Arabidopsis dynamin-related proteins, DRP2A and DRP2B, function coordinately in post-Golgi trafficking

The damage-associated molecular pattern cellotriose alters the phosphorylation pattern of proteins involved in cellulose synthesis and trans-Golgi trafficking in Arabidopsis thaliana

We have recently demonstrated that the cellulose breakdown product cellotriose is a damage-associated molecular pattern (DAMP) which induces responses related to the integrity of the cell wall. Activation of downstream responses requires the Arabidopsis malectin domain-containing CELLOOLIGOMER RECEPTOR KINASE1 (CORK1)¹. The cellotriose/CORK1 pathway induces immune responses, including NADPH oxidase-mediated reactive oxygen species production, mitogen-activated protein kinase 3/6 phosphorylation-dependent defense gene activation, and the biosynthesis of defense hormones. However, apoplastic accumulation of cell wall breakdown products should also activate cell wall repair mechanisms. We demonstrate that the phosphorylation pattern of numerous proteins involved in the accumulation of an active cellulose synthase complex in the plasma membrane and those for protein trafficking to and within the trans-Golgi network (TGN) are altered within minutes after cellotriose application to Arabidopsis roots. The phosphorylation pattern of enzymes involved in hemicellulose or pectin biosynthesis and the transcript levels for polysaccharide-synthesizing enzymes responded barely to cellotriose treatments. Our data show that the phosphorylation pattern of proteins involved in cellulose biosynthesis and trans-Golgi trafficking is an early target of the cellotriose/CORK1 pathway.

Arabidopsis Dynamin-Related Protein AtDRP2A Contributes to Late Flg22-Signaling and Effective Immunity Against Pseudomonas syringae Bacteria

In eukaryotes, dynamins and dynamin-related proteins (DRPs) are high-molecular weight GTPases responsible for mechanochemical fission of organelles or membranes. Of the six DRP subfamilies in Arabidopsis thaliana, AtDRP1 and AtDRP2 family members serve as endocytic accessory proteins in clathrin-mediated endocytosis. Most studies have focused on AtDRP1A and AtDRP2B as critical modulators of plant pattern-triggered immunity (PTI) against pathogenic, flagellated Pseudomonas syringae pv. tomato DC3000 bacteria and immune signaling in response to the bacterial flagellin peptide flg22. Much less is known about AtDRP2A, the closely related paralog of AtDRP2B. AtDRP2A and AtDRP2B are the only classical, or bona fide, dynamins in Arabidopsis, based on their evolutionary conserved domain structure with mammalian dynamins functioning in endocytosis. AtDRP2B but not AtDRP2A is required for robust ligand-induced endocytosis of the receptor kinase FLAGELLIN SENSING2 for dampening of early flg22 signaling. Here, we utilized Arabidopsis drp2a null mutants to identify AtDRP2A as a positive contributor to effective PTI against P. syringae pv. tomato DC3000 bacteria, consistent with reduced PATHOGEN RELATED1 (PR1) messenger RNA accumulation. We provide evidence that AtDRP2A is a novel modulator of late flg22 signaling, contributing positively to PR1 gene induction but negatively to polyglucan callose deposition. AtDRP2A has no apparent roles in flg22-elicited mitogen-activated protein kinase defense marker gene induction. In summary, this study adds the evolutionary conserved dynamin AtDRP2A to a small group of vesicular trafficking proteins with roles as non-canonical contributors in immune responses, likely due to modulating one or both the localization and activity of multiple different proteins with distinct contributions to immune signaling. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Localization and circulation: vesicle trafficking in regulating plant nutrient homeostasis

Nutrient homeostasis is essential for plant growth and reproduction. Plants, therefore, have evolved tightly regulated mechanisms for the uptake, translocation, distribution, and storage of mineral nutrients. Considering that inorganic nutrient transport relies on membrane-based transporters and channels, vesicle trafficking, one of the fundamental cell biological processes, has become a hotspot of plant nutrition studies. In this review, we summarize recent advances in the study of how vesicle trafficking regulates nutrient homeostasis to contribute to the adaptation of plants to heterogeneous environments. We also discuss new perspectives on future studies, which may inspire researchers to investigate new approaches to improve the human diet and health by changing the nutrient quality of crops.

Dynamin-Related Proteins Enhance Tomato Immunity by Mediating Pattern Recognition Receptor Trafficking

Pattern recognition receptor (PRR) trafficking to the plasma membrane and endocytosis plays a crucial role in pattern triggered immunity (PTI). Dynamin-related proteins (DRPs) participate in endocytosis and recycling. In Arabidopsis, DRP1 and DRP2 are involved in plasma membrane scission during endocytosis. They are required for the PRR FLS2 endocytosis induction and PTI activation after elicitation with flg22, the MAMP recognized by FLS2. In tomato, SlDRP2A regulates the PRR LeEIX2 endocytosis and PTI activation in response to EIX, the MAMP recognized by LeEIX2. However, it is unknown if other DRPs participate in these processes. Taking advantage of bioinformatics tools, we selected SlDRP2B among the eight DRP2 tomato orthologues to study its functionality in trafficking and plant immunity. Through transient expression of SlDRP1B and its dominant-negative mutant on Nicotiana benthamiana and Nicotiana tabacum, we analyzed SlDRP1B function. We observed that SlDRP1B is physically associated with the LeEIX2 and modifies LeEIX2 trafficking, increasing its presence in endosomes. An enhancement of EIX-elicitated defense responses accompanies the role of SlDRP1B on LeEIX endocytosis. In addition, SlDRP1B overexpression enhanced flg22-elicited defense response. With these results, we conclude that SlDRP1B regulates PRR trafficking and, therefore, plant immunity, similarly to the SlDRP2A role.

An Update on the Key Factors Required for Plant Golgi Structure Maintenance

Plant Golgi apparatus serves as the central station of the secretory pathway and is the site where protein modification and cell wall matrix polysaccharides synthesis occur. The polarized and stacked cisternal structure is a prerequisite for Golgi function. Our understanding of Golgi structure maintenance and trafficking are largely obtained from mammals and yeast, yet, plant Golgi has many different aspects. In this review, we summarize the key players in Golgi maintenance demonstrated by genetic studies in plants, which function in ER-Golgi, intra-Golgi and post-Golgi transport pathways. Among these, we emphasize on players in intra-Golgi trafficking.

FMT, a protein that affects mitochondrial distribution, interacts with translation-related proteins in Arabidopsis thaliana

Two translation-related proteins are identified as FMT-interacting proteins. However, FMT, unlike mutants of other CLU genes in fly and human, has no clear impact on the accumulation of mitochondrial proteins. Organelle distribution is critical for effective metabolism and stress response and is controlled by various environmental factors. Clustered mitochondria (CLU) superfamily genes affect mitochondrial distribution and their disruptions cause mitochondria to cluster within a cell in various species including yeast, fly, mammals and Arabidopsis. In Arabidopsis thaliana, Friendly mitochondria (FMT) is a CLU gene that is required for normal mitochondrial distribution, but its molecular function is unclear. Here, we demonstrate that FMT interacts with some translation-related proteins (translation initiation factor eIFiso4G1 and glutamyl-tRNA synthetase OVA9), as well as itself. We also show FMT forms dynamic particles in the cytosol that sometimes move with mitochondria, and their movements are mainly controlled by actin filaments but also by microtubules. Similar results have been reported for animal CLU orthologs. However, an fmt mutant, unlike animal clu mutants, did not show any clear decrease of nuclear-encoded mitochondrial protein levels. This difference may reflect a functional divergence of FMT from other CLU superfamily genes.

Longin R-SNARE is retrieved from the plasma membrane by ANTH domain-containing proteins in Arabidopsis

The plasma membrane (PM) acts as the interface between intra- and extracellular environments and is thus important for intercellular communication and extracellular signal perception. The composition and amounts of PM proteins are tightly regulated, by molecular mechanisms that remain largely unknown in plant cells. We identified a pair of ANTH domain-containing proteins functioning as adaptors for the retrieval of VAMP72 members, which are components of the membrane fusion machinery, during clathrin-mediated endocytosis. Our results further indicate that the recycling mechanisms of homologous VAMP7 proteins are different in plants and animals, suggesting a divergence of the endocytosis mechanism between these two kingdoms.

The plasma membrane (PM) acts as the interface between intra- and extracellular environments and exhibits a tightly regulated molecular composition. The composition and amount of PM proteins are regulated by balancing endocytic and exocytic trafficking in a cargo-specific manner, according to the demands of specific cellular states and developmental processes. In plant cells, retrieval of membrane proteins from the PM depends largely on clathrin-mediated endocytosis (CME). However, the mechanisms for sorting PM proteins during CME remain ambiguous. In this study, we identified a homologous pair of ANTH domain-containing proteins, PICALM1a and PICALM1b, as adaptor proteins for CME of the secretory vesicle-associated longin-type R-SNARE VAMP72 group. PICALM1 interacted with the SNARE domain of VAMP72 and clathrin at the PM. The loss of function of PICALM1 resulted in faulty retrieval of VAMP72, whereas general endocytosis was not considerably affected by this mutation. The double mutant of PICALM1 exhibited impaired vegetative development, indicating the requirement of VAMP72 recycling for normal plant growth. In the mammalian system, VAMP7, which is homologous to plant VAMP72, is retrieved from the PM via the interaction with a clathrin adaptor HIV Rev-binding protein in the longin domain during CME, which is not functional in the plant system, whereas retrieval of brevin-type R-SNARE members is dependent on a PICALM1 homolog. These results indicate that ANTH domain-containing proteins have evolved to be recruited distinctly for recycling R-SNARE proteins and are critical to eukaryote physiology.

A plant RNA virus hijacks endocytic proteins to establish its infection in plants

Endocytosis and endosomal trafficking play essential roles in diverse biological processes including responses to pathogen attack. It is well established that animal viruses enter host cells through receptor-mediated endocytosis for infection. However, the role of endocytosis in plant virus infection still largely remains unknown. Plant dynamin-related proteins 1 (DRP1) and 2 (DRP2) are the large, multidomain GTPases that participate together in endocytosis. Recently, we have discovered that DRP2 is co-opted by Turnip mosaic virus (TuMV) for infection in plants. We report here that DRP1 is also required for TuMV infection. We show that overexpression of DRP1 from Arabidopsis thaliana (AtDRP1A) promotes TuMV infection, and AtDRP1A interacts with several viral proteins including VPg and cylindrical inclusion (CI), which are the essential components of the virus replication complex (VRC). AtDRP1A colocalizes with the VRC in TuMV-infected cells. Transient expression of a dominant negative (DN) mutant of DRP1A disrupts DRP1-dependent endocytosis and supresses TuMV replication. As adaptor protein (AP) complexes mediate cargo selection for endocytosis, we further investigated the requirement of AP in TuMV infection. Our data suggest that the medium unit of the AP2 complex (AP2β) is responsible for recognizing the viral proteins as cargoes for endocytosis, and knockout of AP2β impairs intracellular endosomal trafficking of VPg and CI and inhibits TuMV replication. Collectively, our results demonstrate that DRP1 and AP2β are two proviral host factors of TuMV and shed light into the involvement of endocytosis and endosomal trafficking in plant virus infection.

Genome-Wide Analysis of Dynamin Gene Family in cassava (Manihot esculenta Crantz) and Transcriptional Regulation of Family Members ARC5 in Hormonal Treatments

The Dynamin gene family play a significance role in many physiological processes, especially ARC5 (Accumulation and replication of chloroplasts 5) in the process of plastid division. We performed a genome-wide analysis of the cassava Dynamin family based on the published cassava genome sequence and identified ARC5. 23 cassava Dynamins (MeDynamins) were identified and renamed. 23 MeDynamins were further divided into five major groups based on their structural and phylogenetic characteristics. The segmental duplication events have a significant impact on the expansion of MeDynamins. ARC5 expression analysis showed that there were differences between leaves and roots of cassava at different developmental stages. The tissue-specific expression analysis of the MeDynamins showed that most of MeDynamins were expressed in stem apical meristem and embryogenesis, whereas ARC5 was mainly expressed in leaves. The processing of IAA (Indole-3-acetic Acid) and MeJA (Methyl Jasmonate) verified the prediction results of cis-elements, and ACR5 was closely related to plant growth and positively correlated. It also indicated that high concentrations of MeJA treatment caused the cassava defense mechanism to function in advance. In conclusion, these findings provide basic insights for functional validation of the ARC5 genes in exogenous hormonal treatments.

A Molecular Perspective on Mitochondrial Membrane Fusion: From the Key Players to Oligomerization and Tethering of Mitofusin

Mitochondria are dynamic organelles characterized by an ultrastructural organization which is essential in maintaining their quality control and ensuring functional efficiency. The complex mitochondrial network is the result of the two ongoing forces of fusion and fission of inner and outer membranes. Understanding the functional details of mitochondrial dynamics is physiologically relevant as perturbations of this delicate equilibrium have critical consequences and involved in several neurological disorders. Molecular actors involved in this process are large GTPases from the dynamin-related protein family. They catalyze nucleotide-dependent membrane remodeling and are widely conserved from bacteria to higher eukaryotes. Although structural characterization of different family members has contributed in understanding molecular mechanisms of mitochondrial dynamics in more detail, the complete structure of some members as well as the precise assembly of functional oligomers remains largely unknown. As increasing structural data become available, the domain modularity across the dynamin superfamily emerged as a foundation for transfering the knowledge towards less characterized members. In this review, we will first provide an overview of the main actors involved in mitochondrial dynamics. We then discuss recent example of computational methodologies for the study of mitofusin oligomers, and present how the usage of integrative modeling in conjunction with biochemical data can be an asset in progressing the still challenging field of membrane dynamics.

The dynamin-like protein Fzl promotes thylakoid fusion and resistance to light stress in Chlamydomonas reinhardtii

Large GTPases of the Dynamin Related Proteins (DRP) family shape lipid bilayers through membrane fission or fusion processes. Despite the highly organized photosynthetic membranes of thylakoids, a single DRP is known to be targeted inside the chloroplast. Fzl from the land plant Arabidopsis thaliana is inserted in the inner envelope and thylakoid membranes to regulate their morphology. Fzl may promote the fusion of thylakoids but this remains to be proven. Moreover, the physiological requirement for fusing thylakoids is currently unknown. Here, we find that the unicellular microalga Chlamydomonas reinhardtii encodes an Fzl ortholog (CrFzl) that is localized in the chloroplast where it is soluble. To explore its function, the CRISPR/Cas9 technology was employed to generate multiple CrFzl knock out strains. Phenotypic analyzes revealed a specific requirement of CrFzl for survival upon light stress. Consistent with this, strong irradiance lead to increased photoinhibition of photosynthesis in mutant cells. Fluorescence and electron microscopy analysis demonstrated that upon exposure to high light, CrFzl mutants show defects in chloroplast morphology but also large cytosolic vacuoles in close contact with the plastid. We further observe that strong irradiance induces an increased recruitment of the DRP to thylakoid membranes. Most importantly, we show that CrFzl is required for the fusion of thylakoids during mating. Together, our results suggest that thylakoids fusion may be necessary for resistance to light stress.

All eukaryotic cells are composed of compartments with defined functions. Among those, mitochondria generate the main source of energy in human and animal cells. Their capacity to generate and diffuse energy in the cell is regulated by fusion and fragmentation processes.

Together with mitochondria that produce energy from oxygen, plant cells include an additional compartment called the chloroplast that produces energy from light. The machinery that converts light into energy is more precisely located inside the chloroplast within stacks of membranes called the thylakoids. Here, we elucidate the function of CrFzl, a previously uncharacterized protein encoded by the genome of the unicellular alga Chlamydomonas reinhardtii. Algal cells that do not contain CrFzl are impaired in their capacity to grow when they receive too much light and our results indicate that CrFzl promotes the fusion of thylakoids during mating. These results suggest that membrane fusion is an essential tool for energy production in stress conditions by living organisms.

Time-resolved interaction proteomics of the GIGANTEA protein under diurnal cycles in Arabidopsis

The plant-specific protein GIGANTEA (GI) controls many developmental and physiological processes, mediating rhythmic post-translational regulation. GI physically binds several proteins implicated in the circadian clock, photoperiodic flowering, and abiotic stress responses. To understand GI's multifaceted function, we aimed to comprehensively and quantitatively identify potential interactors of GI in a time-specific manner, using proteomics on Arabidopsis plants expressing epitope-tagged GI. We detected previously identified (in)direct interactors of GI, as well as proteins implicated in protein folding, or degradation, and a previously uncharacterized transcription factor, CYCLING DOF FACTOR6 (CDF6). We verified CDF6's direct interaction with GI, and ZEITLUPE/FLAVIN-BINDING, KELCH REPEAT, F-BOX 1/LIGHT KELCH PROTEIN 2 proteins, and demonstrated its involvement in photoperiodic flowering. Extending interaction proteomics to time series provides a data resource of candidate protein targets for GI's post-translational control.

Tomato Dynamin Related Protein 2A Associates With LeEIX2 and Enhances PRR Mediated Defense by Modulating Receptor Trafficking

The endocytic trafficking pathway is employed by the plant to regulate immune responses, and is often targeted by pathogen effectors to promote virulence. The model system of the tomato receptor-like protein (RLP) LeEIX2 and its ligand, the elicitor EIX, employs endocytosis to transmit receptor-mediated signals, with some of the signaling events occurring directly from endosomal compartments. Here, to explore the trafficking mechanism of LeEIX2-mediated immune signaling, we used a proteomic approach to identify LeEIX2-associating proteins. We report the identification of SlDRP2A, a dynamin related protein, as an associating partner for LeEIX2. SlDRP2A localizes at the plasma membrane. Overexpression of SlDRP2A increases the sub-population of LeEIX2 in VHAa1 endosomes, and enhances LeEIX2- and FLS2-mediated defense. The effect of SlDRP2A on induction of plant immunity highlights the importance of endomembrane components and endocytosis in signal propagation during plant immune responses.

Dynamin-Like Proteins of Endocytosis in Plants Are Coopted by Potyviruses To Enhance Virus Infection

Endocytosis and endosomal trafficking regulate the proteins targeted to the plasma membrane and play essential roles in diverse cellular processes, including responses to pathogen attack. Here, we report the identification of Glycine max (soybean) endocytosis dynamin-like protein 5A (GmSDL5A) associated with purified soybean mosaic virus (SMV) virions from soybean using a bottom-up proteomics approach. Knockdown of GmSDL5A and its homologous gene GmSDL12A inhibits SMV infection in soybean. The role of analogous dynamin-like proteins in potyvirus infection was further confirmed and investigated using the Arabidopsis/turnip mosaic virus (TuMV) pathosystem. We demonstrate that dynamin-related proteins 2A and 2B in Arabidopsis thaliana (AtDRP2A, AtDRP2B), homologs of GmSDL5A, are recruited to the virus replication complex (VRC) of TuMV. TuMV infection is inhibited in both A. thalianadrp2a (atdrp2a) and atdrp2b knockout mutants. Overexpression of AtDRP2 promotes TuMV replication and intercellular movement. AtRDP2 interacts with TuMV VPg, CP, CI, and 6K2. Of these viral proteins, VPg, CP, and CI are essential for viral intercellular movement, and 6K2, VPg, and CI are critical components of the VRC. We reveal that VPg and CI are present in the punctate structures labeled by the endocytic tracer FM4-64, suggesting that VPg and CI can be endocytosed. Treatment of plant leaves with a dynamin-specific inhibitor disrupts the delivery of VPg and CI to endocytic structures and suppresses TuMV replication and intercellular movement. Taken together, these data suggest that dynamin-like proteins are novel host factors of potyviruses and that endocytic processes are involved in potyvirus infection.IMPORTANCE It is well known that animal viruses enter host cells via endocytosis, whereas plant viruses require physical assistance, such as human and insect activities, to penetrate the host cell to establish their infection. In this study, we report that the endocytosis pathway is also involved in virus infection in plants. We show that plant potyviruses recruit endocytosis dynamin-like proteins to support their infection. Depletion of them by knockout of the corresponding genes suppresses virus replication, whereas overexpression of them enhances virus replication and intercellular movement. We also demonstrate that the dynamin-like proteins interact with several viral proteins that are essential for virus replication and cell-to-cell movement. We further show that treatment of a dynamin-specific inhibitor disrupts endocytosis and inhibits virus replication and intercellular movement. Therefore, the dynamin-like proteins are novel host factors of potyviruses. The corresponding genes may be manipulated using advanced biotechnology to control potyviral diseases.

miRNAs control HAM1 functions at the single-cell-layer level and are essential for normal embryogenesis in Arabidopsis

KEY MESSAGE: miR171a controls HAM1 functions within the protodermal cells of the embryo, and these controls are essential for normal embryogenesis in Arabidopsis. Arabidopsis thaliana miR171a is known to bind to and cleave mRNAs of three HAIRY MERISTEM (HAM) genes that encode members of the GRAS family transcriptional regulators. The molecular functions of the HAM genes are still being elucidated in Arabidopsis. However, detailed expression patterns of miR171a and the effects of the failure of miR171a to suppress HAM genes were unknown till now. Here, we show the detailed expression patterns of miR171a and HAM1 using green fluorescent protein and confocal scanning microscopy. Our observations revealed that miR171a was expressed in the surface cell layer of the embryo and shoot apical meristem, and it controlled HAM1 functions. To determine the impact of the failure of miR171a to suppress of HAM1, we introduced seven synonymous mutations into the miR171a target site of the HAM1 gene (modified HAM1, mHAM1) and generated transgenic plants that had mHAM1 driven by HAM1 native promoter. The mHAM1 transgenic plants showed organogenic defects. These results indicate that the control of HAM1 functions at the single-cell-layer level by miR171a is essential for proper organ formation in Arabidopsis.

Combined 15N-Labeling and TandemMOAC Quantifies Phosphorylation of MAP Kinase Substrates Downstream of MKK7 in Arabidopsis

Reversible protein phosphorylation is a widespread posttranslational modification that plays a key role in eukaryotic signal transduction. Due to the dynamics of protein abundance, low stoichiometry and transient nature of protein phosphorylation, the detection and accurate quantification of substrate phosphorylation by protein kinases remains a challenge in phosphoproteome research. Here, we combine tandem metal-oxide affinity chromatography (tandemMOAC) with stable isotope ¹⁵N metabolic labeling for the measurement and accurate quantification of low abundant, transiently phosphorylated peptides by mass spectrometry. Since tandemMOAC is not biased toward the enrichment of acidophilic, basophilic, or proline-directed kinase substrates, the method is applicable to identify targets of all these three types of protein kinases. The MKK7-MPK3/6 module, for example, is involved in the regulation of plant development and plant basal and systemic immune responses, but little is known about downstream cascade components. Using our here described phosphoproteomics approach we identified several MPK substrates downstream of the MKK7-MPK3/6 phosphorylation cascade in Arabidopsis. The identification and validation of dynamin-related protein 2 as a novel phosphorylation substrate of the MKK7-MPK3/6 module establishes a novel link between MPK signaling and clathrin-mediated vesicle trafficking.

Increased callose deposition in plants lacking DYNAMIN-RELATED PROTEIN 2B is dependent upon POWDERY MILDEW RESISTANT 4

Callose deposition within the cell wall is a well-documented plant immune response to pathogenic organisms as well as to pathogen-/microbe- associated molecular patterns (P/MAMPs). However, the molecular mechanisms that modulate pathogen-induced callose deposition are less understood. We reported previously that Arabidopsis plants lacking the vesicle trafficking component DYNAMIN-RELATED PROTEIN 2B (DRP2B) display increased callose deposition in response to the PAMP flg22. Here, we show that increased number of flg22-induced callose deposits in drp2b leaves is fully dependent on the callose synthase POWDERY MILDEW RESISTANT 4 (PMR4). We propose that in addition to functioning in flg22-induced endocytosis of the plant receptor, FLAGELLIN SENSING 2, DRP2B may regulate the trafficking of proteins involved in callose synthesis, such as PMR4, and/or callose degradation.

Endocytosis and Endosomal Trafficking in Plants

Endocytosis and endosomal trafficking are essential processes in cells that control the dynamics and turnover of plasma membrane proteins, such as receptors, transporters, and cell wall biosynthetic enzymes. Plasma membrane proteins (cargo) are internalized by endocytosis through clathrin-dependent or clathrin-independent mechanism and delivered to early endosomes. From the endosomes, cargo proteins are recycled back to the plasma membrane via different pathways, which rely on small GTPases and the retromer complex. Proteins that are targeted for degradation through ubiquitination are sorted into endosomal vesicles by the ESCRT (endosomal sorting complex required for transport) machinery for degradation in the vacuole. Endocytic and endosomal trafficking regulates many cellular, developmental, and physiological processes, including cellular polarization, hormone transport, metal ion homeostasis, cytokinesis, pathogen responses, and development. In this review, we discuss the mechanisms that mediate the recognition and sorting of endocytic and endosomal cargos, the vesiculation processes that mediate their trafficking, and their connection to cellular and physiological responses in plants.

Phytophthora infestans RXLR-WY Effector AVR3a Associates with Dynamin-Related Protein 2 Required for Endocytosis of the Plant Pattern Recognition Receptor FLS2

Pathogens utilize effectors to suppress basal plant defense known as PTI (Pathogen-associated molecular pattern-triggered immunity). However, our knowledge of PTI suppression by filamentous plant pathogens, i.e. fungi and oomycetes, remains fragmentary. Previous work revealed that the co-receptor BAK1/SERK3 contributes to basal immunity against the potato pathogen Phytophthora infestans. Moreover BAK1/SERK3 is required for the cell death induced by P. infestans elicitin INF1, a protein with characteristics of PAMPs. The P. infestans host-translocated RXLR-WY effector AVR3a is known to supress INF1-mediated cell death by binding the plant E3 ligase CMPG1. In contrast, AVR3aKI-Y147del, a deletion mutant of the C-terminal tyrosine of AVR3a, fails to bind CMPG1 and does not suppress INF1-mediated cell death. Here, we studied the extent to which AVR3a and its variants perturb additional BAK1/SERK3-dependent PTI responses in N. benthamiana using the elicitor/receptor pair flg22/FLS2 as a model. We found that all tested variants of AVR3a suppress defense responses triggered by flg22 and reduce internalization of activated FLS2. Moreover, we discovered that AVR3a associates with the Dynamin-Related Protein 2 (DRP2), a plant GTPase implicated in receptor-mediated endocytosis. Interestingly, silencing of DRP2 impaired ligand-induced FLS2 internalization but did not affect internalization of the growth receptor BRI1. Our results suggest that AVR3a associates with a key cellular trafficking and membrane-remodeling complex involved in immune receptor-mediated endocytosis. We conclude that AVR3a is a multifunctional effector that can suppress BAK1/SERK3-mediated immunity through at least two different pathways.

Vesicular trafficking and salinity responses in plants

Research spanning three decades has demonstrated that vesicles pinch off from the plasma membrane and traffic through the cytoplasm of plant cells, much as previously reported in animal cells. Although the well-conserved clathrin-mediated mechanism of endocytosis has been well characterized, relatively little is known about clathrin-independent pathways in plants. Modulation of endocytosis by both physical stimuli and chemical ligands has been reported in plants. Here, we review the effect of salinity-one of the most deleterious environmental assaults-on endocytosis and intracellular trafficking.

A moving view: subcellular trafficking processes in pattern recognition receptor-triggered plant immunity

A significant challenge for plants is to induce localized defense responses at sites of pathogen attack. Therefore, host subcellular trafficking processes enable accumulation and exchange of defense compounds, which contributes to the plant on-site defenses in response to pathogen perception. This review summarizes our current understanding of the transport processes that facilitate immunity, the significance of which is highlighted by pathogens reprogramming membrane trafficking through host cell translocated effectors. Prominent immune-related cargos of plant trafficking pathways are the pattern recognition receptors (PRRs), which must be present at the plasma membrane to sense microbes in the apoplast. We focus on the dynamic localization of the FLS2 receptor and discuss the pathways that regulate receptor transport within the cell and their link to FLS2-mediated immunity. One emerging theme is that ligand-induced late endocytic trafficking is conserved across different PRR protein families as well as across different plant species.