Exocyst-Positive Organelles and Autophagosomes Are Distinct Organelles in Plants

Full-length transcriptome sequencing of Arabidopsis plants provided new insights into the autophagic regulation of photosynthesis

Autophagy is a highly conserved eukaryotic pathway and plays a crucial role in cell survival under stress conditions. Here, we applied a full-length transcriptome approach to study an Arabidopsis autophagy mutant (atg5-1) subjected to nitrogen-starvation, using Oxford Nanopore Technologies. A total of 39,033 transcripts were identified, including 11,356 new transcripts. In addition, alternative splicing (AS) events and lncRNAs were also detected between Col-0 (WT) and atg5-1. Differentially expressed transcript enrichment showed that autophagy upregulates the expression of many stress-responsive genes and inhibits the transcription of photosynthesis-associated genes. The qRT-PCR results showed that the expression patterns of photosynthesis-related genes in the atg5-1 differed under the conditions of nitrogen starvation and carbon starvation. Under nitrogen starvation treatment, many genes related to photosynthesis also exhibited AS. Chlorophyll fluorescence images revealed that the Fv/Fm and ΦPSII of old atg5-1 leaves were significantly reduced after nitrogen starvation treatment, but the Y(NPQ) indices were significantly increased compared to those of the WT plants. The results of qRT-PCR suggest that autophagy appears to be involved in the degradation of genes related to photodamage repair in PSII. Taken together, the full-length transcriptiome sequencing provide new insights into how new transcripts, lncRNAs and alternative splicing (AS) are involved in plant autophagy through full-length transcriptome sequencing and suggest a new potential link between autophagy and photosynthesis.

Plant-Derived Extracellular Vesicles: A New Revolutionization of Modern Healthy Diets and Biomedical Applications

Plant-derived extracellular vesicles (PDEVs) have recently emerged as a promising area of research due to their potential health benefits and biomedical applications. Produced by various plant species, these EVs contain diverse bioactive molecules, including proteins, lipids, and nucleic acids. Increasing in vitro and in vivo studies have shown that PDEVs have inherent pharmacological activities that affect cellular processes, exerting anti-inflammatory, antioxidant, and anticancer activities, which can potentially contribute to disease therapy and improve human health. Additionally, PDEVs have shown potential as efficient and biocompatible drug delivery vehicles in treating various diseases. However, while PDEVs serve as a potential rising star in modern healthy diets and biomedical applications, further research is needed to address their underlying knowledge gaps, especially the lack of standardized protocols for their isolation, identification, and large-scale production. Furthermore, the safety and efficacy of PDEVs in clinical applications must be thoroughly evaluated. In this review, we concisely discuss current knowledge in the PDEV field, including their characteristics, biomedical applications, and isolation methods, to provide an overview of the current state of PDEV research. Finally, we discuss the challenges regarding the current and prospective issues for PDEVs. This review is expected to provide new insights into healthy diets and biomedical applications of vegetables and fruits, inspiring new advances in natural food-based science and technology.

The EXO70 inhibitor Endosidin2 alters plasma membrane protein composition in Arabidopsis roots

To sustain normal growth and allow rapid responses to environmental cues, plants alter the plasma membrane protein composition under different conditions presumably by regulation of delivery, stability, and internalization. Exocytosis is a conserved cellular process that delivers proteins and lipids to the plasma membrane or extracellular space in eukaryotes. The octameric exocyst complex contributes to exocytosis by tethering secretory vesicles to the correct site for membrane fusion; however, whether the exocyst complex acts universally for all secretory vesicle cargo or just for specialized subsets used during polarized growth and trafficking is currently unknown. In addition to its role in exocytosis, the exocyst complex is also known to participate in membrane recycling and autophagy. Using a previously identified small molecule inhibitor of the plant exocyst complex subunit EXO70A1, Endosidin2 (ES2), combined with a plasma membrane enrichment method and quantitative proteomic analysis, we examined the composition of plasma membrane proteins in the root of Arabidopsis seedlings, after inhibition of the ES2-targetted exocyst complex, and verified our findings by live imaging of GFP-tagged plasma membrane proteins in root epidermal cells. The abundance of 145 plasma membrane proteins was significantly reduced following short-term ES2 treatments and these likely represent candidate cargo proteins of exocyst-mediated trafficking. Gene Ontology analysis showed that these proteins play diverse functions in cell growth, cell wall biosynthesis, hormone signaling, stress response, membrane transport, and nutrient uptake. Additionally, we quantified the effect of ES2 on the spatial distribution of EXO70A1 with live-cell imaging. Our results indicate that the plant exocyst complex mediates constitutive dynamic transport of subsets of plasma membrane proteins during normal root growth.

Unconventional Pathways of Protein Secretion: Mammals vs. Plants

In eukaryotes, many proteins contain an N-terminal signal peptide that allows their translocation into the endoplasmic reticulum followed by secretion outside the cell according to the classical secretory system. However, an increasing number of secreted proteins lacking the signal peptide sequence are emerging. These proteins, secreted in several alternative ways collectively known as unconventional protein secretion (UPS) pathways, exert extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Pathways for UPS include direct transfer across the plasma membrane, secretion from endosomal/multivesicular body-related components, release within plasma membrane-derived microvesicles, or use of elements of autophagy. In this review we describe the mammals and plants UPS pathways identified so far highlighting commonalities and differences.

The effects of osmotic stress on the cell wall-plasma membrane domains of the unicellular streptophyte, Penium margaritaceum

Penium margaritaceum is a unicellular zygnematophyte (basal Streptophyteor Charophyte) that has been used as a model organism for the study of cell walls of Streptophytes and for elucidating organismal adaptations that were key in the evolution of land plants.. When Penium is incubated in sorbitol-enhance medium, i.e., hyperosmotic medium, 1000-1500 Hechtian strands form within minutes and connect the plasma membrane to the cell wall. As cells acclimate to this osmotic stress over time, further significant changes occur at the cell wall and plasma membrane domains. The homogalacturonan lattice of the outer cell wall layer is significantly reduced and is accompanied by the formation of a highly elongate, "filamentous" phenotype. Distinct peripheral thickenings appear between the CW and plasma membrane and contain membranous components and a branched granular matrix. Monoclonal antibody labeling of these thickenings indicates the presence of rhamnogalacturonan-I epitopes. Acclimatization also results in the proliferation of the cell's vacuolar networks and macroautophagy. Penium's ability to acclimatize to osmotic stress offers insight into the transition of ancient zygnematophytes from an aquatic to terrestrial existence.

Extracellular Vesicles: Emerging Players in Plant Defense Against Pathogens

Communication between plants and interacting microorganisms requires functional molecule trafficking, which is essential for host defense and pathogen virulence. Extracellular vesicles (EVs) are single membrane-bound spheres that carry complex cargos, including lipids, proteins, and nucleic acids. They mediate cell-to-cell communication via the transfer of molecules between cells. Plant EVs have been isolated from many plant species and play a prominent role in immune system modulation and plant defense response. Recent studies have shown that plant EVs are emerging players in cross-kingdom regulation and contribute to plant immunity by mediating the trafficking of regulatory small RNA into pathogens, leading to the silencing of pathogen virulence-related genes. This review summarizes the current understanding of plant EV isolation technologies, the role of plant EVs in plant immunity, and the mechanism of plant EV biogenesis, as well as approaches for how these findings can be developed into innovative strategies for crop protection.

The Exocyst Complex Subunit EXO70E1-V From Haynaldia villosa Interacts With Wheat Powdery Mildew Resistance Gene CMPG1-V

EXO70 belongs to the exocyst complex subunit that plays a critical role in regulating plant cell polarity establishment and defense response. A previous study proved that the E3 ligase CMPG1-V from Haynaldia villosa, a diploid wheat relative, positively regulates the resistance to wheat powdery mildew (Pm), caused by fungus Blumeria graminis f.sp tritici (Bgt). In this study, a member of EXO70 superfamily named EXO70E1-V was isolated from H. villosa, and EXO70E1-V interacted with CMPG1-V were shown by yeast two-hybrid (Y2H), pull-down assay, bimolecular fluorescence complementation (BiFC) assay, and luciferase complementation imaging (LCI) assay. It is localized in various subcellular organs, i.e., plasma membrane (PM) and endoplasmic reticulum. Co-expression of EXO70E1-V and CMPG1-V showed dot-like structure fluorescence signals that were mainly in PM and nucleus. Expression of EXO70E1-V was relatively higher in leaf and was significantly induced by Bgt infection and exogenous application of hormones such as salicylic acid. Transient or stable overexpression of EXO70E1-V could not enhance/decrease the Pm resistance level, suggesting overexpression of EXO70E1-V alone has no impact on Pm resistance in wheat.

Ride to cell wall: Arabidopsis XTH11, XTH29 and XTH33 exhibit different secretion pathways and responses to heat and drought stress

The xyloglucan endotransglucosylase/hydrolases (XTHs) are enzymes involved in cell wall assembly and growth regulation, cleaving and re-joining hemicellulose chains in the xyloglucan-cellulose network. Here, in a homologous system, we compare the secretion patterns of XTH11, XTH33 and XTH29, three members of the Arabidopsis thaliana XTH family, selected for the presence (XTH11 and XTH33) or absence (XTH29) of a signal peptide, and the presence of a transmembrane domain (XTH33). We show that XTH11 and XTH33 reached, respectively, the cell wall and plasma membrane through a conventional protein secretion (CPS) pathway, whereas XTH29 moves towards the apoplast following an unconventional protein secretion (UPS) mediated by exocyst-positive organelles (EXPOs). All XTHs share a common C-terminal functional domain (XET-C) that, for XTH29 and a restricted number of other XTHs (27, 28 and 30), continues with an extraterminal region (ETR) of 45 amino acids. We suggest that this region is necessary for the correct cell wall targeting of XTH29, as the ETR-truncated protein never reaches its final destination and is not recruited by EXPOs. Furthermore, quantitative real-time polymerase chain reaction analyses performed on 4-week-old Arabidopsis seedlings exposed to drought and heat stress suggest a different involvement of the three XTHs in cell wall remodeling under abiotic stress, evidencing stress-, organ- and time-dependent variations in the expression levels. Significantly, XTH29, codifying the only XTH that follows a UPS pathway, is highly upregulated with respect to XTH11 and XTH33, which code for CPS-secreted proteins.

Exocyst subunit Exo70B2 is linked to immune signaling and autophagy

During the immune response, activation of the secretory pathway is key to mounting an effective response, while gauging its output is important to maintain cellular homeostasis. The Exo70 subunit of the exocyst functions as a spatiotemporal regulator by mediating numerous interactions with proteins and lipids. However, a molecular understanding of the exocyst regulation remains challenging. We show that, in Arabidopsis thaliana, Exo70B2 behaves as a bona fide exocyst subunit. Conversely, treatment with the salicylic acid (SA) defence hormone analog benzothiadiazole (BTH), or the immunogenic peptide flg22, induced Exo70B2 transport into the vacuole. We reveal that Exo70B2 interacts with AUTOPHAGY-RELATED PROTEIN 8 (ATG8) via two ATG8-interacting motives (AIMs) and its transport into the vacuole is dependent on autophagy. In line with its role in immunity, we discovered that Exo70B2 interacted with and was phosphorylated by the kinase MPK3. Mimicking phosphorylation had a dual impact on Exo70B2: first, by inhibiting localization at sites of active secretion, and second, it increased the interaction with ATG8. Phosphonull variants displayed higher effector-triggered immunity (ETI) and were hypersensitive to BTH, which induce secretion and autophagy. Our results suggest a molecular mechanism by which phosphorylation diverts Exo70B2 from the secretory into the autophagy pathway for its degradation, to dampen secretory activity.

New insights into AtNBR1 as a selective autophagy cargo receptor in Arabidopsis

Selective autophagy, mediated by cargo receptors and recruiting specific targets to autophagosomes for degradation and recycling, plays an important role in quality control and cellular homeostasis in eukaryotes. The Arabidopsis AtNBR1 shares a similar domain organization with the mammalian autophagic receptors p62 and NBR1. We recently demonstrated that AtNBR1 functions as a selective autophagy receptor for the exocyst component AtExo70E2, a marker for the Exocyst-positive organelle (EXPO), which was achieved via a specific ATG8-AtNBR1-AtExo70E2 interaction in Arabidopsis. Here we further showed that nbr1 CRISPR mutants exhibit an early senescence phenotype under short-day growth conditions, which can be restored by complementation with expression of AtNBR1pro::AtNBR1-GFP in the mutant. Interestingly, in addition to the typical cytosolic and punctate patterns, YFP-AtNBR1 also exhibited a microtubule pattern particularly in the cortical layer. Treatments with the microtubule depolymerizer oryzalin but not the microfilament depolymerizer latrunculin B abolished the microtubule pattern and affected the vacuolar delivery of YFP-AtNBR1 upon autophagy induction. These results indicated that microtubules may be required for AtNBR1 to shuttle its cargos to the vacuole during plant autophagy. The present study thus sheds new light on the recognition and movement pattern of AtNBR1 in selective autophagy in Arabidopsis.

AtNBR1 Is a Selective Autophagic Receptor for AtExo70E2 in Arabidopsis

Selective autophagy is a subcellular process whereby cytoplasmic materials are selectively sequestered into autophagosomes for subsequent delivery to the vacuole for degradation and recycling. Arabidopsis (Arabidopsis thaliana) NBR1 (next to BRCA1 gene 1 protein; AtNBR1) has been proposed to function as a selective autophagy receptor in plants, whereby AtNBR1 anchors the ubiquitinated targets to autophagosomes for degradation. However, the specific cargos of AtNBR1 remain elusive. We previously showed that Arabidopsis exocyst subunit EXO70 family protein E2 (AtExo70E2), a marker for exocyst-positive organelle (EXPO), colocalized with the autophagosome marker Arabidopsis autophagy-related protein8 (AtATG8) and was delivered to the vacuole for degradation upon autophagic induction. Here, through multiple analyses, we demonstrate that AtNBR1 is a selective receptor for AtExo70E2 during autophagy in Arabidopsis. First, two novel loss-of-function nbr1 CRISPR mutants (nbr1-c1 and nbr1-c2) showed an early-senescence phenotype under short-day growth conditions. Second, during autophagic induction, the vacuolar delivery of AtExo70E2 or EXPO was significantly reduced in nbr1 mutants compared to wild-type plants. Third, biochemical and recruitment assays demonstrated that AtNBR1 specifically interacted and recruited AtExo70E2 or its EXPO to AtATG8-positive autophagosomes in a ubiquitin-associated (UBA)-independent manner during autophagy. Taken together, our data indicate that AtNBR1 functions as a selective receptor in mediating vacuolar delivery of AtExo70E2 or EXPO in a UBA-independent manner in plant autophagy.

A Diverse Membrane Interaction Network for Plant Multivesicular Bodies: Roles in Proteins Vacuolar Delivery and Unconventional Secretion

Vesicle trafficking between the membrane-bound organelles in plant cells plays crucial roles in the precise transportation of various materials, and thus supports cell proliferation and cellular polarization. Conventionally, plant prevacuolar compartments (PVCs), identified as multivesicular bodies (MVBs), play important roles in both the secretory pathway as intermediate compartments and the endocytic pathway as late endosomes. In recent years, the PVC/MVBs have been proposed to play important roles in both protein vacuolar delivery and unconventional secretion, but several important questions on the new regulators and environmental cues that coordinate the PVC/MVB-organelle membrane interactions and their biological significances remain. In this review, we first summarize the identity and nature of the plant PVC/MVBs, and then we present an update on our current understanding on the interaction of PVC/MVBs with other organelles in the plant endomembrane system with focus on the vacuole, autophagosome, and plasma membrane (PM) in plant development and stress responses. Finally, we raise some open questions and present future perspectives in the study of PVC/MVB-organelle interactions and associated biological functions.

Plant extracellular vesicles

Exocytosis is a key mechanism for delivering materials into the extracellular space for cell function and communication. In plant cells, conventional protein secretion (CPS) is achieved via an ER (endoplasmic reticulum)-Golgi-TGN (trans-Golgi network)-PM (plasma membrane) pathway. Unconventional protein secretion (UPS) bypassing these secretory organelles is also in operation and can potentially lead to the formation of extracellular vesicles (EVs) in plant cells. Although multiple types of EVs have been identified and shown to play important roles in mediating intercellular communications in mammalian cells, there has been a long debate about the possible existence of EVs in plants because of the presence of the cell wall. However, increasing evidence suggests that plants also release EVs having various functions including unconventional protein secretion, RNA transport, and defense against pathogens. In this review, we present an update on the current knowledge about the nature, secretory mechanism, and function of various types of EVs in plants. The key regulators involved in EV secretion are also summarized and discussed. We pay special attention to the function of EVs in plant defense and symbiosis.

Identification and Characterization of the EXO70 Gene Family in Polyploid Wheat and Related Species

The EXO70 gene family is involved in different biological processes in plants, ranging from plant polar growth to plant immunity. To date, analysis of the EXO70 gene family has been limited in Triticeae species, e.g., hexaploidy Triticum aestivum and its ancestral/related species. By in silico analysis of multiple Triticeae sequence databases, a total of 200 EXO70 members were identified. By homologue cloning approaches, 15 full-length cDNA of EXO70s were cloned from diploid Haynaldia villosa. Phylogenetic relationship analysis of 215 EXO70 members classified them into three groups (EXO70.1, EXO70.2, and EXO70.3) and nine subgroups (EXO70A to EXO70I). The distribution of most EXO70 genes among different species/sub-genomes were collinear, implying their orthologous relationship. The EXO70A subgroup has the most introns (at least five introns), while the remaining seven subgroups have only one intron on average. The expression profiling of EXO70 genes from wheat revealed that 40 wheat EXO70 genes were expressed in at least one tissue (leaf, stem, or root), of which 25 wheat EXO70 genes were in response to at least one biotic stress (stripe rust or powdery mildew) or abiotic stress (drought or heat). Subcellular localization analysis showed that ten EXO70-V proteins had distinct plasma membrane localization, EXO70I1-V showed a distinctive spotted pattern on the membrane. The 15 EXO70-V genes were differentially expressed in three tissue. Apart from EXO70D2-V, the remaining EXO70-V genes were in response to at least one stress (flg22, chitin, powdery mildew, drought, NaCl, heat, or cold) or phytohormones (salicylic acid, methyl jasmonate, ethephon, or abscisic acid) and hydrogen peroxide treatments. This research provides a genome-wide glimpse of the Triticeae EXO70 gene family and those up- or downregulated genes require further validation of their biological roles in response to biotic/abiotic stresses.

Comparative transcriptome analysis provides global insight into gene expression differences between two orchid cultivars

The orchids GL and YL are two cultivars of Cymbidium longibracteatum. YL displays an obviously yellowing rhizome and yellow leaves, while GL ('Longchangsu') shows dark green leaves and greenish rhizome. But the molecular mechanism for the differences between the two cultivars is poorly understood. In the present study, we showed that the structure of chloroplasts was significantly damaged in YL. Biochemical analysis uncovered the contents of chlorophyll a, chlorophyll b, total chlorophyll and carotenoid were notably decreased in YL. Using RNA-Seq technology, more than 38 million clean reads were generated in each pool, and 116,422 unigenes were assembled de novo. 6,660 unigenes with differential expression patterns (FDR≤0.01 and |log2 ratio|≥1) were totally identified between the two cultivars. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of differentially expressed unigenes (DEGs) suggested 33 KEGG pathways were notably enriched, including biological processes such as “phenylpropanoid biosynthesis”, “phagosome”, “starch and sucrose metabolism”, “drug metabolism—cytochrome P450”, “fatty acid elongation”, and “flavone and flavonol biosynthesis”. Further analysis revealed that chlorophyll degeneration related unigene (c48794_g1) and flavonoid biosynthesis related unigenes (c16388_g1, c48963_g1, c63571_g1, c4492_g1, c52282_g1, c78740_g1, c4645_g1) were up-regulated while carotenoid biosynthesis related unigene (c7212_g1) were down-regulated in YL. Additionally, six of NAC, R2R3-MYB, bHLH transcription factors (c42861_g1, c105949_g1, c61265_g1, c42659_g1, c82171_g1, c19158_g1) might be involved in regulation of pigment biosynthesis. The chlorophyll degeneration and the flavonoid biosynthesis related unigenes up-regulation together with the carotenoid biosynthesis related unigenes down-regulation may contribute to the yellowing phenotype of YL.

Increases in activity of proteasome and papain-like cysteine protease in Arabidopsis autophagy mutants: back-up compensatory effect or cell-death promoting effect?

Autophagy is essential for protein degradation, nutrient recycling, and nitrogen remobilization. Autophagy is induced during leaf ageing and in response to nitrogen starvation, and is known to play a fundamental role in nutrient recycling for remobilization and seed filling. Accordingly, ageing leaves of Arabidopsis autophagy mutants (atg) have been shown to over-accumulate proteins and peptides, possibly because of a reduced protein degradation capacity. Surprisingly, atg leaves also displayed higher protease activities. The work reported here aimed at identifying the nature of the proteases and protease activities that accumulated differentially (higher or lower) in the atg mutants. Protease identification was performed using shotgun LC-MS/MS proteome analyses and activity-based protein profiling (ABPP). The results showed that the chloroplast FTSH (FILAMENTATION TEMPERATURE SENSITIVE H) and DEG (DEGRADATION OF PERIPLASMIC PROTEINS) proteases and several extracellular serine proteases [subtilases (SBTs) and serine carboxypeptidase-like (SCPL) proteases] were less abundant in atg5 mutants. By contrast, proteasome-related proteins and cytosolic or vacuole cysteine proteases were more abundant in atg5 mutants. Rubisco degradation assays and ABPP showed that the activities of proteasome and papain-like cysteine protease were increased in atg5 mutants. Whether these proteases play a back-up role in nutrient recycling and remobilization in atg mutants or act to promote cell death is discussed in relation to their accumulation patterns in the atg5 mutant compared with the salicylic acid-depleted atg5/sid2 double-mutant, and in low nitrate compared with high nitrate conditions. Several of the proteins identified are indeed known as senescence- and stress-related proteases or as spontaneous cell-death triggering factors.

RIN4 recruits the exocyst subunit EXO70B1 to the plasma membrane

The exocyst is a conserved vesicle-tethering complex with principal roles in cell polarity and morphogenesis. Several studies point to its involvement in polarized secretion during microbial pathogen defense. In this context, we have found an interaction between the Arabidopsis EXO70B1 exocyst subunit, a protein which was previously associated with both the defense response and autophagy, and RPM1 INTERACTING PROTEIN 4 (RIN4), the best studied member of the NOI protein family and a known regulator of plant defense pathways. Interestingly, fragments of RIN4 mimicking the cleavage caused by the Pseudomonas syringae effector protease, AvrRpt2, fail to interact strongly with EXO70B1. We observed that transiently expressed RIN4, but not the plasma membrane (PM) protein aquaporin PIP2, recruits EXO70B1 to the PM. Unlike EXO70B1, RIN4 does not recruit the core exocyst subunit SEC6 to the PM under these conditions. Furthermore, the AvrRpt2 effector protease delivered by P. syringae is able to release both RIN4 and EXO70B1 to the cytoplasm. We present a model for how RIN4 might regulate the localization and putative function of EXO70B1 and speculate on the role the AvrRpt2 protease might have in the regulation of this defense response.

ATG9 regulates autophagosome progression from the endoplasmic reticulum in Arabidopsis

Autophagy is a conserved pathway for bulk degradation of cytoplasmic material by a double-membrane structure named the autophagosome. The initiation of autophagosome formation requires the recruitment of autophagy-related protein 9 (ATG9) vesicles to the preautophagosomal structure. However, the functional relationship between ATG9 vesicles and the phagophore is controversial in different systems, and the molecular function of ATG9 remains unknown in plants. Here, we demonstrate that ATG9 is essential for endoplasmic reticulum (ER)-derived autophagosome formation in plants. Through a combination of genetic, in vivo imaging and electron tomography approaches, we show that Arabidopsis ATG9 deficiency leads to a drastic accumulation of autophagosome-related tubular structures in direct membrane continuity with the ER upon autophagic induction. Dynamic analyses demonstrate a transient membrane association between ATG9 vesicles and the autophagosomal membrane during autophagy. Furthermore, trafficking of ATG18a is compromised in atg9 mutants during autophagy by forming extended tubules in a phosphatidylinositol 3-phosphate-dependent manner. Taken together, this study provides evidence for a pivotal role of ATG9 in regulating autophagosome progression from the ER membrane in Arabidopsis.

MONENSIN SENSITIVITY1 (MON1)/CALCIUM CAFFEINE ZINC SENSITIVITY1 (CCZ1)-Mediated Rab7 Activation Regulates Tapetal Programmed Cell Death and Pollen Development

Programmed cell death (PCD)-triggered degradation of plant tapetum is essential for microspore development and pollen coat formation; however, little is known about the cellular mechanism regulating tapetal PCD Here, we demonstrate that Rab7-mediated vacuolar transport of tapetum degradation-related cysteine proteases is crucial for tapetal PCD and pollen development in Arabidopsis (Arabidopsis thaliana), with the following evidence: (1) The monensin sensitivity1 (mon1) mutants, which are defective in Rab7 activation, showed impaired male fertility due to a combined defect in both tapetum and male gametophyte development. (2) In anthers, MON1 showed preferential high level expression in tapetal cell layers and pollen. (3) The mon1 mutants exhibited delayed tapetum degeneration and tapetal PCD, resulting in abnormal pollen coat formation and decreased male fertility. (4) MON1/CALCIUM CAFFEINE ZINC SENSITIVITY1 (CCZ1)-mediated Rab7 activation was indispensable for vacuolar trafficking of tapetum degradation-related cysteine proteases, supporting that PCD-triggered tapetum degeneration requires Rab7-mediated vacuolar trafficking of these cysteine proteases. (5) MON1 mutations also resulted in defective pollen germination and tube growth. Taken together, tapetal PCD and pollen development require successful MON1/CCZ1-mediated vacuolar transport in Arabidopsis.

An Overview of Protein Secretion in Plant Cells

The delivery of proteins to the apoplast or protein secretion is an essential process in plant cells. Proteins are secreted to perform various biological functions such as cell wall modification and defense response. Conserved from yeast to mammals, both conventional and unconventional protein secretion pathways have been demonstrated in plants. In the conventional protein secretion pathway, secretory proteins with an N-terminal signal peptide are transported to the extracellular region via the endoplasmic reticulum-Golgi apparatus and the subsequent endomembrane system. By contrast, multiple unconventional protein secretion pathways are proposed to mediate the secretion of the leaderless secretory proteins. In this review, we summarize the recent findings and provide a comprehensive overview of protein secretion pathways in plant cells.

The N-Terminal UND Motif of the Arabidopsis U-Box E3 Ligase PUB18 Is Critical for the Negative Regulation of ABA-Mediated Stomatal Movement and Determines Its Ubiquitination Specificity for Exocyst Subunit Exo70B1

The Arabidopsis thaliana U-box E3 ligases PUB18/PUB19 and PUB22/PUB23 are negative regulators of drought stress responses. PUB18/PUB19 regulate the drought stress response in an abscisic acid (ABA)-dependent manner, whereas PUB22/PUB23 regulate this response in an ABA-independent manner. A major structural difference between PUB18/PUB19 and PUB22/PUB23 is the presence of the UND (U-box N-terminal domain). Here, we focused on elucidating the molecular mechanism that mediates the functional difference between PUB18 and PUB22 and found that the UND^PUB18 was critically involved in the negative regulation of ABA-mediated stomatal movements. Exo70B1, a subunit of the exocyst complex, was identified as a target of PUB18, whereas Exo70B2 was a substrate of PUB22. However, the ∆UND-PUB18 derivative failed to ubiquitinate Exo70B1, but ubiquitinated Exo70B2. By contrast, the UND^PUB18-PUB22 chimeric protein ubiquitinated Exo70B1 instead of Exo70B2, suggesting that the ubiquitination specificities of PUB18 and PUB22 to Exo70B1 and Exo70B2, respectively, are dependent on the presence or absence of the UND^PUB18 motif. The ABA-insensitive phenotypes of the pub18 pub19 exo70b1 triple mutant were reminiscent of those of exo70b1 rather than pub18 pub19, indicating that Exo70B1 functions downstream of PUB18. Overall, our results suggest that the UND^PUB18 motif is crucial for the negative regulation of ABA-dependent stomatal movement and for determination of its ubiquitination specificity to Exo70B1.

Exocyst SEC3 and Phosphoinositides Define Sites of Exocytosis in Pollen Tube Initiation and Growth

Polarized exocytosis is critical for pollen tube growth, but its localization and function are still under debate. The exocyst vesicle-tethering complex functions in polarized exocytosis. Here, we show that a sec3a exocyst subunit null mutant cannot be transmitted through the male gametophyte due to a defect in pollen tube growth. The green fluorescent protein (GFP)-SEC3a fusion protein is functional and accumulates at or proximal to the pollen tube tip plasma membrane. Partial complementation of sec3a resulted in the development of pollen with multiple tips, indicating that SEC3 is required to determine the site of pollen germination pore formation. Time-lapse imaging demonstrated that SEC3a and SEC8 were highly dynamic and that SEC3a localization on the apical plasma membrane predicts the direction of growth. At the tip, polar SEC3a domains coincided with cell wall deposition. Labeling of GFP-SEC3a-expressing pollen with the endocytic marker FM4-64 revealed the presence of subdomains on the apical membrane characterized by extensive exocytosis. In steady-state growing tobacco (Nicotiana tabacum) pollen tubes, SEC3a displayed amino-terminal Pleckstrin homology-like domain (SEC3a-N)-dependent subapical membrane localization. In agreement, SEC3a-N interacted with phosphoinositides in vitro and colocalized with a phosphatidylinositol 4,5-bisphosphate (PIP₂) marker in pollen tubes. Correspondingly, molecular dynamics simulations indicated that SEC3a-N associates with the membrane by interacting with PIP₂ However, the interaction with PIP₂ is not required for polar localization and the function of SEC3a in Arabidopsis (Arabidopsis thaliana). Taken together, our findings indicate that SEC3a is a critical determinant of polar exocytosis during tip growth and suggest differential regulation of the exocytotic machinery depending on pollen tube growth modes.

Protein Dynamics in the Plant Extracellular Space

The extracellular space (ECS or apoplast) is the plant cell compartment external to the plasma membrane, which includes the cell walls, the intercellular space and the apoplastic fluid (APF). The present review is focused on APF proteomics papers and intends to draw information on the metabolic processes occurring in the ECS under abiotic and biotic stresses, as well as under non-challenged conditions. The large majority of the proteins detected are involved in "cell wall organization and biogenesis", "response to stimulus" and "protein metabolism". It becomes apparent that some proteins are always detected, irrespective of the experimental conditions, although with different relative contribution. This fact suggests that non-challenged plants have intrinsic constitutive metabolic processes of stress/defense in the ECS. In addition to the multiple functions ascribed to the ECS proteins, should be considered the interactions established between themselves and with the plasma membrane and its components. These interactions are crucial in connecting exterior and interior of the cell, and even simple protein actions in the ECS can have profound effects on plant performance. The proteins of the ECS are permanently contributing to the high dynamic nature of this plant compartment, which seems fundamental to plant development and adaptation to the environmental conditions.

Tethering Complexes in the Arabidopsis Endomembrane System

Targeting of endomembrane transport containers is of the utmost importance for proper land plant growth and development. Given the immobility of plant cells, localized membrane vesicle secretion and recycling are amongst the main processes guiding proper cell, tissue and whole plant morphogenesis. Cell wall biogenesis and modification are dependent on vectorial membrane traffic, not only during normal development, but also in stress responses and in plant defense against pathogens and/or symbiosis. It is surprising how little we know about these processes in plants, from small GTPase regulation to the tethering complexes that act as their effectors. Tethering factors are single proteins or protein complexes mediating first contact between the target membrane and arriving membrane vesicles. In this review we focus on the tethering complexes of the best-studied plant model—Arabidopsis thaliana. Genome-based predictions indicate the presence of all major tethering complexes in plants that are known from a hypothetical last eukaryotic common ancestor (LECA). The evolutionary multiplication of paralogs of plant tethering complex subunits has produced the massively expanded EXO70 family, indicating a subfunctionalization of the terminal exocytosis machinery in land plants. Interpretation of loss of function (LOF) mutant phenotypes has to consider that related, yet clearly functionally-specific complexes often share some common core subunits. It is therefore impossible to conclude with clarity which version of the complex is responsible for the phenotypic deviations observed. Experimental interest in the analysis of plant tethering complexes is growing and we hope to contribute with this review by attracting even more attention to this fascinating field of plant cell biology.

Unconventional Protein Secretion in Plants

Unconventional protein secretion (UPS) describes secretion pathways that bypass one or several of the canonical secretion pit-stops on the way to the plasma membrane, and/or involve the secretion of leaderless proteins. So far, alternatives to conventional secretion were primarily observed and studied in yeast and animal cells. The sessile lifestyle of plants brings with it unique restraints on how they adapt to adverse conditions and environmental challenges. Recently, attention towards unconventional secretion pathways in plant cells has substantially increased, with the large number of leaderless proteins identified through proteomic studies. While UPS pathways in plants are certainly not yet exhaustively researched, an emerging notion is that induction of UPS pathways is correlated with pathogenesis and stress responses. Given the multitude UPS events observed, comprehensively organizing the routes proteins take to the apoplast in defined UPS categories is challenging. With the establishment of a larger collection of studied plant proteins taking these UPS pathways, a clearer picture of endomembrane trafficking as a whole will emerge. There are several novel enabling technologies, such as vesicle proteomics and chemical genomics, with great potential for dissecting secretion pathways, providing information about the cargo that travels along them and the conditions that induce them.

Unconventional protein secretion in plants: a critical assessment

Unconventional protein secretion (UPS) is a collective term for mechanisms by which cytosolic proteins that lack a signal peptide ("leaderless secretory proteins" (LSPs)) can gain access to the cell exterior. Numerous examples of UPS have been well documented in animal and yeast cells. In contrast, our understanding of the mechanism(s) and function of UPS in plants is very limited. This review evaluates the available literature on this subject. The apparent large numbers of LSPs in the plant secretome suggest that UPS also occurs in plants but is not a proof. Although the direct transport of LSPs across the plant plasma membrane (PM) has not yet been described, it is possible that as in other eukaryotes, exosomes may be released from plant cells through fusion of multivesicular bodies (MVBs) with the PM. In this way, LSPs, but also small RNAs (sRNAs), that are passively taken up from the cytosol into the intraluminal vesicles of MVBs, could reach the apoplast. Another possible mechanism is the recently discovered exocyst-positive organelle (EXPO), a double-membrane-bound compartment, distinct from autophagosomes, which appears to sequester LSPs.

Origin of the Autophagosomal Membrane in Plants

During autophagy, cargo molecules destined for degradation are sequestrated into a double-membrane structure called autophagosome, which subsequently fuses with the vacuole. An isolation membrane structure (also called the phagophore) initiates from the platform termed PAS (phagophore assembly site or preautophagosomal structure), which then elongates and expands to become the completed autophagosome. The origin of the membrane for autophagosome formation has been extensively investigated but remains an enigma in the field of autophagy. In yeast and mammalian cells multiple membrane sources have been suggested to contribute to autophagosome formation at different steps, from initiation through expansion and maturation. Recent studies in plants have provided a significant advance in our understanding of the conserved role of autophagy and the underlying mechanism for autophagosome formation. Here, we will discuss and evaluate these new findings on autophagosome formation in plants, with a particular focus on the origin of plant autophagosomal membranes.