The Actin Cytoskeleton: Functional Arrays for Cytoplasmic Organization and Cell Shape Control

YUCCA3 interacts with ADF4 to regulate Arabidopsis hypocotyl elongation by organizing actin arrays

Hypocotyl elongation is critical for plants emerging from the soil, and serves as a model for investigating cell elongation mechanism. It has been reported that auxin biosynthesis enzyme YUCCAs (YUCs) and the cytoskeleton are involved in the regulation of hypocotyl elongation in Arabidopsis. However, whether and how the cytoskeleton is involved in YUCs-regulated hypocotyl elongation is not well understood. Here, we report that YUC3 directly interacted with Actin Depolymerizing Factor 4 (ADF4) to regulate hypocotyl elongation. The yuc3 mutant seedlings produced shorter hypocotyls, while YUC3-OEs seedlings showed longer hypocotyls. Pharmacological analysis showed that microfilament but not microtubule was involved in YUC3-regulated hypocotyl elongation. Consistent with this, defects in actin arrays were observed in the yuc3 seedlings. In addition, YUC3 interacted with ADF4 but not ADF1 in vitro and in vivo. Knock out of ADF4 partially rescued the defects of yuc3 mutant hypocotyl elongation and actin arrays. In summary, our results demonstrate that YUC3 mediates the organization of actin filaments possibly by interacting with ADF4 and affecting its actin depolymerizing/severing activity in the regulation of hypocotyl elongation.

Aberrant growth and expansion in Penium margaritaceum triggered by disruption of microtubules and the cell wall

Penium margaritaceum, a unicellular zygnematophyte (Streptophyta), was employed to elucidate changes in cell expansion when cells were challenged with the fungal pectinolytic enzyme, pectate lyase, and/or the microtubule-disrupting agent, amiprophos-methyl (APM). Microtubule disruption by APM resulted in significant swelling at expansion zones. These swollen zones provided an easy marker for the location of expansion zones, particularly in cells with altered cell wall pectin. Short-term treatment with pectate lyase showed pectin degradation primarily at the isthmus expansion zone and two satellite bands, corresponding to the location of future expansion in daughter cells. When the homogalacturonan lattice of the cell wall was removed by treatment with pectate lyase during long treatments, cell division was maintained, but daughter cell products were considerably smaller. Treatment of cells with a mixture of both pectate lyase and APM resulted in a distinct phenotype, consisting of 'dumbbell'-shaped cells, as APM-induced swelling occurs at the novel expansion centers exposed by pectate lyase treatment. These cells also presented other curious alterations, including an extensive, chloroplast-free cytoplasmic zone at the center of the cell, a septum containing β-glycan, arabinogalactan and homogalacturonan epitopes, unique stacks of endoplasmic reticulum, displaced Golgi bodies, and an extensive network of vacuoles. These results provide insight into the importance of cell wall integrity in defining the location of cell growth and division in P. margaritaceum. Understanding these processes in a unicellular zygnematophyte may provide insights into steps involved in the evolution of land plants.

Inhibition of mitochondrial energy production leads to reorganization of the plant endomembrane system

Mitochondria have generated the bulk of ATP to fuel cellular activities, including membrane trafficking, since the beginning of eukaryogenesis. How inhibition of mitochondrial energy production will affect the form and function of the endomembrane system and whether such changes are specific in today's cells remain unclear. Here, we treated Arabidopsis thaliana with antimycin A (AA), a potent inhibitor of the mitochondrial electron transport chain (mETC), as well as other mETC inhibitors and an uncoupler. We investigated the effects of AA on different endomembrane organelles connected by vesicle trafficking via anterograde and retrograde routes that heavily rely on ATP and GTP provision for SNARE and RAB/GEF function, respectively, in root cells. Similar to previous reports, AA inhibited root growth mainly by shortening the elongation zone (EZ) in an energy- and auxin-dependent way. We found that PIN-FORMED 2 (PIN2) and REQUIRES HIGH BORON 1 (BOR1), key proteins for EZ establishment and cell expansion, undergo accelerated endocytosis and accumulate at enlarged multivesicular bodies (MVBs) after AA treatment. Such accumulation is consistent with the observation that the central vacuole becomes fragmented and spherical and that the Arabidopsis Rab7 homolog RABG3f, a master regulator of MVB and vacuolar function, localizes to the tonoplast, likely in a GTP-bound form. We further examined organelles and vesicle populations along the secretory pathway and found that the Golgi apparatus-in particular, the endoplasmic reticulum-Golgi intermediate compartment (ERGIC)-cannot be maintained when mETC is inhibited. Our findings reveal the importance and specific impact of mitochondrial energy production on endomembrane homeostasis.

The Ras-related nuclear GTPase RAN1 ensures pollen size and tube growth by maintaining the actin cytoskeleton

Controlling organ size in plants is a complex biological process influenced by various factors, including gene expression, genome ploidy and environmental conditions. Despite its importance for plant growth and development, the mechanisms underlying organ size regulation remain unknown. Here, we investigated the role of RAN1, a member of the Ras-related nuclear GTPase family, in regulating pollen size. A RAN1 knockdown mutant (ran1-1) exhibited a significant reduction in pollen size, accompanied by impaired germination and reduced pollen tube growth. RAN1 mutation caused disruptions in actin filament organization, such as aberrant structure of actin collar due to the dysregulation of expression of actin-binding proteins. Furthermore, we identified that mutation of the transcription activator SHORT HYPOCOTYL UNDER BLUE1 (SHB1) showed similar but milder phenotypes in pollen compared to ran1-1. Genetic evidence suggested that SHB1 acts downstream of RAN1. Transient expression assays in leaves showed that SHB1 was largely retained in the cytoplasm of the ran1-1 mutant, potentially affecting the expression of actin-binding proteins. These findings highlight the pivotal role of RAN1 in modulating pollen size and development, providing valuable insights into cell size regulation.

Morphometric analysis of actin networks

The organization of cytoskeletal elements is pivotal for coordinating intracellular transport in eukaryotic cells. Several quantitative measures based on image analysis have been proposed to characterize morphometric features of fluorescently labeled actin networks. While helpful in detecting differences in actin organization between treatments or genotypes, the accuracy of these measures could not be rigorously assessed due to a lack of ground-truth data to which they could be compared. To overcome this limitation, we utilized coarse-grained computer simulations of actin filaments and cross-linkers to generate synthetic actin networks with varying levels of bundling. We converted the simulated networks into pseudofluorescence images similar to images obtained using confocal microscopy. Using both published and novel analysis procedures, we extracted a series of morphometric parameters and benchmarked them against analogous measures based on the ground-truth actin configurations. Our analysis revealed a set of parameters that reliably reports on actin network density, orientation, ordering, and bundling. Application of these morphometric parameters to root epidermal cells of Arabidopsis thaliana revealed subtle changes in network organization between wild-type and mutant cells. This work provides robust measures that can be used to quantify features of actin networks and characterize changes in actin organization for different experimental conditions.

SPIRRIG is required for BRICK1 stability and salt stress induced root hair developmental plasticity in Arabidopsis

Developmental plasticity is critical for plants to adapt to constantly changing environments. Plant root hairs display dramatic plasticity under different environments and therefore play crucial roles in defense against environmental stressors. Here, we report the isolation of an Arabidopsis mutant, salinity over-sensitive mutant 1-1 (som1-1), also exhibiting root hair developmental defects. Map-based cloning and allelic analyses confirmed that som1-1 is a new mutant allele of SPIRRIG (SPI), which encodes a Beige and Chediak Higashi (BEACH) domain-containing protein. SPI has been reported to facilitate actin dependent root hair development by temporally and spatially regulating the expression of BRICK1 (BRK1), a subunit of the SCAR/WAVE actin nucleating promoting complex. Our living cell imaging examinations revealed that salt stress induces an altered actin organization in root hair that mimics those in the spi mutant, implying SPI may respond to salt stress induced root hair plasticity by modulating actin cytoskeleton organization. Furthermore, we found BRK1 is also involved in root hair developmental change under salt stress, and overexpression of BRK1 resulted in root hairs over-sensitive to salt stress as those in spi mutant. Moreover, based on biochemical analyses, we found BRK1 is unstable and SPI mediates BRK1 stability. Functional loss of SPI results in the accumulation of steady-state of BRK1.

Exploring Genomic Regions Associated with Fruit Traits in Pepper: Insights from Multiple GWAS Models

This study utilized 303 pepper accessions from diverse Capsicum species to explore fruit traits, including length, width, wall thickness, and weight. Descriptive statistics revealed a mean fruit length of 66.19 mm, width of 23.48 mm, wall thickness of 1.89 mm, and weight of 15.29 g, with significant variability, particularly in fruit weight. Correlation analysis demonstrated strong positive relationships between fruit width, weight, and fruit wall thickness (r = 0.89 and r = 0.86, respectively), while fruit length showed weaker correlations with these traits. Analysis of fruit positions revealed that the majority of accessions had a pendent fruit position (156), followed by erect (85) and intermediate (8). In terms of fruit shape, triangular and narrow triangular shapes were the most common, observed in 102 and 98 accessions, respectively. Genome-wide association studies (GWAS) identified significant single nucleotide polymorphisms (SNPs) associated with fruit traits across four models (Blink, FarmCPU, MLM, MLMM). The number of significantly associated SNPs were as follows: fruit length (89), fruit width (55), fruit weight (63), fruit wall thickness (48), fruit shape (151), and fruit position (51). Several genes were also identified where the SNPs are located or adjacent to, providing candidate genes for further exploration of the genetic basis of fruit morphology. Notably, genes such as E3 ubiquitin-protein ligase RGLG1 (associated with fruit width), Homeobox-leucine zipper protein HDG11 (involved in fruit width), Auxin response factor 23 (linked to fruit shape), and ATP-dependent zinc metalloprotease FtsH (related to fruit weight) were identified. These findings enhance our understanding of the genetic basis of fruit morphology in Capsicum, offering valuable insights for breeding and agricultural practices.

MYB52 negatively regulates ADF9-meditated actin filament bundling in Arabidopsis pavement cell morphogenesis

It has been proposed that cortical fine actin filaments are needed for the morphogenesis of pavement cells (PCs). However, the precise role and regulation mechanisms of actin filaments in PC morphogenesis are not well understood. Here, we found that Arabidopsis thaliana ACTIN DEPOLYMERIZING FACTOR9 (ADF9) is required for the morphogenesis of PC, which is negatively regulated by the R2R3 MYELOBLASTOSIS (MYB) transcription factor MYB52. In adf9 mutants, the lobe number of cotyledon PCs was significantly reduced, while the average lobe length did not differ significantly compared to that of wild type (Col-0), except for the variations in cell area and circularity, whereas the PC shapes in ADF9 overexpression seedlings showed different results. ADF9 decorated actin filaments, and colocalized with plasma membrane. The extent of filament bundling and actin filament bundling activity in adf9 mutant decreased. In addition, MYB52 directly targeted the promoter of ADF9 and negatively regulated its expression. The myb52-2 mutant showed increased lobe number and cell area, reduced cell circularity of PCs, and the PC phenotypes were suppressed when ADF9 was knocked out. Taken together, our data demonstrate that actin filaments play an important role in the morphogenesis of PC and reveal a transcriptional mechanism underlying MYB52 regulation of ADF9-mediated actin filament bundling in PC morphogenesis.

PLEIOTROPIC REGULATORY LOCUS1 maintains actin microfilament integrity to regulate pavement cell morphogenesis

Actin dynamics are critical for plant cell morphogenesis, but the underlying signaling mechanisms regulating these dynamics are not well understood. Here, we established that PLEIOTROPIC REGULATORY LOCUS1 (PRL1) modulates leaf pavement cell (PC) morphogenesis in Arabidopsis (Arabidopsis thaliana) by maintaining the dynamic homeostasis of actin microfilaments (MF). Our previous studies indicated that PC shape was determined by antagonistic RHO-RELATED GTPase FROM PLANTS 2 (ROP2) and RHO-RELATED GTPase FROM PLANTS 6 (ROP6) signaling pathways that promote cortical MF and microtubule organization, respectively. Our genetic screen for additional components in ROP6-mediated signaling identified prl1 alleles. Genetic analysis confirmed that PRL1 plays a key role in PC morphogenesis. Mutations in PRL1 caused cortical MF depolymerization, resulting in defective PC morphogenesis. Further genetic analysis revealed that PRL1 is epistatic to ROP2 and ROP6 in PC morphogenesis. Mutations in PRL1 enhanced the effects of ROP2 and ROP6 in PC morphogenesis, leading to a synergistic phenotype in the PCs of ROP2 prl1 and ROP6 prl1. Furthermore, the activities of ROP2 and ROP6 were differentially altered in prl1 mutants, suggesting that ROP2 and ROP6 function downstream of PRL1. Additionally, cortical MF depolymerization in prl1 mutants occurred independently of ROP2 and ROP6, implying that these proteins impact PC morphogenesis in the prl1 mutant through other cellular processes. Our research indicates that PRL1 preserves the structural integrity of actin and facilitates pavement cell morphogenesis in Arabidopsis.

The synergistic effect of multiple organic macromolecules on the formation of calcium oxalate raphides of Musa spp

Needle-like calcium oxalate crystals called raphides are unique structures in the plant kingdom. Multiple biomacromolecules work together in the regulatory and transportation pathways to form raphides; however, the mechanism by which this occurs remains unknown. Using banana (Musa spp.), this study combined in vivo methods including confocal microscopy, transmission electron microscopy, and Q Exactive mass spectrometry to identify the main biomolecules, such as vesicles, together with the compositions of lipids and proteins in the crystal chamber, which is the membrane compartment that surrounds each raphide during its formation. Simulations of the vesicle transportation process and the synthesis of elongated calcium oxalate crystals in vitro were then conducted, and the results suggested that the vesicles carrying amorphous calcium oxalate and proteins embedded in raphides are transported along actin filaments. These vesicles subsequently fuse with the crystal chamber, utilizing the proteins embedded in the raphides as a template for the final formation of the structure. Our findings contribute to the fundamental understanding of the regulation of the diverse biomacromolecules that are crucial for raphide formation. Moreover, the implications of these findings extend to other fields such as materials science, and particularly the synthesis of functionalized materials.

Genome-wide association study for yield-related traits in faba bean (Vicia faba L.)

Yield is the most complex trait to improve crop production, and identifying the genetic determinants for high yield is a major issue in breeding new varieties. In faba bean (Vicia faba L.), quantitative trait loci (QTLs) have previously been detected in studies of biparental mapping populations, but the genes controlling the main trait components remain largely unknown. In this study, we investigated for the first time the genetic control of six faba bean yield-related traits: shattering (SH), pods per plant (PP), seeds per pod (SP), seeds per plant (SPL), 100-seed weight (HSW), and plot yield (PY), using a genome-wide association study (GWAS) on a worldwide collection of 352 homozygous faba bean accessions with the aim of identifying markers associated with them. Phenotyping was carried out in field trials at three locations (Spain, United Kingdom, and Serbia) over 2 years. The faba bean panel was genotyped with the Affymetrix faba bean SNP-chip yielding 22,867 SNP markers. The GWAS analysis identified 112 marker-trait associations (MTAs) in 97 candidate genes, distributed over the six faba bean chromosomes. Eight MTAs were detected in at least two environments, and five were associated with multiple traits. The next step will be to validate these candidates in different genetic backgrounds to provide resources for marker-assisted breeding of faba bean yield.

Synergism of vesicle trafficking and cytoskeleton during regulation of plant growth and development: A mechanistic outlook

The cytoskeleton is a fundamental component found in all eukaryotic organisms, serving as a critical factor in various essential cyto-biological mechanisms, particularly in the locomotion and morphological transformations of plant cells. The cytoskeleton is comprised of three main components: microtubules (MT), microfilaments (MF), and intermediate filaments (IF). The cytoskeleton plays a crucial role in the process of cell wall formation and remodeling throughout the growth and development of cells. It is a highly organized and regulated network composed of filamentous components. In the basic processes of intracellular transport, such as mitosis, cytokinesis, and cell polarity, the plant cytoskeleton plays a crucial role according to recent studies. The major flaws in the organization of the cytoskeletal framework are at the root of the aberrant organogenesis currently observed in plant mutants. The regulation of protein compartmentalization and abundance within cells is predominantly governed by the process of vesicle/membrane transport, which plays a crucial role in several signaling cascades.The regulation of membrane transport in eukaryotic cells is governed by a diverse array of proteins. Recent developments in genomics have provided new tools to study the evolutionary relationships between membrane proteins in different plant species. It is known that members of the GTPases, COP, SNAREs, Rabs, tethering factors, and PIN families play essential roles in vesicle transport between plant, animal, and microbial species. This Review presents the latest research on the plant cytoskeleton, focusing on recent developments related to the cytoskeleton and summarizing the role of various proteins in vesicle transport. In addition, the report predicts future research direction of plant cytoskeleton and vesicle trafficking, potential research priorities, and provides researchers with specific pointers to further investigate the significant link between cytoskeleton and vesicle trafficking.

Over-expression of GhACTIN1 under the control of GhSCFP promoter improves cotton fiber and yield

Actin dynamics is pivotal in controlling cotton fiber elongation and the onset of secondary wall biosynthesis. We report that overexpression of GhACTIN1 under fiber fiber-specific promoter, GhSCFP, improves cotton fiber length, strength, and micronaire value. However, the effect of transgene has a more positive effect on fiber strength and micronaire value than fiber length. F-actin quantification and cellulose contents measurement in transgenic developing cotton fiber during the elongation phase showed an increase of up to 8.7% and 4.7% respectively. Additionally, physiological factors such as water use efficiency showed no significant change in transgenic cotton lines, while stomatal conductance and photosynthetic rate were significantly increased. Moreover, agronomical data determined that lint percentage (GOT) and seed cotton yield also increased up to 4.6% and 29.5% respectively, in transgenic cotton lines compared to the control lines. Our data demonstrate that the GhACTIN1 gene is a strong candidate gene for cotton fiber and yield improvement.

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

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

Transgenic sugarcane with higher levels of BRK1 showed improved drought tolerance

KEY MESSAGE

Transgenic sugarcane overexpressing BRK1 showed improved tolerance to drought stress through modulation of actin polymerization and formation of interlocking marginal lobes in epidermal leaf cells, a typical feature associated with BRK1 expression under drought stress. BRICK1 (BRK1) genes promote leaf epidermal cell morphogenesis and division in plants that involves local actin polymerization. Although the changes in actin filament organization during drought have been reported, the role of BRK in stress tolerance remains unknown. In our previous work, the drought-tolerant Erianthus arundinaceus exhibited high levels of the BRK gene expression under drought stress. Therefore, in the present study, the drought-responsive gene, BRK1 from Saccharum spontaneum, was transformed into sugarcane to test if it conferred drought tolerance in the commercial sugarcane cultivar Co 86032. The transgenic lines were subjected to drought stress, and analyzed using physiological parameters for drought stress. The drought-induced BRK1-overexpressing lines of sugarcane exhibited significantly higher transgene expression compared with the wild-type control and also showed improved physiological parameters. In addition, the formation of interlocking marginal lobes in the epidermal leaf cells, a typical feature associated with BRK1 expression, was observed in all transgenic BRK1 lines during drought stress. This is the first report to suggest that BRK1 plays a role in sugarcane acclimation to drought stress and may prove to be a potential candidate in genetic engineering of plants for enhanced biomass production under drought stress conditions.

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

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

Effects of environmental stress factors on the actin cytoskeleton of fungi and plants: Ionizing radiation and ROS

Actin is an abundant and multifaceted protein in eukaryotic cells that has been detected in the cytoplasm as well as in the nucleus. In cooperation with numerous interacting accessory-proteins, monomeric actin (G-actin) polymerizes into microfilaments (F-actin) which constitute ubiquitous subcellular higher order structures. Considering the extensive spatial dimensions and multifunctionality of actin superarrays, the present study analyses the issue if and to what extent environmental stress factors, specifically ionizing radiation (IR) and reactive oxygen species (ROS), affect the cellular actin-entity. In that context, this review particularly surveys IR-response of fungi and plants. It examines in detail which actin-related cellular constituents and molecular pathways are influenced by IR and related ROS. This comprehensive survey concludes that the general integrity of the total cellular actin cytoskeleton is a requirement for IR-tolerance. Actin's functions in genome organization and nuclear events like chromatin remodeling, DNA-repair, and transcription play a key role. Beyond that, it is highly significant that the macromolecular cytoplasmic and cortical actin-frameworks are affected by IR as well. In response to IR, actin-filament bundling proteins (fimbrins) are required to stabilize cables or patches. In addition, the actin-associated factors mediating cellular polarity are essential for IR-survivability. Moreover, it is concluded that a cellular homeostasis system comprising ROS, ROS-scavengers, NADPH-oxidases, and the actin cytoskeleton plays an essential role here. Consequently, besides the actin-fraction which controls crucial genome-integrity, also the portion which facilitates orderly cellular transport and polarized growth has to be maintained in order to survive IR.

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

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

Contrasting self-recognition rejection systems for self-incompatibility in Brassica and Papaver

Self-incompatibility (SI) plays a pivotal role in whether self-pollen is accepted or rejected. Most SI systems employ two tightly linked loci encoding highly polymorphic pollen (male) and pistil (female) S-determinants that control whether self-pollination is successful or not. In recent years our knowledge of the signalling networks and cellular mechanisms involved has improved considerably, providing an important contribution to our understanding of the diverse mechanisms used by plant cells to recognise each other and elicit responses. Here, we compare and contrast two important SI systems employed in the Brassicaceae and Papaveraceae. Both use 'self-recognition' systems, but their genetic control and S-determinants are quite different. We describe the current knowledge about the receptors and ligands, and the downstream signals and responses utilized to prevent self-seed set. What emerges is a common theme involving the initiation of destructive pathways that block the key processes that are required for compatible pollen-pistil interactions.

Stabilization of membrane topologies by proteinaceous remorin scaffolds

In plants, the topological organization of membranes has mainly been attributed to the cell wall and the cytoskeleton. Additionally, few proteins, such as plant-specific remorins have been shown to function as protein and lipid organizers. Root nodule symbiosis requires continuous membrane re-arrangements, with bacteria being finally released from infection threads into membrane-confined symbiosomes. We found that mutations in the symbiosis-specific SYMREM1 gene result in highly disorganized perimicrobial membranes. AlphaFold modelling and biochemical analyses reveal that SYMREM1 oligomerizes into antiparallel dimers and may form a higher-order membrane scaffolding structure. This was experimentally confirmed when expressing this and other remorins in wall-less protoplasts is sufficient where they significantly alter and stabilize de novo membrane topologies ranging from membrane blebs to long membrane tubes with a central actin filament. Reciprocally, mechanically induced membrane indentations were equally stabilized by SYMREM1. Taken together we describe a plant-specific mechanism that allows the stabilization of large-scale membrane conformations independent of the cell wall.

The actin cytoskeleton: Morphological changes in pre- and fully developed lung cancer

Actin, a primary component of the cell cytoskeleton can have multiple isoforms, each of which can have specific properties uniquely suited for their purpose. These monomers are then bound together to form polymeric filaments utilizing adenosine triphosphate hydrolysis as a source of energy. Proteins, such as Arp2/3, VASP, formin, profilin, and cofilin, serve important roles in the polymerization process. These filaments can further be linked to form stress fibers by proteins called actin-binding proteins, such as α-actinin, myosin, fascin, filamin, zyxin, and epsin. These stress fibers are responsible for mechanotransduction, maintaining cell shape, cell motility, and intracellular cargo transport. Cancer metastasis, specifically epithelial mesenchymal transition (EMT), which is one of the key steps of the process, is accompanied by the formation of thick stress fibers through the Rho-associated protein kinase, MAPK/ERK, and Wnt pathways. Recently, with the advent of "field cancerization," pre-malignant cells have also been demonstrated to possess stress fibers and related cytoskeletal features. Analytical methods ranging from western blot and RNA-sequencing to cryo-EM and fluorescent imaging have been employed to understand the structure and dynamics of actin and related proteins including polymerization/depolymerization. More recent methods involve quantifying properties of the actin cytoskeleton from fluorescent images and utilizing them to study biological processes, such as EMT. These image analysis approaches exploit the fact that filaments have a unique structure (curvilinear) compared to the noise or other artifacts to separate them. Line segments are extracted from these filament images that have assigned lengths and orientations. Coupling such methods with statistical analysis has resulted in development of a new reporter for EMT in lung cancer cells as well as their drug responses.

Thiol-disulfide oxidoreductase PDI1;1 regulates actin structures in Oryza sativa root cells

The polarized and dynamic actin cytoskeleton is essential for root cell growth. Here, we report the key role of thiol-disulfide oxidoreductase PDI1;1 in actin structures. Microscopic analyses revealed that after Oryza sativa roots were exposed to H₂ O₂ , both actin and PDI1;1 were depolarized and arranged in a meshwork. In H₂ O₂ -exposed cells, actin formed intermolecularly disulfide-bonded high-molecular-weight structures, which were thiol-trapped by PDI1;1. Recombinant PDI1;1 exhibited the ability to recognize actin in an in vitro binding assay. During recovery from H₂ O₂ exposure, the amount of disulfide-bonded high-molecular-weight structures of actin decreased over time, but deficiency of PDI1;1 inhibited the decrease. These results suggest a PDI1;1-dependent pathway that reduces disulfide bonds in high-molecular-weight structures of actin, thus promoting their degradation.

Cell twisting during desiccation reveals axial asymmetry in wall organization

Plant cell size and shape are tuned to their function and specified primarily by cellulose microfibril (CMF) patterning of the cell wall. Arabidopsis thaliana leaf trichomes are unicellular structures that act as a physical defense to deter insect feeding. This highly polarized cell type employs a strongly anisotropic cellulose wall to extend and taper, generating sharply pointed branches. During elongation, the mechanisms by which shifts in fiber orientation generate cells with predictable sizes and shapes are unknown. Specifically, the axisymmetric growth of trichome branches is often thought to result from axisymmetric CMF patterning. Here, we analyzed the direction and degree of twist of branches after desiccation to reveal the presence of an asymmetric cell wall organization with a left-hand bias. CMF organization, quantified using computational modeling, suggests a limited reorientation of microfibrils during growth and a maximum branch length limited by the wall axial stiffness. The model provides a mechanism for CMF asymmetry, which occurs after the branch bending stiffness becomes low enough that ambient bending affects the principal stresses. After this stage, the CMF synthesis results in a constant bending stiffness for longer branches. The bending vibration natural frequencies of branches with respect to their length are also discussed.

Process Engineering of Biopharmaceutical Production in Moss Bioreactors via Model-Based Description and Evaluation of Phytohormone Impact

The moss Physcomitrella is an interesting production host for recombinant biopharmaceuticals. Here we produced MFHR1, a synthetic complement regulator which has been proposed for the treatment of diseases associated to the complement system as part of human innate immunity. We studied the impact of different operation modes for the production process in 5 L stirred-tank photobioreactors. The total amount of recombinant protein was doubled by using fed-batch or batch compared to semi-continuous operation, although the maximum specific productivity (mg MFHR1/g FW) increased just by 35%. We proposed an unstructured kinetic model which fits accurately with the experimental data in batch and semi-continuous operation under autotrophic conditions with 2% CO₂ enrichment. The model is able to predict recombinant protein production, nitrate uptake and biomass growth, which is useful for process control and optimization. We investigated strategies to further increase MFHR1 production. While mixotrophic and heterotrophic conditions decreased the MFHR1-specific productivity compared to autotrophic conditions, addition of the phytohormone auxin (NAA, 10 µM) to the medium enhanced it by 470% in shaken flasks and up to 230% and 260%, in batch and fed-batch bioreactors, respectively. Supporting this finding, the auxin-synthesis inhibitor L-kynurenine (100 µM) decreased MFHR1 production significantly by 110% and 580% at day 7 and 18, respectively. Expression analysis revealed that the MFHR1 transgene, driven by the Physcomitrella actin5 (PpAct5) promoter, was upregulated 16 h after NAA addition and remained enhanced over the whole process, whereas the auxin-responsive gene PpIAA1A was upregulated within the first 2 hours, indicating that the effect of auxin on PpAct5 promoter-driven expression is indirect. Furthermore, the day of NAA supplementation was crucial, leading to an up to 8-fold increase of MFHR1-specific productivity (0.82 mg MFHR1/g fresh weight, 150 mg accumulated over 7 days) compared to the productivity reported previously. Our findings are likely to be applicable to other plant-based expression systems to increase biopharmaceutical production and yields.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Stereotactic Body Radiotherapy and Conventional Radiotherapy Induce Cytoskeleton Extension and Enlargement of Cell Morphology in Non-Small Cell Lung Cancer

Stereotactic body radiotherapy (SBRT) is now widely used in cancer therapy. However, the biological effects of SBRT compared with conventional radiotherapy (CRT) are not clear. The cytoskeleton plays an important role in many biological processes and cellular life activities. The effects of SBRT or CRT on the morphology and cytoskeletal structure of non-small cell lung cancer (NSCLC) cells remain unknown. Based on the biologically equivalent dose (BED) formula, we designed SBRT and CRT fractionation regimens with the same BED. The morphology was captured during radiation, and rhodamine-phalloidin immunofluorescence was used to study the cytoskeleton. A lactate dehydrogenase assay kit was used to determine the cell membrane permeability, and western blot was used to detect the cytoskeleton protein expression levels. The morphology and cytoskeleton expanded after SBRT or CRT, with an increase in cell membrane permeability and stable cytoskeleton protein levels. Besides, different dose of SBRT (10,20,30 Gy) induce similar morphology and cytoskeleton enlargement. Our findings indicate that SBRT and CRT can induce cytoskeleton reorganization and the enlargement of cell morphology (at different rates) in NSCLC. The morphology and cytoskeleton enlargement after SBRT are dose independence.

Effect of Salvianolate on Myocardial Ischemia Reperfusion Injury and Its Mechanism

<sec> <title>Background:</title> The paper investigated the H9c2 cardiomyocyte model induced by hypoxia. Cell viability was monitored by real-time unlabeled cell function analyzer to determine the levels of LDH, MDA and SOD in cell supernatant. </sec> <sec> <title>Material and Methods:</title> The cytoskeleton staining was labeled by phalloidin staining. WB was applied to detect the expression of myocardial cytoskeleton microtubuleassociated protein and the expression of HIF-1α protein in each group. After adding AMPK inhibitor Compound C, Hoechst 33342 was employed to detect the apoptosis rate of cardiomyocytes, and WB was applied to detect the expressions of myocardial cytoskeleton-associated protein and p-AMPK. </sec> <sec> <title>Results:</title> Salvianolate can effectively improve cell viability, reduce LDH and MDA levels, increase SOD content, improve skeletal structure damage, reduce nuclear concentration, reduce cell debris, and promote the expressions of microtubule-associated protein, α-tubulin and β-tubulin, MAP4, and microfilament-associated protein MLCK, p-MLC-2 in myocardial cytoskeleton microtubules after ischemia and hypoxia. The addition of AMPK inhibitor can inhibit the expressions of p-AMPK, tubulin MAP4, microfilament protein MLCK and p-MLC-2 up-regulated by Salvianolate. </sec> <sec> <title>Conclusion:</title> Salvianolate can promote the expressions of microtubule-associated protein α-tubulin, β-tubulin,MAP4, microfilament-associated protein MLCK and p-MLC-2 in myocardial cytoskeleton after ischemia and hypoxia, indicating that Salvianolate can protect the myocardial cytoskeleton after ischemia and hypoxia, and may protect the structure and function of microtubules and microfilaments in the myocardial cytoskeleton through the AMPK/MAP4 and AMPK/MLCK pathways. </sec>

Genome Sequence of Chrysotila roscoffensis, a Coccolithphore Contributed to Global Biogeochemical Cycles

Chrysotila is a genus of coccolithophores. Together with Emiliania, it is one of the representative genera in the Haptophyta which have been extensively studied. They are photosynthetic unicellular marine algae sharing the common characteristic of the production of CaCO₃ platelets (coccoliths) on the surface of their cells and are crucial contributors to global biogeochemical cycles. Here, we report the genome assembly of Chrysotila roscoffensis. The assembled genome size was ~636 Mb distributed across 769 scaffolds with N50 of 1.63 Mb, and maximum contig length of ~2.6 Mb. Repetitive elements accounted for approximately 59% of the genome. A total of 23,341 genes were predicted from C. roscoffensis genome. The divergence time between C. roscoffensis and Emiliania huxleyi was estimated to be around 537.6 Mya. Gene families related to cytoskeleton, cellular motility and morphology, and ion transport were expanded. The genome of C. roscoffensis will provide a foundation for understanding the genetic and phenotypic diversification and calcification mechanisms of coccolithophores.

Dynamic Rearrangement and Directional Migration of Tubular Vacuoles are Required for the Asymmetric Division of the Arabidopsis Zygote

In most flowering plants, the asymmetric cell division of zygotes is the initial step that establishes the apical-basal axis. In the Arabidopsis zygote, vacuolar accumulation at the basal cell end is crucial to ensure zygotic division asymmetry. Despite the importance, it was unclear whether this polar vacuolar distribution was achieved by predominant biogenesis at the basal region or by directional movement after biogenesis. Here, we found that apical and basal vacuolar contents are dynamically exchanged via a tubular vacuolar network and the vacuoles gradually migrate toward the basal end. The mutant of a vacuolar membrane protein, SHOOT GRAVITROPISM2 (SGR2), failed to form tubular vacuoles, and the mutant of a putative vacuolar fusion factor, VESICLE TRANSPORT THROUGH INTERACTION WITH T-SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE FUSION PROTEIN ATTACHMENT PROTEIN RECEPTORS (SNARES) 11 (VTI11), could not flexibly rearrange the vacuolar network. Both mutants failed to exchange the apical and basal vacuolar contents and to polarly migrate the vacuoles, resulting in a more symmetric division of zygotes. Additionally, we observed that in contrast to sgr2, the zygotic defects of vti11 were rescued by the pharmacological depletion of phosphatidylinositol 3-phosphate (PI3P), a distinct phospholipid in the vacuolar membrane. Thus, SGR2 and VTI11 have individual sites of action in zygotic vacuolar membrane processes. Further, a mutant of YODA (YDA) mitogen-activated protein kinase kinase kinase, a core component of the embryonic axis formation pathway, generated the proper vacuolar network; however, it failed to migrate the vacuoles toward the basal region, which suggests impaired directional cues. Overall, we conclude that SGR2- and VTI11-dependent vacuolar exchange and YDA-mediated directional migration are necessary to achieve polar vacuolar distribution in the zygote.

Spatial and temporal localization of SPIRRIG and WAVE/SCAR reveal roles for these proteins in actin-mediated root hair development

Root hairs are single-cell protrusions that enable roots to optimize nutrient and water acquisition. These structures attain their tubular shapes by confining growth to the cell apex, a process called tip growth. The actin cytoskeleton and endomembrane systems are essential for tip growth; however, little is known about how these cellular components coordinate their activities during this process. Here, we show that SPIRRIG (SPI), a beige and Chediak Higashi domain-containing protein involved in membrane trafficking, and BRK1 and SCAR2, subunits of the WAVE/SCAR (W/SC) actin nucleating promoting complex, display polarized localizations in Arabidopsis thaliana root hairs during distinct developmental stages. SPI accumulates at the root hair apex via post-Golgi compartments and positively regulates tip growth by maintaining tip-focused vesicle secretion and filamentous-actin integrity. BRK1 and SCAR2 on the other hand, mark the root hair initiation domain to specify the position of root hair emergence. Consistent with the localization data, tip growth was reduced in spi and the position of root hair emergence was disrupted in brk1 and scar1234. BRK1 depletion coincided with SPI accumulation as root hairs transitioned from initiation to tip growth. Taken together, our work uncovers a role for SPI in facilitating actin-dependent root hair development in Arabidopsis through pathways that might intersect with W/SC.

Stress effects on the reactive oxygen species-dependent regulation of plant growth and development

Plant growth is mediated by cell proliferation and expansion. Both processes are controlled by a network of endogenous factors such as phytohormones, reactive oxygen species (ROS), sugars, and other signals, which influence gene expression and post-translational regulation of proteins. Stress resilience requires rapid and appropriate responses in plant growth and development as well as defence. Regulation of ROS accumulation in different cellular compartments influences growth responses to abiotic and biotic stresses. While ROS are essential for growth, they are also implicated in the stress-induced cessation of growth and, in some cases, programmed cell death. It is widely accepted that redox post-translational modifications of key proteins determine the growth changes and cell fate responses to stress, but the molecular pathways and factors involved remain poorly characterized. Here we discuss ROS as a signalling molecule, the mechanisms of ROS-dependent regulation that influence protein-protein interactions, protein function, and turnover, together with the relocation of key proteins to different intracellular compartments in a manner that can alter cell fate. Understanding how the redox interactome responds to stress-induced increases in ROS may provide a road map to tailoring the dynamic ROS interactions that determine growth and cell fate in order to enhance stress resilience.

TRANVIA (TVA) facilitates cellulose synthase trafficking and delivery to the plasma membrane

Cellulose is synthesized at the plasma membrane by cellulose synthase (CESA) complexes (CSCs), which are assembled in the Golgi and secreted to the plasma membrane through the trans-Golgi network (TGN) compartment. However, the molecular mechanisms that guide CSCs through the secretory system and deliver them to the plasma membrane are poorly understood. Here, we identified an uncharacterized gene, TRANVIA (TVA), that is transcriptionally coregulated with the CESA genes required for primary cell wall synthesis. The tva mutant exhibits enhanced sensitivity to cellulose synthesis inhibitors; reduced cellulose content; and defective dynamics, density, and secretion of CSCs to the plasma membrane as compared to wild type. TVA is a plant-specific protein of unknown function that is detected in at least two different intracellular compartments: organelles labeled by markers for the TGN and smaller compartments that deliver CSCs to the plasma membrane. Together, our data suggest that TVA promotes trafficking of CSCs to the plasma membrane by facilitating exit from the TGN and/or interaction of CSC secretory vesicles with the plasma membrane.

Physical phenotype of blood cells is altered in COVID-19

Clinical syndrome coronavirus disease 2019 (COVID-19) induced by severe acute respiratory syndrome coronavirus 2 is characterized by rapid spreading and high mortality worldwide. Although the pathology is not yet fully understood, hyperinflammatory response and coagulation disorders leading to congestions of microvessels are considered to be key drivers of the still-increasing death toll. Until now, physical changes of blood cells have not been considered to play a role in COVID-19 related vascular occlusion and organ damage. Here, we report an evaluation of multiple physical parameters including the mechanical features of five frequent blood cell types, namely erythrocytes, lymphocytes, monocytes, neutrophils, and eosinophils. More than four million blood cells of 17 COVID-19 patients at different levels of severity, 24 volunteers free from infectious or inflammatory diseases, and 14 recovered COVID-19 patients were analyzed. We found significant changes in lymphocyte stiffness, monocyte size, neutrophil size and deformability, and heterogeneity of erythrocyte deformation and size. Although some of these changes recovered to normal values after hospitalization, others persisted for months after hospital discharge, evidencing the long-term imprint of COVID-19 on the body.

Cytoskeletal regulation of primary plant cell wall assembly

The plant cell wall is an extracellular matrix that envelopes cells, gives them structure and shape, constitutes the interface with symbionts, and defends plants against external biotic and abiotic stress factors. The assembly of this matrix is regulated and mediated by the cytoskeleton. Cytoskeletal elements define where new cell wall material is added and how fibrillar macromolecules are oriented in the wall. Inversely, the cytoskeleton is also key in the perception of mechanical cues generated by structural changes in the cell wall as well as the mediation of intracellular responses. We review the delivery processes of the cell wall precursors that are required for the cell wall assembly process and the structural continuity between the inside and the outside of the cell. We provide an overview of the different morphogenetic processes for which cell wall assembly is a crucial element and elaborate on relevant feedback mechanisms.

The nonlinear initiation of side-branching by activator-inhibitor-substrate (Turing) morphogenesis

An understanding of the underlying mechanism of side-branching is paramount in controlling and/or therapeutically treating mammalian organs, such as lungs, kidneys, and glands. Motivated by an activator-inhibitor-substrate approach that is conjectured to dominate the initiation of side-branching in a pulmonary vascular pattern, I demonstrate a distinct transverse front instability in which new fingers grow out of an oscillatory breakup dynamics at the front line without any typical length scale. These two features are attributed to unstable peak solutions in 1D that subcritically emanate from Turing bifurcation and that exhibit repulsive interactions. The results are based on a bifurcation analysis and numerical simulations and provide a potential strategy toward also developing a framework of side-branching for other biological systems, such as plant roots and cellular protrusions.

An Update on the Role of the Actin Cytoskeleton in Plasmodesmata: A Focus on Formins

Cell-to-cell communication in plants is mediated by plasmodesmata (PD) whose permeability is tightly regulated during plant growth and development. The actin cytoskeleton has been implicated in regulating the permeability of PD, but the underlying mechanism remains largely unknown. Recent characterization of PD-localized formin proteins has shed light on the role and mechanism of action of actin in regulating PD-mediated intercellular trafficking. In this mini-review article, we will describe the progress in this area.

Microtubules exert early, partial, and variable control of cotton fiber diameter

Variable cotton fiber diameter is set early in anisotropic elongation by cell-type-specific processes involving the temporal and spatial regulation of microtubules in the apical region. Cotton fibers are single cells that originate from the seed epidermis of Gossypium species. Then, they undergo extreme anisotropic elongation and limited diametric expansion. The details of cellular morphogenesis determine the quality traits that affect fiber uses and value, such as length, strength, and diameter. Lower and more consistent diameter would increase the competitiveness of cotton fiber with synthetic fiber, but we do not know how this trait is controlled. The complexity of the question is indicated by the existence of fibers in two major width classes in the major commercial species: broad and narrow fibers exist in commonly grown G. hirsutum, whereas G. barbadense produces only narrow fiber. To further understand how fiber diameter is controlled, we used ovule cultures, morphology measurements, and microtubule immunofluorescence to observe the effects of microtubule antagonists on fiber morphology, including shape and diameter within 80 µm of the apex. The treatments were applied at either one or two days post-anthesis during different stages of fiber morphogenesis. The results showed that inhibiting the presence and/or dynamic activity of microtubules caused larger diameter tips to form, with greater effects often observed with earlier treatment. The presence and geometry of a microtubule-depleted-zone below the apex were transiently correlated with the apical diameter of the narrow tip types. Similarly, the microtubule antagonists had somewhat different effects between tip types. Overall, the results demonstrate cell-type-specific mechanisms regulating fiber expansion within 80 µm of the apex, with variation in the impact of microtubules between tip types and over developmental time.

Highlighting reactive oxygen species as multitaskers in root development

Reactive oxygen species (ROS) are naturally produced by several redox reactions during plant regular metabolism such as photosynthesis and respiration. Due to their chemical properties and high reactivity, ROS were initially described as detrimental for cells during oxidative stress. However, they have been further recognized as key players in numerous developmental and physiological processes throughout the plant life cycle. Recent studies report the important role of ROS as growth regulators during plant root developmental processes such as in meristem maintenance, in root elongation, and in lateral root, root hair, endodermis, and vascular tissue differentiation. All involve multifaceted interplays between steady-state levels of ROS with transcriptional regulators, phytohormones, and nutrients. In this review, we attempt to summarize recent findings about how ROS are involved in multiple stages of plant root development during cell proliferation, elongation, and differentiation.

Low-dose X-ray irradiation induces morphological changes and cytoskeleton reorganization in osteoblasts

Recently, research into the biological effects of low dose X-ray irradiation (LDI) has been a focus of interest. Numerous studies have suggested that cells exhibit different responses and biological effects to LDI compared with high doses. Preliminary studies have demonstrated that LDI may promote osteoblast proliferation and differentiation in vitro, thereby accelerating fracture healing in mice. However, the exact mechanism of action by which LDI exerts its effects remains unclear. Previous studies using microarrays revealed that LDI promoted the expression of genes associated with the cytoskeleton. In the current study, the effect of X-ray irradiation (0.5 and 5 Gy) on the morphology of MC3T3-E1 cells and fiber actin organization was investigated. Osteoblasts were treated with 0, 0.5 and 5 Gy X- ray irradiation, following which changes in the actin cytoskeleton were observed. The levels of RhoA, ROCK, cofilin and phosphorylated-cofilin were measured by reverse transcription-quantitative PCR and western blotting. Subsequently, osteoblasts were pretreated with ROCK specific inhibitor Y27632 to observe the changes of actin skeleton after X-ray irradiation. The results demonstrated that the cellular morphological changes were closely associated with radiation dose and exposure time. Furthermore, the gene expression levels of small GTPase RhoA and its effectors were increased following LDI. These results indicated that the RhoA/Rho-associated kinase pathway may serve a significant role in regulating LDI-induced osteoblast cytoskeleton reorganization.

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

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

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

The Cytoskeleton and Its Role in Determining Cellulose Microfibril Angle in Secondary Cell Walls of Woody Tree Species

Recent advances in our understanding of the molecular control of secondary cell wall (SCW) formation have shed light on molecular mechanisms that underpin domestication traits related to wood formation. One such trait is the cellulose microfibril angle (MFA), an important wood quality determinant that varies along tree developmental phases and in response to gravitational stimulus. The cytoskeleton, mainly composed of microtubules and actin filaments, collectively contribute to plant growth and development by participating in several cellular processes, including cellulose deposition. Studies in Arabidopsis have significantly aided our understanding of the roles of microtubules in xylem cell development during which correct SCW deposition and patterning are essential to provide structural support and allow for water transport. In contrast, studies relating to SCW formation in xylary elements performed in woody trees remain elusive. In combination, the data reviewed here suggest that the cytoskeleton plays important roles in determining the exact sites of cellulose deposition, overall SCW patterning and more specifically, the alignment and orientation of cellulose microfibrils. By relating the reviewed evidence to the process of wood formation, we present a model of microtubule participation in determining MFA in woody trees forming reaction wood (RW).

Cryo-EM Structure of Actin Filaments from Zea mays Pollen

Actins are among the most abundant and conserved proteins in eukaryotic cells, where they form filamentous structures that perform vital roles in key cellular processes. Although large amounts of data on the biochemical activities, dynamic behaviors, and important cellular functions of plant actin filaments have accumulated, their structural basis remains elusive. Here, we report a 3.9 Å structure of the plant actin filament from Zea mays pollen (ZMPA) using cryo-electron microscopy. The structure shows a right-handed, double-stranded (two parallel strands) and staggered architecture that is stabilized by intra- and interstrand interactions. While the overall structure resembles that of other actin filaments, its DNase I binding loop bends farther outward, adopting an open conformation similar to that of the jasplakinolide- or beryllium fluoride (BeFx)-stabilized rabbit skeletal muscle actin (RSMA) filament. Single-molecule magnetic tweezers analysis revealed that the ZMPA filament can resist a greater stretching force than the RSMA filament. Overall, these data provide evidence that plant actin filaments have greater stability than animal actin filaments, which might be important to their role as tracks for long-distance vesicle and organelle transportation.plantcell;31/12/2855/FX1F1fx1.

Orchestrating cell morphology from the inside out - using polarized cell expansion in plants as a model

Intracellular organization forms the basis of changes in the extracellular matrix. In walled cells, these changes are essential for morphogenesis and growth. The highly polarized cells of mosses and liverworts together with root hairs and pollen tubes are geometrically simple cells that develop in the absence of complex tissue-scale signaling, providing an excellent model to study cell polarity. Recent advances present a unifying theme where the cytoskeleton and its associated motors work in coordination with vesicle trafficking. This coordination results in a recycling system near the cell tip, where endocytosed molecules are sorted and combined with exocytic cargo driving growth. Interestingly, functional similarities between filamentous fungi and plants promise to advance our understanding of cell polarization and growth across kingdoms.

Pectin Chemistry and Cellulose Crystallinity Govern Pavement Cell Morphogenesis in a Multi-Step Mechanism

Simple plant cell morphologies, such as cylindrical shoot cells, are determined by the extensibility pattern of the primary cell wall, which is thought to be largely dominated by cellulose microfibrils, but the mechanism leading to more complex shapes, such as the interdigitated patterns in the epidermis of many eudicotyledon leaves, is much less well understood. Details about the manner in which cell wall polymers at the periclinal wall regulate the morphogenetic process in epidermal pavement cells and mechanistic information about the initial steps leading to the characteristic undulations in the cell borders are elusive. Here, we used genetics and recently developed cell mechanical and imaging methods to study the impact of the spatio-temporal dynamics of cellulose and homogalacturonan pectin distribution during lobe formation in the epidermal pavement cells of Arabidopsis (Arabidopsis thaliana) cotyledons. We show that nonuniform distribution of cellulose microfibrils and demethylated pectin coincides with spatial differences in cell wall stiffness but may intervene at different developmental stages. We also show that lobe period can be reduced when demethyl-esterification of pectins increases under conditions of reduced cellulose crystallinity. Our data suggest that lobe initiation involves a modulation of cell wall stiffness through local enrichment in demethylated pectin, whereas subsequent increase in lobe amplitude is mediated by the stress-induced deposition of aligned cellulose microfibrils. Our results reveal a key role of noncellulosic polymers in the biomechanical regulation of cell morphogenesis.

A Label-free Mass Spectrometry Method to Predict Endogenous Protein Complex Composition

Information on the composition of protein complexes can accelerate mechanistic analyses of cellular systems. Protein complex composition identifies genes that function together and provides clues about regulation within and between cellular pathways. Cytosolic protein complexes control metabolic flux, signal transduction, protein abundance, and the activities of cytoskeletal and endomembrane systems. It has been estimated that one third of all cytosolic proteins in leaves exist in an oligomeric state, yet the composition of nearly all remain unknown. Subunits of stable protein complexes copurify, and combinations of mass-spectrometry-based protein correlation profiling and bioinformatic analyses have been used to predict protein complex subunits. Because of uncertainty regarding the power or availability of bioinformatic data to inform protein complex predictions across diverse species, it would be highly advantageous to predict composition based on elution profile data alone. Here we describe a mass spectrometry-based protein correlation profiling approach to predict the composition of hundreds of protein complexes based on biochemical data. Extracts were obtained from an intact organ and separated in parallel by size and charge under nondenaturing conditions. More than 1000 proteins with reproducible elution profiles across all replicates were subjected to clustering analyses. The resulting dendrograms were used to predict the composition of known and novel protein complexes, including many that are likely to assemble through self-interaction. An array of validation experiments demonstrated that this new method can drive protein complex discovery, guide hypothesis testing, and enable systems-level analyses of protein complex dynamics in any organism with a sequenced genome.

Cytoskeleton Dynamics Are Necessary for Early Events of Lateral Root Initiation in Arabidopsis

How plant cells re-establish differential growth to initiate organs is poorly understood. Morphogenesis of lateral roots relies on the asymmetric cell division of initially symmetric founder cells. This division is preceded by the tightly controlled asymmetric radial expansion of these cells. The cellular mechanisms that license and ensure the coordination of these events are unknown. Here, we quantitatively analyze microtubule and F-actin dynamics during lateral root initiation. Using mutants and pharmacological and tissue-specific genetic perturbations, we show that dynamic reorganization of both microtubule and F-actin networks is necessary for the asymmetric expansion of the founder cells. This cytoskeleton remodeling intertwines with auxin signaling in the pericycle and endodermis in order for founder cells to acquire a basic polarity required for initiating lateral root development. Our results reveal the conservation of cell remodeling and polarization strategies between the Arabidopsis zygote and lateral root founder cells. We propose that coordinated, auxin-driven reorganization of the cytoskeleton licenses asymmetric cell growth and divisions during embryonic and post-embryonic organogenesis.

Low Water Potential and At14a-Like1 (AFL1) Effects on Endocytosis and Actin Filament Organization

At14a-Like1 (AFL1) is a stress-induced protein of unknown function that promotes growth during low water potential stress and drought. Previous analysis indicated that AFL1 may have functions related to endocytosis and regulation of actin filament organization, processes for which the effects of low water potential are little known. We found that low water potential led to a decrease in endocytosis, as measured by uptake of the membrane-impermeable dye FM4-64. Ectopic expression of AFL1 reversed the decrease in FM4-64 uptake seen in wild type, while reduced AFL1 expression led to further inhibition of FM4-64 uptake. Increased AFL1 also made FM4-64 uptake less sensitive to the actin filament disruptor Latrunculin B (LatB). LatB decreased AFL1-Clathrin Light Chain colocalization, further indicating that effects of AFL1 on endocytosis may be related to actin filament organization or stability. Consistent with this hypothesis, ectopic AFL1 expression made actin filaments less sensitive to disruption by LatB or Cytochalasin D and led to increased actin filament skewness and decreased occupancy, indicative of more bundled actin filaments. This latter effect could be partially mimicked by the actin filament stabilizer Jasplakinolide (JASP). However, AFL1 did not substantially inhibit actin filament dynamics, indicating that AFL1 acts via a different mechanism than JASP-induced stabilization. AFL1 partially colocalized with actin filaments but not with microtubules, further indicating actin-filament-related function of AFL1. These data provide insight into endocytosis and actin filament responses to low water potential stress and demonstrate an involvement of AFL1 in these key cellular processes.