Turgor Research Articles

Plasma Membrane-Cell Wall Feedback in Bacteria.

Most bacteria have cell wall peptidoglycan surrounding their plasma membranes. The essential cell wall provides a scaffold for the envelope, protection against turgor pressure and is a proven drug target. Synthesis of the cell wall involves reactions that span cytoplasmic and periplasmic compartments. Bacteria carry out the last steps of cell wall synthesis along their plasma membrane. The plasma membrane in bacteria is heterogeneous and contains membrane compartments. Here, I outline findings that highlight the emerging notion that plasma membrane compartments and the cell wall peptidoglycan are functionally intertwined. I start by providing models of cell wall synthesis compartmentalization within the plasma membrane in mycobacteria, Escherichia coli, and Bacillus subtilis. Then, I revisit literature that supports a role for the plasma membrane and its lipids in modulating enzymatic reactions that synthesize cell wall precursors. I also elaborate on what is known about bacterial lateral organization of the plasma membrane and the mechanisms by which organization is established and maintained. Finally, I discuss the implications of cell wall partitioning in bacteria and highlight how targeting plasma membrane compartmentalization serves as a way to disrupt cell wall synthesis in diverse species.

Long-distance turgor pressure changes induce local activation of plant glutamate receptor-like channels

In Arabidopsis thaliana, local wounding and herbivore feeding provoke leaf-to-leaf propagating Ca2+ waves that are dependent on the activity of members of the glutamate receptor-like channels (GLRs). In systemic tissues, GLRs are needed to sustain the synthesis of jasmonic acid (JA) with the subsequent activation of JA-dependent signaling response required for the plant acclimation to the perceived stress. Even though the role of GLRs is well established, the mechanism through which they are activated remains unclear. Here, we report that invivo, the amino-acid-dependent activation of the AtGLR3.3 channel and systemic responses require a functional ligand-binding domain. By combining imaging and genetics, we show that leaf mechanical injury, such as wounds and burns, as well as hypo-osmotic stress in root cells, induces the systemic apoplastic increase of L-glutamate (L-Glu), which is largely independent of AtGLR3.3 that is instead required for systemic cytosolic Ca2+ elevation. Moreover, by using a bioelectronic approach, we show that the local release of minute concentrations of L-Glu in the leaf lamina fails to induce any long-distance Ca2+ waves.

Pyridoxine biosynthesis protein MoPdx1 affects the development and pathogenicity of Magnaporthe oryzae.

B vitamins are essential micro-organic compounds for the development of humans and animals. Vitamin B6 comprises a group of components including pyridoxine, pyridoxal, and pyridoxamine. In addition, vitamin B6 acts as the coenzymes in amino acid biosynthesis, decarboxylation, racemic reactions, and other biological processes. In this study, we found that the expressions of a gene encoding pyridoxine biosynthesis protein (PDX1) were significantly upregulated in the early infectious stages in M. oryzae. Furthermore, deletion of MoPDX1 slowed vegetative growth on different media, especially on MM media, and the growth defect was rescued when MoPdx1-protein was expressed in mutants strains and when commercial VB6 (pyridoxine) was added exogenously. However, VB6 content in different strains cultured in CM media has no significant difference, suggested that MoPdx1 was involved in de novo VB6 biosynthesis not in uptake process, and VB6 regulates the vegetative growth of M. oryzae. The ΔMopdx1 mutants presented abnormal appressorium turgor, slowed invasive growth and reduced virulence on rice seedlings and sheath cells. MoPdx1 was located in the cytoplasm and present in spore and germ tubes at 14 hours post inoculation (hpi) and then transferred into the appressorium at 24 hpi. Addition of VB6 in the conidial suspentions could rescue the defects of appressorium turgor pressure at 14 hpi or 24 hpi, invasive growth and pathogenicity of the MoPDX1 deletion mutants. Indicated that MoPdx1 affected the appressorium turgor pressure, invasive growth and virulence mainly depended on de novo VB6, and VB6 was biosynthesized in conidia, then transported into the appressorium, which play important roles in substances transportation from conidia to appressorium thus to regulate the appressorium turgor pressure. However, deletion of MoPDX1 did not affect the ability that scavenge ROS produced by rice cells, and the mutant strains were unable to activate host defense responses. In addition, co-immunoprecipitation (Co-IP) assays investigating potential MoPdx1-interacting proteins suggested that MoPdx1 might take part in multiple pathways, especially in the ribosome and in biosynthesis of some substances. These results indicate that vitamins are involved in the development and pathogenicity of M. oryzae.

Rigidity control mechanism by turgor pressure in plants

The bodies of herbaceous plants are slender, thin, and soft. These plants support their bodies through the action of turgor pressure associated with their internal water stores. The purpose of this study was to apply the principles of structural mechanics to clarify the underlying mechanism of rigidity control that is responsible for turgor pressure in plants and the reason behind the self-supporting ability of herbaceous plants. We modeled a plant a horizontally oriented thin-walled cylindrical cantilever with closed ends enclosing a cavity filled with water that is acted on by its own weight and by internal tension generated through turgor pressure. We derived an equation describing the plant’s consequent deflection, introducing a dimensionless parameter to express the decrease in deflection associated with the action of turgor pressure. We found that the mechanical and physical characteristics of herbaceous plants that would appear to be counter-productive from a superficial perspective increase the deflection decreasing effect of turgor pressure.

Role of cell wall anisotropy on guard cell mechanics in Arabidopsis thaliana.

The function of plant stomata is regulated by nanoscale cellular components and turgor pressure in flanking guard cells that control stomatal pore size to facilitate gas exchange and photosynthesis. Cell wall nanostructure plays a critical role in stomatal behavior. Stomatal aperture is regulated by tuning the mechanical properties of guard cell walls and by water uptake or release by the cells. However, the specific mechanical roles of different wall components, especially in newly maturing guard cells, are unclear. Here, we used a combination of molecular genetics, nanoindentation in normal and lateral directions, computational modeling, and microscopy to investigate the influences of wall components and maturation on wall mechanics and turgor pressure in stomatal guard cells. Lateral indentation measurements allow the unique anisotropic properties of each guard cell to be quantified with respect to age and genotype, including mutants with lower cellulose content and with lower or higher pectin molecular weight. Our results highlight the significance of cellulose in modulating the dynamics of guard cells by affecting wall anisotropy and emphasize the role of pectin modification for tuning stomatal responses to environmental stimuli. Our nanoindentation measurements when combined with finite element modeling allow the wall modulus, anisotropy, and turgor pressure to be measured. We also found that both cell wall mechanics and turgor pressure undergo significant changes during stomatal maturation. The role of cell wall architecture in determining anisotropic wall mechanics is also discussed. Together, these results provide new insights of wall mechanics and their effect on stomatal function and growth in plants.

Thicket and mesh: Understanding turgor pressure resistance of outer membrane-cell wall system.

The cell envelope of Gram-negative bacteria is composed of three layers: the outer membrane (OM), the peptidoglycan cell wall (PG), and the inner membrane. Together, the cell envelope provides a physical and chemical barrier that protects against the turgor pressure generated by the solute concentration difference between the inside and outside of bacteria, which can range from 0.2 atm to 5 atm. Previously, we measured the contribution of each component to see how it responds to different strains and determined that most of the cell envelope's strength comes from the OM and PG. However, the combined contribution from the OM and the PG, especially considering the Braun's lipoprotein (Lpp) that connects the two, remains to be elucidated. To address this, we constructed and simulated a combined OM-PG system that contains both OM and PG connected by Lpp. We also imposed different initial PG strain levels by stretching it by 2.0X and 2.5X its original size. The results showed that OM can resist imposed tensions by PG up to 2.0x its resting size. Moreover, we explored how the OM/PG-spanning protein TolC can affect the overall resistance to tension, through both enhancement of the connections between the layers and weakening them individually.

Plant biology: How the humble plant droops its leaves

The humble plant (Mimosa pudica) droops its leaves in response to touch. A new study explains how changes of turgor pressure exerted by protoplasts on surrounding cell walls translate into directional cell deformation that drives leaf movement.

Pollen tube invasive growth is promoted by callose.

Callose, a β-1,3-glucan, lines the pollen tube cell wall except for the apical growing region, and it constitutes the main polysaccharide in pollen tube plugs. These regularly deposited plugs separate the active portion of the pollen tube cytoplasm from the degenerating cell segments. They have been hypothesized to reduce the total amount of cell volume requiring turgor regulation, thus aiding the invasive growth mechanism. To test this, we characterized the growth pattern of Arabidopsis callose synthase mutants with altered callose deposition patterns. Mutant pollen tubes without callose wall lining or plugs had a wider diameter but grew slower compared to their respective wildtype. To probe the pollen tube's ability to perform durotropism in the absence of callose, we performed mechanical assays such as growth in stiffened media and assessed turgor through incipient plasmolysis. We found that mutants lacking plugs had lower invading capacity and higher turgor pressure when faced with a mechanically challenging substrate. To explain this unexpected elevation in turgor pressure in the callose synthase mutants we suspected that it is enabled by feedback-driven increased levels of de-esterified pectin and/or cellulose in the tube cell wall. Through immunolabeling we tested this hypothesis and found that the content and spatial distribution of these cell wall polysaccharides was altered in callose-deficient mutant pollen tubes. Combined, the results reveal how callose contributes to the pollen tube's invasive capacity and thus plays an important role in fertilization. In order to understand, how the pollen tube deposits callose, we examined the involvement of the actin cytoskeleton in the spatial targeting of callose synthases to the cell surface. The spatial proximity of actin with locations of callose deposition and the dramatic effect of pharmacological interference with actin polymerization suggest a potential role for the cytoskeleton in the spatial control of the characteristic wall assembly process in pollen tubes.

Morpho-Physiological and Hormonal Response of Winter Wheat Varieties to Drought Stress at Stem Elongation and Anthesis Stages.

Drought stress can significantly reduce wheat growth and development as well as grain yield. This study investigated morpho-physiological and hormonal (abscisic (ABA) and salicylic (SA) acids) responses of six winter wheat varieties during stem elongation and anthesis stage as well grain yield-related traits were measured after harvest. To examine drought response, plants were exposed to moderate non-lethal drought stress by withholding watering for 45 and 65% of the volumetric soil moisture content (VSMC) for 14 days at separate experiments for each of those two growth stages. During the stem elongation phase, ABA was increased, confirming the stress status of plants, and SA showed a tendency to increase, suggesting their role as stress hormones in the regulation of stress response, such as the increase in the number of leaves and tillers in drought stress conditions, and further keeping turgor pressure and osmotic adjustment in leaves. At the anthesis stage, heavier drought stress resulted in ABA accumulation in flag leaves that generated an integrated response of maturation, where ABA was not positively correlated with any of investigated traits. After harvest, the variety Bubnjar, followed by Pepeljuga and Anđelka, did not significantly decrease the number of grains per ear and 1000 kernel weight (except Anđelka) in drought treatments, thus, declaring them more tolerant to drought. On the other hand, Rujana, Fifi, and particularly Silvija experienced the highest reduction in grain yield-related traits, considering them drought-sensitive varieties.

The ASH1–PEX16 regulatory pathway controls peroxisome biogenesis for appressorium-mediated insect infection by a fungal pathogen

Entomopathogenic fungi infect insects by penetrating through the cuticle into the host body. To breach the host cuticle, some fungal pathogens produce specialized infection cells called appressoria, which develop enormous turgor pressure to allow cuticle penetration. However, regulatory mechanisms underlying appressorium turgor generation are poorly understood. Here, we show that the histone lysine methyltransferase ASH1 in the insecticidal fungus Metarhizium robertsii, which is strongly induced during infection of the mosquito cuticle, regulates appressorium turgor generation and cuticle penetration by activating the peroxin gene Mrpex16 via H3K36 dimethylation. MrPEX16 is required for the biogenesis of peroxisomes that participate in lipid catabolism and further promotes the hydrolysis of triacylglycerols stored in lipid droplets to produce glycerol for turgor generation, facilitating appressorium-mediated insect infection. Together, the ASH1-PEX16 pathway plays a pivotal role in regulating peroxisome biogenesis to promote lipolysis for appressorium turgor generation, providing insights into the molecular mechanisms underlying fungal pathogenesis.

Revisiting the relationship between turgor pressure and plant cell growth.

Growth is central to plant morphogenesis. Plant cells are encased in rigid cell walls, and they must overcome physical confinement to grow to specific sizes and shapes. Cell wall tension and turgor pressure are the main mechanical components impacting plant cell growth. Cell wall mechanics has been the focus of most plant biomechanical studies, and turgor pressure was often considered as a constant and largely passive component. Nevertheless, it is increasingly accepted that turgor pressure plays a significant role in plant growth. Numerous theoretical and experimental studies suggest that turgor pressure can be both spatially inhomogeneous and actively modulated during morphogenesis. Here, we revisit the pressure-growth relationship by reviewing recent advances in investigating the interactions between cellular/tissular pressure and growth.

The COPII subunit MoSec24B is involved in development, pathogenicity and autophagy in the rice blast fungus.

The endoplasmic reticulum (ER) acts as the starting point of the secretory pathway, where approximately one-third of the proteins are correctly folded and modified, loaded into vesicles, and transported to the Golgi for further processing and modification. In this process, COPII vesicles are responsible for transporting cargo proteins from the ER to the Golgi. Here, we identified the inner shell subunit of COPII vesicles (MoSec24B) and explored the importance of MoSec24B in the rice blast fungus. The targeted disruption of MoSec24B led to decreased growth, reduced conidiation, restricted glycogen and lipids utilization, sensitivity to the cell wall and hypertonic stress, the failure of septin-mediated repolarization of appressorium, impaired appressorium turgor pressure, and decreased ability to infect, which resulted in reduced pathogenicity to the host plant. Furthermore, MoSec24B functions in the three mitogen-activated protein kinase (MAPK) signaling pathways by acting with MoMst50. Deletion of MoSec24B caused reduced lipidation of MoAtg8, accelerated degradation of exogenously introduced GFP-MoAtg8, and increased lipidation of MoAtg8 upon treatment with a late inhibitor of autophagy (BafA1), suggesting that MoSec24B regulates the fusion of late autophagosomes with vacuoles. Together, these results suggest that MoSec24B exerts a significant role in fungal development, the pathogenesis of filamentous fungi and autophagy.

Evidence that endosperm turgor pressure both promotes and restricts seed growth and size

In plants, as in animals, organ growth depends on mechanical interactions between cells and tissues, and is controlled by both biochemical and mechanical cues. Here, we investigate the control of seed size, a key agronomic trait, by mechanical interactions between two compartments: the endosperm and the testa. By combining experiments with computational modelling, we present evidence that endosperm pressure plays two antagonistic roles: directly driving seed growth, but also indirectly inhibiting it through tension it generates in the surrounding testa, which promotes wall stiffening. We show that our model can recapitulate wild type growth patterns, and is consistent with the small seed phenotype of the haiku2 mutant, and the results of osmotic treatments. Our work suggests that a developmental regulation of endosperm pressure is required to prevent a precocious reduction of seed growth rate induced by force-dependent seed coat stiffening.

Visualization of Peptidoglycan Structures of Escherichia coli by Quick-Freeze Deep-Etch Electron Microscopy.

Peptidoglycan (PG) is an essential component of the bacterial cell wall that protects the cell from turgor pressure and maintains its shape. In diderm (gram-negative) bacteria, such as Escherichia coli, the PG layer is flexible with a thickness of a 2-6nm, and its visualization is difficult due to the presence of the outer membrane. The quick-freeze deep-etch replica method has been widely used for the visualization of flexible structures in cell interior, such as cell organelles and membrane components. In this technique, a platinum replica on the surface of a specimen fixed by freezing is observed using a transmission electron microscope. In this chapter, we describe the application of this method for visualizing the E. coli PG layer. We expect that these methods will be useful for the visualization of the PG layer in diverse bacterial species.

Mechanical modeling of growth applied to Saccharomyces cerevisiae yeast cells

A theoretical and numerical model is developed to describe the growth of Saccharomyces cerevisiae yeasts. This kind of cells is considered here as an axisymmetrical and deformable structure, the inner surface of which is continuously acted upon by a high turgor pressure. Due to the small ratio between the cellwall thickness and the cell radius, a structural shell approach is used. Moreover, the finite strain range is assumed because of the soft nature of these cells. The adopted kinematics is herein based on the multiplicative decomposition of the deformation gradient into an elastic part Fe and an irreversible part related to the growth Fg, i.e. F = FeFg. The reversible response is described using an hyperelastic model of the Ogden type. In accordance with continuum thermodynamics requirements, a criterion is introduced to control the evolution of the growth phenomenon. In this latter two parameters are involved: a growth stress-like threshold, and a growth characteristic time. Embedded within the finite element framework, an illustrative example shows the growth phenomenon of spherical cells going from yeast bud emergence to the step just before cell division. A parametric study highlights the influence of the above mentioned parameters on the cell responses.

Effect of Urea and KCl Fertilization on the Growth and Results of Gogo Rice of Situ Pateggang Variety

Rice requires nitrogen for the growth of plant vegetative organs. Potassium plays a role in regulating the osmotic pressure and turgor pressure of plants, especially in a dry land. The purpose of this study was to determine the effect of varieties and nitrogen and potassium fertilization on the growth and yield of upland rice. The research was conducted in Bleberan Village, Kab. Gunungkidul and continued at the Laboratory of the Department of Agricultural Cultivation, Faculty of Agriculture, Gadjah Mada University. This study used a split-plot design of three replications. Planting The main plot is nitrogen fertilization which consists of no nitrogen fertilization (0 kg.ha-1 Urea), nitrogen fertilization according to the recommendation (350 kg.ha-1 Urea) and twice recommended nitrogen fertilization (700 kg.ha-1 Urea). Subplots potassium fertilization which consists of no potassium fertilization (0 kg.ha-1), potassium fertilization according to the recommendation (225 kg.ha-1 KCl), and twice recommended potassium fertilization (450 kg.ha-1 KCL). This planting uses polybags, per plot there are 10 polybags containing 3 upland rice seeds. There were 9 treatment combinations, each of which was repeated three times. Analysis of the data using variance, then the Tukey test with a level of 5% was carried out. The results showed that Urea fertilization did not increase the total dry weight of the plant, the number of panicles and 1000 grain weight, and grain yield per clump. KCl fertilization with a dose of 60 g/plot (450 kg/ha) increased the total dry weight of the plant but did not increase the number of panicles and 1000 grain weight and plant yield.

When Do Subpollen Particles Become Relevant for Ice Nucleation Processes in Clouds?

AbstractWhen exposed to sufficiently humid environments, pollen grains burst and release large quantities of small subpollen particles (SPPs) which carry ice nucleating macromolecules. In this study, for the first time we develop a physically based parameterization describing the bursting process of pollen by applying a turgor pressure parameterization and quantify the impact SPPs have on overall ice nucleation in clouds. SPPs are generated from simulated birch pollen emissions over Europe for a 10‐day case study in spring. We found SPP concentrations to surpass pollen grain concentrations by 4–6 orders of magnitude leading to an abundance of biological ice nuclei from SPPs in the range of 103−104 m−3. However, it is found that these concentrations lead to only small changes in hydrometeor number densities and precipitation. Addressing the question when SPPs become relevant for ice nucleation in clouds, we conducted a sensitivity investigation. We find that amplifying ice nucleation efficiency of biological particles by factors greater 100 increases the ice particle numbers by up to 25% (T ≈ 268 K). Strong reductions show in cloud droplet number concentration and water vapor at these temperatures while water vapor is increasing at 600 m. Overall, we found a net reduction of water in the atmosphere as liquid and particularly water vapor density is reduced, while frozen water mass density increases above 257 K. Findings indicate an alteration of mixed‐phase cloud composition and increased precipitation (up to 6.2%) when SPPs are considered as highly efficient biological ice nuclei.

Morphological and ecophysiological responsiveness of Stipa tenacissima L. populations along a Mediterranean climatic gradient

• Stipa tenacissima populations allocate resources differently with climatic gradient. • Populations of arid sites exposed thicker and smaller leaves with lower productivity. • Xeric populations adjust gas exchange and stomatal closure to avoid water lost. • Mesic populations revealed higher photosynthesis with higher leaf productivity. • Stipa showed high tissues adaptation to dehydration and quick recovery after drought. Predicted climate change in the Mediterranean region has raised concerns about the possible impacts on Alphagrass ( Stipa tenacissima L.) steppes. To understand the vulnerability degree and acclimation capacity of S. tenacissima populations to climate warming and the CO 2 mitigation potential of Alfa steppes, we measured the physiological and morphological traits in five populations across a Mediterranean rainfall range from 100 to 600 mm/year. Results showed different response patterns to the precipitation gradient. Major differences were found in the gas exchange and leaf traits in the mesic to xeric sites. Xeric populations exhibited moderate gas exchange rates together with thicker and smaller leaves (i.e., low leaf dry matter content (LDMC), and high leaf thickness (Lth). They also presented differences in several functional traits related to water economy and earlier stomatal closure with drought. Furthermore, internal water use efficiency (IWUE) and apparent carboxylation efficiency (CE) were the lowest, and dark respiration (RD) was the highest in the arid site (Sfax) due to high temperatures. The relative water content at the turgor loss point (RWCtlp) varied from 0.91 to 0.85 from mesic to xeric sites, which demonstrates a high degree of osmotic adjustment. The leaf water retention capacity by osmotic adjustment and high cell turgor with a reduction in gas exchange activity through stomatal closure promote to S. tenacissima a sustain physiological activities under severe drought. These results suggest that S. tenacissima may have high adaptability to drought by regulating physiological activity and maintaining turgor pressure associated with suitable leaf structure.

Anatomy and Biomechanics of Peltate Begonia Leaves-Comparative Case Studies.

Plants are exposed to various external stresses influencing physiology, anatomy, and morphology. Shape, geometry, and size of shoots and leaves are particularly affected. Among the latter, peltate leaves are not very common and so far, only few studies focused on their properties. In this case study, four Begonia species with different leaf shapes and petiole attachment points were analyzed regarding their leaf morphology, anatomy, and biomechanical properties. One to two plants per species were examined. In all four species, the petiole showed differently sized vascular bundles arranged in a peripheral ring and subepidermal collenchyma. These anatomical characteristics, low leaf dry mass, and low amount of lignified tissue in the petiole point toward turgor pressure as crucial for leaf stability. The petiole-lamina transition zone shows a different organization in leaves with a more central (peltate) and lateral petiole insertion. While in non-peltate leaves simple fiber branching is present, peltate leaves show a more complex reticulate fiber arrangement. Tensile and bending tests revealed similar structural Young's moduli in all species for intercostal areas and venation, but differences in the petiole. The analysis of the leaves highlights the properties of petiole and the petiole-lamina transition zone that are needed to resist external stresses.

Signaling Transduction of ABA, ROS, and Ca2+ in Plant Stomatal Closure in Response to Drought

Drought is a global threat that affects agricultural production. Plants have evolved several adaptive strategies to cope with drought. Stomata are essential structures for plants to control water status and photosynthesis rate. Stomatal closure is an efficient way for plants to reduce water loss and improve survivability under drought conditions. The opening and closure of stomata depend on the turgor pressure in guard cells. Three key signaling molecules, including abscisic acid (ABA), reactive oxygen species (ROS), and calcium ion (Ca2+), play pivotal roles in controlling stomatal closure. Plants sense the water-deficit signal mainly via leaves and roots. On the one hand, ABA is actively synthesized in root and leaf vascular tissues and transported to guard cells. On the other hand, the roots sense the water-deficit signal and synthesize CLAVATA3/EMBRYO-SURROUNDING REGION RELATED 25 (CLE25) peptide, which is transported to the guard cells to promote ABA synthesis. ABA is perceived by pyrabactin resistance (PYR)/PYR1-like (PYL)/regulatory components of ABA receptor (RCAR) receptors, which inactivate PP2C, resulting in activating the protein kinases SnRK2s. Many proteins regulating stomatal closure are activated by SnRK2s via protein phosphorylation. ABA-activated SnRK2s promote apoplastic ROS production outside of guard cells and transportation into the guard cells. The apoplastic H2O2 can be directly sensed by a receptor kinase, HYDROGEN PEROXIDE-INDUCED CA2+ INCREASES1 (HPCA1), which induces activation of Ca2+ channels in the cytomembrane of guard cells, and triggers an increase in Ca2+ in the cytoplasm of guard cells, resulting in stomatal closure. In this review, we focused on discussing the signaling transduction of ABA, ROS, and Ca2+ in controlling stomatal closure in response to drought. Many critical genes are identified to have a function in stomatal closure under drought conditions. The identified genes in the process can serve as candidate genes for genetic engineering to improve drought resistance in crops. The review summarizes the recent advances and provides new insights into the signaling regulation of stomatal closure in response to water-deficit stress and new clues on the improvement of drought resistance in crops.