Arabidopsis group Ie formins localize to specific cell membrane domains, interact with actin-binding proteins and cause defects in cell expansion upon aberrant expression

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

Cytoskeleton as a roadmap navigating rhizobia to establish symbiotic root nodulation in legumes

Legumes enter into symbiotic associations with soil nitrogen-fixing rhizobia, culminating in the creation of new organs, root nodules. This complex process relies on chemical and physical interaction between legumes and rhizobia, including early signalling events informing the host legume plant of a potentially beneficial microbe and triggering the nodulation program. The great significance of this plant-microbe interaction rests upon conversion of atmospheric dinitrogen not accessible to plants into a biologically active form of ammonia available to plants. The plant cytoskeleton consists in a highly dynamic network and undergoes rapid remodelling upon sensing various developmental and environmental cues, including response to attachment, internalization, and accommodation of rhizobia in plant root and nodule cells. This dynamic nature is governed by cytoskeleton-associated proteins that modulate cytoskeletal behaviour depending on signal perception and transduction. Precisely localized cytoskeletal rearrangements are therefore essential for the uptake of rhizobia, their targeted delivery, and establishing beneficial root nodule symbiosis. This review summarizes current knowledge about rhizobia-dependent rearrangements and functions of the cytoskeleton in legume roots and nodules. General patterns and nodule type-, nodule stage-, and species-specific aspects of actin filaments and microtubules remodelling are discussed. Moreover, emerging evidence is provided about fine-tuning the root nodulation process through cytoskeleton-associated proteins. We also consider future perspectives on dynamic localization studies of the cytoskeleton during early symbiosis utilizing state of the art molecular and advanced microscopy approaches. Based on acquired detailed knowledge of the mutualistic interactions with microbes, these approaches could contribute to broader biotechnological crop 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.

Automatic extraction of actin networks in plants

The actin cytoskeleton is essential in eukaryotes, not least in the plant kingdom where it plays key roles in cell expansion, cell division, environmental responses and pathogen defence. Yet, the precise structure-function relationships of properties of the actin network in plants are still to be unravelled, including details of how the network configuration depends upon cell type, tissue type and developmental stage. Part of the problem lies in the difficulty of extracting high-quality, quantitative measures of actin network features from microscopy data. To address this problem, we have developed DRAGoN, a novel image analysis algorithm that can automatically extract the actin network across a range of cell types, providing seventeen different quantitative measures that describe the network at a local level. Using this algorithm, we then studied a number of cases in Arabidopsis thaliana, including several different tissues, a variety of actin-affected mutants, and cells responding to powdery mildew. In many cases we found statistically-significant differences in actin network properties. In addition to these results, our algorithm is designed to be easily adaptable to other tissues, mutants and plants, and so will be a valuable asset for the study and future biological engineering of the actin cytoskeleton in globally-important crops.

Formin protein DRT1 affects gross morphology and chloroplast relocation in rice

Plant height and tiller number are two major factors determining plant architecture and yield. However, in rice (Oryza sativa), the regulatory mechanism of plant architecture remains to be elucidated. Here, we reported a recessive rice mutant presenting dwarf and reduced tillering phenotypes (drt1). Map-based cloning revealed that the phenotypes are caused by a single point mutation in DRT1, which encodes the Class I formin protein O. sativa formin homolog 13 (OsFH13), binds with F-actin, and promotes actin polymerization for microfilament organization. DRT1 protein localized on the plasma membrane (PM) and chloroplast (CP) outer envelope. DRT1 interacted with rice phototropin 2 (OsPHOT2), and the interaction was interrupted in drt1. Upon blue light stimulus, PM localized DRT1 and OsPHOT2 were translocated onto the CP membrane. Moreover, deficiency of DRT1 reduced OsPHOT2 internalization and OsPHOT2-mediated CP relocation. Our study suggests that rice formin protein DRT1/OsFH13 is necessary for plant morphology and CP relocation by modulating the actin-associated cytoskeleton network.

Actin depolymerizing factor ADF7 inhibits actin bundling protein VILLIN1 to regulate root hair formation in response to osmotic stress in Arabidopsis

Actin cytoskeleton is essential for root hair formation. However, the underlying molecular mechanisms of actin dynamics in root hair formation in response to abiotic stress are largely undiscovered. Here, genetic analysis showed that actin-depolymerizing protein ADF7 and actin-bundling protein VILLIN1 (VLN1) were positively and negatively involved in root hair formation of Arabidopsis respectively. Moreover, RT-qPCR, GUS staining, western blotting, and genetic analysis revealed that ADF7 played an important role in inhibiting the expression and function of VLN1 during root hair formation. Filament actin (F-actin) dynamics observation and actin pharmacological experiments indicated that ADF7-inhibited-VLN1 pathway led to the decline of F-actin bundling and thick bundle formation, as well as the increase of F-actin depolymerization and turnover to promote root hair formation. Furthermore, the F-actin dynamics mediated by ADF7-inhibited-VLN1 pathway was associated with the reactive oxygen species (ROS) accumulation in root hair formation. Finally, ADF7-inhibited-VLN1 pathway was critical for osmotic stress-induced root hair formation. Our work demonstrates that ADF7 inhibits VLN1 to regulate F-actin dynamics in root hair formation in response to osmotic stress, providing the novel evidence on the F-actin dynamics and their molecular mechanisms in root hair formation and in abiotic stress.

Root hairs are required for plants to absorb nutrients and water. The dynamics of cytoskeleton such as actin filaments (F-actin) are necessary for the formation of root hairs, which is regulated by different kinds of cytoskeleton-binding proteins. At the same time, the dynamics of cytoskeleton are also involved in plant abiotic stress tolerance. However, there are few studies on the underlying molecular mechanisms of F-actin dynamics in root hair formation in response to abiotic stress. Actin depolymerization factor 7 (ADF7) and actin bunding protein Villin 1 (VLN1) are important actin-binding proteins in Arabidopsis. Here, we describe a pathway that ADF7 inhibits VLN1 to regulate F-actin dynamics in root hair formation in response to osmotic stress, providing a new evidence for the studies on the molecular mechanisms of F-actin dynamics in root hair formation and in plant abiotic stress tolerance.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

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

Comprehensive analysis of formin gene family highlights candidate genes related to pollen cytoskeleton and male fertility in wheat (Triticum aestivum L.)

Background

Formin, a highly conserved multi-domain protein, interacts with microfilaments and microtubules. Although specifically expressed formin genes in anthers are potentially significant in research on male sterility and hybrid wheat breeding, similar reports in wheat, especially in thermo-sensitive genic male sterile (TGMS) wheat, remain elusive.

Results

Herein, we systematically characterized the formin genes in TGMS wheat line BS366 named TaFormins (TaFHs) and predicted their functions in inducing stress response. In total, 25 TaFH genes were uncovered, majorly localized in 2A, 2B, and 2D chromosomes. According to the neighbor-joining (NJ) method, all TaFH proteins from wheat and other plants clustered in 6 sub-groups (A-F). The modeled 3D structures of TaFH1-A/B, TaFH2-A/B, TaFH3-A/B and TaFH3-B/D were validated. And different numbers of stress and hormone-responsive regulatory elements in their 1500 base pair promoter regions were contained in the TaFH genes copies. TaFHs had specific temporal and spatial expression characteristics, whereby TaFH1, TaFH4, and TaFH5 were expressed highly in the stamen of BS366. Besides, the accumulation of TaFHs was remarkably lower in a low-temperature sterile condition (Nanyang) than fertile condition (Beijing), particularly at the early stamen development stage. The pollen cytoskeleton of BS366 was abnormal in the three stages under sterile and fertile environments. Furthermore, under different stress levels, TaFHs expression could be induced by drought, salt, abscisic acid (ABA), salicylic acid (SA), methyl jasmonate (MeJA), indole-3-acetic acid (IAA), polyethylene glycol (PEG), and low temperature. Some miRNAs, including miR167, miR1120, and miR172, interacts with TaFH genes; thus, we constructed an interaction network between microRNAs, TaFHs, phytohormone responses, and distribution of cytoskeleton to reveal the regulatory association between upstream genes of TaFH family members and sterile.

Conclusions

Collectively, this comprehensive analysis provides novel insights into TaFHs and miRNA resources for wheat breeding. These findings are, therefore, valuable in understanding the mechanism of TGMS fertility conversion in wheat.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07878-7.

Secretory Vesicles Targeted to Plasma Membrane During Pollen Germination and Tube Growth

Pollen germination and pollen tube growth are important biological events in the sexual reproduction of higher plants, during which a large number of vesicle trafficking and membrane fusion events occur. When secretory vesicles are transported via the F-actin network in proximity to the apex of the pollen tube, the secretory vesicles are tethered and fused to the plasma membrane by tethering factors and SNARE proteins, respectively. The coupling and uncoupling between the vesicle membrane and plasma membrane are also regulated by dynamic cytoskeleton, proteins, and signaling molecules, including small G proteins, calcium, and PIP2. In this review, we focus on the current knowledge regarding secretory vesicle delivery, tethering, and fusion during pollen germination and tube growth and summarize the progress in research on how regulators and signaling molecules participate in the above processes.

The Arabidopsis thaliana Class II Formin FH13 Modulates Pollen Tube Growth

Formins are a large, evolutionarily conserved family of actin-nucleating proteins with additional roles in regulating microfilament, microtubule, and membrane dynamics. Angiosperm formins, expressed in both sporophytic and gametophytic tissues, can be divided into two subfamilies, Class I and Class II, each often exhibiting characteristic domain organization. Gametophytically expressed Class I formins have been documented to mediate plasma membrane-based actin assembly in pollen grains and pollen tubes, contributing to proper pollen germination and pollen tube tip growth, and a rice Class II formin, FH5/RMD, has been proposed to act as a positive regulator of pollen tube growth based on mutant phenotype and overexpression data. Here we report functional characterization of the Arabidopsis thaliana pollen-expressed typical Class II formin FH13 (At5g58160). Consistent with published transcriptome data, live-cell imaging in transgenic plants expressing fluorescent protein-tagged FH13 under the control of the FH13 promoter revealed expression in pollen and pollen tubes with non-homogeneous signal distribution in pollen tube cytoplasm, suggesting that this formin functions in the male gametophyte. Surprisingly, fh13 loss of function mutations do not affect plant fertility but result in stimulation of in vitro pollen tube growth, while tagged FH13 overexpression inhibits pollen tube elongation. Pollen tubes of mutants expressing a fluorescent actin marker exhibited possible minor alterations of actin organization. Our results thus indicate that FH13 controls or limits pollen tube growth, or, more generally, that typical Class II formins should be understood as modulators of pollen tube elongation rather than merely components of the molecular apparatus executing tip growth.

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.

Arabidopsis Class II Formins AtFH13 and AtFH14 Can Form Heterodimers but Exhibit Distinct Patterns of Cellular Localization

Formins are evolutionarily conserved multi-domain proteins participating in the control of both actin and microtubule dynamics. Angiosperm formins form two evolutionarily distinct families, Class I and Class II, with class-specific domain layouts. The model plant Arabidopsis thaliana has 21 formin-encoding loci, including 10 Class II members. In this study, we analyze the subcellular localization of two A. thaliana Class II formins exhibiting typical domain organization, the so far uncharacterized formin AtFH13 (At5g58160) and its distant homolog AtFH14 (At1g31810), previously reported to bind microtubules. Fluorescent protein-tagged full length formins and their individual domains were transiently expressed in Nicotiana benthamiana leaves under the control of a constitutive promoter and their subcellular localization (including co-localization with cytoskeletal structures and the endoplasmic reticulum) was examined using confocal microscopy. While the two formins exhibit distinct and only partially overlapping localization patterns, they both associate with microtubules via the conserved formin homology 2 (FH2) domain and with the periphery of the endoplasmic reticulum, at least in part via the N-terminal PTEN (Phosphatase and Tensin)-like domain. Surprisingly, FH2 domains of AtFH13 and AtFH14 can form heterodimers in the yeast two-hybrid assay-a first case of potentially biologically relevant formin heterodimerization mediated solely by the FH2 domain.

Homeostasis of branched-chain amino acids is critical for the activity of TOR signaling in Arabidopsis

The target of rapamycin (TOR) kinase is an evolutionarily conserved hub of nutrient sensing and metabolic signaling. In plants, a functional connection of TOR activation with glucose availability was demonstrated, while it is yet unclear whether branched-chain amino acids (BCAAs) are a primary input of TOR signaling as they are in yeast and mammalian cells. Here, we report on the characterization of an Arabidopsis mutant over-accumulating BCAAs. Through chemical interventions targeting TOR and by examining mutants of BCAA biosynthesis and TOR signaling, we found that BCAA over-accumulation leads to up-regulation of TOR activity, which causes reorganization of the actin cytoskeleton and actin-associated endomembranes. Finally, we show that activation of TOR is concomitant with alteration of cell expansion, proliferation and specialized metabolism, leading to pleiotropic effects on plant growth and development. These results demonstrate that BCAAs contribute to plant TOR activation and reveal previously uncharted downstream subcellular processes of TOR signaling.

Arabidopsis Class I Formin FH1 Relocates between Membrane Compartments during Root Cell Ontogeny and Associates with Plasmodesmata

Formins are evolutionarily conserved eukaryotic proteins engaged in actin nucleation and other aspects of cytoskeletal organization. Angiosperms have two formin clades with multiple paralogs; typical plant Class I formins are integral membrane proteins that can anchor cytoskeletal structures to membranes. For the main Arabidopsis housekeeping Class I formin, FH1 (At3g25500), plasmalemma localization was documented in heterologous expression and overexpression studies. We previously showed that loss of FH1 function increases cotyledon epidermal pavement cell shape complexity via modification of actin and microtubule organization and dynamics. Here, we employ transgenic Arabidopsis expressing green fluorescent protein-tagged FH1 (FH1-GFP) from its native promoter to investigate in vivo behavior of this formin using advanced microscopy techniques. The fusion protein is functional, since its expression complements the fh1 loss-of-function mutant phenotype. Accidental overexpression of FH1-GFP results in a decrease in trichome branch number, while fh1 mutation has the opposite effect, indicating a general role of this formin in controlling cell shape complexity. Consistent with previous reports, FH1-GFP associates with membranes. However, the protein exhibits surprising actin- and secretory pathway-dependent dynamic localization and relocates between cellular endomembranes and the plasmalemma during cell division and differentiation in root tissues, with transient tonoplast localization at the transition/elongation zones border. FH1-GFP also accumulates in actin-rich regions of cortical cytoplasm and associates with plasmodesmata in both the cotyledon epidermis and root tissues. Together with previous reports from metazoan systems, this suggests that formins might have a shared (ancestral or convergent) role at cell-cell junctions.

Arabidopsis class I formins control membrane-originated actin polymerization at pollen tube tips

A population of dynamic apical actin filaments is required for rapid polarized pollen tube growth. However, the cellular mechanisms driving their assembly remain incompletely understood. It was postulated that formin is a major player in nucleating apical actin assembly, but direct genetic and cytological evidence remains to be firmly established. Here we found that both Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5) are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes, with AtFH3 playing a more dominant role. We found that both formins have plasma membrane (PM) localization signals but exhibit distinct PM localization patterns in the pollen tube, and loss of their function reduces the amount of apical actin filaments. Live-cell imaging revealed that the reduction in filamentous actin is very likely due to the decrease in filament elongation. Furthermore, we found that the rate of tip-directed vesicle transport is reduced and the pattern of apical vesicle accumulation is altered in formin loss-of-function mutant pollen tubes, which explains to some extent the reduction in pollen tube elongation. Thus, we provide direct genetic and cytological evidence showing that formin is an important player in nucleating actin assembly from the PM at pollen tube tips.

Actin polymerization has been implicated in the regulation of rapid polarized pollen tube growth. The important role of actin polymerization is well appreciated, but the mechanisms that regulate rapid actin polymerization in pollen tubes remain incompletely understood. It was postulated that one of the major actin polymerization pathways in pollen tubes involves formin/profilin modules. However, direct genetic and cytological evidence is still required to support the role of formin in this framework. Using state-of-the-art live-cell imaging in combination with reverse genetic approaches, we demonstrate here that two class I formins, Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5), are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes. In support of the role of AtFH3 and AtFH5 in regulating membrane-originated apical actin polymerization, we found that both of them are localized to the plasma membrane (PM) at pollen tube tips. Live-cell imaging revealed that the reduction in filamentous actin is very likely due to the decrease in elongation of actin filaments originating from the apical membrane. We also found that AtFH3 and AtFH5 exhibit distinct PM localization patterns in the pollen tube, suggesting that they might have distinct roles in regulating actin polymerization in pollen tubes. Our study provides direct genetic and cytological evidence that formins act as important players in regulating apical actin assembly in pollen tubes.

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.

The Root Transition Zone: A Hot Spot for Signal Crosstalk

The root transition zone (TZ), located between the apical meristem and basal elongation region, has a unique role in root growth and development. The root TZ is not only the active site for hormone crosstalk, but also the perception site for various environmental cues, such as aluminum (Al) stress and low phosphate (Pi) stress. We propose that the root TZ is a hot spot for the integration of diverse inputs from endogenous (hormonal) and exogenous (sensorial) stimuli to control root growth.

Emerging roles of cortical microtubule-membrane interactions

Plant cortical microtubules have crucial roles in cell wall development. Cortical microtubules are tightly anchored to the plasma membrane in a highly ordered array, which directs the deposition of cellulose microfibrils by guiding the movement of the cellulose synthase complex. Cortical microtubules also interact with several endomembrane systems to regulate cell wall development and other cellular events. Recent studies have identified new factors that mediate interactions between cortical microtubules and endomembrane systems including the plasma membrane, endosome, exocytic vesicles, and endoplasmic reticulum. These studies revealed that cortical microtubule-membrane interactions are highly dynamic, with specialized roles in developmental and environmental signaling pathways. A recent reconstructive study identified a novel function of the cortical microtubule-plasma membrane interaction, which acts as a lateral fence that defines plasma membrane domains. This review summarizes recent advances in our understanding of the mechanisms and functions of cortical microtubule-membrane interactions.

LlFH1-mediated interaction between actin fringe and exocytic vesicles is involved in pollen tube tip growth

Pollen tube tip growth is an extreme form of polarized cell growth, which requires polarized exocytosis based on a dynamic actin cytoskeleton. However, the molecular basis for the connection between actin filaments and exocytic vesicles is unclear. Here, we identified a Lilium longiflorum pollen-specific formin (LlFH1) and found that it localized at the apical vesicles and plasma membrane (PM). Overexpression of LlFH1 induced excessive actin cables in the tube tip region, and downregulation of LlFH1 eliminated the actin fringe. Fluorescence recovery after photobleaching (FRAP) analysis revealed that LlFH1-labeled exocytic vesicles exhibited an initial accumulation at the shoulder of the apex and coincided with the leading edge of the actin fringe. Time-lapse analysis suggested that nascent actin filaments followed the emergence of the apical vesicles, implying that LlFH1 at apical vesicles could initiate actin polymerization. Biochemical assays showed that LlFH1 FH1FH2 could nucleate actin polymerization, but then capped the actin filament at the barbed end and inhibited its elongation. However, in the presence of lily profilins, LlFH1 FH1FH2 could accelerate barbed-end actin elongation. In addition, LlFH1 FH1FH2 was able to bundle actin filaments. Thus, we propose that LlFH1 and profilin coordinate the interaction between the actin fringe and exocytic vesicle trafficking during pollen tube growth of lily.

Genetic Regulation of Transcriptional Variation in Natural Arabidopsis thaliana Accessions

An increased knowledge of the genetic regulation of expression in Arabidopsis thaliana is likely to provide important insights about the basis of the plant’s extensive phenotypic variation. Here, we reanalyzed two publicly available datasets with genome-wide data on genetic and transcript variation in large collections of natural A. thaliana accessions. Transcripts from more than half of all genes were detected in the leaves of all accessions, and from nearly all annotated genes in at least one accession. Thousands of genes had high transcript levels in some accessions, but no transcripts at all in others, and this pattern was correlated with the genome-wide genotype. In total, 2669 eQTL were mapped in the largest population, and 717 of them were replicated in the other population. A total of 646 cis-eQTL-regulated genes that lacked detectable transcripts in some accessions was found, and for 159 of these we identified one, or several, common structural variants in the populations that were shown to be likely contributors to the lack of detectable RNA transcripts for these genes. This study thus provides new insights into the overall genetic regulation of global gene expression diversity in the leaf of natural A. thaliana accessions. Further, it also shows that strong cis-acting polymorphisms, many of which are likely to be structural variations, make important contributions to the transcriptional variation in the worldwide A. thaliana population.

Multifunctional Microtubule-Associated Proteins in Plants

Microtubules (MTs) are involved in key processes in plant cells, including cell division, growth and development. MT-interacting proteins modulate MT dynamics and organization, mediating functional and structural interaction of MTs with other cell structures. In addition to conventional microtubule-associated proteins (MAPs) in plants, there are many other MT-binding proteins whose primary function is not related to the regulation of MTs. This review focuses on enzymes, chaperones, or proteins primarily involved in other processes that also bind to MTs. The MT-binding activity of these multifunctional MAPs is often performed only under specific environmental or physiological conditions, or they bind to MTs only as components of a larger MT-binding protein complex. The involvement of multifunctional MAPs in these interactions may underlie physiological and morphogenetic events, e.g., under specific environmental or developmental conditions. Uncovering MT-binding activity of these proteins, although challenging, may contribute to understanding of the novel functions of the MT cytoskeleton in plant biological processes.

An update on cell surface proteins containing extensin-motifs

In recent years it has become clear that there are several molecular links that interconnect the plant cell surface continuum, which is highly important in many biological processes such as plant growth, development, and interaction with the environment. The plant cell surface continuum can be defined as the space that contains and interlinks the cell wall, plasma membrane and cytoskeleton compartments. In this review, we provide an updated view of cell surface proteins that include modular domains with an extensin (EXT)-motif followed by a cytoplasmic kinase-like domain, known as PERKs (for proline-rich extensin-like receptor kinases); with an EXT-motif and an actin binding domain, known as formins; and with extracellular hybrid-EXTs. We focus our attention on the EXT-motifs with the short sequence Ser-Pro(3-5), which is found in several different protein contexts within the same extracellular space, highlighting a putative conserved structural and functional role. A closer understanding of the dynamic regulation of plant cell surface continuum and its relationship with the downstream signalling cascade is a crucial forthcoming challenge.

Interactions between plant endomembrane systems and the actin cytoskeleton

Membrane trafficking, organelle movement, and morphogenesis in plant cells are mainly controlled by the actin cytoskeleton. Not all proteins that regulate the cytoskeleton and membrane dynamics in animal systems have functional homologs in plants, especially for those proteins that form the bridge between the cytoskeleton and membrane; the membrane-actin adaptors. Their nature and function is only just beginning to be elucidated and this field has been greatly enhanced by the recent identification of the NETWORKED (NET) proteins, which act as membrane-actin adaptors. In this review, we will summarize the role of the actin cytoskeleton and its regulatory proteins in their interaction with endomembrane compartments and where they potentially act as platforms for cell signaling and the coordination of other subcellular events.

Formins: linking cytoskeleton and endomembranes in plant cells

The cytoskeleton plays a central part in spatial organization of the plant cytoplasm, including the endomebrane system. However, the mechanisms involved are so far only partially understood. Formins (FH2 proteins), a family of evolutionarily conserved proteins sharing the FH2 domain whose dimer can nucleate actin, mediate the co-ordination between actin and microtubule cytoskeletons in multiple eukaryotic lineages including plants. Moreover, some plant formins contain transmembrane domains and participate in anchoring cytoskeletal structures to the plasmalemma, and possibly to other membranes. Direct or indirect membrane association is well documented even for some fungal and metazoan formins lacking membrane insertion motifs, and FH2 proteins have been shown to associate with endomembranes and modulate their dynamics in both fungi and metazoans. Here we summarize the available evidence suggesting that formins participate in membrane trafficking and endomembrane, especially ER, organization also in plants. We propose that, despite some methodological pitfalls inherent to in vivo studies based on (over)expression of truncated and/or tagged proteins, formins are beginning to emerge as candidates for the so far somewhat elusive link between the plant cytoskeleton and the endomembrane system.

Across the great divide: the plant cell surface continuum

The plant cell wall, plasma membrane and cytoskeleton exist as a cell surface continuum. This interconnection of organelles forms the interface between the plant cell and the external environment and is important for detecting the presence of a diverse range of stimuli. A plethora of plasma membrane microdomains with putative roles in membrane localized enzymatic or signalling processes have been described. While regulation of cell wall composition is defined by proteins within the plasma membrane, the cell wall has been shown to have an anchoring role on plasma membrane proteins which affects their lateral mobility. This interplay between plasma membrane and cell wall components is necessary for plasma membrane microdomain function. Actin and microtubule cytoskeletons are also involved in maintenance and function of the cell surface continuum. Investigation of the interactions between organellar components of this mechanism are important if we are to understand how cells respond to external signals.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

HopW1 from Pseudomonas syringae disrupts the actin cytoskeleton to promote virulence in Arabidopsis

A central mechanism of virulence of extracellular bacterial pathogens is the injection into host cells of effector proteins that modify host cellular functions. HopW1 is an effector injected by the type III secretion system that increases the growth of the plant pathogen Pseudomonas syringae on the Columbia accession of Arabidopsis. When delivered by P. syringae into plant cells, HopW1 causes a reduction in the filamentous actin (F-actin) network and the inhibition of endocytosis, a known actin-dependent process. When directly produced in plants, HopW1 forms complexes with actin, disrupts the actin cytoskeleton and inhibits endocytosis as well as the trafficking of certain proteins to vacuoles. The C-terminal region of HopW1 can reduce the length of actin filaments and therefore solubilize F-actin in vitro. Thus, HopW1 acts by disrupting the actin cytoskeleton and the cell biological processes that depend on actin, which in turn are needed for restricting P. syringae growth in Arabidopsis.

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.

Formins and membranes: anchoring cortical actin to the cell wall and beyond

Formins are evolutionarily conserved eukaryotic proteins participating in actin and microtubule organization. Land plants have three formin clades, with only two - Class I and II - present in angiosperms. Class I formins are often transmembrane proteins, residing at the plasmalemma and anchoring the cortical cytoskeleton across the membrane to the cell wall, while Class II formins possess a PTEN-related membrane-binding domain. Lower plant Class III and non-plant formins usually contain domains predicted to bind RHO GTPases that are membrane-associated. Thus, some kind of membrane anchorage appears to be a common formin feature. Direct interactions between various non-plant formins and integral or peripheral membrane proteins have indeed been reported, with varying mechanisms and biological implications. Besides of summarizing new data on Class I and Class II formin-membrane relationships, this review surveys such "non-classical" formin-membrane interactions and examines which, if any, of them may be evolutionarily conserved and operating also in plants. FYVE, SH3 and BAR domain-containing proteins emerge as possible candidates for such conserved membrane-associated formin partners.

AtFH16, [corrected] an Arabidopsis type II formin, binds and bundles both microfilaments and microtubules, and preferentially binds to microtubules

Formins are well-known regulators that participate in the organization of the actin cytoskeleton in organisms. The Arabidopsis thaliana L. genome encodes 21 formins, which can be divided into two distinct subfamilies. However, type II formins have to date been less well characterized. Here, we cloned a type II formin, AtFH16, and characterized its biochemical activities on actin and microtubule dynamics. The results show that the FH1FH2 structure of AtFH16 cannot nucleate actin polymerization efficiently, but can bind and bundle microfilaments. AtFH16 FH1FH2 is also able to bind and bundle microtubules, and preferentially binds microtubules over microfilaments in vitro. In addition, AtFH16 FH1FH2 co-localizes with microtubules in onion epidermal cells, indicating a higher binding affinity of AtFH16 FH1FH2 for microtubules rather than microfilaments in vivo. In conclusion, AtFH16 is able to interact with both microfilaments and microtubules, suggesting that AtFH16 probably functions as a bifunctional protein, and may thus participate in plant cellular processes.

Root apex transition zone as oscillatory zone

Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.

Mediation of clathrin-dependent trafficking during cytokinesis and cell expansion by Arabidopsis stomatal cytokinesis defective proteins

Stomatal cytokinesis defective1 (SCD1) encodes a putative Rab guanine nucleotide exchange factor that functions in membrane trafficking and is required for cytokinesis and cell expansion in Arabidopsis thaliana. Here, we show that the loss of SCD2 function disrupts cytokinesis and cell expansion and impairs fertility, phenotypes similar to those observed for scd1 mutants. Genetic and biochemical analyses showed that SCD1 function is dependent upon SCD2 and that together these proteins are required for plasma membrane internalization. Further specifying the role of these proteins in membrane trafficking, SCD1 and SCD2 proteins were found to be associated with isolated clathrin-coated vesicles and to colocalize with clathrin light chain at putative sites of endocytosis at the plasma membrane. Together, these data suggest that SCD1 and SCD2 function in clathrin-mediated membrane transport, including plasma membrane endocytosis, required for cytokinesis and cell expansion.

Formin homology 1 (OsFH1) regulates submergence-dependent root hair development in rice plants

By using a forward genetic approach, a formin homology 1 gene (OsFH1) was identified as a critical regulator of rice root hair development. The phenotypic effect of OsFH1 on root hair development was verified by using three independent mutants, one point mutation and two T-DNA insertions. The study showed that OsFH1 is required for the elongation of root-hairs. However, Osfh1 exhibited growth defect of root hairs only when roots were grown submerged in solution. To understand how OsFH1 impinges on plant responses to root submergence, the growth responses of Osfh1 root hairs to anoxia, carbohydrate supplementation and exogenous hormones (auxin and ethylene) and nutrients (Fe and Pi) were examined. However, none of these treatments rescued the growth defects of Osfhl1 root hairs. This study demonstrates that OsFH1 could be involved in preventing submergence-induced inhibition of root hair growth.

Formin homology 1 (OsFH1) regulates root-hair elongation in rice (Oryza sativa)

The outgrowth of root hairs from the epidermal cell layer is regulated by a strict genetic regulatory system and external growth conditions. Rice plants cultivated in water-logged paddy land are exposed to a soil ecology that differs from the environment surrounding upland plants, such as Arabidopsis and maize. To identify genes that play important roles in root-hair growth, a forward genetics approach was used to screen for short-root-hair mutants. A short-root-hair mutant was identified, and the gene was isolated using map-based cloning and sequencing. The mutant harbored a point mutation at a splicing acceptor site, which led to truncation of OsFH1 (rice formin homology 1). Subsequent analysis of two additional T-DNA mutants verified that OsFH1 is important for root-hair elongation. Further studies revealed that the action of OsFH1 on root-hair growth is dependent on growth conditions. The mutant Osfh1 exhibited root-hair defects when roots were grown submerged in solution, and mutant roots produced normal root hairs in the air. However, root-hair phenotypes of mutants were not influenced by the external supply of hormones or carbohydrates, a deficiency of nutrients, such as Fe or P i , or aeration. This study shows that OsFH1 plays a significant role in root-hair elongation in a growth condition-dependent manner.

Invasive cells in animals and plants: searching for LECA machineries in later eukaryotic life

Invasive cell growth and migration is usually considered a specifically metazoan phenomenon. However, common features and mechanisms of cytoskeletal rearrangements, membrane trafficking and signalling processes contribute to cellular invasiveness in organisms as diverse as metazoans and plants – two eukaryotic realms genealogically connected only through the last common eukaryotic ancestor (LECA). By comparing current understanding of cell invasiveness in model cell types of both metazoan and plant origin (invadopodia of transformed metazoan cells, neurites, pollen tubes and root hairs), we document that invasive cell behavior in both lineages depends on similar mechanisms. While some superficially analogous processes may have arisen independently by convergent evolution (e.g. secretion of substrate- or tissue-macerating enzymes by both animal and plant cells), at the heart of cell invasion is an evolutionarily conserved machinery of cellular polarization and oriented cell mobilization, involving the actin cytoskeleton and the secretory pathway. Its central components - small GTPases (in particular RHO, but also ARF and Rab), their specialized effectors, actin and associated proteins, the exocyst complex essential for polarized secretion, or components of the phospholipid- and redox- based signalling circuits (inositol-phospholipid kinases/PIP2, NADPH oxidases) are aparently homologous among plants and metazoans, indicating that they were present already in LECA.

Reviewer: This article was reviewed by Arcady Mushegian, Valerian Dolja and Purificacion Lopez-Garcia.

Plant formins: membrane anchors for actin polymerization

In plants, the actin cytoskeleton plays a fundamental role in intracellular transport, cell growth, and morphology. Formins are central regulators of actin polymerization and actin-based processes in many eukaryotes. Plants have a diverse family of formins and this diversity arose early in land plant evolution, probably deriving from family expansion and domain acquisition. Recently, formins from different plant lineages have been studied and the focus of these studies is beginning to shift from biochemical characterization to in vivo function. In vivo studies have shown that distinct biochemical activities confer specific cellular functions. Despite these differences, many plant formins have in common a direct link to the plasma membrane, suggesting that formins in plants are important links between the plasma membrane and actin remodeling.

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.

An Arabidopsis class II formin, AtFH19, nucleates actin assembly, binds to the barbed end of actin filaments, and antagonizes the effect of AtFH1 on actin dynamics

Formin is a major protein responsible for regulating the nucleation of actin filaments, and as such, it permits the cell to control where and when to assemble actin arrays. It is encoded by a multigene family comprising 21 members in Arabidopsis thaliana. The Arabidopsis formins can be separated into two phylogenetically-distinct classes: there are 11 class I formins and 10 class II formins. Significant questions remain unanswered regarding the molecular mechanism of actin nucleation and elongation stimulated by each formin isovariant, and how the different isovariants coordinate to regulate actin dynamics in cells. Here, we characterize a class II formin, AtFH19, biochemically. We found that AtFH19 retains all general properties of the formin family, including nucleation and barbed end capping activity. It can also generate actin filaments from a pool of actin monomers bound to profilin. However, both the nucleation and barbed end capping activities of AtFH19 are less efficient compared to those of another well-characterized formin, AtFH1. Interestingly, AtFH19 FH1FH2 competes with AtFH1 FH1FH2 in binding actin filament barbed ends, and inhibits the effect of AtFH1 FH1FH2 on actin. We thus propose a mechanism in which two quantitatively different formins coordinate to regulate actin dynamics by competing for actin filament barbed ends.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

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.

Class II formin targeting to the cell cortex by binding PI(3,5)P(2) is essential for polarized growth

PI(3,5)P₂ is directly linked to generation and remodeling of the cortical actin array by formin during polarized cell growth in plants.

Class II formins are key regulators of actin and are essential for polarized plant cell growth. Here, we show that the class II formin N-terminal phosphatase and tensin (PTEN) domain binds phosphoinositide-3,5-bisphosphate (PI(3,5)P₂). Replacing the PTEN domain with polypeptides of known lipid-binding specificity, we show that PI(3,5)P₂ binding was required for formin-mediated polarized growth. Via PTEN, formin also localized to the cell apex, phragmoplast, and to the cell cortex as dynamic cortical spots. We show that the cortical localization driven by binding to PI(3,5)P₂ was required for function. Silencing the kinases that produce PI(3,5)P₂ reduced cortical targeting of formin and inhibited polarized growth. We show a subset of cortical formin spots moved in actin-dependent linear trajectories. We observed that the linearly moving subpopulation of cortical formin generated new actin filaments de novo and along preexisting filaments, providing evidence for formin-mediated actin bundling in vivo. Taken together, our data directly link PI(3,5)P₂ to generation and remodeling of the cortical actin array.

New insights into the role of plant formins: regulating the organization of the actin and microtubule cytoskeleton

Formins are well-known as important regulators participating in the organization of the actin cytoskeleton in organisms. For many years in the past, research on plant formins is more difficult than that in other eukaryotic formins and is limited to class I formins. Nevertheless, positive progress has been made in plant formin research recently, especially the investigations on class II formins. New functions of plant formins are identified gradually, such as regulating cell division and affecting diffuse cell expansion. More significantly, plant formins are also verified to interact with microtubules in vivo and in vitro. They may probably function as linking proteins between microtubules and microfilaments to participate in various cellular processes.

Control of the actin cytoskeleton in root hair development

The development of root hair includes four stages: bulge site selection, bulge formation, tip growth, and maturation. The actin cytoskeleton is involved in all of these stages and is organized into distinct arrangements in the different stages. In addition to the actin configuration, actin isoforms also play distinct roles in the different stages. The actin cytoskeleton is regulated by actin-binding proteins, such as formin, Arp2/3 complex, profilin, actin depolymerizing factor, and villin. Some upstream signals, i.e. calcium, phospholipids, and small GTPase regulate the activity of these actin-binding proteins to produce the proper actin configuration. We constructed a working model on how the actin cytoskeleton is controlled by actin-binding proteins and upstream signaling in root hair development based on the current literature: at the tip of hairs, actin polymerization appears to be facilitated by Arp2/3 complex that is activated by small GTPase, and profilin that is regulated by phosphatidylinositol 4,5-bisphosphate. Meanwhile, actin depolymerization and turnover are likely mediated by villin and actin depolymerizing factor, which are stimulated by calcium. At the shank, actin cables are produced by formin and villin. Under the complicated interaction, the actin cytoskeleton is controlled spatially and temporally during root hair development.

Protein-protein interactions as a proxy to monitor conformational changes and activation states of the tomato resistance protein I-2

Plant resistance proteins (R) are involved in pathogen recognition and subsequent initiation of defence responses. Their activity is regulated by inter- and intramolecular interactions. In a yeast two-hybrid screen two clones (I2I-1 and I2I-2) specifically interacting with I-2, a Fusarium oxysporum f. sp. lycopersici resistance protein of the CC-NB-LRR family, were identified. Sequence analysis revealed that I2I-1 belongs to the Formin gene family (SlFormin) whereas I2I-2 has homology to translin-associated protein X (SlTrax). SlFormin required only the N-terminal CC I-2 domain for binding, whereas SlTrax required both I-2 CC and part of the NB-ARC domain. Tomato plants stably silenced for these interactors were not compromised in I-2-mediated disease resistance. When extended or mutated forms of I-2 were used as baits, distinct and often opposite, interaction patterns with the two interactors were observed. These interaction patterns correlated with the proposed activation state of I-2 implying that active and inactive R proteins adopt distinct conformations. It is concluded that the yeast two hybrid system can be used as a proxy to monitor these different conformational states.

PDZ domain proteins: 'dark matter' of the plant proteome?

PDZ domain proteins in metazoans function in diverse roles, and in conjunction with PDZ domain-binding proteins form macromolecular complexes for signaling at synapses and cell junctions. Bioinformatics approaches using the SMART tool indicate there are only a modest number of Arabidopsis PDZ proteins. However, there are hundreds of proteins predicted to possess PDZ domain-binding motifs, suggesting that there are many PDZ domain proteins not detectable by conventional bioinformatic approaches. Our Scansite analysis of PDZ domain-binding proteins indicates that PDZ domain proteins may play key roles in cytoskeletal organization including actin microfilaments, microtubules, and nuclear cytoskeletal proteins, and in the organization of macromolecular complexes involved in cell-to-cell signaling, transport, and cell wall formation.

Building bridges: formin1 of Arabidopsis forms a connection between the cell wall and the actin cytoskeleton

Actin microfilament (MF) organization and remodelling is critical to cell function. The formin family of actin binding proteins are involved in nucleating MFs in Arabidopsis thaliana. They all contain formin homology domains in the intracellular, C-terminal half of the protein that interacts with MFs. Formins in class I are usually targeted to the plasma membrane and this is true of Formin1 (AtFH1) of A. thaliana. In this study, we have investigated the extracellular domain of AtFH1 and we demonstrate that AtFH1 forms a bridge from the actin cytoskeleton, across the plasma membrane and is anchored within the cell wall. AtFH1 has a large, extracellular domain that is maintained by purifying selection and that contains four conserved regions, one of which is responsible for immobilising the protein. Protein anchoring within the cell wall is reduced in constructs that express truncations of the extracellular domain and in experiments in protoplasts without primary cell walls. The 18 amino acid proline-rich extracellular domain that is responsible for AtFH1 anchoring has homology with cell-wall extensins. We also have shown that anchoring of AtFH1 in the cell wall promotes actin bundling within the cell and that overexpression of AtFH1 has an inhibitory effect on organelle actin-dependant dynamics. Thus, the AtFH1 bridge provides stable anchor points for the actin cytoskeleton and is probably a crucial component of the signalling response and actin-remodelling mechanisms.