The Specification of Geometric Edges by a Plant Rab GTPase Is an Essential Cell-Patterning Principle During Organogenesis in Arabidopsis

The role of Rab GTPase in Plant development and stress

Small GTPase is a type of crucial regulator in eukaryotes. It acts as a molecular switch by binding with GTP and GDP in cytoplasm, affecting various cellular processes. Small GTPase were divided into five subfamilies based on sequence, structure and function: Ras, Rho, Rab, Arf/Sar and Ran, with Rab being the largest subfamily. Members of the Rab subfamily play an important role in regulating complex vesicle transport and microtubule system activity. Plant cells are composed of various membrane-bound organelles, and vesicle trafficking is fundamental to the existence of plants. At present, the function of some Rab members, such as RabA1a, RabD2b/c and RabF2, has been well characterized in plants. This review summarizes the role of Rab GTPase in regulating plant tip growth, morphogenesis, fruit ripening and stress response, and briefly describes the regulatory mechanisms involved. It provides a reference for further alleviating environmental stress, improving plant resistance and even improving fruit quality.

A self-regulatory cell-wall-sensing module at cell edges controls plant growth

Morphogenesis of multicellular organs requires coordination of cellular growth. In plants, cell growth is determined by turgor pressure and the mechanical properties of the cell wall, which also glues cells together. Because plants have to integrate tissue-scale mechanical stresses arising through growth in a fixed tissue topology, they need to monitor cell wall mechanical status and adapt growth accordingly. Molecular factors have been identified, but whether cell geometry contributes to wall sensing is unknown. Here we propose that plant cell edges act as cell-wall-sensing domains during growth. We describe two Receptor-Like Proteins, RLP4 and RLP4-L1, which occupy a unique polarity domain at cell edges established through a targeted secretory transport pathway. We show that RLP4s associate with the cell wall at edges via their extracellular domain, respond to changes in cell wall mechanics and contribute to directional growth control in Arabidopsis.

Divided by fate: The interplay between division orientation and cell shape underlying lateral root initiation in Arabidopsis

The development of lateral roots starts with a round of anticlinal, asymmetric cell divisions in lateral root founder cells in the pericycle, deep within the root. The reorientation of the cell division plane occurs in parallel with changes in cell shape and needs to be coordinated with its direct neighbor, the endodermis. This accommodation response requires the integration of biochemical and mechanical signals in both cell types. Recently, it was reported that dynamic changes in the cytoskeleton and possibly the cell wall are part of the molecular mechanism required to correctly orient and position the cell division plane. Here we discuss the latest progress made towards our understanding of the regulation of cell shape and division plane orientation underlying lateral root initiation in Arabidopsis.

Emerging role of small GTPases and their interactome in plants to combat abiotic and biotic stress

Plants are frequently subjected to abiotic and biotic stress which causes major impediments in their growth and development. It is emerging that small guanosine triphosphatases (small GTPases), also known as monomeric GTP-binding proteins, assist plants in managing environmental stress. Small GTPases function as tightly regulated molecular switches that get activated with the aid of guanosine triphosphate (GTP) and deactivated by the subsequent hydrolysis of GTP to guanosine diphosphate (GDP). All small GTPases except Rat sarcoma (Ras) are found in plants, including Ras-like in brain (Rab), Rho of plant (Rop), ADP-ribosylation factor (Arf) and Ras-like nuclear (Ran). The members of small GTPases in plants interact with several downstream effectors to counteract the negative effects of environmental stress and disease-causing pathogens. In this review, we describe processes of stress alleviation by developing pathways involving several small GTPases and their associated proteins which are important for neutralizing fungal infections, stomatal regulation, and activation of abiotic stress-tolerant genes in plants. Previous reviews on small GTPases in plants were primarily focused on Rab GTPases, abiotic stress, and membrane trafficking, whereas this review seeks to improve our understanding of the role of all small GTPases in plants as well as their interactome in regulating mechanisms to combat abiotic and biotic stress. This review brings to the attention of scientists recent research on small GTPases so that they can employ genome editing tools to precisely engineer economically important plants through the overexpression/knock-out/knock-in of stress-related small GTPase genes.

RHO GTPase of plants regulates polarized cell growth and cell division orientation during morphogenesis

Cell polarity-broadly defined as the asymmetric distribution of cellular activities and subcellular components within a cell-determines the geometry of cell growth and division during development. RHO GTPase proteins regulate the establishment of cell polarity and are conserved among eukaryotes. RHO of plant (ROP) proteins are a subgroup of RHO GTPases that are required for cellular morphogenesis in plants. However, how ROP proteins modulate the geometry of cell growth and division during the morphogenesis of plant tissues and organs is not well understood. To investigate how ROP proteins function during tissue development and organogenesis, we characterized the function of the single-copy ROP gene of the liverwort Marchantia polymorpha (MpROP). M. polymorpha develops morphologically complex three-dimensional tissues and organs exemplified by air chambers and gemmae, respectively. Mprop loss-of-function mutants form defective air chambers and gemmae, indicating ROP function is required for tissue development and organogenesis. During air chamber and gemma development in wild type, the MpROP protein is enriched to sites of polarized growth at the cell surface and accumulates at the expanding cell plate of dividing cells. Consistent with these observations, polarized cell growth is lost and cell divisions are misoriented in Mprop mutants. We propose that ROP regulates both polarized cell growth and cell division orientation in a coordinated manner to orchestrate tissue development and organogenesis in land plants.

Discretizing the cellular bases of plant morphogenesis: Emerging properties from subcellular and noisy patterning

A fundamental question in biology is how multicellular organisms robustly shape their organs. In the past decade, much progress has been made not just in identifying biochemical and biophysical factors underpinning morphogenesis, but also in analyzing their spatio-temporal dynamics. A remarkable outcome of such analyses is that morphogenesis involves high levels of heterogeneity and fluctuations at local scales. Although this could be considered as white noise to be averaged over time, there is increasing evidence that these heterogeneities and fluctuations are instructive cues for development. In this review, we highlight some of the new questions that such heterogeneities raise for plant morphogenesis. We also investigate their effects across scales, focusing on how subcellular heterogeneities contribute to organ shape robustness and evolvability.

Expression dynamics and a loss-of-function of Arabidopsis RabC1 GTPase unveil its role in plant growth and seed development

Transcript isoform dynamics, spatiotemporal expression, and mutational analysis uncover that Arabidopsis RabC1 GTPase is required for root length, flowering time, seed size, and seed mucilage. Rab GTPases are crucial regulators for moving different molecules to their specific compartments according to the needs of the cell. In this work, we illustrate the role of RabC1 GTPase in Arabidopsis growth and seed development. We identify and analyze the expression pattern of three transcript isoforms of RabC1 in different development stages, along with their tissue-specific transcript abundance. The promoter activity of RabC1 using promoter-GUS fusion shows that it is widely expressed during the growth of Arabidopsis, particularly in seed tissues such as chalazal seed coat and chalazal endosperm. Lack of RabC1 function led to shorter roots, lesser biomass, delayed flowering, and sluggish plant development. The mutants had smaller seeds than the wildtype, less seed mass, and lower seed coat permeability. Developing seeds also revealed a smaller endosperm cavity and shorter integument cells. Additionally, we found that the knock-out mutant had downregulated expression of genes implicated in the transit of sugars and amino acids from maternal tissue to developing seed. The seeds of the loss-of-function mutant had reduced seed mucilage. All the observed mutant phenotypes were restored in the complemented lines confirming the function of RabC1 in seed development and plant growth.

Rab GTPases, tethers, and SNAREs work together to regulate Arabidopsis cell plate formation

Cell plates are transient structures formed by the fusion of vesicles at the center of the dividing plane; furthermore, these are precursors to new cell walls and are essential for cytokinesis. Cell plate formation requires a highly coordinated process of cytoskeletal rearrangement, vesicle accumulation and fusion, and membrane maturation. Tethering factors have been shown to interact with the Ras superfamily of small GTP binding proteins (Rab GTPases) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are essential for cell plate formation during cytokinesis and are fundamental for maintaining normal plant growth and development. In Arabidopsis thaliana, members of the Rab GTPases, tethers, and SNAREs are localized in cell plates, and mutations in the genes encoding these proteins result in typical cytokinesis-defective phenotypes, such as the formation of abnormal cell plates, multinucleated cells, and incomplete cell walls. This review highlights recent findings on vesicle trafficking during cell plate formation mediated by Rab GTPases, tethers, and SNAREs.

An actin remodeling role for Arabidopsis processing bodies revealed by their proximity interactome

Cellular condensates can comprise membrane-less ribonucleoprotein assemblies with liquid-like properties. These cellular condensates influence various biological outcomes, but their liquidity hampers their isolation and characterization. Here, we investigated the composition of the condensates known as processing bodies (PBs) in the model plant Arabidopsis thaliana through a proximity-biotinylation proteomics approach. Using in situ protein-protein interaction approaches, genetics and high-resolution dynamic imaging, we show that processing bodies comprise networks that interface with membranes. Surprisingly, the conserved component of PBs, DECAPPING PROTEIN 1 (DCP1), can localize to unique plasma membrane subdomains including cell edges and vertices. We characterized these plasma membrane interfaces and discovered a developmental module that can control cell shape. This module is regulated by DCP1, independently from its role in decapping, and the actin-nucleating SCAR-WAVE complex, whereby the DCP1-SCAR-WAVE interaction confines and enhances actin nucleation. This study reveals an unexpected function for a conserved condensate at unique membrane interfaces.

RAB GTPases and SNAREs at the trans-Golgi network in plants

Membrane traffic is a fundamental cellular system to exchange proteins and membrane lipids among single membrane-bound organelles or between an organelle and the plasma membrane in order to keep integrity of the endomembrane system. RAB GTPases and SNARE proteins, the key regulators of membrane traffic, are conserved broadly among eukaryotic species. However, genome-wide analyses showed that organization of RABs and SNAREs that regulate the post-Golgi transport pathways is greatly diversified in plants compared to other model eukaryotes. Furthermore, some organelles acquired unique properties in plant lineages. Like in other eukaryotic systems, the trans-Golgi network of plants coordinates secretion and vacuolar transport; however, uniquely in plants, it also acts as a platform for endocytic transport and recycling. In this review, we focus on RAB GTPases and SNAREs that function at the TGN, and summarize how these regulators perform to control different transport pathways at the plant TGN. We also highlight the current knowledge of RABs and SNAREs' role in regulation of plant development and plant responses to environmental stimuli.

Screening of Differentially Expressed Genes and Localization Analysis of Female Gametophyte at the Free Nuclear Mitosis Stage in Pinus tabuliformis Carr

Female sterility is a common phenomenon in the plant world, and systematic research has not been carried out in gymnosperms. In this study, the ovules of No. 28 sterile line and No. 15 fertile line Pinus tabuliformis were used as materials, and a total of 18 cDNA libraries were sequenced by the HiSeqTM 4000 platform to analyze the differentially expressed genes (DEGs) and simple sequence repeats (SSRs) between the two lines. In addition, this study further analyzed the DEGs involved in the signal transduction of plant hormones, revealing that the signal pathways related to auxin, cytokinin, and gibberellin were blocked in the sterile ovule. Additionally, real-time fluorescent quantitative PCR verified that the expression trend of DEGs related to plant hormones was consistent with the results of high-throughput sequencing. Frozen sections and fluorescence in situ hybridization (FISH) were used to study the temporal and spatial expression patterns of PtRab in the ovules of P. tabuliformis. It was found that PtRab was significantly expressed in female gametophytes and rarely expressed in the surrounding diploid tissues. This study further explained the molecular regulation mechanism of female sterility in P. tabuliformis, preliminarily mining the key factors of ovule abortion in gymnosperms at the transcriptional level.

Fifteen compelling open questions in plant cell biology

As scientists, we are at least as excited about the open questions-the things we do not know-as the discoveries. Here, we asked 15 experts to describe the most compelling open questions in plant cell biology. These are their questions: How are organelle identity, domains, and boundaries maintained under the continuous flux of vesicle trafficking and membrane remodeling? Is the plant cortical microtubule cytoskeleton a mechanosensory apparatus? How are the cellular pathways of cell wall synthesis, assembly, modification, and integrity sensing linked in plants? Why do plasmodesmata open and close? Is there retrograde signaling from vacuoles to the nucleus? How do root cells accommodate fungal endosymbionts? What is the role of cell edges in plant morphogenesis? How is the cell division site determined? What are the emergent effects of polyploidy on the biology of the cell, and how are any such "rules" conditioned by cell type? Can mechanical forces trigger new cell fates in plants? How does a single differentiated somatic cell reprogram and gain pluripotency? How does polarity develop de-novo in isolated plant cells? What is the spectrum of cellular functions for membraneless organelles and intrinsically disordered proteins? How do plants deal with internal noise? How does order emerge in cells and propagate to organs and organisms from complex dynamical processes? We hope you find the discussions of these questions thought provoking and inspiring.

Cell biology of primary cell wall synthesis in plants

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

The role of pectin phase separation in plant cell wall assembly and growth

A rapidly increasing body of literature suggests that many biological processes are driven by phase separation within polymer mixtures. Liquid-liquid phase separation can lead to the formation of membrane-less organelles, which are thought to play a wide variety of roles in cell metabolism, gene regulation or signaling. One of the characteristics of these systems is that they are poised at phase transition boundaries, which makes them perfectly suited to elicit robust cellular responses to often very small changes in the cell's "environment". Recent observations suggest that, also in the semi-solid environment of plant cell walls, phase separation not only plays a role in wall patterning, hydration and stress relaxation during growth, but also may provide a driving force for cell wall expansion. In this context, pectins, the major polyanionic polysaccharides in the walls of growing cells, appear to play a critical role. Here, we will discuss (i) our current understanding of the structure-function relationship of pectins, (ii) in vivo evidence that pectin modification can drive critical phase transitions in the cell wall, (iii) how such phase transitions may drive cell wall expansion in addition to turgor pressure and (iv) the periodic cellular processes that may control phase transitions underlying cell wall assembly and expansion.

Multiple functions of the vacuole in plant growth and fruit quality

Vacuoles are organelles in plant cells that play pivotal roles in growth and developmental regulation. The main functions of vacuoles include maintaining cell acidity and turgor pressure, regulating the storage and transport of substances, controlling the transport and localization of key proteins through the endocytic and lysosomal-vacuolar transport pathways, and responding to biotic and abiotic stresses. Further, proteins localized either in the tonoplast (vacuolar membrane) or inside the vacuole lumen are critical for fruit quality. In this review, we summarize and discuss some of the emerging functions and regulatory mechanisms associated with plant vacuoles, including vacuole biogenesis, vacuole functions in plant growth and development, fruit quality, and plant-microbe interaction, as well as some innovative research technology that has driven advances in the field. Together, the functions of plant vacuoles are important for plant growth and fruit quality. The investigation of vacuole functions in plants is of great scientific significance and has potential applications in agriculture.

Microtubule associated protein WAVE DAMPENED2-LIKE (WDL) controls microtubule bundling and the stability of the site of tip-growth in Marchantia polymorpha rhizoids

Tip-growth is a mode of polarized cell expansion where incorporation of new membrane and wall is stably restricted to a single, small domain of the cell surface resulting in the formation of a tubular projection that extends away from the body of the cell. The organization of the microtubule cytoskeleton is conserved among tip-growing cells of land plants: bundles of microtubules run longitudinally along the non-growing shank and a network of fine microtubules grow into the apical dome where growth occurs. Together, these microtubule networks control the stable positioning of the growth site at the cell surface. This conserved dynamic organization is required for the spatial stability of tip-growth, as demonstrated by the formation of sinuous tip-growing cells upon treatment with microtubule-stabilizing or microtubule-destabilizing drugs. Microtubule associated proteins (MAPs) that either stabilize or destabilize microtubule networks are required for the maintenance of stable tip-growth in root hairs of flowering plants. NIMA RELATED KINASE (NEK) is a MAP that destabilizes microtubule growing ends in the apical dome of tip-growing rhizoid cells in the liverwort Marchantia polymorpha. We hypothesized that both microtubule stabilizing and destabilizing MAPs are required for the maintenance of the stable tip-growth in liverworts. To identify genes encoding microtubule-stabilizing and microtubule-destabilizing activities we generated 120,000 UV-B mutagenized and 336,000 T-DNA transformed Marchantia polymorpha plants and screened for defective rhizoid phenotypes. We identified 119 mutants and retained 30 mutants in which the sinuous rhizoid phenotype was inherited. The 30 mutants were classified into at least 4 linkage groups. Characterisation of two of the linkage groups showed that MAP genes–WAVE DAMPENED2-LIKE (WDL) and NIMA-RELATED KINASE (NEK)–are required to stabilize the site of tip growth in elongating rhizoids. Furthermore, we show that MpWDL is required for the formation of a bundled array of parallel and longitudinally orientated microtubules in the non-growing shank of rhizoids where MpWDL-YFP localizes to microtubule bundles. We propose a model where the opposite functions of MpWDL and MpNEK on microtubule bundling are spatially separated and promote tip-growth spatial stability.

Plant cells control where they grow by adding membrane and cell wall material to a defined area of their surface. In particular, filamentous rooting cells develop the cellular projections essential to their function by restricting cell expansion to a stable domain of their surface. The spatial stability of this mechanism known as tip-growth defines the final shape of the cellular projections–straight projections form from stable tip-growth, while wavy or bifurcating projections form from unstable tip-growth. Microtubules are known to regulate tip-growth stability. Both microtubule stabilisation and destabilisation leads to unstable tip-growth. We have discovered two proteins that associate with microtubules, control their stability and are required for stabilizing tip-growth in the common liverwort. The first protein is known to destabilize microtubules in the tip of filamentous rooting cells of the common liverwort, and we found the second protein to stabilize, or bundle, microtubules in their shank. This is important because it is the first protein found to stabilize microtubules in the common liverwort and because it is the first time a protein stabilizing microtubules in rooting cells of plants is shown to localize separately from proteins that destabilizes microtubules. We propose that tip-growth stability requires the opposite functions of these two microtubule associated protein to be spatially separated.

Plant RABs: Role in Development and in Abiotic and Biotic Stress Responses

Endosomal trafficking plays an integral role in various eukaryotic cellular activities and is vital for higher-order functions in multicellular organisms. RAB GTPases are important proteins that influence various aspects of membrane traffic, which consequently influence many cellular functions and responses. Compared to yeast and mammals, plants have evolved a unique set of plant-specific RABs that play a significant role in their development. RABs form the largest family of small guanosine triphosphate (GTP)-binding proteins, and are divided into eight sub-families named RAB1, RAB2, RAB5, RAB6, RAB7, RAB8, RAB11 and RAB18. Recent studies on different species suggest that RAB proteins play crucial roles in intracellular trafficking and cytokinesis, in autophagy, plant microbe interactions and in biotic and abiotic stress responses. This review recaptures and summarizes the roles of RABs in plant cell functions and in enhancing plant survival under stress conditions.

Rab-dependent vesicular traffic affects female gametophyte development in Arabidopsis

Eukaryotic cells rely on the accuracy and efficiency of vesicular traffic. In plants, disturbances in vesicular trafficking are well studied in quickly dividing root meristem cells or polar growing root hairs and pollen tubes. The development of the female gametophyte, a unique haploid reproductive structure located in the ovule, has received far less attention in studies of vesicular transport. Key molecules providing the specificity of vesicle formation and its subsequent recognition and fusion with the acceptor membrane are Rab proteins. Rabs are anchored to membranes by covalently linked geranylgeranyl group(s) that are added by the Rab geranylgeranyl transferase (RGT) enzyme. Here we show that Arabidopsis plants carrying mutations in the gene encoding the β-subunit of RGT (rgtb1) exhibit severely disrupted female gametogenesis and this effect is of sporophytic origin. Mutations in rgtb1 lead to internalization of the PIN1 and PIN3 proteins from the basal membranes to vesicles in provascular cells of the funiculus. Decreased transport of auxin out of the ovule is accompanied by auxin accumulation in tissue surrounding the growing gametophyte. In addition, female gametophyte development arrests at the uni- or binuclear stage in a significant portion of the rgtb1 ovules. These observations suggest that communication between the sporophyte and the developing female gametophyte relies on Rab-dependent vesicular traffic of the PIN1 and PIN3 transporters and auxin efflux out of the ovule.

Mapping cellular nanoscale viscoelasticity and relaxation times relevant to growth of living Arabidopsis thaliana plants using multifrequency AFM

The shapes of living organisms are formed and maintained by precise control in time and space of growth, which is achieved by dynamically fine-tuning the mechanical (viscous and elastic) properties of their hierarchically built structures from the nanometer up. Most organisms on Earth including plants grow by yield (under pressure) of cell walls (bio-polymeric matrices equivalent to extracellular matrix in animal tissues) whose underlying nanoscale viscoelastic properties remain unknown. Multifrequency atomic force microscopy (AFM) techniques exist that are able to map properties to a small subgroup of linear viscoelastic materials (those obeying the Kelvin-Voigt model), but are not applicable to growing materials, and hence are of limited interest to most biological situations. Here, we extend existing dynamic AFM methods to image linear viscoelastic behaviour in general, and relaxation times of cells of multicellular organisms in vivo with nanoscale resolution (~80 nm pixel size in this study), featuring a simple method to test the validity of the mechanical model used to interpret the data. We use this technique to image cells at the surface of living Arabidopsis thaliana hypocotyls to obtain topographical maps of storage E' = 120-200 MPa and loss E″ = 46-111 MPa moduli as well as relaxation times τ = 2.2-2.7 µs of their cell walls. Our results demonstrate that (taken together with previous studies) cell walls, despite their complex molecular composition, display a striking continuity of simple, linear, viscoelastic behaviour across scales-following almost perfectly the standard linear solid model-with characteristic nanometer scale patterns of relaxation times, elasticity and viscosity, whose values correlate linearly with the speed of macroscopic growth. We show that the time-scales probed by dynamic AFM experiments (microseconds) are key to understand macroscopic scale dynamics (e.g. growth) as predicted by physics of polymer dynamics.

The AGCVIII kinase Dw2 modulates cell proliferation, endomembrane trafficking, and MLG/xylan cell wall localization in elongating stem internodes of Sorghum bicolor

Stems of bioenergy sorghum (Sorghum bicolor L. Moench.), a drought-tolerant C4 grass, contain up to 50 nodes and internodes of varying length that span 4-5 m and account for approximately 84% of harvested biomass. Stem internode growth impacts plant height and biomass accumulation and is regulated by brassinosteroid signaling, auxin transport, and gibberellin biosynthesis. In addition, an AGCVIII kinase (Dw2) regulates sorghum stem internode growth, but the underlying mechanism and signaling network are unknown. Here we provide evidence that mutation of Dw2 reduces cell proliferation in internode intercalary meristems, inhibits endocytosis, and alters the distribution of heteroxylan and mixed linkage glucan in cell walls. Phosphoproteomic analysis showed that Dw2 signaling influences the phosphorylation of proteins involved in lipid signaling (PLDδ), endomembrane trafficking, hormone, light, and receptor signaling, and photosynthesis. Together, our results show that Dw2 modulates endomembrane function and cell division during sorghum internode growth, providing insight into the regulation of monocot stem development.

Expression Levels of Genes Encoding Proteins Involved in the Cell Wall-Plasma Membrane-Cytoskeleton Continuum Are Associated With the Maturation-Related Adventitious Rooting Competence of Pine Stem Cuttings

Stem cutting recalcitrance to adventitious root formation is a major limitation for the clonal propagation or micropropagation of elite genotypes of many forest tree species, especially at the adult stage of development. The interaction between the cell wall-plasma membrane and cytoskeleton may be involved in the maturation-related decline of adventitious root formation. Here, pine homologs of several genes encoding proteins involved in the cell wall-plasma membrane-cytoskeleton continuum were identified, and the expression levels of 70 selected genes belonging to the aforementioned group and four genes encoding auxin carrier proteins were analyzed during adventitious root formation in rooting-competent and non-competent cuttings of Pinus radiata. Variations in the expression levels of specific genes encoding cell wall components and cytoskeleton-related proteins were detected in rooting-competent and non-competent cuttings in response to wounding and auxin treatments. However, the major correlation of gene expression with competence for adventitious root formation was detected in a family of genes encoding proteins involved in sensing the cell wall and membrane disturbances, such as specific receptor-like kinases (RLKs) belonging to the lectin-type RLKs, wall-associated kinases, Catharanthus roseus RLK1-like kinases and leucine-rich repeat RLKs, as well as downstream regulators of the small guanosine triphosphate (GTP)-binding protein family. The expression of these genes was more affected by organ and age than by auxin and time of induction.

The liverwort oil body is formed by redirection of the secretory pathway

Eukaryotic cells acquired novel organelles during evolution through mechanisms that remain largely obscure. The existence of the unique oil body compartment is a synapomorphy of liverworts that represents lineage-specific acquisition of this organelle during evolution, although its origin, biogenesis, and physiological function are yet unknown. We find that two paralogous syntaxin-1 homologs in the liverwort Marchantia polymorpha are distinctly targeted to forming cell plates and the oil body, suggesting that these structures share some developmental similarity. Oil body formation is regulated by an ERF/AP2-type transcription factor and loss of the oil body increases M. polymorpha herbivory. These findings highlight a common strategy for the acquisition of organelles with distinct functions in plants, via periodical redirection of the secretory pathway depending on cellular phase transition.

A Comprehensive Toolkit for Quick and Easy Visualization of Marker Proteins, Protein-Protein Interactions and Cell Morphology in Marchantia polymorpha

Even though stable genomic transformation of sporelings and thalli of Marchantia polymorpha is straightforward and efficient, numerous problems can arise during critical phases of the process such as efficient spore production, poor selection capacity of antibiotics or low transformation efficiency. It is therefore also desirable to establish quick methods not relying on stable transgenics to analyze the localization, interactions and functions of proteins of interest. The introduction of foreign DNA into living cells via biolistic mechanisms has been first reported roughly 30 years ago and has been commonly exploited in established plant model species such as Arabidopsis thaliana or Nicotiana benthamiana. Here, we report the fast and reliable transient biolistic transformation of Marchantia thallus epidermal cells using fluorescent protein fusions. We present a catalog of fluorescent markers which can be readily used for tagging of a variety of subcellular compartments. Moreover, we report the functionality of the bimolecular fluorescence complementation (BiFC) in M. polymorpha with the example of the p-body markers MpDCP1/2. Finally, we provide standard staining procedures for live cell imaging in M. polymorpha, applicable to visualize cell boundaries or cellular structures, to complement or support protein localizations and to understand how results gained by transient transformations can be embedded in cell architecture and dynamics. Taken together, we offer a set of easy and quick tools for experiments that aim at understanding subcellular localization, protein-protein interactions and thus functions of proteins of interest in the emerging early diverging land plant model M. polymorpha.

Salt-induced expression of intracellular vesicle trafficking genes, CaRab-GTP, and their association with Na+ accumulation in leaves of chickpea (Cicer arietinum L.)

BACKGROUND

Chickpea is an important legume and is moderately tolerant to salinity stress during the growing season. However, the level and mechanisms for salinity tolerance can vary among accessions and cultivars. A large family of CaRab-GTP genes, previously identified in chickpea, is homologous to intracellular vesicle trafficking superfamily genes that play essential roles in response to salinity stress in plants.

RESULTS

To determine which of the gene family members are involved in the chickpea salt response, plants from six selected chickpea accessions (Genesis 836, Hattrick, ICC12726, Rupali, Slasher and Yubileiny) were exposed to salinity stress and expression profiles resolved for the major CaRab-GTP gene clades after 5, 9 and 15 days of salt exposure. Gene clade expression profiles (using degenerate primers targeting all members of each clade) were tested for their relationship to salinity tolerance measures, namely plant biomass and Na⁺ accumulation. Transcripts representing 11 out of the 13 CaRab clades could be detected by RT-PCR, but only six (CaRabA2, -B, -C, -D, -E and -H) could be quantified using qRT-PCR due to low expression levels or poor amplification efficiency of the degenerate primers for clades containing several gene members. Expression profiles of three gene clades, CaRabB, -D and -E, were very similar across all six chickpea accessions, showing a strongly coordinated network. Salt-induced enhancement of CaRabA2 expression at 15 days showed a very strong positive correlation (R² = 0.905) with Na⁺ accumulation in leaves. However, salinity tolerance estimated as relative plant biomass production compared to controls, did not correlate with Na⁺ accumulation in leaves, nor with expression profiles of any of the investigated CaRab-GTP genes.

CONCLUSION

A coordinated network of CaRab-GTP genes, which are likely involved in intracellular trafficking, are important for the salinity stress response of chickpea plants.

The importance of being edgy: cell geometric edges as an emerging polar domain in plant cells

Polarity is an essential feature of multicellular organisms and underpins growth and development as well as physiological functions. In polyhedral plant cells, polar domains at different faces have been studied in detail. In recent years, cell edges (where two faces meet) have emerged as discrete spatial domains with distinct biochemical identities. Here, we review and discuss recent advances in our understanding of cell edges as functional polar domains in plant cells and other organisms, highlighting conceptual parallels and open questions regarding edge polarity.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

Spatio-temporal control of post-Golgi exocytic trafficking in plants

A complex and dynamic endomembrane system is a hallmark of eukaryotic cells and underpins the evolution of specialised cell types in multicellular organisms. Endomembrane system function critically depends on the ability of the cell to (1) define compartment and pathway identity, and (2) organise compartments and pathways dynamically in space and time. Eukaryotes possess a complex molecular machinery to control these processes, including small GTPases and their regulators, SNAREs, tethering factors, motor proteins, and cytoskeletal elements. Whereas many of the core components of the eukaryotic endomembrane system are broadly conserved, there have been substantial diversifications within different lineages, possibly reflecting lineage-specific requirements of endomembrane trafficking. This Review focusses on the spatio-temporal regulation of post-Golgi exocytic transport in plants. It highlights recent advances in our understanding of the elaborate network of pathways transporting different cargoes to different domains of the cell surface, and the molecular machinery underpinning them (with a focus on Rab GTPases, their interactors and the cytoskeleton). We primarily focus on transport in the context of growth, but also highlight how these pathways are co-opted during plant immunity responses and at the plant-pathogen interface.

Cellular perspectives for improving mesophyll conductance

After entering the leaf, CO2 faces an intricate pathway to the site of photosynthetic fixation embedded within the chloroplasts. The efficiency of CO2 flux is hindered by a number of structural and biochemical barriers which, together, define the ease of flow of the gas within the leaf, termed mesophyll conductance. Previous authors have identified the key elements of this pathway, raising the prospect of engineering the system to improve CO2 flux and, thus, to increase leaf photosynthetic efficiency. In this review, we provide a perspective on the potential for improving the individual elements that contribute to this complex parameter. We lay particular emphasis on generation of the cellular architecture of the leaf which sets the initial boundaries of a number of mesophyll conductance parameters, incorporating an overview of the molecular transport processes which have been proposed as major facilitators of CO2 flux across structural boundaries along the pathway. The review highlights the research areas where future effort might be invested to increase our fundamental understanding of mesophyll conductance and leaf function and, consequently, to enable translation of these findings to improve the efficiency of crop photosynthesis.

Differentiation of Trafficking Pathways at Golgi Entry Core Compartments and Post-Golgi Subdomains

Eukaryotic cells have developed specialized membrane structures called organelles, which compartmentalize cellular functions and chemical reactions. Recent improvements in microscopy and membrane compartment isolation techniques are now sophisticating our view. Emerging evidences support that there are distinct sub-populations or subdomains, which are spatially and/or temporally segregated within one type of organelle, contributing to specify differential sorting of various cargos to distinct destinations of the cell. In plant cells, the Golgi apparatus represents a main trafficking hub in which entry occurs through a Golgi Entry Core Compartment (GECCO), that remains to be further characterized, and sorting of cargos is mediated through multiple transport pathways with different sets of regulator proteins at the post-Golgi compartment trans-Golgi network (TGN). Both GECCO and TGN are differentiated sub-populations as compared to the rest of Golgi, and moreover, further subdomain formation within TGN is suggested to play a key role for cargo sorting. In this review, we will summarize recent findings obtained on organelle subdomains, and their relationship with cargo entry at and exit from the Golgi apparatus.

RAB GTPases and their effectors in plant endosomal transport

The plant endomembrane system comprises distinctive membrane-bounded organelles connected with one another by the membrane trafficking system. The RAB GTPase is a key component of the membrane trafficking machinery that regulates the targeting and tethering of trafficking vesicles to target compartments by acting as a molecular switch cycling between active and inactive states. The functions of RAB GTPases are fulfilled through their interactions with several classes of interacting factors, including guanine nucleotide exchange factors (GEFs) and effector proteins. Effector proteins for plant RAB GTPases consist of evolutionarily conserved and plant-unique factors, which are involved in various membrane trafficking events in plant cells in ways unique to plants. In this review, we summarize recent findings on the functions of endosomal RAB GTPases that underwent unique diversification during plant evolution, with a special focus on RAB5/RABF and RAB11/RABA.

Interactions between Transport Protein Particle (TRAPP) complexes and Rab GTPases in Arabidopsis

Transport Protein Particle II (TRAPPII) is essential for exocytosis, endocytosis, protein sorting and cytokinesis. In spite of a considerable understanding of its biological role, little information is known about Arabidopsis TRAPPII complex topology and molecular function. In this study, independent proteomic approaches initiated with TRAPP components or Rab-A GTPase variants converge on the TRAPPII complex. We show that the Arabidopsis genome encodes the full complement of 13 TRAPPC subunits, including four previously unidentified components. A dimerization model is proposed to account for binary interactions between TRAPPII subunits. Preferential binding to dominant negative (GDP-bound) versus wild-type or constitutively active (GTP-bound) RAB-A2a variants discriminates between TRAPPII and TRAPPIII subunits and shows that Arabidopsis complexes differ from yeast but resemble metazoan TRAPP complexes. Analyzes of Rab-A mutant variants in trappii backgrounds provide genetic evidence that TRAPPII functions upstream of RAB-A2a, allowing us to propose that TRAPPII is likely to behave as a guanine nucleotide exchange factor (GEF) for the RAB-A2a GTPase. GEFs catalyze exchange of GDP for GTP; the GTP-bound, activated, Rab then recruits a diverse local network of Rab effectors to specify membrane identity in subsequent vesicle fusion events. Understanding GEF-Rab interactions will be crucial to unravel the co-ordination of plant membrane traffic.

Two mechanisms regulate directional cell growth in Arabidopsis lateral roots

Morphogenesis in plants depends critically on directional (anisotropic) growth. This occurs principally perpendicular to the net orientation of cellulose microfibrils (CMFs), which is in turn controlled by cortical microtubules (CMTs). In young lateral roots of Arabidopsis thaliana, growth anisotropy also depends on RAB-A5c, a plant-specific small GTPase that specifies a membrane trafficking pathway to the geometric edges of cells. Here we investigate the functional relationship between structural anisotropy at faces and RAB-A5c activity at edges during lateral root development. We show that surprisingly, inhibition of RAB-A5c function is associated with increased CMT/CMF anisotropy. We present genetic, pharmacological, and modelling evidence that this increase in CMT/CMF anisotropy partially compensates for loss of an independent RAB-A5c-mediated mechanism that maintains anisotropic growth in meristematic cells. We show that RAB-A5c associates with CMTs at cell edges, indicating that CMTs act as an integration point for both mechanisms controlling cellular growth anisotropy in lateral roots.

Universal Methods for Transgene Induction Using the Dexamethasone-Inducible Transcription Activation System pOp6/LhGR in Arabidopsis and Other Plant Species

Use of chemically inducible systems for transgene expression is a crucial requirement for modern plant biology research, as it allows (1) expression of transgenes that compromise plant viability or fertility when constitutively expressed and (2) spatiotemporal control of transgene expression levels. We describe the stringently regulated and highly responsive dexamethasone-inducible gene expression system pOp6/LhGR, which comprises the chimeric transcription activator LhGR and the corresponding pOp6 promoter. Upon induction, the LhGR activator binds to the pOp6 promoter and induces expression of the target gene of interest. We provide detailed protocols for inducing transgene expression at different developmental stages and in different plant species and discuss dexamethasone stability and use of its analogs. We also introduce new, versatile, GATEWAY-compatible binary vectors that are now available for the pOp6/LhGR system. © 2019 by John Wiley & Sons, Inc.

Cellular basis of growth in plants: geometry matters

The growth of individual cells underlies the development of biological forms. In plants, cells are interconnected by rigid walls, fixing their position with respect to one another and generating mechanical feedbacks between cells. Current research is shedding new light on how plant growth is controlled by physical inputs at the level of individual cells and growing tissues. In this review, we discuss recent progress in our understanding of the cellular basis of growth from a biomechanical perspective. We describe the role of the cell wall and turgor pressure in growth and highlight the often-overlooked role of cell geometry in this process. It is becoming apparent that a combination of experimental and theoretical approaches is required to answer new emerging questions in the biomechanics of plant morphogenesis. We summarise how this multidisciplinary approach brings us closer to a unified understanding of the generation of biological forms in plants.

Independent yet overlapping pathways ensure the robustness and responsiveness of trans-Golgi network functions in Arabidopsis

The trans-Golgi-network (TGN) has essential housekeeping functions in secretion, endocytosis and protein sorting, but also more specialized functions in plant development. How the robustness of basal TGN function is ensured while specialized functions are differentially regulated is poorly understood. Here, we investigate two key regulators of TGN structure and function, ECHIDNA and the Transport Protein Particle II (TRAPPII) tethering complex. An analysis of physical, network and genetic interactions suggests that two network communities are implicated in TGN function and that ECHIDNA and TRAPPII belong to distinct yet overlapping pathways. Whereas ECHIDNA and TRAPPII colocalized at the TGN in interphase cells, their localization diverged in dividing cells. Moreover, ECHIDNA and TRAPPII localization patterns were mutually independent. TGN structure, endocytosis and sorting decisions were differentially impacted in echidna and trappii mutants. Our analyses point to a partitioning of specialized TGN functions, with ECHIDNA being required for cell elongation and TRAPPII for cytokinesis. Two independent pathways able to compensate for each other might contribute to the robustness of TGN housekeeping functions and to the responsiveness and fine tuning of its specialized functions.

Meeting report - Cellular gateways: expanding the role of endocytosis in plant development

The occasion of The Company of Biologists' workshop 'Cellular gateways: expanding the role of endocytosis in plant development' on 22-25 April 2018, at Wiston House, an Elizabethan mansion in West Sussex, England, witnessed stimulating and lively discussions on the mechanism and functions of endocytosis in plant cells. The workshop was organized by Jenny Russinova, Daniël Van Damme (both VIB/University of Ghent, Belgium) and Takashi Ueda (National Institute for Basic Biology, Okazaki, Japan), and aimed to bridge the gap in knowledge about the endocytic machinery and its cargos in the plant field.

A Lignin Molecular Brace Controls Precision Processing of Cell Walls Critical for Surface Integrity in Arabidopsis

The cell wall, a defining feature of plants, provides a rigid structure critical for bonding cells together. To overcome this physical constraint, plants must process cell wall linkages during growth and development. However, little is known about the mechanism guiding cell-cell detachment and cell wall remodeling. Here, we identify two neighboring cell types in Arabidopsis that coordinate their activities to control cell wall processing, thereby ensuring precise abscission to discard organs. One cell type produces a honeycomb structure of lignin, which acts as a mechanical "brace" to localize cell wall breakdown and spatially limit abscising cells. The second cell type undergoes transdifferentiation into epidermal cells, forming protective cuticle, demonstrating de novo specification of epidermal cells, previously thought to be restricted to embryogenesis. Loss of the lignin brace leads to inadequate cuticle formation, resulting in surface barrier defects and susceptible to infection. Together, we show how plants precisely accomplish abscission.

RAB GTPases in the Basal Land Plant Marchantia polymorpha

The RAB GTPase is an evolutionarily conserved machinery component of membrane trafficking, which is the fundamental system for cell viability and higher order biological functions. The composition of RAB GTPases in each organism is closely related to the complexity and organization of the membrane trafficking pathway, which has been developed uniquely to realize the organism-specific membrane trafficking system. Comparative genomics has suggested that terrestrialization and/or multicellularization were associated with the expansion of membrane trafficking pathways in green plants, which has yet to be validated in basal land plant lineages. To obtain insight into the diversification of membrane trafficking systems in green plants, we analyzed RAB GTPases encoded in the genome of the liverwort Marchantia polymorpha in a comprehensive manner. We isolated all genes for RAB GTPases in Marchantia and analyzed their expression patterns and subcellular localizations in thallus cells. While a majority of MpRAB GTPases exhibited a ubiquitous expression pattern, specific exceptions were also observed; MpRAB2b, which contains a sequence similar to an intraflagellar transport protein at the C-terminal region; and MpRAB23, which has been secondarily lost in angiosperms, were specifically expressed in the male reproductive organ. MpRAB21, which is another RAB GTPase whose homolog is absent in Arabidopsis, exhibited endosomal localization with RAB5 members in Marchantia. These results suggest that Marchantia possesses unique membrane trafficking pathways involving a unique repertoire of RAB GTPases.

Seeing the endomembrane system for the trees: Evolutionary analysis highlights the importance of plants as models for eukaryotic membrane-trafficking

Plant cells show many signs of a unique evolutionary history. This is seen in the system of intracellular organelles and vesicle transport pathways plants use to traffic molecular cargo. Bioinformatic and cell biological work in this area is beginning to tackle the question of how plant cells have evolved, and what this tells us about the evolution of other eukaryotes. Key protein families with membrane trafficking function, including Rabs, SNAREs, vesicle coat proteins, and ArfGAPs, show patterns of evolution that indicate both specialization and conservation in plants. These changes are accompanied by changes at the level of organelles and trafficking pathways between them. Major specializations include losses of several ancient Rabs, novel functions of many proteins, and apparent modification of trafficking in endocytosis and cytokinesis. Nevertheless, plants show extensive conservation of ancestral membrane trafficking genes, and conservation of their ancestral function in most duplicates. Moreover, plants have retained several ancient membrane trafficking genes lost in the evolution of animals and fungi. Considering this, plants such as Arabidopsis are highly valuable for investigating not only plant-specific aspects of membrane trafficking, but also general eukaryotic mechanisms.

Exocytic trafficking pathways in plants: why and how they are redirected

The membrane trafficking system is responsible for precise transportation and localization of proteins, lipids, and polysaccharides among single membrane-bound organelles, the plasma membrane, and the extracellular space. While the exocytic trafficking pathway is considered to be a default transport pathway in many organisms, including land plants, research has shown that evolutionary processes led to an increase in the number of machinery components involved in the plant exocytic pathway. This study provides an overview of the diversification of exocytic trafficking pathways in plants, which mediate the formation and maintenance of cell polarity, interaction with symbiotic and pathogenic microbes, and cytokinesis. To fulfill these functions, distinct strategies have been employed to reroute secretory/exocytic transport during land plant evolution.

Root Cell-Specific Regulators of Phosphate-Dependent Growth

Cellular specialization in abiotic stress responses is an important regulatory feature driving plant acclimation. Our in silico approach of iterative coexpression, interaction, and enrichment analyses predicted root cell-specific regulators of phosphate starvation response networks in Arabidopsis (Arabidopsis thaliana). This included three uncharacterized genes termed Phosphate starvation-induced gene interacting Root Cell Enriched (PRCE1, PRCE2, and PRCE3). Root cell-specific enrichment of 12 candidates was confirmed in promoter-GFP lines. T-DNA insertion lines of 11 genes showed changes in phosphate status and growth responses to phosphate availability compared with the wild type. Some mutants (cbl1, cipk2, prce3, and wdd1) displayed strong biomass gain irrespective of phosphate supply, while others (cipk14, mfs1, prce1, prce2, and s6k2) were able to sustain growth under low phosphate supply better than the wild type. Notably, root or shoot phosphate accumulation did not strictly correlate with organ growth. Mutant response patterns markedly differed from those of master regulators of phosphate homeostasis, PHOSPHATE STARVATION RESPONSE1 (PHR1) and PHOSPHATE2 (PHO2), demonstrating that negative growth responses in the latter can be overcome when cell-specific regulators are targeted. RNA sequencing analysis highlighted the transcriptomic plasticity in these mutants and revealed PHR1-dependent and -independent regulatory circuits with gene coexpression profiles that were highly correlated to the quantified physiological traits. The results demonstrate how in silico prediction of cell-specific, stress-responsive genes uncovers key regulators and how their manipulation can have positive impacts on plant growth under abiotic stress.

A Simple Chamber for Long-term Confocal Imaging of Root and Hypocotyl Development

Several aspects of plant development, such as lateral root morphogenesis, occur on time spans of several days. To study underlying cellular and subcellular processes, high resolution time-lapse microscopy strategies that preserve physiological conditions are required. Plant tissues must have adequate nutrient and water supply with sustained gaseous exchange but, when submerged and immobilized under a coverslip, they are particularly susceptible to anoxia. One strategy that has been successfully employed is the use of a perfusion system to maintain a constant supply of oxygen and nutrients. However, such arrangements can be complicated, cumbersome, and require specialized equipment. Presented here is an alternative strategy for a simple imaging system using perfluorodecalin as an immersion medium. This system is easy to set up, requires minimal equipment, and is easily mounted on a microscope stage, allowing several imaging chambers to be set up and imaged in parallel. In this system, lateral root growth rates are indistinguishable from growth rates under standard conditions on agar plates for the first two days, and lateral root growth continues at reduced rates for at least another day. Plant tissues are supplied with nutrients via an agar slab that can be used also to administer a range of pharmacological compounds. The system was established to monitor lateral root development but is readily adaptable to image other plant organs such as hypocotyls and primary roots.

Coordination of plant cell growth and division: collective control or mutual agreement?

Plant tissue growth requires the interdependent cellular processes of cytoplasmic growth, cell wall extension and cell division, but the feedbacks that link these processes are poorly understood. Recent papers have revealed developmentally regulated coupling between plant cell growth and progression through both mitotic cycles and endocycles. Modeling has given insight into the effects of cell geometry and tissue mechanics on the orientation of cell divisions. Developmental inputs by auxin have been highlighted in the control of cell turgor, vacuole function and the microtubule dynamics that underlies oriented growth and division. Overall, recent work emphasizes growth and proliferation as processes that are negotiated within and between cells, rather than imposed on cells across tissues.

Subcellular localizations of Arabidopsis myotubularins MTM1 and MTM2 suggest possible functions in vesicular trafficking between ER and cis-Golgi

The two Arabidopsis genes AtMTM1 and AtMTM2 encode highly similar phosphoinositide 3-phosphatases from the myotubularin family. Despite the high-level conservation of structure and biochemical activities, their physiological roles have significantly diverged. The nature of a membrane and the concentrations of their membrane-anchored substrates (PtdIns3P or PtdIns3,5P2) and/or products (PtdIns5P and PtdIns) are considered critical for determining the functional specificity of myotubularins. We have performed comprehensive analyses of the subcellular localization of AtMTM1 and AtMTM2 using a variety of specific constructs transiently expressed in Nicotiana benthamiana leaf epidermal cells under the control of 35S promoter. AtMTM1 co-localized preferentially with cis-Golgi membranes, while AtMTM2 associated predominantly with ER membranes. In a stark contrast with animal/human MTMs, neither AtMTM1 nor AtMTM2 co-localizes with early or late endosomes or with TGN/EE compartments, making them unlikely participants in the endosomal trafficking system. Localization of the AtMTM2 is sensitive to cold and osmotic stress challenges. In contrast to animal myotubularins, Arabidopsis myotubularins do not associate with endosomes. Our results suggest that Arabidopsis myotubularins play a role in the vesicular trafficking between ER exit sites and cis-Golgi elements. The significance of these results is discussed also in the context of stress biology and plant autophagy.

Plant Cell Shape: Trafficking Gets Edgy

Polyhedral-shaped plant cells have faces, corners, and edges that can have different material properties. As Kirchhelle et al. (2016) now show, RAB-A5c reveals a trafficking compartment that localizes to the edges where two cell walls meet, with a potential role in mediating local wall stiffness.

ENDOSOMAL RAB EFFECTOR WITH PX-DOMAIN, an Interacting Partner of RAB5 GTPases, Regulates Membrane Trafficking to Protein Storage Vacuoles in Arabidopsis

RAB5 GTPases act as molecular switches that regulate various endosomal functions in animal cells, including homotypic fusion of early endosomes, endosomal motility, endosomal signaling, and subcompartmentalization of the endosomal membrane. RAB5 proteins fulfill these diverse functions through interactions with downstream effector molecules. Two canonical RAB5 members, ARA7 and RAB HOMOLOG1 (RHA1), are encoded in the Arabidopsis thaliana genome. ARA7 and RHA1 play crucial roles in endocytic and vacuolar trafficking pathways. Plant RAB5 GTPases function via interactions with effector molecules, whose identities and functions are currently unclear. In this study, we searched for canonical RAB5 effector molecules of Arabidopsis and identified a candidate, which we called ENDOSOMAL RAB EFFECTOR WITH PX-DOMAIN (EREX). The intimate genetic interaction between EREX and RAB5 members, the results from subcellular colocalization experiments, and the direct interaction observed in an in vitro pull-down assay strongly suggest that EREX is a genuine effector of canonical RAB5s in Arabidopsis. We further found that close homologs of EREX play partially redundant functions with EREX in the transport of seed storage proteins. Our results indicate that canonical plant RAB5s acquired distinct effector molecules from those of non-plant systems to fulfill their functions.