A decade of pollen transcriptomics

KEY MESSAGE

Overview of pollen transcriptome studies. Pollen development is driven by gene expression, and knowledge of the molecular events underlying this process has undergone a quantum leap in the last decade through studies of the transcriptome. Here, we outline historical evidence for male haploid gene expression and review the wealth of pollen transcriptome data now available. Knowledge of the transcriptional capacity of pollen has progressed from genetic studies to the direct analysis of RNA and from gene-by-gene studies to analyses on a genomic scale. Microarray and/or RNA-seq data can now be accessed for all phases and cell types of developing pollen encompassing 10 different angiosperms. These growing resources have accelerated research and will undoubtedly inspire new directions and the application of system-based research into the mechanisms that govern the development, function and evolution of angiosperm pollen.

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.

Microspore embryogenesis: establishment of embryo identity and pattern in culture

The developmental plasticity of plants is beautifully illustrated by the competence of the immature male gametophyte to change its developmental fate from pollen to embryo development when exposed to stress treatments in culture. This process, referred to as microspore embryogenesis, is widely exploited in plant breeding, but also provides a unique system to understand totipotency and early cell fate decisions. We summarize the major concepts that have arisen from decades of cell and molecular studies on microspore embryogenesis and put these in the context of recent experiments, as well as results obtained from the study of pollen and zygotic embryo development.

Chromatin remodelling during male gametophyte development

The plant life cycle alternates between a diploid sporophytic phase and haploid gametophytic phase, with the latter giving rise to the gametes. Male gametophyte development encompasses two mitotic divisions that results in a simple three-celled structure knows as the pollen grain, in which two sperm cells are encased within a larger vegetative cell. Both cell types exhibit a very different type of chromatin organization - highly condensed in sperm cell nuclei and highly diffuse in the vegetative cell. Distinct classes of histone variants have dynamic and differential expression in the two cell lineages of the male gametophyte. Here we review how the dynamics of histone variants are linked to reprogramming of chromatin activities in the male gametophyte, compaction of the sperm cell genome and zygotic transitions post-fertilization.

Gametophytic and zygotic selection leads to segregation distortion through in vivo induction of a maternal haploid in maize

Production of maternal haploids via a male inducer can greatly accelerate maize breeding and is an interesting biological phenomenon in double fertilization. However, the mechanism behind haploid induction remains elusive. Segregation distortion, which is increasingly recognized as a potentially powerful evolutionary force, has recently been observed during maternal haploid induction in maize. The results present here showed that both male gametophytic and zygotic selection contributed to severe segregation distortion of a locus, named segregation distortion 1 (sed1), during maternal haploid induction in maize. Interestingly, analysis of reciprocal crosses showed that sed1 is expressed in the male gametophyte. A novel mapping strategy based on segregation distortion has been used to fine-map this locus. Strong selection for the presence of the sed1 haplotype from inducers in kernels with haploid formation and defects could be detected in the segregating population. Dual-pollination experiments showed that viable pollen grains from inducers had poor pollen competitive ability against pollen from normal genotypes. Although defective kernels and haploids have different phenotypes, they are most probably caused by the sed1 locus, and possible mechanisms for production of maternal haploids and the associated segregation distortion are discussed. This research also provides new insights into the process of double fertilization.

Proline is required for male gametophyte development in Arabidopsis

BACKGROUND

In crosses between the proline-deficient mutant homozygous for p5cs1 and heterozygous for p5cs2 (p5cs1 p5cs2/P5CS2), used as male, and different Arabidopsis mutants, used as females, the p5cs2 mutant allele was rarely transmitted to the outcrossed progeny, suggesting that the fertility of the male gametophyte carrying mutations in both P5CS1 and P5CS2 is severely compromised.

RESULTS

To confirm the fertility defects of pollen from p5cs1 p5cs2/P5CS2 mutants, transmission of mutant alleles through pollen was tested in two ways. First, the number of progeny inheriting a dominant sulfadiazine resistance marker linked to p5cs2 was determined. Second, the number of p5cs2/p5cs2 embryos was determined. A ratio of resistant to susceptible plantlets close to 50%, and the absence of aborted embryos were consistent with the hypothesis that the male gametophyte carrying both p5cs1 and p5cs2 alleles is rarely transmitted to the offspring. In addition, in reciprocal crosses with wild type, about 50% of the p5cs2 mutant alleles were transmitted to the sporophytic generation when p5cs1 p5cs2/P5CS2 was used as a female, while less than 1% of the p5cs2 alleles could be transmitted to the outcrossed progeny when p5cs1 p5cs2/P5CS2 was used as a male. Morphological and functional analysis of mutant pollen revealed a population of small, degenerated, and unviable pollen grains, indicating that the mutant homozygous for p5cs1 and heterozygous for p5cs2 is impaired in pollen development, and suggesting a role for proline in male gametophyte development. Consistent with these findings, we found that pollen from p5cs1 homozygous mutants, display defects similar to, but less pronounced than pollen from p5cs1 p5cs2/P5CS2 mutants. Finally, we show that pollen from p5cs1 p5cs2/P5CS2 plants contains less proline than wild type and that exogenous proline supplied from the beginning of another development can partially complement both morphological and functional pollen defects.

CONCLUSIONS

Our data show that the development of the male gametophyte carrying mutations in both P5CS1 and P5CS2 is severely compromised, and indicate that proline is required for pollen development and transmission.

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.

Cytokinin receptors in sporophytes are essential for male and female functions in Arabidopsis thaliana

Arabidopsis has three cytokinin receptors genes: CRE1, AHK2, and AHK3. Availability of plants that are homozygous mutant for these three genes indicates that cytokinin receptors in the haploid cells are dispensable for the development of male and female gametophytes. The triple mutants form a few flowers but never set seed, indicating that reproductive growth is impaired. We investigated which reproductive processes are affected in the triple mutants. Anthers of mutant plants contained fewer pollen grains and did not dehisce. Pollen in the anthers completed the formation of the one vegetative nucleus and the two sperm nuclei, as seen in wild type. The majority of the ovules were abnormal: 78% lacked the embryo sac, 10% carried a female gametophyte that terminated its development before completing three rounds of nuclear division, and about 12% completed three rounds of nuclear division but the gametophytes were smaller than those of the wild type. Reciprocal crosses between the wild type and the triple mutants indicated that pollen from mutant plants did not germinate on wild-type stigmas, and wild-type pollen did not germinate on mutant stigmas. These results suggest that cytokinin receptors in the sporophyte are indispensable for anther dehiscence, pollen maturation, induction of pollen germination by the stigma, and female gametophyte formation and maturation.

The Arabidopsis eukaryotic translation initiation factor 3, subunit F (AteIF3f), is required for pollen germination and embryogenesis

Previous studies have shown that subunits E (eIF3e), F (eIF3f) and H (elF3h) of eukaryotic translation initiation factor 3 play important roles in cell development in humans and yeast. eIF3e and eIF3h have also been reported to be important for normal cell growth in Arabidopsis. However, the functions of subunit eIF3f remain largely unknown in plant species. Here we report characterization of mutants for the Arabidopsis eIF3f (AteIF3f) gene. AteIF3f encodes a protein that is highly expressed in pollen grains, developing embryos and root tips, and interacts with Arabidopsis eIF3e and eIF3h proteins. A Ds insertional mutation in AteIF3f disrupted pollen germination and embryo development. Expression of some of the genes that are essential for pollen tube growth and embryogenesis is down-regulated in ateif3f-1 homozygous seedlings obtained by pollen rescue. These results suggested that AteIF3f might play important roles in Arabidopsis cell growth and differentiation in combination with eIF3e and eIF3h.

Complete blockage of the mevalonate pathway results in male gametophyte lethality

Plants have two isoprenoid biosynthetic pathways: the cytosolic mevalonate (MVA) pathway and the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Since the discovery of the MEP pathway, possible metabolic cross-talk between these pathways has prompted intense research. Although many studies have shown the existence of such cross-talk using feeding experiments, it remains to be determined if native cross-talk, rather than exogenously applied metabolites, can compensate for complete blockage of the MVA pathway. Previously, Arabidopsis mutants for HMG1 and HMG2 encoding HMG-CoA reductase (HMGR) were isolated. Although it was shown that HMGR1 is a functional HMGR, the enzyme activity of HMGR2 has not been confirmed. It is demonstrated here that HMG2 encodes a functional reductase with similar activity to HMGR1, using enzyme assays and complementation experiments. To estimate the contribution of native cross-talk, an attempt was made to block the MVA pathway by making double mutants lacking both HMG1 and HMG2, but no double homozygotes were detected in the progeny of self-pollinated HMG1/hmg1 hmg2/hmg2 plants. hmg1 hmg2 male gametophytes appeared to be lethal based on crossing experiments, and microscopy indicated that approximately 50% of the microspores from the HMG1/hmg1 hmg2/hmg2 plant appeared shrunken and exhibited poorly defined endoplasmic reticulum membranes. In situ hybridization showed that HMG1 transcripts were expressed in both the tapetum and microspores, while HMG2 mRNA appeared only in microspores. It is concluded that native cross-talk from the plastid cannot compensate for complete blockage of the MVA pathway, at least during male gametophyte development, because either HMG1 or HMG2 is required for male gametophyte development.

IRE1, a component of the unfolded protein response signaling pathway, protects pollen development in Arabidopsis from heat stress

The unfolded protein response (UPR) is activated by various stresses during vegetative development in Arabidopsis, but is constitutively active in anthers of unstressed plants. To understand the role of the UPR during reproductive development, we analyzed a double mutant, ire1a ire1b. The double mutant knocks out the RNA-splicing arm of the UPR signaling pathway. It is fertile at room temperature but male sterile at modestly elevated temperature (ET). The conditional male sterility in the mutant is a sporophytic trait, and when the double mutant was grown at ET, defects appeared in the structure of the tapetum. As a result, the tapetum in the double mutant failed to properly deposit the pollen coat at ET, which made pollen grains clump and prevented their normal dispersal. IRE1 is a dual protein kinase/ribonuclease involved in the splicing of bZIP60 mRNA, and through complementation analysis of various mutant forms of IRE1b it was demonstrated that the ribonuclease activity of IRE1 was required for protecting male fertility from ET. It was also found that overexpression of SEC31A rescued the conditional male sterility in the double mutant. SEC31A is involved in trafficking from the endoplasmic reticulum to Golgi and a major target of the IRE1-mediated UPR signaling in stressed seedlings. Thus, IRE1, a major component of the UPR, plays an important role in protecting pollen development from ET.

Arabidopsis PLC2 is involved in auxin-modulated reproductive development

Phospholipase C (PLC) is an enzyme that plays crucial roles in various signal transduction pathways in mammalian cells. However, the role of PLC in plant development is poorly understood. Here we report involvement of PLC2 in auxin-mediated reproductive development in Arabidopsis. Disruption of PLC2 led to sterility, indicating a significant role for PLC2 in reproductive development. Development of both male and female gametophytes was severely perturbed in plc2 mutants. Moreover, elevated auxin levels were observed in plc2 floral tissues, suggesting that the infertility of plc2 plants may be associated with increased auxin concentrations in the reproductive organs. We show that expression levels of the auxin reporters DR5:GUS and DR5:GFP were elevated in plc2 anthers and ovules. In addition, we found that expression of the auxin biosynthetic YUCCA genes was increased in plc2 plants. We conclude that PLC2 is involved in auxin biosynthesis and signaling, thus modulating development of both male and female gametophytes in Arabidopsis.

The Long Journey of Pollen Tube in the Pistil

In non-cleistogamous plants, the male gametophyte, the pollen grain is immotile and exploits various agents, such as pollinators, wind, and even water, to arrive to a receptive stigma. The complex process of pollination involves a tubular structure, i.e., the pollen tube, which delivers the two sperm cells to the female gametophyte to enable double fertilization. The pollen tube has to penetrate the stigma, grow in the style tissues, pass through the septum, grow along the funiculus, and navigate to the micropyle of the ovule. It is a long journey for the pollen tube and its two sperm cells before they meet the female gametophyte, and it requires very accurate regulation to perform successful fertilization. In this review, we update the knowledge of molecular dialogues of pollen-pistil interaction, especially the progress of pollen tube activation and guidance, and give perspectives for future research.

Chemical Analysis of Pollen by FT-Raman and FTIR Spectroscopies

Pollen studies are important for the assessment of present and past environment, including biodiversity, sexual reproduction of plants and plant-pollinator interactions, monitoring of aeroallergens, and impact of climate and pollution on wild communities and cultivated crops. Although information on chemical composition of pollen is of importance in all of those research areas, pollen chemistry has been rarely measured due to complex and time-consuming analyses. Vibrational spectroscopies, coupled with multivariate data analysis, have shown great potential for rapid chemical characterization, identification and classification of pollen. This study, comprising 219 species from all principal taxa of seed plants, has demonstrated that high-quality Raman spectra of pollen can be obtained by Fourier transform (FT) Raman spectroscopy. In combination with Fourier transform infrared spectroscopy (FTIR), FT-Raman spectroscopy is obtaining comprehensive information on pollen chemistry. Presence of all the main biochemical constituents of pollen, such as proteins, lipids, carbohydrates, carotenoids and sporopollenins, have been identified and detected in the spectra, and the study shows approaches to measure relative and absolute content of these constituents. The results show that FT-Raman spectroscopy has clear advantage over standard dispersive Raman measurements, in particular for measurement of pollen samples with high pigment content. FT-Raman spectra are strongly biased toward chemical composition of pollen wall constituents, namely sporopollenins and pigments. This makes Raman spectra complementary to FTIR spectra, which over-represent chemical constituents of the grain interior, such as lipids and carbohydrates. The results show a large variability in pollen chemistry for families, genera and even congeneric species, revealing wide range of reproductive strategies, from storage of nutrients to variation in carotenoids and phenylpropanoids. The information on pollen's chemical patterns for major plant taxa should be of outstanding value for various studies in plant biology and ecology, including aerobiology, palaeoecology, forensics, community ecology, plant-pollinator interactions, and climate effects on plants.

The Role of Endoplasmic Reticulum Stress Response in Pollen Development and Heat Stress Tolerance

Endoplasmic reticulum (ER) stress is defined by a protracted disruption in protein folding and accumulation of unfolded or misfolded proteins in the ER. This accumulation of unfolded proteins can result from excessive demands on the protein folding machinery triggered by environmental and cellular stresses such as nutrient deficiencies, oxidative stress, pathogens, and heat. The cell responds to ER stress by activating a protective pathway termed unfolded protein response (UPR), which comprises cellular mechanisms targeted to maintain cellular homeostasis by increasing the ER's protein folding capacity. The UPR is especially significant for plants as being sessile requires them to adapt to multiple environmental stresses. While multiple stresses trigger the UPR at the vegetative stage, it appears to be active constitutively in the anthers of unstressed plants. Transcriptome analysis reveals significant upregulation of ER stress-related transcripts in diploid meiocytes and haploid microspores. Interestingly, several ER stress-related genes are specifically upregulated in the sperm cells. The analysis of gene knockout mutants in Arabidopsis has revealed that defects in ER stress response lead to the failure of normal pollen development and enhanced susceptibility of male gametophyte to heat stress conditions. In this mini-review, we provide an overview of the role of ER stress and UPR in pollen development and its protective roles in maintaining male fertility under heat stress conditions.

Pollen--tiny and ephemeral but not forgotten: New ideas on their ecology and evolution

Ecologists and evolutionary biologists have been interested in the functional biology of pollen since the discovery in the 1800s that pollen grains encompass tiny plants (male gametophytes) that develop and produce sperm cells. After the discovery of double fertilization in flowering plants, botanists in the early 1900s were quick to explore the effects of temperature and maternal nutrients on pollen performance, while evolutionary biologists began studying the nature of haploid selection and pollen competition. A series of technical and theoretic developments have subsequently, but usually separately, expanded our knowledge of the nature of pollen performance and how it evolves. Today, there is a tremendous diversity of interests that touch on pollen performance, ranging from the ecological setting on the stigma, structural and physiological aspects of pollen germination and tube growth, the form of pollen competition and its role in sexual selection in plants, virus transmission, mating system evolution, and inbreeding depression. Given the explosion of technical knowledge of pollen cell biology, computer modeling, and new methods to deal with diversity in a phylogenetic context, we are now more than ever poised for a new era of research that includes complex functional traits that limit or enhance the evolution of these deceptively simple organisms.

Pectin Synthesis and Pollen Tube Growth in Arabidopsis Involves Three GAUT1 Golgi-Anchoring Proteins: GAUT5, GAUT6, and GAUT7

The major cell wall pectic glycan homogalacturonan (HG) is crucial for plant growth, development, and reproduction. HG synthesis occurs in the Golgi and is catalyzed by members of the galacturonosyltransferase (GAUT) family with GAUT1 being the archetypal and best studied family member. In Arabidopsis suspension culture cells and tobacco leaves, the Golgi localization of Arabidopsis GAUT1 has been shown to require protein-protein interactions with its homolog GAUT7. Here we show that in pollen tubes GAUT5 and GAUT6, homologs of GAUT7, also target GAUT1 to the Golgi apparatus. Pollen tube germination and elongation in double homozygous knock-out mutants (gaut5 gaut6, gaut5 gaut7, and gaut6 gaut7) are moderately impaired, whereas gaut5 ^-/- gaut6 ^-/- gaut7 ^+/- triple mutant is severely impaired and male infertile. Amounts and distributions of methylesterified HG in the pollen tube tip were severely distorted in the double and heterozygous triple mutants. A chimeric protein comprising GAUT1 and a non-cleavable membrane anchor domain was able to partially restore pollen tube germination and elongation and to reverse male sterility in the triple mutant. These results indicate that GAUT5, GAUT6, and GAUT7 are required for synthesis of native HG in growing pollen tubes and have critical roles in pollen tube growth and male fertility in Arabidopsis.

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.

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.

Diverse ecological relations of male gametophyte populations in stylar environments

PREMISE OF STUDY

Pollen on a stigma represents a local population of male gametophytes vying for access to female gametophytes in the associated ovary. As in most populations, density-independent and density-dependent survival depend on intrinsic characteristics of male gametophytes and environmental (pistil) conditions. These characteristics and conditions could differ among flowers, plants, populations, and species, creating diverse male-gametophyte population dynamics, which can influence seed siring and production.

METHODS

For nine species, we characterized the relations of both the mean and standard deviation of pollen-tube number at the style base to pollen receipt with nonlinear regression. Models represented asymptotic or peaked relations, providing information about the incidence and magnitude of facilitation and competition, the spatial and temporal characteristics of competition, and the intensity and relative timing of density-independent mortality.

KEY RESULTS

We infer that pollen tubes of most species competed sequentially, their tips ceasing growth if earlier tubes had depleted stylar space/resources; although two species experienced simultaneous competition. Tube success of three species revealed positive density dependence (facilitation) at low density. For at least four species, density-independent mortality preceded competition. Tube success varied mostly within plants, rather than among plants or conspecific populations. Pollen quality influenced tube success for two of three species; affecting density-independent survival in one and density-dependent performance in the other.

CONCLUSIONS

The diverse relations of pollen-tube success to pollen receipt evident among just nine species indicate significant contributions of the processes governing pollen germination and tube growth to the reproductive diversity of angiosperms.

The essential role of the phosphorylated pathway of serine biosynthesis in Arabidopsis

In plants, 3 different pathways of serine biosynthesis have been described: the Glycolate pathway, which is associated with photorespiration, and 2 non-photorespiratory pathways, the Glycerate and the Phosphorylated pathways. The Phosphorylated Pathway of Serine Biosynthesis (PPSB) has been known since the 1950s, but has been studied relatively little, probably because it was considered of minor significance as compared with the Glycolate pathway. In the associated study (1), we described for the first time in plants the in vivo functional characterization of the PPSB, by targeting the phosphoserine phosphatase (PSP1), the last enzyme of the pathway. Following a gain- and loss-of-function approach in Arabidopsis, we provided genetic and molecular evidence for the essential role of PSP1 for embryo and pollen development, and for proper root growth. A metabolomics study indicated that the PPSB affects glycolysis, the Krebs cycle, and the biosynthesis of several amino acids, which suggests that this pathway is an important link connecting metabolism and development. The mechanisms underlying the essential functions of PSP1 are discussed.

Elimination of Viroids from Tobacco Pollen Involves a Decrease in Propagation Rate and an Increase of the Degradation Processes

Some viroids-single-stranded, non-coding, circular RNA parasites of plants-are not transmissible through pollen to seeds and to next generation. We analyzed the cause for the elimination of apple fruit crinkle viroid (AFCVd) and citrus bark cracking viroid (CBCVd) from male gametophyte cells of Nicotiana tabacum by RNA deep sequencing and molecular methods using infected and transformed tobacco pollen tissues at different developmental stages. AFCVd was not transferable from pollen to seeds in reciprocal pollinations, due to a complete viroid eradication during the last steps of pollen development and fertilization. In pollen, the viroid replication pathway proceeds with detectable replication intermediates, but is dramatically depressed in comparison to leaves. Specific and unspecific viroid degradation with some preference for (-) chains occurred in pollen, as detected by analysis of viroid-derived small RNAs, by quantification of viroid levels and by detection of viroid degradation products forming "comets" on Northern blots. The decrease of viroid levels during pollen development correlated with mRNA accumulation of several RNA-degrading factors, such as AGO5 nuclease, DICER-like and TUDOR S-like nuclease. In addition, the functional status of pollen, as a tissue with high ribosome content, could play a role during suppression of AFCVd replication involving transcription factors IIIA and ribosomal protein L5.

For things to stay the same, things must change: polyploidy and pollen tube growth rates

BACKGROUND AND AIMS

Pollen tube growth rate (PTGR) is an important single-cell performance trait that may evolve rapidly under haploid selection. Angiosperms have experienced repeated cycles of polyploidy (whole genome duplication), and polyploidy has cell-level phenotypic consequences arising from increased bulk DNA amount and numbers of genes and their interactions. We sought to understand potential effects of polyploidy on several underlying determinants of PTGR - pollen tube dimensions and construction rates - by comparing diploid-polyploid near-relatives in Betula (Betulaceae) and Handroanthus (Bignoniaceae).

METHODS

We performed intraspecific, outcrossed hand-pollinations on pairs of flowers. In one flower, PTGR was calculated from the longest pollen tube per time of tube elongation. In the other, styles were embedded in glycol methacrylate, serial-sectioned in transverse orientation, stained and viewed at 1000× to measure tube wall thicknesses (W) and circumferences (C). Volumetric growth rate (VGR) and wall production rate (WPR) were then calculated for each tube by multiplying cross-sectional tube area (πr2) or wall area (W × C), by the mean PTGR of each maternal replicate respectively.

KEY RESULTS

In Betula and Handroanthus, the hexaploid species had significantly wider pollen tubes (13 and 25 %, respectively) and significantly higher WPRs (22 and 18 %, respectively) than their diploid congeners. PTGRs were not significantly different in both pairs, even though wider polyploid tubes were predicted to decrease PTGRs by 16 and 20 %, respectively.

CONCLUSIONS

The larger tube sizes of polyploids imposed a substantial materials cost on PTGR, but polyploids also exhibited higher VGRs and WPRs, probably reflecting the evolution of increased metabolic activity. Recurrent cycles of polyploidy followed by genome reorganization may have been important for the evolution of fast PTGRs in angiosperms, involving a complex interplay between correlated changes in ploidy level, genome size, cell size and pollen tube energetics.

Establishment of the male germline and sperm cell movement during pollen germination and tube growth in maize

Two sperm cells are required to achieve double fertilization in flowering plants (angiosperms). In contrast to animals and lower plants such as mosses and ferns, sperm cells of flowering plants (angiosperms) are immobile and are transported to the female gametes (egg and central cell) via the pollen tube. The two sperm cells arise from the generative pollen cell either within the pollen grain or after germination inside the pollen tube. While pollen tube growth and sperm behaviour has been intensively investigated in model plant species such as tobacco and lily, little is know about sperm dynamics and behaviour during pollen germination, tube growth and sperm release in grasses. In the March issue of Journal of Experimental Botany, we have reported about the sporophytic and gametophytic control of pollen tube germination, growth and guidance in maize.1 Five progamic phases were distinguished involving various prezygotic crossing barriers before sperm cell delivery inside the female gametophyte takes place. Using live cell imaging and a generative cell-specific promoter driving α-tubulin-YFP expression in the male germline, we report here the formation of the male germline inside the pollen grain and the sperm behaviour during pollen germination and their movement dynamics during tube growth in maize.

An Unexplored Side of Regeneration Niche: Seed Quantity and Quality Are Determined by the Effect of Temperature on Pollen Performance

In 1977, Peter Grubb introduced the regeneration niche concept, which assumes that a plant species cannot persist if the environmental conditions are only suitable for adult plant growth and survival, but not for seed production, dispersal, germination, and seedling establishment. During the last decade, this concept has received considerable research attention as it helps to better understand community assembly, population dynamics, and plant responses to environmental changes. Yet, in its present form, it focuses too much on the post-fertilization stages of plant sexual reproduction, neglecting the fact that the environment can operate as a constraint at many points in the chain of processes necessary for successful regeneration. In this review, we draw the attention of the plant ecology research community to the pre-fertilization stages of plant sexual reproduction, an almost ignored but important aspect of the regeneration niche, and their potential consequences for successful seed production. Particularly, we focus on how temperature affects pollen performance and determines plant reproduction success by playing an important role in the temporal and spatial variations in seed quality and quantity. We also review the pollen adaptations to temperature stresses at different levels of plant organization and discuss the plasticity of the performance of pollen under changing temperature conditions. The reviewed literature demonstrates that pre-fertilization stages of seed production, particularly the extreme sensitivity of male gametophyte performance to temperature, are the key determinants of a species' regeneration niche. Thus, we suggest that previous views stating that the regeneration niche begins with the production of seeds should be modified to include the preceding stages. Lastly, we identify several gaps in pollen-related studies revealing a framework of opportunities for future research, particularly how these findings could be used in the field of plant biology and ecology.