A Functional Study of AUXILIN-LIKE1 and 2, Two Putative Clathrin Uncoating Factors in Arabidopsis

Structural and Evolutionary Aspects of Plant Endocytosis

Endocytosis is an essential eukaryotic process that maintains the homeostasis of the plasma membrane proteome by vesicle-mediated internalization. Its predominant mode of operation utilizes the polymerization of the scaffold protein clathrin forming a coat around the vesicle; therefore, it is termed clathrin-mediated endocytosis (CME). Throughout evolution, the machinery that mediates CME is marked by losses, multiplications, and innovations. CME employs a limited number of conserved structural domains and folds, whose assembly and connections are species dependent. In plants, many of the domains are grouped into an ancient multimeric complex, the TPLATE complex, which occupies a central position as an interaction hub for the endocytic machinery. In this review, we provide an overview of the current knowledge regarding the structural aspects of plant CME, and we draw comparisons to other model systems. To do so, we have taken advantage of recent developments with respect to artificial intelligence-based protein structure prediction.

SH3Ps recruit auxilin-like vesicle uncoating factors for clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is an essential process of cargo uptake operating in all eukaryotes. In animals and yeast, BAR-SH3 domain proteins, endophilins and amphiphysins, function at the conclusion of CME to recruit factors for vesicle scission and uncoating. Arabidopsis thaliana contains the BAR-SH3 domain proteins SH3P1-SH3P3, but their role is poorly understood. Here, we identify SH3Ps as functional homologs of endophilin/amphiphysin. SH3P1-SH3P3 bind to discrete foci at the plasma membrane (PM), and SH3P2 recruits late to a subset of clathrin-coated pits. The SH3P2 PM recruitment pattern is nearly identical to its interactor, a putative uncoating factor, AUXILIN-LIKE1. Notably, SH3P1-SH3P3 are required for most of AUXILIN-LIKE1 recruitment to the PM. This indicates a plant-specific modification of CME, where BAR-SH3 proteins recruit auxilin-like uncoating factors rather than the uncoating phosphatases, synaptojanins. SH3P1-SH3P3 act redundantly in overall CME with the plant-specific endocytic adaptor TPLATE complex but not due to an SH3 domain in its TASH3 subunit.

Ectopic assembly of an auxin efflux control machinery shifts developmental trajectories

Polar auxin transport in the Arabidopsis (Arabidopsis thaliana) root tip maintains high auxin levels around the stem cell niche that gradually decrease in dividing cells but increase again once they transition toward differentiation. Protophloem differentiates earlier than other proximal tissues and employs a unique auxin "canalization" machinery that is thought to balance auxin efflux with retention. It consists of a proposed activator of PIN-FORMED (PIN) auxin efflux carriers, the cAMP-, cGMP- and Calcium-dependent (AGC) kinase PROTEIN KINASE ASSOCIATED WITH BRX (PAX); its inhibitor, BREVIS RADIX (BRX); and PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K) enzymes, which promote polar PAX and BRX localization. Because of a dynamic PAX-BRX-PIP5K interplay, the net cellular output of this machinery remains unclear. In this study, we deciphered the dosage-sensitive regulatory interactions among PAX, BRX, and PIP5K by their ectopic expression in developing xylem vessels. The data suggest that the dominant collective output of the PAX-BRX-PIP5K module is a localized reduction in PIN abundance. This requires PAX-stimulated clathrin-mediated PIN endocytosis upon site-specific phosphorylation, which distinguishes PAX from other AGC kinases. An ectopic assembly of the PAX-BRX-PIP5K module is sufficient to cause cellular auxin retention and affects root growth vigor by accelerating the trajectory of xylem vessel development. Our data thus provide direct evidence that local manipulation of auxin efflux alters the timing of cellular differentiation in the root.

Biomolecular condensation orchestrates clathrin-mediated endocytosis in plants

Clathrin-mediated endocytosis is an essential cellular internalization pathway involving the dynamic assembly of clathrin and accessory proteins to form membrane-bound vesicles. The evolutionarily ancient TSET-TPLATE complex (TPC) plays an essential, but ill-defined role in endocytosis in plants. Here we show that two highly disordered TPC subunits, AtEH1 and AtEH2, function as scaffolds to drive biomolecular condensation of the complex. These condensates specifically nucleate on the plasma membrane through interactions with anionic phospholipids, and facilitate the dynamic recruitment and assembly of clathrin, as well as early- and late-stage endocytic accessory proteins. Importantly, condensation promotes ordered clathrin assemblies. TPC-driven biomolecular condensation thereby facilitates dynamic protein assemblies throughout clathrin-mediated endocytosis. Furthermore, we show that a disordered region of AtEH1 controls the material properties of endocytic condensates in vivo. Alteration of these material properties disturbs the recruitment of accessory proteins, influences endocytosis dynamics and impairs plant responsiveness. Our findings reveal how collective interactions shape endocytosis.

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.

Developmental patterning function of GNOM ARF-GEF mediated from the cell periphery

The GNOM (GN) Guanine nucleotide Exchange Factor for ARF small GTPases (ARF-GEF) is among the best studied trafficking regulators in plants, playing crucial and unique developmental roles in patterning and polarity. The current models place GN at the Golgi apparatus (GA), where it mediates secretion/recycling, and at the plasma membrane (PM) presumably contributing to clathrin-mediated endocytosis (CME). The mechanistic basis of the developmental function of GN, distinct from the other ARF-GEFs including its closest homologue GNOM-LIKE1 (GNL1), remains elusive. Insights from this study largely extend the current notions of GN function. We show that GN, but not GNL1, localizes to the cell periphery at long-lived structures distinct from clathrin-coated pits, while CME and secretion proceed normally in gn knockouts. The functional GN mutant variant GNfewerroots, absent from the GA, suggests that the cell periphery is the major site of GN action responsible for its developmental function. Following inhibition by Brefeldin A, GN, but not GNL1, relocates to the PM likely on exocytic vesicles, suggesting selective molecular associations en route to the cell periphery. A study of GN-GNL1 chimeric ARF-GEFs indicates that all GN domains contribute to the specific GN function in a partially redundant manner. Together, this study offers significant steps toward the elucidation of the mechanism underlying unique cellular and development functions of GNOM.

Analysis of Plant-Specific ANTH Domain-Containing Protein in Marchantia polymorpha

Membrane trafficking is a fundamental mechanism for protein and lipid transport in eukaryotic cells and exhibits marked diversity among eukaryotic lineages with distinctive body plans and lifestyles. Diversification of the membrane trafficking system is associated with the expansion and secondary loss of key machinery components, including RAB GTPases, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and adaptor proteins, during plant evolution. The number of AP180 N-terminal homology (ANTH) proteins, an adaptor family that regulates vesicle formation and cargo sorting during clathrin-mediated endocytosis, increases during plant evolution. In the genome of Arabidopsis thaliana, 18 genes for ANTH proteins have been identified, a higher number than that in yeast and animals, suggesting a distinctive diversification of ANTH proteins. Conversely, the liverwort Marchantia polymorpha possesses a simpler repertoire; only two genes encoding canonical ANTH proteins have been identified in its genome. Intriguingly, a non-canonical ANTH protein is encoded in the genome of M. polymorpha, which also harbors a putative kinase domain. Similar proteins have been detected in sporadic lineages of plants, suggesting their ancient origin and multiple secondary losses during evolution. We named this unique ANTH group phosphatidylinositol-binding clathrin assembly protein-K (PICALM-K) and characterized it in M. polymorpha using genetic, cell biology-based and artificial intelligence (AI)-based approaches. Our results indicate a flagella-related function of MpPICALM-K in spermatozoids, which is distinct from that of canonical ANTH proteins. Therefore, ANTH proteins have undergone significant functional diversification during evolution, and PICALM-K represents a plant-unique ANTH protein that is delivered by neofunctionalization through exon shuffling.

The Arabidopsis PHOSPHATE 1 exporter undergoes constitutive internalization via clathrin-mediated endocytosis

Inorganic phosphate (Pi) homeostasis is essential for plant growth and depends on the transport of Pi across cells. In Arabidopsis thaliana, PHOSPHATE 1 (PHO1) is present in the root pericycle and xylem parenchyma where it exports Pi into the xylem apoplast for its transfer to shoots. PHO1 consists of a cytosolic SPX domain followed by membrane‐spanning α‐helices and ends with the EXS domain, which participates in the steady‐state localization of PHO1 to the Golgi and trans‐Golgi network (TGN). However, PHO1 exports Pi across the plasma membrane (PM), making its localization difficult to reconcile with its function. To investigate whether PHO1 transiently associates with the PM, we inhibited clathrin‐mediated endocytosis (CME) by overexpressing AUXILIN‐LIKE 2 or HUB1. Inhibiting CME resulted in PHO1 re‐localization from the Golgi/TGN to the PM when PHO1 was expressed in Arabidopsis root pericycle or epidermis or Nicotiana benthamiana leaf epidermal cells. A fusion protein between the PHO1 EXS region and GFP was stabilized at the PM by CME inhibition, indicating that the EXS domain plays an important role in sorting PHO1 to/from the PM. PHO1 internalization from the PM occurred independently of AP2 and was not influenced by Pi deficiency, the ubiquitin‐conjugating E2 PHO2, or the potential ubiquitination of cytosolic lysines in the EXS domain. PM‐stabilized PHO1 showed reduced root‐to‐shoot Pi export activity, indicating that CME of PHO1 may be important for its optimal Pi export activity and plant Pi homeostasis.

Phosphorylation of ADAPTOR PROTEIN-2 μ-adaptin by ADAPTOR-ASSOCIATED KINASE1 regulates the tropic growth of Arabidopsis roots

ADAPTOR-ASSOCIATED PROTEIN KINASE1 (AAK1) is a known regulator of clathrin-mediated endocytosis in mammals. Human AAK1 phosphorylates the μ2 subunit of the ADAPTOR PROTEIN-2 (AP-2) complex (AP2M) and plays important roles in cell differentiation and development. Previous interactome studies discovered the association of AAK1 with AP-2 in Arabidopsis (Arabidopsis thaliana), but its function was unclear. Here, genetic analysis revealed that the Arabidopsis aak1 and ap2m mutants both displayed altered root tropic growth, including impaired touch- and gravity-sensing responses. In Arabidopsis, AAK1-phosphorylated AP2M on Thr-163, and expression of the phospho-null version of AP2M in the ap2m mutant led to an aak1-like phenotype, whereas the phospho-mimic forms of AP2M rescued the aak1 mutant. In addition, we found that the AAK1-dependent phosphorylation state of AP2M modulates the frequency distribution of endocytosis. Our data indicate that the phosphorylation of AP2M on Thr-163 by AAK1 fine-tunes endocytosis in the Arabidopsis root to control its tropic growth.

Salicylic acid inhibits rice endocytic protein trafficking mediated by OsPIN3t and clathrin to affect root growth

Salicylic acid (SA) plays important roles in different aspects of plant development, including root growth, where auxin is also a major player by means of its asymmetric distribution. However, the mechanism underlying the effect of SA on the development of rice roots remains poorly understood. Here, we show that SA inhibits rice root growth by interfering with auxin transport associated with the OsPIN3t- and clathrin-mediated gene regulatory network (GRN). SA inhibits root growth as well as Brefeldin A-sensitive trafficking through a non-canonical SA signaling mechanism. Transcriptome analysis of rice seedlings treated with SA revealed that the OsPIN3t auxin transporter is at the center of a GRN involving the coat protein clathrin. The root growth and endocytic trafficking in both the pin3t and clathrin heavy chain mutants were SA insensitivity. SA inhibitory effect on the endocytosis of OsPIN3t was dependent on clathrin; however, the root growth and endocytic trafficking mediated by tyrphostin A23 (TyrA23) were independent of the pin3t mutant under SA treatment. These data reveal that SA affects rice root growth through the convergence of transcriptional and non-SA signaling mechanisms involving OsPIN3t-mediated auxin transport and clathrin-mediated trafficking as key components.

Spatial proteomics of vesicular trafficking: coupling mass spectrometry and imaging approaches in membrane biology

In plants, membrane compartmentalization requires vesicle trafficking for communication among distinct organelles. Membrane proteins involved in vesicle trafficking are highly dynamic and can respond rapidly to changes in the environment and to cellular signals. Capturing their localization and dynamics is thus essential for understanding the mechanisms underlying vesicular trafficking pathways. Quantitative mass spectrometry and imaging approaches allow a system-wide dissection of the vesicular proteome, the characterization of ligand-receptor pairs and the determination of secretory, endocytic, recycling and vacuolar trafficking pathways. In this review, we highlight major proteomics and imaging methods employed to determine the location, distribution and abundance of proteins within given trafficking routes. We focus in particular on methodologies for the elucidation of vesicle protein dynamics and interactions and their connections to downstream signalling outputs. Finally, we assess their biological applications in exploring different cellular and subcellular processes.

Adaptor protein complex interaction map in Arabidopsis identifies P34 as a common stability regulator

Adaptor protein (AP) complexes are evolutionarily conserved vesicle transport regulators that recruit coat proteins, membrane cargoes and coated vesicle accessory proteins. As in plants endocytic and post-Golgi trafficking intersect at the trans-Golgi network, unique mechanisms for sorting cargoes of overlapping vesicular routes are anticipated. The plant AP complexes are part of the sorting machinery, but despite some functional information, their cargoes, accessory proteins and regulation remain largely unknown. Here, by means of various proteomics approaches, we generated the overall interactome of the five AP and the TPLATE complexes in Arabidopsis thaliana. The interactome converged on a number of hub proteins, including the thus far unknown adaptin binding-like protein, designated P34. P34 interacted with the clathrin-associated AP complexes, controlled their stability and, subsequently, influenced clathrin-mediated endocytosis and various post-Golgi trafficking routes. Altogether, the AP interactome network offers substantial resources for further discoveries of unknown endomembrane trafficking regulators in plant cells.

Volleying plasma membrane proteins from birth to death: Role of J-domain proteins

The function, stability, and turnover of plasma membrane (PM) proteins are crucial for cellular homeostasis. Compared to soluble proteins, quality control of plasma membrane proteins is extremely challenging. Failure to meet the high quality control standards is detrimental to cellular and organismal health. J-domain proteins (JDPs) are among the most diverse group of chaperones that collaborate with other chaperones and protein degradation machinery to oversee cellular protein quality control (PQC). Although fragmented, the available literature from different models, including yeast, mammals, and plants, suggests that JDPs assist PM proteins with their synthesis, folding, and trafficking to their destination as well as their degradation, either through endocytic or proteasomal degradation pathways. Moreover, some JDPs interact directly with the membrane to regulate the stability and/or functionality of proteins at the PM. The deconvoluted picture emerging is that PM proteins are relayed from one JDP to another throughout their life cycle, further underscoring the versatility of the Hsp70:JDP machinery in the cell.

The Arabidopsis SAC9 enzyme is enriched in a cortical population of early endosomes and restricts PI(4,5)P2 at the plasma membrane

Membrane lipids, and especially phosphoinositides, are differentially enriched within the eukaryotic endomembrane system. This generates a landmark code by modulating the properties of each membrane. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] specifically accumulates at the plasma membrane in yeast, animal, and plant cells, where it regulates a wide range of cellular processes including endocytic trafficking. However, the functional consequences of mispatterning PI(4,5)P₂ in plants are unknown. Here, we functionally characterized the putative phosphoinositide phosphatase SUPPRESSOR OF ACTIN9 (SAC9) in Arabidopsis thaliana (Arabidopsis). We found that SAC9 depletion led to the ectopic localization of PI(4,5)P₂ on cortical intracellular compartments, which depends on PI4P and PI(4,5)P₂ production at the plasma membrane. SAC9 localizes to a subpopulation of trans-Golgi Network/early endosomes that are enriched in a region close to the cell cortex and that are coated with clathrin. Furthermore, it interacts and colocalizes with Src Homology 3 Domain Protein 2 (SH3P2), a protein involved in endocytic trafficking. In the absence of SAC9, SH3P2 localization is altered and the clathrin-mediated endocytosis rate is reduced. Together, our results highlight the importance of restricting PI(4,5)P₂ at the plasma membrane and illustrate that one of the consequences of PI(4,5)P₂ misspatterning in plants is to impact the endocytic trafficking.

Advances in structural, spatial, and temporal mechanics of plant endocytosis

Endocytic trafficking underlies processes essential for plant growth and development, including the perception of and response to abiotic and extracellular stimuli, post-Golgi and exocytic trafficking, and cytokinesis. Protein adaptors and regulatory factors of clathrin-mediated endocytosis that contribute to the formation of endocytic clathrin-coated vesicles are evolutionarily conserved. Yet, work of the last ten years has identified differences between the endocytic mechanisms of plants and Opisthokonts involving the endocytic adaptor TPLATE complex, the requirement of actin during CME, and the function of clathrin-independent endocytosis in the uptake of plant-specific plasma membrane proteins. Here, we review clathrin-mediated and -independent pathways in plants and describe recent advances enabled by new proteomic and imaging methods, and conditional perturbation of endocytosis. In addition, we summarize the formation and trafficking of clathrin-coated vesicles based on temporal and structural data garnered from high-resolution quantitative imaging studies. Finally, new information about the cross-talk between endocytosis and other endomembrane trafficking pathways and organelles will also be discussed.

Mechanistic insights into an atypical interaction between ATG8 and SH3P2 in Arabidopsis thaliana

In selective macroautophagy/autophagy, cargo receptors are recruited to the forming autophagosome by interacting with Atg8 (autophagy-related 8)-family proteins and facilitate the selective sequestration of specific cargoes for autophagic degradation. In addition, Atg8 interacts with a number of adaptors essential for autophagosome biogenesis, including ATG and non-ATG proteins. The majority of these adaptors and receptors are characterized by an Atg8-family interacting motif (AIM) for binding to Atg8. However, the molecular basis for the interaction mode between ATG8 and regulators or cargo receptors in plants remains largely unclear. In this study, we unveiled an atypical interaction mode for Arabidopsis ATG8f with a plant unique adaptor protein, SH3P2 (SH3 domain-containing protein 2), but not with the other two SH3 proteins. By structure analysis of the unbound form of ATG8f, we identified the unique conformational changes in ATG8f upon binding to the AIM sequence of a plant known autophagic receptor, NBR1. To compare the binding affinity of SH3P2-ATG8f with that of ATG8f-NBR1, we performed a gel filtration assay to show that ubiquitin-associated domain of NBR1 outcompetes the SH3 domain of SH3P2 for ATG8f interaction. Biochemical and cellular analysis revealed that distinct interfaces were employed by ATG8f to interact with NBR1 and SH3P2. Further subcellular analysis showed that the AIM-like motif of SH3P2 is essential for its recruitment to the phagophore membrane but is dispensable for its trafficking in endocytosis. Taken together, our study provides an insightful structural basis for the ATG8 binding specificity toward a plant-specific autophagic adaptor and a conserved autophagic receptor.Abbreviations: ATG, autophagy-related; AIM, Atg8-family interacting motif; BAR, Bin-Amphiphysin-Rvs; BFA, brefeldin A; BTH, benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester; CCV, clathrin-coated-vesicle; CLC2, clathrin light chain 2; Conc A, concanamycin A; ER, endoplasmic reticulum; LDS, LIR docking site; MAP1LC3/LC3, microtubule associated protein 1 light chain 3; LIR, LC3-interacting region; PE, phosphatidylethanolamine; SH3P2, SH3 domain containing protein 2; SH3, Src-Homology-3; UBA, ubiquitin-associated; UIM, ubiquitin-interacting motif.

Proteomic characterization of isolated Arabidopsis clathrin-coated vesicles reveals evolutionarily conserved and plant-specific components

In eukaryotes, clathrin-coated vesicles (CCVs) facilitate the internalization of material from the cell surface as well as the movement of cargo in post-Golgi trafficking pathways. This diversity of functions is partially provided by multiple monomeric and multimeric clathrin adaptor complexes that provide compartment and cargo selectivity. The adaptor-protein assembly polypeptide-1 (AP-1) complex operates as part of the secretory pathway at the trans-Golgi network (TGN), while the AP-2 complex and the TPLATE complex jointly operate at the plasma membrane to execute clathrin-mediated endocytosis. Key to our further understanding of clathrin-mediated trafficking in plants will be the comprehensive identification and characterization of the network of evolutionarily conserved and plant-specific core and accessory machinery involved in the formation and targeting of CCVs. To facilitate these studies, we have analyzed the proteome of enriched TGN/early endosome-derived and endocytic CCVs isolated from dividing and expanding suspension-cultured Arabidopsis (Arabidopsis thaliana) cells. Tandem mass spectrometry analysis results were validated by differential chemical labeling experiments to identify proteins co-enriching with CCVs. Proteins enriched in CCVs included previously characterized CCV components and cargos such as the vacuolar sorting receptors in addition to conserved and plant-specific components whose function in clathrin-mediated trafficking has not been previously defined. Notably, in addition to AP-1 and AP-2, all subunits of the AP-4 complex, but not AP-3 or AP-5, were found to be in high abundance in the CCV proteome. The association of AP-4 with suspension-cultured Arabidopsis CCVs is further supported via additional biochemical data.

Proteomic characterization of isolated Arabidopsis clathrin-coated vesicles reveals evolutionarily conserved and plant-specific components

In eukaryotes, clathrin-coated vesicles (CCVs) facilitate the internalization of material from the cell surface as well as the movement of cargo in post-Golgi trafficking pathways. This diversity of functions is partially provided by multiple monomeric and multimeric clathrin adaptor complexes that provide compartment and cargo selectivity. The adaptor-protein assembly polypeptide-1 (AP-1) complex operates as part of the secretory pathway at the trans-Golgi network (TGN), while the AP-2 complex and the TPLATE complex jointly operate at the plasma membrane to execute clathrin-mediated endocytosis. Key to our further understanding of clathrin-mediated trafficking in plants will be the comprehensive identification and characterization of the network of evolutionarily conserved and plant-specific core and accessory machinery involved in the formation and targeting of CCVs. To facilitate these studies, we have analyzed the proteome of enriched TGN/early endosome-derived and endocytic CCVs isolated from dividing and expanding suspension-cultured Arabidopsis (Arabidopsis thaliana) cells. Tandem mass spectrometry analysis results were validated by differential chemical labeling experiments to identify proteins co-enriching with CCVs. Proteins enriched in CCVs included previously characterized CCV components and cargos such as the vacuolar sorting receptors in addition to conserved and plant-specific components whose function in clathrin-mediated trafficking has not been previously defined. Notably, in addition to AP-1 and AP-2, all subunits of the AP-4 complex, but not AP-3 or AP-5, were found to be in high abundance in the CCV proteome. The association of AP-4 with suspension-cultured Arabidopsis CCVs is further supported via additional biochemical data.

Membrane trafficking functions of the ANTH/ENTH/VHS domain-containing proteins in plants

Subcellular localization of proteins acting on the endomembrane system is primarily regulated via membrane trafficking. To obtain and maintain the correct protein composition of the plasma membrane and membrane-bound organelles, the loading of selected cargos into transport vesicles is critically regulated at donor compartments by adaptor proteins binding to the donor membrane, the cargo molecules, and the coat-protein complexes, including the clathrin coat. The ANTH/ENTH/VHS domain-containing protein superfamily generally comprises a structurally related ENTH, ANTH, or VHS domain in the N-terminal region and a variable C-terminal region, which is thought to act as an adaptor during transport vesicle formation. This protein family is involved in various plant processes, including pollen tube growth, abiotic stress response, and development. In this review, we provide an overview of the recent findings on ANTH/ENTH/VHS domain-containing proteins in plants.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

Fine Mapping and Candidate Gene Identification for the CapUp Locus Controlling Fruit Orientation in Pepper (Capsicum spp.)

The orientation of fruits is a distinguishing morphological feature of pepper (Capsicum spp.) varieties. The pendent (downward curved) growth of the fruit stalks, known as pedicels, is highly correlated with fruit weight and pedicel length. A previous genetic analysis revealed that the pendent fruit orientation is governed by a dominant gene, and incomplete inheritance is also observed in some Capsicum accessions. To identify and localize this gene, a single quantitative trait locus (QTL) analysis was performed on one F₂ and two recombinant inbred line (RIL) populations, and a genome-wide association study (GWAS) was performed using a core collection. Common QTL regions associated with fruit orientation were detected on chromosome 12. A total of 187,966 SNPs were identified in a genotyping-by-sequencing (GBS) for GWAS analysis of 196 Capsicum annuum, 25 Capsicum baccatum, 21 Capsicum chinense, and 14 Capsicum frutescens accessions, representing the germplasm collection of South Korea. The results of these analyses enabled us to narrow down the CapUp region of interest to 200-250 Mbp on chromosome 12. Seven candidate genes were found to be located between two markers that were completely cosegregated with the fruit orientation phenotype. The findings and markers developed in this study will be helpful for additional understanding of pepper fruit development and breeding for fruit orientation.

Systematic analysis of specific and nonspecific auxin effects on endocytosis and trafficking

The phytohormone auxin and its directional transport through tissues are intensively studied. However, a mechanistic understanding of auxin-mediated feedback on endocytosis and polar distribution of PIN auxin transporters remains limited due to contradictory observations and interpretations. Here, we used state-of-the-art methods to reexamine the auxin effects on PIN endocytic trafficking. We used high auxin concentrations or longer treatments versus lower concentrations and shorter treatments of natural indole-3-acetic acid (IAA) and synthetic naphthalene acetic acid (NAA) auxins to distinguish between specific and nonspecific effects. Longer treatments of both auxins interfere with Brefeldin A-mediated intracellular PIN2 accumulation and also with general aggregation of endomembrane compartments. NAA treatment decreased the internalization of the endocytic tracer dye, FM4-64; however, NAA treatment also affected the number, distribution, and compartment identity of the early endosome/trans-Golgi network, rendering the FM4-64 endocytic assays at high NAA concentrations unreliable. To circumvent these nonspecific effects of NAA and IAA affecting the endomembrane system, we opted for alternative approaches visualizing the endocytic events directly at the plasma membrane (PM). Using total internal reflection fluorescence microscopy, we saw no significant effects of IAA or NAA treatments on the incidence and dynamics of clathrin foci, implying that these treatments do not affect the overall endocytosis rate. However, both NAA and IAA at low concentrations rapidly and specifically promoted endocytosis of photo-converted PIN2 from the PM. These analyses identify a specific effect of NAA and IAA on PIN2 endocytosis, thus, contributing to its polarity maintenance and furthermore illustrate that high auxin levels have nonspecific effects on trafficking and endomembrane compartments.

Manganese triggers phosphorylation-mediated endocytosis of the Arabidopsis metal transporter NRAMP1

The NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1) transporter guarantees plant survival of manganese (Mn) deficiency by mediating Mn entry into root cells. Unlike other high-affinity metal transporters, NRAMP1 is only slightly regulated at the transcriptional level. We show here that adequate Mn content in tissues is safeguarded through a tight control of the quantity of NRAMP1 present at the surface of root cells. Depending on Mn availability, an NRAMP1-GFP fusion protein cycles dynamically between the plasma membrane (PM) and endosomal compartments. This involves a clathrin-mediated endocytosis pathway, as disrupting this pathway in auxilin-overexpressor lines prevents NRAMP1 internalization. Mutation of the phosphorylated serine residues 20, 22 and 24 in the cytosol-exposed N terminus of NRAMP1 alters its membrane distribution. Indeed, a phospho-dead mutation stabilizes NRAMP1 at the PM, regardless of the Mn regime, and dramatically reduces plant tolerance to Mn toxicity. Conversely a phosphomimetic mutant is constitutively internalized into endosomes. Together, these data establish that phosphorylation of NRAMP1 is the trigger for its Mn-induced endocytosis and represents the main level of regulation of this transporter. Furthermore, the extent of Mn toxicity observed when interrupting NRAMP1 membrane cycling undermines the dogma that Mn is only marginally toxic to plants.

Visualizing protein-protein interactions in plants by rapamycin-dependent delocalization

Identifying protein-protein interactions (PPIs) is crucial for understanding biological processes. Many PPI tools are available, yet only some function within the context of a plant cell. Narrowing down even further, only a few tools allow complex multi-protein interactions to be visualized. Here, we present a conditional in vivo PPI tool for plant research that meets these criteria. Knocksideways in plants (KSP) is based on the ability of rapamycin to alter the localization of a bait protein and its interactors via the heterodimerization of FKBP and FRB domains. KSP is inherently free from many limitations of other PPI systems. This in vivo tool does not require spatial proximity of the bait and prey fluorophores and it is compatible with a broad range of fluorophores. KSP is also a conditional tool and therefore the visualization of the proteins in the absence of rapamycin acts as an internal control. We used KSP to confirm previously identified interactions in Nicotiana benthamiana leaf epidermal cells. Furthermore, the scripts that we generated allow the interactions to be quantified at high throughput. Finally, we demonstrate that KSP can easily be used to visualize complex multi-protein interactions. KSP is therefore a versatile tool with unique characteristics and applications that complements other plant PPI methods.

The Sterol Trafficking Pathway in Arabidopsis thaliana

In plants, the trafficking mechanisms by which sterols move through the plant and into target cells are unknown. Earlier studies identified endosomes as primary candidates for internalization of sterols in plants, but these results have come into question. Here, we show that in elongating root cells, the internalization of sterol occurs primarily by a non-endocytic mechanism. Added fluorescent sterols [dehydroergosterol (DHE) and BODIPY-cholesterol (BCh)] do not initially label endosomes identified by fluorescent protein markers or by internalized FM4-64. Instead, the nuclear envelope, an organelle not associated with the endocytic pathway but part of the endoplasmic reticulum (ER), becomes labeled. This result is supported by experiments with the inducible overexpression of auxilin-2-like protein (AUX2 line), which blocks most endocytosis upon induction. Internalization and nuclear envelope labeling still occur in induced AUX2 cells. Longer-term incubation labels the oil body, a site involved in sterol storage. Although the first site of localization, the nuclear envelope, is part of the ER, other domains of the ER do not accumulate the label. The trafficking pathway differs from vesicular endocytosis and points toward a different pathway of sterol transport possibly involving other mechanisms, such as ER-plasma membrane contact sites and cytoplasmic transport.

A kinetic view of clathrin assembly and endocytic cargo sorting

Specificity and sensitivity in biochemical reactions can be achieved through regulation of equilibrium binding affinity or through proofreading mechanisms that allow for the dissociation of unwanted intermediates. In this essay, we aim to provide our perspectives on how the concept of kinetic proofreading might apply in the context of cargo sorting in clathrin-mediated endocytosis.

Conditional destabilization of the TPLATE complex impairs endocytic internalization

In plants, endocytosis is essential for many developmental and physiological processes, including regulation of growth and development, hormone perception, nutrient uptake, and defense against pathogens. Our toolbox to modulate this process is, however, rather limited. Here, we report a conditional tool to impair endocytosis. We generated a partially functional TPLATE allele by substituting the most conserved domain of the TPLATE subunit of the endocytic TPLATE complex (TPC). This substitution destabilizes TPC and dampens the efficiency of endocytosis. Short-term heat treatment increases TPC destabilization and reversibly delocalizes TPLATE from the plasma membrane to aggregates in the cytoplasm. This blocks FM uptake and causes accumulation of various known endocytic cargoes at the plasma membrane. Short-term heat treatment therefore transforms the partially functional TPLATE allele into an effective conditional tool to impair endocytosis. Next to their role in endocytosis, several TPC subunits are also implicated in actin-regulated autophagosomal degradation. Inactivating TPC via the WDX mutation, however, does not impair autophagy, thus enabling specific and reversible modulation of endocytosis in planta.

Attracted to membranes: lipid-binding domains in plants

Membranes are essential for cells and organelles to function. As membranes are impermeable to most polar and charged molecules, they provide electrochemical energy to transport molecules across and create compartmentalized microenvironments for specific enzymatic and cellular processes. Membranes are also responsible for guided transport of cargoes between organelles and during endo- and exocytosis. In addition, membranes play key roles in cell signaling by hosting receptors and signal transducers and as substrates and products of lipid second messengers. Anionic lipids and their specific interaction with target proteins play an essential role in these processes, which are facilitated by specific lipid-binding domains. Protein crystallography, lipid-binding studies, subcellular localization analyses, and computer modeling have greatly advanced our knowledge over the years of how these domains achieve precision binding and what their function is in signaling and membrane trafficking, as well as in plant development and stress acclimation.

Nanobody-Dependent Delocalization of Endocytic Machinery in Arabidopsis Root Cells Dampens Their Internalization Capacity

Plant cells perceive and adapt to an ever-changing environment by modifying their plasma membrane (PM) proteome. Whereas secretion deposits new integral membrane proteins, internalization by endocytosis removes membrane proteins and associated ligands, largely with the aid of adaptor protein (AP) complexes and the scaffolding molecule clathrin. Two AP complexes function in clathrin-mediated endocytosis at the PM in plant cells, the heterotetrameric AP-2 complex and the hetero-octameric TPLATE complex (TPC). Whereas single subunit mutants in AP-2 develop into viable plants, genetic mutation of a single TPC subunit causes fully penetrant male sterility and silencing single subunits leads to seedling lethality. To address TPC function in somatic root cells, while minimizing indirect effects on plant growth, we employed nanobody-dependent delocalization of a functional, GFP-tagged TPC subunit, TML, in its respective homozygous genetic mutant background. In order to decrease the amount of functional TPC at the PM, we targeted our nanobody construct to the mitochondria and fused it to TagBFP2 to visualize it independently of its bait. We furthermore limited the effect of our delocalization to those tissues that are easily accessible for live-cell imaging by expressing it from the PIN2 promoter, which is active in root epidermal and cortex cells. With this approach, we successfully delocalized TML from the PM. Moreover, we also show co-recruitment of TML-GFP and AP2A1-TagRFP to the mitochondria, suggesting that our approach delocalized complexes, rather than individual adaptor complex subunits. In line with the specific expression domain, we only observed minor effects on root growth, yet realized a clear reduction of endocytic flux in epidermal root cells. Nanobody-dependent delocalization in plants, here exemplified using a TPC subunit, has the potential to be widely applicable to achieve specific loss-of-function analysis of otherwise lethal mutants.

AGC kinases and MAB4/MEL proteins maintain PIN polarity by limiting lateral diffusion in plant cells

Polar subcellular localization of the PIN exporters of the phytohormone auxin is a key determinant of directional, intercellular auxin transport and thus a central topic of both plant cell and developmental biology. Arabidopsis mutants lacking PID, a kinase that phosphorylates PINs, or the MAB4/MEL proteins of unknown molecular function display PIN polarity defects and phenocopy pin mutants, but mechanistic insights into how these factors convey PIN polarity are missing. Here, by combining protein biochemistry with quantitative live-cell imaging, we demonstrate that PINs, MAB4/MELs, and AGC kinases interact in the same complex at the plasma membrane. MAB4/MELs are recruited to the plasma membrane by the PINs and in concert with the AGC kinases maintain PIN polarity through limiting lateral diffusion-based escape of PINs from the polar domain. The PIN-MAB4/MEL-PID protein complex has self-reinforcing properties thanks to positive feedback between AGC kinase-mediated PIN phosphorylation and MAB4/MEL recruitment. We thus uncover the molecular mechanism by which AGC kinases and MAB4/MEL proteins regulate PIN localization and plant development.

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MAB4/MEL proteins are recruited to the plasma membrane by PINs

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PINs, MAB4/MELs, and AGC kinases directly interact in a multiprotein complex

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PIN phosphorylation and MAB4/MEL recruitment form a positive feedback loop

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MAB4/MELs and AGC kinases maintain PIN polarity by limiting PIN lateral diffusion

Induction of pre-chorismate, jasmonate and salicylate pathways by Burkholderia sp. RR18 in peanut seedlings

AIMS

To characterize the mechanisms by which bacteria in the peanut rhizosphere promote plant growth and suppress Aspergillus niger, the fungus that causes collar rot of peanut.

METHODS AND RESULTS

In all, 131 isolates cultured from the peanut rhizosphere were assayed for growth promotion in a seedling germination assay. The most effective isolate, RR18, was identified as Burkholderia sp. by 16S sequencing analysis. RR18 reduced collar rot disease incidence and increased the germination rate and biomass of peanut seeds, and had broad-spectrum antifungal activity. Quantitative analyses showed that RR18 induced long-lasting accumulation of jasmonic acid, salicylic acid and phenols, and triggered the activity of six defence enzymes related to these changes. Comparative proteomic analysis of treated and untreated seedlings revealed a clear induction of four abundant proteins, including a member of the pre-chorismate pathway, a regulator of clathrin-coated vesicles, a transcription factor and a hypothetical protein.

CONCLUSION

Burkholderia sp. RR18 promotes peanut growth and disease resistance, and stably induces two distinct defence pathways associated with systemic resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates that a strain of the Burkholderia cepacia complex can elicit both salicylic- and jasmonic-acid-mediated defences, in addition to having numerous other beneficial properties.

Nanobody-Dependent Delocalization of Endocytic Machinery in Arabidopsis Root Cells Dampens Their Internalization Capacity

Plant cells perceive and adapt to an ever-changing environment by modifying their plasma membrane (PM) proteome. Whereas secretion deposits new integral membrane proteins, internalization by endocytosis removes membrane proteins and associated ligands, largely with the aid of adaptor protein (AP) complexes and the scaffolding molecule clathrin. Two AP complexes function in clathrin-mediated endocytosis at the PM in plant cells, the heterotetrameric AP-2 complex and the hetero-octameric TPLATE complex (TPC). Whereas single subunit mutants in AP-2 develop into viable plants, genetic mutation of a single TPC subunit causes fully penetrant male sterility and silencing single subunits leads to seedling lethality. To address TPC function in somatic root cells, while minimizing indirect effects on plant growth, we employed nanobody-dependent delocalization of a functional, GFP-tagged TPC subunit, TML, in its respective homozygous genetic mutant background. In order to decrease the amount of functional TPC at the PM, we targeted our nanobody construct to the mitochondria and fused it to TagBFP2 to visualize it independently of its bait. We furthermore limited the effect of our delocalization to those tissues that are easily accessible for live-cell imaging by expressing it from the PIN2 promoter, which is active in root epidermal and cortex cells. With this approach, we successfully delocalized TML from the PM. Moreover, we also show co-recruitment of TML-GFP and AP2A1-TagRFP to the mitochondria, suggesting that our approach delocalized complexes, rather than individual adaptor complex subunits. In line with the specific expression domain, we only observed minor effects on root growth, yet realized a clear reduction of endocytic flux in epidermal root cells. Nanobody-dependent delocalization in plants, here exemplified using a TPC subunit, has the potential to be widely applicable to achieve specific loss-of-function analysis of otherwise lethal mutants.

Non-steroidal Anti-inflammatory Drugs Target TWISTED DWARF1-Regulated Actin Dynamics and Auxin Transport-Mediated Plant Development

The widely used non-steroidal anti-inflammatory drugs (NSAIDs) are derivatives of the phytohormone salicylic acid (SA). SA is well known to regulate plant immunity and development, whereas there have been few reports focusing on the effects of NSAIDs in plants. Our studies here reveal that NSAIDs exhibit largely overlapping physiological activities to SA in the model plant Arabidopsis. NSAID treatments lead to shorter and agravitropic primary roots and inhibited lateral root organogenesis. Notably, in addition to the SA-like action, which in roots involves binding to the protein phosphatase 2A (PP2A), NSAIDs also exhibit PP2A-independent effects. Cell biological and biochemical analyses reveal that many NSAIDs bind directly to and inhibit the chaperone activity of TWISTED DWARF1, thereby regulating actin cytoskeleton dynamics and subsequent endosomal trafficking. Our findings uncover an unexpected bioactivity of human pharmaceuticals in plants and provide insights into the molecular mechanism underlying the cellular action of this class of anti-inflammatory compounds.

Endocytosis of BRASSINOSTEROID INSENSITIVE1 Is Partly Driven by a Canonical Tyr-Based Motif

Clathrin-mediated endocytosis (CME) and its core endocytic machinery are evolutionarily conserved across all eukaryotes. In mammals, the heterotetrameric adaptor protein complex-2 (AP-2) sorts plasma membrane (PM) cargoes into vesicles via the recognition of motifs based on Tyr or di-Leu in their cytoplasmic tails. However, in plants, very little is known about how PM proteins are sorted for CME and whether similar motifs are required. In Arabidopsis (Arabidopsis thaliana), the brassinosteroid (BR) receptor BR INSENSITIVE1 (BRI1) undergoes endocytosis, which depends on clathrin and AP-2. Here, we demonstrate that BRI1 binds directly to the medium AP-2 subunit (AP2M). The cytoplasmic domain of BRI1 contains five putative canonical surface-exposed Tyr-based endocytic motifs. The Tyr-to-Phe substitution in Y₈₉₈KAI reduced BRI1 internalization without affecting its kinase activity. Consistently, plants carrying the BRI1^Y898F mutation were hypersensitive to BRs. Our study demonstrates that AP-2-dependent internalization of PM proteins via the recognition of functional Tyr motifs also operates in plants.

Covalent Self-Labeling of Tagged Proteins with Chemical Fluorescent Dyes in BY-2 Cells and Arabidopsis Seedlings

Synthetic chemical fluorescent dyes promise to be useful for many applications in biology. Covalent, targeted labeling, such as with a SNAP-tag, uses synthetic dyes to label specific proteins in vivo for studying processes such as endocytosis or for imaging via super-resolution microscopy. Despite its potential, such chemical tagging has not been used effectively in plants. A major drawback has been the limited knowledge regarding cell wall and membrane permeability of the available synthetic dyes. Of 31 synthetic dyes tested here, 23 were taken up into BY-2 cells, while eight were not. This creates sets of dyes that can serve to measure endocytosis. Three of the dyes that were able to enter the cells, SNAP-tag ligands of diethylaminocoumarin, tetramethylrhodamine, and silicon-rhodamine 647, were used to SNAP-tag α-tubulin. Successful tagging was verified by live cell imaging and visualization of microtubule arrays in interphase and during mitosis in Arabidopsis (Arabidopsis thaliana) seedlings. Fluorescence activation-coupled protein labeling with DRBG-488 was used to observe PIN-FORMED2 (PIN2) endocytosis and delivery to the vacuole as well as preferential delivery of newly synthesized PIN2 to the actively forming cell plate during mitosis. Together, the data demonstrate that specific self-labeling of proteins can be used effectively in plants to study a wide variety of cellular and biological processes.

Experimental toolbox for quantitative evaluation of clathrin-mediated endocytosis in the plant model Arabidopsis

Clathrin-mediated endocytosis (CME) is a crucial cellular process implicated in many aspects of plant growth, development, intra- and intercellular signaling, nutrient uptake and pathogen defense. Despite these significant roles, little is known about the precise molecular details of how CME functions in planta To facilitate the direct quantitative study of plant CME, we review current routinely used methods and present refined, standardized quantitative imaging protocols that allow the detailed characterization of CME at multiple scales in plant tissues. These protocols include: (1) an efficient electron microscopy protocol for the imaging of Arabidopsis CME vesicles in situ, thus providing a method for the detailed characterization of the ultrastructure of clathrin-coated vesicles; (2) a detailed protocol and analysis for quantitative live-cell fluorescence microscopy to precisely examine the temporal interplay of endocytosis components during single CME events; (3) a semi-automated analysis to allow the quantitative characterization of global internalization of cargos in whole plant tissues; and (4) an overview and validation of useful genetic and pharmacological tools to interrogate the molecular mechanisms and function of CME in intact plant samples.This article has an associated First Person interview with the first author of the paper.

Visualising endocytosis in plants: past, present, and future

Chris Hawes had a lively fascination for the immensely complex organisation of the endomembrane system, including the process of endocytosis. This is the method by which eukaryotic cells internalise membrane proteins, lipids, carbohydrates, and cell wall enzymes from the cell surface through membrane bound vesicles. Endocytosis occurs progressively, starting with early membrane deformation, scission, and finally the release of the vesicle into the cytoplasm. Next to secretion, endocytosis allows the cell to control the proteome composition of its inner and outer surface membrane and as such, its communication with the outside world. Whereas endocytosis was initially considered theoretically impossible in plants due to their high turgor pressure, it is now established as essential for plant life. Furthermore, endocytosis remains a highly active field of research, both in yeast, animal, and plant model systems. Over the past three decades, the tools and techniques used to visualise, quantify, and characterise endocytosis have resulted in an increasingly higher spatiotemporal understanding of this process. Here we provide a brief history of plant endocytosis research from the time when Chris Hawes was investigating the process, to the current state-of-the-art in the field. We will end this chapter with a discussion on some promising future developments for plant endocytosis research. LAY DESCRIPTION: Endocytosis is a key process whereby eukaryotic cells can selectively take up membrane proteins, extracellular material and lipids. As this process controls the abundance and protein composition of the plasma membrane, it also controls the communication of the cell with the outside world. Whereas endocytosis was initially considered theoretically impossible in plants due to their high turgor pressure, it is now established as essential for plant life. Today, endocytosis remains a highly active field of research, both in yeast, animal, and plant model systems. Endocytosis was one of the favourite research topics of Chris Hawes, which is why this mini-review is part of the Festschrift issue in his honour. We provide here a brief history of plant endocytosis research from the time when Chris Hawes was investigating the process, to the current state-of-the-art in the field. Over the past three decades, the tools and techniques that were developed to visualise, quantify, and characterise endocytosis have allowed to achieve an increasingly higher spatiotemporal understanding of this process. We end this chapter with a discussion on some promising future developments for plant endocytosis research.

All Roads Lead to Auxin: Post-translational Regulation of Auxin Transport by Multiple Hormonal Pathways

Auxin is a key hormonal regulator, that governs plant growth and development in concert with other hormonal pathways. The unique feature of auxin is its polar, cell-to-cell transport that leads to the formation of local auxin maxima and gradients, which coordinate initiation and patterning of plant organs. The molecular machinery mediating polar auxin transport is one of the important points of interaction with other hormones. Multiple hormonal pathways converge at the regulation of auxin transport and form a regulatory network that integrates various developmental and environmental inputs to steer plant development. In this review, we discuss recent advances in understanding the mechanisms that underlie regulation of polar auxin transport by multiple hormonal pathways. Specifically, we focus on the post-translational mechanisms that contribute to fine-tuning of the abundance and polarity of auxin transporters at the plasma membrane and thereby enable rapid modification of the auxin flow to coordinate plant growth and development.

Arabidopsis Flippases Cooperate with ARF GTPase Exchange Factors to Regulate the Trafficking and Polarity of PIN Auxin Transporters

Cell polarity is a fundamental feature of all multicellular organisms. PIN auxin transporters are important cell polarity markers that play crucial roles in a plethora of developmental processes in plants. Here, to identify components involved in cell polarity establishment and maintenance in plants, we performed a forward genetic screening of PIN2:PIN1-HA;pin2 Arabidopsis (Arabidopsis thaliana) plants, which ectopically express predominantly basally localized PIN1 in root epidermal cells, leading to agravitropic root growth. We identified the regulator of PIN polarity 12 (repp12) mutation, which restored gravitropic root growth and caused a switch in PIN1-HA polarity from the basal to apical side of root epidermal cells. Next Generation Sequencing and complementation experiments established the causative mutation of repp12 as a single amino acid exchange in Aminophospholipid ATPase3 (ALA3), a phospholipid flippase predicted to function in vesicle formation. repp12 and ala3 T-DNA mutants show defects in many auxin-regulated processes, asymmetric auxin distribution, and PIN trafficking. Analysis of quintuple and sextuple mutants confirmed the crucial roles of ALA proteins in regulating plant development as well as PIN trafficking and polarity. Genetic and physical interaction studies revealed that ALA3 functions together with the ADP ribosylation factor GTPase exchange factors GNOM and BIG3 in regulating PIN polarity, trafficking, and auxin-mediated development.

Clathrin-mediated trafficking and PIN trafficking are required for auxin canalization and vascular tissue formation in Arabidopsis

The flexible development of plants is characterized by a high capacity for post-embryonic organ formation and tissue regeneration, processes, which require tightly regulated intercellular communication and coordinated tissue (re-)polarization. The phytohormone auxin, the main driver for these processes, is able to establish polarized auxin transport channels, which are characterized by the expression and polar, subcellular localization of the PIN1 auxin transport proteins. These channels are demarcating the position of future vascular strands necessary for organ formation and tissue regeneration. Major progress has been made in the last years to understand how PINs can change their polarity in different contexts and thus guide auxin flow through the plant. However, it still remains elusive how auxin mediates the establishment of auxin conducting channels and the formation of vascular tissue and which cellular processes are involved. By the means of sophisticated regeneration experiments combined with local auxin applications in Arabidopsis thaliana inflorescence stems we show that (i) PIN subcellular dynamics, (ii) PIN internalization by clathrin-mediated trafficking and (iii) an intact actin cytoskeleton required for post-endocytic trafficking are indispensable for auxin channel formation, de novo vascular formation and vascular regeneration after wounding. These observations provide novel insights into cellular mechanism of coordinated tissue polarization during auxin canalization.

Protein sorting into protein bodies during barley endosperm development is putatively regulated by cytoskeleton members, MVBs and the HvSNF7s

Cereal endosperm is a short-lived tissue adapted for nutrient storage, containing specialized organelles, such as protein bodies (PBs) and protein storage vacuoles (PSVs), for the accumulation of storage proteins. During development, protein trafficking and storage require an extensive reorganization of the endomembrane system. Consequently, endomembrane-modifying proteins will influence the final grain quality and yield. However, little is known about the molecular mechanism underlying endomembrane system remodeling during barley grain development. By using label-free quantitative proteomics profiling, we quantified 1,822 proteins across developing barley grains. Based on proteome annotation and a homology search, 94 proteins associated with the endomembrane system were identified that exhibited significant changes in abundance during grain development. Clustering analysis allowed characterization of three different development phases; notably, integration of proteomics data with in situ subcellular microscopic analyses showed a high abundance of cytoskeleton proteins associated with acidified PBs at the early development stages. Moreover, endosomal sorting complex required for transport (ESCRT)-related proteins and their transcripts are most abundant at early and mid-development. Specifically, multivesicular bodies (MVBs), and the ESCRT-III HvSNF7 proteins are associated with PBs during barley endosperm development. Together our data identified promising targets to be genetically engineered to modulate seed storage protein accumulation that have a growing role in health and nutritional issues.

Evolutionarily unique mechanistic framework of clathrin-mediated endocytosis in plants

In plants, clathrin mediated endocytosis (CME) represents the major route for cargo internalisation from the cell surface. It has been assumed to operate in an evolutionary conserved manner as in yeast and animals. Here we report characterisation of ultrastructure, dynamics and mechanisms of plant CME as allowed by our advancement in electron microscopy and quantitative live imaging techniques. Arabidopsis CME appears to follow the constant curvature model and the bona fide CME population generates vesicles of a predominantly hexagonal-basket type; larger and with faster kinetics than in other models. Contrary to the existing paradigm, actin is dispensable for CME events at the plasma membrane but plays a unique role in collecting endocytic vesicles, sorting of internalised cargos and directional endosome movement that itself actively promote CME events. Internalized vesicles display a strongly delayed and sequential uncoating. These unique features highlight the independent evolution of the plant CME mechanism during the autonomous rise of multicellularity in eukaryotes.

Novel Role of JAC1 in Influencing Photosynthesis, Stomatal Conductance, and Photooxidative Stress Signalling Pathway in Arabidopsis thaliana

Regulation of light absorption under variable light conditions is essential to optimize photosynthetic and acclimatory processes in plants. Light energy absorbed in excess has a damaging effect on chloroplasts and can lead to cell death. Therefore, plants have evolved protective mechanisms against excess excitation energy that include chloroplast accumulation and avoidance responses. One of the proteins involved in facilitating chloroplast movements in Arabidopsis thaliana is the J domain-containing protein required for chloroplast accumulation response 1 (JAC1). The function of JAC1 relates to the chloroplast actin filaments appearance and disappearance. So far, the role of JAC1 was studied mainly in terms of chloroplasts photorelocation. Here, we demonstrate that the function of JAC1 is more complex, since it influences the composition of photosynthetic pigments, the efficiency of photosynthesis, and the CO₂ uptake rate. JAC1 has positive effect on water use efficiency (WUE) by reducing stomatal aperture and water vapor conductance. Importantly, we show that the stomatal aperture regulation is genetically coupled with JAC1 activity. In addition, our data demonstrate that JAC1 is involved in the fine-tuning of H₂O₂ foliar levels, antioxidant enzymes activities and cell death after UV-C photooxidative stress. This work uncovers a novel function for JAC1 in affecting photosynthesis, CO₂ uptake, and photooxidative stress responses.

The Beginning of the End: Initial Steps in the Degradation of Plasma Membrane Proteins

The plasma membrane (PM), as border between the inside and the outside of a cell, is densely packed with proteins involved in the sensing and transmission of internal and external stimuli, as well as transport processes and is therefore vital for plant development as well as quick and accurate responses to the environment. It is consequently not surprising that several regulatory pathways participate in the tight regulation of the spatiotemporal control of PM proteins. Ubiquitination of PM proteins plays a key role in directing their entry into the endo-lysosomal system, serving as a signal for triggering endocytosis and further sorting for degradation. Nevertheless, a uniting picture of the different roles of the respective types of ubiquitination in the consecutive steps of down-regulation of membrane proteins is still missing. The trans-Golgi network (TGN), which acts as an early endosome (EE) in plants receives the endocytosed cargo, and here the decision is made to either recycled back to the PM or further delivered to the vacuole for degradation. A multi-complex machinery, the endosomal sorting complex required for transport (ESCRT), concentrates ubiquitinated proteins and ushers them into the intraluminal vesicles of multi-vesicular bodies (MVBs). Several ESCRTs have ubiquitin binding subunits, which anchor and guide the cargos through the endocytic degradation route. Basic enzymes and the mode of action in the early degradation steps of PM proteins are conserved in eukaryotes, yet many plant unique components exist, which are often essential in this pathway. Thus, deciphering the initial steps in the degradation of ubiquitinated PM proteins, which is the major focus of this review, will greatly contribute to the larger question of how plants mange to fine-tune their responses to their environment.

SH3Ps-Evolution and Diversity of a Family of Proteins Engaged in Plant Cytokinesis

SH3P2 (At4g34660), an Arabidopsis thaliana SH3 and Bin/amphiphysin/Rvs (BAR) domain-containing protein, was reported to have a specific role in cell plate assembly, unlike its paralogs SH3P1 (At1g31440) and SH3P3 (At4g18060). SH3P family members were also predicted to interact with formins—evolutionarily conserved actin nucleators that participate in microtubule organization and in membrane–cytoskeleton interactions. To trace the origin of functional specialization of plant SH3Ps, we performed phylogenetic analysis of SH3P sequences from selected plant lineages. SH3Ps are present in charophytes, liverworts, mosses, lycophytes, gymnosperms, and angiosperms, but not in volvocal algae, suggesting association of these proteins with phragmoplast-, but not phycoplast-based cell division. Separation of three SH3P clades, represented by SH3P1, SH3P2, and SH3P3 of A. thaliana, appears to be a seed plant synapomorphy. In the yeast two hybrid system, Arabidopsis SH3P3, but not SH3P2, binds the FH1 and FH2 domains of the formin FH5 (At5g54650), known to participate in cytokinesis, while an opposite binding specificity was found for the dynamin homolog DRP1A (At5g42080), confirming earlier findings. This suggests that the cytokinetic role of SH3P2 is not due to its interaction with FH5. Possible determinants of interaction specificity of SH3P2 and SH3P3 were identified bioinformatically.

Never Walk Alone: Clathrin-Coated Vesicle (CCV) Components in Plant Immunity

At the host-pathogen interface, the protein composition of the plasma membrane (PM) has important implications for how a plant cell perceives and responds to invading microbial pathogens. A plant's ability to modulate its PM composition is critical for regulating the strength, duration, and integration of immune responses. One mechanism by which plant cells reprogram their cell surface is vesicular trafficking, including secretion and endocytosis. These trafficking processes add or remove cargo proteins (such as pattern-recognition receptors, transporters, and other proteins with immune functions) to or from the PM via small, membrane-bound vesicles. Clathrin-coated vesicles (CCVs) that form at the PM and trans-Golgi network/early endosomes have emerged as the prominent vesicle type in the regulation of plant immune responses. In this review, we discuss the roles of the CCV core, adaptors, and accessory components in plant defense signaling and immunity against various microbial pathogens.

The Nuts and Bolts of PIN Auxin Efflux Carriers

The plant-specific proteins named PIN-FORMED (PIN) efflux carriers facilitate the direction of auxin flow and thus play a vital role in the establishment of local auxin maxima within plant tissues that subsequently guide plant ontogenesis. They are membrane integral proteins with two hydrophobic regions consisting of alpha-helices linked with a hydrophilic loop, which is usually longer for the plasma membrane-localized PINs. The hydrophilic loop harbors molecular cues important for the subcellular localization and thus auxin efflux function of those transporters. The three-dimensional structure of PIN has not been solved yet. However, there are scattered but substantial data concerning the functional characterization of amino acid strings that constitute these carriers. These sequences include motifs vital for vesicular trafficking, residues regulating membrane diffusion, cellular polar localization, and activity of PINs. Here, we summarize those bits of information striving to provide a reference to structural motifs that have been investigated experimentally hoping to stimulate the efforts toward unraveling of PIN structure-function connections.

PIN2 Polarity Establishment in Arabidopsis in the Absence of an Intact Cytoskeleton

Cell polarity is crucial for the coordinated development of all multicellular organisms. In plants, this is exemplified by the PIN-FORMED (PIN) efflux carriers of the phytohormone auxin: The polar subcellular localization of the PINs is instructive to the directional intercellular auxin transport, and thus to a plethora of auxin-regulated growth and developmental processes. Despite its importance, the regulation of PIN polar subcellular localization remains poorly understood. Here, we have employed advanced live-cell imaging techniques to study the roles of microtubules and actin microfilaments in the establishment of apical polar localization of PIN2 in the epidermis of the Arabidopsis root meristem. We report that apical PIN2 polarity requires neither intact actin microfilaments nor microtubules, suggesting that the primary spatial cue for polar PIN distribution is likely independent of cytoskeleton-guided endomembrane trafficking.

Pinstatic Acid Promotes Auxin Transport by Inhibiting PIN Internalization

Polar auxin transport plays a pivotal role in plant growth and development. PIN-FORMED (PIN) auxin efflux carriers regulate directional auxin movement by establishing local auxin maxima, minima, and gradients that drive multiple developmental processes and responses to environmental signals. Auxin has been proposed to modulate its own transport by regulating subcellular PIN trafficking via processes such as clathrin-mediated PIN endocytosis and constitutive recycling. Here, we further investigated the mechanisms by which auxin affects PIN trafficking by screening auxin analogs and identified pinstatic acid (PISA) as a positive modulator of polar auxin transport in Arabidopsis (Arabidopsis thaliana). PISA had an auxin-like effect on hypocotyl elongation and adventitious root formation via positive regulation of auxin transport. PISA did not activate SCF^TIR1/AFB signaling and yet induced PIN accumulation at the cell surface by inhibiting PIN internalization from the plasma membrane. This work demonstrates PISA to be a promising chemical tool to dissect the regulatory mechanisms behind subcellular PIN trafficking and auxin transport.

Mapping and identification of CsUp, a gene encoding an Auxilin-like protein, as a putative candidate gene for the upward-pedicel mutation (up) in cucumber

BACKGROUND

Pedicel orientation can affect the female flower orientation and seed yield in cucumber. A spontaneous mutant possessing upward growth of pedicels was identified in the wild type inbred strain 9930 and named upward-pedicel (up). The morphological and genetic analyses of up were performed in this study. In order to clone the up gene, 933 F₂ individuals and 524 BC₁ individuals derived from C-8-6 (WT) and up were used for map-based cloning.

RESULTS

up was mapped to a 35.2 kb physical interval on chromosome 1, which contains three predicted genes. Sequencing analysis revealed that a 5-bp deletion was found in the second exon of Csa1G535800, and it led to a frameshift mutation resulting in a premature stop codon. The candidate gene of CsUp (Csa1G535800) was further confirmed via genomic and cDNA sequencing in biparental and natural cucumber populations. Sequencing data showed that a 4-bp deletion was found in the sixth exon of Csa1G535800 in CGN19839, another inbred line, and there was also a mutation of an amino acid in Csa1G535800 that could contribute to the upward growth of pedicels in CGN19839. Moreover, it was found that Csa1G535800 exhibited strong expression in the pedicel of WT, suggesting its important role in development of pedicel orientation. Thus, Csa1G535800 was considered to be the candidate gene of CsUp.

CONCLUSIONS

CsUp encodes an Auxilin-like protein and controls pedicel orientation in cucumber. The identification of CsUp may help us to understand the mechanism of pedicel orientation development and allow for investigation of novel functions of Auxilin-like proteins in cucumber.