Male gametophyte development: a molecular perspective

Calreticulin expression and localization in relation to exchangeable Ca2+ during pollen development in Petunia

BACKGROUND

Pollen development in the anther in angiosperms depends on complicated cellular interactions associated with the expression of gametophytic and sporophytic genes which control fundamental processes during microsporo/gametogenesis, such as exo/endocytosis, intracellular transport, cell signaling, chromatin remodeling, and cell division. Most if not all of these cellular processes depend of local concentration of calcium ions (Ca²⁺). Work from our laboratory and others provide evidence that calreticulin (CRT), a prominent Ca²⁺-binding/buffering protein in the endoplasmic reticulum (ER) of eukaryotic cells, may be involved in pollen formation and function. Here, we show for the first time the expression pattern of the PhCRT1 gene and CRT accumulation in relation to exchangeable Ca²⁺ in Petunia hybrida developing anther, and discuss probable roles for this protein in the male gametophyte development.

RESULTS

Using northern hybridization, western blot analysis, fluorescent in situ hybridization (FISH), immunocytochemistry, and potassium antimonate precipitation, we report that PhCRT1 is highly expressed in the anther and localization pattern of the CRT protein correlates with loosely bound (exchangeable) Ca²⁺ during the successive stages of microsporo/gametogenesis. We confirmed a permanent presence of both CRT and exchangeable Ca²⁺ in the germ line and tapetal cells, where these factors preferentially localized to the ER which is known to be the most effective intracellular Ca²⁺ store in eukaryotic cells. In addition, our immunoblots revealed a gradual increase in CRT level from the microsporocyte stage through the meiosis and the highest CRT level at the microspore stage, when both microspores and tapetal cells show extremely high secretory activity correlated with the biogenesis of the sporoderm.

CONCLUSION

Our present data provide support for a key role of CRT in developing anther of angiosperms - regulation of Ca²⁺ homeostasis during pollen grains formation. This Ca²⁺-buffering chaperone seems to be essential for pollen development and maturation since a high rate of protein synthesis and protein folding within the ER as well as intracellular Ca²⁺ homeostasis are strictly required during the multi-step process of pollen development.

Holistic insights from pollen omics: co-opting stress-responsive genes and ER-mediated proteostasis for male fertility

Sexual reproduction in flowering plants takes place without an aqueous environment. Sperm are carried by pollen through air to reach the female gametophyte, though the molecular basis underlying the protective strategy of the male gametophyte is poorly understood. Here we compared the published transcriptomes of Arabidopsis thaliana pollen, and of heat-responsive genes, and uncovered insights into how mature pollen (MP) tolerates desiccation, while developing and germinating pollen are vulnerable to heat stress. Germinating pollen expresses molecular chaperones or "heat shock proteins" in the absence of heat stress. Furthermore, pollen tubes that grew through pistils at basal temperature showed induction of the endoplasmic reticulum (ER) stress response, which is a characteristic of stressed vegetative tissues. Recent studies show MP contains mRNA-protein (mRNP) aggregates that resemble "stress" granules triggered by heat or other stresses to protect cells. Based on these observations, we postulate that mRNP particles are formed in maturing pollen in response to developmentally programmed dehydration. Dry pollen can withstand harsh conditions as it is dispersed in air. We propose that, when pollen lands on a compatible pistil and hydrates, mRNAs stored in particles are released, aided by molecular chaperones, to become translationally active. Pollen responds to osmotic, mechanical, oxidative, and peptide cues that promote ER-mediated proteostasis and membrane trafficking for tube growth and sperm discharge. Unlike vegetative tissues, pollen depends on stress-protection strategies for its normal development and function. Thus, heat stress during reproduction likely triggers changes that interfere with the normal pollen responses, thereby compromising male fertility. This holistic perspective provides a framework to understand the basis of heat-tolerant strains in the reproduction of crops.

Arabidopsis ADP-RIBOSYLATION FACTOR-A1s mediate tapetum-controlled pollen development

Propagation of angiosperms mostly relies on sexual reproduction, in which gametophytic development is a pre-requisite. Male gametophytic development requires both gametophytic and sporophytic factors, most importantly early secretion and late programmed cell death of the tapetum. In addition to transcriptional factors, proteins at endomembrane compartments, such as receptor-like kinases and vacuolar proteases, control tapetal function. The cellular machinery that regulates their distribution is beginning to be revealed. We report here that ADP-RIBOSYLATION FACTOR-A1s (ArfA1s) are critical for tapetum-controlled pollen development. All six ArfA1s in the Arabidopsis genome are expressed during anther development, among which ArfA1b is specific to the tapetum and developing microspores. Although the ArfA1b loss-of-function mutant showed no pollen defects, probably due to redundancy, interference with ArfA1s by a dominant negative approach in the tapetum resulted in tapetal dysfunction and pollen abortion. We further showed that all six ArfA1s are associated with the Golgi and the trans-Golgi network/early endosome, suggesting that they have roles in regulating post-Golgi trafficking to the plasma membrane or to vacuoles. Indeed, we demonstrated that the expression of ArfA1b^DN interfered with the targeting of proteins critical for tapetal development. The results presented here demonstrate a key role of ArfA1s in tapetum-controlled pollen development by mediating protein targeting through post-Golgi trafficking routes.

The impact of stress combination on reproductive processes in crops

Historically, extended droughts combined with heat waves caused severe reductions in crop yields estimated at billions of dollars annually. Because global warming and climate change are driving an increase in the frequency and intensity of combined water-deficit and heat stress episodes, understanding how these episodes impact yield is critical for our efforts to develop climate change-resilient crops. Recent studies demonstrated that a combination of water-deficit and heat stress exacerbates the impacts of water-deficit or heat stress on reproductive processes of different cereals and legumes, directly impacting grain production. These studies identified several different mechanisms potentially underlying the effects of stress combination on anthers, pollen, and stigma development and function, as well as fertilization. Here we review some of these findings focusing on unbalanced reactive oxygen accumulation, altered sugar concentrations, and conflicting functions of different hormones, as contributing to the reduction in yield during a combination of water-deficit and heat stress. Future studies focused on the effects of water-deficit and heat stress combination on reproduction of different crops are likely to unravel additional mechanisms, as well as reveal novel ways to develop stress combination-resilient crops. These could mitigate some of the potentially devastating impacts of this stress combination on agriculture.

RH17 restricts reproductive fate and represses autonomous seed coat development in sexual Arabidopsis

Plant sexual and asexual reproduction through seeds (apomixis) is tightly controlled by complex gene regulatory programs, which are not yet fully understood. Recent findings suggest that RNA helicases are required for plant germline development. This resembles their crucial roles in animals, where they are involved in controlling gene activity and the maintenance of genome integrity. Here, we identified previously unknown roles of Arabidopsis RH17 during reproductive development. Interestingly, RH17 is involved in repression of reproductive fate and of elements of seed development in the absence of fertilization. In lines carrying a mutant rh17 allele, development of supernumerary reproductive cell lineages in the female flower tissues (ovules) was observed, occasionally leading to formation of two embryos per seed. Furthermore, seed coat, and putatively also endosperm development, frequently initiated autonomously. Such induction of several features phenocopying distinct elements of apomixis by a single mutation is unusual and suggests that RH17 acts in regulatory control of plant reproductive development. Furthermore, an in-depth understanding of its action might be of use for agricultural applications.

A dynamic intron retention program regulates the expression of several hundred genes during pollen meiosis

Intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during early stages of pollen development. To improve our understanding of the gene regulatory mechanisms that drive developmental processes, we performed a genome-wide study of alternative splicing and isoform switching during five key stages of pollen development in field mustard, Brassica rapa. Surprisingly, for several hundred genes (12.3% of the genes analysed), isoform switching results in stage-specific expression of intron-retaining transcripts at the meiotic stage of pollen development. In such cases, we report temporally regulated switching between expression of a canonical, translatable isoform and an intron-retaining transcript that is predicted to produce a truncated and presumably inactive protein. The results suggest a new pervasive mechanism underlying modulation of protein levels in a plant developmental program. The effect is not based on gene expression induction but on the type of transcript produced. We conclude that intron retention is a stage-specific mechanism of functional attenuation of a subset of co-regulated, functionally related genes during meiosis, especially genes related to ribosome biogenesis, mRNA transport and nuclear envelope architecture. We also propose that stage-specific expression of a non-functional isoform of Brassica rapa BrSDG8, a non-redundant member of histone methyltransferase gene family, linked to alternative splicing regulation, may contribute to the intron retention observed.

Proline-rich extensin-like receptor kinases PERK5 and PERK12 are involved in pollen tube growth

Proline-rich extensin-like receptor kinases (PERKs) belong to the hydroxyproline-rich glycoprotein (HRGP) superfamily known to be involved in many plant developmental processes. Here, we characterized two pollen-expressed PERKs from Arabidopsis thaliana, PERK5 and PERK12. Pollen tube growth was impaired in single and double perk5-1 perk12-1 loss of function mutants, with an impact on seed production. When the segregation was analysed, a male gametophytic defect was found, indicating that perk5-1 and perk12-1 mutants carry deficient pollen transmission. Furthermore, perk5-1 perk12-1 displayed an excessive accumulation of pectins and cellulose at the cell wall of the pollen tubes. Our results indicate that PERK5 and PERK12 are necessary for proper pollen tube growth, highlighting their role in cell wall assembly and reactive oxygen species homeostasis.

Sporophytic control of anther development and male fertility by glucose-6-phosphate/phosphate translocator 1 (OsGPT1) in rice

Coordination between the sporophytic tissue and the gametic pollen within anthers is tightly controlled to achieve the optimal pollen fitness. Glucose-6-phosphate/phosphate translocator (GPT) transports glucose-6-phosphate, a key precursor of starch and/or fatty acid biosynthesis, into plastids. Here, we report the functional characterization of OsGPT1 in the rice anther development and pollen fertility. Pollen grains from homozygous osgpt1 mutant plants fail to accumulate starch granules, resulting in pollen sterility. Genetic analyses reveal a sporophytic effect for this mutation. OsGPT1 is highly expressed in the tapetal layer of rice anther. Degeneration of the tapetum, an important process to provide cellular contents to support pollen development, is impeded in osgpt1 plants. In addition, defective intine and exine are observed in the pollen from osgpt1 plants. Expression levels of multiple genes that are important to tapetum degeneration or pollen wall formation are significantly decreased in osgpt1 anthers. Previously, we reported that AtGPT1 plays a gametic function in the accumulation of lipid bodies in Arabidopsis pollen. This report highlights a sporophytic role of OsGPT1 in the tapetum degeneration and pollen development. The divergent functions of OsGPT1 and AtGPT1 in pollen development might be a result of their independent evolution after monocots and dicots diverged.

Morpho-Physiological and Transcriptome Changes in Tomato Anthers of Different Developmental Stages under Drought Stress

Drought limits the growth and productivity of plants. Reproductive development is sensitive to drought but the underlying physiological and molecular mechanisms remain unclear in tomatoes. Here, we investigated the effect of drought on tomato floral development using morpho-physiological and transcriptome analyses. Drought-induced male sterility through abnormal anther development includes pollen abortion, inadequate pollen starch accumulation and anther indehiscence which caused floral bud and opened flower abortions and reduced fruit set/yield. Under drought stress (DS), pollen mother cell to meiotic (PMC-MEI) anthers survived whereas tetrad to vacuolated uninucleate microspore (TED-VUM) anthers aborted. PMC-MEI anthers had lower ABA increase, reduced IAA and elevated sugar contents under DS relative to well-watered tomato plants. However, TED-VUM anthers had higher ABA increase and IAA levels, and lower accumulation of soluble sugars, indicating abnormal carbohydrate and hormone metabolisms when exposed to drought-stress conditions. Moreover, RNA-Seq analysis identified altogether >15,000 differentially expressed genes that were assigned to multiple pathways, suggesting that tomato anthers utilize complicated mechanisms to cope with drought. In particular, we found that tapetum development and ABA homeostasis genes were drought-induced while sugar utilization and IAA metabolic genes were drought-repressed in PMC-MEI anthers. Our results suggest an important role of phytohormones metabolisms in anther development under DS and provide novel insight into the molecular mechanism underlying drought resistance in tomatoes.

Pollen development in cotton (Gossypium hirsutum) is highly sensitive to heat exposure during the tetrad stage

The development of gametes in plants is acutely susceptible to heatwaves as brief as a few days, adversely affecting pollen maturation and reproductive success. Pollen in cotton (Gossypium hirsutum) was differentially affected when tetrad and binucleate stages were exposed to heat, revealing new insights into the interaction between heat and pollen development. Squares were tagged and exposed to 36/25°C (day/night, moderate heat) or 40/30°C (day/night, extreme heat) for 5 days. Mature pollen grains and leaves were collected for physiological and proteomic responses. While photosynthetic competence was not compromised even at 40°C, leaf tissues became leakier. In contrast, pollen grains were markedly smaller after the tetrad stage was exposed to 40°C and boll production was reduced by 65%. Sugar levels in pollen grains were elevated after exposure to heat, eliminating carbohydrate deficits as a likely cause of poor reproductive capacity. Proteomic analysis of pure pollen samples revealed a particularly high abundance of 70-kDa heat shock (Hsp70s) and cytoskeletal proteins. While short-term bursts of heat had a minor impact on leaves, male gametophyte development was profoundly damaged. Cotton acclimates to maxima of 36°C at both the vegetative and reproductive stages but 5-days exposure to 40°C significantly impairs reproductive development.

MircroRNA Profiles of Early Rice Inflorescence Revealed a Specific miRNA5506 Regulating Development of Floral Organs and Female Megagametophyte in Rice

The developmental process of inflorescence and gametophytes is vital for sexual reproduction in rice. Multiple genes and conserved miRNAs have been characterized to regulate the process. The changes of miRNAs expression during the early development of rice inflorescence remain unknown. In this study, the analysis of miRNAs profiles in the early stage of rice inflorescence development identified 671 miRNAs, including 67 known and 44 novel differentially expressed miRNAs (DEMs). Six distinct clusters of miRNAs expression patterns were detected, and Cluster 5 comprised 110 DEMs, including unconserved, rice-specific osa-miR5506. Overexpression of osa-miR5506 caused pleiotropic abnormalities, including over- or under-developed palea, various numbers of floral organs and spikelet indeterminacy. In addition, the defects of ovaries development were frequently characterized by multiple megasporocytes, ovule-free ovary, megasporocyte degenerated and embryo sac degenerated in the transgenic lines. osa-miR5506 targeted REM transcription factor LOC_Os03g11370. Summarily, these results demonstrated that rice-specific osa-miR5506 plays an essential role in the regulation of floral organ number, spikelet determinacy and female gametophyte development in rice.

Proteomic Analysis of Generative and Vegetative Nuclei Reveals Molecular Characteristics of Pollen Cell Differentiation in Lily

In plants, the cell fates of a vegetative cell (VC) and generative cell (GC) are determined after the asymmetric division of the haploid microspore. The VC exits the cell cycle and grows a pollen tube, while the GC undergoes further mitosis to produce two sperm cells for double fertilization. However, our understanding of the mechanisms underlying their fate differentiation remains limited. One major advantage of the nuclear proteome analysis is that it is the only method currently able to uncover the systemic differences between VC and GC due to GC being engulfed within the cytoplasm of VC, limiting the use of transcriptome. Here, we obtained pure preparations of the vegetative cell nuclei (VNs) and generative cell nuclei (GNs) from germinating lily pollens. Utilizing these high-purity VNs and GNs, we compared the differential nucleoproteins between them using state-of-the-art quantitative proteomic techniques. We identified 720 different amount proteins (DAPs) and grouped the results in 11 fate differentiation categories. Among them, we identified 29 transcription factors (TFs) and 10 cell fate determinants. Significant differences were found in the molecular activities of vegetative and reproductive nuclei. The TFs in VN mainly participate in pollen tube development. In comparison, the TFs in GN are mainly involved in cell differentiation and male gametogenesis. The identified novel TFs may play an important role in cell fate differentiation. Our data also indicate differences in nuclear pore complexes and epigenetic modifications: more nucleoporins synthesized in VN; more histone variants and chaperones; and structural maintenance of chromosome (SMC) proteins, chromatin remodelers, and DNA methylation-related proteins expressed in GN. The VC has active macromolecular metabolism and mRNA processing, while GC has active nucleic acid metabolism and translation. Moreover, the members of unfolded protein response (UPR) and programmed cell death accumulate in VN, and DNA damage repair is active in GN. Differences in the stress response of DAPs in VN vs. GN were also found. This study provides a further understanding of pollen cell differentiation mechanisms and also a sound basis for future studies of the molecular mechanisms behind cell fate differentiation.

The Arabidopsis R-SNARE protein YKT61 is essential for gametophyte development

Gametophyte development is a pre-requisite for plant reproduction and seed yield; therefore, studies of gametophyte development help us understand fundamental biological questions and have potential applications in agriculture. The biogenesis and dynamics of endomembrane compartments are critical for cell survival, and their regulatory mechanisms are just beginning to be revealed. Here, we report that the Arabidopsis thaliana SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) protein YKT61 is essential for both male and female gametogenesis. By using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-based genome editing, we demonstrated that male and female gametophytes carrying YKT61 loss-of-function alleles do not survive. Specifically, loss of YKT61 function resulted in the arrest of male gametophytic development at pollen mitosis I and the degeneration of female gametophytes. A three-base-pair deletion in YKT61 in the ykt61-3 mutant resulted in a single-amino acid deletion in the longin domain of YKT61; the resulting mutant protein does not interact with multiple SNAREs and showed substantially reduced membrane association, suggesting that the N-terminal longin domain of YKT61 plays multiple roles in its function. This study demonstrates that Arabidopsis YKT61 is essential for male and female gametogenesis and sets an example for functional characterization of essential genes with the combination of Cas9-mediated editing and expression from a Cas9-resistant transgene.

An L-type lectin receptor-like kinase promotes starch accumulation during rice pollen maturation

Starch accumulation is key for the maturity of rice pollen grains; however, the regulatory mechanism underlying this process remains unknown. Here, we have isolated a male-sterile rice mutant, abnormal pollen 1 (ap1), which produces nonviable pollen grains with defective starch accumulation. Functional analysis revealed that AP1 encodes an active L-type lectin receptor-like kinase (L-LecRLK). AP1 is localized to the plasma membrane and its transcript is highly accumulated in pollen during the starch synthesis phase. RNA-seq and phosphoproteomic analysis revealed that the expression/phosphorylation levels of numerous genes/proteins involved in starch and sucrose metabolism pathway were significantly altered in the mutant pollen, including a known rice UDP-glucose pyrophosphorylase (OsUGP2). We further found that AP1 physically interacts with OsUGP2 to elevate its enzymatic activity, likely through targeted phosphorylation. These findings revealed a novel role of L-LecRLK in controlling pollen maturity via modulating sucrose and starch metabolism.

Male gametophyte development in flowering plants: A story of quarantine and sacrifice

Over 160 years ago, scientists made the first microscopic observations of angiosperm pollen. Unlike in animals, male meiosis in angiosperms produces a haploid microspore that undergoes one asymmetric division to form a vegetative cell and a generative cell. These two cells have distinct fates: the vegetative cell exits the cell cycle and elongates to form a tip-growing pollen tube; the generative cell divides once more in the pollen grain or within the growing pollen tube to form a pair of sperm cells. The concept that male germ cells are less active than the vegetative cell came from biochemical analyses and pollen structure anatomy early in the last century and is supported by the pollen transcriptome data of the last decade. However, the mechanism of how and when the transcriptional repression in male germ cells occurs is still not fully understood. In this review, we provide a brief account of the cytological and metabolic differentiation between the vegetative cell and male germ cells, with emphasis on the role of temporary callose walls, dynamic nuclear pore density, transcription repression, and histone variants. We further discuss the intercellular movement of small interfering RNA (siRNA) derived from transposable elements (TEs) and reexamine the function of TE expression in male germ cells.

Implications of ionizing radiation on pollen performance in comparison with diverse models of polar cell growth

Research concerning the effects of ionizing radiation (IR) on plant systems is essential for numerous aspects of human society, as for instance, in terms of agriculture and plant breeding, but additionally for elucidating consequences of radioactive contamination of the ecosphere. This comprehensive survey analyses effects of x- and γ-irradiation on male gametophytes comprising primarily in vitro but also in vivo data of diverse plant species. The IR-dose range for pollen performance was compiled and 50% inhibition doses (ID₅₀ ) for germination and tube growth were comparatively related to physiological characteristics of the microgametophyte. Factors influencing IR-susceptibility of mature pollen and polarized tube growth were evaluated, such as dose-rate, environmental conditions, or species-related variations. In addition, all available reports suggesting bio-positive IR-effects particularly on pollen performance were examined. Most importantly, for the first time influences of IR specifically on diverse phylogenetic models of polar cell growth were comparatively analysed, and thus demonstrated that the gametophytic system of pollen is extremely resistant to IR, more than plant sporophytes and especially much more than comparable animal cells. Beyond that, this study develops hypotheses regarding a molecular basis for the extreme IR-resistance of the plant microgametophyte and highlights its unique rank among organismal systems.

Epigenetic reprogramming rewires transcription during the alternation of generations in Arabidopsis

Alternation between morphologically distinct haploid and diploid life forms is a defining feature of most plant and algal life cycles, yet the underlying molecular mechanisms that govern these transitions remain unclear. Here, we explore the dynamic relationship between chromatin accessibility and epigenetic modifications during life form transitions in Arabidopsis. The diploid-to-haploid life form transition is governed by the loss of H3K9me2 and DNA demethylation of transposon-associated cis-regulatory elements. This event is associated with dramatic changes in chromatin accessibility and transcriptional reprogramming. In contrast, the global loss of H3K27me3 in the haploid form shapes a chromatin accessibility landscape that is poised to re-initiate the transition back to diploid life after fertilisation. Hence, distinct epigenetic reprogramming events rewire transcription through major reorganisation of the regulatory epigenome to guide the alternation of generations in flowering plants.

Genome-wide identification of GH3 genes in Brassica oleracea and identification of a promoter region for anther-specific expression of a GH3 gene

Background

The Gretchen Hagen 3 (GH3) genes encode acyl acid amido synthetases, many of which have been shown to modulate the amount of active plant hormones or their precursors. GH3 genes, especially Group III subgroup 6 GH3 genes, and their expression patterns in economically important B. oleracea var. oleracea have not been systematically identified.

Results

As a first step to understand regulation and molecular functions of Group III subgroup 6 GH3 genes, 34 GH3 genes including four subgroup 6 genes were identified in B. oleracea var. oleracea. Synteny found around subgroup 6 GH3 genes in B. oleracea var. oleracea and Arabidopsis thaliana indicated that these genes are evolutionarily related. Although expression of four subgroup 6 GH3 genes in B. oleracea var. oleracea is not induced by auxin, gibberellic acid, or jasmonic acid, the genes show different organ-dependent expression patterns. Among subgroup 6 GH3 genes in B. oleracea var. oleracea, only BoGH3.13–1 is expressed in anthers when microspores, polarized microspores, and bicellular pollens are present, similar to two out of four syntenic A. thaliana subgroup 6 GH3 genes. Detailed analyses of promoter activities further showed that BoGH3.13–1 is expressed in tapetal cells and pollens in anther, and also expressed in leaf primordia and floral abscission zones.

Conclusions

Sixty-two base pairs (bp) region (− 340 ~ − 279 bp upstream from start codon) and about 450 bp region (− 1489 to − 1017 bp) in BoGH3.13–1 promoter are important for expressions in anther and expressions in leaf primordia and floral abscission zones, respectively. The identified anther-specific promoter region can be used to develop male sterile transgenic Brassica plants.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07345-9.

Potential Association of Reactive Oxygen Species With Male Sterility in Peach

Male sterility is an important agronomic trait for hybrid vigor utilization and hybrid seed production, but its underlying mechanisms remain to be uncovered. Here, we investigated the mechanisms of male sterility in peach using a combined cytology, physiology, and molecular approach. Cytological features of male sterility include deformed microspores and tapetum cells along with absence of pollen grains. Microspores had smaller nucleus at the mononuclear stage and were compressed into belts and subsequently disappeared in the anther cavity, whereas tapetum cells were swollen and vacuolated, with a delayed degradation to flowering time. Male sterile anthers had an ROS burst and lower levels of major antioxidants, which may cause abnormal development of microspores and tapetum, leading to male sterility in peach. In addition, the male sterility appears to be cytoplasmic in peach, which could be due to sequence variation in the mitochondrial genome. Our results are helpful for further investigation of the genetic mechanisms underlying male sterility in peach.

LINC-complex mediated positioning of the vegetative nucleus is involved in calcium and ROS signaling in Arabidopsis pollen tubes

Nuclear movement and positioning play a role in developmental processes throughout life. Nuclear movement and positioning are mediated primarily by linker of nucleoskeleton and cytoskeleton (LINC) complexes. LINC complexes are comprised of the inner nuclear membrane SUN proteins and the outer nuclear membrane (ONM) KASH proteins. In Arabidopsis pollen tubes, the vegetative nucleus (VN) maintains a fixed distance from the pollen tube tip during growth, and the VN precedes the sperm cells (SCs). In pollen tubes of wit12 and wifi, mutants deficient in the ONM component of a plant LINC complex, the SCs precede the VN during pollen tube growth and the fixed VN distance from the tip is lost. Subsequently, pollen tubes frequently fail to burst upon reception. In this study, we sought to determine if the pollen tube reception defect observed in wit12 and wifi is due to decreased sensitivity to reactive oxygen species (ROS). Here, we show that wit12 and wifi are hyposensitive to exogenous H₂O₂, and that this hyposensitivity is correlated with decreased proximity of the VN to the pollen tube tip. Additionally, we report the first instance of nuclear Ca²⁺ peaks in growing pollen tubes, which are disrupted in the wit12 mutant. In the wit12 mutant, nuclear Ca²⁺ peaks are reduced in response to exogenous ROS, but these peaks are not correlated with pollen tube burst. This study finds that VN proximity to the pollen tube tip is required for both response to exogenous ROS, as well as internal nuclear Ca²⁺ fluctuations.

Loss of Small-RNA-Directed DNA Methylation in the Plant Cell Cycle Promotes Germline Reprogramming and Somaclonal Variation

5-methyl cytosine is widespread in plant genomes in both CG and non-CG contexts. During replication, hemi-methylation on parental DNA strands guides symmetric CG methylation on nascent strands, but non-CG methylation requires modified histones and small RNA guides. Here, we used immortalized Arabidopsis cell suspensions to sort replicating nuclei and determine genome-wide cytosine methylation dynamics during the plant cell cycle. We find that symmetric mCG and mCHG are selectively retained in actively dividing cells in culture, whereas mCHH is depleted. mCG becomes transiently asymmetric during S phase but is rapidly restored in G2, whereas mCHG remains asymmetric throughout the cell cycle. Hundreds of loci gain ectopic CHG methylation, as well as 24-nt small interfering RNAs (siRNAs) and histone H3 lysine dimethylation (H3K9me2), without gaining CHH methylation. This suggests that spontaneous epialleles that arise in plant cell cultures are stably maintained by siRNA and H3K9me2 independent of the canonical RNA-directed DNA methylation (RdDM) pathway. In contrast, loci that fail to produce siRNA may be targeted for demethylation when the cell cycle arrests. Comparative analysis with methylomes of various tissues and cell types suggests that loss of small-RNA-directed non-CG methylation during DNA replication promotes germline reprogramming and epigenetic variation in plants propagated as clones.

Recent advances in understanding the biological roles of the plant nuclear envelope

The functional organization of the plant nuclear envelope is gaining increasing attention through new connections made between nuclear envelope-associated proteins and important plant biological processes. Animal nuclear envelope proteins play roles in nuclear morphology, nuclear anchoring and movement, chromatin tethering and mechanical signaling. However, how these roles translate to functionality in a broader biological context is often not well understood. A surprising number of plant nuclear envelope-associated proteins are plant-unique, suggesting that separate functionalities evolved after the split of Opisthokonta and Streptophyta. Significant progress has now been made in discovering broader biological roles of plant nuclear envelope proteins, increasing the number of known plant nuclear envelope proteins, and connecting known proteins to chromatin organization, gene expression, and the regulation of nuclear calcium. The interaction of viruses with the plant nuclear envelope is another emerging theme. Here, we survey the recent developments in this still relatively new, yet rapidly advancing field.

The Importance of Cytokinins during Reproductive Development in Arabidopsis and Beyond

Fertilization and seed formation are fundamental events in the life cycle of flowering plants. The seed is a functional unit whose main purpose is to propagate the plant. The first step in seed development is the formation of male and female gametophytes and subsequent steps culminate in successful fertilization. The detailed study of this process is highly relevant because it directly impacts human needs, such as protecting biodiversity and ensuring sustainable agriculture to feed the increasing world population. Cytokinins comprise a class of phytohormones that play many important roles during plant growth and development and in recent years, the role of this class of phytohormones during reproduction has become clear. Here, we review the role of cytokinins during ovule, pollen and seed formation at the genetic and molecular levels. The expansion of knowledge concerning the molecular mechanisms that control plant reproduction is extremely important to optimise seed production.

Sulfur or Pollen? Chemical, Biological, and Toxicological Basis for the Correct Risk Communication of Urban Yellow Dust Deposition

Urban yellow dust deposition is a common phenomenon in many parts of the world, which is sometimes called "sulfur shower," "sulfur rain," or "pollen storm." Most people, especially those living in the vicinity of industrial facilities, wrongly perceive the yellow dust as sulfur when in fact it is pollen. The misunderstanding increases risk perception as people believe the "yellow powder" is a serious threat to their health. Based on simple observations, it is virtually impossible to differentiate sulfur from pollen, so risk communication should consider the chemical, biological, and toxicological aspects of these agents. In this review, we clarify that industrial emissions of sulfur are under the form of sulfides, oxides, and other volatile compounds which are gaseous and noncolored, and we explain that it is chemically impossible for gaseous sulfur to become solid yellow sulfur under normal environmental conditions. We also describe pollen and its release from trees, shrubs, and herbs a process influenced by atmospheric conditions. We suggest take-home messages that risk communicators may use when explaining the phenomenon to their communities.

Obtaining Mutant Pollen for Phenotypic Analysis and Pollen Tube Dual Staining

Mutant phenotype observation is the most useful and important method to study which biological process a gene-of-interest is involved in. In flowering plants, excessive pollen grains land and germinate on the stigma, then pollen tubes grow through the transmitting tract to reach the ovules, eventually enter the micropyle to complete double fertilization. First, for mutants whose homozygotes could not be obtained due to pollen tube defects, it is difficult to observe the defect phenotype since the pollen grains of different genotypes are mixed together. Here, we provide a detailed protocol to pick out mutant pollen grains from the heterozygous mutant plants in Arabidopsis thaliana. By using this method, we could obtain sufficient mutant pollen grains for phenotypic analysis. Second, it is difficult to compare the pollen/pollen tube behavior of two different genotypes/species in vivo in a same pistil. Here, we develop a new dual staining method which combines GUS staining with aniline blue staining. By using this method, we can analyze the competence of the two different pollen tubes in the same pistil.

Analysis of Pollen Grains by Immunostaining and FISH in Triticeae Species

In this chapter we describe protocols for immunolabeling and FISH of pollen grains undergoing postmeiotic mitosis using Aegilops speltoides, Secale cereale, and Hordeum vulgare as models. Tissue sectioning of pollen overcomes the problem of the pollen grain wall impermeability that interferes with immunolocalization and in situ hybridization. The crucial element of the protocol is the generation and immobilization of tissue sections. Our method facilitates the investigation of the microspore cell divisions and pollen grain maturation.

Haploid Selection in “Diploid” Organisms

Evolutionary rates and strength of selection differ markedly between haploid and diploid genomes. Any genes expressed in a haploid state will be directly exposed to selection, whereas alleles in a diploid state may be partially or fully masked by a homologous allele. This difference may shape key evolutionary processes, including rates of adaptation and inbreeding depression, but also the evolution of sex chromosomes, heterochiasmy, and stable sex ratio biases. All diploid organisms carry haploid genomes, most notably the haploid genomes in gametes produced by every sexually reproducing eukaryote. Furthermore, haploid expression occurs in genes with monoallelic expression, in sex chromosomes, and in organelles, such as mitochondria and plastids. A comparison of evolutionary rates among these haploid genomes reveals striking parallels. Evidence suggests that haploid selection has the potential to shape evolution in predominantly diploid organisms, and taking advantage of the rapidly developing technologies, we are now in the position to quantify the importance of such selection on haploid genomes.

Pollen competition is the mechanism underlying a variety of evolutionary phenomena in dioecious plants

It has long been known that more pollen grains often arrive on stigmas than there are ovules to fertilize, resulting in pollen competition. Moreover, this competition among pollen grains (gametophytes) depends, in part, on their extensive haploid gene expression. Here I review how this leads to a variety of phenomena in dioecious plants of interest to evolutionary biologists. For example, pollen competition can lead to extreme female-biased sex ratios. In addition, gene expression by individual pollen grains can slow mutation accumulation and degeneration of the Y chromosome. Lastly, I review work on how the haploid selection resulting from pollen competition has been proposed to influence which alleles are linked to the Y chromosome, and some recent empirical evidence in support of this theory.

Nondisjunction and unequal spindle organization accompany the drive of Aegilops speltoides B chromosomes

B chromosomes (Bs) are supernumerary chromosomes, which are often preferentially inherited. When transmission rates of chromosomes are higher than 0.5, not obeying the Mendelian law of equal segregation, the resulting transmission advantage is collectively referred to as 'chromosome drive'. Here we analysed the drive mechanism of Aegilops speltoides Bs. The repeat AesTR-183 of A. speltoides Bs, which also can be detected on the Bs of Aegilops mutica and rye, was used to track Bs during pollen development. Nondisjunction of CENH3-positive, tubulin interacting B sister chromatids and an asymmetric spindle during first pollen grain mitosis are key for the accumulation process. A quantitative flow cytometric approach revealed that, independent of the number of Bs present in the mother plant, Bs accumulate in the generative nuclei to > 93%. Nine out of 11 tested (peri)centromeric repeats were shared by A and B chromosomes. Our findings provide new insights into the process of chromosome drive. Quantitative flow cytometry is a useful and reliable method to study the drive frequency of Bs. Nondisjunction and unequal spindle organization accompany during first pollen mitosis the drive of A. speltoides Bs. The prerequisites for the drive process seems to be common in Poaceae.

The Solanum chacoense Fertilization-Related Kinase 3 (ScFRK3) is involved in male and female gametophyte development

BACKGROUND

The Fertilization-related kinases (FRK) form a class that belongs to the MEKK subfamily of plant MAPKKKs. It was recently shown that FRK class kinases expanded during angiosperm evolution, reaching their maximum numbers in the lineage leading to solanaceous species and culminating in the Solanum genus where they account for more than 40% of the total MEKKs. The first members studied, ScFRK1 and ScFRK2 were shown to play a pivotal role in gametophyte development in the wild potato species Solanum chacoense.

RESULTS

ScFRK3 is also involved in gametophyte development. ScFRK3 is expressed in developing pollen and young ovules, reaching its highest level immediately after meiosis and during the mitosis steps in both gametophytes. Hence, three independent lines of ScFRK3 RNAi mutant plants showed decreased number of seeds per fruit. We also observed an important number of degenerated embryo sac in mature ovary. Analysis of ovule development showed that most embryo sac did not enter mitosis I in ScFRK3 RNAi mutant plants. Severe lethality was also observed during male gametophyte development, pollen being arrested before mitosis I, as observed in the female gametophyte. Obvious defects in vegetative organs were not observed, emphasizing the reproductive roles of the FRK class kinases. To isolate MAP kinases acting downstream of ScFRK3, a de novo S. chacoense transcriptome from male and female reproductive organs was assembled. Of the five ScMKKs and 16 ScMPKs retrieved, only the ScMKK3 interacted with ScFRK3, while only the ScMPK13 interacted with ScMKK3, leading to an apparent single three-tiered canonical MAP kinase cascade combination involving ScFRK3-ScMKK3-ScMPK13.

CONCLUSIONS

The ScFRK3 MAPKKK is involved in a signaling cascade that regulates both male and female gamete development, and most probably act upstream of ScMKK3 and ScMPK13.

BICELLULAR POLLEN 1 is a modulator of DNA replication and pollen development in Arabidopsis

During male gametogenesis in Arabidopsis, the haploid microspore undergoes an asymmetric division to produce a vegetative and a generative cell, the latter of which continues to divide symmetrically to form two sperms. This simple system couples cell cycle with cell fate specification. Here we addressed the role of DNA replication in male gametogenesis using a mutant bicellular pollen 1 (bice1), which produces bicellular, rather than tricellular, pollen grains as in the wild-type plant at anthesis. The mutation prolonged DNA synthesis of the generative cell, which resulted in c. 40% of pollen grains arrested at the two-nucleate stage. The extended S phase did not impact the cell fate of the generative cell as shown by cell-specific markers. BICE1 encodes a plant homolog of human D123 protein that is required for G1 progression, but the underlying mechanism is unknown. Here we showed that BICE1 interacts with MCM4 and MCM7 of the pre-replication complex. Consistently, double mutations in BICE1 and MCM4, or MCM7, also led to bicellular pollen and condensed chromosomes. These suggest that BICE1 plays a role in modulating DNA replication via interaction with MCM4 and MCM7.

A simple and robust protocol for immunostaining Arabidopsis pollen nuclei

Pollen represents the male sexual lineage in flowering plants. At maturity, pollen grains are composed of a companion vegetative cell with embedded sperm. During pollen development, these two cell types acquire vastly differing cell fates. Underlying this differential fate acquisition is dramatic reconfiguration of pollen chromatin that is highly evident at a cytological level. The precise link between histone mark deposition and fate acquisition remains largely unexplored, which in part has been hindered by the difficulty in working with pollen in model plant species like Arabidopsis. Here, we describe a simple and robust protocol to isolate Arabidopsis pollen nuclei and immunostain for histone marks. Plant growth aside, the protocol can be performed over 2 days with few Arabidopsis plants, thus allowing multiple genotypes to be analysed in parallel. We also describe a method to de-mask epitopes through antigen retrieval, which vastly improves the signal for antibodies that target heterochromatic histone marks.

Mycotoxins and Mycotoxin Producing Fungi in Pollen: Review

Due to its divergent chemical composition and good nutritional properties, pollen is not only important as a potential food supplement but also as a good substrate for the development of different microorganisms. Among such microorganisms, toxigenic fungi are extremely dangerous as they can synthesize mycotoxins as a part of their metabolic pathways. Furthermore, favorable conditions that enable the synthesis of mycotoxins (adequate temperature, relative humidity, pH, and aw values) are found frequently during pollen collection and/or production process. Internationally, several different mycotoxins have been identified in pollen samples, with a noted predominance of aflatoxins, ochratoxins, fumonisins, zearalenone, deoxynivalenol, and T-2 toxin. Mycotoxins are, generally speaking, extremely harmful for humans and other mammals. Current EU legislation contains guidelines on the permissible content of this group of compounds, but without information pertaining to the content of mycotoxins in pollen. Currently only aflatoxins have been researched and discussed in the literature in regard to proposed limits. Therefore, the aim of this review is to give information about the presence of different mycotoxins in pollen samples collected all around the world, to propose possible aflatoxin contamination pathways, and to emphasize the importance of a regular mycotoxicological analysis of pollen. Furthermore, a suggestion is made regarding the legal regulation of pollen as a food supplement and the proposed tolerable limits for other mycotoxins.

Regulation of pollen lipid body biogenesis by MAP kinases and downstream WRKY transcription factors in Arabidopsis

Signaling pathways that control the activities in non-photosynthetic plastids, important sites of plant metabolism, are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Loss of function of WRKY2/WRKY34 results in reduced GPT1 expression, and concomitantly, reduced accumulation of lipid bodies in mature pollen, which leads to compromised pollen viability, germination, pollen tube growth, and male transmission in Arabidopsis. Pollen-specific overexpression of GPT1 rescues the pollen defects of wrky2 wrky34 double mutant. Furthermore, gain-of-function activation of MPK3/MPK6 enhances GPT1 expression; whereas GPT1 expression is reduced in mkk4 mkk5 double mutant. Together, this study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

Plastids are important sites of plant metabolism including fatty acid and starch biosynthesis. At present, how the activities in the plastids are coordinated with those in the cytoplasm and the signaling pathway(s) involved are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 is localized on the membrane of plastids and transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Genetic analyses demonstrated that WRKY2/WRKY34 and their upstream MPK3/MPK6 are involved in regulating GPT1 expression, therefore, the accumulation of lipid bodies in mature pollen, which is critical to pollen viability, pollen germination, pollen tube growth, and male transmission in Arabidopsis. This study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

Feeling the force: how pollen tubes deal with obstacles

Physical forces are involved in the regulation of plant development and morphogenesis by translating mechanical stress into the modification of physiological processes, which, in turn, can affect cellular growth. Pollen tubes respond rapidly to external stimuli and provide an ideal system to study the effect of mechanical cues at the single-cell level. Here, pollen tubes were exposed to mechanical stress while monitoring the reconfiguration of their growth and recording the generated forces in real-time. We combined a lab-on-a-chip device with a microelectromechanical systems (MEMS)-based capacitive force sensor to mimic and quantify the forces that are involved in pollen tube navigation upon confronting mechanical obstacles. Several stages of obstacle avoidance were identified, including force perception, growth adjustment and penetration. We have experimentally determined the perceptive force threshold, which is the force threshold at which the pollen tube reacts to an obstacle, for Lilium longiflorum and Arabidopsis thaliana. In addition, the method we developed provides a way to calculate turgor pressure based on force and optical data. Pollen tubes sense physical barriers and actively adjust their growth behavior to overcome them. Furthermore, our system offers an ideal platform to investigate intracellular activity during force perception and growth adaption in tip growing cells.

A chemical treatment method for obtaining clean and intact pollen shells of different species

Pollen grains and plant spores have emerged as a novel biomaterial for a broad range of applications including oral drug and vaccine delivery, catalyst support, and removal of heavy metals. However, before pollens can be used, their intrinsic biomolecules, which occupy a large part of the pollen inner cavity must be removed not only to create empty space but because they have potential to cause allergies when used in vivo. These intrinsic materials in the pollen core can be extracted through a chemical treatment to generate clean pollen shells. The commonly used method involves a series of sequential treatments with organic solvents, alkalis, and acids to remove the native pollen biomolecules. This method, though successful for treating lycopodium (Lycopodium clavatum) spores, fails for other species of pollens such as common ragweed (Ambrosia elatior) and thus prevents widespread investigation of different pollens. Herein, we report a new chemical treatment for obtaining clean pollen shells from multiple plant species. This new method involves sequential treatment with acetone, phosphoric acid, and potassium hydroxide. Scanning electron micrographs and protein quantification have shown that the new method can successfully produce clean, intact, and hollow shells from many pollen species including ragweed, sunflower, black alder, and lamb's quarters. These results demonstrate the broad applicability of this method to clean pollens of different species, and paves the way to start investigating them for various applications.

The Evolutionary Consequences of Selection at the Haploid Gametic Stage

As an immediate consequence of sexual reproduction, biphasic life cycles with alternating diploid and haploid phases are a common characteristic of sexually reproducing eukaryotes. Much of our focus in evolutionary biology has been directed toward dynamics in diploid or haploid populations, but we rarely consider selection occurring during both phases when studying evolutionary processes. One of the reasons for this apparent omission is the fact that many flowering plants and metazoans are predominantly diploid with a very short haploid gametic phase. While this gametic phase may be short, it can play a crucial role in fundamental processes including the rate of adaptation, the load of mutation, and the evolution of features such as recombination. In addition, if selection acts in different directions between the two phases, a genetic conflict will occur, impacting the maintenance of genetic variation. Here we provide an overview of theoretical and empirical studies investigating the importance of selection at the haploid gametic phase in predominantly diploid organisms and discuss future directions to improve our understanding of the underlying dynamics and the general implications of haploid selection.

Auxin: a molecular trigger of seed development

This review by Figueiredo and Köhler describes the molecular mechanisms driving seed development. They review the role of the hormone auxin for the initial development of the three seed structures and as a trigger of fertilization-independent seed development.

The evolution of seeds defines a remarkable landmark in the history of land plants. A developing seed contains three genetically distinct structures: the embryo, the nourishing tissue, and the seed coat. While fertilization is necessary to initiate seed development in most plant species, apomicts have evolved mechanisms allowing seed formation independently of fertilization. Despite their socio–economical relevance, the molecular mechanisms driving seed development have only recently begun to be understood. Here we review the current knowledge on the role of the hormone auxin for the initial development of the three seed structures and as a trigger of fertilization-independent seed development.

Haploid selection, sex ratio bias, and transitions between sex-determining systems

Sex determination is remarkably dynamic; many taxa display shifts in the location of sex-determining loci or the evolution of entirely new sex-determining systems. Predominant theories for why we observe such transitions generally conclude that novel sex-determining systems are favoured by selection if they equalise the sex ratio or increase linkage with a locus that experiences different selection in males versus females. We use population genetic models to extend these theories in two ways: (1) We consider the dynamics of loci very tightly linked to the ancestral sex-determining loci, e.g., within the nonrecombining region of the ancestral sex chromosomes. Variation at such loci can favour the spread of new sex-determining systems in which the heterogametic sex changes (XY to ZW or ZW to XY) and the new sex-determining region is less closely linked (or even unlinked) to the locus under selection. (2) We consider selection upon haploid genotypes either during gametic competition (e.g., pollen competition) or meiosis (i.e., nonmendelian segregation), which can cause the zygotic sex ratio to become biased. Haploid selection can drive transitions between sex-determining systems without requiring selection to act differently in diploid males versus females. With haploid selection, we find that transitions between male and female heterogamety can evolve so that linkage with the sex-determining locus is either strengthened or weakened. Furthermore, we find that sex ratio biases may increase or decrease with the spread of new sex chromosomes, which implies that transitions between sex-determining systems cannot be simply predicted by selection to equalise the sex ratio. In fact, under many conditions, we find that transitions in sex determination are favoured equally strongly in cases in which the sex ratio bias increases or decreases. Overall, our models predict that sex determination systems should be highly dynamic, particularly when haploid selection is present, consistent with the evolutionary lability of this trait in many taxa.

WRKY2/34-VQ20 Modules in Arabidopsis thaliana Negatively Regulate Expression of a Trio of Related MYB Transcription Factors During Pollen Development

Male gametogenesis in plants is tightly controlled and involves the complex and precise regulation of transcriptional reprogramming. Interactions between WRKY proteins and VQ motif-containing proteins are required to control these complicated transcriptional networks. However, our understanding of the mechanisms by which these complexes affect downstream gene expression is quite limited. In this study, we found that WRKY2 and WKRY34 repress MYB97, MYB101, and MYB120 expression during male gametogenesis. MYB expression was up-regulated in the wrky2-1 wrky34-1 vq20-1 triple mutant during male gametogenesis. The expression levels of six potential targets of the three MYBs increased the most in the wrky2-1 wrky34-1 vq20-1 triple mutant, followed by the wrky2-1 wrky34-1 double mutant, compared with in wild-type. Yeast one-hybrid and dual luciferase reporter assays indicated that WRKY2 and WRKY34 recognized the MYB97 promoter by binding to its W-boxes. MYB97 overexpression caused defects in pollen germination and pollen tube length, which impacted male fertility. Thus, WRKY2/34-VQ20 complexes appear to negatively regulate the expression of certain MYBs during plant male gametogenesis.

Arabinogalactan proteins and their sugar chains: functions in plant reproduction, research methods, and biosynthesis

The arabinogalactan protein (AGP) family is one of the most complex protein families and is ubiquitous in the plant kingdom. Moreover, it has been demonstrated to play various roles during plant reproduction. A typical AGP contains a hydroxyproline-rich core protein with high heterogeneity and varying numbers of polysaccharide side chains. However, the functions of the polysaccharide components (i.e. AG sugar chains) remain largely unknown due to the general difficulties associated with studying sugar chains in glycobiology. In recent years, methodological breakthroughs have resulted in substantial progress in AGP research. Here, we summarise the multiple roles of AGPs during plant gametophyte development and male-female communication, with a focus on recent advances. In addition, we discuss the analytical tools used in AGP research, and the biosynthesis and function of AG sugar chains. A comprehensive understanding of the AGP family will help clarify the mechanisms precisely controlling reproductive processes.

Pollen differentiation as well as pollen tube guidance and discharge are independent of the presence of gametes

After meiosis, an unequal cell division generates the male gamete lineage in flowering plants. The generative cell will undergo a second division, giving rise to the two gametes, i.e. the sperm cells. The other cell will develop into the vegetative cell that plays a crucial role in pollen tube formation and sperm delivery. Recently, the vegetative cell has been suggested to be important for programming of the chromatin state in sperm cells and/or the resulting fertilization products. Blocking the initial unequal division genetically, we first highlight that the default differentiation state after male meiosis is a vegetative fate, which is consistent with earlier work. We find that uni-nucleated mutant microspores differentiated as wild-type vegetative cells, including chromatin remodeling and the transcriptional activation of transposable elements. Moreover, live-cell imaging revealed that this vegetative cell is sufficient for the correct guidance of the pollen tube to the female gametes. Hence, we conclude that vegetative cell differentiation and function does not depend on the formation or presence of the actual gametes but rather on external signals or a cell-autonomous pace keeper.

Summary: Cell biological analyses in Arabidopsis show that the vegetative cell differentiates without the presence of the actual gametes, and is solely sufficient for pollen tube germination, guidance, ovule penetration and pollen tube discharge.

SHI/STY Genes Affect Pre- and Post-meiotic Anther Processes in Auxin Sensing Domains in Arabidopsis

In flowering plants, mature sperm cells are enclosed in pollen grains formed in structures called anthers. Several cell layers surrounding the central sporogenous cells of the anther are essential for directing the developmental processes that lead to meiosis, pollen formation, and the subsequent pollen release. The specification and function of these tissues are regulated by a large number of genetic factors. Additionally, the plant hormone auxin has previously been shown to play important roles in the later phases of anther development. Using the R2D2 auxin sensor system we here show that auxin is sensed also in the early phases of anther cell layer development, suggesting that spatiotemporal regulation of auxin levels is important for early anther morphogenesis. Members of the SHI/STY transcription factor family acting as direct regulators of YUC auxin biosynthesis genes have previously been demonstrated to affect early anther patterning. Using reporter constructs we show that SHI/STY genes are dynamically active throughout anther development and their expression overlaps with those of three additional downstream targets, PAO5, EOD3 and PGL1. Characterization of anthers carrying mutations in five SHI/STY genes clearly suggests that SHI/STY transcription factors affect anther organ identity. In addition, their activity is important to repress periclinal cell divisions as well as premature entrance into programmed cell death and cell wall lignification, which directly influences the timing of anther dehiscence and the pollen viability. The SHI/STY proteins also prevent premature pollen germination suggesting that they may play a role in the induction or maintenance of pollen dormancy.

ARF2-ARF4 and ARF5 are Essential for Female and Male Gametophyte Development in Arabidopsis

The plant hormone auxin plays critical roles in plant growth and development. Auxin response factors (ARFs) are a class of transcription factors which regulate auxin-mediated gene expression. While the functions of ARFs in sporophytic development have been well characterized, their functions specific to gametophytic development have not been studied extensively. In this study, Arabidopsis ARF genes were selectively down-regulated in gametophytes by misexpression of targeted microRNAs (amiRARF234, amiRARFMP and MIR167a) to silence AtARF2-AtAEF4, AtARF5, AtARF6 and AtARF8. Embryo sacs in amiRARF234- and amiRARFMP-expressing plants exhibited identity defects in cells at the micropylar pole, such as formation of two cells with egg cell-like morphology, concomitant with loss of synergid marker expression and seed abortion. The pollen grains of the transgenic plants were morphologically aberrant and unviable, and the inclusions and nuclei were lost in the abnormal pollen grains. However, plants misexpressing MIR167a showed no obvious abnormal phenotypes in the embryo sacs and pollen grains. Overall, these results provide evidence that AtARF2-AtARF4 and AtARF5 play significant roles in regulating both female and male gametophyte development in Arabidopsis.

Haploid Selection Favors Suppressed Recombination Between Sex Chromosomes Despite Causing Biased Sex Ratios

To date, research on the evolution of sex chromosomes has focused on sexually antagonistic selection among diploids, which has been shown to be a potent driver of the strata and reduced recombination that characterize many sex chromosomes. However, significant selection can also occur on haploid genotypes during less conspicuous life cycle stages, e.g., competition among sperm/pollen or meiotic drive during gamete/spore production. These haploid selective processes are typically sex-specific, e.g., gametic/gametophytic competition typically occurs among sperm/pollen, and meiotic drive typically occurs during either spermatogenesis or oogenesis. We use models to investigate whether sex-specific selection on haploids could drive the evolution of recombination suppression on the sex chromosomes, as has been demonstrated for sex-specific selection among diploids. A potential complication is that zygotic sex-ratios become biased when haploid selected loci become linked to the sex-determining region because the zygotic sex ratio is determined by the relative number and fitness of X- vs. Y-bearing sperm. Despite causing biased zygotic sex-ratios, we find that a period of sex-specific haploid selection generally favors recombination suppression on the sex chromosomes. Suppressed recombination is favored because it allows associations to build up between haploid-beneficial alleles and the sex that experiences haploid selection most often (e.g., pollen beneficial alleles become strongly associated with the male determining region, Y or Z). Haploid selected loci can favor recombination suppression even in the absence of selective differences between male and female diploids. Overall, we expand our view of the sex-specific life cycle stages that can drive sex chromosome evolution to include gametic competition and meiotic drive. Based on our models, sex chromosomes should become enriched for genes that experience haploid selection, as is expected for genes that experience sexually antagonistic selection. Thus, we generate a number of predictions that can be evaluated in emerging sex chromosome systems.

Central metabolite and sterol profiling divides tobacco male gametophyte development and pollen tube growth into eight metabolic phases

While changes in the transcriptome and proteome of developing pollen have been investigated in tobacco and other species, the metabolic consequences remain rather unclear. Here, a broad range of metabolites was investigated in close succession of developmental stages. Thirteen stages of tobacco male gametophyte development were collected, ranging from tetrads to pollen tubes. Subsequently, the central metabolome and sterol composition were analyzed by GC-mass spectrometry (MS), monitoring 77 metabolites and 29 non-identified analytes. The overall results showed that development and tube growth could be divided into eight metabolic phases with the phase including mitosis I being most distinct. During maturation, compounds such as sucrose and proline accumulated. These were degraded after rehydration, while γ-aminobutyrate transiently increased, possibly deriving from proline breakdown. Sterol analysis revealed that tetrads harbor similar sterols as leaves, but throughout maturation unusual sterols increased. Lastly, two further sterols exclusively accumulated in pollen tubes. This study allows a deeper look into metabolic changes during the development of a quasi-single cell type. Metabolites accumulating during maturation might accelerate pollen germination and tube growth, protect from desiccation, and feed pollinators. Future studies of the underlying processes orchestrating the changes in metabolite levels might give valuable insights into cellular regulation of plant metabolism.

Arabidopsis WRKY2 and WRKY34 transcription factors interact with VQ20 protein to modulate pollen development and function

Plant male gametogenesis is tightly regulated, and involves complex and precise regulations of transcriptional reprogramming. WRKY transcription factors have been demonstrated to play critical roles in plant development and stress responses. Several members of this family physically interact with VQ motif-containing proteins (VQ proteins) to mediate a plethora of programs in Arabidopsis; however, the involvement of WRKY-VQ complexes in plant male gametogenesis remains largely unknown. In this study, we found that WRKY2 and WKRY34 interact with VQ20 both in vitro and in vivo. Further experiments displayed that the conserved VQ motif of VQ20 is responsible for their physical interactions. The VQ20 protein localizes in the nucleus and specifically expresses in pollens. Phenotypic analysis showed that WRKY2, WRKY34 and VQ20 are crucial for pollen development and function. Mutations of WRKY2, WRKY34 and VQ20 simultaneously resulted in male sterility, with defects in pollen development, germination and tube growth. Further investigation revealed that VQ20 affects the transcriptional functions of its interacting WRKY partners. Complementation evidence supported that the VQ motif of VQ20 is essential for pollen development, as a mutant form of VQ20 in which LVQK residues in the VQ motif were replaced by EDLE did not rescue the phenotype of the w2-1 w34-1 vq20-1 triple-mutant plants. Further expression analysis indicated that WRKY2, WRKY34 and VQ20 co-modulate multiple genes involved in pollen development, germination and tube growth. Taken together, our study provides evidence that VQ20 acts as a key partner of WRKY2 and WKRY34 in plant male gametogenesis.