The pollen tube clear zone: clues to the mechanism of polarized growth

VAMP726 and VAMP725 regulate vesicle secretion and pollen tube growth in Arabidopsis

KEY MESSAGE

VAMP726/VAMP725 and SYP131 can form a part of a SNARE complex to mediate vesicle secretion at the pollen tube apex. Secretory vesicle fusion with the plasma membrane of the pollen tube tip is a key step in pollen tube growth. Membrane fusion was mediated by SNAREs. However, little is known about the composition and function of the SNARE complex during pollen tube tip growth. In this study, we constructed a double mutant vamp725 vamp726 via CRISPR‒Cas9. Fluorescence labeling combined with microscopic observation, luciferase complementation imaging, co-immunoprecipitation and GST pull-down were applied in the study. We show that double mutation of the R-SNAREs VAMP726 and VAMP725 significantly inhibits pollen tube growth in Arabidopsis and slows vesicle exocytosis at the apex of the pollen tube. GFP-VAMP726 and VAMP725-GFP localize mainly to secretory vesicles and the plasma membrane at the apex of the pollen tube. In addition, fluorescence recovery after photobleaching (FRAP) experiments showed that mCherry-VAMP726 colocalizes with Qa-SNARE SYP131 in the central region of the pollen tube apical plasma membrane. Furthermore, we found that VAMP726 and VAMP725 can interact with the SYP131. Based on these results, we suggest that VAMP726/VAMP725 and SYP131 can form a part of a SNARE complex to mediate vesicle secretion at the pollen tube apex, and vesicle secretion may mainly occur at the central region of the pollen tube apical plasma membrane.

Exploring the Role of the Plant Actin Cytoskeleton: From Signaling to Cellular Functions

The plant actin cytoskeleton is characterized by the basic properties of dynamic array, which plays a central role in numerous conserved processes that are required for diverse cellular functions. Here, we focus on how actins and actin-related proteins (ARPs), which represent two classical branches of a greatly diverse superfamily of ATPases, are involved in fundamental functions underlying signal regulation of plant growth and development. Moreover, we review the structure, assembly dynamics, and biological functions of filamentous actin (F-actin) from a molecular perspective. The various accessory proteins known as actin-binding proteins (ABPs) partner with F-actin to finely tune actin dynamics, often in response to various cell signaling pathways. Our understanding of the significance of the actin cytoskeleton in vital cellular activities has been furthered by comparison of conserved functions of actin filaments across different species combined with advanced microscopic techniques and experimental methods. We discuss the current model of the plant actin cytoskeleton, followed by examples of the signaling mechanisms under the supervision of F-actin related to cell morphogenesis, polar growth, and cytoplasmic streaming. Determination of the theoretical basis of how the cytoskeleton works is important in itself and is beneficial to future applications aimed at improving crop biomass and production efficiency.

Basally distributed actin array drives embryonic hypocotyl elongation during the seed-to-seedling transition in Arabidopsis

Seed germination is a vital developmental transition for the production of progeny by sexual reproduction in spermatophytes. The seed-to-seedling transition is predominately driven by hypocotyl cell elongation. However, the mechanism that underlies hypocotyl growth remains largely unknown. In this study, we characterized the actin array reorganization in embryonic hypocotyl epidermal cells. Live-cell imaging revealed a basally organized actin array formed during hypocotyl cell elongation. This polarized actin assembly is a barrel-shaped network, which comprises a backbone of longitudinally aligned actin cables and a fine actin cap linking these cables. We provide genetic evidence that the basal actin array formation requires formin-mediated actin polymerization and directional movement of actin filaments powered by myosin XIs. In fh1-1 and xi3ko mutants, actin filaments failed to reorganize into the basal actin array, and the hypocotyl cell elongation was inhibited compared with wild-type plants. Collectively, our work uncovers the molecular mechanisms for basal actin array assembly and demonstrates the connection between actin polarization and hypocotyl elongation during seed-to-seedling transition.

Actin cytoskeleton in the control of vesicle transport, cytoplasmic organization, and pollen tube tip growth

Pollen tubes extend rapidly via tip growth. This process depends on a dynamic actin cytoskeleton, which has been implicated in controlling organelle movements, cytoplasmic streaming, vesicle trafficking, and cytoplasm organization in pollen tubes. In this update review, we describe the progress in understanding the organization and regulation of the actin cytoskeleton and the function of the actin cytoskeleton in controlling vesicle traffic and cytoplasmic organization in pollen tubes. We also discuss the interplay between ion gradients and the actin cytoskeleton that regulates the spatial arrangement and dynamics of actin filaments and the organization of the cytoplasm in pollen tubes. Finally, we describe several signaling components that regulate actin dynamics in pollen tubes.

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.

At the Nexus between Cytoskeleton and Vacuole: How Plant Cytoskeletons Govern the Dynamics of Large Vacuoles

Large vacuoles are a predominant cell organelle throughout the plant body. They maximally account for over 90% of cell volume and generate turgor pressure that acts as a driving force of cell growth, which is essential for plant development. The plant vacuole also acts as a reservoir for sequestering waste products and apoptotic enzymes, thereby enabling plants to rapidly respond to fluctuating environments. Vacuoles undergo dynamic transformation through repeated enlargement, fusion, fragmentation, invagination, and constriction, eventually resulting in the typical 3-dimensional complex structure in each cell type. Previous studies have indicated that such dynamic transformations of plant vacuoles are governed by the plant cytoskeletons, which consist of F-actin and microtubules. However, the molecular mechanism of cytoskeleton-mediated vacuolar modifications remains largely unclear. Here we first review the behavior of cytoskeletons and vacuoles during plant development and in response to environmental stresses, and then introduce candidates that potentially play pivotal roles in the vacuole-cytoskeleton nexus. Finally, we discuss factors hampering the advances in this research field and their possible solutions using the currently available cutting-edge technologies.

Visualization and Quantification of the Dynamics of Actin Filaments in Arabidopsis Pollen Tubes

The actin cytoskeleton plays an essential role in the regulation of polarized pollen tube growth, and its functions are dictated by its spatial organization and dynamics. Here we describe an assay to monitor the dynamics of actin filaments decorated with Lifeact-mEGFP in Arabidopsis pollen tubes using spinning disk confocal microscopy and measuring the parameters associated with their dynamics. The method allows us to assess the dynamics of actin filaments in growing Arabidopsis pollen tubes.

Sterols and Sphingolipids as New Players in Cell Wall Building and Apical Growth of Nicotiana tabacum L. Pollen Tubes

Pollen tubes are tip-growing cells that create safe routes to convey sperm cells to the embryo sac for double fertilization. Recent studies have purified and biochemically characterized detergent-insoluble membranes from tobacco pollen tubes. These microdomains, called lipid rafts, are rich in sterols and sphingolipids and are involved in cell polarization in organisms evolutionarily distant, such as fungi and mammals. The presence of actin in tobacco pollen tube detergent-insoluble membranes and the preferential distribution of these domains on the apical plasma membrane encouraged us to formulate the intriguing hypothesis that sterols and sphingolipids could be a "trait d'union" between actin dynamics and polarized secretion at the tip. To unravel the role of sterols and sphingolipids in tobacco pollen tube growth, we used squalestatin and myriocin, inhibitors of sterol and sphingolipid biosynthesis, respectively, to determine whether lipid modifications affect actin fringe morphology and dynamics, leading to changes in clear zone organization and cell wall deposition, thus suggesting a role played by these lipids in successful fertilization.

Low Concentration of Aluminum-Stimulated Pollen Tube Growth of Apples (Malus domestica)

Aluminum (Al) is an important element in soil constitution. Previous studies have shown that high concentration of Al affects the normal growth of crops, resulting in crop yield reduction and inferior quality. Nevertheless, Al has also been referred to as a beneficial element, especially when used at low concentrations, but the cytological mechanism is not clear. Influences of low concentration AlCl₃ on the pollen tube growth of apple (Malus domestica) and its possible cytological mechanism were investigated in this study. The results showed that 20 μM AlCl₃ promoted pollen germination and tube elongation; 20 μM AlCl₃ enhanced Ca²⁺ influx but did not affect [Ca²⁺]c of the pollen tube tip; and 20 μM AlCl₃ decreased acid pectins in pollen tubes but increased esterified pectins and arabinan pectins in pollen tubes. According to the information provided in this research, 20 μM AlCl₃ stimulated growth of pollen tubes by enhancing Ca²⁺ influx and changing cell wall components.

SEC1A is a major Arabidopsis Sec1/Munc18 gene in vesicle trafficking during pollen tube tip growth

Pollen tubes (PTs) grow by the targeted secretion of new cell wall material to their expanding tip region. Sec1/Munc18 (SM) proteins promote membrane fusion through regulation of the SNARE complex. We have previously shown that disruption of protein glycosylation in the Arabidopsis thaliana hpat1 hpat3 double mutant leads to PT growth defects that can be suppressed by reducing secretion. Here, we identified five point mutant alleles of the SM protein SEC1A as hpat1/3 suppressors. The suppressors increased seed set, reduced PT growth defects and reduced the rate of glycoprotein secretion. In the absence of the hpat mutations, sec1a reduced pollen germination and PT elongation producing shorter and wider PTs. Consistent with a defect in membrane fusion, sec1a PTs accumulated secretory vesicles. Though sec1a had significantly reduced male transmission, homozygous sec1a plants maintained full seed set, demonstrating that SEC1A was ultimately dispensable for pollen fertility. However, when combined with a mutation in another SEC1-like SM gene, keule, pollen fertility was totally abolished. Mutation in sec1b, the final member of the Arabidopsis SEC1 clade, did not enhance the sec1a phenotype. Thus, SEC1A is the major SM protein promoting pollen germination and tube elongation, but in its absence KEULE can partially supply this activity. When we examined the expression of the SM protein family in other species for which pollen expression data were available, we found that at least one Sec1-like protein was highly expressed in pollen samples, suggesting a conserved role in pollen fertility in other species.

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.

RNAi-Mediated Knockdown of Calreticulin3a Impairs Pollen Tube Growth in Petunia

Pollen tube growth depends on several complex processes, including exo/endocytosis, cell wall biogenesis, intracellular transport, and cell signaling. Our previous results provided evidence that calreticulin (CRT)—a prominent calcium (Ca²⁺)-buffering molecular chaperone in the endoplasmic reticulum (ER) lumen—is involved in pollen tube formation and function. We previously cloned and characterized the CRT gene belonging to the CRT1/2 subgroup from Petunia hybrida (PhCRT1/2), and found that post-transcriptional silencing of PhCRT1/2 expression strongly impaired pollen tube growth in vitro. Here, we report cloning of a new PhCRT3a homolog; we identified the full-length cDNA sequence and described its molecular characteristics and phylogenetic relationships to other plant CRT3 genes. Using an RNA interference (RNAi) strategy, we found that knockdown of PhCRT3a gene expression caused numerous defects in the morphology and ultrastructure of cultivated pollen tubes, including disorganization of the actin cytoskeleton and loss of cytoplasmic zonation. Elongation of siPhCRT3a pollen tubes was disrupted, and some of them ruptured. Our present data provide the first evidence that PhCRT3a expression is required for normal pollen tube growth. Thus, we discuss relationships between diverse CRT isoforms in several interdependent processes driving the apical growth of the pollen tube, including actomyosin-dependent cytoplasmic streaming, organelle positioning, vesicle trafficking, and cell wall biogenesis.

Arabidopsis annexin 5 is involved in maintenance of pollen membrane integrity and permeability

In Arabidopsis, a dry stigma surface enables a gradual hydration of pollen grains by a controlled release of water. Occasionally the grains may be exposed to extreme precipitations that cause rapid water influx and swelling, eventually leading to pollen membrane rupture. In metazoans, calcium- and phospholipid-binding proteins, referred to as annexins, participate in the repair of plasma membrane damages. It remains unclear, however, how this process is conducted in plants. Here, we examined whether plant annexin 5 (ANN5), the most abundant member of the annexin family in pollen, is involved in the restoration of pollen membrane integrity. We analyzed the cellular dynamics of ANN5 in pollen grains undergoing hydration in favorable or stress conditions. We observed a transient association of ANN5 with the pollen membrane during in vitro hydration that did not occur in the pollen grains being hydrated on the stigma. To simulate a rainfall, we performed spraying of the pollinated stigma with deionized water that induced ANN5 accumulation at the pollen membrane. Interestingly, calcium or magnesium application affected pollen membrane properties differently, causing rupture or shrinkage of pollen membrane, respectively. Both treatments, however, induced ANN5 recruitment to the pollen membrane. Our data suggest a model in which ANN5 is involved in the maintenance of membrane integrity in pollen grains exposed to osmotic or ionic imbalances.

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.

Myosin XI-B is involved in the transport of vesicles and organelles in pollen tubes of Arabidopsis thaliana

The movement of organelles and vesicles in pollen tubes depends on F-actin. However, the molecular mechanism through which plant myosin XI drives the movement of organelles is still controversial, and the relationship between myosin XI and vesicle movement in pollen tubes is also unclear. In this study, we found that the siliques of the myosin xi-b/e mutant were obviously shorter than those of the wild-type (WT) and that the seed set of the mutant was severely deficient. The pollen tube growth of myosin xi-b/e was significantly inhibited both in vitro and in vivo. Fluorescence recovery after photobleaching showed that the velocity of vesicle movement in the pollen tube tip of the myosin xi-b/e mutant was lower than that of the WT. It was also found that peroxisome movement was significantly inhibited in the pollen tubes of the myosin xi-b/e mutant, while the velocities of the Golgi stack and mitochondrial movement decreased relatively less in the pollen tubes of the mutant. The endoplasmic reticulum streaming in the pollen tube shanks was not significantly different between the WT and the myosin xi-b/e mutant. In addition, we found that myosin XI-B-GFP colocalized obviously with vesicles and peroxisomes in the pollen tubes of Arabidopsis. Taken together, these results indicate that myosin XI-B may bind mainly to vesicles and peroxisomes, and drive their movement in pollen tubes. These results also suggest that the mechanism by which myosin XI drives organelle movement in plant cells may be evolutionarily conserved compared with other eukaryotic cells.

Lipid flippases as key players in plant adaptation to their environment

Lipid flippases (P4 ATPases) are active transporters that catalyse the translocation of lipids between the two sides of the biological membranes in the secretory pathway. This activity modulates biological membrane properties, contributes to vesicle formation, and is the trigger for lipid signalling events, which makes P4 ATPases essential for eukaryotic cell survival. Plant P4 ATPases (also known as aminophospholipid ATPases (ALAs)) are crucial for plant fertility and proper development, and are involved in key adaptive responses to biotic and abiotic stress, including chilling tolerance, heat adaptation, nutrient deficiency responses and pathogen defence. While ALAs present many analogies to mammalian and yeast P4 ATPases, they also show characteristic features as the result of their independent evolution. In this Review, the main properties, roles, regulation and mechanisms of action of ALA proteins are discussed.

Transcriptome and translatome changes in germinated pollen under heat stress uncover roles of transporter genes involved in pollen tube growth

Plant reproduction is one key biological process that is very sensitive to heat stress and, as a result, enhanced global warming becomes a serious threat to agriculture. In this work, we have studied the effects of heat on germinated pollen of Arabidopsis thaliana both at the transcriptional and translational level. We have used a high-resolution ribosome profiling technology to provide a comprehensive study of the transcriptome and the translatome of germinated pollen at permissive and restrictive temperatures. We have found significant down-regulation of key membrane transporters required for pollen tube growth by heat, thus uncovering heat-sensitive targets. A subset of the heat-repressed transporters showed coordinated up-regulation with canonical heat-shock genes at permissive conditions. We also found specific regulations at the translational level and we have uncovered the presence of ribosomes on sequences annotated as non-coding. Our results demonstrate that heat impacts mostly on membrane transporters thus explaining the deleterious effects of heat stress on pollen growth. The specific regulations at the translational level and the presence of ribosomes on non-coding RNAs highlights novel regulatory aspects on plant fertilization.

Transcriptome and translatome changes in germinated pollen under heat stress uncover roles of transporter genes involved in pollen tube growth

Plant reproduction is one key biological process that is very sensitive to heat stress and, as a result, enhanced global warming becomes a serious threat to agriculture. In this work, we have studied the effects of heat on germinated pollen of Arabidopsis thaliana both at the transcriptional and translational level. We have used a high-resolution ribosome profiling technology to provide a comprehensive study of the transcriptome and the translatome of germinated pollen at permissive and restrictive temperatures. We have found significant down-regulation of key membrane transporters required for pollen tube growth by heat, thus uncovering heat-sensitive targets. A subset of the heat-repressed transporters showed coordinated up-regulation with canonical heat-shock genes at permissive conditions. We also found specific regulations at the translational level and we have uncovered the presence of ribosomes on sequences annotated as non-coding. Our results demonstrate that heat impacts mostly on membrane transporters thus explaining the deleterious effects of heat stress on pollen growth. The specific regulations at the translational level and the presence of ribosomes on non-coding RNAs highlights novel regulatory aspects on plant fertilization.

Plasma membrane nano-organization specifies phosphoinositide effects on Rho-GTPases and actin dynamics in tobacco pollen tubes

Pollen tube growth requires coordination of cytoskeletal dynamics and apical secretion. The regulatory phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) is enriched in the subapical plasma membrane of pollen tubes of Arabidopsis thaliana and tobacco (Nicotiana tabacum) and can influence both actin dynamics and secretion. How alternative PtdIns(4,5)P2 effects are specified is unclear. In tobacco pollen tubes, spinning disc microscopy (SD) reveals dual distribution of a fluorescent PtdIns(4,5)P2-reporter in dynamic plasma membrane nanodomains vs. apparent diffuse membrane labeling, consistent with spatially distinct coexisting pools of PtdIns(4,5)P2. Several PI4P 5-kinases (PIP5Ks) can generate PtdIns(4,5)P2 in pollen tubes. Despite localizing to one membrane region, the PIP5Ks AtPIP5K2-EYFP and NtPIP5K6-EYFP display distinctive overexpression effects on cell morphologies, respectively related to altered actin dynamics or membrane trafficking. When analyzed by SD, AtPIP5K2-EYFP associated with nanodomains, whereas NtPIP5K6-EYFP localized diffusely. Chimeric AtPIP5K2-EYFP and NtPIP5K6-EYFP variants with reciprocally swapped membrane-associating domains evoked reciprocally shifted effects on cell morphology upon overexpression. Overall, active PI4P 5-kinase variants stabilized actin when targeted to nanodomains, suggesting a role of nanodomain-associated PtdIns(4,5)P2 in actin regulation. This notion is further supported by interaction and proximity of nanodomain-associated AtPIP5K2 with the Rho-GTPase NtRac5, and by its functional interplay with elements of Rho of plants signaling. Plasma membrane nano-organization may thus aid the specification of PtdIns(4,5)P2 functions to coordinate cytoskeletal dynamics and secretion.

Membrane imaging in the plant endomembrane system

Recent studies on membrane imaging in the plant endomembrane system by 2-D/3-D CLSM and TEM provide future perspectives of whole-cell ET and cryo-FIB-aided cryo-ET analysis.

GORI, encoding the WD40 domain protein, is required for pollen tube germination and elongation in rice

Successful delivery of sperm cells to the embryo sac in higher plants is mediated by pollen tube growth. The molecular mechanisms underlying pollen germination and tube growth in crop plants remain rather unclear, although these mechanisms are crucial to plant reproduction and seed formation. By screening pollen-specific gene mutants in rice (Oryza sativa), we identified a T-DNA insertional mutant of Germinating modulator of rice pollen (GORI) that showed a one-to-one segregation ratio for wild type (WT) to heterozygous. GORI encodes a seven-WD40-motif protein that is homologous to JINGUBANG/REN4 in Arabidopsis. GORI is specifically expressed in rice pollen, and its protein is localized in the nucleus, cytosol and plasma membrane. Furthermore, a homozygous mutant, gori-2, created through CRISPR-Cas9 clearly exhibited male sterility with disruption of pollen tube germination and elongation. The germinated pollen tube of gori-2 exhibited decreased actin filaments and altered pectin distribution. Transcriptome analysis revealed that 852 pollen-specific genes were downregulated in gori-2 compared with the WT, and Gene Ontology enrichment analysis indicated that these genes are strongly associated with cell wall modification and clathrin coat assembly. Based on the molecular features of GORI, phenotypical observation of the gori mutant and its interaction with endocytic proteins and Rac GTPase, we propose that GORI plays key roles in forming endo-/exocytosis complexes that could mediate pollen tube growth in rice.

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.

Rab-E and its interaction with myosin XI are essential for polarised cell growth

The fundamental process of polarised exocytosis requires the interconnected activity of molecular motors trafficking vesicular cargo within a dynamic cytoskeletal network. In plants, few mechanistic details are known about how molecular motors, such as myosin XI, associate with their secretory cargo to support the ubiquitous processes of polarised growth and cell division. Live-cell imaging coupled with targeted gene knockouts and a high-throughput RNAi assay enabled the first characterisation of the loss of Rab-E function. Yeast two-hybrid and subsequent in silico structural prediction uncovered a specific interaction between Rab-E and myosin XI that is conserved between P. patens and A. thaliana. Rab-E co-localises with myosin XI at sites of active exocytosis, and at the growing tip both proteins are spatiotemporally coupled. Rab-E is required for normal plant growth in P. patens and the rab-E and myosin XI phenotypes are rescued by A. thaliana's Rab-E1c and myosin XI-K/E, respectively. Both PpMyoXI and AtMyoXI-K interact with PpRabE14, and the interaction is specifically mediated by PpMyoXI residue V1422. This interaction is required for polarised growth. Our results suggest that the interaction of Rab-E and myosin XI is a conserved feature of polarised growth in plants.

The Arabidopsis thaliana Class II Formin FH13 Modulates Pollen Tube Growth

Formins are a large, evolutionarily conserved family of actin-nucleating proteins with additional roles in regulating microfilament, microtubule, and membrane dynamics. Angiosperm formins, expressed in both sporophytic and gametophytic tissues, can be divided into two subfamilies, Class I and Class II, each often exhibiting characteristic domain organization. Gametophytically expressed Class I formins have been documented to mediate plasma membrane-based actin assembly in pollen grains and pollen tubes, contributing to proper pollen germination and pollen tube tip growth, and a rice Class II formin, FH5/RMD, has been proposed to act as a positive regulator of pollen tube growth based on mutant phenotype and overexpression data. Here we report functional characterization of the Arabidopsis thaliana pollen-expressed typical Class II formin FH13 (At5g58160). Consistent with published transcriptome data, live-cell imaging in transgenic plants expressing fluorescent protein-tagged FH13 under the control of the FH13 promoter revealed expression in pollen and pollen tubes with non-homogeneous signal distribution in pollen tube cytoplasm, suggesting that this formin functions in the male gametophyte. Surprisingly, fh13 loss of function mutations do not affect plant fertility but result in stimulation of in vitro pollen tube growth, while tagged FH13 overexpression inhibits pollen tube elongation. Pollen tubes of mutants expressing a fluorescent actin marker exhibited possible minor alterations of actin organization. Our results thus indicate that FH13 controls or limits pollen tube growth, or, more generally, that typical Class II formins should be understood as modulators of pollen tube elongation rather than merely components of the molecular apparatus executing tip growth.

ROS and Ions in Cell Signaling during Sexual Plant Reproduction

Pollen grain is a unique haploid organism characterized by two key physiological processes: activation of metabolism upon exiting dormancy and polar tube growth. In gymnosperms and flowering plants, these processes occur in different time frames and exhibit important features; identification of similarities and differences is still in the active phase. In angiosperms, the growth of male gametophyte is directed and controlled by its microenvironment, while in gymnosperms it is relatively autonomous. Recent reviews have detailed aspects of interaction between angiosperm female tissues and pollen such as interactions between peptides and their receptors; however, accumulated evidence suggests low-molecular communication, in particular, through ion exchange and ROS production, equally important for polar growth as well as for pollen germination. Recently, it became clear that ROS and ionic currents form a single regulatory module, since ROS production and the activity of ion transport systems are closely interrelated and form a feedback loop.

Identification of pollen and pistil polygalacturonases in Nicotiana tabacum and their function in interspecific stigma compatibility

KEY MESSAGE

The pollen and pistil polygalacturonases in Nicotiana tabacum were identified and found to regulate pollen tube growth and interspecific compatibility. Polygalacturonase (PG) is one of the enzymes catalyzing the hydrolysis of pectin. This process plays important roles in the pollen and pistil. In this research, the pollen and pistil PGs in Nicotiana tabacum (NtPGs) were identified, and their expression, localization and the potential function in the pollen and interspecific stigma incompatibility were explored. The results showed that 118 NtPGs were retrieved from the genome of N. tabacum. The phylogenetic tree and RT-qPCR analysis led to the identification of 10 pollen PGs; among them, two, seven and one showed specifically higher expression levels in the early development of anthers, during pollen maturation and in mature anthers, respectively, indicating their function difference. Immunofluorescence analysis showed that PGs were located in the cytoplasm of (1) mature pollen and (2) in vitro grown pollen tubes, as well as in the wall of in vivo grown pollen tubes. Four NtPGs in clade A were identified as the pistil PGs, and the pistil PGs were not found in clade E. Significantly higher PGs expression was recorded after incompatible pollination in comparison with the compatible stigma, indicating a potential function of PGs in regulating stigma incompatibility. The influence of PGs on pollen tube growth was explored in vitro and partly in vivo, showing that high PGs activity inhibited pollen tube growth. The application of PGs on the otherwise compatible stigma resulted in pollen tube growth inhibition or failure of germination. These results further supported that increased PGs expression in incompatible stigma might be partially responsible for the interspecific stigma incompatibility in Nicotiana.

Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.

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

BACKGROUND AND AIMS

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

METHODS

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

KEY RESULTS

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

CONCLUSIONS

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

Exocytosis and endocytosis: coordinating and fine-tuning the polar tip growth domain in pollen tubes

Pollen tubes rapidly elongate, penetrate, and navigate through multiple female tissues to reach ovules for sperm delivery by utilizing a specialized form of polar growth known as tip growth. This process requires a battery of cellular activities differentially occurring at the apical growing region of the plasma membrane (PM), such as the differential cellular signaling involving calcium (Ca2+), phospholipids, and ROP-type Rho GTPases, fluctuation of ions and pH, exocytosis and endocytosis, and cell wall construction and remodeling. There is an emerging understanding of how at least some of these activities are coordinated and/or interconnected. The apical active ROP modulates exocytosis to the cell apex for PM and cell wall expansion differentially occurring at the tip. The differentiation of the cell wall involves at least the preferential distribution of deformable pectin polymers to the apex and non-deformable pectin polymers to the shank of pollen tubes, facilitating the apical cell expansion driven by high internal turgor pressure. Recent studies have generated inroads into how the ROP GTPase-based intracellular signaling is coordinated spatiotemporally with the external wall mechanics to maintain the tubular cell shape and how the apical cell wall mechanics are regulated to allow rapid tip growth while maintaining the cell wall integrity under the turgor pressure. Evidence suggests that exocytosis and endocytosis play crucial but distinct roles in this spatiotemporal coordination. In this review, we summarize recent advances in the regulation and coordination of the differential pectin distribution and the apical domain of active ROP by exocytosis and endocytosis in pollen tubes.

Coordinated Localization and Antagonistic Function of NtPLC3 and PI4P 5-Kinases in the Subapical Plasma Membrane of Tobacco Pollen Tubes

Polar tip growth of pollen tubes is regulated by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂), which localizes in a well-defined region of the subapical plasma membrane. How the PtdIns(4,5)P₂ region is maintained is currently unclear. In principle, the formation of PtdIns(4,5)P₂ by PI4P 5-kinases can be counteracted by phospholipase C (PLC), which hydrolyzes PtdIns(4,5)P₂. Here, we show that fluorescence-tagged tobacco NtPLC3 displays a subapical plasma membrane distribution which frames that of fluorescence-tagged PI4P 5-kinases, suggesting that NtPLC3 may modulate PtdIns(4,5)P₂-mediated processes in pollen tubes. The expression of a dominant negative NtPLC3 variant resulted in pollen tube tip swelling, consistent with a delimiting effect on PtdIns(4,5)P₂ production. When pollen tube morphologies were assessed as a quantitative read-out for PtdIns(4,5)P₂ function, NtPLC3 reverted the effects of a coexpressed PI4P 5-kinase, demonstrating that NtPLC3-mediated breakdown of PtdIns(4,5)P₂ antagonizes the effects of PtdIns(4,5)P₂ overproduction in vivo. When analyzed by spinning disc microscopy, fluorescence-tagged NtPLC3 displayed discontinuous membrane distribution omitting punctate areas of the membrane, suggesting that NtPLC3 is involved in the spatial restriction of plasma membrane domains also at the nanodomain scale. Together, the data indicate that NtPLC3 may contribute to the spatial restriction of PtdIns(4,5)P₂ in the subapical plasma membrane of pollen tubes.

RNA-Seq analysis of compatible and incompatible styles of Pyrus species at the beginning of pollination

At the early stage of pollination, the difference in gene expression between compatibility and incompatibility is highly significant about the pollen-specific expression of the LRR gene, resistance, and defensin genes. In Rosaceae, incompatible pollen can penetrate into the style during the gametophytic self-incompatibility response. It is therefore considered a stylar event rather than a stigmatic event. In this study, we explored the differences in gene expression between compatibility and incompatibility in the early stage of pollination. The self-compatible pear variety "Jinzhuili" is a naturally occurring bud mutant from "Yali", a leading Chinese native cultivar exhibiting typical gametophytic self-incompatibility. We collected the styles of 'Yali' and 'Jinzhuili' at 0.5 and 2 h after self-pollination and then performed high-throughput sequencing. According to the KEGG analysis of the differentially expressed genes, several metabolic pathways, such as "Plant hormone signal transduction", "Plant-pathogen interaction", are the main pathways was the most represented pathway. Quantitative PCR was used to validate these differential genes. The expression levels of genes related to pollen growth and disease inhibition, such as LRR (Leucine-rich repeat extensin), resistance, defensin, and auxin, differed significantly between compatible and incompatible pollination. Interestingly, at 0.5 h, most of these genes were upregulated in the compatible pollination system compared with the incompatible pollination system. Calcium transport, which requires ATPase, also demonstrated upregulated expression. In summary, the self-incompatibility reaction was initiated when the pollen land on the stigma.

LORELEI-LIKE GPI-ANCHORED PROTEINS 2/3 Regulate Pollen Tube Growth as Chaperones and Coreceptors for ANXUR/BUPS Receptor Kinases in Arabidopsis

Pollen tube growth is crucial for successful fertilization. In Arabidopsis thaliana, the ANXUR (ANX)/BUPS receptor kinase complex controls and maintains pollen tube growth in response to autocrine rapid alkalinization factor 4/19 (RALF4/19) signaling; however, the molecular and cellular mechanisms underlying the ANX/BUPS-mediated regulation of pollen tube growth remain unclear. In this study, we found that pollen-specific LORELEI-like GPI-anchored proteins 2 and 3 (LLG2/3) promote pollen tube growth in vitro and in vivo. LLG2/3 interacte with ANX/BUPS in a RALF4-concentration-dependent manner, suggesting that ANX/BUPS and LLG2/3 might form a receptor-coreceptor complex for perceiving RALF peptide signals. Disruption of the ANX/BUPS-LLG2/3 interaction led to the cytoplasmic retention of ANX1/2, in either llg2/3 knockdown mutants or in anx1/2 mutants lacking the J region, which mediates the ANX/BUPS-LLG2/3 interaction. Moreover, we found that RALF4 induced the production of reactive oxygen species (ROS), which stimulate pollen tube growth and reduce pollen burst rate. ROS levels are reduced in the pollen tubes of LLG2/3 RNAi lines, and application of exogenous H₂O₂ could partially rescue the defective pollen tube growth of LLG2/3 RNAi lines. Taken together, our study identifies LLG2/3 as novel regulatory components of pollen tube growth, and shows that they chaperone ANX/BUPS for secretion to the apical plasma membrane of pollen tube and act as coreceptors of ANX/BUPS in the activation of ROS production for promoting pollen tube growth.

ZmGLR, a cell membrane localized microtubule-associated protein, mediated leaf morphogenesis in maize

Microtubule arrays play notable roles in cell division, cell movement, cell morphogenesis and signal transduction. Due to their important regulation of microtubule dynamic instability and array-ordering processes, microtubule-associated proteins have been a cutting-edge issue in research. Here, a new maize microtubule-associated protein, ZmGLR (Zea mays glutamic acid- and lysine-rich), was found. ZmGLR bundles microtubules in vitro and targets the cell membrane through an interaction between 24 conserved N-terminal amino acids and specific phosphatidylinositol phosphates (PtdInsPs). Increased Ca²⁺ levels in the cytoplasm lead to ZmGLR partially dissociating from the cell membrane and moving into the cytoplasm to associate with microtubule. Overexpression and RNAi of ZmGLR both resulted in misoriented microtubule arrays, which led to dwarf maize plants and curved leaves. In addition, the expression of ZmGLR was regulated by BR and auxin through ZmBES1 and ZmARF9, respectively. This study reveals that the microtubule-associated protein ZmGLR plays a crucial role in cortical microtubule reorientation and maize leaf morphogenesis.

Chloride as a Beneficial Macronutrient in Higher Plants: New Roles and Regulation

Chloride (Cl-) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3-), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl- has gone from being considered a harmful ion, accidentally absorbed through NO3- transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl- determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl- and NO3- molecules, the NO3-/Cl- plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl- influx (ZmNPF6.4 and ZmNPF6.6), Cl- efflux (AtSLAH3 and AtSLAH1), and Cl- compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl-, as well as for cell turgor regulation and stress tolerance in plants.

A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization

In flowering plants, pollen tubes undergo tip growth to deliver two nonmotile sperm to the ovule where they fuse with an egg and central cell to achieve double fertilization. This extended journey involves rapid growth and changes in gene activity that manage compatible interactions with at least seven different cell types. Nearly half of the genome is expressed in haploid pollen, which facilitates genetic analysis, even of essential genes. These unique attributes make pollen an ideal system with which to study plant cell-cell interactions, tip growth, cell migration, the modulation of cell wall integrity, and gene expression networks. We highlight the signaling systems required for pollen tube navigation and the potential roles of Ca²⁺ signals. The dynamics of pollen development make sexual reproduction highly sensitive to heat stress. Understanding this vulnerability may generate strategies to improve seed crop yields that are under threat from climate change.

Quantitative Analysis of Tip Growth, Phototropic Responses, and Other Blue Light-Dependent Photoresponses of Vaucheria

The coenocytic tip-growing alga Vaucheria exhibits positive and negative phototropism, apical expansion, polarotropism, and branch induction from the illuminated region of the cell, all of which are caused by blue light. The bending response of Vaucheria is a blue light-mediated growth response. Differently from diffuse-growing cells or organs, the apical hemispherical dome of the Vaucheria cell is the site of not only maximum growth activity but also the site of blue light perception. Thence the phototropic response is initiated by the bulging mechanism: that is, a quick shift of the growth center to the adjacent subapical flank region. Since tip growth is driven by localized exocytosis, both phototropic bending and branch induction are considered to be closely related blue light-responses. Here I describe first how to prepare a highly useful culture medium for most freshwater algae, to establish unialgal and axenic culture of Vaucheria, and then describe several simple illumination systems using ordinary and/or inverted microscopes for the measurements of tip growth and for analyses of phototropism, polarotropism, and blue light-induced branching. Brief information is also included concerning the nature and function of aureochrome, the newly discovered, ochrophyte-specific blue light receptor. Aureochrome mediates blue light-induced branching, but its role in the phototropic response is still not elucidated.

Arabidopsis thaliana plants lacking the ARP2/3 complex show defects in cell wall assembly and auxin distribution

BACKGROUND AND AIM

The cytoskeleton plays an important role in the synthesis of plant cell walls. Both microtubules and actin cytoskeleton are known to be involved in the morphogenesis of plant cells through their role in cell wall building. The role of ARP2/3-nucleated actin cytoskeleton in the morphogenesis of cotyledon pavement cells has been described before. Seedlings of Arabidopsis mutants lacking a functional ARP2/3 complex display specific cell wall-associated defects.

METHODS

In three independent Arabidopsis mutant lines lacking subunits of the ARP2/3 complex, phenotypes associated with the loss of the complex were analysed throughout plant development. Organ size and anatomy, cell wall composition, and auxin distribution were investigated.

KEY RESULTS

ARP2/3-related phenotype is associated with changes in cell wall composition, and the phenotype is manifested especially in mature tissues. Cell walls of mature plants contain less cellulose and a higher amount of homogalacturonan, and display changes in cell wall lignification. Vascular bundles of mutant inflorescence stems show a changed pattern of AUX1-YFP expression. Plants lacking a functional ARP2/3 complex have decreased basipetal auxin transport.

CONCLUSIONS

The results suggest that the ARP2/3 complex has a morphogenetic function related to cell wall synthesis and auxin transport.

Microtubules play a role in trafficking prevacuolar compartments to vacuoles in tobacco pollen tubes

Fine regulation of exocytosis and endocytosis plays a basic role in pollen tube growth. Excess plasma membrane secreted during pollen tube elongation is known to be retrieved by endocytosis and partially reused in secretory pathways through the Golgi apparatus. Dissection of endocytosis has enabled distinct degradation pathways to be identified in tobacco pollen tubes and has shown that microtubules influence the transport of plasma membrane internalized in the tip region to vacuoles. Here, we used different drugs affecting the polymerization state of microtubules together with SYP21, a marker of prevacuolar compartments, to characterize trafficking of prevacuolar compartments in Nicotiana tabacum pollen tubes. Ultrastructural and biochemical analysis showed that microtubules bind SYP21-positive microsomes. Transient transformation of pollen tubes with LAT52-YFP-SYP21 revealed that microtubules play a key role in the delivery of prevacuolar compartments to tubular vacuoles.

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.

Apple S-RNase interacts with an actin-binding protein, MdMVG, to reduce pollen tube growth by inhibiting its actin-severing activity at the early stage of self-pollination induction

In S-RNase-mediated self-incompatibility, S-RNase secreted from the style destroys the actin cytoskeleton of the self-pollen tubes, eventually halting their growth, but the mechanism of this process remains unclear. In vitro biochemical assays revealed that S-RNase does not bind or sever filamentous actin (F-actin). In apple (Malus domestica), we identified an actin-binding protein containing myosin, villin and GRAM (MdMVG), that physically interacts with S-RNase and directly binds and severs F-actin. Immunofluorescence assays and total internal reflection fluorescence microscopy indicated that S-RNase inhibits the F-actin-severing activity of MdMVG in vitro. In vivo, the addition of S-RNase to self-pollen tubes increased the fluorescence intensity of actin microfilaments and reduced the severing frequency of microfilaments and the rate of pollen tube growth in self-pollination induction in the presence of MdMVG overexpression. By generating 25 single-, double- and triple-point mutations in the amino acid motif E-E-K-E-K of MdMVG via mutagenesis and testing the resulting mutants with immunofluorescence, we identified a triple-point mutant, MdMVG^{(E167A/E171A/K185A)} , that no longer has F-actin-severing activity or interacts with any of the four S-haplotype S-RNases, indicating that all three amino acids (E167, E171 and K185) are essential for the severing activity of MdMVG and its interaction with S-RNases. We conclude that apple S-RNase interacts with MdMVG to reduce self-pollen tube growth by inhibiting its F-actin-severing activity.

Calcium dynamics in tomato pollen tubes using the Yellow Cameleon 3.6 sensor

In vitro tomato pollen tubes show a cytoplasmic calcium gradient that oscillates with the same period as growth. Pollen tube growth requires coordination between the tip-focused cytoplasmic calcium concentration ([Ca²⁺]_cyt) gradient and the actin cytoskeleton. This [Ca²⁺]_cyt gradient is necessary for exocytosis of small vesicles, which contributes to the delivery of new membrane and cell wall at the pollen tube tip. The mechanisms that generate and maintain this [Ca²⁺]_cyt gradient are not completely understood. Here, we studied calcium dynamics in tomato (Solanum lycopersicum) pollen tubes using transgenic tomato plants expressing the Yellow Cameleon 3.6 gene under the pollen-specific promoter LAT52. We use tomato as an experimental model because tomato is a Solanaceous plant that is easy to transform, and has an excellent genomic database and genetic stock center, and unlike Arabidopsis, tomato pollen is a good system to do biochemistry. We found that tomato pollen tubes showed an oscillating tip-focused [Ca²⁺]_cyt gradient with the same period as growth. Then, we used a pharmacological approach to disturb the intracellular Ca²⁺ homeostasis, evaluating how the [Ca²⁺]_cyt gradient, pollen germination and in vitro pollen tube growth were affected. We found that cyclopiazonic acid (CPA), a drug that inhibits plant P_IIA-type Ca²⁺-ATPases, increased [Ca²⁺]_cyt in the subapical zone, leading to the disappearance of the Ca²⁺ oscillations and inhibition of pollen tube growth. In contrast, 2-aminoethoxydiphenyl borate (2-APB), an inhibitor of Ca²⁺ released from the endoplasmic reticulum to the cytoplasm in animals cells, completely reduced [Ca²⁺]_cyt at the tip of the tube, blocked the gradient and arrested pollen tube growth. Although both drugs have antagonistic effects on [Ca²⁺]_cyt, both inhibited pollen tube growth triggering the disappearance of the [Ca²⁺]_cyt gradient. When CPA and 2-APB were combined, their individual inhibitory effects on pollen tube growth were partially compensated. Finally, we found that GsMTx-4, a peptide from spider venom that blocks stretch-activated Ca²⁺ channels, inhibited tomato pollen germination and had a heterogeneous effect on pollen tube growth, suggesting that these channels are also involved in the maintenance of the [Ca²⁺]_cyt gradient. All these results indicate that tomato pollen tube is an excellent model to study calcium dynamics.

Glycolysis regulates pollen tube polarity via Rho GTPase signaling

As a universal energy generation pathway utilizing carbon metabolism, glycolysis plays an important housekeeping role in all organisms. Pollen tubes expand rapidly via a mechanism of polarized growth, known as tip growth, to deliver sperm for fertilization. Here, we report a novel and surprising role of glycolysis in the regulation of growth polarity in Arabidopsis pollen tubes via impingement of Rho GTPase-dependent signaling. We identified a cytosolic phosphoglycerate kinase (pgkc-1) mutant with accelerated pollen germination and compromised pollen tube growth polarity. pgkc-1 mutation greatly diminished apical exocytic vesicular distribution of REN1 RopGAP (Rop GTPase activating protein), leading to ROP1 hyper-activation at the apical plasma membrane. Consequently, pgkc-1 pollen tubes contained higher amounts of exocytic vesicles and actin microfilaments in the apical region, and showed reduced sensitivity to Brefeldin A and Latrunculin B, respectively. While inhibition of mitochondrial respiration could not explain the pgkc-1 phenotype, the glycolytic activity is indeed required for PGKc function in pollen tubes. Moreover, the pgkc-1 pollen tube phenotype was mimicked by the inhibition of another glycolytic enzyme. These findings highlight an unconventional regulatory function for a housekeeping metabolic pathway in the spatial control of a fundamental cellular process.

Glycolysis, which breaks down glucose to produce energy, has long been considered a “housekeeping” pathway in living cells, i.e., it helps maintain basic cellular functions. Here, we found that the glycolysis pathway plays an unconventional regulatory role in cell polarity, i.e., the intrinsic asymmetry in the shape, structure, and organization of cellular components. Mutation in the gene encoding the glycolytic enzyme cytosolic phosphoglycerate kinase (PGKc) leads to swollen and shorter pollen tubes in Arabidopsis thaliana, which is associated with the over-activation of Rho GTPase—a master regulator of cell polarity. Our results suggest that this phenomenon is caused by a specific regulatory role of cytosolic glycolysis rather than the global energy supply or moonlighting functions of glycolytic enzymes that modulate pollen tube growth polarity. Our findings shed light on the diverse biological roles of glycolysis in plants beyond simple “housekeeping” functions.

Simultaneous imaging and functional studies reveal a tight correlation between calcium and actin networks

Tip-growing cells elongate in a highly polarized manner via focused secretion of flexible cell-wall material. Calcium has been implicated as a vital factor in regulating the deposition of cell-wall material. However, deciphering the molecular and mechanistic calcium targets in vivo has remained challenging. Here, we investigated intracellular calcium dynamics in the moss Physcomitrella patens, which provides a system with an abundant source of genetically identical tip-growing cells, excellent cytology, and a large molecular genetic tool kit. To visualize calcium we used a genetically encoded cytosolic FRET probe, revealing a fluctuating tipward gradient with a complex oscillatory profile. Wavelet analysis coupled with a signal-sifting algorithm enabled the quantitative comparison of the calcium behavior in cells where growth was inhibited mechanically, pharmacologically, or genetically. We found that cells with suppressed growth have calcium oscillatory profiles with longer frequencies, suggesting that there is a feedback between the calcium gradient and growth. To investigate the mechanistic basis for this feedback we simultaneously imaged cytosolic calcium and actin, which has been shown to be essential for tip growth. We found that high cytosolic calcium promotes disassembly of a tip-focused actin spot, while low calcium promotes assembly. In support of this, abolishing the calcium gradient resulted in dramatic actin accumulation at the tip. Together these data demonstrate that tipward calcium is quantitatively linked to actin accumulation in vivo and that the moss P. patens provides a powerful system to uncover mechanistic links between calcium, actin, and growth.

Ethylene promotes pollen tube growth by affecting actin filament organization via the cGMP-dependent pathway in Arabidopsis thaliana

Ethylene and cGMP are key regulators of plant developmental processes. In this study, we demonstrate that ethylene or cGMP promote pollen tube growth in a dose-dependent manner. The etr1-1 mutant was found to be insensitive to ethylene with regard to pollen tube growth, while the growth-promoting effect of ethylene in etr2-2, ein4-4, or ein4-7 did not change, suggesting that ethylene signaling was mainly perceived by ETR1. However, the function of cGMP was not inhibited in etr1-1 and pollen tubes became insensitive to ethylene when the endogenous cGMP level was artificially decreased. This shows that cGMP is necessary for the control of pollen tube growth and that it might be a downstream component of ETR1 in the ethylene signaling pathway. Our study also found that ethylene or cGMP increase the actin bundles and elevated the percentage of relative amount of F-actin, while removal of cGMP decreased actin bundles abundance and altered the ratio of F-actin in the tip and base regions of pollen tubes. In conclusion, our data suggests that ethylene functions as the upstream signal of cGMP, and that both signals promote pollen germination and tube growth by regulating F-actin, which is essential for vesicular transport and cytoplasmic streaming.

A Ca2+-dependent remodelled actin network directs vesicle trafficking to build wall ingrowth papillae in transfer cells

The transport function of transfer cells is conferred by an enlarged plasma membrane area, enriched in nutrient transporters, that is supported on a scaffold of wall ingrowth (WI) papillae. Polarized plumes of elevated cytosolic Ca2+ define loci at which WI papillae form in developing adaxial epidermal transfer cells of Vicia faba cotyledons that are induced to trans-differentiate when the cotyledons are placed on culture medium. We evaluated the hypothesis that vesicle trafficking along a Ca2+-regulated remodelled actin network is the mechanism that underpins this outcome. Polarized to the outer periclinal cytoplasm, a Ca2+-dependent remodelling of long actin bundles into short, thin bundles was found to be essential for assembling WI papillae but not the underlying uniform wall layer. The remodelled actin network directed polarized vesicle trafficking to sites of WI papillae construction, and a pharmacological study indicated that both exo- and endocytosis contributed to assembly of the papillae. Potential candidates responsible for the Ca2+-dependent actin remodelling, along with those underpinning polarized exo- and endocyotosis, were identified in a transcriptome RNAseq database generated from the trans-differentiating epidermal cells. Of most significance, endocytosis was controlled by up-regulated expression of a dynamin-like isoform. How a cycle of localized exo- and endocytosis, regulated by Ca2+-dependent actin remodelling, assembles WI papillae is discussed.

Spermine Regulates Pollen Tube Growth by Modulating Ca2+-Dependent Actin Organization and Cell Wall Structure

Proper growth of the pollen tube depends on an elaborate mechanism that integrates several molecular and cytological sub-processes and ensures a cell shape adapted to the transport of gametes. This growth mechanism is controlled by several molecules among which cytoplasmic and apoplastic polyamines. Spermine (Spm) has been correlated with various physiological processes in pollen, including structuring of the cell wall and modulation of protein (mainly cytoskeletal) assembly. In this work, the effects of Spm on the growth of pear pollen tubes were analyzed. When exogenous Spm (100 μM) was supplied to germinating pollen, it temporarily blocked tube growth, followed by the induction of apical swelling. This reshaping of the pollen tube was maintained also after growth recovery, leading to a 30-40% increase of tube diameter. Apical swelling was also accompanied by a transient increase in cytosolic calcium concentration and alteration of pH values, which were the likely cause for major reorganization of actin filaments and cytoplasmic organelle movement. Morphological alterations of the apical and subapical region also involved changes in the deposition of pectin, cellulose, and callose in the cell wall. Thus, results point to the involvement of Spm in cell wall construction as well as cytoskeleton organization during pear pollen tube growth.

Arabidopsis shaker pollen inward K+ channel SPIK functions in SnRK1 complex-regulated pollen hydration on the stigma

Pollen hydration is a critical step that determines pollen germination on the stigma. KINβγ is a plant-specific subunit of the SNF1-related protein kinase 1 complex (SnRK1 complex). In pollen of the Arabidopsis kinβγ mutant, the levels of reactive oxygen species were decreased which lead to compromised hydration of the mutant pollen on the stigma. In this study, we analyzed gene expression in kinβγ mutant pollen by RNA-seq and found the expression of inward shaker K⁺ channel SPIK was down-regulated in the kinβγ pollen. Furthermore, we showed that the pollen hydration of the Arabidopsis spik mutant was defective on the wild-type stigma, although the mutant pollen demonstrated normal hydration in vitro. Additionally, the defective hydration of spik mutant pollen could not be rescued by the wild-type pollen on the stigma, indicating that the spik mutation deprived the capability of pollen absorption on the stigma. Our results suggest that the Arabidopsis SnRK1 complex regulates SPIK expression, which functions in determining pollen hydration on the stigma.