Mechanosensation Dynamically Coordinates Polar Growth and Cell Wall Assembly to Promote Cell Survival

Conserved physical mechanisms of cell and tissue elongation

Living organisms have the ability to self-shape into complex structures appropriate for their function. The genetic and molecular mechanisms that enable cells to do this have been extensively studied in several model and non-model organisms. In contrast, the physical mechanisms that shape cells and tissues have only recently started to emerge, in part thanks to new quantitative in vivo measurements of the physical quantities guiding morphogenesis. These data, combined with indirect inferences of physical characteristics, are starting to reveal similarities in the physical mechanisms underlying morphogenesis across different organisms. Here, we review how physics contributes to shape cells and tissues in a simple, yet ubiquitous, morphogenetic transformation: elongation. Drawing from observed similarities across species, we propose the existence of conserved physical mechanisms of morphogenesis.

MoMkk1 and MoAtg1 dichotomously regulating autophagy and pathogenicity through MoAtg9 phosphorylation in Magnaporthe oryzae

Autophagy is a central biodegradation pathway critical in eliminating intracellular cargo to maintain cellular homeostasis and improve stress resistance. At the same time, the key component of the mitogen-activated protein kinase cascade regulating cell wall integrity signaling MoMkk1 has an essential role in the autophagy of the rice blast fungus Magnaporthe oryzae. Still, the mechanism of how MoMkk1 regulates autophagy is unclear. Interestingly, we found that MoMkk1 regulates the autophagy protein MoAtg9 through phosphorylation. MoAtg9 is a transmembrane protein subjected to phosphorylation by autophagy-related protein kinase MoAtg1. Here, we provide evidence demonstrating that MoMkk1-dependent MoAtg9 phosphorylation is required for phospholipid translocation during isolation membrane stages of autophagosome formation, an autophagic process essential for the development and pathogenicity of the fungus. In contrast, MoAtg1-dependent phosphorylation of MoAtg9 negatively regulates this process, also impacting growth and pathogenicity. Our studies are the first to demonstrate that MoAtg9 is subject to MoMkk1 regulation through protein phosphorylation and that MoMkk1 and MoAtg1 dichotomously regulate autophagy to underlie the growth and pathogenicity of M. oryzae.IMPORTANCEMagnaporthe oryzae utilizes multiple signaling pathways to promote colonization of host plants. MoMkk1, a cell wall integrity signaling kinase, plays an essential role in autophagy governed by a highly conserved autophagy kinase MoAtg1-mediated pathway. How MoMkk1 regulates autophagy in coordination with MoAtg1 remains elusive. Here, we provide evidence that MoMkk1 phosphorylates MoAtg9 to positively regulate phospholipid translocation during the isolation membrane or smaller membrane structures stage of autophagosome formation. This is in contrast to the negative regulation of MoAtg9 by MoAtg1 for the same process. Intriguingly, MoMkk1-mediated MoAtg9 phosphorylation enhances the fungal infection of rice, whereas MoAtg1-dependant MoAtg9 phosphorylation significantly attenuates it. Taken together, we revealed a novel mechanism of autophagy and virulence regulation by demonstrating the dichotomous functions of MoMkk1 and MoAtg1 in the regulation of fungal autophagy and pathogenicity.

Importance of Non-Covalent Interactions in Yeast Cell Wall Molecular Organization

This review covers a group of non-covalently associated molecules, particularly proteins (NCAp), incorporated in the yeast cell wall (CW) with neither disulfide bridges with proteins covalently attached to polysaccharides nor other covalent bonds. Most NCAp, particularly Bgl2, are polysaccharide-remodeling enzymes. Either directly contacting their substrate or appearing as CW lipid-associated molecules, such as in vesicles, they represent the most movable enzymes and may play a central role in CW biogenesis. The absence of the covalent anchoring of NCAp allows them to be there where and when it is necessary. Another group of non-covalently attached to CW molecules are polyphosphates (polyP), the universal regulators of the activity of many enzymes. These anionic polymers are able to form complexes with metal ions and increase the diversity of non-covalent interactions through charged functional groups with both proteins and polysaccharides. The mechanism of regulation of polysaccharide-remodeling enzyme activity in the CW is unknown. We hypothesize that polyP content in the CW is regulated by another NCAp of the CW-acid phosphatase-which, along with post-translational modifications, may thus affect the activity, conformation and compartmentalization of Bgl2 and, possibly, some other polysaccharide-remodeling enzymes.

Quantifying turgor pressure in budding and fission yeasts based upon osmotic properties

Walled cells, such as plants, fungi, and bacteria cells, possess a high internal hydrostatic pressure, termed turgor pressure, that drives volume growth and contributes to cell shape determination. Rigorous measurement of turgor pressure, however, remains challenging, and reliable quantitative measurements, even in budding yeast are still lacking. Here, we present a simple and robust experimental approach to access turgor pressure in yeasts based upon the determination of isotonic concentration using protoplasts as osmometers. We propose three methods to identify the isotonic condition - three-dimensional cell volume, cytoplasmic fluorophore intensity, and mobility of a cytGEMs nano-rheology probe - that all yield consistent values. Our results provide turgor pressure estimates of 1.0 ± 0.1 MPa for Schizosaccharomyces pombe, 0.49 ± 0.01 MPa for Schizosaccharomyces japonicus, 0.5 ± 0.1 MPa for Saccharomyces cerevisiae W303a and 0.31 ± 0.03 MPa for Saccharomyces cerevisiae BY4741. Large differences in turgor pressure and nano-rheology measurements between the Saccharomyces cerevisiae strains demonstrate how fundamental biophysical parameters can vary even among wild-type strains of the same species. These side-by-side measurements of turgor pressure in multiple yeast species provide critical values for quantitative studies on cellular mechanics and comparative evolution.

The Ustilago maydis AA10 LPMO is active on fungal cell wall chitin

Lytic polysaccharide monooxygenases (LPMOs) can perform oxidative cleavage of glycosidic bonds in carbohydrate polymers (e.g., cellulose, chitin), making them more accessible to hydrolytic enzymes. While most studies have so far mainly explored the role of LPMOs in a (plant) biomass conversion context, alternative roles and paradigms begin to emerge. The AA10 LPMOs are active on chitin and/or cellulose and mostly found in bacteria and in some viruses and archaea. Interestingly, AA10-encoding genes are also encountered in some pathogenic fungi of the Ustilaginomycetes class, such as Ustilago maydis, responsible for corn smut disease. Transcriptomic studies have shown the overexpression of the AA10 gene during the infectious cycle of U. maydis. In fact, U. maydis has a unique AA10 gene that codes for a catalytic domain appended with a C-terminal disordered region. To date, there is no public report on fungal AA10 LPMOs. In this study, we successfully produced the catalytic domain of this LPMO (UmAA10_cd) in Pichia pastoris and carried out its biochemical characterization. Our results show that UmAA10_cd oxidatively cleaves α- and β-chitin with C1 regioselectivity and boosts chitin hydrolysis by a GH18 chitinase from U. maydis (UmGH18A). Using a biologically relevant substrate, we show that UmAA10_cd exhibits enzymatic activity on U. maydis fungal cell wall chitin and promotes its hydrolysis by UmGH18A. These results represent an important step toward the understanding of the role of LPMOs in the fungal cell wall remodeling process during the fungal life cycle.IMPORTANCELytic polysaccharide monooxygenases (LPMOs) have been mainly studied in a biotechnological context for the efficient degradation of recalcitrant polysaccharides. Only recently, alternative roles and paradigms begin to emerge. In this study, we provide evidence that the AA10 LPMO from the phytopathogen Ustilago maydis is active against fungal cell wall chitin. Given that chitin-active LPMOs are commonly found in microbes, it is important to consider fungal cell wall as a potential target for this enigmatic class of enzymes.

Quantifying turgor pressure in budding and fission yeasts based upon osmotic properties

Walled cells, such as plants, fungi, and bacteria cells, possess a high internal hydrostatic pressure, termed turgor pressure, that drives volume growth and contributes to cell shape determination. Rigorous measurement of turgor pressure, however, remains challenging, and reliable quantitative measurements, even in budding yeast are still lacking. Here, we present a simple and robust experimental approach to access turgor pressure in yeasts based upon the determination of isotonic concentration using protoplasts as osmometers. We propose three methods to identify the isotonic condition - 3D cell volume, cytoplasmic fluorophore intensity, and mobility of a cytGEMs nano-rheology probe - that all yield consistent values. Our results provide turgor pressure estimates of 1.0 ± 0.1 MPa for S. pombe, 0.49 ± 0.01 MPa for S. japonicus, 0.5 ± 0.1 MPa for S. cerevisiae W303a and 0.31 ± 0.03 MPa for S. cerevisiae BY4741. Large differences in turgor pressure and nano-rheology measurements between the S. cerevisiae strains demonstrate how fundamental biophysical parameters can vary even among wildtype strains of the same species. These side-by-side measurements of turgor pressure in multiple yeast species provide critical values for quantitative studies on cellular mechanics and comparative evolution.

Elevated levels of sphingolipid MIPC in the plasma membrane disrupt the coordination of cell growth with cell wall formation in fission yeast

Coupling cell wall expansion with cell growth is a universal challenge faced by walled organisms. Mutations in Schizosaccharomyces pombe css1, which encodes a PM inositol phosphosphingolipid phospholipase C, prevent cell wall expansion but not synthesis of cell wall material. To probe how Css1 modulates cell wall formation we used classical and chemical genetics coupled with quantitative mass spectrometry. We found that elevated levels of the sphingolipid biosynthetic pathway's final product, mannosylinositol phosphorylceramide (MIPC), specifically correlated with the css1-3 phenotype. We also found that an apparent indicator of sphingolipids and a sterol biosensor accumulated at the cytosolic face of the PM at cell tips and the division site of css1-3 cells and, in accord, the PM in css1-3 was less dynamic than in wildtype cells. Interestingly, disrupting the protein glycosylation machinery recapitulated the css1-3 phenotype and led us to investigate Ghs2, a glycosylated PM protein predicted to modify cell wall material. Disrupting Ghs2 function led to aberrant cell wall material accumulation suggesting Ghs2 is dysfunctional in css1-3. We conclude that preventing an excess of MIPC in the S. pombe PM is critical to the function of key PM-localized proteins necessary for coupling growth with cell wall formation.

Architecture of the dynamic fungal cell wall

The fungal cell wall is essential for growth and survival, and is a key target for antifungal drugs and the immune system. The cell wall must be robust but flexible, protective and shielding yet porous to nutrients and membrane vesicles and receptive to exogenous signals. Most fungi have a common inner wall skeleton of chitin and β-glucans that functions as a flexible viscoelastic frame to which a more diverse set of outer cell wall polymers and glycosylated proteins are attached. Whereas the inner wall largely determines shape and strength, the outer wall confers properties of hydrophobicity, adhesiveness, and chemical and immunological heterogeneity. The spatial organization and dynamic regulation of the wall in response to prevailing growth conditions enable fungi to thrive within changing, diverse and often hostile environments. Understanding this architecture provides opportunities to develop diagnostics and drugs to combat life-threatening fungal infections.

Vast heterogeneity in cytoplasmic diffusion rates revealed by nanorheology and Doppelgänger simulations

The cytoplasm is a complex, crowded, actively driven environment whose biophysical characteristics modulate critical cellular processes such as cytoskeletal dynamics, phase separation, and stem cell fate. Little is known about the variance in these cytoplasmic properties. Here, we employed particle-tracking nanorheology on genetically encoded multimeric 40 nm nanoparticles (GEMs) to measure diffusion within the cytoplasm of individual fission yeast (Schizosaccharomyces pombe) cellscells. We found that the apparent diffusion coefficients of individual GEM particles varied over a 400-fold range, while the differences in average particle diffusivity among individual cells spanned a 10-fold range. To determine the origin of this heterogeneity, we developed a Doppelgänger simulation approach that uses stochastic simulations of GEM diffusion that replicate the experimental statistics on a particle-by-particle basis, such that each experimental track and cell had a one-to-one correspondence with their simulated counterpart. These simulations showed that the large intra- and inter-cellular variations in diffusivity could not be explained by experimental variability but could only be reproduced with stochastic models that assume a wide intra- and inter-cellular variation in cytoplasmic viscosity. The simulation combining intra- and inter-cellular variation in viscosity also predicted weak nonergodicity in GEM diffusion, consistent with the experimental data. To probe the origin of this variation, we found that the variance in GEM diffusivity was largely independent of factors such as temperature, the actin and microtubule cytoskeletons, cell-cyle stage, and spatial locations, but was magnified by hyperosmotic shocks. Taken together, our results provide a striking demonstration that the cytoplasm is not "well-mixed" but represents a highly heterogeneous environment in which subcellular components at the 40 nm size scale experience dramatically different effective viscosities within an individual cell, as well as in different cells in a genetically identical population. These findings carry significant implications for the origins and regulation of biological noise at cellular and subcellular levels.

Cell wall dynamics stabilize tip growth in a filamentous fungus

Hyphal tip growth allows filamentous fungi to colonize space, reproduce, or infect. It features remarkable morphogenetic plasticity including unusually fast elongation rates, tip turning, branching, or bulging. These shape changes are all driven from the expansion of a protective cell wall (CW) secreted from apical pools of exocytic vesicles. How CW secretion, remodeling, and deformation are modulated in concert to support rapid tip growth and morphogenesis while ensuring surface integrity remains poorly understood. We implemented subresolution imaging to map the dynamics of CW thickness and secretory vesicles in Aspergillus nidulans. We found that tip growth is associated with balanced rates of CW secretion and expansion, which limit temporal fluctuations in CW thickness, elongation speed, and vesicle amount, to less than 10% to 20%. Affecting this balance through modulations of growth or trafficking yield to near-immediate changes in CW thickness, mechanics, and shape. We developed a model with mechanical feedback that accounts for steady states of hyphal growth as well as rapid adaptation of CW mechanics and vesicle recruitment to different perturbations. These data provide unprecedented details on how CW dynamics emerges from material secretion and expansion, to stabilize fungal tip growth as well as promote its morphogenetic plasticity.

Fission Yeast Rho1p-GEFs: From Polarity and Cell Wall Synthesis to Genome Stability

Rho1p is a membrane-associated protein that belongs to the Rho family of small GTPases. These proteins coordinate processes such as actin remodelling and polarised secretion to maintain the shape and homeostasis of yeast cells. In response to extracellular stimuli, Rho1p undergoes conformational switching between a guanosine triphosphate (GTP)-bound active state and a guanosine diphosphate (GDP)-bound inactive state. Cycling is improved with guanine nucleotide exchange factor (GEF) activity necessary to activate signalling and GTPase activating protein (GAP) activity required for subsequent signal depletion. This review focuses on fission yeast Rho1p GEFs, Rgf1p, Rgf2p, and Rgf3p that belong to the family of DH-PH domain-containing Dbl-related GEFs. They are multi-domain proteins that detect biological signals that induce or inhibit their catalytic activity over Rho1p. Each of them activates Rho1p in different places and times. Rgf1p acts preferentially during polarised growth. Rgf2p is required for sporulation, and Rgf3p plays an essential function in septum synthesis. In addition, we outline the noncanonical roles of Rho1p-GEFs in genomic instability.

The Cdc42 GAP Rga6 promotes monopolar outgrowth of spores

The molecular mechanisms underlying the establishment of the monopolar growth of fission yeast spores have been less characterized. Here, we report that the Cdc42 GTPase-activating protein (GAP) Rga6 is required for promoting monopolar growth during spore germination. The absence of Rga6 increases the number of spores that grow in a bipolar fashion. Rga6 decorates the non-growing cortical region, binds phosphatidylinositol 4,5-bisphosphate, and colocalizes with the phosphatidylinositol 4,5-bisphosphate-binding protein Opy1. Overexpression of Opy1 diminishes the cortical localization of Rga6. The characteristic localization of Rga6 on the cell cortex depends on the C-terminal PBR region of Rga6. Moreover, engineered chimera composed of the Rga6 C-terminal PBR region fused to the GAP domain of Rga3 or Rga4 are sufficient to rescue the spore growth phenotype caused by the absence of Rga6. Hence, our work establishes a paradigm in which the lipid composition of the plasma membrane directs polarized cell growth by specifying the cortical localization of a GAP protein.

Mechanobiology of the cell wall – insights from tip-growing plant and fungal cells

The cell wall (CW) is a thin and rigid layer encasing the membrane of all plant and fungal cells. It ensures mechanical integrity by bearing mechanical stresses derived from large cytoplasmic turgor pressure, contacts with growing neighbors or growth within restricted spaces. The CW is made of polysaccharides and proteins, but is dynamic in nature, changing composition and geometry during growth, reproduction or infection. Such continuous and often rapid remodeling entails risks of enhanced stress and consequent damages or fractures, raising the question of how the CW detects and measures surface mechanical stress and how it strengthens to ensure surface integrity? Although early studies in model fungal and plant cells have identified homeostatic pathways required for CW integrity, recent methodologies are now allowing the measurement of pressure and local mechanical properties of CWs in live cells, as well as addressing how forces and stresses can be detected at the CW surface, fostering the emergence of the field of CW mechanobiology. Here, using tip-growing cells of plants and fungi as case study models, we review recent progress on CW mechanosensation and mechanical regulation, and their implications for the control of cell growth, morphogenesis and survival.

Bending stiffness of Candida albicans hyphae as a proxy of cell wall properties

The cell wall is a key component of fungi. It constitutes a highly regulated viscoelastic shell which counteracts internal cell turgor pressure. Its mechanical properties thus contribute to define cell morphology. Measurements of the elastic moduli of the fungal cell wall have been carried out in many species including Candida albicans, a major human opportunistic pathogen. They mainly relied on atomic force microscopy, and mostly considered the yeast form. We developed a parallelized pressure-actuated microfluidic device to measure the bending stiffness of hyphae. We found that the cell wall stiffness lies in the MPa range. We then used three different ways to disrupt cell wall physiology: inhibition of beta-glucan synthesis, a key component of the inner cell wall; application of a hyperosmotic shock triggering a sudden decrease of the hyphal diameter; deletion of two genes encoding GPI-modified cell wall proteins resulting in reduced cell wall thickness. The bending stiffness values were affected to different extents by these environmental stresses or genetic modifications. Overall, our results support the elastic nature of the cell wall and its ability to remodel at the scale of the entire hypha over minutes.

Modern Biophysics Redefines Our Understanding of Fungal Cell Wall Structure, Complexity, and Dynamics

Even though the cell wall has been recognized as a crucial protective organelle for fungi, essential for its virulence and a unique emblem of this kingdom, biosynthesis of this organelle remains poorly understood. Our knowledge was based mainly in the past on the chemical analysis of cell wall mutants and on the biochemical study of a few synthases and transglycosidases. Recent developments in biophysical equipment and methods, such as solid-state nuclear magnetic resonance or cryo-electron microscopy, have promoted a better appreciation of the spatiotemporal dynamics of cell wall biosynthesis. The new information will be presented here with the cell wall of the human opportunistic pathogen Aspergillus fumigatus.

Cellulose accumulation mediated by PbrCSLD5, a cellulose synthase-like protein, results in cessation of pollen tube growth in Pyrus bretschneideri

Cellulose, a key component of the cell wall, plays an important role in maintaining the growth of pollen tubes. However, the molecular mechanism of cellulose participating in the cessation of pear pollen tube growth remains unclear. Here, we reported that at 15 h post-cultured (HPC), the slow-growth pear pollen tubes showed thickened cell walls and cellulose accumulation in the inner wall. Transcriptome data and quantitative real-time PCR analysis showed that PbrCSLD5, a cellulose synthesis-like gene, was highly expressed in the 15 HPC pear pollen tubes. Knockdown of PbrCSLD5 caused a decrease in cellulose content in pear pollen tubes. Moreover, PbrCSLD5 overexpression in Arabidopsis resulted in the accumulation of cellulose and disruption of normal pollen tube growth. Transcription factor PbrMADS52 was found to bind to the promoter of PbrCSLD5 and enhanced its expression. Our results suggested that the PbrMADS52-PbrCSLD5 signaling pathway led to increased cellulose content in the pear pollen tube cell wall, thereby inhibiting pollen tube growth. These results provided new insights into the regulation of pollen tube growth.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Fission yeast polycystin Pkd2p promotes cell size expansion and antagonizes the Hippo-related SIN pathway

Polycystins are conserved mechanosensitive channels whose mutations lead to the common human renal disorder autosomal dominant polycystic kidney disease (ADPKD). Previously, we discovered that the plasma membrane-localized fission yeast polycystin homolog Pkd2p is an essential protein required for cytokinesis; however, its role remains unclear. Here, we isolated a novel temperature-sensitive pkd2 mutant, pkd2-B42. Among the strong growth defects of this mutant, the most striking was that many mutant cells often lost a significant portion of their volume in just 5 min followed by a gradual recovery, a process that we termed 'deflation'. Unlike cell lysis, deflation did not result in plasma membrane rupture and occurred independently of cell cycle progression. The tip extension of pkd2-B42 cells was 80% slower than that of wild-type cells, and their turgor pressure was 50% lower. Both pkd2-B42 and the hypomorphic depletion mutant pkd2-81KD partially rescued mutants of the septation initiation network (SIN), a yeast Hippo-related signaling pathway, by preventing cell lysis, enhancing septum formation and doubling the number of Sid2p and Mob1p molecules at the spindle pole bodies. We conclude that Pkd2p promotes cell size expansion during interphase by regulating turgor pressure and antagonizes the SIN during cytokinesis. This article has an associated First Person interview with the first author of the paper.

Cells under pressure: how yeast cells respond to mechanical forces

In their natural habitats, unicellular fungal microbes are exposed to a myriad of mechanical cues such as shear forces from fluid flow, osmotic changes, and contact forces arising from microbial expansion in confined niches. While the rigidity of the cell wall is critical to withstand such external forces and balance high internal turgor pressure, it poses mechanical challenges during physiological processes such as cell growth, division, and mating that require cell wall remodeling. Thus, even organisms as simple as yeast have evolved complex signaling networks to sense and respond to intrinsic and extrinsic mechanical forces. In this review, we summarize the type and origin of mechanical forces experienced by unicellular yeast and discuss how these forces reorganize cell polarity and how pathogenic fungi exploit polarized assemblies to track weak spots in host tissues for successful penetration. We then describe mechanisms of force-sensing by conserved sets of mechanosensors. Finally, we elaborate downstream mechanotransduction mechanisms that orchestrate appropriate cellular responses, leading to improved mechanical fitness.

The Fission Yeast Cell Integrity Pathway: A Functional Hub for Cell Survival upon Stress and Beyond

The survival of eukaryotic organisms during environmental changes is largely dependent on the adaptive responses elicited by signal transduction cascades, including those regulated by the Mitogen-Activated Protein Kinase (MAPK) pathways. The Cell Integrity Pathway (CIP), one of the three MAPK pathways found in the simple eukaryote fission of yeast Schizosaccharomyces pombe, shows strong homology with mammalian Extracellular signal-Regulated Kinases (ERKs). Remarkably, studies over the last few decades have gradually positioned the CIP as a multi-faceted pathway that impacts multiple functional aspects of the fission yeast life cycle during unperturbed growth and in response to stress. They include the control of mRNA-stability through RNA binding proteins, regulation of calcium homeostasis, and modulation of cell wall integrity and cytokinesis. Moreover, distinct evidence has disclosed the existence of sophisticated interplay between the CIP and other environmentally regulated pathways, including Stress-Activated MAP Kinase signaling (SAPK) and the Target of Rapamycin (TOR). In this review we present a current overview of the organization and underlying regulatory mechanisms of the CIP in S. pombe, describe its most prominent functions, and discuss possible targets of and roles for this pathway. The evolutionary conservation of CIP signaling in the dimorphic fission yeast S. japonicus will also be addressed.

Candida albicans Can Survive Antifungal Surface Coatings on Surfaces with Microcone Topography

This study demonstrates the ability of Candida albicans, a medically significant human fungal pathogen, to minimize contact with an antifungal surface coating that on a flat surface is lethal on contact by growing on and between micron-sized surface topographical features, thus minimizing the contact area. Scanning electron microscopy showed that cells contacting the "floor" between microcones were killed, whereas cells attached to microcones survived and formed hyphal filaments. These spanned space between cones and avoided contact with the flat surface in-between cones. Thus, fungal cells managed to attach and grow despite the antifungal coating. This ability of Candida albicans to exploit topography features to minimize surface contact yet utilize the solid surface for anchoring reduces the effectiveness of the grafted antifungal surface coating. This suggests that biomedical devices with rough surfaces might be more challenging to protect against fungal biofilm formation via application of an antifungal coating.

Detection of surface forces by the cell-wall mechanosensor Wsc1 in yeast

Surface receptors of animal cells, such as integrins, promote mechanosensation by forming clusters as signaling hubs that transduce tensile forces. Walled cells of plants and fungi also feature surface sensors, with long extracellular domains that are embedded in their cell walls (CWs) and are thought to detect injuries and promote repair. How these sensors probe surface forces remains unknown. By studying the conserved CW sensor Wsc1 in fission yeast, we uncovered the formation of micrometer-sized clusters at sites of force application onto the CW. Clusters assembled within minutes of CW compression, in dose dependence with mechanical stress and disassembled upon relaxation. Our data support that Wsc1 accumulates to sites of enhanced mechanical stress through reduced lateral diffusivity, mediated by the binding of its extracellular WSC domain to CW polysaccharides, independent of canonical polarity, trafficking, and downstream CW regulatory pathways. Wsc1 may represent an autonomous module to detect and transduce local surface forces onto the CW.

The Role of the Cell Integrity Pathway in Septum Assembly in Yeast

Cytokinesis divides a mother cell into two daughter cells at the end of each cell cycle and proceeds via the assembly and constriction of a contractile actomyosin ring (CAR). Ring constriction promotes division furrow ingression, after sister chromatids are segregated to opposing sides of the cleavage plane. Cytokinesis contributes to genome integrity because the cells that fail to complete cytokinesis often reduplicate their chromosomes. While in animal cells, the last steps of cytokinesis involve extracellular matrix remodelling and mid-body abscission, in yeast, CAR constriction is coupled to the synthesis of a polysaccharide septum. To preserve cell integrity during cytokinesis, fungal cells remodel their cell wall through signalling pathways that connect receptors to downstream effectors, initiating a cascade of biological signals. One of the best-studied signalling pathways is the cell wall integrity pathway (CWI) of the budding yeast Saccharomyces cerevisiae and its counterpart in the fission yeast Schizosaccharomyces pombe, the cell integrity pathway (CIP). Both are signal transduction pathways relying upon a cascade of MAP kinases. However, despite strong similarities in the assembly of the septa in both yeasts, there are significant mechanistic differences, including the relationship of this process with the cell integrity signalling pathways.

Antibiotic Resistance Gene Transformation and Ultrastructural Alterations of Lettuce (Lactuca sativa L.) Resulting from Sulfadiazine Accumulation in Culture Solution

The research herein explored the possible mechanism of toxicity of the antibiotic sulfadiazine (SD) and the related antibiotic resistance gene transformation in lettuce by systematically investigating its growth responses, ultrastructural changes, and antibiotic resistance gene transformation via solution culture experiments. The results showed that SD mainly accumulated in the roots of lettuce at concentrations ranging from 6.48 to 120.87 μg/kg, which were significantly higher than those in leaves (3.90 to 16.74 μg/kg). Lower concentrations of SD (0.5 and 2.0 mg/L) in the culture nutrient solution exerted little effect on lettuce growth, while at SD concentrations higher than 10 mg/L, the growth of lettuce was significantly inhibited, manifesting as shorter root length and lower dry matter yield of whole lettuce plants. Compared with that for the control group, the absolute abundance of bacteria in the root endophyte, rhizosphere, and phyllosphere communities under different concentrations of SD treatment decreased significantly. sul1 and sul2 mainly accumulated in the root endophyte community, at levels significantly higher than those in the leaf endophyte community. Studies of electrolyte leakage and ultrastructural characteristics of root and leaf cells indicated that lettuce grown in culture solutions with high SD concentrations suffered severe damage and disintegration of the cell walls of organs, especially chloroplasts, in leaves. Furthermore, the possible mechanism of SD toxicity in lettuce was confirmed to start with the roots, followed by a free flow of SD into the leaves to destroy the chloroplasts in the leaf cells, which ultimately reduced photosynthesis and decreased plant growth. Studies have shown that antibiotic residues have negative effects on the growth of lettuce and highlight a potential risk of the development and spread of antibiotic resistance in vegetable endophyte systems.

Soft but Not Too Soft-How a Rigid Tube Expands without Breaking

Fungi grow by apical extension of their hyphae. The continuous growth requires constant delivery of vesicles, which fuse with the membrane and secrete cell wall biosynthesis enzymes. The growth mechanism requires the fungal cytoskeleton and turgor pressure. In a recent study by Fukuda et al. (mBio 12:e03196-20, 2021, https://doi.org/10.1128/mBio.03196-20), hyphal growth was studied in microfluidic devices with channels smaller than the hyphal diameter. The authors discovered that fast-growing fungi like Neurospora crassa enter the channels, but hyphal tips become fragile and rupture frequently, whereas slower-growing fungi like Aspergillus nidulans adapt their hyphal diameter and grow without problems through the channels. This study suggests two different growth mechanisms and a tradeoff between hyphal plasticity and growth speed.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Rapid adaptation of endocytosis, exocytosis and eisosomes after an acute increase in membrane tension in yeast cells

During clathrin-mediated endocytosis (CME) in eukaryotes, actin assembly is required to overcome large membrane tension and turgor pressure. However, the molecular mechanisms by which the actin machinery adapts to varying membrane tension remain unknown. In addition, how cells reduce their membrane tension when they are challenged by hypotonic shocks remains unclear. We used quantitative microscopy to demonstrate that cells rapidly reduce their membrane tension using three parallel mechanisms. In addition to using their cell wall for mechanical protection, yeast cells disassemble eisosomes to buffer moderate changes in membrane tension on a minute time scale. Meanwhile, a temporary reduction in the rate of endocytosis for 2–6 min and an increase in the rate of exocytosis for at least 5 min allow cells to add large pools of membrane to the plasma membrane. We built on these results to submit the cells to abrupt increases in membrane tension and determine that the endocytic actin machinery of fission yeast cells rapidly adapts to perform CME. Our study sheds light on the tight connection between membrane tension regulation, endocytosis, and exocytosis.

Characterization of the nanomechanical properties of the fission yeast (Schizosaccharomyces pombe) cell surface by atomic force microscopy

Variations in cell wall composition and biomechanical properties can contribute to the cellular plasticity required during complex processes such as polarized growth and elongation in microbial cells. This study utilizes atomic force microscopy (AFM) to map the cell surface topography of fission yeast, Schizosaccharomyces pombe, at the pole regions and to characterize the biophysical properties within these regions under physiological, hydrated conditions. High-resolution images acquired from AFM topographic scanning reveal decreased surface roughness at the cell poles. Force extension curves acquired by nanoindentation probing with AFM cantilever tips under low applied force revealed increased cell wall deformation and decreased cellular stiffness (cellular spring constant) at cell poles (17 ± 4 mN/m) relative to the main body of the cell that is not undergoing growth and expansion (44 ± 10 mN/m). These findings suggest that the increased deformation and decreased stiffness at regions of polarized growth at fission yeast cell poles provide the plasticity necessary for cellular extension. This study provides a direct biophysical characterization of the S. pombe cell surface by AFM, and it provides a foundation for future investigation of how the surface topography and local nanomechanical properties vary during different cellular processes.

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.

Balancing of the mitotic exit network and cell wall integrity signaling governs the development and pathogenicity in Magnaporthe oryzae

The fungal cell wall plays an essential role in maintaining cell morphology, transmitting external signals, controlling cell growth, and even virulence. Relaxation and irreversible stretching of the cell wall are the prerequisites of cell division and development, but they also inevitably cause cell wall stress. Both Mitotic Exit Network (MEN) and Cell Wall Integrity (CWI) are signaling pathways that govern cell division and cell stress response, respectively, how these pathways cross talk to govern and coordinate cellular growth, development, and pathogenicity remains not fully understood. We have identified MoSep1, MoDbf2, and MoMob1 as the conserved components of MEN from the rice blast fungus Magnaporthe oryzae. We have found that blocking cell division results in abnormal CWI signaling. In addition, we discovered that MoSep1 targets MoMkk1, a conserved key MAP kinase of the CWI pathway, through protein phosphorylation that promotes CWI signaling. Moreover, we provided evidence demonstrating that MoSep1-dependent MoMkk1 phosphorylation is essential for balancing cell division with CWI that maintains the dynamic stability required for virulence of the blast fungus.

The cell wall is a relatively rigid structure for supporting the cell shape and against extracellular stresses. However, it also maintains plasticity to cope with cell division, growth, and differentiation. In the rice blast pathogenic fungus Magnaporthe oryzae, such differentiation corresponds directly to its virulence. Thus, how to balance the “strong for shaping” with the “malleable for growth and virulence” poses as an important question of both basic- and applied science of significance. We here report that the protein kinase MoSep1 links the Mitotic Exit Network (MEN) to the Cell Wall Integrity (CWI) signaling through the phosphorylation of the CWI MAP kinase kinase MoMkk1. We found that the MoSep1-dependent phosphorylation of MoMkk1 relieves the cell wall stress caused by cell division and that the MEN-CWI-mediated balance of rigid and remodeling of the cell wall is important in the growth, development, and virulence of the blast fungus. Our study provides a new evidence on how the blast fungus adapts to self-generated stress for growth and virulence and it sheds new light on the crosstalk between MEN and CWI signaling.

Coordinating cell polarization and morphogenesis through mechanical feedback

Many cellular processes require cell polarization to be maintained as the cell changes shape, grows or moves. Without feedback mechanisms relaying information about cell shape to the polarity molecular machinery, the coordination between cell polarization and morphogenesis, movement or growth would not be possible. Here we theoretically and computationally study the role of a genetically-encoded mechanical feedback (in the Cell Wall Integrity pathway) as a potential coordination mechanism between cell morphogenesis and polarity during budding yeast mating projection growth. We developed a coarse-grained continuum description of the coupled dynamics of cell polarization and morphogenesis as well as 3D stochastic simulations of the molecular polarization machinery in the evolving cell shape. Both theoretical approaches show that in the absence of mechanical feedback (or in the presence of weak feedback), cell polarity cannot be maintained at the projection tip during growth, with the polarization cap wandering off the projection tip, arresting morphogenesis. In contrast, for mechanical feedback strengths above a threshold, cells can robustly maintain cell polarization at the tip and simultaneously sustain mating projection growth. These results indicate that the mechanical feedback encoded in the Cell Wall Integrity pathway can provide important positional information to the molecular machinery in the cell, thereby enabling the coordination of cell polarization and morphogenesis.

Association between the Microsatellite Ap243, AC117 and SV185 Polymorphisms and Nosema Disease in the Dark Forest Bee Apis mellifera mellifera

The microsporidian Nosema parasites, primarily Nosema ceranae, remain critical threats to the health of the honey bee Apis mellifera. One promising intervention approach is the breeding of Nosema-resistant honey bee colonies using molecular technologies, for example marker-assisted selection (MAS). For this, specific genetic markers used in bee selection should be developed. The objective of the paper is to search for associations between some microsatellite markers and Nosema disease in a dark forest bee Apis mellifera mellifera. For the dark forest bee, the most promising molecular genetic markers for determining resistance to nosemosis are microsatellite loci AC117, Ap243 and SV185, the alleles of which (“177”, “263” and “269”, respectively) were associated with a low level of Nosema infection. This article is the first associative study aimed at finding DNA loci of resistance to nosemosis in the dark forest bee. Nevertheless, microsatellite markers identified can be used to predict the risk of developing the Nosema disease.

An image analysis method to survey the dynamics of polar protein abundance in the regulation of tip growth

Tip growth is critical for the lifestyle of many walled cells. In yeast and fungi, this process is typically associated with the polarized deposition of conserved tip factors, including landmarks, Rho GTPases, cytoskeleton regulators, and membrane and cell wall remodelers. Because tip growth speeds may vary extensively between life cycles or species, we asked whether the local amount of specific polar elements could determine or limit tip growth speeds. Using the model fission yeast, we developed a quantitative image analysis pipeline to dynamically correlate single tip elongation speeds and polar protein abundance in large data sets. We found that polarity landmarks are typically diluted by growth. In contrast, tip growth speed is positively correlated with the local amount of factors related to actin, secretion or cell wall remodeling, but, surprisingly, exhibits long saturation plateaus above certain concentrations of those factors. Similar saturation observed for Spitzenkörper components in much faster growing fungal hyphae suggests that elements independent of canonical surface remodelers may limit single tip growth. This work provides standardized methods and resources to decipher the complex mechanisms that control cell growth.This article has an associated First Person interview with Sarah Taheraly, joint first author of the paper.

The principles of cellular geometry scaling

Cellular dimensions profoundly influence cellular physiology. For unicellular organisms, this has direct bearing on their ecology and evolution. The morphology of a cell is governed by scaling rules. As it grows, the ratio of its surface area to volume is expected to decrease. Similarly, if environmental conditions force proliferating cells to settle on different size optima, cells of the same type may exhibit size-dependent variation in cellular processes. In fungi, algae and plants where cells are surrounded by a rigid wall, division at smaller size often produces immediate changes in geometry, decreasing cell fitness. Here, we discuss how cells interpret their size, buffer against changes in shape and, if necessary, scale their polarity to maintain optimal shape at different cell volumes.

The Perinuclear ER Scales Nuclear Size Independently of Cell Size in Early Embryos

Nuclear size plays pivotal roles in gene expression, embryo development, and disease. A central hypothesis in organisms ranging from yeast to vertebrates is that nuclear size scales to cell size. This implies that nuclei may reach steady-state sizes set by limiting cytoplasmic pools of size-regulating components. By monitoring nuclear dynamics in early sea urchin embryos, we found that nuclei undergo substantial growth in each interphase, reaching a maximal size prior to mitosis that declined steadily over the course of development. Manipulations of cytoplasmic volume through multiple chemical and physical means ruled out cell size as a major determinant of nuclear size and growth. Rather, our data suggest that the perinuclear endoplasmic reticulum, accumulated through dynein activity, serves as a limiting membrane pool that sets nuclear surface growth rate. Partitioning of this local pool at each cell division modulates nuclear growth kinetics and dictates size scaling throughout early development.

Single-cell biophysical study reveals deformability and internal ordering relationship in T cells

Deformability and internal ordering are key features related to cell function, particularly critical for cells that routinely undergo large deformations, like T cells during extravasation and migration. In the measurement of cell deformability, a considerable variability is typically obtained, masking the identification of possible interrelationships between deformability, internal ordering and cell function. We report the development of a single-cell methodology that combines measurements of living-cell deformability, using micropipette aspiration, and three-dimensional confocal analysis of the nucleus and cytoskeleton. We show that this single-cell approach can serve as a powerful tool to identify appropriate parameters that characterize deformability within a population of cells, not readably discernable in population-averaged data. By applying this single-cell methodology to mouse CD4+ T cells, our results demonstrate that the relative size of the nucleus, better than other geometrical or cytoskeletal features, effectively determines the overall deformability of the cells within the population.

Crosstalk and spatiotemporal regulation between stress-induced MAP kinase pathways and pheromone signaling in budding yeast

Budding yeast, Saccharomyces cerevisiae, has been widely used as a model system to study cellular signaling in response to internal and external cues. Yeast was among the first organisms in which the architecture, feedback mechanisms and physiological responses of various MAP kinase signaling cascades were studied in detail. Although these MAP kinase pathways are activated by different signals and elicit diverse cellular responses, such as adaptation to stress and mating, they function as an interconnected signaling network, as they influence each other and, in some cases, even share components. Indeed, various stress signaling pathways interfere with pheromone signaling that triggers a distinct cellular differentiation program. However, the molecular mechanisms responsible for this crosstalk are still poorly understood. Here, we review the general topology of the yeast MAP kinase signaling network and highlight recent and new data revealing how conflicting intrinsic and extrinsic signals are interpreted to orchestrate appropriate cellular responses.

To preserve or to destroy, that is the question: the role of the cell wall integrity pathway in pollen tube growth

In plants, cell-shape is defined by the cell wall, a complex network of polymers located outside the plasma membrane. During cell growth, cell wall properties have to be adjusted, assuring cell expansion without compromising cell integrity. Plasma membrane-located receptors sense cell wall properties, transducing extracellular signals into intracellular cascades through the cell wall integrity (CWI) pathway that, in turn, leads to adjustments in the regulation and composition of the cell wall. Using pollen tube growth as a single celled model system, we describe the importance of RAPID ALKALINIZATION FACTOR (RALF) peptides as sensors of cell wall integrity. RALF peptides can mediate the communication between cell wall components and plasma membrane-localized receptor-like kinases (RLKs) of the CrRLK1L family. The subsequent activation of intracellular pathways regulates H⁺, Ca²⁺, and ROS levels in the cell and apoplast, thereby modulating cell wall integrity. Interestingly, the RALF-CrRLK1L module and some of the components working up- and downstream of the RLK is conserved in many other developmental and physiological signaling processes.

Decoupling of Rates of Protein Synthesis from Cell Expansion Leads to Supergrowth

Cell growth is a complex process in which cells synthesize cellular components while they increase in size. It is generally assumed that the rate of biosynthesis must somehow be coordinated with the rate of growth in order to maintain intracellular concentrations. However, little is known about potential feedback mechanisms that could achieve proteome homeostasis or the consequences when this homeostasis is perturbed. Here, we identify conditions in which fission yeast cells are prevented from volume expansion but nevertheless continue to synthesize biomass, leading to general accumulation of proteins and increased cytoplasmic density. Upon removal of these perturbations, this biomass accumulation drove cells to undergo a multi-generational period of "supergrowth" wherein rapid volume growth outpaced biosynthesis, returning proteome concentrations back to normal within hours. These findings demonstrate a mechanism for global proteome homeostasis based on modulation of volume growth and dilution.

Cell wall integrity maintenance during plant development and interaction with the environment

Cell walls are highly dynamic structures that provide mechanical support for plant cells during growth, development and adaptation to a changing environment. Thus, it is important for plants to monitor the state of their cell walls and ensure their functional integrity at all times. This monitoring involves perception of physical forces at the cell wall-plasma membrane interphase. These forces are altered during cell division and morphogenesis, as well as in response to various abiotic and biotic stresses. Mechanisms responsible for the perception of physical stimuli involved in these processes have been difficult to separate from other regulatory mechanisms perceiving chemical signals such as hormones, peptides or cell wall fragments. However, recently developed technologies in combination with more established genetic and biochemical approaches are beginning to open up this exciting field of study. Here, we will review our current knowledge of plant cell wall integrity signalling using selected recent findings and highlight how the cell wall-plasma membrane interphase can act as a venue for sensing changes in the physical forces affecting plant development and stress responses. More importantly, we discuss how these signals may be integrated with chemical signals derived from established signalling cascades to control specific adaptive responses during exposure to biotic and abiotic stresses.

Systematic mapping of cell wall mechanics in the regulation of cell morphogenesis

Walled cells of plants, fungi, and bacteria come with a large range of shapes and sizes, which are ultimately dictated by the mechanics of their cell wall. This stiff and thin polymeric layer encases the plasma membrane and protects the cells mechanically by opposing large turgor pressure derived mechanical stresses. To date, however, we still lack a quantitative understanding for how local and/or global mechanical properties of the wall support cell morphogenesis. Here, we combine subresolution imaging and laser-mediated wall relaxation to quantitate subcellular values of wall thickness (h) and bulk elastic moduli (Y) in large populations of live mutant cells and in conditions affecting cell diameter in the rod-shaped model fission yeast. We find that lateral wall stiffness, defined by the surface modulus, σ = hY, robustly scales with cell diameter. This scaling is valid across tens of mutants spanning various functions-within the population of individual isogenic strains, along single misshaped cells, and even across the fission yeasts clade. Dynamic modulations of cell diameter by chemical and/or mechanical means suggest that the cell wall can rapidly adapt its surface mechanics, rendering stretched wall portions stiffer than unstretched ones. Size-dependent wall stiffening constrains diameter definition and limits size variations; it may also provide an efficient means to keep elastic strains in the wall below failure strains, potentially promoting cell survival. This quantitative set of data impacts our current understanding of the mechanics of cell walls and its contribution to morphogenesis.

Fission yeast TRP channel Pkd2p localizes to the cleavage furrow and regulates cell separation during cytokinesis

Force plays a central role in separating daughter cells during cytokinesis, the last stage of cell division. However, the mechanism of force sensing during cytokinesis remains unknown. Here we discovered that Pkd2p, a putative force-sensing transient receptor potential channel, localizes to the cleavage furrow during cytokinesis of the fission yeast, Schizosaccharomyces pombe. Pkd2p, whose human homologues are associated with autosomal polycystic kidney disease, is an essential protein whose localization depends on the contractile ring and the secretory pathway. We identified and characterized a novel pkd2 mutant pkd2-81KD. The pkd2 mutant cells show signs of osmotic stress, including temporary shrinking, paused turnover of the cytoskeletal structures, and hyperactivated mitogen-activated protein kinase signaling. During cytokinesis, although the contractile ring constricts more rapidly in the pkd2 mutant than the wild-type cells (50% higher), the cell separation in the mutant is slower and often incomplete. These cytokinesis defects are also consistent with misregulated turgor pressure. Finally, the pkd2 mutant exhibits strong genetic interactions with two mutants of the septation initiation network pathway, a signaling cascade essential for cytokinesis. We propose that Pkd2p modulates osmotic homeostasis and is potentially a novel regulator of cytokinesis.

Gazing at Cell Wall Expansion under a Golden Light

In plants, cell growth is constrained by a stiff cell wall, at least this is the way textbooks usually present it. Accordingly, many studies have focused on the elasticity and plasticity of the cell wall as prerequisites for expansion during growth. With their specific evolutionary history, cell wall composition, and environment, brown algae present a unique configuration offering a new perspective on the involvement of the cell wall, viewed as an inert material yet with intrinsic mechanical properties, in growth. In light of recent findings, we explore here how much of the functional relationship between cell wall chemistry and intrinsic mechanics on the one hand, and growth on the other hand, has been uncovered in brown algae.

The brown algal mode of tip growth: Keeping stress under control

Tip growth has been studied in pollen tubes, root hairs, and fungal and oomycete hyphae and is the most widely distributed unidirectional growth process on the planet. It ensures spatial colonization, nutrient predation, fertilization, and symbiosis with growth speeds of up to 800 μm h-1. Although turgor-driven growth is intuitively conceivable, a closer examination of the physical processes at work in tip growth raises a paradox: growth occurs where biophysical forces are low, because of the increase in curvature in the tip. All tip-growing cells studied so far rely on the modulation of cell wall extensibility via the polarized excretion of cell wall-loosening compounds at the tip. Here, we used a series of quantitative measurements at the cellular level and a biophysical simulation approach to show that the brown alga Ectocarpus has an original tip-growth mechanism. In this alga, the establishment of a steep gradient in cell wall thickness can compensate for the variation in tip curvature, thereby modulating wall stress within the tip cell. Bootstrap analyses support the robustness of the process, and experiments with fluorescence recovery after photobleaching (FRAP) confirmed the active vesicle trafficking in the shanks of the apical cell, as inferred from the model. In response to auxin, biophysical measurements change in agreement with the model. Although we cannot strictly exclude the involvement of a gradient in mechanical properties in Ectocarpus morphogenesis, the viscoplastic model of cell wall mechanics strongly suggests that brown algae have evolved an alternative strategy of tip growth. This strategy is largely based on the control of cell wall thickness rather than fluctuations in cell wall mechanical properties.

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.

Elongation and shape changes in organisms with cell walls: A dialogue between experiments and models

The generation of anisotropic shapes occurs during morphogenesis of almost all organisms. With the recent renewal of the interest in mechanical aspects of morphogenesis, it has become clear that mechanics contributes to anisotropic forms in a subtle interaction with various molecular actors. Here, we consider plants, fungi, oomycetes, and bacteria, and we review the mechanisms by which elongated shapes are generated and maintained. We focus on theoretical models of the interplay between growth and mechanics, in relation with experimental data, and discuss how models may help us improve our understanding of the underlying biological mechanisms.