Knockin' on pollen's door: live cell imaging of early polarization events in germinating Arabidopsis pollen

The male germ unit association is independently regulated of GUM in Arabidopsis thaliana

Cytoplasmic projections (CPs) formed by the generative and sperm cells link the male gametes with the vegetative cell (VC) nucleus, which are required to build the male germ unit (MGU) assemblage in the angiosperm pollen grain. As molecular and genetic controls underlying CP development and formation of the MGU are unknown, it was hypothesized that physical association between germ cells and the VC nucleus might be lost in germ unit malformed (gum) mutants or in those which either block generative cell (GC) division or that additionally prevent gamete differentiation. In vivo, analysis of marked cellular components demonstrated a linkage of sperm cells (SCs) and the VC nucleus in gum mutant alleles despite their increased physical separation. Similarly, for several independent classes of bicellular pollen mutants, undivided GCs were associated with the VC nucleus like GCs in wild-type pollen. We conclude that the early formation of GC CPs to establish the MGU is regulated independently of DUO1-DAZ1 and DUO3 transcription factors as well as cyclin-dependent kinase function (CDKA;1). As the absence of cytoplasmic protrusion was expected in the gum mutants in Arabidopsis, early histological studies reported temporal disappearance of cytoplasmic protrusion in several organisms. Our findings demonstrated the striking importance of live imaging to verify the broad conservation of the persistent MGU contact in all the angiosperms and its important role in successful double fertilization.

Distinct functions of microtubules and actin filaments in the transportation of the male germ unit in pollen

Flowering plants rely on the polarized growth of pollen tubes to deliver sperm cells (SCs) to the embryo sac for double fertilization. In pollen, the vegetative nucleus (VN) and two SCs form the male germ unit (MGU). However, the mechanism underlying directional transportation of MGU is not well understood. In this study, we provide the first full picture of the dynamic interplay among microtubules, actin filaments, and MGU during pollen germination and tube growth. Depolymerization of microtubules and inhibition of kinesin activity result in an increased velocity and magnified amplitude of VN's forward and backward movement. Pharmacological washout experiments further suggest that microtubules participate in coordinating the directional movement of MGU. In contrast, suppression of the actomyosin system leads to a reduced velocity of VN mobility but without a moving pattern change. Moreover, detailed observation shows that the direction and velocity of VN's movement are in close correlations with those of the actomyosin-driven cytoplasmic streaming surrounding VN. Therefore, we propose that while actomyosin-based cytoplasmic streaming influences on the oscillational movement of MGU, microtubules and kinesins avoid MGU drifting with the cytoplasmic streaming and act as the major regulator for fine-tuning the proper positioning and directional migration of MGU in pollen.

Transcriptomic Insight into the Pollen Tube Growth of Olea europaea L. subsp. europaea Reveals Reprogramming and Pollen-Specific Genes Including New Transcription Factors

The pollen tube is a key innovation of land plants that is essential for successful fertilisation. Its development and growth have been profusely studied in model organisms, but in spite of the economic impact of olive trees, little is known regarding the genome-wide events underlying pollen hydration and growth in this species. To fill this gap, triplicate mRNA samples at 0, 1, 3, and 6 h of in vitro germination of olive cultivar Picual pollen were analysed by RNA-seq. A bioinformatics R workflow called RSeqFlow was developed contemplating the best practices described in the literature, covering from expression data filtering to differential expression and clustering, to finally propose hub genes. The resulting olive pollen transcriptome consisted of 22,418 reliable transcripts, where 5364 were differentially expressed, out of which 173 have no orthologue in plants and up to 3 of them might be pollen-specific transcription factors. Functional enrichment revealed a deep transcriptional reprogramming in mature olive pollen that is also dependent on protein stability and turnover to allow pollen tube emergence, with many hub genes related to heat shock proteins and F-box-containing proteins. Reprogramming extends to the first 3 h of growth, including processes consistent with studies performed in other plant species, such as global down-regulation of biosynthetic processes, vesicle/organelle trafficking and cytoskeleton remodelling. In the last stages, growth should be maintained from persistent transcripts. Mature pollen is equipped with transcripts to successfully cope with adverse environments, even though the in vitro growth seems to induce several stress responses. Finally, pollen-specific transcription factors were proposed as probable drivers of pollen germination in olive trees, which also shows an overall increased number of pollen-specific gene isoforms relative to other plants.

Thermosensitivity of pollen: a molecular perspective

A current trend in climate comprises adverse weather anomalies with more frequent and intense temperature events. Heatwaves are a serious threat to global food security because of the susceptibility of crop plants to high temperatures. Among various developmental stages of plants, even a slight rise in temperature during reproductive development proves detrimental, thus making sexual reproduction heat vulnerable. In this context, male gametophyte or pollen development stages are the most sensitive ones. High-temperature exposure induces pollen abortion, reducing pollen viability and germination rate with a concomitant effect on seed yield. This review summarizes the ultrastructural, morphological, biochemical, and molecular changes underpinning high temperature-induced aberrations in male gametophytes. Specifically, we highlight the temperature sensing cascade operating in pollen, involving reactive oxygen species (ROS), heat shock factors (HSFs), a hormones and transcriptional regulatory network. We also emphasize integrating various omics approaches to decipher the molecular events triggered by heat stress in pollen. The knowledge of genes, proteins, and metabolites conferring thermotolerance in reproductive tissues can be utilized to breed/engineer thermotolerant crops to ensure food security.

Functional non-equivalence of pollen ADF isovariants in Arabidopsis

ADF/cofilin is a central regulator of actin dynamics. We previously demonstrated that two closely related Arabidopsis class IIa ADF isovariants, ADF7 and ADF10, are involved in the enhancement of actin turnover in pollen, but whether they have distinct functions remains unknown. Here, we further demonstrate that they exhibit distinct functions in regulating actin turnover both in vitro and in vivo. We found that ADF7 binds to ADP-G-actin with lower affinity, and severs and depolymerizes actin filaments less efficiently in vitro than ADF10. Accordingly, in pollen grains, ADF7 more extensively decorates actin filaments and is less freely distributed in the cytoplasm compared to ADF10. We further demonstrate that ADF7 and ADF10 show distinct intracellular localizations during pollen germination, and they have non-equivalent functions in promoting actin turnover in pollen. We thus propose that cooperation and labor division of ADF7 and ADF10 enable pollen cells to achieve exquisite control of the turnover of different actin structures to meet different cellular needs.

A Decade of Pollen Phosphoproteomics

Angiosperm mature pollen represents a quiescent stage with a desiccated cytoplasm surrounded by a tough cell wall, which is resistant to the suboptimal environmental conditions and carries the genetic information in an intact stage to the female gametophyte. Post pollination, pollen grains are rehydrated, activated, and a rapid pollen tube growth starts, which is accompanied by a notable metabolic activity, synthesis of novel proteins, and a mutual communication with female reproductive tissues. Several angiosperm species (Arabidopsis thaliana, tobacco, maize, and kiwifruit) were subjected to phosphoproteomic studies of their male gametophyte developmental stages, mostly mature pollen grains. The aim of this review is to compare the available phosphoproteomic studies and to highlight the common phosphoproteins and regulatory trends in the studied species. Moreover, the pollen phosphoproteome was compared with root hair phosphoproteome to pinpoint the common proteins taking part in their tip growth, which share the same cellular mechanisms.

The nuclear envelope protein KAKU4 determines the migration order of the vegetative nucleus and sperm cells in pollen tubes

A putative component protein of the nuclear lamina, KAKU4, modulates nuclear morphology in Arabidopsis thaliana seedlings, but its physiological significance is unknown. KAKU4 was highly expressed in mature pollen grains, each of which has a vegetative cell and two sperm cells. KAKU4 protein was highly abundant on the envelopes of vegetative nuclei and less abundant on the envelopes of sperm cell nuclei in pollen grains and elongating pollen tubes. Vegetative nuclei are irregularly shaped in wild-type pollen. However, KAKU4 deficiency caused them to become more spherical. After a pollen grain germinates, the vegetative nuclei and sperm cells enter and move along the pollen tube. In the wild type, the vegetative nucleus preceded the sperm cell nuclei in >90% of the pollen tubes, whereas, in kaku4 mutants, the vegetative nucleus preceded the sperm cell nuclei in only about half of the pollen tubes. kaku4 pollen was less competitive for fertilization than wild-type pollen after pollination. These results led us to hypothesize that the nuclear shape in vegetative cells of pollen grains affects the orderly migration of the vegetative nucleus and sperm cells in pollen tubes.

Live-cell imaging of early events following pollen perception in self-incompatible Arabidopsis thaliana

Early events occurring at the surface of the female organ are critical for plant reproduction, especially in species with a dry stigma. After landing on the stigmatic papilla cells, the pollen hydrates and germinates a tube, which penetrates the cell wall and grows towards the ovules to convey the male gametes to the embryo sac. In self-incompatible species within the Brassicaceae, these processes are blocked when the stigma encounters an incompatible pollen. Based on the generation of self-incompatible Arabidopsis lines and by setting up a live imaging system, we showed that control of pollen hydration has a central role in pollen selectivity. The faster the pollen pumps water from the papilla during an initial period of 10 min, the faster it germinates. Furthermore, we found that the self-incompatibility barriers act to block the proper hydration of incompatible pollen and, when hydration is promoted by high humidity, an additional control prevents pollen tube penetration into the stigmatic wall. In papilla cells, actin bundles focalize at the contact site with the compatible pollen but not with the incompatible pollen, raising the possibility that stigmatic cells react to the mechanical pressure applied by the invading growing tube.

Twice the fun, double the trouble: gamete interactions in flowering plants

During sexual reproduction two gametes of opposite sex unite to produce a zygote. Gamete fusion is a highly controlled process and it has become evident that, across species, common concepts apply to this ancient and fundamental event. Sexual reproduction in flowering plants is even more complex in that two sperm cells fertilize two female reproductive cells (egg and central cell) in a process called double fertilization. Due to the coordinated developmental progression and mutual dependency of the two fertilization products (embryo and endosperm), the success and timing of the two fusion events substantially affects seed set. So far, four proteins are known to act on the surfaces of Arabidopsis gametes to accomplish double fertilization. The molecular and evolutionary characteristics of these players prove that flowering plants integrate plant-specific and widely conserved mechanisms to accomplish the timely fusion of each sperm cell with one female reproductive cell.

Actin Polymerization Mediated by AtFH5 Directs the Polarity Establishment and Vesicle Trafficking for Pollen Germination in Arabidopsis

The process of pollen germination is crucial for flowering plant reproduction, but the mechanisms through which pollen grains establish polarity and select germination sites are not well understood. In this study, we report that a formin family protein, AtFH5, is localized to the vesicles and rotates ahead of Lifeact-mEGFP-labeled actin filaments during pollen germination. The translocation of AtFH5 to the plasma membrane initiates the assembly of a collar-like actin structure at the prospective germination site prior to germination. Genetic and pharmacological evidence further revealed an interdependent relationship between the mobility of AtFH5-labeled vesicles and the polymerization of actin filaments: vesicle-localized AtFH5 promotes actin assembly, and the polymerization and elongation of actin filaments, in turn, is essential for the mobility of AtFH5-labeled vesicles in pollen grains. Taken together, our work revealed a molecular mechanism underlying the polarity establishment and vesicle mobility during pollen germination.

Electromechanics of polarized cell growth

One of the most challenging questions in cell and developmental biology is how molecular signals are translated into mechanical forces that ultimately drive cell growth and motility. Despite an impressive body of literature demonstrating the importance of cytoskeletal and motor proteins as well as osmotic stresses for cell developmental mechanics, a host of dissenting evidence strongly suggests that these factors per se cannot explain growth mechanics even at the level of a single tip-growing cell. The present study addresses this issue by exploring fundamental interrelations between electrical and mechanical fields operating in cells. In the first instance, we employ a simplified but instructive model of a quiescent cell to demonstrate that even in a quasi-equilibrium state, ion transport processes are conditioned principally by mechanical tenets. Then we inquire into the electromechanical conjugacy in growing pollen tubes as biologically relevant and physically tractable developmental systems owing to their extensively characterized growth-associated ionic fluxes and strikingly polarized growth and morphology. A comprehensive analysis of the multifold stress pattern in the growing apices of pollen tubes suggests that tip-focused ionic fluxes passing through the polyelectrolyte-rich apical cytoplasm give rise to electrokinetic flows that actualize otherwise isotropic intracellular turgor into anisotropic stress field. The stress anisotropy can be then imparted from the apical cytoplasm to the abutting frontal cell wall to induce its local extension and directional cell growth. Converging lines of evidence explored in the concluding sections attest that tip-focused ionic fluxes and associated interfacial transport phenomena are not specific for pollen tubes but are also employed by a vast variety of algal, plant, fungal and animal cells, rendering their cytoplasmic stress fields essentially anisotropic and ultimately instrumental in cell shaping, growth and motility.

Interplay between Ions, the Cytoskeleton, and Cell Wall Properties during Tip Growth

Tip growth is a focused and tightly regulated apical explosion that depends on the interconnected activities of ions, the cytoskeleton, and the cell wall.

Male gametophyte development and function in angiosperms: a general concept

Overview of pollen development. Male gametophyte development of angiosperms is a complex process that requires coordinated activity of different cell types and tissues of both gametophytic and sporophytic origin and the appropriate specific gene expression. Pollen ontogeny is also an excellent model for the dissection of cellular networks that control cell growth, polarity, cellular differentiation and cell signaling. This article describes two sequential phases of angiosperm pollen ontogenesis-developmental phase leading to the formation of mature pollen grains, and a functional or progamic phase, beginning with the impact of the grains on the stigma surface and ending at double fertilization. Here we present an overview of important cellular processes in pollen development and explosive pollen tube growth stressing the importance of reserves accumulation and mobilization and also the mutual activation of pollen tube and pistil tissues, pollen tube guidance and the communication between male and female gametophytes. We further describe the recent advances in regulatory mechanisms involved such as posttranscriptional regulation (including mass transcript storage) and posttranslational modifications to modulate protein function, intracellular metabolic signaling, ionic gradients such as Ca(2+) and H(+) ions, cell wall synthesis, protein secretion and intercellular signaling within the reproductive tissues.

Phosphoproteomics Profiling of Tobacco Mature Pollen and Pollen Activated in vitro

Tobacco mature pollen has extremely desiccated cytoplasm, and is metabolically quiescent. Upon re-hydration it becomes metabolically active and that results in later emergence of rapidly growing pollen tube. These changes in cytoplasm hydration and metabolic activity are accompanied by protein phosphorylation. In this study, we subjected mature pollen, 5-min-activated pollen, and 30-min-activated pollen to TCA/acetone protein extraction, trypsin digestion and phosphopeptide enrichment by titanium dioxide. The enriched fraction was subjected to nLC-MS/MS. We identified 471 phosphopeptides that carried 432 phosphorylation sites, position of which was exactly matched by mass spectrometry. These 471 phosphopeptides were assigned to 301 phosphoproteins, because some proteins carried more phosphorylation sites. Of the 13 functional groups, the majority of proteins were put into these categories: transcription, protein synthesis, protein destination and storage, and signal transduction. Many proteins were of unknown function, reflecting the fact that male gametophyte contains many specific proteins that have not been fully functionally annotated. The quantitative data highlighted the dynamics of protein phosphorylation during pollen activation; the identified phosphopeptides were divided into seven groups based on the regulatory trends. The major group comprised mature pollen-specific phosphopeptides that were dephosphorylated during pollen activation. Several phosphopeptides representing the same phosphoprotein had different regulation, which pinpointed the complexity of protein phosphorylation and its clear functional context. Collectively, we showed the first phosphoproteomics data on activated pollen where the position of phosphorylation sites was clearly demonstrated and regulatory kinetics was resolved.

Actin3 promoter reveals undulating F-actin bundles at shanks and dynamic F-actin meshworks at tips of tip-growing pollen tubes

The dynamic actin cytoskeleton of pollen tubes is both the driver of the tip growth and the organizer of cell polarity. In order to understand this fast re-arranging cytoskeletal system, we need reliable constructs expressed under relevant promoters. Here we are reporting that the Lifeact reporter, expressed under the pollen-specific Actin3 promoter, visualizes very dynamic F-actin elements both in germinating pollen grains and tip-growing pollen tubes. Importantly, we have documented very active actin polymerization at the cell periphery, especially in the bulging area during pollen germination and in the apical clear zone. Expression of the Lifeact reporter under control of the pollen-specific Actin3 promoter revealed 2 new aspects: (i) long F-actin bundles in pollen tube shanks are dynamic, showing undulating movements, (ii) subapical 'actin collars' or 'fringes' are absent.

F-actin forms mobile and unwinding ring-shaped structures in germinating Arabidopsis pollen expressing Lifeact

The flowering plant pollen tube is the fastest elongating plant cell and transports the sperm cells for double fertilization. The highly dynamic formation and reorganization of the actin cytoskeleton is essential for pollen germination and pollen tube growth. To drive pollen-specific expression of fluorescent marker proteins, commonly the strong Lat52 promoter is used. Here we show by quantitative fluorescent analysis that the gametophyte-specific ARO1 promoter from Arabidopsis drives an about 3.5 times weaker transgene expression than the Lat52 promoter. In one third of the pollen of F-actin-labeled ARO1p:tagRFP-T-Lifeact transgenic lines we observed mobile ring-shaped actin structures in pollen grains and pollen tubes. Pollen tube growth, transgene transmission and seed production were not affected by tagRFP-T-Lifeact expression. F-actin rings were able to integrate into emerging actin filaments and they may reflect a particular physiological state of the pollen or a readily available storage form provided for rapid actin network remodeling.