Oscillatory ROP GTPase activation leads the oscillatory polarized growth of pollen tubes

RHO OF PLANTS signalling and the activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS: specificity in cellular signal transduction in plants

Every cell constantly receives signals from its neighbours or the environment. In plants, most signals are perceived by RECEPTOR-LIKE KINASEs (RLKs) and then transmitted into the cell. The molecular switches RHO OF PLANTS (ROP) are critical proteins for polar signal transduction and regulate multiple cell polarity processes downstream of RLKs. Many ROP-regulating proteins and scaffold proteins of the ROP complex are known. However, the spatiotemporal ROP signalling complex composition is not yet understood. Moreover, how specificity is achieved in different ROP signalling pathways within one cell still needs to be determined. This review gives an overview of recent advances in ROP signalling and how specificity by downstream scaffold proteins can be achieved. The composition of the ROP signalling complexes is discussed, focusing on the possibility of the simultaneous presence of ROP activators and inactivators within the same complex to balance ROP activity. Furthermore, this review highlights the function of plant-specific ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS polarizing ROP signalling and defining the specificity of the initiated ROP signalling pathway.

The Arp2/3 complex promotes periodic removal of Pak1-mediated negative feedback to facilitate anticorrelated Cdc42 oscillations

The conserved GTPase Cdc42 is a major regulator of polarized growth in most eukaryotes. Cdc42 periodically cycles between active and inactive states at sites of polarized growth. These periodic cycles are caused by positive feedback and time-delayed negative feedback loops. In the bipolar yeast S. pombe, both growing ends must regulate Cdc42 activity. At each cell end, Cdc42 activity recruits the Pak1 kinase which prevents further Cdc42 activation thus establishing negative feedback. It is unclear how Cdc42 activation returns to the end after Pak1-dependent negative feedback. Using genetic and chemical perturbations, we find that disrupting branched actin-mediated endocytosis disables Cdc42 reactivation at the cell ends. With our experimental data and mathematical models, we show that endocytosis-dependent Pak1 removal from the cell ends allows the Cdc42 activator Scd1 to return to that end to enable reactivation of Cdc42. Moreover, we show that Pak1 elicits its own removal via activation of endocytosis. In agreement with these observations, our model and experimental data show that in each oscillatory cycle, Cdc42 activation increases followed by an increase in Pak1 recruitment at that end. These findings provide a deeper insight into the self-organization of Cdc42 regulation and reveal previously unknown feedback with endocytosis in the establishment of cell polarity.

Investigation of ROP GTPase Activity and Cytoskeleton Dynamics During Tip Growth in Root Hairs and Pollen Tubes

Pollen tubes and root hairs are typical tip-growing cells and are employed as model systems to study plant cell polarity. Previous studies have shown that the Rho family ROP GTPase plays a critical role in the regulation of pollen tube and root hair growth. Periodically, activated ROP GTPase coordinates with the tip-focused calcium gradient, to regulate actin dynamics and vesicle trafficking. Moreover, microtubules are also involved in organelle movement and growth directionality. Here, we describe methods for analyzing the spatiotemporal localization and activity of ROP, cortical microtubule organization, and F-actin dynamics in pollen tubes and/or root hairs.

Barley guanine nucleotide exchange factor HvGEF14 is an activator of the susceptibility factor HvRACB and supports host cell entry by Blumeria graminis f. sp. hordei

In barley (Hordeum vulgare), signalling rat sarcoma homolog (RHO) of plants guanosine triphosphate hydrolases (ROP GTPases) support the penetration success of Blumeria graminis f. sp. hordei but little is known about ROP activation. Guanine nucleotide exchange factors (GEFs) facilitate the exchange of ROP-bound GDP for GTP and thereby turn ROPs into a signalling-activated ROP-GTP state. Plants possess a unique class of GEFs harbouring a plant-specific ROP nucleotide exchanger domain (PRONE). Here, we performed phylogenetic analyses and annotated barley PRONE-GEFs. The leaf epidermal-expressed PRONE-GEF HvGEF14 undergoes a transcriptional down-regulation on inoculation with B. graminis f. sp. hordei and directly interacts with the ROP GTPase and susceptibility factor HvRACB in yeast and in planta. Overexpression of activated HvRACB or of HvGEF14 led to the recruitment of ROP downstream interactor HvRIC171 to the cell periphery. HvGEF14 further supported direct interaction of HvRACB with a HvRACB-GTP-binding CRIB (Cdc42/Rac Interactive Binding motif) domain-containing HvRIC171 truncation. Finally, the overexpression of HvGEF14 caused enhanced susceptibility to fungal entry, while HvGEF14 RNAi provoked a trend to more penetration resistance. HvGEF14 might therefore play a role in the activation of HvRACB in barley epidermal cells during fungal penetration.

The legacy of kinesins in the pollen tube thirty years later

The pollen tube is fundamental in the reproduction of seed plants. Particularly in angiosperms, we now have much information about how it grows, how it senses extracellular signals, and how it converts them into a directional growth mechanism. The expansion of the pollen tube is also related to dynamic cytoplasmic processes based on the cytoskeleton (such as polymerization/depolymerization of microtubules and actin filaments) or motor activity along with the two cytoskeletal systems and is dependent on motor proteins. While a considerable amount of information is available for the actomyosin system in the pollen tube, the role of microtubules in the transport of organelles or macromolecular structures is still quite uncertain despite that 30 years ago the first work on the presence of kinesins in the pollen tube was published. Since then, progress has been made in elucidating the role of kinesins in plant cells. However, their role within the pollen tube is still enigmatic. In this review, I will postulate some roles of kinesins in the pollen tube 30 years after their initial discovery based on information obtained in other plant cells in the meantime. The most concrete hypotheses predict that kinesins in the pollen tube enable the short movement of specific organelles or contribute to generative cell or sperm cell transport, as well as mediate specific steps in the process of endocytosis.

Let's shape again: the concerted molecular action that builds the pollen tube

The pollen tube is being subjected to control by a complex network of communication that regulates its shape and the misfunction of a single component causes specific deformations. In flowering plants, the pollen tube is a tubular extension of the pollen grain required for successful sexual reproduction. Indeed, maintaining the unique shape of the pollen tube is essential for the pollen tube to approach the embryo sac. Many processes and molecules (such as GTPase activity, phosphoinositides, Ca²⁺ gradient, distribution of reactive oxygen species and nitric oxide, nonuniform pH values, organization of the cytoskeleton, balance between exocytosis and endocytosis, and cell wall structure) play key and coordinated roles in maintaining the cylindrical shape of pollen tubes. In addition, the above factors must also interact with each other so that the cell shape is maintained while the pollen tube follows chemical signals in the pistil that guide it to the embryo sac. Any intrinsic changes (such as erroneous signals) or extrinsic changes (such as environmental stresses) can affect the above factors and thus fertilization by altering the tube morphology. In this review, the processes and molecules that enable the development and maintenance of the unique shape of pollen tubes in angiosperms are presented emphasizing their interaction with specific tube shape. Thus, the purpose of the review is to investigate whether specific deformations in pollen tubes can help us to better understand the mechanism underlying pollen tube shape.

ROPGAP-dependent interaction between brassinosteroid and ROP2-GTPase signaling controls pavement cell shape in Arabidopsis

The epidermal pavement cell shape in Arabidopsis is driven by chemical and mechanical cues that direct partitioning mechanisms required for the establishment of the lobe- and indentation-defining polar sites. Brassinosteroid (BR) hormones regulate pavement cell morphogenesis, but the underlying mechanism remains unclear. Here, we identified two PLECKSTRIN HOMOLOGY GTPase-ACTIVATING proteins (PHGAPs) as substrates of the GSK3-like kinase BR-INSENSITIVE2 (BIN2). The phgap1phgap2 mutant displayed severe epidermal cell shape phenotypes, and the PHGAPs were markedly enriched in the anticlinal face of the pavement cell indenting regions. BIN2 phosphorylation of PHGAPs was required for their stability and polarization. BIN2 inhibition activated ROP2-GTPase signaling specifically in the lobes because of PHGAP degradation, while the PHGAPs restrained ROP2 activity in the indentations. Hence, we connect BR and ROP2-GTPase signaling pathways via the regulation of PHGAPs and put forward the importance of spatiotemporal control of BR signaling for pavement cell interdigitation.

Calmodulin Domain Protein Kinase PiCDPK1 Regulates Pollen Tube Growth Polarity through Interaction with RhoGDI

The pollen-specific calcium-dependent protein kinase PiCDPK1 of Petunia inflata has previously been shown to regulate polarity in tip growth in pollen tubes. Here we report the identification of a Rho Guanine Dissociation Inhibitor (PiRhoGDI1) as a PiCDPK1 interacting protein. We demonstrate that PiRhoGDI1 and PiCDPK1 interact in a yeast 2-hybrid assay, as well as in an in vitro pull-down assay, and that PiRhoGDI1 is phosphorylated by PiCDPK1 in vitro. We further demonstrate the PiRhoGDI1 is capable of rescuing the loss of growth polarity phenotype caused by over-expressing PiCDPK1 in vivo using stable transgenic plants. We confirmed that PiRhoGDI1 interacts with a pollen-expressed ROP GTPase isoform consistent with the established role of RhoGDIs in negatively regulating GTPases through their membrane removal and locking them in an inactive cytosolic complex. ROP is a central regulator of polarity in tip growth, upstream of Ca²⁺, and PiCDPK1 over-expression has been previously reported to lead to dramatic elevation of cytosolic Ca²⁺ through a positive feedback loop. The discovery that PiCDPK1 impacts ROP regulation via PiRhoGDI1 suggests that PiCDPK1 acts as RhoGDI displacement factor and leads us to propose a model which we hypothesize regulates the rapid recycling of ROP GTPase at the pollen tube tip.

Signaling at Physical Barriers during Pollen-Pistil Interactions

In angiosperms, double fertilization requires pollen tubes to transport non-motile sperm to distant egg cells housed in a specialized female structure known as the pistil, mediating the ultimate fusion between male and female gametes. During this journey, the pollen tube encounters numerous physical barriers that must be mechanically circumvented, including the penetration of the stigmatic papillae, style, transmitting tract, and synergid cells as well as the ultimate fusion of sperm cells to the egg or central cell. Additionally, the pollen tube must maintain structural integrity in these compact environments, while responding to positional guidance cues that lead the pollen tube to its destination. Here, we discuss the nature of these physical barriers as well as efforts to genetically and cellularly identify the factors that allow pollen tubes to successfully, specifically, and quickly circumnavigate them.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

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

GTPase ROP6 negatively modulates phosphate deficiency through inhibition of PHT1;1 and PHT1;4 in Arabidopsis thaliana

Phosphorus, an essential macroelement for plant growth and development, is a major limiting factor for sustainable crop yield. The Rho of plant (ROP) GTPase is involved in regulating multiple signal transduction processes in plants, but potentially including the phosphate deficiency signaling pathway remains unknown. Here, we identified that the rop6 mutant exhibited a dramatic tolerant phenotype under Pi-deficient conditions, with higher phosphate accumulation and lower anthocyanin content. In contrast, the rop6 mutant was more sensitive to arsenate (As(V)) toxicity, the analog of Pi. Immunoblot analysis displayed that the ROP6 protein was rapidly degraded through ubiquitin/26S proteasome pathway under Pi-deficient conditions. In addition, pull-down assay using GST-RIC1 demonstrated that the ROP6 activity was decreased obviously under Pi-deficient conditions. Strikingly, protein-protein interaction and two-voltage clamping assays demonstrated that ROP6 physically interacted with and inhibited the key phosphate uptake transporters PHT1;1 and PHT1;4 in vitro and in vivo. Moreover, genetic analysis showed that ROP6 functioned upstream of PHT1;1 and PHT1;4. Thus, we conclude that GTPase ROP6 modulates the uptake of phosphate by inhibiting the activities of PHT1;1 and PHT1;4 in Arabidopsis.

Eng2, a new player involved in feedback loop regulation of Cdc42 activity in fission yeast

Cell polarity and morphogenesis are regulated by the small GTPase Cdc42. Even though major advances have been done in the field during the last years, the molecular details leading to its activation in particular cellular contexts are not completely understood. In fission yeast, the β(1,3)-glucanase Eng2 is a "moonlighting protein" with a dual function, acting as a hydrolase during spore dehiscence, and as component of the endocytic machinery in vegetative cells. Here, we report that Eng2 plays a role in Cdc42 activation during polarized growth through its interaction with the scaffold protein Scd2, which brings Cdc42 together with its guanine nucleotide exchange factor (GEF) Scd1. eng2Δ mutant cells have defects in activation of the bipolar growth (NETO), remaining monopolar during all the cell cycle. In the absence of Eng2 the accumulation of Scd1 and Scd2 at the poles is reduced, the levels of Cdc42 activation decrease, and the Cdc42 oscillatory behavior, associated with bipolar growth in wild type cells, is altered. Furthermore, overexpression of Eng2 partially rescues the growth and polarity defects of a cdc42-L160S mutant. Altogether, our work unveils a new factor regulating the activity of Cdc42, which could potentially link the polarity and endocytic machineries.

The quest for the central players governing pollen tube growth and guidance

Recent insights into the mechanism of pollen tube growth and guidance point to the importance of H⁺ dynamics, which are regulated by the plasma membrane H⁺-ATPase.

Membrane receptor-mediated mechano-transduction maintains cell integrity during pollen tube growth within the pistil

Invasive or penetrative growth is critical for developmental and reproductive processes (e.g., pollen tube penetration of pistils) and disease progression (e.g., cancer metastasis and fungal hyphae invasion). The invading or penetrating cells experience drastic changes in mechanical pressure from the surroundings and must balance growth with cell integrity. Here, we show that Arabidopsis pollen tubes sense and/or respond to mechanical changes via a cell-surface receptor kinase Buddha's Paper Seal 1 (BUPS1) while emerging from compressing female tissues. BUPS1-defective pollen tubes fail to maintain cell integrity after emergence from these tissues. The mechano-transduction function of BUPS1 is established by using a microfluidic channel device mimicking the mechanical features of the in vivo growth path. BUPS1-based mechano-transduction activates Rho-like GTPase from Plant 1 (ROP1) GTPase to promote exocytosis that facilitates secretion of BUPS1's ligands for mechanical signal amplification and cell wall rigidification in pollen tubes. These findings uncover a membrane receptor-based mechano-transduction system for cells to cope with the physical challenges during invasive or penetrative growth.

A positive feedback circuit for ROP-mediated polar growth

Tip growth is a special type of polarized growth in which a single and unique polarization site is established and maintained. Rho of Plants (ROP) proteins, which represent the only class of Rho GTPases in plants, regulate tip growth. The dynamic and asymmetric distribution of ROPs is critical for the establishment and maintenance of tip growth, and requires at least one positive feedback loop, which is still elusive. Here, we report a positive feedback circuit essential for tip growth of root hairs, in which ROPs, ROP activators and effectors, and AGC1.5 subfamily kinases are interconnected by sequential oligomerization and phosphorylation. AGC1.5 subfamily kinases interact with and phosphorylate two guanine nucleotide exchange factors (GEFs) of ROPs, RopGEF4 and RopGEF10. They also interact with two ROP effectors, ICR2/RIP3 and MIDD1/RIP4, which bridge active ROPs with AGC1.5. Functional loss of the AGC1.5 subfamily kinases or ICR2 and MIDD1 compromised root hair growth due to reduced ROP signaling. We found that asymmetric targeting of RopGEF4 and RopGEF10 is controlled by AGC1.5-dependent phosphorylation. Interestingly, we discovered that the ROP effectors recruit AGC1.5 to active ROP domains at the plasma membrane during root hair growth and are critical for AGC1.5-dependent phosphorylation of RopGEFs. Given the large number of AGC kinases in plants, this positive feedback circuit may be a universal theme for plant cell polar growth.

A Fully Functional ROP Fluorescent Fusion Protein Reveals Roles for This GTPase in Subcellular and Tissue-Level Patterning

Rho of Plants (ROPs) are GTPases that regulate polarity and patterned wall deposition in plants. As these small, globular proteins have many interactors, it has been difficult to ensure that methods to visualize ROP in live cells do not affect ROP function. Here, motivated by work in fission yeast (Schizosaccharomyces pombe), we generated a fluorescent moss (Physcomitrium [Physcomitrella] patens) ROP4 fusion protein by inserting mNeonGreen after Gly-134. Plants harboring tagged ROP4 and no other ROP genes were phenotypically normal. Plants lacking all four ROP genes comprised an unpatterned clump of spherical cells that were unable to form gametophores, demonstrating that ROP is essentially for spatial patterning at the cellular and tissue levels. The functional ROP fusion protein formed a steep gradient at the apical plasma membranes of growing tip cells. ROP also predicted the site of branch formation in the apical cell at the onset of mitosis, which occurs one to two cell cycles before a branch cell emerges. While fluorescence recovery after photobleaching studies demonstrated that ROP dynamics do not depend on the cytoskeleton, acute depolymerization of the cytoskeleton removed ROP from the membrane only in recently divided cells, pointing to a feedback mechanism between the cell cycle, cytoskeleton, and ROP.

Rho GTPase ROP1 Interactome Analysis Reveals Novel ROP1-Associated Pathways for Pollen Tube Polar Growth in Arabidopsis

ROP (Rho-like GTPases from plants) GTPases are polarly localized key regulators of polar growth in pollen tubes and other cells in plants. However, how ROP GTPases are regulated and how they control polar growth remains to be fully understood. To gain new insights into ROP-dependent mechanisms underlying polar cell growth, we characterized the interactome of ROP1 GTPase that controls Arabidopsis pollen tube (PT) tip growth, an extreme form of polar cell growth. We established an efficient method for culturing Arabidopsis pollen tubes in liquid medium, which was used for immunoprecipitation/mass spectrometry-based identification of ROP1-associated proteins. A total of 654 candidates were isolated from the ROP1 interactome in Arabidopsis pollen tubes, and GO (Gene Ontology) classification and pathway analysis revealed multiple uncharacterized ROP1-dependent processes including translation, cell wall modification, post transcriptional modification, and ion homeostasis, in addition to known ROP1-dependent pathways. The ROP1-interactome data was further supported by the co-expression of the candidate interactors in highly mature pollen with PT germination and growth defects being discovered in 25% (8/32) of the candidate mutant genes. Taken together, our work uncovers valuable information for the identification and functional elucidation of ROP-associated proteins in the regulation of polar growth, and provides a reliable reference to identify critical regulators of polar cell growth in the future.

Cellular self-organization: generating order from the abyss

While the organization of inanimate systems such as gases or liquids is predominantly thermodynamically driven—a mixture of two gases will tend to mix until they reach equilibrium—biological systems frequently exhibit organization that is far from a well-mixed equilibrium. The anisotropies displayed by cells are evident in some of the dynamic processes that constitute life including cell development, movement, and division. These anisotropies operate at different length-scales, from the meso- to the nanoscale, and are proposed to reflect self-organization, a characteristic of living systems that is becoming accessible to reconstitution from purified components, and thus a more thorough understanding. Here, some examples of self-organization underlying cellular anisotropies at the cellular level are reviewed, with an emphasis on Rho-family GTPases operating at the plasma membrane. Given the technical challenges of studying these dynamic proteins, some of the successful approaches that are being employed to study their self-organization will also be considered.

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.

Regulation of Cdc42 for polarized growth in budding yeast

The Rho GTPase Cdc42 is a central regulator of cell polarity in diverse cell types. The activity of Cdc42 is dynamically controlled in time and space to enable distinct polarization events, which generally occur along a single axis in response to spatial cues. Our understanding of the mechanisms underlying Cdc42 polarization has benefited largely from studies of the budding yeast Saccharomyces cerevisiae, a genetically tractable model organism. In budding yeast, Cdc42 activation occurs in two temporal steps in the G1 phase of the cell cycle to establish a proper growth site. Here, we review findings in budding yeast that reveal an intricate crosstalk among polarity proteins for biphasic Cdc42 regulation. The first step of Cdc42 activation may determine the axis of cell polarity, while the second step ensures robust Cdc42 polarization for growth. Biphasic Cdc42 polarization is likely to ensure the proper timing of events including the assembly and recognition of spatial landmarks and stepwise assembly of a new ring of septins, cytoskeletal GTP-binding proteins, at the incipient bud site. Biphasic activation of GTPases has also been observed in mammalian cells, suggesting that biphasic activation could be a general mechanism for signal-responsive cell polarization. Cdc42 activity is necessary for polarity establishment during normal cell division and development, but its activity has also been implicated in the promotion of aging. We also discuss negative polarity signaling and emerging concepts of Cdc42 signaling in cellular aging.

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.

Villin controls the formation and enlargement of punctate actin foci in pollen tubes

Self-incompatibility (SI) in the poppy Papaver rhoeas triggers dramatic alterations in actin within pollen tubes. However, how these actin alterations are mechanistically achieved remains largely unexplored. Here, we used treatment with the Ca2+ ionophore A23187 to mimic the SI-induced elevation in cytosolic Ca2+ and trigger formation of the distinctive F-actin foci. Live-cell imaging revealed that this remodeling involves F-actin fragmentation and depolymerization, accompanied by the rapid formation of punctate actin foci and subsequent increase in their size. We established that actin foci are generated and enlarged from crosslinking of fragmented actin filament structures. Moreover, we show that villins associate with actin structures and are involved in this actin reorganization process. Notably, we demonstrate that Arabidopsis VILLIN5 promotes actin depolymerization and formation of actin foci by fragmenting actin filaments, and controlling the enlargement of actin foci via bundling of actin filaments. Our study thus uncovers important novel insights about the molecular players and mechanisms involved in forming the distinctive actin foci in pollen tubes.

Spatiotemporal dynamics of a reaction-diffusion model of pollen tube tip growth

A reaction-diffusion model is proposed to describe the mechanisms underlying the spatial distributions of ROP1 and calcium on the pollen tube tip. The model assumes that the plasma membrane ROP1 activates itself through positive feedback loop, while the cytosolic calcium ions inhibit ROP1 via a negative feedback loop. Furthermore it is proposed that lateral movement of molecules on the plasma membrane are depicted by diffusion. It is shown that bistable or oscillatory dynamics could exist even in the non-spatial model, and stationary and oscillatory spatiotemporal patterns are found in the full spatial model which resemble the experimental data of pollen tube tip growth.

Signaling in Pollen Tube Growth: Beyond the Tip of the Polarity Iceberg

The coordinated growth of pollen tubes through floral tissues to deliver the sperm cells to the egg and facilitate fertilization is a highly regulated process critical to the Angiosperm life cycle. Studies suggest that the concerted action of a variety of signaling pathways underlies the rapid polarized tip growth exhibited by pollen tubes. Ca2+ and small GTPase-mediated pathways have emerged as major players in the regulation of pollen tube growth. Evidence suggests that these two signaling pathways not only integrate with one another but also with a variety of other important signaling events. As we continue to elucidate the mechanisms involved in pollen tube growth, there is a growing importance in taking a holistic approach to studying these pathways in order to truly understand how tip growth in pollen tubes is orchestrated and maintained. This review considers our current state of knowledge of Ca2+-mediated and GTPase signaling pathways in pollen tubes, how they may intersect with one another, and other signaling pathways involved. There will be a particular focus on recent reports that have extended our understanding in these areas.

Distinct RopGEFs Successively Drive Polarization and Outgrowth of Root Hairs

Root hairs are tubular protrusions of the root epidermis that significantly enlarge the exploitable soil volume in the rhizosphere. Trichoblasts, the cell type responsible for root hair formation, switch from cell elongation to tip growth through polarization of the growth machinery to a predefined root hair initiation domain (RHID) at the plasma membrane. The emergence of this polar domain resembles the establishment of cell polarity in other eukaryotic systems [1-3]. Rho-type GTPases of plants (ROPs) are among the first molecular determinants of the RHID [4, 5], and later play a central role in polar growth [6]. Numerous studies have elucidated mechanisms that position the RHID in the cell [7-9] or regulate ROP activity [10-18]. The molecular players that target ROPs to the RHID and initiate outgrowth, however, have not been identified. We dissected the timing of the growth machinery assembly in polarizing hair cells and found that positioning of molecular players and outgrowth are temporally separate processes that are each controlled by specific ROP guanine nucleotide exchange factors (GEFs). A functional analysis of trichoblast-specific GEFs revealed GEF3 to be required for normal ROP polarization and thus efficient root hair emergence, whereas GEF4 predominantly regulates subsequent tip growth. Ectopic expression of GEF3 induced the formation of spatially confined, ROP-recruiting domains in other cell types, demonstrating the role of GEF3 to serve as a membrane landmark during cell polarization.

Analysis of the Localization of Fluorescent PpROP1 and PpROP-GEF4 Fusion Proteins in Moss Protonemata Based on Genomic "Knock-In" and Estradiol-Titratable Expression

Tip growth of pollen tubes, root hairs, and apical cells of moss protonemata is controlled by ROP (Rho of plants) GTPases, which were shown to accumulate at the apical plasma membrane of these cells. However, most ROP localization patterns reported in the literature are based on fluorescent protein tagging and need to be interpreted with caution, as ROP fusion proteins were generally overexpressed at undefined levels, in many cases without assessing effects on tip growth. ROP-GEFs, important regulators of ROP activity, were also described to accumulate at the apical plasma membrane during tip growth. However, to date only the localization of fluorescent ROP-GEF fusion proteins strongly overexpressed using highly active promoters have been investigated. Here, the intracellular distributions of fluorescent PpROP1 and PpROP-GEF4 fusion proteins expressed at essentially endogenous levels in apical cells of Physcomitrella patens "knock-in" protonemata were analyzed. Whereas PpROP-GEF4 was found to associate with a small apical plasma membrane domain, PpROP1 expression was below the detection limit. Estradiol-titratable expression of a fluorescent PpROP1 fusion protein at the lowest detectable level, at which plant development was only marginally affected, was therefore employed to show that PpROP1 also accumulates at the apical plasma membrane, although within a substantially larger domain. Interestingly, RNA-Seq data indicated that the majority of all genes active in protonemata are expressed at lower levels than PpROP1, suggesting that estradiol-titratable expression may represent an important alternative to "knock-in" based analysis of the intracellular distribution of fluorescent fusion proteins in protonemal cells.

Actin Cytoskeleton as Actor in Upstream and Downstream of Calcium Signaling in Plant Cells

In plant cells, calcium (Ca²⁺) serves as a versatile intracellular messenger, participating in several fundamental and important biological processes. Recent studies have shown that the actin cytoskeleton is not only an upstream regulator of Ca²⁺ signaling, but also a downstream regulator. Ca²⁺ has been shown to regulates actin dynamics and rearrangements via different mechanisms in plants, and on this basis, the upstream signaling encoded within the Ca²⁺ transient can be decoded. Moreover, actin dynamics have also been proposed to act as an upstream of Ca²⁺, adjust Ca²⁺ oscillations, and establish cytosolic Ca²⁺ ([Ca²⁺]_cyt) gradients in plant cells. In the current review, we focus on the advances in uncovering the relationship between the actin cytoskeleton and calcium in plant cells and summarize our current understanding of this relationship.

AGC1.5 Kinase Phosphorylates RopGEFs to Control Pollen Tube Growth

Double fertilization in angiosperms requires the targeted delivery of immotile sperm to the eggs through pollen tubes. The polarity of tip-growing pollen tubes is maintained through dynamic association of active Rho GTPases of plants (ROP-GTP) with the apical plasma membrane. Guanine nucleotide exchange factors for ROPs (RopGEFs) catalyze the activation of ROPs and thereby affect spatiotemporal ROP signaling. Whereas RopGEFs have been found to be phosphorylated proteins, the kinases responsible for their phosphorylation in vivo and biological consequences of RopGEF phosphorylation in pollen tube growth remain unclear. We report here that the Arabidopsis AGC1.5 subfamily of cytoplasmic kinases is critical for the restricted localization of ROP-GTP during pollen tube growth. Loss of AGC1.5 and AGC1.7 functions resulted in the mistargeting of active ROPs and defective events downstream of ROP signaling in pollen tubes. AGC1.5 interacts with RopGEFs via their catalytic PRONE domain and phosphorylates RopGEFs at a conserved Ser residue of PRONE domain. Loss of AGC1.5 and AGC1.7 functions resulted in the mistargeting of RopGEFs in pollen tubes, similar to the phenotype caused by the mutation that renders RopGEFs non-phosphorylatable by AGC1.5. Collectively, our results provide mechanistic insights into the spatiotemporal activation of ROPs during the polar growth of pollen tubes.

Calcium imaging in Arabidopsis pollen cells using G-CaMP5

Calcium (Ca²⁺ ) signaling has been implicated in pollen germination and pollen tube growth. To date, however, we still know very little about how exactly Ca²⁺ signaling links to various physiological subcellular processes during pollen germination and pollen tube growth. Given that Ca²⁺ signaling is tightly related to the cytosolic concentration and dynamics of Ca²⁺ , it is vital to trace the dynamic changes in Ca²⁺ levels in order to decode Ca²⁺ signaling. Here, we demonstrate that G-CaMP5 serves well as an indicator for monitoring cytosolic Ca²⁺ dynamics in pollen cells. Using this probe, we show that cytosolic Ca²⁺ changes dramatically during pollen germination, and, as reported previously, Ca²⁺ forms a tip-focused gradient in the pollen tube and undergoes oscillation in the tip region during pollen tube growth. In particular, using G-CaMP5 allowed us to capture the dynamic changes in the cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ) in pollen tubes in response to various exogenous treatments. Our data suggest that G-CaMP5 is a suitable probe for monitoring the dynamics of [Ca²⁺ ]_cyt in pollen cells.

Lily Pollen Tubes Pulse According to a Simple Spatial Oscillator

Polar growth is a fundamental mode of cell morphogenesis observed in nearly all major groups of organisms. Among polarly growing cells, the angiosperm pollen tubes have emerged as powerful experimental systems in large part because of their oscillatory growth, which provides a window into the network of interactions regulating morphogenesis. Empirical studies of oscillatory pollen tubes have sought to uncover the temporal sequence of cellular and molecular events that constitutes an oscillatory cycle. Here we show that in lily pollen tubes the distance or wavelength (λ = 6.3 ± 1.7 μm) over which an oscillatory cycle unfolds is more robust than the period of oscillation (τ = 39.1 ± 17.6 s) (n = 159 cells). Moreover, the oscillatory cycle is divided into slow and fast phases, with each phase unfolding over precisely one half of the wavelength. Using these observations, we show that a simple spatial bi-oscillator predicts the most common modes of oscillation observed in pollen tubes. These results call into question the traditional view of pollen tube morphogenesis as a temporal succession of cellular events. Space, not time, may be the most natural metric to inteprete the morphogenetic dynamics of these cells.

The REN4 rheostat dynamically coordinates the apical and lateral domains of Arabidopsis pollen tubes

The dynamic maintenance of polar domains in the plasma membrane (PM) is critical for many fundamental processes, e.g., polar cell growth and growth guidance but remains poorly characterized. Rapid tip growth of Arabidopsis pollen tubes requires dynamic distribution of active ROP1 GTPase to the apical domain. Here, we show that clathrin-mediated endocytosis (CME) coordinates lateral REN4 with apical ROP1 signaling. REN4 interacted with but antagonized active ROP1. REN4 also interacts and co-localizes with CME components, but exhibits an opposite role to CME, which removes both REN4 and active ROP1 from the PM. Mathematical modeling shows that REN4 restrains the spatial distribution of active ROP1 and is important for the robustness of polarity control. Hence our results indicate that REN4 acts as a spatiotemporal rheostat by interacting with ROP1 to initiate their removal from the PM by CME, thereby coordinating a dynamic demarcation between apical and lateral domains during rapid tip growth.

Spatio-temporal Aspects of Ca2+ Signalling: Lessons from Guard Cells and Pollen Tubes

Changes in cytosolic Ca2+ concentration ([Ca2+]cyt) serve to transmit information in eukaryotic cells. The involvement of this second messenger in plant cell growth as well as osmotic- and water relations is well established. After almost 40 years of intense research on the coding and decoding of plant Ca2+ signals, numerous proteins involved in Ca2+ action have been identified. However, we are still far from understanding the complexity of Ca2+ networks. New in vivo Ca2+ imaging techniques combined with molecular genetics allow visualisation of spatio-temporal aspects of Ca2+ signalling. In parallel, cell biology together with protein biochemistry and electrophysiology are able to dissect information processing by this second messenger in space and time. Here we focus on the time-resolved changes in cellular events upon Ca2+ signals, concentrating on the two best-studied cell types, pollen tubes and guard cells. We put their signalling networks side by side, compare them with those of other cell types and discuss rapid signalling in the context of Ca2+ transients and oscillations to regulate ion homeostasis.

Inhibition of Ras activity coordinates cell fusion with cell-cell contact during yeast mating

In the fission yeast Schizosaccharomyces pombe, pheromone signaling engages a signaling pathway composed of a G protein-coupled receptor, Ras, and a mitogen-activated protein kinase (MAPK) cascade that triggers sexual differentiation and gamete fusion. Cell-cell fusion requires local cell wall digestion, which relies on an initially dynamic actin fusion focus that becomes stabilized upon local enrichment of the signaling cascade on the structure. We constructed a live-reporter of active Ras1 (Ras1-guanosine triphosphate [GTP]) that shows Ras activity at polarity sites peaking on the fusion structure before fusion. Remarkably, constitutive Ras1 activation promoted fusion focus stabilization and fusion attempts irrespective of cell pairing, leading to cell lysis. Ras1 activity was restricted by the guanosine triphosphatase-activating protein Gap1, which was itself recruited to sites of Ras1-GTP and was essential to block untimely fusion attempts. We propose that negative feedback control of Ras activity restrains the MAPK signal and couples fusion with cell-cell engagement.

Activation of ROP6 GTPase by Phosphatidylglycerol in Arabidopsis

Plant Rho-like GTPases (ROPs) are switch-like proteins which play essential roles in controlling cell polarity development and cellular activities. ROPs are regulated by many factors, such as auxin, light, and RopGEFs and RopGAPs proteins. However, it has not been reported yet whether small molecules play a role in the regulation of ROP activity. Here, we showed that AtROP6 specially bound to a phospholipid, phosphatidylglycerol (PG), by the protein-lipid overlay and liposome sedimentation assays, and further MST assay gave a dissociation constant (Kd) of 4.8 ± 0.4 μM for binding of PG to His-AtROP6. PG profile analysis in Arabidopsis revealed that PG existed both in leaves and roots but with distinctive fatty acyl chain patterns. By evaluating AtROP6 activity using RIC1 effector binding-based assay, we found that PG stimulated AtROP6 activity. In the FM4-64 uptake experiment, PG inhibited AtROP6-mediated endocytosis process. By evaluating internalization of PIN2, PG was shown to regulate endocytosis process coordinately with NAA. Further root gravitropism experiment revealed that PG enhanced the AtROP6-mediated root gravity response. These results suggest that the phospholipid PG physically binds AtROP6, stimulates its activity and influences AtROP6-mediated root gravity response in Arabidopsis.

Finite Element Modeling of Shape Changes in Plant Cells

Mechanical modeling of plant cells using finite element methods serves to simulate the behavior of complex cell shapes with the aim to understand biological functioning

Arabidopsis PRK6 interacts specifically with AtRopGEF8/12 and induces depolarized growth of pollen tubes when overexpressed

The pollen receptor kinases (PRK) are critical regulators of pollen tube growth. The Arabidopsis genome encodes eight PRK genes, of which six are highly expressed in pollen tubes. The potential functions of AtPRK1 through AtPRK5, but not of AtPRK6, in pollen growth were analyzed in tobacco. Herein, AtPRK6 was cloned, and its function was identified. AtPRK6 was expressed specifically in pollen tubes. A yeast two-hybrid screen of AtPRK6 against 14 Arabidopsis Rop guanine nucleotide exchange factors (RopGEFs) showed that AtPRK6 interacted with AtRopGEF8 and AtRopGEF12. These interactions were confirmed in Arabidopsis mesophyll protoplasts. The interactions between AtPRK6 and AtRopGEF8/12 were mediated by the C-termini of AtRopGEF8/12 and by the juxtamembrane and kinase domain of AtPRK6, but were not dependent on the kinase activity. In addition, transient overexpression of AtPRK6::GFP in Arabidopsis protoplasts revealed that AtPRK6 was localized to the plasma membrane. Tobacco pollen tubes overexpressing AtPRK6 exhibited shorter tubes with enlarged tips. This depolarized tube growth required the kinase domain of AtPRK6 and was not dependent on kinase activity. Taken together, the results show that AtPRK6, through its juxtamembrane and kinase domains (KD), interacts with AtRopGEF8/12 and plays crucial roles in polarized growth of pollen tubes.

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.

Oscillatory fungal cell growth

Cells are dynamic systems, the state of which undergoes constant alteration that results in morphological changes and movement. Many dynamic cellular processes that appear continuous are driven by underlying mechanisms that oscillate with distinct periods. For example eukaryotic cells do not grow continuously, but rather by pulsed extension of the periphery. Stepwise cell extension at the hyphal tips of several filamentous fungi was discovered 20 years ago, but only a few molecular details of the mechanism have been clarified since then. A recent study has provided evidence for correlations among intracellular Ca²⁺ levels, actin assembly, exocytosis and cell extension in growing hyphal tips. This suggests that pulsed Ca²⁺ influxes coordinate the temporal control of actin assembly and exocytosis, which results in stepwise cell extension. The coordinated oscillation of these machineries are likely to be ubiquitous among all eukaryotes. Indeed, intracellular Ca²⁺ levels and/or actin polymerization oscillate in mammalian and plant cells. This review summarizes the mechanisms of oscillation in several systems.

Exocytosis-coordinated mechanisms for tip growth underlie pollen tube growth guidance

Many tip-growing cells are capable of responding to guidance cues, during which cells precisely steer their growth toward the source of guidance signals. Though several players in signal perception have been identified, little is known about the downstream signaling that controls growth direction during guidance. Here, using combined modeling and experimental studies, we demonstrate that the growth guidance of Arabidopsis pollen tubes is regulated by the signaling network that controls tip growth. Tip-localized exocytosis plays a key role in this network by integrating guidance signals with the ROP1 Rho GTPase signaling and coordinating intracellular signaling with cell wall mechanics. This model reproduces the high robustness and responsiveness of pollen tube guidance and explains the connection between guidance efficiency and the parameters of the tip growth system. Hence, our findings establish an exocytosis-coordinated mechanism underlying the cellular pathfinding guided by signal gradients and the mechanistic linkage between tip growth and guidance.

Oscillatory signatures underlie growth regimes in Arabidopsis pollen tubes: computational methods to estimate tip location, periodicity, and synchronization in growing cells

Oscillations in pollen tubes have been reported for many cellular processes, including growth, extracellular ion fluxes, and cytosolic ion concentrations. However, there is a shortage of quantitative methods to measure and characterize the different dynamic regimes observed. Herein, a suite of open-source computational methods and original algorithms were integrated into an automated analysis pipeline that we employed to characterize specific oscillatory signatures in pollen tubes of Arabidopsis thaliana (Col-0). Importantly, it enabled us to detect and quantify a Ca2+ spiking behaviour upon growth arrest and synchronized oscillations involving growth, extracellular H+ fluxes, and cytosolic Ca2+, providing the basis for novel hypotheses. Our computational approach includes a new tip detection method with subpixel resolution using linear regression, showing improved ability to detect oscillations when compared to currently available methods. We named this data analysis pipeline 'Computational Heuristics for Understanding Kymographs and aNalysis of Oscillations Relying on Regression and Improved Statistics', or CHUKNORRIS. It can integrate diverse data types (imaging, electrophysiology), extract quantitative and time-explicit estimates of oscillatory characteristics from isolated time series (period and amplitude) or pairs (phase relationships and delays), and evaluate their synchronization state. Here, its performance is tested with ratiometric and single channel kymographs, ion flux data, and growth rate analysis.

Oscillatory signatures underlie growth regimes in Arabidopsis pollen tubes: computational methods to estimate tip location, periodicity, and synchronization in growing cells

Oscillations in pollen tubes have been reported for many cellular processes, including growth, extracellular ion fluxes, and cytosolic ion concentrations. However, there is a shortage of quantitative methods to measure and characterize the different dynamic regimes observed. Herein, a suite of open-source computational methods and original algorithms were integrated into an automated analysis pipeline that we employed to characterize specific oscillatory signatures in pollen tubes of Arabidopsis thaliana (Col-0). Importantly, it enabled us to detect and quantify a Ca2+ spiking behaviour upon growth arrest and synchronized oscillations involving growth, extracellular H+ fluxes, and cytosolic Ca2+, providing the basis for novel hypotheses. Our computational approach includes a new tip detection method with subpixel resolution using linear regression, showing improved ability to detect oscillations when compared to currently available methods. We named this data analysis pipeline 'Computational Heuristics for Understanding Kymographs and aNalysis of Oscillations Relying on Regression and Improved Statistics', or CHUKNORRIS. It can integrate diverse data types (imaging, electrophysiology), extract quantitative and time-explicit estimates of oscillatory characteristics from isolated time series (period and amplitude) or pairs (phase relationships and delays), and evaluate their synchronization state. Here, its performance is tested with ratiometric and single channel kymographs, ion flux data, and growth rate analysis.

The Microtubule-Associated Protein MAP18 Affects ROP2 GTPase Activity during Root Hair Growth

Establishment and maintenance of the polar site are important for root hair tip growth. We previously reported that Arabidopsis (Arabidopsis thaliana) MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) functions in controlling the direction of pollen tube growth and root hair elongation. Additionally, the Rop GTPase ROP2 was reported as a positive regulator of both root hair initiation and tip growth in Arabidopsis. Both loss of function of ROP2 and knockdown of MAP18 lead to a decrease in root hair length, whereas overexpression of either MAP18 or ROP2 causes multiple tips or a branching hair phenotype. However, it is unclear whether MAP18 and ROP2 coordinately regulate root hair growth. In this study, we demonstrate that MAP18 and ROP2 interact genetically and functionally. MAP18 interacts physically with ROP2 in vitro and in vivo and preferentially binds to the inactive form of the ROP2 protein. MAP18 promotes ROP2 activity during root hair tip growth. Further investigation revealed that MAP18 competes with RhoGTPase GDP DISSOCIATION INHIBITOR1/SUPERCENTIPEDE1 for binding to ROP2, in turn affecting the localization of active ROP2 in the plasma membrane of the root hair tip. These results reveal a novel function of MAP18 in the regulation of ROP2 activation during root hair growth.

Controlling contractile instabilities in the actomyosin cortex

The actomyosin cell cortex is an active contractile material for driving cell- and tissue morphogenesis. The cortex has a tendency to form a pattern of myosin foci, which is a signature of potentially unstable behavior. How a system that is prone to such instabilities can rveliably drive morphogenesis remains an outstanding question. Here, we report that in the Caenorhabditis elegans zygote, feedback between active RhoA and myosin induces a contractile instability in the cortex. We discover that an independent RhoA pacemaking oscillator controls this instability, generating a pulsatory pattern of myosin foci and preventing the collapse of cortical material into a few dynamic contracting regions. Our work reveals how contractile instabilities that are natural to occur in mechanically active media can be biochemically controlled to robustly drive morphogenetic events.

Salicylic Acid Regulates Pollen Tip Growth through an NPR3/NPR4-Independent Pathway

Tip growth is a common strategy for the rapid elongation of cells to forage the environment and/or to target to long-distance destinations. In the model tip growth system of Arabidopsis pollen tubes, several small-molecule hormones regulate their elongation, but how these rapidly diffusing molecules control extremely localized growth remains mysterious. Here we show that the interconvertible salicylic acid (SA) and methylated SA (MeSA), well characterized for their roles in plant defense, oppositely regulate Arabidopsis pollen tip growth with SA being inhibitory and MeSA stimulatory. The effect of SA and MeSA was independent of known NPR3/NPR4 SA receptor-mediated signaling pathways. SA inhibited clathrin-mediated endocytosis in pollen tubes associated with an increased accumulation of less stretchable demethylated pectin in the apical wall, whereas MeSA did the opposite. Furthermore, SA and MeSA alter the apical activation of ROP1 GTPase, a key regulator of tip growth in pollen tubes, in an opposite manner. Interestingly, both MeSA methylesterase and SA methyltransferase, which catalyze the interconversion between SA and MeSA, are localized at the apical region of pollen tubes, indicating of the tip-localized production of SA and MeSA and consistent with their effects on the apical cellular activities. These findings suggest that local generation of a highly diffusible signal can regulate polarized cell growth, providing a novel mechanism of cell polarity control apart from the one involving protein and mRNA polarization.

Arabidopsis RhoGDIs Are Critical for Cellular Homeostasis of Pollen Tubes

Rhos of plants (ROPs) play a key role in plant cell morphogenesis, especially in tip-growing pollen tubes and root hairs, by regulating an array of intracellular activities such as dynamic polymerization of actin microfilaments. ROPs are regulated by guanine nucleotide exchange factors (RopGEFs), GTPase activating proteins (RopGAPs), and guanine nucleotide dissociation inhibitors (RhoGDIs). RopGEFs and RopGAPs play evolutionarily conserved function in ROP signaling. By contrast, although plant RhoGDIs regulate the membrane extraction and cytoplasmic sequestration of ROPs, less clear are their positive roles in ROP signaling as do their yeast and metazoan counterparts. We report here that functional loss of all three Arabidopsis (Arabidopsis thaliana) GDIs (tri-gdi) significantly reduced male transmission due to impaired pollen tube growth in vitro and in vivo. We demonstrate that ROPs were ectopically activated at the lateral plasma membrane of the tri-gdi pollen tubes. However, total ROPs were reduced posttranslationally in the tri-gdi mutant, resulting in overall dampened ROP signaling. Indeed, a ROP5 mutant that was unable to interact with GDIs failed to induce growth, indicating the importance of the ROP-GDI interaction for ROP signaling. Functional loss of GDIs impaired cellular homeostasis, resulting in excess apical accumulation of wall components in pollen tubes, similar to that resulting from ectopic phosphatidylinositol 4,5-bisphosphate signaling. GDIs and phosphatidylinositol 4,5-bisphosphate may antagonistically coordinate to maintain cellular homeostasis during pollen tube growth. Our results thus demonstrate a more complex role of GDIs in ROP-mediated pollen tube growth.

Signals fly when kinases meet Rho-of-plants (ROP) small G-proteins

Rho-type small GTP-binding plant proteins function as two-state molecular switches in cellular signalling. There is accumulating evidence that Rho-of-plants (ROP) signalling is positively controlled by plant receptor kinases, through the ROP guanine nucleotide exchange factor proteins. These signalling modules regulate cell polarity, cell shape, hormone responses, and pathogen defence, among other things. Other ROP-regulatory proteins might also be subjected to protein phosphorylation by cellular kinases (e.g., mitogen-activated protein kinases or calcium-dependent protein kinases), in order to integrate various cellular signalling pathways with ROP GTPase-dependent processes. In contrast to the role of kinases in upstream ROP regulation, much less is known about the potential link between ROP GTPases and downstream kinase signalling. In other eukaryotes, Rho-type G-protein-activated kinases are widespread and have a key role in many cellular processes. Recent data indicate the existence of structurally different ROP-activated kinases in plants, but their ROP-dependent biological functions still need to be validated. In addition to these direct interactions, ROPs may also indirectly control the activity of mitogen-activated protein kinases or calcium-dependent protein kinases. These kinases may therefore function as upstream as well as downstream kinases in ROP-mediated signalling pathways, such as the phosphatidylinositol monophosphate kinases involved in cell polarity establishment.

Live imaging of root hairs

Root hairs are single cells specialized in the absorption of water and nutrients. Growing root hairs requires intensive cell wall changes to accommodate cell expansion at the apical end by a process known as tip growth. The cell wall of plants is a very rigid structure comprised largely of polysaccharides and hydroxyproline-rich O-glycoproteins. The importance of root hairs stems from their capacity to expand the surface of interaction between the root and the environment, in search for the necessary nutrients and water to allow plant growth. Therefore, it becomes crucial to deepen our knowledge of them, particularly in the light of the applicability in agriculture by allowing the expansion of croplands. Root hair growth is an extremely fast process, reaching growth rates of up to 1 μm/min and it also is a dynamic process; there can be situations in which the final length might not be affected but the growth rate is. Consequently, in this chapter we focus on a method for studying growth dynamics and rates during a time course. This method is versatile allowing for it to be used in other plant organs such as lateral root, hypocotyl, etc., and also in various conditions.

Focusing on the focus: what else beyond the master switches for polar cell growth?

Cell polarity, often associated with polarized cell expansion/growth in plants, describes the uneven distribution of cellular components, such as proteins, nucleic acids, signaling molecules, vesicles, cytoskeletal elements, and organelles, which may ultimately modulate cell shape, structure, and function. Pollen tubes and root hairs are model cell systems for studying the molecular mechanisms underlying sustained tip growth. The formation of intercalated epidermal pavement cells requires excitatory and inhibitory pathways to coordinate cell expansion within single cells and between cells in contact. Strictly controlled cell expansion is linked to asymmetric cell division in zygotes and stomatal lineages, which require integrated processes of pre-mitotic cellular polarization and division asymmetry. While small GTPase ROPs are recognized as fundamental signaling switches for cell polarity in various cellular and developmental processes in plants, the broader molecular machinery underpinning polarity establishment required for asymmetric division remains largely unknown. Here, we review the widely used ROP signaling pathways in cell polar growth and the recently discovered feedback loops with auxin signaling and PIN effluxers. We discuss the conserved phosphorylation and phospholipid signaling mechanisms for regulating uneven distribution of proteins, as well as the potential roles of novel proteins and MAPKs in the polarity establishment related to asymmetric cell division in plants.

Arabidopsis RIC1 Severs Actin Filaments at the Apex to Regulate Pollen Tube Growth

Pollen tubes deliver sperms to the ovule for fertilization via tip growth. The rapid turnover of F-actin in pollen tube tips plays an important role in this process. In this study, we demonstrate that Arabidopsis thaliana RIC1, a member of the ROP-interactive CRIB motif-containing protein family, regulates pollen tube growth via its F-actin severing activity. Knockout of RIC1 enhanced pollen tube elongation, while overexpression of RIC1 dramatically reduced tube growth. Pharmacological analysis indicated that RIC1 affected F-actin dynamics in pollen tubes. In vitro biochemical assays revealed that RIC1 directly bound and severed F-actin in the presence of Ca(2+) in addition to interfering with F-actin turnover by capping F-actin at the barbed ends. In vivo, RIC1 localized primarily to the apical plasma membrane (PM) of pollen tubes. The level of RIC1 at the apical PM oscillated during pollen tube growth. The frequency of F-actin severing at the apex was notably decreased in ric1-1 pollen tubes but was increased in pollen tubes overexpressing RIC1. We propose that RIC1 regulates F-actin dynamics at the apical PM as well as the cytosol by severing F-actin and capping the barbed ends in the cytoplasm, establishing a novel mechanism that underlies the regulation of pollen tube growth.