Arabidopsis VILLIN2 and VILLIN3 are required for the generation of thick actin filament bundles and for directional organ growth

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes

For successful double fertilization in flowering plants (angiosperms), pollen tubes deliver 2 nonmotile sperm cells toward female gametes (egg and central cell, respectively). Heatwaves, especially during the reproduction period, threaten male gametophyte (pollen) development, resulting in severe yield losses. Using maize (Zea mays) as a crop and grass model system, we found strong seed set reduction when moderate heat stress was applied for 2 d during the uni- and bicellular stages of pollen development. We show that heat stress accelerates pollen development and impairs pollen germination capabilities when applied at the unicellular stage. Heat stress at the bicellular stage impairs sperm cell development and transport into pollen tubes. To understand the course of the latter defects, we used marker lines and analyzed the transcriptomes of isolated sperm cells. Heat stress affected the expression of genes associated with transcription, RNA processing and translation, DNA replication, and the cell cycle. This included the genes encoding centromeric histone 3 (CENH3) and α-tubulin. Most genes that were misregulated encode proteins involved in the transition from metaphase to anaphase during pollen mitosis II. Heat stress also activated spindle assembly check point and meta- to anaphase transition genes in sperm cells. In summary, misregulation of the identified genes during heat stress at the bicellular stage results in sperm cell development and transport defects ultimately leading to sterility.

Genome-Wide Analysis of VILLIN Gene Family Associated with Stress Responses in Cotton (Gossypium spp.)

The VILLIN (VLN) protein plays a crucial role in regulating the actin cytoskeleton, which is involved in numerous developmental processes, and is crucial for plant responses to both biotic and abiotic factors. Although various plants have been studied to understand the VLN gene family and its potential functions, there has been limited exploration of VLN genes in Gossypium and fiber crops. In the present study, we characterized 94 VLNs from Gossypium species and 101 VLNs from related higher plants such as Oryza sativa and Zea mays and some fungal, algal, and animal species. By combining these VLN sequences with other Gossypium spp., we classified the VLN gene family into three distinct groups, based on their phylogenetic relationships. A more in-depth examination of Gossypium hirsutum VLNs revealed that 14 GhVLNs were distributed across 12 of the 26 chromosomes. These genes exhibit specific structures and protein motifs corresponding to their respective groups. GhVLN promoters are enriched with cis-elements related to abiotic stress responses, hormonal signals, and developmental processes. Notably, a significant number of cis-elements were associated with the light responses. Additionally, our analysis of gene-expression patterns indicated that most GhVLNs were expressed in various tissues, with certain members exhibiting particularly high expression levels in sepals, stems, and tori, as well as in stress responses. The present study potentially provides fundamental insights into the VLN gene family and could serve as a valuable reference for further elucidating the diverse functions of VLN genes in cotton.

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.

Genome-Wide Identification and Expression Analysis of the VILLIN Gene Family in Soybean

The VILLIN (VLN) protein is an important regulator of the actin cytoskeleton, which orchestrates many developmental processes and participates in various biotic and abiotic responses in plants. Although the VLN gene family and their potential functions have been analyzed in several plants, knowledge of VLN genes in soybeans and legumes remains rather limited. In this study, a total of 35 VLNs were characterized from soybean and five related legumes. Combining with the VLN sequences from other nine land plants, we categorized the VLN gene family into three groups according to phylogenetic relationships. Further detailed analysis of the soybean VLNs indicated that the ten GmVLNs were distributed on 10 of the 20 chromosomes, and their gene structures and protein motifs showed high group specificities. The expression pattern analysis suggested that most GmVLNs are widely expressed in various tissues, but three members have a very high level in seeds. Moreover, we observed that the cis-elements enriched in the promoters of GmVLNs are mainly related to abiotic stresses, hormone signals, and developmental processes. The largest number of cis-elements were associated with light responses, and two GmVLNs, GmVLN5a, and GmVLN5b were significantly increased under the long light condition. This study not only provides some basic information about the VLN gene family but also provides a good reference for further characterizing the diverse functions of VLN genes in soybeans.

VILLIN2 regulates cotton defense against Verticillium dahliae by modulating actin cytoskeleton remodeling

The active structural change of actin cytoskeleton is a general host response upon pathogen attack. This study characterized the function of the cotton (Gossypium hirsutum) actin-binding protein VILLIN2 (GhVLN2) in host defense against the soilborne fungus Verticillium dahliae. Biochemical analysis demonstrated that GhVLN2 possessed actin-binding, -bundling, and -severing activities. A low concentration of GhVLN2 could shift its activity from actin bundling to actin severing in the presence of Ca2+. Knockdown of GhVLN2 expression by virus-induced gene silencing reduced the extent of actin filament bundling and interfered with the growth of cotton plants, resulting in the formation of twisted organs and brittle stems with a decreased cellulose content of the cell wall. Upon V. dahliae infection, the expression of GhVLN2 was downregulated in root cells, and silencing of GhVLN2 enhanced the disease tolerance of cotton plants. The actin bundles were less abundant in root cells of GhVLN2-silenced plants than in control plants. However, upon infection by V. dahliae, the number of actin filaments and bundles in the cells of GhVLN2-silenced plants was raised to a comparable level as those in control plants, with the dynamic remodeling of the actin cytoskeleton appearing several hours in advance. GhVLN2-silenced plants exhibited a higher incidence of actin filament cleavage in the presence of Ca2+, suggesting that pathogen-responsive downregulation of GhVLN2 could activate its actin-severing activity. These data indicate that the regulated expression and functional shift of GhVLN2 contribute to modulating the dynamic remodeling of the actin cytoskeleton in host immune responses against V. dahliae.

Machine learning and feature analysis of the cortical microtubule organization of Arabidopsis cotyledon pavement cells

The measurement of cytoskeletal features can provide valuable insights into cell biology. In recent years, digital image analysis of cytoskeletal features has become an important research tool for quantitative evaluation of cytoskeleton organization. In this study, we examined the utility of a supervised machine learning approach with digital image analysis to distinguish different cellular organizational patterns. We focused on the jigsaw puzzle-shaped pavement cells of Arabidopsis thaliana. Measurements of three features of cortical microtubules in these cells (parallelness, density, and the coefficient of variation of the intensity distribution of fluorescently labeled cytoskeletons [as an indicator of microtubule bundling]) were obtained from microscopic images. A random forest machine learning model was then used with these images to differentiate mutant and wild type, and Taxol-treated and control cells. Using these three metrics, we were able to distinguish wild type from bpp125 triple mutant cells, with approximately 80% accuracy; classification accuracy was 88% for control and Taxol-treated cells. Different features contributed most to the classification, namely, coefficient of variation for the wild-type/mutant cells and parallelness for the Taxol-treated/control cells. The random forest method used enabled quantitative evaluation of the contribution of features to the classification, and partial dependence plots showed the relationships between metric values and classification accuracy. While further improvements to the method are needed, our small-scale analysis shows the potential for this approach in large-scale screening analyses.

Genome-wide association and RNA-seq analyses identify loci for pod orientation in rapeseed (Brassica napus)

Spatial distribution and orientation of pods on the main raceme (stem) and branches could affect rapeseed yield. However, genomic regions underlying the pod orientation were not described in Brassica species. Here, we determined the extent of genetic variation in pod orientation, described as the angles of pedicel on raceme (APR) and angles of the pod on pedicel (APP) among 136 rapeseed accessions grown across three environments of the upper, middle and lower Yangtze River in China. The APR ranged from 59° to 109°, while the APP varied from 142° to 178°. Statistical analysis showed that phenotypic variation was due to genotypic (G) and environmental (E) effects. Using the genome-wide association analysis (GWAS) approach, two QTLs for APR (qBnAPR.A02 and qBnAPR.C02) and two for APP (qBnAPP.A05 and qBnAPP.C05), having minor to moderate allelic effects (4.30% to 19.47%) were identified. RNA-seq analysis revealed 606 differentially expressed genes (DEGs) in two rapeseed accessions representing the extreme phenotypes for pod orientation and different alleles at the QTLs of APR. Three DEGs (BnLAZY4.A02, BnSAUR32.A02, and BnSAUR32.C02) were identified as the most likely candidates responsible for variation in pod orientation (APR). This study elucidates the genomic regions and putative candidate genes underlying pod orientation in B. napus.

Dissection of the genetic architecture of peduncle vascular bundle-related traits in maize by a genome-wide association study

The peduncle vascular system of maize is critical for the transport of photosynthetic products, nutrients, and water from the roots and leaves to the ear. Accordingly, it positively affects the grain yield. However, the genetic basis of peduncle vascular bundle (PVB)-related traits in maize remains unknown. Thus, 15 PVB-related traits of 386 maize inbred lines were investigated at three locations (Yongcheng, 17YC; Kaifeng, 20KF; and Yuanyang, 20YY). The repeatability for the 15 traits ranged from 35.53% to 92.13%. A genome-wide association study was performed and 69 non-redundant quantitative trait loci (QTL) were detected, including 9, 41, and 27 QTL identified at 17YC, 20KF, and 20YY, respectively. These QTL jointly explained 4.72% (SLL) to 37.30% (NSVB) of the phenotypic variation. Eight QTL were associated with the same trait at two locations. Furthermore, four pleiotropic QTL were identified. Moreover, one QTL (qPVB44), associated with NSVB_20KF, was co-localized with a previously reported locus related to kernel width, implying qPVB44 may affect the kernel width by modulating the number of small vascular bundles. Examinations of the 69 QTL identified 348 candidate genes that were classified in five groups. Additionally, 26 known VB-related homologous genes (e.g. VLN2, KNOX1, and UGT72B3) were detected in 20 of the 69 QTL. A comparison of the NSVB between a Zmvln2 EMS mutant and its wild type elucidated the function of the candidate gene ZmVLN2. These results are important for clarifying the genetic basis of PVB-related traits and may be useful for breeding new high-yielding maize cultivars.

Arabidopsis ADF1 is Regulated by MYB73 and is Involved in Response to Salt Stress Affecting Actin Filament Organization

Actin cytoskeleton and transcription factors play key roles in plant response to salt stress; however, little is known about the link between the two regulators in response to salt stress. Actin-depolymerizing factors (ADFs) are conserved actin-binding proteins in eukaryotes. Here, we revealed that the expression level of ADF1 was induced by salt stress. The adf1 mutants showed significantly reduced survival rate, increased percentage of actin cable and reduced density of actin filaments, while ADF1 overexpression seedlings displayed the opposite results when compared with WT under the same condition. Furthermore, biochemical assays revealed that MYB73, a R2R3 MYB transcription factor, binds to the promoter of ADF1 and represses its expression via the MYB-binding site core motif ACCTAC. Taken together, our results indicate that ADF1 participates in salt stress by regulating actin organization and may also serve as a potential downstream target of MYB73, which is a negative regulator of salt stress.

Sexual and Apogamous Species of Woodferns Show Different Protein and Phytohormone Profiles

The gametophyte of ferns reproduces either by sexual or asexual means. In the latter, apogamy represents a peculiar case of apomixis, in which an embryo is formed from somatic cells. A proteomic and physiological approach was applied to the apogamous fern Dryopteris affinis ssp. affinis and its sexual relative D. oreades. The proteomic analysis compared apogamous vs. female gametophytes, whereas the phytohormone study included, in addition to females, three apogamous stages (filamentous, spatulate, and cordate). The proteomic profiles revealed a total of 879 proteins and, after annotation, different regulation was found in 206 proteins of D. affinis and 166 of its sexual counterpart. The proteins upregulated in D. affinis are mostly associated to protein metabolism (including folding, transport, and proteolysis), ribosome biogenesis, gene expression and translation, while in the sexual counterpart, they account largely for starch and sucrose metabolism, generation of energy and photosynthesis. Likewise, ultra-performance liquid chromatography-tandem spectrometry (UHPLC-MS/MS) was used to assess the levels of indol-3-acetic acid (IAA); the cytokinins: 6-benzylaminopurine (BA), trans-Zeatine (Z), trans-Zeatin riboside (ZR), dyhidrozeatine (DHZ), dyhidrozeatin riboside (DHZR), isopentenyl adenine (iP), isopentenyl adenosine (iPR), abscisic acid (ABA), the gibberellins GA₃ and GA₄, salicylic acid (SA), and the brassinosteroids: brassinolide (BL) and castasterone (CS). IAA, the cytokinins Z, ZR, iPR, the gibberellin GA₄, the brassinosteoids castasterone, and ABA accumulated more in the sexual gametophyte than in the apogamous one. When comparing the three apogamous stages, BA and SA peaked in filamentous, GA₃ and BL in spatulate and DHRZ in cordate gametophytes. The results point to the existence of large metabolic differences between apogamous and sexual gametophytes, and invite to consider the fern gametophyte as a good experimental system to deepen our understanding of plant reproduction.

MPK3- and MPK6-mediated VLN3 phosphorylation regulates actin dynamics during stomatal immunity in Arabidopsis

Upon perception of pathogens, plants can rapidly close their stomata to restrict pathogen entry into internal tissue, leading to stomatal immunity as one aspect of innate immune responses. The actin cytoskeleton is required for plant defense against microbial invaders. However, the precise functions of host actin during plant immunity remain largely unknown. Here, we report that Arabidopsis villin3 (VLN3) is critical for plant resistance to bacteria by regulating stomatal immunity. Our in vitro and in vivo phosphorylation assays show that VLN3 is a physiological substrate of two pathogen-responsive mitogen-activated protein kinases, MPK3/6. Quantitative analyses of actin dynamics and genetic studies reveal that VLN3 phosphorylation by MPK3/6 modulates actin remodeling to activate stomatal defense in Arabidopsis.

Plants can rapidly close stomata to restrict pathogen entry into leaves. Here the authors show that phosphorylation of villin3 by mitogen-activated protein kinases modulates actin remodeling to activate stomatal defense in Arabidopsis.

TALEN-Based HvMPK3 Knock-Out Attenuates Proteome and Root Hair Phenotypic Responses to flg22 in Barley

Mitogen activated protein kinases (MAPKs) integrate elicitor perception with both early and late responses associated with plant defense and innate immunity. Much of the existing knowledge on the role of plant MAPKs in defense mechanisms against microbes stems from extensive research in the model plant Arabidopsis thaliana. In the present study, we investigated the involvement of barley (Hordeum vulgare) MPK3 in response to flagellin peptide flg22, a well-known bacterial elicitor. Using differential proteomic analysis we show that TALEN-induced MPK3 knock-out lines of barley (HvMPK3 KO) exhibit constitutive downregulation of defense related proteins such as PR proteins belonging to thaumatin family and chitinases. Further analyses showed that the same protein families were less prone to flg22 elicitation in HvMPK3 KO plants compared to wild types. These results were supported and validated by chitinase activity analyses and immunoblotting for HSP70. In addition, differential proteomes correlated with root hair phenotypes and suggested tolerance of HvMPK3 KO lines to flg22. In conclusion, our study points to the specific role of HvMPK3 in molecular and root hair phenotypic responses of barley to flg22.

GhVLN4 is involved in multiple stress responses and required for resistance to Verticillium wilt

As structural and signaling platform in plant cell, the actin cytoskeleton is regulated by diverse actin binding proteins (ABPs). Villins are one type of major ABPs responsible for microfilament bundling, which have proved to play important roles in plant growth and development. However, the function of villins in stress tolerance is poorly understood. Here, we report the function of cotton GhVLN4 in Verticillium wilt resistance and abiotic stress tolerance. The expression of GhVLN4 was up-regulated by gibberellin, ethylene, ABA, salicylic acid, jasmonate, NaCl, PEG, and Verticillium dahliae treatment, suggesting the involvement of GhVLN4 in multiple stress and hormone responses and signaling. Virus-induced gene silencing GhVLN4 made cotton more susceptible to V. dahliae characterized by the preferential colonization and rapid growth of the fungus in both phloem and xylem of the infected stems. Arabidopsis overexpressing GhVLN4 exhibited higher resistance to V. dahliae, salt and drought than the wild-type plants. The enhanced resistance to V. dahliae is likely related to the upregulated components in SA signaling pathway; the improved tolerance to salt and drought is characterized by upregulation of the components both in ABA- related and ABA-independent signal pathways, along with altered stomatal aperture under drought. Our findings demonstrate that GhVLN4 may play important roles in regulating plant tolerance to both biotic and abiotic stresses.

Bundling up the Role of the Actin Cytoskeleton in Primary Root Growth

Primary root growth is required by the plant to anchor in the soil and reach out for nutrients and water, while dealing with obstacles. Efficient root elongation and bending depends upon the coordinated action of environmental sensing, signal transduction, and growth responses. The actin cytoskeleton is a highly plastic network that constitutes a point of integration for environmental stimuli and hormonal pathways. In this review, we present a detailed compilation highlighting the importance of the actin cytoskeleton during primary root growth and we describe how actin-binding proteins, plant hormones, and actin-disrupting drugs affect root growth and root actin. We also discuss the feedback loop between actin and root responses to light and gravity. Actin affects cell division and elongation through the control of its own organization. We remark upon the importance of longitudinally oriented actin bundles as a hallmark of cell elongation as well as the role of the actin cytoskeleton in protein trafficking and vacuolar reshaping during this process. The actin network is shaped by a plethora of actin-binding proteins; however, there is still a large gap in connecting the molecular function of these proteins with their developmental effects. Here, we summarize their function and known effects on primary root growth with a focus on their high level of specialization. Light and gravity are key factors that help us understand root growth directionality. The response of the root to gravity relies on hormonal, particularly auxin, homeostasis, and the actin cytoskeleton. Actin is necessary for the perception of the gravity stimulus via the repositioning of sedimenting statoliths, but it is also involved in mediating the growth response via the trafficking of auxin transporters and cell elongation. Furthermore, auxin and auxin analogs can affect the composition of the actin network, indicating a potential feedback loop. Light, in its turn, affects actin organization and hence, root growth, although its precise role remains largely unknown. Recently, fundamental studies with the latest techniques have given us more in-depth knowledge of the role and organization of actin in the coordination of root growth; however, there remains a lot to discover, especially in how actin organization helps cell shaping, and therefore root growth.

Coefficient of variation as an image-intensity metric for cytoskeleton bundling

The evaluation of cytoskeletal bundling is a fundamental experimental method in the field of cell biology. Although the skewness of the pixel intensity distribution derived from fluorescently-labeled cytoskeletons has been widely used as a metric to evaluate the degree of bundling in digital microscopy images, its versatility has not been fully validated. Here, we applied the coefficient of variation (CV) of intensity values as an alternative metric, and compared its performance with skewness. In synthetic images representing extremely bundled conditions, the CV successfully detected degrees of bundling that could not be distinguished by skewness. On actual microscopy images, CV was better than skewness, especially on variable-angle epifluorescence microscopic images or stimulated emission depletion and confocal microscopy images of very small areas of around 1 μm2. When blur or noise was added to synthetic images, CV was found to be robust to blur but deleteriously affected by noise, whereas skewness was robust to noise but deleteriously affected by blur. For confocal images, CV and skewness showed similar sensitivity to noise, possibly because optical blurring is often present in microscopy images. Therefore, in practical use with actual microscopy images, CV may be more appropriate than skewness, unless the image is extremely noisy.

Parallel global profiling of plant TOR dynamics reveals a conserved role for LARP1 in translation

Target of rapamycin (TOR) is a protein kinase that coordinates eukaryotic metabolism. In mammals, TOR specifically promotes translation of ribosomal protein (RP) mRNAs when amino acids are available to support protein synthesis. The mechanisms controlling translation downstream from TOR remain contested, however, and are largely unexplored in plants. To define these mechanisms in plants, we globally profiled the plant TOR-regulated transcriptome, translatome, proteome, and phosphoproteome. We found that TOR regulates ribosome biogenesis in plants at multiple levels, but through mechanisms that do not directly depend on 5' oligopyrimidine tract motifs (5'TOPs) found in mammalian RP mRNAs. We then show that the TOR-LARP1-5'TOP signaling axis is conserved in plants and regulates expression of a core set of eukaryotic 5'TOP mRNAs, as well as new, plant-specific 5'TOP mRNAs. Our study illuminates ancestral roles of the TOR-LARP1-5'TOP metabolic regulatory network and provides evolutionary context for ongoing debates about the molecular function of LARP1.

GLABRA2 Regulates Actin Bundling Protein VILLIN1 in Root Hair Growth in Response to Osmotic Stress

Actin binding proteins and transcription factors are essential in regulating plant root hair growth in response to various environmental stresses; however, the interaction between these two factors in regulating root hair growth remains poorly understood. Apical and subapical thick actin bundles are necessary for terminating rapid elongation of root hair cells. Here, we show that Arabidopsis (Arabidopsis thaliana) actin-bundling protein Villin1 (VLN1) decorates filaments in shank, subapical, and apical hairs. vln1 mutants displayed significantly longer hairs with longer hair growing time and defects in the thick actin bundles and bundling activities in the subapical and apical regions, whereas seedlings overexpressing VLN1 showed different results. Genetic analysis showed that the transcription factor GLABRA2 (Gl2) played a regulatory role similar to that of VLN1 in hair growth and actin dynamics. Moreover, further analyses demonstrated that VLN1 overexpression suppresses the gl2 mutant phenotypes regarding hair growth and actin dynamics; GL2 directly recognizes the promoter of VLN1 and positively regulates VLN1 expression in root hairs; and the GL2-mediated VLN1 pathway is involved in the root hair growth response to osmotic stress. Our results demonstrate that the GL2-mediated VLN1 pathway plays an important role in the root hair growth response to osmotic stress, and they describe a transcriptional mechanism that regulates actin dynamics and thereby modulates cell tip growth in response to environmental signals.

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

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

Mapping proteome-wide targets of protein kinases in plant stress responses

Protein kinases are major regulatory components in almost all cellular processes in eukaryotic cells. By adding phosphate groups, protein kinases regulate the activity, localization, protein-protein interactions, and other features of their target proteins. It is known that protein kinases are central components in plant responses to environmental stresses such as drought, high salinity, cold, and pathogen attack. However, only a few targets of these protein kinases have been identified. Moreover, how these protein kinases regulate downstream biological processes and mediate stress responses is still largely unknown. In this study, we introduce a strategy based on isotope-labeled in vitro phosphorylation reactions using in vivo phosphorylated peptides as substrate pools and apply this strategy to identify putative substrates of nine protein kinases that function in plant abiotic and biotic stress responses. As a result, we identified more than 5,000 putative target sites of osmotic stress-activated SnRK2.4 and SnRK2.6, abscisic acid-activated protein kinases SnRK2.6 and casein kinase 1-like 2 (CKL2), elicitor-activated protein kinase CDPK11 and MPK6, cold-activated protein kinase MPK6, H₂O₂-activated protein kinase OXI1 and MPK6, and salt-induced protein kinase SOS1 and MPK6, as well as the low-potassium-activated protein kinase CIPK23. These results provide comprehensive information on the role of these protein kinases in the control of cellular activities and could be a valuable resource for further studies on the mechanisms underlying plant responses to environmental stresses.

An Auxin Transport Inhibitor Targets Villin-Mediated Actin Dynamics to Regulate Polar Auxin Transport

Auxin transport inhibitors are essential tools for understanding auxin-dependent plant development. One mode of inhibition affects actin dynamics; however, the underlying mechanisms remain unclear. In this study, we characterized the action of 2,3,5-triiodobenzoic acid (TIBA) on actin dynamics in greater mechanistic detail. By surveying mutants for candidate actin-binding proteins with reduced TIBA sensitivity, we determined that Arabidopsis (Arabidopsis thaliana) villins contribute to TIBA action. By directly interacting with the C-terminal headpiece domain of villins, TIBA causes villin to oligomerize, driving excessive bundling of actin filaments. The resulting changes in actin dynamics impair auxin transport by disrupting the trafficking of PIN-FORMED auxin efflux carriers and reducing their levels at the plasma membrane. Collectively, our study provides mechanistic insight into the link between the actin cytoskeleton, vesicle trafficking, and auxin transport.

Live Cell Imaging of Arabidopsis Root Hairs

Root hairs are tubular extensions from the root surface that expand by tip growth. This highly focused type of cell expansion, combined with position of root hairs on the surface of the root, makes them ideal cells for microscopic observation. This chapter describes the method that is routinely used in our laboratory for live cell imaging of Arabidopsis root hairs.

A Comparison of sun, ovate, fs8.1 and Auxin Application on Tomato Fruit Shape and Gene Expression

Elongated tomato fruit shape is the result of the action of the fruit shape genes possibly in coordination with the phytohormone auxin. To investigate the possible link between auxin and the fruit shape genes, a series of auxin (2,4-D) treatments were performed on the wild-type and the fruit shape near-isogenic lines (NILs) in Solanum pimpinellifolium accession LA1589 background. Morphological and histological analyses indicated that auxin application approximately 3 weeks before anthesis led to elongated pear-shaped ovaries and fruits, which was mainly attributed to the increase of ovary/fruit proximal end caused by the increase of both cell number and cell size. Fruit shape changes caused by SUN, OVATE and fs8.1 were primarily due to the alterations of cell number along different growth axes. Particularly, SUN caused elongation by extending cell number along the entire proximal-distal axis, whereas OVATE caused fruit elongation in the proximal area, which was most similar to the effect of auxin on ovary shape. Expression analysis of flower buds at different stages in fruit shape NILs indicated that SUN had a stronger impact on the transcriptome than OVATE and fs8.1. The sun NIL differentially expressed genes were enriched in several biological processes, such as lipid metabolism, ion transmembrane and actin cytoskeleton organization. Additionally, SUN also shifted the expression of the auxin-related genes, including those involved in auxin biosynthesis, homeostasis, signal transduction and polar transport, indicating that SUN may regulate ovary/fruit shape through modifying the expression of auxin-related genes very early during the formation of the ovary in the developing flower.

Proteomes of Leaf-Growing Zones in Rice Genotypes with Contrasting Drought Tolerance

Plants require a distinctive cohort of enzymes to coordinate cell division and expansion. Proteomic analysis now enables interrogation of immature leaf bases where these processes occur. Hence, proteins in tissues sampled from leaves of a drought-tolerant rice (IAC1131) are investigated to provide insights into the effect of soil drying on gene expression relative to the drought-sensitive genotype Nipponbare. Shoot growth zones are dissected to estimate the proportion of dividing cells and extract protein for subsequent tandem mass tags quantitative proteomic analysis. Gene ontology annotations of differentially expressed proteins provide insights into responses of Nipponbare and IAC1131 to drought. Soil drying does not affect the percentage of mitotic cells in IAC1131. More than 800 proteins across most functional categories increase in drought (and decrease on rewatering) in IAC1131, including proteins involved in "organizing the meristem" and "new cell formation". On the other hand, the percentage of dividing cells in Nipponbare is severely impaired during drought and fewer than 200 proteins respond in abundance when growing zones undergo a drying cycle. Remarkably, the proteomes of the growing zones of each genotype respond in a highly distinctive manner, reflecting their contrasting drought tolerance even at the earliest stages of leaf development.

How to Investigate the Role of the Actin-Myosin Cytoskeleton in Organ Straightening

Although plant organ segments bend in response to environmental stimuli such as gravity and light, they stop bending and subsequently straighten during the course of tropic responses. The straightening phenomenon can clearly be observed by setting the bent organs under microgravity and dark conditions. It has recently become clear that the straightening mechanism requires the activity of the actin-myosin XI cytoskeleton. A clinostat device makes it possible to simulate microgravity conditions by counteracting the Earth's unilateral gravitational pull. Here, we describe a method for assessing the straightening ability of organs by clinostat analysis using Arabidopsis thaliana inflorescence stems of actin and myosin xi mutants as examples.

Visualizing and Quantifying In Vivo Cortical Cytoskeleton Structure and Dynamics

The cortical microtubule and actin meshworks play a central role in the shaping of plant cells. Transgenic plants expressing fluorescent protein markers specifically tagging the two main cytoskeletal systems are available, allowing noninvasive in vivo studies. Advanced microscopy techniques, in particular confocal laser scanning microscopy (CLSM), spinning disk confocal microscopy (SDCM), and variable angle epifluorescence microscopy (VAEM), can be nowadays used for imaging the cortical cytoskeleton of living cells with unprecedented spatial and temporal resolution. With the aid of free computing tools based on the publicly available ImageJ software package, quantitative information can be extracted from microscopic images and video sequences, providing insight into both architecture and dynamics of the cortical cytoskeleton.

An Immune-Responsive Cytoskeletal-Plasma Membrane Feedback Loop in Plants

Cell wall appositions (CWAs) are produced reactively by the plant immune system to arrest microbial invasion through the local inversion of plant cell growth. This process requires the controlled invagination of the plasma membrane (PM) in coordination with the export of barrier material to the volume between the plant PM and cell wall. Plant actin dynamics are essential to this response, but it remains unclear how exocytosis and the cytoskeleton are linked in space and time to form functional CWAs. Here, we show that actin-dependent trafficking to immune response sites of Arabidopsis thaliana delivers membrane-integrated FORMIN4, which in turn contributes to local cytoskeletal dynamics. Total internal reflection fluorescence (TIRF) microscopy combined with controlled induction of FORMIN4-GFP expression reveals a dynamic population of vesicular bodies that accumulate to form clusters at the PM through an actin-dependent process. Deactivation of FORMIN4 and its close homologs partially compromises subsequent defense and alters filamentous actin (F-actin) distribution at mature CWAs. The localization of FORMIN4 is stable and segregated from the dynamic traffic of the endosomal network. Moreover, the tessellation of FORMIN4 at the PM with meso-domains of PEN3 reveals a fine spatial segregation of destinations for actin-dependent immunity cargo. Together, our data suggest a model where FORMIN4 is a spatial feedback element in a multi-layered, temporally defined sequence of cytoskeletal response. This positional feedback makes a significant contribution to the distribution of actin filaments at the dynamic CWA boundary and to the outcomes of pre-invasion defense.

Interplay between Ions, the Cytoskeleton, and Cell Wall Properties during Tip Growth

Tip growth is a focused and tightly regulated apical explosion that depends on the interconnected activities of ions, the cytoskeleton, and the cell wall.

Integrative RNA- and miRNA-Profile Analysis Reveals a Likely Role of BR and Auxin Signaling in Branch Angle Regulation of B. napus

Oilseed rape (Brassica napus L.) is the second largest oilseed crop worldwide and one of the most important oil crops in China. As a component of plant architecture, branch angle plays an important role in yield performance, especially under high-density planting conditions. However, the mechanisms underlying the regulation of branch angle are still largely not understood. Two oilseed rape lines with significantly different branch angles were used to conduct RNA- and miRNA-profiling at two developmental stages, identifying differential expression of a large number of genes involved in auxin- and brassinosteroid (BR)-related pathways. Many auxin response genes, including AUX1, IAA, GH3, and ARF, were enriched in the compact line. However, a number of genes involved in BR signaling transduction and biosynthesis were down-regulated. Differentially expressed miRNAs included those involved in auxin signaling transduction. Expression patterns of most target genes were fine-tuned by related miRNAs, such as miR156, miR172, and miR319. Some miRNAs were found to be differentially expressed at both developmental stages, including three miR827 members. Our results provide insight that auxin- and BR-signaling may play a pivotal role in branch angle regulation.

A new kymogram-based method reveals unexpected effects of marker protein expression and spatial anisotropy of cytoskeletal dynamics in plant cell cortex

BACKGROUND

Cytoskeleton can be observed in live plant cells in situ with high spatial and temporal resolution using a combination of specific fluorescent protein tag expression and advanced microscopy methods such as spinning disc confocal microscopy (SDCM) or variable angle epifluorescence microscopy (VAEM). Existing methods for quantifying cytoskeletal dynamics are often either based on laborious manual structure tracking, or depend on costly commercial software. Current automated methods also do not readily allow separate measurements of structure lifetime, lateral mobility, and spatial anisotropy of these parameters.

RESULTS

We developed a new freeware-based, operational system-independent semi-manual technique for analyzing VAEM or SDCM data, QuACK (Quantitative Analysis of Cytoskeletal Kymograms), and validated it on data from Arabidopsis thaliana fh1 formin mutants, previously shown by conventional methods to exhibit altered actin and microtubule dynamics compared to the wild type. Besides of confirming the published mutant phenotype, QuACK was used to characterize surprising differential effects of various fluorescent protein tags fused to the Lifeact actin probe on actin dynamics in A. thaliana cotyledon epidermis. In particular, Lifeact-YFP slowed down actin dynamics compared to Lifeact-GFP at marker expression levels causing no macroscopically noticeable phenotypic alterations, although the two fluorophores are nearly identical. We could also demonstrate the expected, but previously undocumented, anisotropy of cytoskeletal dynamics in elongated epidermal cells of A. thaliana petioles and hypocotyls.

CONCLUSIONS

Our new method for evaluating plant cytoskeletal dynamics has several advantages over existing techniques. It is intuitive, rapid compared to fully manual approaches, based on the free ImageJ software (including macros we provide here for download), and allows measurement of multiple parameters. Our approach was already used to document unexpected differences in actin mobility in transgenic A. thaliana expressing Lifeact fusion proteins with different fluorophores, highlighting the need for cautious interpretation of experimental results, as well as to reveal hitherto uncharacterized anisotropy of cytoskeletal mobility in elongated plant cells.

Protein Phosphatase 2Cs and Microtubule-Associated Stress Protein 1 Control Microtubule Stability, Plant Growth, and Drought Response

Plant growth is coordinated with environmental factors, including water availability during times of drought. Microtubules influence cell expansion; however, the mechanisms by which environmental signals impinge upon microtubule organization and whether microtubule-related factors limit growth during drought remains unclear. We found that three Clade E Growth-Regulating (EGR) Type 2C protein phosphatases act as negative growth regulators to restrain growth during drought. Quantitative phosphoproteomics indicated that EGRs target cytoskeleton and plasma membrane-associated proteins. Of these, Microtubule-Associated Stress Protein 1 (MASP1), an uncharacterized protein, increased in abundance during stress treatment and could bind, bundle, and stabilize microtubules in vitro. MASP1 overexpression enhanced growth, in vivo microtubule stability, and recovery of microtubule organization during drought acclimation. These MASP1 functions in vivo were dependent on phosphorylation of a single serine. For all EGR and MASP1 mutants and transgenic lines examined, enhanced microtubule recovery and stability were associated with increased growth during drought stress. The EGR-MASP1 system selectively regulates microtubule recovery and stability to adjust plant growth and cell expansion in response to changing environmental conditions. Modification of EGR-MASP1 signaling may be useful to circumvent negative growth regulation limiting plant productivity. EGRs are likely to regulate additional proteins involved in microtubule stability and stress signaling.

Proteomic analysis of the mature Brassica stigma reveals proteins with diverse roles in vegetative and reproductive development

The stigma, the specialized apex of the Brassicaceae gynoecium, plays a role in pollen capture, discrimination, hydration, germination, and guidance. Despite this crucial role in reproduction, the global proteome underlying Brassicaceae stigma development and function remains largely unknown. As a contribution towards the characterization of the Brassicaceae dry stigma global proteome, more than 2500 Brassica napus mature stigma proteins were identified using three different gel-based proteomics approaches. Most stigma proteins participated in Metabolic Processes, Responses to Stimulus or Stress, Cellular or Developmental Processes, and Transport. The stigma was found to express a wide variety of proteins with demonstrated roles in cellular and organ development including proteins known to be involved in cellular expansion and morphogenesis, embryo development, as well as gynoecium and stigma development. Comparisons to a corresponding proteome from a very morphologically different Poaceae dry stigma showed a very similar distribution of proteins among different functional categories, but also revealed evident distinctions in protein composition especially in glucosinolate and carotenoid metabolism, photosynthesis, and self-incompatibility. To our knowledge, this study reports the largest Brassicaceae stigma protein dataset described to date.

TWISTED DWARF1 Mediates the Action of Auxin Transport Inhibitors on Actin Cytoskeleton Dynamics

Plant growth and architecture is regulated by the polar distribution of the hormone auxin. Polarity and flexibility of this process is provided by constant cycling of auxin transporter vesicles along actin filaments, coordinated by a positive auxin-actin feedback loop. Both polar auxin transport and vesicle cycling are inhibited by synthetic auxin transport inhibitors, such as 1-N-naphthylphthalamic acid (NPA), counteracting the effect of auxin; however, underlying targets and mechanisms are unclear. Using NMR, we map the NPA binding surface on the Arabidopsis thaliana ABCB chaperone TWISTED DWARF1 (TWD1). We identify ACTIN7 as a relevant, although likely indirect, TWD1 interactor, and show TWD1-dependent regulation of actin filament organization and dynamics and that TWD1 is required for NPA-mediated actin cytoskeleton remodeling. The TWD1-ACTIN7 axis controls plasma membrane presence of efflux transporters, and as a consequence act7 and twd1 share developmental and physiological phenotypes indicative of defects in auxin transport. These can be phenocopied by NPA treatment or by chemical actin (de)stabilization. We provide evidence that TWD1 determines downstream locations of auxin efflux transporters by adjusting actin filament debundling and dynamizing processes and mediating NPA action on the latter. This function appears to be evolutionary conserved since TWD1 expression in budding yeast alters actin polarization and cell polarity and provides NPA sensitivity.

Functions of plant-specific myosin XI: from intracellular motility to plant postures

The plant-specific protein motor class myosin XI is known to function in rapid bulk flow of the cytoplasm (cytoplasmic streaming) and in organellar movements. Recent studies unveiled a wide range of physiological functions of myosin XI motors, from intracellular motility to organ movements. Arabidopsis thaliana has 13 members of myosin XI class. In vegetative organs, myosins XIk, XI1, and XI2 primarily contribute to dynamics and spatial configurations of endoplasmic reticulum that develops a tubular network in the cell periphery and thick strand-like structures in the inner cell regions. Myosin XI-i forms a nucleocytoplasmic linker and is responsible for nuclear movement and shape. In addition to these intracellular functions, myosin XIf together with myosin XIk is involved in the fundamental nature of plants; the actin-myosin XI cytoskeleton regulates organ straightening to adjust plant posture.

VLN2 Regulates Plant Architecture by Affecting Microfilament Dynamics and Polar Auxin Transport in Rice

As a fundamental and dynamic cytoskeleton network, microfilaments (MFs) are regulated by diverse actin binding proteins (ABPs). Villins are one type of ABPs belonging to the villin/gelsolin superfamily, and their function is poorly understood in monocotyledonous plants. Here, we report the isolation and characterization of a rice (Oryza sativa) mutant defective in VILLIN2 (VLN2), which exhibits malformed organs, including twisted roots and shoots at the seedling stage. Cellular examination revealed that the twisted phenotype of the vln2 mutant is mainly caused by asymmetrical expansion of cells on the opposite sides of an organ. VLN2 is preferentially expressed in growing tissues, consistent with a role in regulating cell expansion in developing organs. Biochemically, VLN2 exhibits conserved actin filament bundling, severing and capping activities in vitro, with bundling and stabilizing activity being confirmed in vivo. In line with these findings, the vln2 mutant plants exhibit a more dynamic actin cytoskeleton network than the wild type. We show that vln2 mutant plants exhibit a hypersensitive gravitropic response, faster recycling of PIN2 (an auxin efflux carrier), and altered auxin distribution. Together, our results demonstrate that VLN2 plays an important role in regulating plant architecture by modulating MF dynamics, recycling of PIN2, and polar auxin transport.

Calcium is an organizer of cell polarity in plants

Cell polarity is a fundamental property of pro- and eukaryotic cells. It is necessary for coordination of cell division, cell morphogenesis and signaling processes. How polarity is generated and maintained is a complex issue governed by interconnected feed-back regulations between small GTPase signaling and membrane tension-based signaling that controls membrane trafficking, and cytoskeleton organization and dynamics. Here, we will review the potential role for calcium as a crucial signal that connects and coordinates the respective processes during polarization processes in plants. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Plant villins: versatile actin regulatory proteins

Regulation of actin dynamics is a central theme in cell biology that is important for different aspects of cell physiology. Villin, a member of the villin/gelsolin/fragmin superfamily of proteins, is an important regulator of actin. Villins contain six gelsolin homology domains (G1-G6) and an extra headpiece domain. In contrast to their mammalian counterparts, plant villins are expressed widely, implying that plant villins play a more general role in regulating actin dynamics. Some plant villins have a defined role in modifying actin dynamics in the pollen tube; most of their in vivo activities remain to be ascertained. Recently, our understanding of the functions and mechanisms of action for plant villins has progressed rapidly, primarily due to the advent of Arabidopsis thaliana genetic approaches and imaging capabilities that can visualize actin dynamics at the single filament level in vitro and in living plant cells. In this review, we focus on discussing the biochemical activities and modes of regulation of plant villins. Here, we present current understanding of the functions of plant villins. Finally, we highlight some of the key unanswered questions regarding the functions and regulation of plant villins for future research.

Live cell imaging of Arabidopsis root hairs

Root hairs are tubular extensions from the root surface that expand by tip growth. This highly focused type of cell expansion, combined with position of root hairs on the surface of the root, makes them ideal cells for microscopic observation. This chapter describes the method that is routinely used in our laboratory for live cell imaging of Arabidopsis root hairs.

No stress! Relax! Mechanisms governing growth and shape in plant cells

The mechanisms through which plant cells control growth and shape are the result of the coordinated action of many events, notably cell wall stress relaxation and turgor-driven expansion. The scalar nature of turgor pressure would drive plant cells to assume spherical shapes; however, this is not the case, as plant cells show an amazing variety of morphologies. Plant cell walls are dynamic structures that can display alterations in matrix polysaccharide composition and concentration, which ultimately affect the wall deformation rate. The wide varieties of plant cell shapes, spanning from elongated cylinders (as pollen tubes) and jigsaw puzzle-like epidermal cells, to very long fibres and branched stellate leaf trichomes, can be understood if the underlying mechanisms regulating wall biosynthesis and cytoskeletal dynamics are addressed. This review aims at gathering the available knowledge on the fundamental mechanisms regulating expansion, growth and shape in plant cells by putting a special emphasis on the cell wall-cytoskeleton system continuum. In particular, we discuss from a molecular point of view the growth mechanisms characterizing cell types with strikingly different geometries and describe their relationship with primary walls. The purpose, here, is to provide the reader with a comprehensive overview of the multitude of events through which plant cells manage to expand and control their final shapes.

Identification of novel PAMP-triggered phosphorylation and dephosphorylation events in Arabidopsis thaliana by quantitative phosphoproteomic analysis

Signaling cascades rely strongly on protein kinase-mediated substrate phosphorylation. Currently a major challenge in signal transduction research is to obtain high confidence substrate phosphorylation sites and assign them to specific kinases. In response to bacterial flagellin, a pathogen-associated molecular pattern (PAMP), we searched for rapidly phosphorylated proteins in Arabidopsis thaliana by combining multistage activation (MSA) and electron transfer dissociation (ETD) fragmentation modes, which generate complementary spectra and identify phosphopeptide sites with increased reliability. Of a total of 825 phosphopeptides, we identified 58 to be differentially phosphorylated. These peptides harbor kinase motifs of mitogen-activated protein kinases (MAPKs) and calcium-dependent protein kinases (CDPKs), as well as yet unknown protein kinases. Importantly, 12 of the phosphopeptides show reduced phosphorylation upon flagellin treatment. Since protein abundance levels did not change, these results indicate that flagellin induces not only various protein kinases but also protein phosphatases, even though a scenario of inhibited kinase activity may also be possible.

Visualizing and quantifying the in vivo structure and dynamics of the Arabidopsis cortical cytoskeleton using CLSM and VAEM

The cortical microtubules, and to some extent also the actin meshwork, play a central role in the shaping of plant cells. Transgenic plants expressing fluorescent protein markers specifically tagging the two main cytoskeletal systems are available, allowing noninvasive in vivo studies. Advanced microscopy techniques, in particular confocal laser scanning microscopy (CLSM) and variable angle epifluorescence microscopy (VAEM), can be nowadays used for imaging the cortical cytoskeleton of living cells with unprecedented spatial and temporal resolution. With the aid of suitable computing techniques, quantitative information can be extracted from microscopic images and video sequences, providing insight into both architecture and dynamics of the cortical cytoskeleton.

The actin cytoskeleton in root hairs: all is fine at the tip

Filamentous actin forms characteristic bundles in plant cells that facilitate cytoplasmic streaming. In contrast, networks of actin exhibiting fast turnover are found especially near sites of rapid cell expansion. These networks may serve various functions including delivering and retaining vesicles while preventing penetration of organelles into the area where cell growth occurs thereby allowing fast turnover of vesicles to and from the plasma membrane. Root hairs elongate by polarized growth at their tips and the local accumulation of fine F-actin near the tip has provided valuable insight into the organization of these networks. Here we will sequentially focus on the role of the actin cytoskeleton in root hair tip growth and on how activities of different actin binding proteins in the apical part of growing root hairs contribute to build the fine F-actin configuration that correlates with tip growth.

Arabidopsis actin-depolymerizing factor7 severs actin filaments and regulates actin cable turnover to promote normal pollen tube growth

Actin filaments are often arranged into higher-order structures, such as the longitudinal actin cables that generate the reverse fountain cytoplasmic streaming pattern present in pollen tubes. While several actin binding proteins have been implicated in the generation of these cables, the mechanisms that regulate their dynamic turnover remain largely unknown. Here, we show that Arabidopsis thaliana actin-depolymerizing factor7 (ADF7) is required for turnover of longitudinal actin cables. In vitro biochemical analyses revealed that ADF7 is a typical ADF that prefers ADP-G-actin over ATP-G-actin. ADF7 inhibits nucleotide exchange on actin and severs filaments, but its filament severing and depolymerizing activities are less potent than those of the vegetative ADF1. ADF7 primarily decorates longitudinal actin cables in the shanks of pollen tubes. Consistent with this localization pattern, the severing frequency and depolymerization rate of filaments significantly decreased, while their maximum lifetime significantly increased, in adf7 pollen tube shanks. Furthermore, an ADF7-enhanced green fluorescent protein fusion with defective severing activity but normal G-actin binding activity could not complement adf7, providing compelling evidence that the severing activity of ADF7 is vital for its in vivo functions. These observations suggest that ADF7 evolved to promote turnover of longitudinal actin cables by severing actin filaments in pollen tubes.

Arabidopsis villins promote actin turnover at pollen tube tips and facilitate the construction of actin collars

Apical actin filaments are crucial for pollen tube tip growth. However, the specific dynamic changes and regulatory mechanisms associated with actin filaments in the apical region remain largely unknown. Here, we have investigated the quantitative dynamic parameters that underlie actin filament growth and disappearance in the apical regions of pollen tubes and identified villin as the major player that drives rapid turnover of actin filaments in this region. Downregulation of Arabidopsis thaliana VILLIN2 (VLN2) and VLN5 led to accumulation of actin filaments at the pollen tube apex. Careful analysis of single filament dynamics showed that the severing frequency significantly decreased, and the lifetime significantly increased in vln2 vln5 pollen tubes. These results indicate that villin-mediated severing is critical for turnover and departure of actin filaments originating in the apical region. Consequently, the construction of actin collars was affected in vln2 vln5 pollen tubes. In addition to the decrease in severing frequency, actin filaments also became wavy and buckled in the apical cytoplasm of vln2 vln5 pollen tubes. These results suggest that villin confers rigidity upon actin filaments. Furthermore, an observed decrease in skewness of actin filaments in the subapical region of vln2 vln5 pollen tubes suggests that villin-mediated bundling activity may also play a role in the construction of actin collars. Thus, our data suggest that villins promote actin turnover at pollen tube tips and facilitate the construction of actin collars.

Regulation of plant gravity sensing and signaling by the actin cytoskeleton

Gravitropism is a process by which plant organs readjust their growth toward or away from the gravity vector when the plant is reoriented. The actin cytoskeleton has often been a significant component of models explaining gravitropism, but its role in this process has become somewhat controversial in light of reports showing that actin inhibitors enhance the gravitropic response. The work with inhibitors implies that actin might function as a negative regulator of gravitropism. In this article, possibilities for how such a role might be accomplished are presented. First, the organization of actin in statocytes is revisited in an attempt to rationalize how compressive forces exerted by statoliths on membranes can lead to enhanced gravity sensing. Second, recent genetic work in the model plant Arabidopsis thaliana is discussed, focusing on the potential involvement of the protein degradation machinery in actin-mediated control of statolith dynamics and on the intriguing possibility that an actin-regulated, ligand-receptor mechanism for gravity signal transduction might operate in higher plants. Third, modifications in the trafficking of auxin efflux transporters are considered as possible mechanisms for the enhanced gravity responses observed in plant organs when the actin cytoskeleton is disrupted by chemical inhibitors. The various possibilities presented in this review emphasize the large amount of research that remains to be done before we can fully understand how the actin cytoskeleton modulates tropisms in higher plants.

AtFH1 formin mutation affects actin filament and microtubule dynamics in Arabidopsis thaliana

Plant cell growth and morphogenesis depend on remodelling of both actin and microtubule cytoskeletons. AtFH1 (At5g25500), the main housekeeping Arabidopsis formin, is targeted to membranes and known to nucleate and bundle actin. The effect of mutations in AtFH1 on root development and cytoskeletal dynamics was examined. Consistent with primarily actin-related formin function, fh1 mutants showed increased sensitivity to the actin polymerization inhibitor latrunculin B (LatB). LatB-treated mutants had thicker, shorter roots than wild-type plants. Reduced cell elongation and morphological abnormalities were observed in both trichoblasts and atrichoblasts. Fluorescently tagged cytoskeletal markers were used to follow cytoskeletal dynamics in wild-type and mutant plants using confocal microscopy and VAEM (variable-angle epifluorescence microscopy). Mutants exhibited more abundant but less dynamic F-actin bundles and more dynamic microtubules than wild-type seedlings. Treatment of wild-type seedlings with a formin inhibitor, SMIFH2, mimicked the root growth and cell expansion phenotypes and cytoskeletal structure alterations observed in fh1 mutants. The results suggest that besides direct effects on actin organization, the in vivo role of AtFH1 also includes modulation of microtubule dynamics, possibly mediated by actin-microtubule cross-talk.

Arabidopsis VILLIN2 and VILLIN3 act redundantly in sclerenchyma development via bundling of actin filaments

The organization of the actin cytoskeleton has been implicated in sclerenchyma development. However, the molecular mechanisms linking the actin cytoskeleton to this process remain poorly understood. In particular, there have been no studies showing that direct genetic manipulation of the actin cytoskeleton affects sclerenchyma development. Villins belong to the villin/gelsolin/fragmin superfamily and are versatile actin-modifying proteins. Several recent studies have implicated villins in tip growth of single cells, but how villins act in multicellular plant development remains largely unknown. Here, we found that two closely related villin isovariants from Arabidopsis, VLN2 and VLN3, act redundantly in sclerenchyma development. Detailed analysis of cross-sections from inflorescence stems of vln2 vln3 double mutant plants revealed a reduction in stem size and in the number of vascular bundles; however, no defects in synthesis of the secondary cell wall were detected. Surprisingly, the vln2 vln3 double mutation did not affect cell elongation of inter-fascicular fibers. Biochemical analyses showed that recombinant VLN2 was able to cap, sever and bundle actin filaments, similar to VLN3. Consistent with these biochemical activities, loss of function of VLN2 and VLN3 resulted in a decrease in the amount of F-actin and actin bundles in plant cells. Collectively, our findings demonstrate that VLN2 and VLN3 act redundantly in sclerenchyma development via bundling of actin filaments.

Bundling actin filaments from membranes: some novel players

Progress in live-cell imaging of the cytoskeleton has significantly extended our knowledge about the organization and dynamics of actin filaments near the plasma membrane of plant cells. Noticeably, two populations of filamentous structures can be distinguished. On the one hand, fine actin filaments which exhibit an extremely dynamic behavior basically characterized by fast polymerization and prolific severing events, a process referred to as actin stochastic dynamics. On the other hand, thick actin bundles which are composed of several filaments and which are comparatively more stable although they constantly remodel as well. There is evidence that the actin cytoskeleton plays critical roles in trafficking and signaling at both the cell cortex and organelle periphery but the exact contribution of actin bundles remains unclear. A common view is that actin bundles provide the long-distance tracks used by myosin motors to deliver their cargo to growing regions and accordingly play a particularly important role in cell polarization. However, several studies support that actin bundles are more than simple passive highways and display multiple and dynamic roles in the regulation of many processes, such as cell elongation, polar auxin transport, stomatal and chloroplast movement, and defense against pathogens. The list of identified plant actin-bundling proteins is ever expanding, supporting that plant cells shape structurally and functionally different actin bundles. Here I review the most recently characterized actin-bundling proteins, with a particular focus on those potentially relevant to membrane trafficking and/or signaling.