The KEEP ON GOING protein of Arabidopsis regulates intracellular protein trafficking and is degraded during fungal infection

More than meets the eye: knowns and unknowns of the trafficking of small secreted proteins in Arabidopsis

Small proteins represent a significant portion of the cargo transported through plant secretory pathways, playing crucial roles in developmental processes, fertilization, and responses to environmental stresses. Despite the importance of small secreted proteins, substantial knowledge gaps persist regarding the regulatory mechanisms governing their trafficking along the secretory pathway, and their ultimate localization or destination. To address these gaps, we conducted a comprehensive literature review, focusing particularly on trafficking and localization of Arabidopsis small secreted proteins with potential biochemical and/or signaling roles in the extracellular space, typically those within the size range of 101-200 amino acids. Our investigation reveals that while at least six members of the 21 mentioned families have a confirmed extracellular localization, eight exhibit intracellular localization, including cytoplasmic, nuclear, and chloroplastic locations, despite the presence of N-terminal signal peptides. Further investigation into the trafficking and secretion mechanisms of small protein cargo could not only deepen our understanding of plant cell biology and physiology but also provide a foundation for genetic manipulation strategies leading to more efficient plant cultivation.

Small secreted proteins and exocytosis regulators: do they go along?

Small secreted proteins play an important role in plant development, as well as in reactions to changes in the environment. In Arabidopsis thaliana, they are predominantly members of highly expanded families, such as the pathogenesis-related (PR) 1-like protein family, whose most studied member PR1 is involved in plant defense responses by a so far unknown mechanism, or Clavata3/Endosperm Surrounding Region (CLE) protein family, whose members' functions in the development are well described. Our survey of the existing literature for the two families showed a lack of details on their localization, trafficking, and exocytosis. Therefore, in order to uncover the modes of their secretion, we tested the hypothesis that a direct link between the secreted cargoes and the secretion regulators such as Rab GTPases, SNAREs, and exocyst subunits could be established using in silico co-expression and clustering approaches. We employed several independent techniques to uncover that only weak co-expression links could be found for limited numbers of secreted cargoes and regulators. We propose that there might be particular spatio-temporal requirements for PR1 and CLE proteins to be synthesized and secreted, and efforts to experimentally cover these discrepancies should be invested along with functional studies.

The functional and structural characterization of Xanthomonas campestris pv. campestris core effector XopP revealed a new kinase activity

Exo70B1 is a protein subunit of the exocyst complex with a crucial role in a variety of cell mechanisms, including immune responses against pathogens. The calcium-dependent kinase 5 (CPK5) of Arabidopsis thaliana (hereafter Arabidopsis), phosphorylates AtExo70B1 upon functional disruption. We previously reported that, the Xanthomonas campestris pv. campestris effector XopP compromises AtExo70B1, while bypassing the host's hypersensitive response, in a way that is still unclear. Herein we designed an experimental approach, which includes biophysical, biochemical, and molecular assays and is based on structural and functional predictions, utilizing AplhaFold and DALI online servers, respectively, in order to characterize the in vivo XccXopP function. The interaction between AtExo70B1 and XccXopP was found very stable in high temperatures, while AtExo70B1 appeared to be phosphorylated at XccXopP-expressing transgenic Arabidopsis. XccXopP revealed similarities with known mammalian kinases and phosphorylated AtExo70B1 at Ser107, Ser111, Ser248, Thr309, and Thr364. Moreover, XccXopP protected AtExo70B1 from AtCPK5 phosphorylation. Together these findings show that XccXopP is an effector, which not only functions as a novel serine/threonine kinase upon its host target AtExo70B1 but also protects the latter from the innate AtCPK5 phosphorylation, in order to bypass the host's immune responses. Data are available via ProteomeXchange with the identifier PXD041405.

Evolution of the Tóxicos en Levadura 63 (TL63) gene family in plants and functional characterization of Arabidopsis thaliana TL63 under oxidative stress

MAIN CONCLUSION

TL63 orthologs were angiosperm specific and had undergone motifs loss and gain, and increased purifying selection. AtTL63 was involved in the response of yeast and Arabidopsis plants to oxidative stress. The Tóxicos en Levadura (TL) family, a class of E3 ubiquitin ligases with typical RING-H2 type zinc finger structure, plays a pivotal role in mediating physiological processes and responding to stress in plants. However, the evolution and function of TL63 remain unclear. In this study, TL63 homologs were dated roughly back to the origin of land plants and confirmed to have subjected to the gain and loss of motifs and increased purifying selection. Phylogenetic analysis displayed that 279 TL63s could be divided into four main clades (Clade A-D). Notably, the ancestral tandem TL40/41 cluster contributed to the expansion of modern Brassicaceae TL40/41. The substitution rate tests revealed that the TL63 lineage was evidently different from other lineages. The codon usage index exhibited that monocotyledons preferred to use not A3s and T3s, but C3s, G3s, CAI, CBI and Fop. Sequence analysis showed that the TL63 homologs had conserved TM and GLD motifs and RING-H2 domain whose key amino acid residues accounted for the high average abundance. Particularly, Arabidopsis thaliana TL63 (AtTL63) was located in the nuclei, cell membranes and peroxisomes and expressed universally and significantly throughout A. thaliana development. Under H2O2 treatment, low or moderate expression of the AtTL63 held beneficial effects on the growth and viability of yeast cells and the mutation or overexpression of the AtTL63 positively affected the growth of A. thaliana plants. In brief, this study could supply useful insight into the evolution of the plant TL63s and the AtTL63 functions under oxidative stress.

Traffic Control: Subversion of Plant Membrane Trafficking by Pathogens

Membrane trafficking pathways play a prominent role in plant immunity. The endomembrane transport system coordinates membrane-bound cellular organelles to ensure that immunological components are utilized effectively during pathogen resistance. Adapted pathogens and pests have evolved to interfere with aspects of membrane transport systems to subvert plant immunity. To do this, they secrete virulence factors known as effectors, many of which converge on host membrane trafficking routes. The emerging paradigm is that effectors redundantly target every step of membrane trafficking from vesicle budding to trafficking and membrane fusion. In this review, we focus on the mechanisms adopted by plant pathogens to reprogram host plant vesicle trafficking, providing examples of effector-targeted transport pathways and highlighting key questions for the field to answer moving forward.

The Plant Ubiquitin-Proteasome System as a Target for Microbial Manipulation

The plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin-proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.

The host exocyst complex is targeted by a conserved bacterial type-III effector that promotes virulence

For most Gram-negative bacteria, pathogenicity largely depends on the type-III secretion system that delivers virulence effectors into eukaryotic host cells. The subcellular targets for the majority of these effectors remain unknown. Xanthomonas campestris, the causal agent of black rot disease of crucifers such as Brassica spp., radish, and turnip, delivers XopP, a highly conserved core-effector protein produced by X. campestris, which is essential for virulence. Here, we show that XopP inhibits the function of the host-plant exocyst complex by direct targeting of Exo70B, a subunit of the exocyst complex, which plays a significant role in plant immunity. XopP interferes with exocyst-dependent exocytosis and can do this without activating a plant NOD-like receptor that guards Exo70B in Arabidopsis. In this way, Xanthomonas efficiently inhibits the host's pathogen-associated molecular pattern (PAMP)-triggered immunity by blocking exocytosis of pathogenesis-related protein-1A, callose deposition, and localization of the FLAGELLIN SENSITIVE2 (FLS2) immune receptor to the plasma membrane, thus promoting successful infection. Inhibition of exocyst function without activating the related defenses represents an effective virulence strategy, indicating the ability of pathogens to adapt to host defenses by avoiding host immunity responses.

Ubiquitin ligases at the nexus of plant responses to biotic and abiotic stresses

Plants must cope with an ever-changing environment, including concurrent biotic and abiotic stresses. The ubiquitin-proteasome system (UPS) is intricately involved in regulating signaling events that facilitate cellular changes required to mitigate the detrimental effects of environmental stress. A key component of the UPS are ubiquitin ligases (or E3s) that catalyze the attachment of ubiquitin molecules to select substrate proteins, which are then recognized by the 26S proteasome for degradation. With the identification of substrate proteins, a growing number of E3s are shown to differentially regulate responses to abiotic as well as bioitic stresses. The review discusses select E3s to illustrate the role of ubiquitin ligases as negative and/or positive regulators of responses to both biotic and abiotic stresses.

Vacuoles in Bryophytes: Properties, Biogenesis, and Evolution

Vacuoles are the most conspicuous organelles in plants for their indispensable functions in cell expansion, solute storage, water balance, etc. Extensive studies on angiosperms have revealed that a set of conserved core molecular machineries orchestrate the formation of vacuoles from multiple pathways. Usually, vacuoles in seed plants are classified into protein storage vacuoles and lytic vacuoles for their distinctive morphology and physiology function. Bryophytes represent early diverged non-vascular land plants, and are of great value for a better understanding of plant science. However, knowledge about vacuole morphology and biogenesis is far less characterized in bryophytes. In this review, first we summarize known knowledge about the morphological and metabolic constitution properties of bryophytes' vacuoles. Then based on known genome information of representative bryophytes, we compared the conserved molecular machinery for vacuole biogenesis among different species including yeast, mammals, Arabidopsis and bryophytes and listed out significant changes in terms of the presence/absence of key machinery genes which participate in vacuole biogenesis. Finally, we propose the possible conserved and diverged mechanism for the biogenesis of vacuoles in bryophytes compared with seed plants.

Functional role of formate dehydrogenase 1 (FDH1) for host and nonhost disease resistance against bacterial pathogens

Nonhost disease resistance is the most common type of plant defense mechanism against potential pathogens. In the present study, the metabolic enzyme formate dehydrogenase 1 (FDH1) was identified to associate with nonhost disease resistance in Nicotiana benthamiana and Arabidopsis thaliana. In Arabidopsis, AtFDH1 was highly upregulated in response to both host and nonhost bacterial pathogens. The Atfdh1 mutants were compromised in nonhost resistance, basal resistance, and gene-for-gene resistance. The expression patterns of salicylic acid (SA) and jasmonic acid (JA) marker genes after pathogen infections in Atfdh1 mutant indicated that both SA and JA are involved in the FDH1-mediated plant defense response to both host and nonhost bacterial pathogens. Previous studies reported that FDH1 localizes to mitochondria, or both mitochondria and chloroplasts. Our results showed that the AtFDH1 mainly localized to mitochondria, and the expression level of FDH1 was drastically increased upon infection with host or nonhost pathogens. Furthermore, we identified the potential co-localization of mitochondria expressing FDH1 with chloroplasts after the infection with nonhost pathogens in Arabidopsis. This finding suggests the possible role of FDH1 in mitochondria and chloroplasts during defense responses against bacterial pathogens in plants.

Immunity functions of Arabidopsis pathogenesis-related 1 are coupled but not confined to its C-terminus processing and trafficking

The pathogenesis-related 1 (PR1) proteins are members of the cross-kingdom conserved CAP superfamily (from Cysteine-rich secretory protein, Antigen 5, and PR1 proteins). PR1 mRNA expression is frequently used for biotic stress monitoring in plants; however, the molecular mechanisms of its cellular processing, localization, and function are still unknown. To analyse the localization and immunity features of Arabidopsis thaliana PR1, we employed transient expression in Nicotiana benthamiana of the tagged full-length PR1 construct, and also disrupted variants with C-terminal truncations or mutations. We found that en route from the endoplasmic reticulum, the PR1 protein transits via the multivesicular body and undergoes partial proteolytic processing, dependent on an intact C-terminal motif. Importantly, only nonmutated or processing-mimicking variants of PR1 are secreted to the apoplast. The C-terminal proteolytic cleavage releases a protein fragment that acts as a modulator of plant defence responses, including localized cell death control. However, other parts of PR1 also have immunity potential unrelated to cell death. The described modes of the PR1 contribution to immunity were found to be tissue-localized and host plant ontogenesis dependent.

Identification and expression analysis of PUB genes in tea plant exposed to anthracnose pathogen and drought stresses

The plant U-box (PUB) gene family, one of the major ubiquitin ligase families in plants, plays important roles in multiple cellular processes including environmental stress responses and resistance. The function of U-box genes has been well characterized in Arabidopsis and other plants. However, little is known about the tea plant (Camellia sinensis) PUB genes. Here, 89 U-box proteins were identified from the chromosome-scale referenced genome of tea plant. According to the domain organization and phylogenetic analysis, the tea plant PUB family were classified into ten classes, named Class I to X, respectively. Using previously released stress-related RNA-seq data in tea plant, we identified 34 stress-inducible CsPUB genes. Specifically, eight CsPUB genes were expressed differentially under both anthracnose pathogen and drought stresses. Moreover, six of the eight CsPUBs were upregulated in response to these two stresses. Expression profiling performed by qRT-PCR was consistent with the RNA-seq analysis, and stress-related cis-acting elements were identified in the promoter regions of the six upregulated CsPUB genes. These results strongly implied the putative functions of U-box ligase genes in response to biotic and abiotic stresses in tea plant.

Factors that affect protein abundance of a positive regulator of abscisic acid signalling, the basic leucine zipper transcription factor ABRE-binding factor 2 (ABF2)

Most members of basic leucine zipper (bZIP) transcription factor (TF) subgroup A play important roles as positive effectors in abscisic acid (ABA) signaling during germination and/or in vegetative stress responses. In multiple plant species, one member, ABA insensitive 5 (ABI5), is a major TF that promotes seed maturation and blocks early seeding growth in response to ABA. Other members, referred to as either ABRE-binding factors (ABFs), ABRE-binding proteins (AREBs), or D3 protein-binding factors (DPBFs), are implicated as major players in stress responses during vegetative growth. Studies on the proteolytic regulation of ABI5, ABF1, and ABF3 in Arabidopsis thaliana have shown that the proteins have moderate degradation rates and accumulate in the presence of the proteasome inhibitor MG132. Exogenous ABA slows their degradation and the ubiquitin E3 ligase called KEEP ON GOING (KEG) is important for their degradation. However, there are some reported differences in degradation among subgroup A members. The conserved C-terminal sequences (referred to as the C4 region) enhance degradation of ABI5 but stabilize ABF1 and ABF3. To better understand the proteolytic regulation of the ABI5/ABFs and determine whether there are differences between vegetative ABFs and ABI5, we studied the degradation of an additional family member, ABF2, and compared its in vitro degradation to that of ABI5. As previously seen for ABI5, ABF1, and ABF3, epitope-tagged constitutively expressed ABF2 degrades in seedlings treated with cycloheximide and is stabilized following treatment with the proteasome inhibitor MG132. Tagged ABF2 protein accumulates when seedlings are treated with ABA, but its mRNA levels do not increase, suggesting that the protein is stabilized in the presence of ABA. ABF2 is also an in vitro ubiquitination substrate of the E3 ligase KEG and recombinant ABF2 is stable in keg lysates. ABF2 with a C4 deletion degrades more quickly in vitro than full-length ABF2, as previously observed for ABF1 and ABF3, suggesting that the conserved C4 region contributes to its stability. In contrast to ABF2 and consistent with previously published work, ABI5 with C terminal deletions including an analogous C4 deletion is stabilized in vitro compared to full length ABI5. In vivo expression of an ABF1 C4 deletion protein appears to have reduced activity compared to equivalent levels of full length ABF1. Additional group A family members show similar proteolytic regulation by MG132 and ABA. Altogether, these results together with other work on ABI5 regulation suggest that the vegetative ABFs share proteolytic regulatory mechanisms that are not completely shared with ABI5.

The Arabidopsis thaliana E3 Ubiquitin Ligase BRIZ Functions in Abscisic Acid Response

The ubiquitin system is essential for multiple hormone signaling pathways in plants. Here, we show that the Arabidopsis thaliana E3 ligase BRIZ, a heteromeric ligase that consists minimally of BRIZ1 and BRIZ2 proteins, functions in abscisic acid (ABA) signaling or response. briz1 and briz2 homozygous mutants either fail to germinate or emerge later than wild-type seedlings, with little cotyledon expansion or root elongation and no visible greening. Viability staining indicates that briz1 and briz2 embryos are alive but growth-arrested. Germination of briz mutants is improved by addition of the carotenoid biosynthetic inhibitor fluridone or gibberellic acid (GA3), and briz mutants have improved development in backgrounds deficient in ABA synthesis (gin1-3/aba2) or signaling (abi5-7). Endogenous ABA is not higher in briz2 seeds compared to wild-type seeds, and exogenous ABA does not affect BRIZ mRNAs in imbibed seeds. These results indicate that briz embryos are hypersensitive to ABA and that under normal growth conditions, BRIZ acts to suppress ABA signaling or response. ABA signaling and sugar signaling are linked, and we found that briz1 and briz2 mutants excised from seed coats are hypersensitive to sucrose. Although briz single mutants do not grow to maturity, we were able to generate mature briz2-3 abi5-7 double mutant plants that produced seeds. These seeds are more sensitive to exogenous sugar and are larger than seeds from sibling abi5-7 BRIZ2/briz2-3 plants, suggesting that BRIZ has a parental effect on seed development. From these data, we propose a model in which the BRIZ E3 ligase suppresses ABA responses during seed maturation and germination and early seedling establishment.

CERBERUS is critical for stabilization of VAPYRIN during rhizobial infection in Lotus japonicus

CERBERUS (also known as LIN) and VAPYRIN (VPY) are essential for infection of legumes by rhizobia and arbuscular mycorrhizal fungi (AMF). Medicago truncatula LIN (MtLIN) was reported to interact with MtVPY, but the significance of this interaction is unclear and the function of VPY in Lotus japonicus has not been studied. We demonstrate that CERBERUS has auto-ubiquitination activity in vitro and is localized within distinct motile puncta in L. japonicus root hairs and in Nicotiana benthamiana leaves. CERBERUS colocalized with the trans-Golgi network/early endosome markers. In L. japonicus, two VPY orthologs (LjVPY1 and LjVPY2) were identified. CERBERUS interacted with and colocalized with both LjVPY1 and LjVPY2. Co-expression of CERBERUS with LjVPY1 or LjVPY2 in N. benthamiana led to increased protein levels of LjVPY1 and LjVPY2, which accumulated as mobile punctate bodies in the cytoplasm. Conversely, LjVPY2 protein levels decreased in cerberus roots after rhizobial inoculation. Mutant analysis indicates that LjVPY1 and LjVPY2 are required for rhizobial infection and colonization by AMF. Our data suggest that CERBERUS stabilizes LjVPY1 and LjVPY2 within the trans-Golgi network/early endosome, where they might function to regulate endocytic trafficking and/or the formation or recycling of signaling complexes during rhizobial and AMF symbiosis.

Loss of the Acetyltransferase NAA50 Induces Endoplasmic Reticulum Stress and Immune Responses and Suppresses Growth

Stress signaling in plants is carefully regulated to ensure proper development and reproductive fitness. Overactive defense signaling can result in dwarfism as well as developmental defects. In addition to requiring a substantial amount of energy, plant stress responses place a burden upon the cellular machinery, which can result in the accumulation of misfolded proteins and endoplasmic reticulum (ER) stress. Negative regulators of stress signaling, such as ENHANCED DISEASE RESISTANCE 1 (EDR1), ensure that stress responses are properly suspended when they are not needed, thereby conserving energy for growth and development. Here, we describe the role of an uncharacterized N-terminal acetyltransferase, NAA50, in the regulation of plant development and stress responses in Arabidopsis (Arabidopsis thaliana). Our results demonstrate that NAA50, an interactor of EDR1, plays an important role in regulating the tradeoff between plant growth and defense. Plants lacking NAA50 display severe developmental defects as well as induced stress responses. Reduction of NAA50 expression results in arrested stem and root growth as well as senescence. Furthermore, our results demonstrate that the loss of NAA50 results in constitutive ER stress signaling, indicating that NAA50 may be required for the suppression of ER stress. This work establishes NAA50 as essential for plant development and the suppression of stress responses, potentially through the regulation of ER stress.

Global profiling of plant nuclear membrane proteome in Arabidopsis

The nuclear envelope (NE) is structurally and functionally vital for eukaryotic cells, yet its protein constituents and their functions are poorly understood in plants. Here, we combined subtractive proteomics and proximity-labelling technology coupled with quantitative mass spectrometry to understand the landscape of NE membrane proteins in Arabidopsis. We identified ~200 potential candidates for plant NE transmembrane (PNET) proteins, which unravelled the compositional diversity and uniqueness of the plant NE. One of the candidates, named PNET1, is a homologue of human TMEM209, a critical driver for lung cancer. A functional investigation revealed that PNET1 is a bona fide nucleoporin in plants. It displays both physical and genetic interactions with the nuclear pore complex (NPC) and is essential for embryo development and reproduction in different NPC contexts. Our study substantially enlarges the plant NE proteome and sheds new light on the membrane composition and function of the NPC.

The Arabidopsis exocyst subunits EXO70B1 and EXO70B2 regulate FLS2 homeostasis at the plasma membrane

The plasma membrane (PM)-localized receptor kinase FLAGELLIN SENSING 2 (FLS2) recognizes bacterial flagellin or its immunogenic epitope flg22, and initiates microbe-associated molecular pattern-triggered immunity, which inhibits infection by bacterial pathogens. The localization, abundance and activity of FLS2 are under dynamic control. Here, we demonstrate that Arabidopsis thaliana EXO70B1, a subunit of the exocyst complex, plays a critical role in FLS2 signaling that is independent of the truncated Toll/interleukin-1 receptor-nucleotide binding sequence protein TIR-NBS2 (TN2). In the exo70B1-3 mutant, the abundance of FLS2 protein at the PM is diminished, consistent with the impaired flg22 response of this mutant. EXO70B1-GFP plants showed increased FLS2 accumulation at the PM and therefore enhanced FLS2 signaling. The EXO70B1-mediated trafficking of FLS2 to the PM is partially independent of the PENETRATION 1 (PEN1)-containing secretory pathway. In addition, EXO70B1 interacts with EXO70B2, a close homolog of EXO70B1, and both proteins associate with FLS2 and contribute to the accumulation of FLS2 at the PM. Taken together, our data suggest that the exocyst complex subunits EXO70B1 and EXO70B2 regulate the trafficking of FLS2 to the PM, which represents a new layer of regulation of FLS2 function in plant immunity.

Proximity labeling proteomics reveals critical regulators for inner nuclear membrane protein degradation in plants

The inner nuclear membrane (INM) selectively accumulates proteins that are essential for nuclear functions; however, overaccumulation of INM proteins results in a range of rare genetic disorders. So far, little is known about how defective, mislocalized, or abnormally accumulated membrane proteins are actively removed from the INM, especially in plants and animals. Here, via analysis of a proximity-labeling proteomic profile of INM-associated proteins in Arabidopsis, we identify critical components for an INM protein degradation pathway. We show that this pathway relies on the CDC48 complex for INM protein extraction and 26S proteasome for subsequent protein degradation. Moreover, we show that CDC48 at the INM may be regulated by a subgroup of PUX proteins, which determine the substrate specificity or affect the ATPase activity of CDC48. These PUX proteins specifically associate with the nucleoskeleton underneath the INM and physically interact with CDC48 proteins to negatively regulate INM protein degradation in plants.

Vacuole Biogenesis in Plants: How Many Vacuoles, How Many Models?

Vacuoles are the largest membrane-bounded organelles and have essential roles in plant growth and development, but several important questions on the biogenesis and dynamics of lytic vacuoles (LVs) remain. Here, we summarize and discuss recent research and models of vacuole formation, and propose, with testable hypotheses, that besides inherited vacuoles, plant cells can also synthesize LVs de novo from multiple organelles and routes in response to growth and development or external factors. Therefore, LVs may be further classified into different subgroups and/or populations with different pH, cargos, and functions, among which multivesicular body (MVB)-derived small vacuoles are the main source for central vacuole formation in arabidopsis root cortical cells.

EPSIN1 Modulates the Plasma Membrane Abundance of FLAGELLIN SENSING2 for Effective Immune Responses

The plasma membrane (PM) provides a critical interface between plant cells and their environment to control cellular responses. To perceive the bacterial flagellin peptide flg22 for effective defense signaling, the immune receptor FLAGELLIN SENSING2 (FLS2) needs to be at its site of function, the PM, in the correct abundance. However, the intracellular machinery that controls PM accumulation of FLS2 remains largely undefined. The Arabidopsis (Arabidopsis thaliana) clathrin adaptor EPSIN1 (EPS1) is implicated in clathrin-coated vesicle formation at the trans-Golgi network (TGN), likely aiding the transport of cargo proteins from the TGN for proper location; but EPS1's impact on physiological responses remains elusive. Here, we identify EPS1 as a positive regulator of flg22 signaling and pattern-triggered immunity against Pseudomonas syringae pv tomato DC3000. We provide evidence that EPS1 contributes to modulating the PM abundance of defense proteins for effective immune signaling because in eps1, impaired flg22 signaling correlated with reduced PM accumulation of FLS2 and its coreceptor BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1). The eps1 mutant also exhibited reduced responses to the pathogen/damage-associated molecular patterns elf26 and AtPep1, which are perceived by the coreceptor BAK1 and cognate PM receptors. Furthermore, quantitative proteomics of enriched PM fractions revealed that EPS1 was required for proper PM abundance of a discrete subset of proteins with different cellular functions. In conclusion, our study expands the limited understanding of the physiological roles of EPSIN family members in plants and provides novel insight into the TGN-associated clathrin-coated vesicle trafficking machinery that impacts plant PM-derived defense processes.

Plant Cells under Attack: Unconventional Endomembrane Trafficking during Plant Defense

Since plants lack specialized immune cells, each cell has to defend itself independently against a plethora of different pathogens. Therefore, successful plant defense strongly relies on precise and efficient regulation of intracellular processes in every single cell. Smooth trafficking within the plant endomembrane is a prerequisite for a diverse set of immune responses. Pathogen recognition, signaling into the nucleus, cell wall enforcement, secretion of antimicrobial proteins and compounds, as well as generation of reactive oxygen species, all heavily depend on vesicle transport. In contrast, pathogens have developed a variety of different means to manipulate vesicle trafficking to prevent detection or to inhibit specific plant responses. Intriguingly, the plant endomembrane system exhibits remarkable plasticity upon pathogen attack. Unconventional trafficking pathways such as the formation of endoplasmic reticulum (ER) bodies or fusion of the vacuole with the plasma membrane are initiated and enforced as the counteraction. Here, we review the recent findings on unconventional and defense-induced trafficking pathways as the plant´s measures in response to pathogen attack. In addition, we describe the endomembrane system manipulations by different pathogens, with a focus on tethering and fusion events during vesicle trafficking.

The plastid phosphorylase as a multiple-role player in plant metabolism

The physiological roles of the plastidial phosphorylase in starch metabolism of higher plants have been debated for decades. While estimated physiological substrate levels favor a degradative role, genetic evidence indicates that the plastidial phosphorylase (Pho1) plays an essential role in starch initiation and maturation of the starch granule in developing rice grains. The plastidial enzyme contains a unique peptide domain, up to 82 residues in length depending on the plant species, not found in its cytosolic counterpart or glycogen phosphorylases. The role of this extra peptide domain is perplexing, as its complete removal does not significantly affect the in vitro catalytic or enzymatic regulatory properties of rice Pho1. This peptide domain may have a regulatory function as it contains potential phosphorylation sites and, in some plant Pho1s, a PEST motif, a substrate for proteasome-mediated degradation. We discuss the potential roles of Pho1 and its L80 domain in starch biosynthesis and photosynthesis.

Turnip Mosaic Virus Components Are Released into the Extracellular Space by Vesicles in Infected Leaves

Turnip mosaic virus (TuMV) reorganizes the endomembrane system of the infected cell to generate endoplasmic-reticulum-derived motile vesicles containing viral replication complexes. The membrane-associated viral protein 6K2 plays a key role in the formation of these vesicles. Using confocal microscopy, we observed that this viral protein, a marker for viral replication complexes, localized in the extracellular space of infected Nicotiana benthamiana leaves. Previously, we showed that viral RNA is associated with multivesicular bodies (MVBs). Here, using transmission electron microscopy, we observed the proliferation of MVBs during infection and their fusion with the plasma membrane that resulted in the release of their intraluminal vesicles in the extracellular space. Immunogold labeling with a monoclonal antibody that recognizes double-stranded RNA indicated that the released vesicles contained viral RNA. Focused ion beam-extreme high-resolution scanning electron microscopy was used to generate a three-dimensional image that showed extracellular vesicles in the cell wall. The presence of TuMV proteins in the extracellular space was confirmed by proteomic analysis of purified extracellular vesicles from N benthamiana and Arabidopsis (Arabidopsis thaliana). Host proteins involved in biotic defense and in interorganelle vesicular exchange were also detected. The association of extracellular vesicles with viral proteins and RNA emphasizes the implication of the plant extracellular space in viral infection.

Targeting the pattern-triggered immunity pathway to enhance resistance to Fusarium graminearum

Fusarium head blight (FHB) is a disease of the floral tissues of wheat and barley for which highly resistant varieties are not available. Thus, there is a need to identify genes/mechanisms that can be targeted for the control of this devastating disease. Fusarium graminearum is the primary causal agent of FHB in North America. In addition, it also causes Fusarium seedling blight. Fusarium graminearum can also cause disease in the model plant Arabidopsis thaliana. The Arabidopsis-F. graminearum pathosystem has facilitated the identification of targets for the control of disease caused by this fungus. Here, we show that resistance against F. graminearum can be enhanced by flg22, a bacterial microbe-associated molecular pattern (MAMP). flg22-induced resistance in Arabidopsis requires its cognate pattern recognition receptor (PRR) FLS2, and is accompanied by the up-regulation of WRKY29. The expression of WRKY29, which is associated with pattern-triggered immunity (PTI), is also induced in response to F. graminearum infection. Furthermore, WRKY29 is required for basal resistance as well as flg22-induced resistance to F. graminearum. Moreover, constitutive expression of WRKY29 in Arabidopsis enhances disease resistance. The PTI pathway is also activated in response to F. graminearum infection of wheat. Furthermore, flg22 application and ectopic expression of WRKY29 enhance FHB resistance in wheat. Thus, we conclude that the PTI pathway provides a target for the control of FHB in wheat. We further show that the ectopic expression of WRKY29 in wheat results in shorter stature and early heading time, traits that are important to wheat breeding.

Cation Specificity of Vacuolar NHX-Type Cation/H+ Antiporters

Cation/H⁺ (NHX-type) antiporters are important regulators of intracellular ion homeostasis and are critical for cell expansion and plant stress acclimation. In Arabidopsis (Arabidopsis thaliana), four distinct NHX isoforms, named AtNHX1 to AtNHX4, locate to the tonoplast. To determine the concerted roles of all tonoplast NHXs on vacuolar ion and pH homeostasis, we examined multiple knockout mutants lacking all but one of the four vacuolar isoforms and quadruple knockout plants lacking any vacuolar NHX activity. The nhx triple and quadruple knockouts displayed reduced growth phenotypes. Exposure to sodium chloride improved growth while potassium chloride was deleterious to some knockouts. Kinetic analysis of K⁺ and Na⁺ transport indicated that AtNHX1 and AtNHX2 are the main contributors to both vacuolar pH and K⁺ and Na⁺ uptake, while AtNHX3 and AtNHX4 differ in Na⁺/K⁺ selectivity. The lack of any vacuolar NHX activity resulted in no K⁺ uptake, highly acidic vacuoles, and reduced but not abolished vacuolar Na⁺ uptake. Additional K⁺/H⁺ and Na⁺/H⁺ exchange activity assays in the quadruple knockout indicated Na⁺ uptake that was not H⁺ coupled, suggesting the existence of an alternative, cation/H⁺-independent, Na⁺ conductive pathway in vacuoles. These results highlight the importance of NHX-type cation/H⁺ antiporters in the maintenance of cellular cation homeostasis and in growth and development.

Root Adaptation to H2O2-Induced Oxidative Stress by ARF-GEF BEN1- and Cytoskeleton-Mediated PIN2 Trafficking

Abiotic stress poses constant challenges for plant survival and is a serious problem for global agricultural productivity. On a molecular level, stress conditions result in elevation of reactive oxygen species (ROS) production causing oxidative stress associated with oxidation of proteins and nucleic acids as well as impairment of membrane functions. Adaptation of root growth to ROS accumulation is facilitated through modification of auxin and cytokinin hormone homeostasis. Here, we report that in Arabidopsis root meristem, ROS-induced changes of auxin levels correspond to decreased abundance of PIN auxin efflux carriers at the plasma membrane (PM). Specifically, increase in H2O2 levels affects PIN2 endocytic recycling. We show that the PIN2 intracellular trafficking during adaptation to oxidative stress requires the function of the ADP-ribosylation factor (ARF)-guanine-nucleotide exchange factor (GEF) BEN1, an actin-associated regulator of the trafficking from the PM to early endosomes and, presumably, indirectly, trafficking to the vacuoles. We propose that H2O2 levels affect the actin dynamics thus modulating ARF-GEF-dependent trafficking of PIN2. This mechanism provides a way how root growth acclimates to stress and adapts to a changing environment.

Plant and animal PR1 family members inhibit programmed cell death and suppress bacterial pathogens in plant tissues

A role for programmed cell death (PCD) has been established as the basis for plant-microbe interactions. A functional plant-based cDNA library screen identified possible anti-PCD genes, including one member of the PR1 family, designated P14a, from tomato. Members of the PR1 family have been subject to extensive research in view of their possible role in resistance against pathogens. The PR1 family is represented in every plant species studied to date and homologues have been found in animals, fungi and insects. However, the biological function of the PR1 protein from plants has remained elusive in spite of extensive research regarding a role in the response of plants to disease. Constitutive expression of P14a in transgenic tomato roots protected the roots against PCD triggered by Fumonisin B1, as did the human orthologue GLIPR1, indicating a kingdom crossing function for PR1. Tobacco plants transformed with a P14a-GFP fusion construct and inoculated with Pseudomonas syringae pv. tabaci revealed that the mRNA was abundant throughout the leaves, but the fusion protein was restricted to the lesion margins, where cell death and bacterial spread were arrested. Vitus vinifera grapes expressing the PR1 homologue P14a as a transgene were protected against the cell death symptoms of Pierce's disease. A pull-down assay identified putative PR1-interacting proteins, including members of the Rac1 immune complex, known to function in innate immunity in rice and animal systems. The findings herein are consistent with a role of PR1 in the suppression of cell death-dependent disease symptoms and a possible mode of action.

Ubiquitylation in plants: signaling hub for the integration of environmental signals

A fundamental question in biology is how organisms integrate the plethora of environmental cues that they perceive to trigger a co-ordinated response. The regulation of protein stability, which is largely mediated by the ubiquitin-proteasome system in eukaryotes, plays a pivotal role in these processes. Due to their sessile lifestyle and the need to respond rapidly to a multitude of environmental factors, plants are thought to be especially dependent on proteolysis to regulate cellular processes. In this review, we present the complexity of the ubiquitin system in plants, and discuss the relevance of the proteolytic and non-proteolytic roles of this system in the regulation and co-ordination of plant responses to environmental signals. We also discuss the role of the ubiquitin system as a key regulator of plant signaling pathways. We focus more specifically on the functions of E3 ligases as regulators of the jasmonic acid (JA), salicylic acid (SA), and ethylene hormone signaling pathways that play important roles to mount a co-ordinated response to multiple environmental stresses. We also provide examples of new players in this field that appear to integrate different cues and signaling pathways.

The Cytokinin Oxidase/Dehydrogenase CKX1 Is a Membrane-Bound Protein Requiring Homooligomerization in the Endoplasmic Reticulum for Its Cellular Activity

Degradation of the plant hormone cytokinin is controlled by cytokinin oxidase/dehydrogenase (CKX) enzymes. The molecular and cellular behavior of these proteins is still largely unknown. In this study, we show that CKX1 is a type II single-pass membrane protein that localizes predominantly to the endoplasmic reticulum (ER) in Arabidopsis (Arabidopsis thaliana). This indicates that this CKX isoform is a bona fide ER protein directly controlling the cytokinin, which triggers the signaling from the ER. By using various approaches, we demonstrate that CKX1 forms homodimers and homooligomers in vivo. The amino-terminal part of CKX1 was necessary and sufficient for the protein oligomerization as well as for targeting and retention in the ER. Moreover, we show that protein-protein interaction is largely facilitated by transmembrane helices and depends on a functional GxxxG-like interaction motif. Importantly, mutations rendering CKX1 monomeric interfere with its steady-state localization in the ER and cause a loss of the CKX1 biological activity by increasing its ER-associated degradation. Therefore, our study provides evidence that oligomerization is a crucial parameter regulating CKX1 biological activity and the cytokinin concentration in the ER. The work also lends strong support for the cytokinin signaling from the ER and for the functional relevance of the cytokinin pool in this compartment.

Degradation of the stress-responsive enzyme formate dehydrogenase by the RING-type E3 ligase Keep on Going and the ubiquitin 26S proteasome system

KEG is involved in mediating the proteasome-dependent degradation of FDH, a stress-responsive enzyme. The UPS may function to suppress FDH mediated stress responses under favorable growth conditions. Formate dehydrogenase (FDH) has been studied in bacteria and yeasts for the purpose of industrial application of NADH co-factor regeneration. In plants, FDH is regarded as a universal stress protein involved in responses to various abiotic and biotic stresses. Here we show that FDH abundance is regulated by the ubiquitin proteasome system (UPS). FDH is ubiquitinated in planta and degraded by the 26S proteasome. Interaction assays identified FDH as a potential substrate for the RING-type ubiquitin ligase Keep on Going (KEG). KEG is capable of attaching ubiquitin to FDH in in vitro assays and the turnover of FDH was increased when co-expressed with a functional KEG in planta, suggesting that KEG contributes to FDH degradation. Consistent with a role in regulating FDH abundance, transgenic plants overexpressing KEG were more sensitive to the inhibitory effects of formate. In addition, FDH is a phosphoprotein and dephosphorylation was found to increase the stability of FDH in degradation assays. Based on results from this and previous studies, we propose a model where KEG mediates the ubiquitination and subsequent degradation of phosphorylated FDH and, in response to unfavourable growth conditions, reduction in FDH phosphorylation levels may prohibit turnover allowing the stabilized FDH to facilitate stress responses.

Tomato Prenylated RAB Acceptor Protein 1 Modulates Trafficking and Degradation of the Pattern Recognition Receptor LeEIX2, Affecting the Innate Immune Response

Plants recognize microbial/pathogen associated molecular patterns (MAMP/PAMP) through pattern recognition receptors (PRRs) triggering an immune response against pathogen progression. MAMP/PAMP triggered immune response requires PRR endocytosis and trafficking for proper deployment. LeEIX2 is a well-known Solanum lycopersicum RLP-PRR, able to recognize and respond to the fungal MAMP/PAMP ethylene-inducing xylanase (EIX), and its function is highly dependent on intracellular trafficking. Identifying protein machinery components regulating LeEIX2 intracellular trafficking is crucial to our understanding of LeEIX2 mediated immune responses. In this work, we identified a novel trafficking protein, SlPRA1A, a predicted regulator of RAB, as an interactor of LeEIX2. Overexpression of SlPRA1A strongly decreases LeEIX2 endosomal localization, as well as LeEIX2 protein levels. Accordingly, the innate immune responses to EIX are markedly reduced by SlPRA1A overexpression, presumably due to a decreased LeEIX2 availability. Studies into the role of SlPRA1A in LeEIX2 trafficking revealed that LeEIX2 localization in multivesicular bodies/late endosomes is augmented by SlPRA1A. Furthermore, inhibiting vacuolar function prevents the LeEIX2 protein level reduction mediated by SlPRA1A, suggesting that SlPRA1A may redirect LeEIX2 trafficking to the vacuole for degradation. Interestingly, SlPRA1A overexpression reduces the amount of several RLP-PRRs, but does not affect the protein level of receptor-like kinase PRRs, suggesting a specific role of SlPRA1A in RLP-PRR trafficking and degradation.

Conventional and unconventional ubiquitination in plant immunity

Ubiquitination is one of the most abundant types of protein post-translational modification (PTM) in plant cells. The importance of ubiquitination in the regulation of many aspects of plant immunity has been increasingly appreciated in recent years. Most of the studies linking ubiquitination to the plant immune system, however, have been focused on the E3 ubiquitin ligases and the conventional ubiquitination that leads to the degradation of the substrate proteins by the 26S proteasome. By contrast, our knowledge about the role of unconventional ubiquitination that often serves as non-degradative, regulatory signal remains a significant gap. We discuss, in this review, the recent advances in our understanding of ubiquitination in the modulation of plant immunity, with a particular focus on the E3 ubiquitin ligases. We approach the topic from a perspective of two broadly defined types of ubiquitination in an attempt to highlight the importance, yet current scarcity, in our knowledge about the regulation of plant immunity by unconventional ubiquitination.

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.

Non-26S Proteasome Endomembrane Trafficking Pathways in ABA Signaling

The phytohormone abscisic acid (ABA) is a vital endogenous messenger that regulates diverse physiological processes in plants. The regulation of ABA signaling has been well studied at both the transcriptional and translational levels. Post-translational modification of key regulators in ABA signaling by the 26S ubiquitin proteasome pathway is well known. Recently, increasing evidence demonstrates that atypical turnover of key regulators by the endocytic trafficking pathway and autophagy also play vital roles in ABA perception, signaling, and action. We summarize and synthesize here recent findings in the field of ABA signaling.

Membrane Trafficking in Plant Immunity

Plants employ sophisticated mechanisms to interact with pathogenic as well as beneficial microbes. Of those, membrane trafficking is key in establishing a rapid and precise response. Upon interaction with pathogenic microbes, surface-localized immune receptors undergo endocytosis for signal transduction and activity regulation while cell wall components, antimicrobial compounds, and defense proteins are delivered to pathogen invasion sites through polarized secretion. To sustain mutualistic associations, host cells also reprogram the membrane trafficking system to accommodate invasive structures of symbiotic microbes. Here, we provide an analysis of recent advances in understanding the roles of secretory and endocytic membrane trafficking pathways in plant immune activation. We also discuss strategies deployed by adapted microbes to manipulate these pathways to subvert or inhibit plant defense.

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

Deposition of cell wall-reinforcing papillae is an integral component of the plant immune response. The Arabidopsis PENETRATION 3 (PEN3) ATP binding cassette (ABC) transporter plays a role in defense against numerous pathogens and is recruited to sites of pathogen detection where it accumulates within papillae. However, the trafficking pathways and regulatory mechanisms contributing to recruitment of PEN3 and other defenses to the host-pathogen interface are poorly understood. Here, we report a confocal microscopy-based screen to identify mutants with altered localization of PEN3-GFP after inoculation with powdery mildew fungi. We identified a mutant, aberrant localization of PEN3 3 (alp3), displaying accumulation of the normally plasma membrane (PM)-localized PEN3-GFP in endomembrane compartments. The mutant was found to be disrupted in the P₄-ATPase AMINOPHOSPHOLIPID ATPASE 3 (ALA3), a lipid flippase that plays a critical role in vesicle formation. We provide evidence that PEN3 undergoes continuous endocytic cycling from the PM to the trans-Golgi network (TGN). In alp3, PEN3 accumulates in the TGN, causing delays in recruitment to the host-pathogen interface. Our results indicate that PEN3 and other defense proteins continuously cycle through the TGN and that timely exit of these proteins from the TGN is critical for effective pre-invasive immune responses against powdery mildews.

Subcellular Localization of Arabidopsis Pathogenesis-Related 1 (PR1) Protein

The Arabidopsisthaliana pathogenesis-related 1 (PR1) is an important defense protein, so far it has only been detected in extracellular space and its subcellular sorting and transport remain unexplained. Using a green fluorescent protein (GFP) tagged full length, as well as a C-terminus truncated version of PR1, we observed that when expressed ectopically in Nicotiana benthamiana leaves, PR1 co-localizes only partially with Golgi markers, and much more prominently with the late endosome (LE)/multivesicular body (MVB) FYVE marker. The C-truncated version PR1ΔC predominantly localized to the endoplasmic reticulum (ER). The same localizations were found for stable Arabidopsis transformants with expression of PR1 and PR1ΔC driven by the native promoter. We conclude that the A. thaliana PR1 (AtPR1) undergoes an unconventional secretion pathway, starting from the C-terminus-dependent sorting from the ER, and utilizing further transportation via phosphatidyl-inositol-3-phosphate (PI(3)P) positive LE/MVB-like vesicles. The homology model of the PR1 structure shows that the cluster of positively charged amino acid residues (arginines 60, 67, 137, and lysine 135) could indeed interact with negatively charged phospholipids of cellular membranes. It remains to be resolved whether Golgi and LE/MVB localization reflects an alternative sorting or trafficking succession, and what the role of lipid interactions in it will be.

Subcellular Localization of Arabidopsis Pathogenesis-Related 1 (PR1) Protein

The Arabidopsisthaliana pathogenesis-related 1 (PR1) is an important defense protein, so far it has only been detected in extracellular space and its subcellular sorting and transport remain unexplained. Using a green fluorescent protein (GFP) tagged full length, as well as a C-terminus truncated version of PR1, we observed that when expressed ectopically in Nicotiana benthamiana leaves, PR1 co-localizes only partially with Golgi markers, and much more prominently with the late endosome (LE)/multivesicular body (MVB) FYVE marker. The C-truncated version PR1ΔC predominantly localized to the endoplasmic reticulum (ER). The same localizations were found for stable Arabidopsis transformants with expression of PR1 and PR1ΔC driven by the native promoter. We conclude that the A. thaliana PR1 (AtPR1) undergoes an unconventional secretion pathway, starting from the C-terminus-dependent sorting from the ER, and utilizing further transportation via phosphatidyl-inositol-3-phosphate (PI(3)P) positive LE/MVB-like vesicles. The homology model of the PR1 structure shows that the cluster of positively charged amino acid residues (arginines 60, 67, 137, and lysine 135) could indeed interact with negatively charged phospholipids of cellular membranes. It remains to be resolved whether Golgi and LE/MVB localization reflects an alternative sorting or trafficking succession, and what the role of lipid interactions in it will be.

Functional characterization of the chaperon-like protein Cdc48 in cryptogein-induced immune response in tobacco

Cdc48, a molecular chaperone conserved in different kingdoms, is a member of the AAA+ family contributing to numerous processes in mammals including proteins quality control and degradation, vesicular trafficking, autophagy and immunity. The functions of Cdc48 plant orthologues are less understood. We previously reported that Cdc48 is regulated by S-nitrosylation in tobacco cells undergoing an immune response triggered by cryptogein, an elicitin produced by the oomycete Phytophthora cryptogea. Here, we inv estigated the function of NtCdc48 in cryptogein signalling and induced hypersensitive-like cell death. NtCdc48 was found to accumulate in elicited cells at both the protein and transcript levels. Interestingly, only a small proportion of the overall NtCdc48 population appeared to be S-nitrosylated. Using gel filtration in native conditions, we confirmed that NtCdc48 was present in its hexameric active form. An immunoprecipitation-based strategy following my mass spectrometry analysis led to the identification of about a hundred NtCdc48 partners and underlined its contribution in cellular processes including targeting of ubiquitylated proteins for proteasome-dependent degradation, subcellular trafficking and redox regulation. Finally, the analysis of cryptogein-induced events in NtCdc48-overexpressing cells highlighted a correlation between NtCdc48 expression and hypersensitive cell death. Altogether, this study identified NtCdc48 as a component of cryptogein signalling and plant immunity.

An Aphid Effector Targets Trafficking Protein VPS52 in a Host-Specific Manner to Promote Virulence

Plant- and animal-feeding insects secrete saliva inside their hosts, containing effectors, which may promote nutrient release and suppress immunity. Although for plant pathogenic microbes it is well established that effectors target host proteins to modulate host cell processes and promote disease, the host cell targets of herbivorous insects remain elusive. Here, we show that the existing plant pathogenic microbe effector paradigm can be extended to herbivorous insects in that effector-target interactions inside host cells modify critical host processes to promote plant susceptibility. We showed that the effector Mp1 from Myzus persicae associates with the host Vacuolar Protein Sorting Associated Protein52 (VPS52). Using natural variants, we provide a strong link between effector virulence activity and association with VPS52, and show that the association is highly specific to Mpersicae-host interactions. Also, coexpression of Mp1, but not Mp1-like variants, specifically with host VPS52s resulted in effector relocalization to vesicle-like structures that associate with prevacuolar compartments. We show that high VPS52 levels negatively impact virulence, and that aphids are able to reduce VPS52 levels during infestation, indicating that VPS52 is an important virulence target. Our work is an important step forward in understanding, at the molecular level, how a major agricultural pest promotes susceptibility during infestation of crop plants. We give evidence that an herbivorous insect employs effectors that interact with host proteins as part of an effective virulence strategy, and that these effectors likely function in a species-specific manner.

Extracellular Vesicles Isolated from the Leaf Apoplast Carry Stress-Response Proteins

Exosomes are extracellular vesicles (EVs) that play a central role in intercellular signaling in mammals by transporting proteins and small RNAs. Plants are also known to produce EVs, particularly in response to pathogen infection. The contents of plant EVs have not been analyzed, however, and their function is unknown. Here, we describe a method for purifying EVs from the apoplastic fluids of Arabidopsis (Arabidopsis thaliana) leaves. Proteomic analyses of these EVs revealed that they are highly enriched in proteins involved in biotic and abiotic stress responses. Consistent with this finding, EV secretion was enhanced in plants infected with Pseudomonas syringae and in response to treatment with salicylic acid. These findings suggest that EVs may represent an important component of plant immune responses.

The Kinase Activity of Calcineurin B-like Interacting Protein Kinase 26 (CIPK26) Influences Its Own Stability and that of the ABA-regulated Ubiquitin Ligase, Keep on Going (KEG)

The Really Interesting New Gene (RING)-type E3 ligase, Keep on Going (KEG) plays a critical role in Arabidopsis growth after germination and the connections between KEG and hormone signaling pathways are expanding. With regards to abscisic acid (ABA) signaling, KEG targets ABA-responsive transcription factors abscisic acid insensitive 5, ABF1 and ABF3 for ubiquitination and subsequent degradation through the 26S proteasome. Regulation of E3 ligases through self-ubiquitination is common to RING-type E3 ligases and ABA promotes KEG self-ubiquitination and degradation. ABA-mediated degradation of KEG is phosphorylation-dependent; however, upstream signaling proteins that may regulate KEG stability have not been characterized. In this report, we show that CBL-Interacting Protein Kinase (CIPK) 26 can phosphorylate KEG in vitro. Using both in vitro and in planta degradation assays we provide evidence which suggests that the kinase activity of CIPK26 promotes the degradation of KEG. Furthermore, we found that the kinase activity of CIPK26 also influences its own stability; a constitutively active version is more stable than a wild type or a kinase dead version. Our results suggest a reciprocal regulation model wherein an activated and stable CIPK26 phosphorylates KEG to promote degradation of the E3.

Different Endomembrane Trafficking Pathways Establish Apical and Basal Polarities

The endomembrane system is an interconnected network required to establish signal transduction, cell polarity, and cell shape in response to developmental or environmental stimuli. In the model plant Arabidopsis thaliana, there are numerous markers to visualize polarly localized plasma membrane proteins utilizing endomembrane trafficking. Previous studies have shown that the large ARF-GEF GNOM plays a key role in the establishment of basal (rootward) polarity, whereas the apically (shootward) polarized membrane proteins undergo sorting via different routes. However, the mechanism that maintains apical polarity is largely unknown. Here, we used a chemical genomic approach and identified the compound endosidin 16 (ES16), which perturbed apically localized plasma membrane proteins without affecting basal polarity. We demonstrated that ES16 is an inhibitor for recycling of apical, lateral, and nonpolar plasma membrane proteins as well as biosynthetic secretion, leaving the basal proteins as the only exceptions not subject to ES16 inhibition. Further evidence from pharmaceutical and genetic data revealed that ES16 effects are mediated through the regulation of small GTPase RabA proteins and that RabA GTPases work in concert with the BIG clade ARF-GEF to modulate the nonbasal trafficking. Our results reveal that ES16 defines a distinct pathway for endomembrane sorting routes and is essential for the establishment of cell polarity.

TWS1, a Novel Small Protein, Regulates Various Aspects of Seed and Plant Development

Small proteins have long been overlooked due to their poor annotation and the experimental challenges they pose. However, in recent years, their role in various processes has started to emerge, opening new research avenues. Here, we present the isolation and characterization of two allelic mutants, twisted seed1-1 (tws1-1) and tws1-2, which exhibit an array of developmental and biochemical phenotypes in Arabidopsis (Arabidopsis thaliana) seeds. We have identified AT5G01075 as the subtending gene encoding a small protein of 81 amino acids localized in the endoplasmic reticulum. TWS1 is strongly expressed in seeds, where it regulates both embryo development and accumulation of storage compounds. TWS1 loss-of-function seeds exhibit increased starch, sucrose, and protein accumulation at the detriment of fatty acids. TWS1 is also expressed in vegetative and reproductive tissues, where it is responsible for proper epidermal cell morphology and overall plant growth. At the cellular level, TWS1 is responsible for cuticle deposition on epidermal cells and organization of the endomembrane system. Finally, we show that TWS1 is a single-copy gene in Arabidopsis, and it is specifically conserved among angiosperms.

The Multivesicular Bodies (MVBs)-Localized AAA ATPase LRD6-6 Inhibits Immunity and Cell Death Likely through Regulating MVBs-Mediated Vesicular Trafficking in Rice

Previous studies have shown that multivesicular bodies (MVBs)/endosomes-mediated vesicular trafficking may play key roles in plant immunity and cell death. However, the molecular regulation is poorly understood in rice. Here we report the identification and characterization of a MVBs-localized AAA ATPase LRD6-6 in rice. Disruption of LRD6-6 leads to enhanced immunity and cell death in rice. The ATPase activity and homo-dimerization of LRD6-6 is essential for its regulation on plant immunity and cell death. An ATPase inactive mutation (LRD6-6^E315Q) leads to dominant-negative inhibition in plants. The LRD6-6 protein co-localizes with the MVBs marker protein RabF1/ARA6 and interacts with ESCRT-III components OsSNF7 and OsVPS2. Further analysis reveals that LRD6-6 is required for MVBs-mediated vesicular trafficking and inhibits the biosynthesis of antimicrobial compounds. Collectively, our study shows that the AAA ATPase LRD6-6 inhibits plant immunity and cell death most likely through modulating MVBs-mediated vesicular trafficking in rice.

Plants have evolved sophistical immunity system in fighting against pathogenic micro-organisms including bacteria, fungi and oomycetes. Upon perception of pathogens, the immune system activates rapid cell death, characterized as a form of hypersensitive response typically in and around the infection sites to restrict pathogen invasion and prevent disease development. Recent studies have suggested that MVBs-mediated vesicular trafficking might play key roles in plant immunity and cell death. However, the molecular regulation is poorly known. By using the lesion resembling disease (lrd) mutant, lrd6-6, which exhibits autoimmunity and spontaneous cell death, we characterized LRD6-6 as a MVBs-localized AAA ATPase. We found that the ATPase LRD6-6 was required for MVBs-mediated vesicular trafficking and inhibited the biosynthesis of antimicrobial compounds for immune response in rice. Both the ATPase activity and homo-dimerization of LRD6-6 were essential for its inhibition on immunity and cell death. The catalytically inactive ATPase, LRD6-6^E315Q, played dominant-negative effect on inhibition of immunity in plants. In addition, the LRD6-6 protein co-localized with the MVBs-spread marker protein RabF1/ARA6 and also interacted with ESCRT-III components OsSNF7 and OsVPS2. In summary, our study has shown that the AAA ATPase LRD6-6 inhibits plant immunity and cell death most likely through modulating MVBs-mediated vesicular trafficking in rice.

Nuclear Pore Permeabilization Is a Convergent Signaling Event in Effector-Triggered Immunity

Nuclear transport of immune receptors, signal transducers, and transcription factors is an essential regulatory mechanism for immune activation. Whether and how this process is regulated at the level of the nuclear pore complex (NPC) remains unclear. Here, we report that CPR5, which plays a key inhibitory role in effector-triggered immunity (ETI) and programmed cell death (PCD) in plants, is a novel transmembrane nucleoporin. CPR5 associates with anchors of the NPC selective barrier to constrain nuclear access of signaling cargos and sequesters cyclin-dependent kinase inhibitors (CKIs) involved in ETI signal transduction. Upon activation by immunoreceptors, CPR5 undergoes an oligomer to monomer conformational switch, which coordinates CKI release for ETI signaling and reconfigures the selective barrier to allow significant influx of nuclear signaling cargos through the NPC. Consequently, these coordinated NPC actions result in simultaneous activation of diverse stress-related signaling pathways and constitute an essential regulatory mechanism specific for ETI/PCD induction.

Biotrophy at Its Best: Novel Findings and Unsolved Mysteries of the Arabidopsis-Powdery Mildew Pathosystem

It is generally accepted in plant-microbe interactions research that disease is the exception rather than a common outcome of pathogen attack. However, in nature, plants with symptoms that signify colonization by obligate biotrophic powdery mildew fungi are omnipresent. The pervasiveness of the disease and the fact that many economically important plants are prone to infection by powdery mildew fungi drives research on this interaction. The competence of powdery mildew fungi to establish and maintain true biotrophic relationships renders the interaction a paramount example of a pathogenic plant-microbe biotrophy. However, molecular details underlying the interaction are in many respects still a mystery. Since its introduction in 1990, the Arabidopsis-powdery mildew pathosystem has become a popular model to study molecular processes governing powdery mildew infection. Due to the many advantages that the host Arabidopsis offers in terms of molecular and genetic tools this pathosystem has great capacity to answer some of the questions of how biotrophic pathogens overcome plant defense and establish a persistent interaction that nourishes the invader while in parallel maintaining viability of the plant host.

Degradation of the ABA co-receptor ABI1 by PUB12/13 U-box E3 ligases

Clade A protein phosphatase 2Cs (PP2Cs) are abscisic acid (ABA) co-receptors that block ABA signalling by inhibiting the downstream protein kinases. ABA signalling is activated after PP2Cs are inhibited by ABA-bound PYR/PYL/RCAR ABA receptors (PYLs) in Arabidopsis. However, whether these PP2Cs are regulated by other factors remains unknown. Here, we report that ABI1 (ABA-INSENSITIVE 1) can interact with the U-box E3 ligases PUB12 and PUB13, but is ubiquitinated only when it interacts with ABA receptors in an in vitro assay. A mutant form of ABI1-1 that is unable to interact with PYLs is more stable than the wild-type protein. Both ABI1 degradation and all tested ABA responses are reduced in pub12 pub13 mutants compared with the wild type. Introducing the abi1-3 loss-of-function mutation into pub12 pub13 mutant recovers the ABA-insensitive phenotypes of the pub12 pub13 mutant. We thus uncover an important regulatory mechanism for regulating ABI1 levels by PUB12 and PUB13.