An Arabidopsis Lipid Flippase Is Required for Timely Recruitment of Defenses to the Host-Pathogen Interface at the Plant Cell Surface

Vesicle trafficking pathways in defence-related cell wall modifications: papillae and encasements

Filamentous pathogens that cause plant diseases such as powdery mildew, rust, anthracnose, and late blight continue to represent an enormous challenge for farmers worldwide. Interestingly, these pathogens, although phylogenetically distant, initiate pathogenesis in a very similar way by penetrating the cell wall and establishing a feeding structure inside the plant host cell. To prevent pathogen ingress, the host cell responds by forming defence structures known as papillae and encasements that are thought to mediate pre- and post-invasive immunity, respectively. This form of defence is evolutionarily conserved in land plants and is highly effective and durable against a broad selection of non-adapted filamentous pathogens. As most pathogens have evolved strategies to overcome the defences of only a limited range of host plants, the papilla/encasement response could hold the potential to become an optimal transfer of resistance from one plant species to another. In this review I lay out current knowledge of the involvement of membrane trafficking that forms these important defence structures and highlight some of the questions that still need to be resolved.

Deficiencies in cluster-2 ALA lipid flippases result in salicylic acid-dependent growth reductions

P4 ATPases (i.e., lipid flippases) are eukaryotic enzymes that transport lipids across membrane bilayers. In plants, P4 ATPases are named Aminophospholipid ATPases (ALAs) and are organized into five phylogenetic clusters. Here we generated an Arabidopsis mutant lacking all five cluster-2 ALAs (ala8/9/10/11/12), which is the most highly expressed ALA subgroup in vegetative tissues. Plants harboring the quintuple knockout (KO) show rosettes that are 2.2-fold smaller and display chlorotic lesions. A similar but less severe phenotype was observed in an ala10/11 double KO. The growth and lesion phenotypes of ala8/9/10/11/12 mutants were reversed by expressing a NahG transgene, which encodes an enzyme that degrades salicylic acid (SA). A role for SA in promoting the lesion phenotype was further supported by quantitative PCR assays showing increased mRNA abundance for an SA-biosynthesis gene ISOCHORISMATE SYNTHASE 1 (ICS1) and two SA-responsive genes PATHOGENESIS-RELATED GENE 1 (PR1) and PR2. Lesion phenotypes were also reversed by growing plants in liquid media containing either low calcium (~0.1 mM) or high nitrogen concentrations (~24 mM), which are conditions known to suppress SA-dependent autoimmunity. Yeast-based fluorescent lipid uptake assays revealed that ALA10 and ALA11 display overlapping substrate specificities, including the transport of LysoPC signaling lipids. Together, these results establish that the biochemical functions of ALA8-12 are at least partially overlapping, and that deficiencies in cluster-2 ALAs result in an SA-dependent autoimmunity phenotype that has not been observed for flippase mutants with deficiencies in other ALA clusters.

Preparing Arabidopsis thaliana root protoplasts for cryo electron tomography

The use of protoplasts in plant biology has become a convenient tool for the application of transient gene expression. This model system has allowed the study of plant responses to biotic and abiotic stresses, protein location and trafficking, cell wall dynamics, and single-cell transcriptomics, among others. Although well-established protocols for isolating protoplasts from different plant tissues are available, they have never been used for studying plant cells using cryo electron microscopy (cryo-EM) and cryo electron tomography (cryo-ET). Here we describe a workflow to prepare root protoplasts from Arabidopsis thaliana plants for cryo-ET. The process includes protoplast isolation and vitrification on EM grids, and cryo-focused ion beam milling (cryo-FIB), with the aim of tilt series acquisition. The whole workflow, from growing the plants to the acquisition of the tilt series, may take a few months. Our protocol provides a novel application to use plant protoplasts as a tool for cryo-ET.

Arabidopsis flippase ALA3 is required for adjustment of early subcellular trafficking in plant response to osmotic stress

To compensate for their sessile lifestyle, plants developed several responses to exogenous changes. One of the previously investigated and not yet fully understood adaptations occurs at the level of early subcellular trafficking, which needs to be rapidly adjusted to maintain cellular homeostasis and membrane integrity under osmotic stress conditions. To form a vesicle, the membrane needs to be deformed, which is ensured by multiple factors, including the activity of specific membrane proteins, such as flippases from the family of P4-ATPases. The membrane pumps actively translocate phospholipids from the exoplasmic/luminal to the cytoplasmic membrane leaflet to generate curvature, which might be coupled with recruitment of proteins involved in vesicle formation at specific sites of the donor membrane. We show that lack of the AMINOPHOSPHOLIPID ATPASE3 (ALA3) flippase activity caused defects at the plasma membrane and trans-Golgi network, resulting in altered endocytosis and secretion, processes relying on vesicle formation and movement. The mentioned cellular defects were translated into decreased intracellular trafficking flexibility failing to adjust the root growth on osmotic stress-eliciting media. In conclusion, we show that ALA3 cooperates with ARF-GEF BIG5/BEN1 and ARF1A1C/BEX1 in a similar regulatory pathway to vesicle formation, and together they are important for plant adaptation to osmotic stress.

Mini-review: Lipid flippases as putative targets for biotechnological crop improvement

An increasing world population and drastic changes in weather conditions are challenging agricultural production. To face these challenges and ensure sustainable food production in the future, crop plants need to be improved to withstand several different biotic and abiotic stresses. Commonly, breeders select varieties that can tolerate a specific type of stress and then cross these varieties to stack beneficial traits. This strategy is time-consuming and strictly dependent on the stacked traits been genetically unlinked. Here, we revise the role of plant lipid flippases of the P4 ATPase family in stress-related responses with a special focus on the pleiotropic nature of their functions and discuss their suitability as biotechnological targets for crop improvement.

Plant transbilayer lipid asymmetry and the role of lipid flippases

Many biological membranes present an asymmetric lipid distribution between the two leaflets that is known as the transbilayer lipid asymmetry. This asymmetry is essential for cell survival and its loss is related to apoptosis. In mammalian and yeast cells, ATP-dependent transport of lipids to the cytosolic side of the biological membranes, carried out by so-called lipid flippases, contributes to the transbilayer lipid asymmetry. Most of these lipid flippases belong to the P4-ATPase protein family, which is also present in plants. In this review, we summarize the relatively scarce literature concerning the presence of transbilayer lipid asymmetry in different plant cell membranes and revise the potential role of lipid flippases of the P4-ATPase family in generation and/or maintenance of this asymmetry.

Arabidopsis GOLD36/MVP1/ERMO3 Is Required for Powdery Mildew Penetration Resistance and Proper Targeting of the PEN3 Transporter

The Arabidopsis PENETRATION 3 (PEN3) ATP binding cassette (ABC) transporter contributes to penetration resistance against nonadapted powdery mildew fungi and is targeted to papillae deposited at sites of interaction with the fungus. Timely recruitment of PEN3 and other components of penetration resistance to the host-pathogen interface is important for successful defense against this biotrophic pathogen. A forward genetic screen was previously carried out to identify Arabidopsis mutants that mistarget the PEN3 transporter or fail to accumulate PEN3 at sites of attempted powdery mildew penetration. This study focuses on PEN3 mistargeting in the aberrant localization of PEN3 4 (alp4) mutant and identification of the causal gene. In the alp4 mutant, PEN3 accumulates within the endomembrane system in an apparently abnormal endoplasmic reticulum and is not exported into papillae at powdery mildew penetration sites. This targeting defect compromises defenses at the host-pathogen interface, resulting in increased penetration success by a nonadapted powdery mildew. Genetic mapping identified alp4 as an allele of GOLGI DEFECTS 36 (GOLD36), a gene encoding a GDSL-lipase/esterase family protein that is involved in maintaining normal morphology and organization of multiple endomembrane compartments. Genetic complementation confirmed that mutation in GOLD36 is responsible for the PEN3 targeting and powdery mildew penetration resistance defects in alp4. These results reinforce the importance of endomembrane trafficking in resistance to haustorium-forming phytopathogens such as powdery mildew fungi.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Polarized NORTIA accumulation in response to pollen tube arrival at synergids promotes fertilization

Signal-mediated regulation of protein trafficking is an elegant mechanism for controlling the delivery of molecules to a precise location for critical signaling events that occur over short time frames. During plant reproduction, the FERONIA receptor complex is critical for intercellular communication that leads to gamete delivery; however, the impact of the FERONIA signal transduction cascade on protein trafficking in synergid cells remains unknown. Live imaging of pollen tube reception has revealed that a key outcome of FERONIA signaling is polar accumulation of the MLO protein NORTIA at the filiform apparatus in response to signals from an arriving pollen tube. Artificial delivery of NORTIA to the filiform apparatus is sufficient to bypass the FERONIA signaling pathway and to promote interspecific pollen tube reception. We propose that polar accumulation of NORTIA leads to the production of a secondary booster signal to ensure that pollen tubes burst to deliver the sperm cells for double fertilization.

Arabidopsis P4 ATPase-mediated cell detoxification confers resistance to Fusarium graminearum and Verticillium dahliae

Many toxic secondary metabolites produced by phytopathogens can subvert host immunity, and some of them are recognized as pathogenicity factors. Fusarium head blight and Verticillium wilt are destructive plant diseases worldwide. Using toxins produced by the causal fungi Fusarium graminearum and Verticillium dahliae as screening agents, here we show that the Arabidopsis P4 ATPases AtALA1 and AtALA7 are responsible for cellular detoxification of mycotoxins. Through AtALA1-/AtALA7-mediated vesicle transport, toxins are sequestered in vacuoles for degradation. Overexpression of AtALA1 and AtALA7 significantly increases the resistance of transgenic plants to F. graminearum and V. dahliae, respectively. Notably, the concentration of deoxynivalenol, a mycotoxin harmful to the health of humans and animals, was decreased in transgenic Arabidopsis siliques and maize seeds. This vesicle-mediated cell detoxification process provides a strategy to increase plant resistance against different toxin-associated diseases and to reduce the mycotoxin contamination in food and feed.

Protoplast: A Valuable Toolbox to Investigate Plant Stress Perception and Response

Plants are constantly facing abiotic and biotic stresses. To continue to thrive in their environment, they have developed many sophisticated mechanisms to perceive these stresses and provide an appropriate response. There are many ways to study these stress signals in plant, and among them, protoplasts appear to provide a unique experimental system. As plant cells devoid of cell wall, protoplasts allow observations at the individual cell level. They also offer a prime access to the plasma membrane and an original view on the inside of the cell. In this regard, protoplasts are particularly useful to address essential biological questions regarding stress response, such as protein signaling, ion fluxes, ROS production, and plasma membrane dynamics. Here, the tools associated with protoplasts to comprehend plant stress signaling are overviewed and their potential to decipher plant defense mechanisms is discussed.

The ARP2/3 complex, acting cooperatively with Class I formins, modulates penetration resistance in Arabidopsis against powdery mildew invasion

The actin cytoskeleton regulates an array of diverse cellular activities that support the establishment of plant-microbe interactions and plays a critical role in the execution of plant immunity. However, molecular and cellular mechanisms regulating the assembly and rearrangement of actin filaments (AFs) at plant-pathogen interaction sites remain largely elusive. Here, using live-cell imaging, we show that one of the earliest cellular responses in Arabidopsis thaliana upon powdery mildew attack is the formation of patch-like AF structures beneath fungal invasion sites. The AFs constituting actin patches undergo rapid turnover, which is regulated by the actin-related protein (ARP)2/3 complex and its activator, the WAVE/SCAR regulatory complex (W/SRC). The focal accumulation of phosphatidylinositol-4,5-bisphosphate at fungal penetration sites appears to be a crucial upstream modulator of the W/SRC-ARP2/3 pathway-mediated actin patch formation. Knockout of W/SRC-ARP2/3 pathway subunits partially compromised penetration resistance with impaired endocytic recycling of the defense-associated t-SNARE protein PEN1 and its deposition into apoplastic papillae. Simultaneously knocking out ARP3 and knocking down the Class I formin (AtFH1) abolished actin patch formation, severely impaired the deposition of cell wall appositions, and promoted powdery mildew entry into host cells. Our results demonstrate that the ARP2/3 complex and formins, two actin-nucleating systems, act cooperatively and contribute to Arabidopsis penetration resistance to fungal invasion.

Lipid flippases as key players in plant adaptation to their environment

Lipid flippases (P4 ATPases) are active transporters that catalyse the translocation of lipids between the two sides of the biological membranes in the secretory pathway. This activity modulates biological membrane properties, contributes to vesicle formation, and is the trigger for lipid signalling events, which makes P4 ATPases essential for eukaryotic cell survival. Plant P4 ATPases (also known as aminophospholipid ATPases (ALAs)) are crucial for plant fertility and proper development, and are involved in key adaptive responses to biotic and abiotic stress, including chilling tolerance, heat adaptation, nutrient deficiency responses and pathogen defence. While ALAs present many analogies to mammalian and yeast P4 ATPases, they also show characteristic features as the result of their independent evolution. In this Review, the main properties, roles, regulation and mechanisms of action of ALA proteins are discussed.

Dynamic membranes: the multiple roles of P4 and P5 ATPases

The lipid bilayer of biological membranes has a complex composition, including high chemical heterogeneity, the presence of nanodomains of specific lipids, and asymmetry with respect to lipid composition between the two membrane leaflets. In membrane trafficking, membrane vesicles constantly bud off from one membrane compartment and fuse with another, and both budding and fusion events have been proposed to require membrane lipid asymmetry. One mechanism for generating asymmetry in lipid bilayers involves the action of the P4 ATPase family of lipid flippases; these are biological pumps that use ATP as an energy source to flip lipids from one leaflet to the other. The model plant Arabidopsis (Arabidopsis thaliana) contains 12 P4 ATPases (AMINOPHOSPHOLIPID ATPASE1-12; ALA1-12), many of which are functionally redundant. Studies of P4 ATPase mutants have confirmed the essential physiological functions of these pumps and pleiotropic mutant phenotypes have been observed, as expected when genes required for basal cellular functions are disrupted. For instance, phenotypes associated with ala3 (dwarfism, pollen defects, sensitivity to pathogens and cold, and reduced polar cell growth) can be related to membrane trafficking problems. P5 ATPases are evolutionarily related to P4 ATPases, and may be the counterpart of P4 ATPases in the endoplasmic reticulum. The absence of P4 and P5 ATPases from prokaryotes and their ubiquitous presence in eukaryotes make these biological pumps a defining feature of eukaryotic cells. Here, we review recent advances in the field of plant P4 and P5 ATPases.

Chemokine-like MDL proteins modulate flowering time and innate immunity in plants

Human macrophage migration inhibitory factor (MIF) is an atypical chemokine implicated in intercellular signaling and innate immunity. MIF orthologs (MIF/D-DT-like proteins, MDLs) are present throughout the plant kingdom, but remain experimentally unexplored in these organisms. Here, we provide an in planta characterization and functional analysis of the three-member gene/protein MDL family in Arabidopsis thaliana. Subcellular localization experiments indicated a nucleo-cytoplasmic distribution of MDL1 and MDL2, while MDL3 is localized to peroxisomes. Protein–protein interaction assays revealed the in vivo formation of MDL1, MDL2, and MDL3 homo-oligomers, as well as the formation of MDL1-MDL2 hetero-oligomers. Functionally, Arabidopsismdl mutants exhibited a delayed transition from vegetative to reproductive growth (flowering) under long-day conditions, but not in a short-day environment. In addition, mdl mutants were more resistant to colonization by the bacterial pathogen Pseudomonas syringae pv. maculicola. The latter phenotype was compromised by the additional mutation of SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2), a gene implicated in the defense-induced biosynthesis of the key signaling molecule salicylic acid. However, the enhanced antibacterial immunity was not associated with any constitutive or pathogen-induced alterations in the levels of characteristic phytohormones or defense-associated metabolites. Interestingly, bacterial infection triggered relocalization and accumulation of MDL1 and MDL2 at the peripheral lobes of leaf epidermal cells. Collectively, our data indicate redundant functionality and a complex interplay between the three chemokine-like Arabidopsis MDL proteins in the regulation of both developmental and immune-related processes. These insights expand the comparative cross-kingdom analysis of MIF/MDL signaling in human and plant systems.

The transport mechanism of P4 ATPase lipid flippases

P4 ATPase lipid flippases are ATP-driven transporters that translocate specific lipids from the exoplasmic to the cytosolic leaflet of biological membranes, thus establishing a lipid gradient between the two leaflets that is essential for many cellular processes. While substrate specificity, subcellular and tissue-specific expression, and physiological functions have been assigned to a number of these transporters in several organisms, the mechanism of lipid transport has been a topic of intense debate in the field. The recent publication of a series of structural models based on X-ray crystallography and cryo-EM studies has provided the first glimpse into how P4 ATPases have adapted the transport mechanism used by the cation-pumping family members to accommodate a substrate that is at least an order of magnitude larger than cations.

A potential pathway for flippase-facilitated glucosylceramide catabolism in plants

The Aminophospholipid ATPase (ALA) family of plant lipid flippases is involved in the selective transport of lipids across membrane bilayers. Recently, we demonstrated that double mutants lacking both ALA4 and -5 are severely dwarfed. Dwarfism in ala4/5 mutants was accompanied by cellular elongation defects and various lipidomic perturbations, including a 1.4-fold increase in the accumulation of glucosylceramides (GlcCers) relative to total sphingolipid content. Here, we present a potential model for flippase-facilitated GlcCer catabolism in plants, where a combination of ALA flippases transport GlcCers to cytosolic membrane surfaces where they are degraded by Glucosylceramidases (GCDs). GCDs remove the glucose headgroup from GlcCers to produce a ceramide (Cer) backbone, which can be further degraded to sphingoid bases (Sphs, e.g, phytosphingosine) and fatty acids (FAs). In the absence of GlcCer-transporting flippases, GlcCers are proposed to accumulate on extracytoplasmic (i.e., apoplastic) or lumenal membrane surfaces. As GlcCers are potential precursors for Sph production, impaired GlcCer catabolism might also result in the decreased production of the secondary messenger Sph-1-phosphate (Sph-1-P, e.g., phytosphingosine-1-P), a regulator of cell turgor. Importantly, we postulate that either GlcCer accumulation or reduced Sph-1-P signaling might contribute to the growth reductions observed in ala4/5 mutants. Similar catabolic pathways have been proposed for humans and yeast, suggesting flippase-facilitated GlcCer catabolism is conserved across eukaryotes.

Protein Profiles of Lipid Droplets during the Hypersensitive Defense Response of Arabidopsis against Pseudomonas Infection

Lipid droplets (LDs) have classically been viewed as seed storage particles, yet they are now emerging as dynamic organelles associated with developmental and stress responses. Nevertheless, their involvement in plant immunity has still been little studied. Here, we found LD accumulation in Arabidopsis thaliana leaves that induced a hypersensitive response (HR) after Pseudomonas infection. We established a protocol to reproducibly isolate LDs and to analyze their protein content. The expression of GFP fusion proteins in Nicotiana benthamiana and in transgenic Arabidopsis lines validated the LD localization of glycerol-3-phosphate acyltransferase 4 (GPAT4) and 8 (GPAT8), required for cutin biosynthesis. Similarly, we showed LD localization of α-dioxygenase1 (α-DOX1) and caleosin3 (CLO3), involved in the synthesis of fatty acid derivatives, and that of phytoalexin-deficient 3 (PAD3), which is involved in camalexin synthesis. We found evidence suggesting the existence of different populations of LDs, with varying protein contents and distributions. GPAT4 and GPAT8 were associated with LDs inside stomata and surrounding cells of untreated leaves, yet they were mainly confined to LDs in guard cells after bacterial inoculation. By contrast, α-DOX1 and PAD3 were associated with LDs in the epidermal cells of HR-responding leaves, with PAD3 mostly restricted to cells near dead tissue, while CLO3 had a more ubiquitous distribution. As such, the nature of the proteins identified, together with the phenotypic examination of selected mutants, suggests that LDs participate in lipid changes and in the production and transport of defense components affecting the interaction of plants with invading pathogens.

The mysterious life of the plant trans-Golgi network: advances and tools to understand it better

By being at the interface of the exocytic and endocytic pathways, the plant trans-Golgi network (TGN) is a multitasking and highly diversified organelle. Despite governing vital cellular processes, the TGN remains one of the most uncharacterized organelle of plant cells. In this review, we highlight recent studies that have contributed new insights and to the generation of markers needed to answer several important questions on the plant TGN. Several drugs specifically affecting proteins critical for the TGN functions have been extremely useful for the identification of mutants of the TGN in the pursuit to understand how the morphology and the function of this organelle are controlled. In addition to these chemical tools, we review emerging microscopy techniques that help visualize the TGN at an unpreceded resolution and appreciate the heterogeneity and dynamics of this organelle in plant cells.

Catch You on the Flip Side: A Critical Review of Flippase Mutant Phenotypes

Lipid flippases are integral membrane proteins that use ATP hydrolysis to power the generation of phospholipid asymmetry between the two leaflets of biological membranes, a process essential for cell survival. Although the first report of a plant lipid flippase was published in 2000, progress in the field has been slow, partially due to the high level of redundancy in this gene family. However, recently an increasing number of reports have examined the physiological function of lipid flippases, mainly in Arabidopsis thaliana. In this review we aim to summarize recent findings on the physiological relevance of lipid flippases in plant adaptation to a changing environment and caution against misinterpretation of pleiotropic effects in genetic studies of flippases.

Could vesicular transport of Na+ and Cl- be a feature of salt tolerance in halophytes?

Background

Halophytes tolerate external salt concentrations of 200 mm and more, accumulating salt concentrations of 500 mm and more in their shoots; some, recretohalophytes, excrete salt through glands on their leaves. Ions are accumulated in central vacuoles, but the pathway taken by these ions from the outside of the roots to the vacuoles inside the cells is poorly understood. Do the ions cross membranes through ion channels and transporters or move in vesicles, or both? Vesicular transport from the plasma membrane to the vacuole would explain how halophytes avoid the toxicity of high salt concentrations on metabolism. There is also a role for vesicles in the export of ions via salt glands.

Scope and Methods

We have collected data on the fluxes of sodium and chloride ions in halophytes, based on the weight of the transporting organs and on the membrane area across which the flux occurs; the latter range from 17 nmol m-2 s-1 to 4.2 μmol m-2 s-1 and values up to 1 μmol m-2 s-1 need to be consistent with whatever transport system is in operation. We have summarized the sizes and rates of turnover of vesicles in plants, where clathrin-independent vesicles are 100 nm or more in diameter and can merge with the plasma membrane at rates of 100 s-1. We gathered evidence for vesicular transport of ions in halophytes and evaluated whether vesicular transport could account for the observable fluxes.

Conclusions

There is strong evidence in favour of vesicular transport in plants and circumstantial evidence in favour of the movement of ions in vesicles. Estimated rates of vesicle turnover could account for ion transport at the lower reported fluxes (around 20 nmol m-2 s-1), but the higher fluxes may require vesicles of the order of 1 μm or more in diameter. The very high fluxes reported in some salt glands might be an artefact of the way they were measured.

Outer, inner and planar polarity in the Arabidopsis root

Plant roots control uptake of water and nutrients and cope with environmental challenges. The root epidermis provides the first selective interface for nutrient absorption, while the endodermis produces the main apoplastic diffusion barrier in the form of a structure called the Casparian strip. The positioning of root hairs on epidermal cells, and of the Casparian strip around endodermal cells, requires asymmetries along cellular axes (cell polarity). Cell polarity is termed planar polarity, when coordinated within the plane of a given tissue layer. Here, we review recent molecular advances towards understanding both the polar positioning of the proteo-lipid membrane domain instructing root hair initiation, and the cytoskeletal, trafficking and polar tethering requirements of proteins at outer or inner plasma membrane domains. Finally, we highlight progress towards understanding mechanisms of Casparian strip formation and underlying endodermal cell polarity.

Trans-Golgi network/early endosome: a central sorting station for cargo proteins in plant immunity

In plants, the trans-Golgi network (TGN) functionally overlaps with the early endosome (EE), serving as a central sorting hub to direct newly synthesized and endocytosed cargo to the cell surface or vacuole. Here, we focus on the emerging role of the TGN/EE in sorting of immune cargo proteins for effective plant immunity against pathogenic bacteria and fungi. Specific vesicle coat and regulatory components at the TGN/EE ensure that immune cargoes are correctly sorted and transported to the location of their cellular functions. Our understanding of the identity of immune cargoes and the underlying cellular mechanisms regulating their sorting are still rudimentary, but this knowledge is essential to understanding the physiological contribution of the TGN/EE to effective immune responses.

Vesicle trafficking in plant immunity

To defend against extracellular pathogens, plants primarily depend on cell-autonomous innate immunity due to the lack of the circulatory immune system including mobile immune cells. To extracellularly restrict or kill the pathogens, plant cells dump out antimicrobials. However, since antimicrobials are also toxic to plant cells themselves, they have to be safely delivered to the target sites in a separate vesicular compartment. In addition, because immune responses often requires energy otherwise used for the other metabolic processes, it is very important to properly control the duration and strength of immune responses depending on pathogen types. This can be achieved by regulating the sensing of immune signals and the delivery/discharge of extracellular immune molecules, all of which are controlled by membrane trafficking in plant cells. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are now considered as the minimal factors that can merge two distinct membranes of cellular compartments. Hence, in this review, known and potential immune functions of SNAREs as well as regulatory proteins will be discussed.

Arabidopsis BTB/POZ protein-dependent PENETRATION3 trafficking and disease susceptibility

The outermost cell layer of plant roots (epidermis) constantly encounters environmental challenges. The epidermal outer plasma membrane domain harbours the PENETRATION3 (PEN3)/ABCG36/PDR8 ATP-binding cassette transporter that confers non-host resistance to several pathogens. Here, we show that the Arabidopsis ENDOPLASMIC RETICULUM-ARRESTED PEN3 (EAP3) BTB/POZ-domain protein specifically mediates PEN3 exit from the endoplasmic reticulum and confers resistance to a root-penetrating fungus, providing prime evidence for BTB/POZ-domain protein-dependent membrane trafficking underlying disease resistance.