recently. Our complementation and expression analyses showed that expression divergence (i.e. regulatory subfunctionalization), rather than functional divergence, explains the retention of LLG paralogs. Interestingly, all but one monocot and all eudicot species examined had an LLG copy with preferential expression in male reproductive tissues, while the other duplicate copies showed highest levels of expression in female or vegetative tissues. The single LLG copy in Amborella trichopoda is expressed vastly higher in male compared to in female reproductive or vegetative tissues. We propose that expression divergence plays an important role in retention of LLG duplicates in angiosperms.

Function of Rice High-Affinity Potassium Transporters in Pollen Development and Fertility

Plant High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) transporters have been predicted as membrane H+-K+ symporters in facilitating K+ uptake and distribution, while their role in seed production remains to be elucidated. In this study, we report that OsHAK26 is preferentially expressed in anthers and seed husks and located in the Golgi apparatus. Knockout of either OsHAK26 or plasma membrane located H+-K+ symporter gene OsHAK1 or OsHAK5 in both Nipponbare and Dongjin cultivars caused distorted anthers, reduced number and germination rate of pollen grains. Seed-setting rate assay by reciprocal cross-pollination between the mutants of oshak26, oshak1, oshak5 and their wild types confirmed that each HAK transporter is foremost for pollen viability, seed-setting and grain yield. Intriguingly, the pollens of oshak26 showed much thinner wall and were more vulnerable to desiccation than those of oshak1 or oshak5. In vitro assay revealed that the pollen germination rate of oshak5 was dramatically affected by external K+ concentration. The results suggest that the role of OsHAK26 in maintaining pollen development and fertility may relate to its proper cargo sorting for construction of pollen walls, while the role of OsHAK1 and OsHAK5 in maintaining seed production likely relates to their transcellular K+ transport activity.

Dynamic imaging of cell wall polysaccharides by metabolic click-mediated labeling of pectins in living elongating cells

Protein tracking in living plant cells has become routine with the emergence of reporter genes encoding fluorescent tags. Unfortunately, this imaging strategy is not applicable to glycans because they are not directly encoded by the genome. Indeed, complex glycans result from sequential additions and/or removals of monosaccharides by the glycosyltransferases and glycosidases of the cell's biosynthetic machinery. Currently, the imaging of cell wall polymers mainly relies on the use of antibodies or dyes that exhibit variable specificities. However, as immunolocalization typically requires sample fixation, it does not provide access to the dynamics of living cells. The development of click chemistry in plant cell wall biology offers an alternative for live-cell labeling. It consists of the incorporation of a carbohydrate containing a bio-orthogonal chemical reporter into the target polysaccharide using the endogenous biosynthetic machinery of the cell. Once synthesized and deposited in the cell wall, the polysaccharide containing the analog monosaccharide is covalently coupled to an exogenous fluorescent probe. Here, we developed a metabolic click labeling approach which allows the imaging of cell wall polysaccharides in living and elongating cells without affecting cell viability. The protocol was established using the pollen tube, a useful model to follow cell wall dynamics due to its fast and tip-polarized growth, but was also successfully tested on Arabidopsis root cells and root hairs. This method offers the possibility of imaging metabolically incorporated sugars of viable and elongating cells, allowing the study of the long-term dynamics of labeled extracellular polysaccharides.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

Here we review, from a quantitative point of view, the cell biology of protonemal tip growth in the model moss Physcomitrium patens. We focus on the role of the cytoskeleton, vesicle trafficking, and cell wall mechanics, including reviewing some of the existing mathematical models of tip growth. We provide a primer for existing cell biological tools that can be applied to the future study of tip growth in moss. Polarized cell growth is a ubiquitous process throughout the plant kingdom in which the cell elongates in a self-similar manner. This process is important for nutrient uptake by root hairs, fertilization by pollen, and gametophyte development by the protonemata of bryophytes and ferns. In this review, we will focus on the tip growth of moss cells, emphasizing the role of cytoskeletal organization, cytoplasmic zonation, vesicle trafficking, cell wall composition, and dynamics. We compare some of the existing knowledge on tip growth in protonemata against what is known in pollen tubes and root hairs, which are better-studied tip growing cells. To fully understand how plant cells grow requires that we deepen our knowledge in a variety of forms of plant cell growth. We focus this review on the model plant Physcomitrium patens, which uses tip growth as the dominant form of growth at its protonemal stage. Because mosses and vascular plants shared a common ancestor more than 450 million years ago, we anticipate that both similarities and differences between tip growing plant cells will provide mechanistic information of tip growth as well as of plant cell growth in general. Towards this mechanistic understanding, we will also review some of the existing mathematical models of plant tip growth and their applicability to investigate protonemal morphogenesis. We attempt to integrate the conclusions and data across cell biology and physical modeling to our current state of knowledge of polarized cell growth in P. patens and highlight future directions in the field.

A Compendium of in vitro Germination Media for Pollen Research

The correct choice of in vitro pollen germination media (PGM) is crucial in basic and applied pollen research. However, the methodological gaps (e.g., strong focus of current research on model species and cultivated plants along with the lack of general rules for developing a PGM) makes experimenting with pollen difficult. We closed these gaps by compiling a compendium of optimized in vitro PGM recipes from more than 1800 articles published in English, German, and Russian from 1926 to 2019. The compendium includes 1572 PGM recipes successfully used to germinate pollen grains or produce pollen tubes in 816 species representing 412 genera and 114 families (both monocots and dicots). Among the 110 components recorded from the different PGM recipes, sucrose (89% of species), H₃BO₃ (77%), Ca²⁺ (59%), Mg²⁺ (44%), and K⁺ (39%) were the most commonly used PGM components. PGM pH was reported in 35% of all studies reviewed. Also, we identified some general rules for creating PGM for various groups of species differing in area of research (wild and cultivated species), phylogenetic relatedness (angiosperms vs. gymnosperms, dicots vs. monocots), pollen physiology (bi- and tri-cellular), biochemistry (starchy vs. starchless pollen grains), and stigma properties (dry vs. wet), and compared the component requirements. Sucrose, calcium, and magnesium concentrations were significantly different across most categories indicating that pollen sensitivity to sugar and mineral requirements in PGM is highly group-specific and should be accounted for when composing new PGM. This compendium is an important data resource on PGM and can facilitate future pollen research.

Pollen Germination and Pollen Tube Growth in Gymnosperms

Pollen germination and pollen tube growth are common to all seed plants, but these processes first developed in gymnosperms and still serve for their successful sexual reproduction. The main body of data on the reproductive physiology, however, was obtained on flowering plants, and one should be careful to extrapolate the discovered patterns to gymnosperms. In recent years, physiological studies of coniferous pollen have been increasing, and both the features of this group and the similarities with flowering plants have already been identified. The main part of the review is devoted to physiological studies carried out on conifer pollen. The main properties and diversity of pollen grains and pollination strategies in gymnosperms are described.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

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.

Reproductive ecology and postpollination development in the hydrophilous monocot Ruppia maritima

PREMISE

Water-pollination (hydrophily) is a rare but important pollination mechanism that has allowed angiosperms to colonize marine and aquatic habitats. Hydrophilous plants face unique reproductive challenges, and many have evolved characteristic pollen traits and pollination strategies that may have downstream consequences for pollen performance. However, little is known about reproductive development in the life history stage between pollination and fertilization (the progamic phase) in hydrophilous plants. The purpose of this study was to characterize reproductive ecology and postpollination development in water-pollinated Ruppia maritima L.

METHODS

Naturally pollinated inflorescences of R. maritima were collected from the field. Experimental pollinations using both putatively outcross and self pollen were conducted in the greenhouse and inflorescences were collected at appropriate intervals after pollination. Pollen reception, pollen germination, pollen tube growth, and carpel morphology were characterized.

RESULTS

Ruppia maritima exhibits incomplete protogyny, allowing for delayed selfing. Pollen germinated within 15 min after pollination. The average shortest possible pollen tube pathway was 425 μm and pollen tubes first reached the ovule at 45 min after pollination. The mean adjusted pollen tube growth rate was 551 μm/h.

CONCLUSIONS

Ruppia pollen is adapted for rapid pollen germination, which is likely advantageous in an aquatic habitat. Small effective pollen loads suggest that pollen competition intensity is low. Selection for traits such as a long period of stigma receptivity, fast pollen germination, and carpel morphology likely played a larger role in shaping postpollination reproductive development in Ruppia than evolution in pollen tube growth rates.

How does genome size affect the evolution of pollen tube growth rate, a haploid performance trait?

PREMISE

Male gametophytes of most seed plants deliver sperm to eggs via a pollen tube. Pollen tube growth rates (PTGRs) of angiosperms are exceptionally rapid, a pattern attributed to more effective haploid selection under stronger pollen competition. Paradoxically, whole genome duplication (WGD) has been common in angiosperms but rare in gymnosperms. Pollen tube polyploidy should initially accelerate PTGR because increased heterozygosity and gene dosage should increase metabolic rates. However, polyploidy should also independently increase tube cell size, causing more work which should decelerate growth. We asked how genome size changes have affected the evolution of seed plant PTGRs.

METHODS

We assembled a phylogenetic tree of 451 species with known PTGRs. We then used comparative phylogenetic methods to detect effects of neo-polyploidy (within-genus origins), DNA content, and WGD history on PTGR, and correlated evolution of PTGR and DNA content.

RESULTS

Gymnosperms had significantly higher DNA content and slower PTGR optima than angiosperms, and their PTGR and DNA content were negatively correlated. For angiosperms, 89% of model weight favored Ornstein-Uhlenbeck models with a faster PTGR optimum for neo-polyploids, whereas PTGR and DNA content were not correlated. For within-genus and intraspecific-cytotype pairs, PTGRs of neo-polyploids < paleo-polyploids.

CONCLUSIONS

Genome size increases should negatively affect PTGR when genetic consequences of WGDs are minimized, as found in intra-specific autopolyploids (low heterosis) and gymnosperms (few WGDs). But in angiosperms, the higher PTGR optimum of neo-polyploids and non-negative PTGR-DNA content correlation suggest that recurrent WGDs have caused substantial PTGR evolution in a non-haploid state.

Bipolar pollen germination in blue spruce (Picea pungens)

Direct growth of a pollen tube is an effective mechanism of sperm delivery characteristic for the majority of seed plants. In most cases, only one tube grows from one grain to perform the delivery function; meanwhile in Picea the appearance of two tubes from a single pollen grain is quite common during in vitro germination. Here, we describe the phenomenon of bipolar germination and test two hypotheses on its nature and possible role in gametophyte functioning. The hypothesis on "trophic" function of multiple tubes provoked by poor nutrition discussed in literature was not confirmed by in vitro growth tests; bipolar germination strongly decreased with lowering sucrose availability. The highest proportion of bipolar germination occurred in optimal conditions. We then assumed that bipolar germination occurs because turgor pressure is a non-directional force and effective systems of cell wall mechanical regulation are lacking. In hypertonic medium, bipolar germination was sufficiently lower than in isotonic medium, which was consistent with prediction of the «mechanical» hypothesis. Scanning electron microscopy and fluorescence microscopy analysis of pollen morphology and cell wall dynamics during both types of germination showed that the appearance of a single tube or bipolar germination depends on the extension of exine rupture. Cell wall softening by short-term ·OH treatment sufficiently decreased the percent of bipolar germination without affecting total germination efficiency. We concluded that mechanical properties of the cell wall and turgor pressure could shift the balance towards one of the germination patterns.

Complex reproductive secretions occur in all extant gymnosperm lineages: a proteomic survey of gymnosperm pollination drops

KEY MESSAGE

Complex protein-containing reproductive secretions are a conserved trait amongst all extant gymnosperms; the pollination drops of most groups include carbohydrate-modifying enzymes and defence proteins. Pollination drops are aqueous secretions that receive pollen and transport it to the ovule interior in gymnosperms (Coniferales, Cycadales, Ginkgoales, Gnetales). Proteins are well established as components of pollination drops in conifers (Coniferales) and Ephedra spp. (Gnetales), but it is unknown whether proteins are also present in the pollination drops of cycads (Cycadales), Ginkgo (Ginkgoales), Gnetum (Gnetales), or in the pollination drops produced by sterile ovules occurring on pollen plants in the Gnetales. We used liquid chromatography-tandem mass spectrometry followed by database-derived protein identification to conduct proteomic surveys of pollination drops collected from: Ceratozamia hildae, Zamia furfuracea and Cycas rumphii (Cycadales); Ginkgo biloba (Ginkgoales); Gnetum gnemon and Welwitschia mirabilis, including pollination drops from both microsporangiate and ovulate plants (Gnetales). We identified proteins in all samples: C. hildae (61), Z. furfuracea (40), C. rumphii (9), G. biloba (57), G. gnemon ovulate (17) and sterile ovules from microsporangiate plants (25) and W. mirabilis fertile ovules (1) and sterile ovules from microsporangiate plants (138). Proteins involved in defence and carbohydrate modification occurred in the drops of most groups, indicating conserved functions for proteins in pollination drops. Our study demonstrates that all extant gymnosperm groups produce complex reproductive secretions containing proteins, an ancient trait that likely contributed to the evolutionary success of seed plants.

Anther and pollen development in sweet cherry (Prunus avium L.) in relation to winter dormancy

Anther and pollen development is a highly conserved process in angiosperms, but while pollen formation in annual plants occurs in a few days, in temperate woody perennials, it requires several months. How anther and pollen development is framed in terms of seasonality plays a clear part in reproductive success. In this study, seasonal anther and pollen development is characterized in two sweet cherry cultivars over 2 years, paying special attention to the period of dormancy and unveiling the role of starch in this process. We evaluated starch content from the autumn until bud burst with the help of an image analysis system fitted to a light microscope. Microscope observations allowed the temporal relationship of pollen development to the phenological stages of flower and bud development to be determined. In both cultivars and years, anther and pollen development followed the same pattern. Development was halted by dormancy, when the anthers showed no morphological changes until several weeks after chilling fulfillment, until the milder temperatures reactivated development. After dormancy, starch was accumulated in the connective tissue until tracheary element differentiation. Quantification of starch in the connective tissue of anthers revealed its importance in supporting pollen meiosis and subsequent anther growth.

Autonomous self-fertilization in Linum sulcatum, a native annual with a previously unknown mating system

The mating systems of wild plant populations have profound effects on their genetic structure and evolution, yet remain unknown or incompletely described for many species. One such species, the herbaceous native annual Linum sulcatum Riddell (Linaceae), is thought to be self-compatible, but there has been no experimental evidence to date to support this claim. To assess the breeding system of this species, seeds were collected from wild populations and reared in a controlled environment. Floral manipulations and controlled pollinations were conducted to determine the degree of self-compatibility of this species and to distinguish between autonomous and facilitated modes of selfing. Additional controlled within- and between-population crosses were conducted to determine the relative degree to which this species can outcross. This study showed that self-fertilization was highly successful and can occur autonomously. In contrast, outcrossing success, both within and between populations, was very limited, suggesting this species may exhibit an extreme degree of cross-incompatibility. A pollen tube growth experiment showed that self-pollination resulted in the formation of more pollen tubes relative to cross-pollination and that complete pollen tube growth can occur less than 2 h following self-pollination. This information is relevant to the future persistence of this species, as much of its remaining habitat is distributed among small, highly fragmented patches subjected to current and future environmental stressors.

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.

Evolution of development of pollen performance

With the origin of pollination in ancient seed plants, the male gametophyte ("pollen") began to evolve a new and unique life history stage, the progamic phase, a post-pollination period in which pollen sexual maturation occurs in interaction with sporophyte-derived tissues. Pollen performance traits mediate the timing of the fertilization process, often in competition with other pollen, via the speed of pollen germination, sperm development, and pollen tube growth. Studies of pollen development rarely address the issue of performance or its evolution, which involves linking variation in developmental rates to relative fitness within populations or to adaptations on a macroevolutionary scale. Modifications to the pollen tube pathway and changes in the intensity of pollen competition affect the direction and strength of selection on pollen performance. Hence, pollen developmental evolution is always contextual-it involves both the population biology of pollen reaching stigmas and the co-evolution of sporophytic traits, such as the pollen tube pathway and mating system. For most species, performance evolution generally reflects a wandering history of periods of directional selection and relaxed selection, channeled by developmental limitations, a pattern that favors the accumulation of diversity and redundancy in developmental mechanisms and the genetic machinery. Developmental biologists are focused on finding universal mechanisms that underlie pollen function, and these are largely mechanisms that have evolved through their effects on performance. Here, we suggest ways in which studies of pollen performance or function could progress by cross-fertilization between the "evo" and "devo" fields.

The Evolution of Sexual Fluids in Gymnosperms From Pollination Drops to Nectar

A current synthesis of data from modern and fossil plants paints a new picture of sexual fluids, including nectar, as a foundational component of gymnosperm reproductive evolution. We review the morpho-anatomical adaptations, their accompanying secretions, and the functional compounds involved. We discuss two types of secretions: (1) those involved in fertilization fluids produced by gametophytes and archegonia of zooidogamous gymnosperms, i.e., Ginkgo and cycads, and (2) those involved in pollen capture mechanisms (PCMs), i.e., pollination drops. Fertilization fluids provide both liquid in which sperm swim, as well as chemotactic signals that direct sperm to the egg. Such fertilization fluids were probably found among many extinct plants such as ancient cycads and others with swimming sperm, but were subsequently lost upon the evolution of siphonogamy (direct delivery of sperm to the egg by pollen tubes), as found in modern gnetophytes, conifers, and Pinaceae. Pollination drops are discussed in terms of three major types of PCMs and the unique combinations of morphological and biochemical adaptations that define each. These include their amino acids, sugars, calcium, phosphate and proteins. The evolution of PCMs is also discussed with reference to fossil taxa. The plesiomorphic state of extant gymnosperms is a sugar-containing pollination drop functioning as a pollen capture surface, and an in ovulo pollen germination medium. Additionally, these drops are involved in ovule defense, and provide nectar for pollinators. Pollination drops in anemophilous groups have low sugar concentrations that are too low to provide insects with a reward. Instead, they appear to be optimized for defense and microgametophyte development. In insect-pollinated modern Gnetales a variety of tissues produce sexual fluids that bear the biochemical signature of nectar. Complete absence of fluid secretions is restricted to a few, poorly studied modern conifers, and is presumably derived. Aspects of pollination drop dynamics, e.g., regulation of secretion and retraction, are reviewed. Lastly, we discuss pollination drops' control of pollen germination. Large gaps in our current knowledge include the composition of fertilization fluids, the pollination drops of Podocarpaceae, and the overall hydrodynamics of sexual fluids in general.

The evolution and patterning of male gametophyte development

The reproductive adaptations of land plants have played a key role in their terrestrial colonization and radiation. This encompasses mechanisms used for the production, dispersal and union of gametes to support sexual reproduction. The production of small motile male gametes and larger immotile female gametes (oogamy) in specialized multicellular gametangia evolved in the charophyte algae, the closest extant relatives of land plants. Reliance on water and motile male gametes for sexual reproduction was retained by bryophytes and basal vascular plants, but was overcome in seed plants by the dispersal of pollen and the guided delivery of non-motile sperm to the female gametes. Here we discuss the evolutionary history of male gametogenesis in streptophytes (green plants) and the underlying developmental biology, including recent advances in bryophyte and angiosperm models. We conclude with a perspective on research trends that promise to deliver a deeper understanding of the evolutionary and developmental mechanisms of male gametogenesis in plants.

Actin fringes of polar cell growth

The eukaryotic actin cytoskeleton is a highly dynamic framework that is involved in many biological processes, such as cell growth, division, morphology, and motility. G-actin polymerizes into microfilaments that associate into bundles, patches, and networks, which, in turn, organize into higher order structures that are fundamental for the course of important physiological events. Actin rings are an example for such higher order actin entities, but this term represents an actually diverse set of subcellular structures that are involved in various processes. This review especially sheds light on a crucial type of non-constricting ring-like actin networks, and categorizes them under the term 'actin fringe'. These 'actin fringes' are visualized as highly dynamic and yet steady structures in the tip of various polarized growing cells. The present comprehensive overview compares the actin fringe characteristics of rapidly elongating pollen tubes with several related actin arrays in other cell types of diverse species. The current state of knowledge about various actin fringe functions is summarized, and the key role of this structure in the polar growth process is discussed.

Economy, efficiency, and the evolution of pollen tube growth rates

PREMISE

Pollen tube growth rate (PTGR) is an important aspect of male gametophyte performance because of its central role in the fertilization process. Theory suggests that under intense competition, PTGRs should evolve to be faster, especially if PTGR accurately reflects gametophyte quality. Oddly, we know remarkably little about how effectively the work of tube construction is translated to elongation (growth and growth rate). Here we test the prediction that pollen tubes grow equally efficiently by comparing the scaling of wall production rate (WPR) to PTGR in three water lilies that flower concurrently: Nymphaea odorata, Nuphar advena and Brasenia schreberi.

METHODS

Single-donor pollinations on flower or carpel pairs were fixed just after pollen germination (time A) and 45 min later (time B). Mean PTGR was calculated as the average increase in tube length over that growth period. Tube circumferences (C) and wall thicknesses (W) were measured at time B. For each donor, WPR = mean (C × W) × mean PTGR.

KEY RESULTS

Within species, pollen tubes maintained a constant WPR to PTGR ratio, but species had significantly different ratios. N. odorata and N. advena had similar PTGRs, but for any given PTGR, they had the lowest and highest WPRs, respectively.

CONCLUSIONS

We showed that growth rate efficiencies evolved by changes in the volume of wall material used for growth and in how that material was partitioned between lateral and length dimensions. The economics of pollen tube growth are determined by tube design, which is consequent on trade-offs between efficient growth and other pollen tube functions.

Pollen performance traits reveal prezygotic nonrandom mating and interference competition in Arabidopsis thaliana

PREMISE

The lack of ability to measure pollen performance traits in mixed pollinations has been a major hurdle in understanding the mechanisms of differential success of compatible pollen donors. In previous work, we demonstrated that nonrandom mating between two accessions of Arabidopsis thaliana, Columbia (Col) and Landsberg (Ler), is mediated by the male genotype. Despite these genetic insights, it was unclear at what stage of reproduction these genes were acting. Here, we used an experimental strategy that allowed us to differentiate different pollen populations in mixed pollinations to ask: (1) What pollen performance traits differed between Col and Ler accessions that direct nonrandom mating? (2) Is there evidence of interference competition?

METHODS

We used genetically marked pollen that can be visualized colorimetrically to quantify pollen performance of single populations of pollen in mixed pollinations. We used this and other assays to measure pollen viability, germination, tube growth, patterns of fertilization, and seed abortion. Finally, we assessed interference competition.

RESULTS

In mixed pollinations on Col pistils, Col pollen sired significantly more seeds than Ler pollen. Col pollen displayed higher pollen viability, faster and greater pollen germination, and faster pollen tube growth. We saw no evidence of nonrandom seed abortion. Finally, we found interference competition occurs in mixed pollinations.

CONCLUSION

The lack of differences in postzygotic processes coupled with direct observation of pollen performance traits indicates that nonrandom mating in Arabidopsis thaliana is prezygotic, due mostly to differential pollen germination and pollen tube growth rates. Finally, this study unambiguously demonstrates the existence of interference competition.

Pollen competition in style: Effects of pollen size on siring success in the hermaphroditic common morning glory, Ipomoea purpurea

PREMISE OF THE STUDY

Pollen size varies greatly among flowering plant species and has been shown to influence the delivery of sperm cells to the eggs. Relatively little is known, however, about the functional significance of within-species genetic variation in pollen size. This study tests whether pollen size influences the relative siring success of a pollen donor during in vivo pollen competition experiments.

METHODS

We used two groups of Ipomoea purpurea plants genetically divergent in their pollen sizes and applied equal number of pollen grains from one large-pollen and one small-pollen donor onto the same stigma. Using microsatellite genetic markers, we identified the pollen parent of each of the resulting progeny to determine the relative siring success of the competing donors. Competitions between donors of equal-sized pollen served as a control.

KEY RESULTS

Differences in pollen size significantly affected the relative siring success of a pollen donor; larger-grained individuals outcompeted smaller-grained competitors but not equal-sized competitors. Relative siring success, however, sometimes varied across different pollen recipients.

CONCLUSIONS

Pollen size can influence the relative siring success of different individuals competing on the same stigma during postpollination processes. However, other factors, such as pollen-pistil interaction and environmental conditions, are likely to influence these competitions as well.

The Diversity of the Pollen Tube Pathway in Plants: Toward an Increasing Control by the Sporophyte

Plants, unlike animals, alternate multicellular diploid, and haploid generations in their life cycle. While this is widespread all along the plant kingdom, the size and autonomy of the diploid sporophyte and the haploid gametophyte generations vary along evolution. Vascular plants show an evolutionary trend toward a reduction of the gametophyte, reflected both in size and lifespan, together with an increasing dependence from the sporophyte. This has resulted in an overlooking of the importance of the gametophytic phase in the evolution of higher plants. This reliance on the sporophyte is most notorious along the pollen tube journey, where the male gametophytes have to travel a long way inside the sporophyte to reach the female gametophyte. Along evolution, there is a change in the scenery of the pollen tube pathway that favors pollen competition and selection. This trend, toward apparently making complicated what could be simple, appears to be related to an increasing control of the sporophyte over the gametophyte with implications for understanding plant evolution.

Pollen structure and development in Nymphaeales: insights into character evolution in an ancient angiosperm lineage

PREMISE OF THE STUDY

A knowledge of pollen characters in early-diverging angiosperm lineages is essential for understanding pollen evolution and the role of pollen in angiosperm diversification. In this paper, we report and synthesize data on mature pollen and pollen ontogeny from all genera of Nymphaeales within a comparative, phylogenetic context and consider pollen evolution in this early-diverging angiosperm lineage. We describe mature pollen characters for Euryale, Barclaya, and Nymphaea ondinea, taxa for which little to no structural data exist.

METHODS

We studied mature pollen for all nymphaealean genera using light, scanning electron, and transmission electron microscopy. We reviewed published reports of nymphaealean pollen to provide a comprehensive discussion of pollen characters in water lilies.

KEY RESULTS

Nymphaeales exhibit diversity in key pollen characters, including dispersal unit size, ornamentation, aperture morphology, and tapetum type. All Nymphaeales pollen are tectate-columellate, exhibiting one of two distinct patterns of infratectal ultrastructure-a thick infratectal space with robust columellae or a thin infratectal space with thin columellae. All genera have pollen with a lamellate endexine that becomes compressed in the proximal, but not distal wall. This endexine ultrastructure supports the operculate hypothesis for aperture origin. Nymphaeaceae pollen exhibit a membranous granular layer, which is a synapomorphy of the family.

CONCLUSIONS

Variation in pollen characters indicates that significant potential for lability in pollen development was present in Nymphaeales at the time of its divergence from the rest of angiosperms. Structural and ontogenetic data are essential for interpreting pollen characters, such as infratectum and endexine ultrastructure in Nymphaeales.

Fossil evidence for a herbaceous diversification of early eudicot angiosperms during the Early Cretaceous

Eudicot flowering plants comprise roughly 70% of land plant species diversity today, but their early evolution is not well understood. Fossil evidence has been largely restricted to their distinctive tricolpate pollen grains and this has limited our understanding of the ecological strategies that characterized their primary radiation. I describe megafossils of an Early Cretaceous eudicot from the Potomac Group in Maryland and Virginia, USA that are complete enough to allow reconstruction of important life-history traits. I draw on quantitative and qualitative analysis of functional traits, phylogenetic analysis and sedimentological evidence to reconstruct the biology of this extinct species. These plants were small and locally rare but widespread, fast-growing herbs. They had complex leaves and they were colonizers of bright, wet, disturbance-prone habitats. Other early eudicot megafossils appear to be herbaceous rather than woody, suggesting that this habit was characteristic of their primary radiation. A mostly herbaceous initial diversification of eudicots could simultaneously explain the heretofore sparse megafossil record as well as their rapid diversification during the Early Cretaceous because the angiosperm capacity for fast reproduction and fast evolution is best expressed in herbs.

Using theories of sexual selection and sexual conflict to improve our understanding of plant ecology and evolution

Today it is accepted that the theories of sexual selection and sexual conflict are general and can be applied to both animals and plants. However, potentially due to a controversial history, plant studies investigating sexual selection and sexual conflict are relatively rare. Moreover, these theories and concepts are seldom implemented in research fields investigating related aspects of plant ecology and evolution. Even though these theories are complex, and can be difficult to study, we suggest that several fields in plant biology would benefit from incorporating and testing the impact of selection pressures generated by sexual selection and sexual conflict. Here we give examples of three fields where we believe such incorporation would be particularly fruitful, including (i) mechanisms of pollen-pistil interactions, (ii) mating-system evolution in hermaphrodites and (iii) plant immune responses to pests and pathogens.

Expression patterns of conserved microRNAs in the male gametophyte of loblolly pine (Pinus taeda)

MicroRNAs (miRNAs) are small RNAs that regulate genes involved in various aspects of plant development, but their presence and expression patterns in the male gametophytes of gymnosperms have not yet been established. Therefore, this study identified and compared the expression patterns of conserved miRNAs from two stages of the male gametophyte of loblolly pine (Pinus taeda), which are the mature (ungerminated) and germinated pollen. Microarray was used to identify conserved miRNAs that varied in expression between these two stages of the loblolly pine male gametophyte. Forty-seven conserved miRNAs showed significantly different expression levels between mature and germinated loblolly pine pollen. In particular, miRNAs representing 14 and 8 families were up- and down-regulated in germinated loblolly pine pollen, respectively. qRT-PCR was used to validate their expression patterns using representative miRNAs. Target genes and proteins were identified using psRNATarget program. Predicted targets of the 22 miRNA families belong mostly to classes of genes involved in defense/stress response, metabolism, regulation, and signaling. qRT-PCR was also used to validate the expression patterns of representative target genes. This study shows that conserved miRNAs are expressed in mature and germinated loblolly pine pollen. Many of these miRNAs are differentially expressed, which indicates that the two stages of the male gametophyte examined are regulated at the miRNA level. This study also expands our knowledge of the male gametophytes of seed plants by providing insights on some similarities and differences in the types and expression patterns of conserved miRNAs between loblolly pine with those of rice and Arabidopsis.

Repeated evolution of tricellular (and bicellular) pollen

PREMISE OF STUDY

Male gametophytes of seed plants are sexually immature at the time they are dispersed as pollen, but approximately 30% of flowering plants have tricellular pollen containing fully formed sperm at anthesis. The classic study of Brewbaker (1967: American Journal of Botany 54: 1069-1083) provided a powerful confirmation of the long-standing hypothesis that tricellular pollen had many parallel and irreversible origins within angiosperms. We readdressed the main questions of that study with modern comparative phylogenetic methods.

METHODS

We used our own and more recent reports to greatly expand the Brewbaker data set. We modeled trait evolution for 2511 species on a time-calibrated angiosperm phylogeny using (1) Binary State Speciation and Extinction (BiSSE), which accounts for the effect of species diversification rates on character transition rates and, (2) the hidden rates model (HRM), which incorporates variation in transition rates across a phylogeny.

KEY RESULTS

Seventy percent of species had bicellular pollen. BiSSE found a 1.9-fold higher bicellular to tricellular transition rate than in the reverse direction, and bicellular lineages had a 1.8-fold higher diversification rate than tricellular lineages. HRM found heterogeneity in evolutionary rates, with bidirectional transition rates in three of four rate classes.

CONCLUSIONS

The tricellular condition is not irreversible. Pollen cell numbers are maintained at intermediate frequencies because lower net diversification rates of tricellular lineages are counterbalanced by slower state shifts to the bicellular condition. That tricellular lineages diversify slowly and give rise to bicellular lineages slowly reflects a linkage between the evolution of sporophyte lifestyles and the developmental lability of male gametophytes.

No stress! Relax! Mechanisms governing growth and shape in plant cells

The mechanisms through which plant cells control growth and shape are the result of the coordinated action of many events, notably cell wall stress relaxation and turgor-driven expansion. The scalar nature of turgor pressure would drive plant cells to assume spherical shapes; however, this is not the case, as plant cells show an amazing variety of morphologies. Plant cell walls are dynamic structures that can display alterations in matrix polysaccharide composition and concentration, which ultimately affect the wall deformation rate. The wide varieties of plant cell shapes, spanning from elongated cylinders (as pollen tubes) and jigsaw puzzle-like epidermal cells, to very long fibres and branched stellate leaf trichomes, can be understood if the underlying mechanisms regulating wall biosynthesis and cytoskeletal dynamics are addressed. This review aims at gathering the available knowledge on the fundamental mechanisms regulating expansion, growth and shape in plant cells by putting a special emphasis on the cell wall-cytoskeleton system continuum. In particular, we discuss from a molecular point of view the growth mechanisms characterizing cell types with strikingly different geometries and describe their relationship with primary walls. The purpose, here, is to provide the reader with a comprehensive overview of the multitude of events through which plant cells manage to expand and control their final shapes.

Glycoprotein composition along the pistil of Malus x domestica and the modulation of pollen tube growth

BACKGROUND

The characteristics of pollen tube growth are not constant, but display distinct patterns of growth within the different tissues of the pistil. In the stigma, the growth rate is slow and autotrophic, whereas in the style, it is rapid and heterotrophic. Very little is known about the interactions between these distinct maternal tissues and the traversing pollen tube and the role of this interaction on the observed metabolism. In this work we characterise pollen tube growth in the apple flower and look for differences in glycoprotein epitope localization between two different maternal tissues, the stigma and the style.

RESULTS

While immunocytochemically-detected arabinogalactan proteins were present at high levels in the stigma, they were not detected in the transmitting tissue of the style, where extensins were abundant. Whereas extensins remained at high levels in unpollinated pistils, they were no longer present in the style following pollen tube passage. Similarily, while abundant in unpollinated styles, insoluble polysaccharides such as β-glucans, were depleted in pollinated pistils.

CONCLUSIONS

The switch from autotropic to heterotrophic pollen tube growth correlates spatially with a change of glycoprotein epitopes between the stigma and the style. The depletion of extensins and polysaccharides following pollen tube passage in the style suggest a possible contribution to the acceleration of heterotrophic pollen tube growth, which would imply an active contribution of female tissues on prezygotic male-female crosstalk.

Application of proteomics to the study of pollination drops

PREMISE OF THE STUDY

Pollination drops are a formative component in gymnosperm pollen-ovule interactions. Proteomics offers a direct method for the discovery of proteins associated with this early stage of sexual reproduction. •

METHODS

Pollination drops were sampled from eight gymnosperm species: Chamaecyparis lawsoniana (Port Orford cedar), Ephedra monosperma, Ginkgo biloba, Juniperus oxycedrus (prickly juniper), Larix ×marschlinsii, Pseudotsuga menziesii (Douglas-fir), Taxus ×media, and Welwitschia mirabilis. Drops were collected by micropipette using techniques focused on preventing sample contamination. Drop proteins were separated using both gel and gel-free methods. Tandem mass spectrometric methods were used including a triple quadrupole and an Orbitrap. •

RESULTS

Proteins are present in all pollination drops. Consistency in the protein complement over time was shown in L. ×marschlinsii. Representative mass spectra from W. mirabilis chitinase peptide and E. monosperma serine carboxypeptidase peptide demonstrated high quality results. We provide a summary of gymnosperm pollination drop proteins that have been discovered to date via proteomics. •

DISCUSSION

Using proteomic methods, a dozen classes of proteins have been identified to date. Proteomics presents a way forward in deepening our understanding of the biological function of pollination drops.

The evolution of pollen germination timing in flowering plants: Austrobaileya scandens (Austrobaileyaceae)

BACKGROUND AND AIMS

The pollination to fertilization process (progamic phase) is thought to have become greatly abbreviated with the origin of flowering plants. In order to understand what developmental mechanisms enabled the speeding of fertilization, comparative data are needed from across the group, especially from early-divergent lineages. I studied the pollen germination process of Austrobaileya scandens, a perennial vine endemic to the Wet Tropics area of northeastern Queensland, Australia, and a member of the ancient angiosperm lineage, Austrobaileyales.

METHODOLOGY

I used in vivo and in vitro hand pollinations and timed collections to study development from late pollen maturation to just after germination. Then I compared the contribution of pollen germination timing to progamic phase duration in 131 angiosperm species (65 families).

PRINCIPAL FINDINGS

Mature pollen of Austrobaileya was bicellular, starchless and moderately dehydrated-water content was 31.5 % by weight and volume increased by 57.9 % upon hydration. A callose layer in the inner intine appeared only after pollination. In vivo pollen germination followed a logarithmic curve, rising from 28 % at 1 hour after pollination (hap) to 97 % at 12 hap (R(2) = 0.98). Sufficient pollen germination to fertilize all ovules was predicted to have occurred within 62 min. Across angiosperms, pollen germination ranged from 1 min to >60 h long and required 8.3 ± 9.8 % of the total duration of the progamic phase.

SIGNIFICANCE

Pollen of Austrobaileya has many plesiomorphic features that are thought to prolong germination. Yet its germination is quite fast for species with desiccation-tolerant pollen (range: <1 to 60 h). Austrobaileya and other early-divergent angiosperms have relatively rapid pollen germination and short progamic phases, comparable to those of many insect-pollinated monocots and eudicots. These results suggest that both the pollen germination and pollen tube growth periods were marked by acceleration of developmental processes early in angiosperm history.