Arabidopsis SNAREs SYP61 and SYP121 coordinate the trafficking of plasma membrane aquaporin PIP2;7 to modulate the cell membrane water permeability

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

A Phytophthora sojae CRN effector mediates phosphorylation and degradation of plant aquaporin proteins to suppress host immune signaling

Phytophthora genomes encode a myriad of Crinkler (CRN) effectors, some of which contain putative kinase domains. Little is known about the host targets of these kinase-domain-containing CRNs and their infection-promoting mechanisms. Here, we report the host target and functional mechanism of a conserved kinase CRN effector named CRN78 in a notorious oomycete pathogen, Phytophthora sojae. CRN78 promotes Phytophthora capsici infection in Nicotiana benthamiana and enhances P. sojae virulence on the host plant Glycine max by inhibiting plant H₂O₂ accumulation and immunity-related gene expression. Further investigation reveals that CRN78 interacts with PIP2-family aquaporin proteins including NbPIP2;2 from N. benthamiana and GmPIP2-13 from soybean on the plant plasma membrane, and membrane localization is necessary for virulence of CRN78. Next, CRN78 promotes phosphorylation of NbPIP2;2 or GmPIP2-13 using its kinase domain in vivo, leading to their subsequent protein degradation in a 26S-dependent pathway. Our data also demonstrates that NbPIP2;2 acts as a H₂O₂ transporter to positively regulate plant immunity and reactive oxygen species (ROS) accumulation. Phylogenetic analysis suggests that the phosphorylation sites of PIP2 proteins and the kinase domains of CRN78 homologs are highly conserved among higher plants and oomycete pathogens, respectively. Therefore, this study elucidates a conserved and novel pathway used by effector proteins to inhibit host cellular defenses by targeting and hijacking phosphorylation of plant aquaporin proteins.

CRN effectors are conserved in diverse pathogens of plants, animals, and insects, and highly expanded in Phytophthora species. Nevertheless, little is known about their functions, targets, and action mechanisms. Here, we characterized a kinase-domain-containing CRN effector (CRN78) in a notorious oomycete pathogen, P. sojae. CRN78 is a virulence-essential effector of P. sojae infection, and acts via suppression of plant H₂O₂ accumulation and defense gene expressions. We demonstrated that CRN78 might interact with plant PIP2-family aquaporin proteins, including N. benthamiana NbPIP2;2 and soybean GmPIP2-13, and regulate their phosphorylation, resulting in subsequent 26S-dependent protein degradation. Furthermore, we revealed that NbPIP2;2 was an apoplast-to-cytoplast H₂O₂ transporter and positively regulated plant immunity and ROS accumulation. Importantly, this phosphorylation may be highly conserved in many plant aquaporin proteins. Thus, this study identifies a virulence-related effector from P. sojae and a novel plant immunity-related gene, and reveals a detailed mechanism of effector-mediated phosphorylation and degradation of plant aquaporin proteins.

Current Understanding of Role of Vesicular Transport in Salt Secretion by Salt Glands in Recretohalophytes

Soil salinization is a serious and growing problem around the world. Some plants, recognized as the recretohalophytes, can normally grow on saline-alkali soil without adverse effects by secreting excessive salt out of the body. The elucidation of the salt secretion process is of great significance for understanding the salt tolerance mechanism adopted by the recretohalophytes. Between the 1950s and the 1970s, three hypotheses, including the osmotic potential hypothesis, the transfer system similar to liquid flow in animals, and vesicle-mediated exocytosis, were proposed to explain the salt secretion process of plant salt glands. More recently, increasing evidence has indicated that vesicular transport plays vital roles in salt secretion of recretohalophytes. Here, we summarize recent findings, especially regarding the molecular evidence on the functional roles of vesicular trafficking in the salt secretion process of plant salt glands. A model of salt secretion in salt gland is also proposed.

Wild soybean SNARE proteins BET1s mediate the subcellular localization of the cytoplasmic receptor-like kinases CRCK1s to modulate salt stress responses

Plants have evolved numerous receptor-like kinases (RLKs) that modulate environmental stress responses. However, little is known regarding soybean (Glycine max) RLKs. We have previously identified that Glycine soja Ca2+ /CAM-binding RLK (GsCBRLK) is involved in salt tolerance. Here, we report that soluble NSF attachment protein receptor proteins BET1s mediate subcellular localization of calmodulin-binding receptor-like cytoplasmic kinases CRCK1s to modulate salt stress responses. Direct interaction between GsCBRLK and GsBET11a was initially identified via yeast two-hybrid and bimolecular fluorescence complementation assays. Further analysis demonstrated conserved interaction between BET1s and CRCK1s. GsCBRLK interacted with all BET1 proteins in wild soybean (Glycine soja) and Arabidopsis, and GsBET11a strongly associated with GsCRCK1a-1d, but slightly with AtCRCK1. In addition, GsBET11a interacted with GsCBRLK via its C-terminal transmembrane domain (TMD), where the entire TMD, not the sequence, was critical for the interaction. Moreover, the N-terminal variable domain (VD) of GsCBRLK was responsible for interacting with GsBET11a, and the intensity of interaction between GsCBRLK/AtCRCK1 and GsBET11a was dependent on VD. Furthermore, GsBET11a was able to mediate the GsCBRLK subcellular localization via direct interaction with VD. Additionally, knockout of AtBET11 or AtBET12 individually did not alter GsCBRLK localization, while GsBET11a expression caused partial internalization of GsCBRLK from the plasma membrane (PM). We further suggest the necessity of GsCBRLK VD for its PM localization via N-terminal truncation assays. Finally, GsBET11a was shown to confer enhanced salt stress tolerance when overexpressed in Arabidopsis and soybean. These results revealed the conserved and direct interaction between BET1s and CRCK1s, and suggested their involvement in salt stress responses.

Interaction Between the SNARE SYP121 and the Plasma Membrane Aquaporin PIP2;7 Involves Different Protein Domains

Plasma membrane intrinsic proteins (PIPs) are channels facilitating the passive diffusion of water and small solutes. Arabidopsis PIP2;7 trafficking occurs through physical interaction with SNARE proteins including the syntaxin SYP121, a plasma membrane Qa-SNARE involved in membrane fusion. To better understand the interaction mechanism, we aimed at identifying the interaction motifs in SYP121 and PIP2;7 using ratiometric bimolecular fluorescence complementation assays in Nicotiana benthamiana. SYP121 consists of four regions, N, H, Q, and C, and sequential deletions revealed that the C region, containing the transmembrane domain, as well as the H and Q regions, containing the Habc and Qa-SNARE functional domains, interact with PIP2;7. Neither the linker between the Habc and the Qa-SNARE domains nor the H or Q regions alone could fully restore the interaction with PIP2;7, suggesting that the interacting motif depends on the conformation taken by the HQ region. When investigating the interacting motif(s) in PIP2;7, we observed that deletion of the cytosolic N- and/or C- terminus led to a significant decrease in the interaction with SYP121. Shorter deletions revealed that at the N-terminal amino acid residues 18-26 were involved in the interaction. Domain swapping experiments between PIP2;7 and PIP2;6, a PIP isoform that does not interact with SYP121, showed that PIP2;7 N-terminal part up to the loop C was required to restore the full interaction signal, suggesting that, as it is the case for SYP121, the interaction motif(s) in PIP2;7 depend on the protein conformation. Finally, we also showed that PIP2;7 physically interacted with other Arabidopsis SYP1s and SYP121 orthologs.

Controversial Regulation of Gene Expression and Protein Transduction of Aquaporins under Drought and Salinity Stress

Enhancement of the passage of water through membranes is one of the main mechanisms via which cells can maintain their homeostasis under stress conditions, and aquaporins are the main participants in this process. However, in the last few years, a number of studies have reported discrepancies between aquaporin messenger RNA (mRNA) expression and the number of aquaporin proteins synthesised in response to abiotic stress. These observations suggest the existence of post-transcriptional mechanisms which regulate plasma membrane intrinsic protein (PIP) trafficking to the plasma membrane. This indicates that the mRNA synthesis of some aquaporins could be modulated by the accumulation of the corresponding encoded protein, in relation to the turnover of the membranes. This aspect is discussed in terms of the results obtained: on the one hand, with isolated vesicles, in which the level of proteins present provides the membranes with important characteristics such as resistance and stability and, on the other, with isolated proteins reconstituted in artificial liposomes as an in vitro method to address the in vivo physiology of the entire plant.

Plasma membrane aquaporins interact with the endoplasmic reticulum resident VAP27 proteins at ER-PM contact sites and endocytic structures

Plasma membrane (PM) intrinsic proteins (PIPs) are aquaporins facilitating the diffusion of water and small solutes. The functional importance of the PM organisation of PIPs in the interaction with other cellular structures is not completely understood. We performed a pull-down assay using maize (Zea mays) suspension cells expressing YFP-ZmPIP2;5 and validated the protein interactions by yeast split-ubiquitin and bimolecular fluorescence complementation assays. We expressed interacting proteins tagged with fluorescent proteins in Nicotiana benthamiana leaves and performed water transport assays in oocytes. Finally, a phylogenetic analysis was conducted. The PM-located ZmPIP2;5 physically interacts with the endoplasmic reticulum (ER) resident ZmVAP27-1. This interaction requires the ZmVAP27-1 cytoplasmic major sperm domain. ZmPIP2;5 and ZmVAP27-1 localise in close vicinity in ER-PM contact sites (EPCSs) and endocytic structures upon exposure to salt stress conditions. This interaction enhances PM water permeability in oocytes. Similarly, the Arabidopsis ZmVAP27-1 paralogue, AtVAP27-1, interacts with the AtPIP2;7 aquaporin. Together, these data indicate that the PIP2-VAP27 interaction in EPCSs is evolutionarily conserved, and suggest that VAP27 might stabilise the aquaporins and guide their endocytosis in response to salt stress.

Dynamic Control of the High-Affinity Iron Uptake Complex in Root Epidermal Cells

In plants, iron uptake from the soil is tightly regulated to ensure optimal growth and development. Iron absorption in Arabidopsis root epidermal cells requires the IRT1 transporter that also allows the entry of certain non-iron metals, such as Zn, Mn, and Co. Recent work demonstrated that IRT1 endocytosis and degradation are controlled by IRT1 non-iron metal substrates in a ubiquitin-dependent manner. To better understand how metal uptake is regulated, we identified IRT1-interacting proteins in Arabidopsis roots by mass spectrometry and established an interactome of IRT1. Interestingly, the AHA2 proton pump and the FRO2 reductase, both of which work in concert with IRT1 in the acidification-reduction-transport strategy of iron uptake, were part of this interactome. We confirmed that IRT1, FRO2, and AHA2 associate through co-immunopurification and split-ubiquitin analyses, and uncovered that they form tripartite direct interactions. We characterized the dynamics of the iron uptake complex and showed that FRO2 and AHA2 ubiquitination is independent of the non-iron metal substrates transported by IRT1. In addition, FRO2 and AHA2 are not largely endocytosed in response to non-iron metal excess, unlike IRT1. Indeed, we provide evidence that the phosphorylation of IRT1 in response to high levels of non-iron metals likely triggers dissociation of the complex. Overall, we propose that a dedicated iron-acquisition protein complex exists at the cell surface of Arabidopsis root epidermal cells to optimize iron uptake.

Role of Arabidopsis RAB5 GEF vps9a in maintaining potassium levels under sodium chloride stress

Salt stress is one of the major factors impacting crop productivity worldwide. Through a variety of effector and signaling pathways, plants achieve survival under salinity stress by maintaining high cytosolic potassium/sodium ion (K+/Na+) ratios, preventing Na+ cytotoxicity, and retaining osmotic balance. Ras-related protein 5 (Rab5) members are involved in the trafficking of endosomes to the vacuole or plasma membrane (PM). The vacuolar protein sorting- associated protein 9 (vps9a) encodes the single guanine nucleotide exchange factor (GEF) that activates all three known Rab5 proteins in Arabidopsis thaliana. Previous work from our group has reported the critical function of vps9a for the operation of salt-induced endocytic pathway, as well as the expansion of endomembrane compartments under saline stress conditions. Here we show an additional role of vps9a in plant response to salt stress via maintenance of K+ status of the cell rather than Na+ homeostasis. Our results show that roots from vps9a-2 mutant, subjected to 100 mM NaCl, display alterations in transcript levels of genes involved in the K+ homeostasis pathway. Concurrent with the observed sensitivity of vps9a-2 mutant under NaCl stress, exposure to low K+ environments resulted in growth retardation, and reduced rate of endocytosis. Furthermore, vps9a-2 mutant displays reduced expression of auxin reporter, Direct Repeat-5 (DR5), and alterations in polarity and abundance of auxin efflux carrier PIN- FORMED2 (PIN2). Imposition of NaCl stress was found to be restrictive to the elongation capacity of cells in the root elongation zone of vps9a-2 mutant. Together our results indicate that alterations in K+ homeostasis and associated cellular changes causing increased cell wall pH, contribute to diminished root growth and compromised survival of vps9a-2 mutant under salt stress conditions.

SNAREs in Plant Biotic and Abiotic Stress Responses

In eukaryotes, membraneous cellular compartmentation essentially requires vesicle trafficking for communications among distinct organelles. A donor organelle-generated vesicle releases its cargo into a target compartment by fusing two distinct vesicle and target membranes. Vesicle fusion, the final step of vesicle trafficking, is driven intrinsically by complex formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Although SNAREs are well-conserved across eukaryotes, genomic studies revealed that plants have dramatically increased the number of SNARE genes than other eukaryotes. This increase is attributed to the sessile nature of plants, likely for more sensitive and harmonized responses to environmental stresses. In this review, we therefore try to summarize and discuss the current understanding of plant SNAREs function in responses to biotic and abiotic stresses.

Functions of the Plant Qbc SNARE SNAP25 in Cytokinesis and Biotic and Abiotic Stress Responses

Eukaryotes transport biomolecules between intracellular organelles and between cells and the environment via vesicle trafficking. Soluble N -ethylmaleimide-sensitive factor attachment protein receptors (SNARE proteins) play pivotal roles in vesicle and membrane trafficking. These proteins are categorized as Qa, Qb, Qc, and R SNAREs and form a complex that induces vesicle fusion for targeting of vesicle cargos. As the core components of the SNARE complex, the SNAP25 Qbc SNAREs perform various functions related to cellular homeostasis. The Arabidopsis thaliana SNAP25 homolog AtSNAP33 interacts with Qa and R SNAREs and plays a key role in cytokinesis and in triggering innate immune responses. However, other Arabidopsis SNAP25 homologs, such as AtSNAP29 and AtSNAP30, are not well studied; this includes their localization, interactions, structures, and functions. Here, we discuss three biological functions of plant SNAP25 orthologs in the context of AtSNAP33 and highlight recent findings on SNAP25 orthologs in various plants. We propose future directions for determining the roles of the less well-characterized AtSNAP29 and AtSNAP30 proteins.

Are Aquaporins Expressed in Stomatal Complexes Promising Targets to Enhance Stomatal Dynamics?

The opening and closure of stomata depend on the turgor pressure adjustment by the influx or efflux of ions and water in guard cells. In this process, aquaporins may play important roles by facilitating the transport of water and other small molecules. In this perspective, we consider the potential roles of aquaporins in the membrane diffusion of different molecules (H₂O, CO₂, and H₂O₂), processes dependent on abscisic acid and CO₂ signaling in guard cells. While the limited data already available emphasizes the roles of aquaporins in stomatal movement, we propose additional approaches to elucidate the specific roles of single or several aquaporin isoforms in the stomata and evaluate the perspectives aquaporins might offer to improve stomatal dynamics.

Versatile roles of aquaporin in physiological processes and stress tolerance in plants

Aquaporins are pore-forming transmembrane proteins that facilitate the movement of water and many other small neutral solutes across the cells and intracellular compartments. Plants exhibits high diversity in aquaporin isoforms and broadly classified into five different subfamilies on the basis of phylogenetic distribution and subcellular occurrence: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like proteins (NIPs), small basic intrinsic proteins (SIPs) and uncharacterized intrinsic proteins (XIPs). The gating mechanism of aquaporin channels is tightly regulated by post-translational modifications such as phosphorylation, methylation, acetylation, glycosylation, and deamination. Aquaporin expression and transport functions are also modulated by the various phytohormones-mediated signalling in plants. Combined physiology and transcriptome analysis revealed the role of aquaporins in regulating hydraulic conductance in roots and leaves. The present review mainly focused on aquaporin functional activity during solute transport, plant development, abiotic stress response, and plant-microbe symbiosis. Genetically modified plants overexpressing aquaporin-encoding genes display improved agronomic and abiotic stress tolerance.

Quantitative ubiquitylomics approach for characterizing the dynamic change and extensive modulation of ubiquitylation in rice seed germination

During seed germination, cells embark on extensive post-transcriptional and post-translational modifications (PTM), providing a perfect platform to study these events in embryo rebooting from relative quiescenct to highly active state. PR-619, a deubiquitylase inhibitor, delayed the rice seed germination and resulted in the accumulation of ubiquitylated proteins, which indicated the protein ubiquitylation is involved in this process. Using the K-Ɛ-GG antibody enrichment method integrated with high-resolution mass spectrometry, a list of 2576 lysine ubiquitylated (Kub) sites in 1171 proteins was compiled for rice embryos at 0, 12 and 24 h after imbibition (HAI). Of these, the abundance of 1419 Kub sites in 777 proteins changed significantly. Most of them substantially increased within the first 12 HAI, which is similar to the dynamic state previously observed for protein phosphorylation, implying that the first 12 HAI are essential for subsequent switch during rice seed germination. We also quantitatively analyzed the embryo proteome in these samples. Generally, a specific protein's abundance in the ubiquitylome was uncorrelated to that in the proteome. The differentially ubiquitinated proteins were greatly enriched in the categories of protein processing, DNA and RNA processing/regulation related, signaling, and transport. The DiGly footprint of the Kub sites was significantly reduced on K48, a linkage typically associated with proteasome-mediated degradation. These observations suggest ubiquitylation may modulate the protein function more than providing 26S degradation signals in the early stage of rice seed germination. Revealing this comprehensive ubiquitylome greatly increases our understanding of this critical PTM during seed germination.

The roles of endomembrane trafficking in plant abiotic stress responses

Endomembrane trafficking is a fundamental cellular process in all eukaryotic cells and its regulatory mechanisms have been extensively studied. In plants, the endomembrane trafficking system needs to be constantly adjusted to adapt to the ever-changing environment. Evidence has accumulated supporting the idea that endomembrane trafficking is tightly linked to stress signaling pathways to meet the demands of rapid changes in cellular processes and to ensure the correct delivery of stress-related cargo molecules. However, the underlying mechanisms remain unknown. In this review, we summarize the recent findings on the functional roles of both secretory trafficking and endocytic trafficking in different types of abiotic stresses. We also highlight and discuss the unique properties of specific regulatory molecules beyond their conventional functions in endosomal trafficking during plant growth under stress conditions.

OsmiR528 regulates rice-pollen intine formation by targeting an uclacyanin to influence flavonoid metabolism

The intine layer of pollen is essential for pollen grain maturation and pollen tube germination. Abnormal intine development causes pollen sterility and affects seed-setting; therefore, the identification of regulators of intine formation is important for elucidating the mechanisms of pollen formation and function, especially for crop breeding. Here, we report a microRNA, OsmiR528, which regulates pollen intine formation and male fertility in rice (Oryza sativa). OsmiR528 directly targets the uclacyanin family member OsUCL23 to regulate flavonoid metabolism and pollen intine development. This study revealed the function of OsmiR528 and an uclacyanin in pollen development.

The intine, the inner layer of the pollen wall, is essential for the normal development and germination of pollen. However, the composition and developmental regulation of the intine in rice (Oryza sativa) remain largely unknown. Here, we identify a microRNA, OsmiR528, which regulates the formation of the pollen intine and thus male fertility in rice. The mir528 knockout mutant aborted pollen development at the late binucleate pollen stage, significantly decreasing the seed-setting rate. We further demonstrated that OsmiR528 affects pollen development by directly targeting the uclacyanin gene OsUCL23 (encoding a member of the plant-specific blue copper protein family of phytocyanins) and regulating intine deposition. OsUCL23 overexpression phenocopied the mir528 mutant. The OsUCL23 protein localized in the prevacuolar compartments (PVCs) and multivesicular bodies (MVBs). We further revealed that OsUCL23 interacts with a member of the proton-dependent oligopeptide transport (POT) family of transporters to regulate various metabolic components, especially flavonoids. We propose a model in which OsmiR528 regulates pollen intine formation by directly targeting OsUCL23 and in which OsUCL23 interacts with the POT protein on the PVCs and MVBs to regulate the production of metabolites during pollen development. The study thus reveals the functions of OsmiR528 and an uclacyanin during pollen development.

At the intersection of exocytosis and endocytosis in plants

Vesicle exocytosis and endocytosis control the activities and turnover of plasma membrane proteins required for signaling triggering or attenuating at the cell surface. In recent years, the diverse exocytic and endocytic trafficking pathways have been uncovered in plants. The balance between conventional and unconventional protein secretion provides an efficient strategy to respond to stress conditions. Similarly, clathrin-dependent and -independent endocytosis cooperatively regulate the dynamics of membrane proteins in response to environmental cues. In fact, many aspects of plant growth and development, such as tip growth, immune response, and protein polarity establishment, involve the tight deployment of exo-endocytic trafficking. However, our understanding of their intersection is limited. Here, we discuss recent advances in the molecular factors coupling plant exo-endocytic trafficking and the biological significance of balance between exocytosis and endocytosis in plants.

Remove, Recycle, Degrade: Regulating Plasma Membrane Protein Accumulation

Interactions between plant cells and the environment rely on modulation of protein receptors, transporters, channels, and lipids at the plasma membrane (PM) to facilitate intercellular communication, nutrient uptake, environmental sensing, and directional growth. These functions are fine-tuned by cellular pathways maintaining or reducing particular proteins at the PM. Proteins are endocytosed, and their fate is decided between recycling and degradation to modulate localization, abundance, and activity. Selective autophagy is another pathway regulating PM protein accumulation in response to specific conditions or developmental signals. The mechanisms regulating recycling, degradation, and autophagy have been studied extensively, yet we are just now addressing their regulation and coordination. Here, we (1) provide context concerning regulation of protein accumulation, recycling, or degradation by overviewing endomembrane trafficking; (2) discuss pathways regulating recycling and degradation in terms of cellular roles and cargoes; (3) review plant selective autophagy and its physiological significance; (4) focus on two decision-making mechanisms: regulation of recycling versus degradation of PM proteins and coordination between autophagy and vacuolar degradation; and (5) identify future challenges.

Potassium in Root Growth and Development

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K+ in root growth, development of root system architecture, cellular functions, and specific plant responses to K+ shortage. K+ transport is crucial for its physiological role. A wide range of K+ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K+ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K+ among important general regulatory factors of root growth. K+ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K+ uptake as a crucial mechanism for plant stress response. K+-related regulatory mechanisms, K+ transporters, K+ acquisition efficiency, and phenotyping for selection of K+ efficient plants/cultivars are highlighted in this review.

AtTRAPPC11 is involved in TRAPPIII mediated control of post-Golgi protein trafficking

The plant trans-Golgi Network/Early Endosome (TGN/EE), as an organizer of vesicle trafficking, fulfills a crucial role for plant development and adaptation. Because it coordinates the transport of cell material along different routes, it is expected that a number of TGN/EE associated factors function in the rapid organization of post-Golgi trafficking to ensure that proteins reach their destination. The roles of Transport Protein Particle (TRAPP) complexes in the regulation of plant post-Golgi trafficking start to emerge. We previously demonstrated that the plant TRAPPIII complex is involved in maintenance of TGN organization and function and has a role in endocytic trafficking mediated by the SYP61 TGN/EE compartment. Here we show that attrappc11 mutants display accumulation of the plasma membrane resident proteins CESA6, BRI1 and PIP1;4 in aberrant intracellular compartments. This adds further insights into the functions of TRAPPIII as a regulators of post-Golgi/endosomal traffic.

The MCTP-SNARE Complex Regulates Florigen Transport in Arabidopsis

Multiple flowering pathways in Arabidopsis (Arabidopsis thaliana) converge on the transcriptional regulation of FLOWERING LOCUS T (FT), encoding a mobile floral stimulus that moves from leaves to the shoot apex. Despite our progress in understanding FT movement, the mechanisms underlying its transport along the endoplasmic reticulum-plasmalemma pathway in phloem companion cells remain largely unclear. Here, we show that the plasma membrane-resident syntaxin-like glutamine-soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor (Q-SNARE), SYNTAXIN OF PLANTS121 (SYP121), interacts with QUIRKY (QKY), a member of the family of multiple C2 domain and transmembrane region proteins (MCTPs), to mediate FT transport in Arabidopsis. QKY and SYP121 coordinately regulate FT movement to the plasmalemma through the endosomal trafficking pathway and are required for FT export from companion cells to sieve elements, thus affecting FT transport through the phloem to the shoot apical meristem. These findings suggest that MCTP-SNARE complex-mediated endosomal trafficking is essential for the export of florigen from phloem companion cells to sieve elements to induce flowering.plantcell;31/10/2475/FX1F1fx1.

Genome-wide characterization and expression analysis of aquaporins in salt cress (Eutrema salsugineum)

Aquaporins (AQPs) serve as water channel proteins and belong to major intrinsic proteins (MIPs) family, functioning in rapidly and selectively transporting water and other small solutes across biological membranes. Importantly, AQPs have been shown to play a critical role in abiotic stress response pathways of plants. As a species closely related to Arabidopsis thaliana, Eutrema salsugineum has been proposed as a model for studying salt resistance in plants. Here we surveyed 35 full-length AQP genes in E. salsugineum, which could be grouped into four subfamilies including 12 plasma membrane intrinsic proteins (PIPs), 11 tonoplast intrinsic proteins (TIPs), nine NOD-like intrinsic proteins (NIPs), and three small basic intrinsic proteins (SIPs) by phylogenetic analysis. EsAQPs were comprised of 237-323 amino acids, with a theoretical molecular weight (MW) of 24.31-31.80 kDa and an isoelectric point (pI) value of 4.73-10.49. Functional prediction based on the NPA motif, aromatic/arginine (ar/R) selectivity filter, Froger's position and specificity-determining position suggested quite differences in substrate specificities of EsAQPs. EsAQPs exhibited global expressions in all organs as shown by gene expression profiles and should be play important roles in response to salt, cold and drought stresses. This study provides comprehensive bioinformation on AQPs in E. salsugineum, which would be helpful for gene function analysis for further studies.

Identification of Sugarcane Host Factors Interacting with the 6K2 Protein of the Sugarcane Mosaic Virus

The 6K2 protein of potyviruses plays a key role in the viral infection in plants. In the present study, the coding sequence of 6K2 was cloned from Sugarcane mosaic virus (SCMV) strain FZ1 into pBT3-STE to generate the plasmid pBT3-STE-6K2, which was used as bait to screen a cDNA library prepared from sugarcane plants infected with SCMV based on the DUALmembrane system. One hundred and fifty-seven positive colonies were screened and sequenced, and the corresponding full-length genes were cloned from sugarcane cultivar ROC22. Then, 24 genes with annotations were obtained, and the deduced proteins were classified into three groups, in which eight proteins were involved in the stress response, 12 proteins were involved in transport, and four proteins were involved in photosynthesis based on their biological functions. Of the 24 proteins, 20 proteins were verified to interact with SCMV-6K2 by yeast two-hybrid assays. The possible roles of these proteins in SCMV infection on sugarcane are analyzed and discussed. This is the first report on the interaction of SCMV-6K2 with host factors from sugarcane, and will improve knowledge on the mechanism of SCMV infection in sugarcane.

Comparative Transcriptome Analysis of Waterlogging-Sensitive and Tolerant Zombi Pea (Vigna Vexillata) Reveals Energy Conservation and Root Plasticity Controlling Waterlogging Tolerance

Vigna vexillata (zombi pea) is an underutilized legume crop considered to be a potential gene source in breeding for abiotic stress tolerance. This study focuses on the molecular characterization of mechanisms controlling waterlogging tolerance using two zombi pea varieties with contrasting waterlogging tolerance. Morphological examination revealed that in contrast to the sensitive variety, the tolerant variety was able to grow, maintain chlorophyll, form lateral roots, and develop aerenchyma in hypocotyl and taproots under waterlogging. To find the mechanism controlling waterlogging tolerance in zombi pea, comparative transcriptome analysis was performed using roots subjected to short-term waterlogging. Functional analysis indicated that glycolysis and fermentative genes were strongly upregulated in the sensitive variety, but not in the tolerant one. In contrast, the genes involved in auxin-regulated lateral root initiation and formation were expressed only in the tolerant variety. In addition, cell wall modification, aquaporin, and peroxidase genes were highly induced in the tolerant variety under waterlogging. Our findings suggest that energy management and root plasticity play important roles in mitigating the impact of waterlogging in zombi pea. The basic knowledge obtained from this study can be used in the molecular breeding of waterlogging-tolerant legume crops in the future.

SCARECROW promoter-driven expression of a bacterial mercury transporter MerC in root endodermal cells enhances mercury accumulation in Arabidopsis shoots

Mercury accumulation in Arabidopsis shoots is accelerated by endodermis specific expression of fusion proteins of a bacterial mercury transporter MerC and a plant SNARE SYP121 under control of SCARECROW promoter. We previously demonstrated that the CaMV 35S RNA promoter (p35S)-driven ubiquitous expression of a bacterial mercury transporter MerC, fused with SYP121, an Arabidopsis SNARE protein increases mercury accumulation of Arabidopsis. To establish an improved fine-tuned mercury transport system in plants for phytoremediation, the present study generated and characterized transgenic Arabidopsis plants expressing MerC-SYP121 specifically in the root endodermis, which is a crucial cell type for root element uptake. We generated four independent transgenic Arabidopsis lines expressing a transgene encoding mCherry-MerC-SYP121 under the control of the endodermis-specific SCARECROW promoter (hereafter pSCR lines). Quantitative real-time PCR analysis showed that expression levels of the transgene in roots of the pSCR lines were 3-23% of the p35S driven-overexpressing line. Confocal microscopy analysis showed that mCherry-MerC-SYP121 was dominantly expressed in the endodermis of the meristematic zone as well as in the mature zone of the pSCR roots. Mercury accumulation in shoots of the pSCR lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the p35S over-expressing line. These results suggest that endodermis-specific expression of the MerC-SYP121 fusion proteins in plant roots sufficiently enhances mercury uptake and accumulation into shoots, which would be an ideal phenotype for phytoremediation of mercury-contaminated environments.

Dehydrin MtCAS31 promotes autophagic degradation under drought stress

Drought stress seriously affects crop yield, and the mechanism underlying plant resistance to drought stress via macroautophagy/autophagy is not clear. Here, we show that a dehydrin, Medicago truncatula MtCAS31 (cold acclimation-specific 31), a positive regulator of drought response, plays a key role in autophagic degradation. A GFP cleavage assay and treatment with an autophagy-specific inhibitor indicated that MtCAS31 participates in the autophagic degradation pathway and that overexpressing MtCAS31 promotes autophagy under drought stress. Furthermore, we discovered that MtCAS31 interacts with the autophagy-related protein ATG8a in the AIM-like motif YXXXI, supporting its function in autophagic degradation. In addition, we identified a cargo protein of MtCAS31, the aquaporin MtPIP2;7, by screening an M. truncatula cDNA library. We found that MtPIP2;7 functions as a negative regulator of drought response. Under drought stress, MtCAS31 facilitated the autophagic degradation of MtPIP2;7 and reduced root hydraulic conductivity, thus reducing water loss and improving drought tolerance. Taken together, our results reveal a novel function of dehydrins in promoting the autophagic degradation of proteins, which extends our knowledge of the function of dehydrins.Abbreviations: AIM: ATG8-interacting motif; ATG: autophagy-related; ATI1: ATG8-interacting protein1; BiFC: Biomolecular fluorescence complementation; CAS31: cold acclimation-specific 31; ConcA: concanamycin A; DSK2: dominant suppressor of KAR2; ER: endoplasmic reticulum; ERAD: ER-associated degradation; NBR1: next to BRCA1 gene 1; PM: plasma membrane; PIPs: plasma membrane intrinsic proteins; TALEN: transcription activator-like effector nuclease; TSPO: tryptophan-rich sensory protein/translocator; UPR: unfolded protein response; VC: vector control.

VAPYRIN Marks an Endosomal Trafficking Compartment Involved in Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhiza (AM) is a symbiosis between plants and AM fungi that requires the intracellular accommodation of the fungal partner in the host. For reciprocal nutrient exchange, AM fungi form intracellular arbuscules that are surrounded by the peri-arbuscular membrane. This membrane, together with the fungal plasma membrane, and the space in between, constitute the symbiotic interface, over which nutrients are exchanged. Intracellular establishment of AM fungi requires the VAPYRIN protein which is induced in colonized cells, and which localizes to numerous small mobile structures of unknown identity (Vapyrin-bodies). In order to characterize the identity and function of the Vapyrin-bodies we pursued a dual strategy. First, we co-expressed fluorescently tagged VAPYRIN with a range of subcellular marker proteins, and secondly, we employed biochemical tools to identify interacting partner proteins of VAPYRIN. As an important tool for the quantitative analysis of confocal microscopic data sets from co-expression of fluorescent proteins, we developed a semi-automated image analysis pipeline that allows for precise spatio-temporal quantification of protein co-localization and of the dynamics of organelle association from movies. Taken together, these experiments revealed that Vapyrin-bodies have an endosomal identity with trans-Golgi features, and that VAPYRIN interacts with a symbiotic R-SNARE of the VAMP721 family, that localizes to the same compartment.

Regulation of a plant aquaporin by a Casparian strip membrane domain protein-like

The absorption of soil water by roots allows plants to maintain their water status. At the endodermis, water transport can be affected by initial formation of a Casparian strip and further deposition of suberin lamellas and regulated by the function of aquaporins. Four Casparian strip membrane domain protein-like (CASPL; CASPL1B1, CASPL1B2, CASPL1D1, and CASPL1D2) were previously shown to interact with PIP2;1. The present work shows that CASPL1B1, CASPL1B2, and CASPL1D2 are exclusively expressed in suberized endodermal cells, suggesting a cell-specific role in suberization and/or water transport regulation. When compared with wild-type plants, and by contrast to caspl1b1*caspl1b2 double loss of function, caspl1d1*caspl1d2 double mutants showed, in some control or NaCl stress experiments and not upon abscisic acid (ABA) treatment, a weak enlargement of the continuous suberization zone. None of the mutants showed root hydraulic conductivity (Lp_r ) phenotype, whether in control, NaCl, or ABA treatment conditions. The data suggest a slight negative role for CASPL1D1 and CASPL1D2 in suberization under control or salt stress conditions, with no major impact on whole root transport functions. At the molecular level, CASPL1B1 was able to physically interact with PIP2;1 and potentially could influence the regulation of aquaporins by acting on their phosphorylated form.

Manipulating Endoplasmic Reticulum-Plasma Membrane Tethering in Plants Through Fluorescent Protein Complementation

The bimolecular fluorescence complementation (BiFC) assay has been widely used to examine interactions between integral and peripheral proteins within putative plasma membrane (PM) microdomains. In the course of using BiFC assays to examine the co-localization of plasma membrane (PM) targeted receptor-like kinases (RLKs), such as FLS2, with PM micro-domain proteins such as remorins, we unexpectedly observed heterogeneous distribution patterns of fluorescence on the PM of Nicotiana benthamiana leaf cortical cells. These patterns appeared to co-localize with the endoplasmic reticulum (ER) and with ER-PM contact sites, and closely resembled patterns caused by over-expression of the ER-PM tether protein Synaptotagmin1 (SYT1). Using domain swap experiments with SYT1, we inferred that non-specific dimerization between FLS2-VenusN and VenusC-StRem1.3 could create artificial ER-PM tether proteins analogous to SYT1. The same patterns of ER-PM tethering were produced when a representative set of integral membrane proteins were partnered in BiFC complexes with PM-targeted peripheral membrane proteins, including PtdIns(4)P-binding proteins. We inferred that spontaneous formation of mature fluorescent proteins caused the BiFC complexes to trap the integral membrane proteins in the ER during delivery to the PM, producing a PM-ER tether. This phenomenon could be a useful tool to deliberately manipulate ER-PM tethering or to test protein membrane localization. However, this study also highlights the risk of using the BiFC assay to study membrane protein interactions in plants, due to the possibility of alterations in cellular structures and membrane organization, or misinterpretation of protein-protein interactions. A number of published studies using this approach may therefore need to be revisited.

GmSYP24, a putative syntaxin gene, confers osmotic/drought, salt stress tolerances and ABA signal pathway

As major environment factors, drought or high salinity affect crop growth, development and yield. Transgenic approach is an effective way to improve abiotic stress tolerance of crops. In this study, we comparatively analyzed gene structures, genome location, and the evolution of syntaxin proteins containing late embryogenesis abundant (LEA2) domain. GmSYP24 was identified as a dehydration-responsive gene. Our study showed that the GmSYP24 protein was located on the cell membrane. The overexpression of GmSYP24 (GmSYP24ox) in soybean and heteroexpression of GmSYP24 (GmSYP24hx) in Arabidopsis exhibited insensitivity to osmotic/drought and high salinity. However, wild type soybean, Arabidopsis, and the mutant of GmSYP24 homologous gene of Arabidopsis were sensitive to the stresses. Under the abiotic stresses, transgenic soybean plants had greater water content and higher activities of POD, SOD compared with non-transgenic controls. And the leaf stomatal density and opening were reduced in transgenic Arabidopsis. The sensitivity to ABA was decreased during seed germination of GmSYP24ox and GmSYP24hx. GmSYP24hx induced up-regulation of ABA-responsive genes. GmSYP24ox alters the expression of some aquaporins under osmotic/drought, salt, or ABA treatment. These results demonstrated that GmSYP24 played an important role in osmotic/drought or salt tolerance in ABA signal pathway.

Low Water Potential and At14a-Like1 (AFL1) Effects on Endocytosis and Actin Filament Organization

At14a-Like1 (AFL1) is a stress-induced protein of unknown function that promotes growth during low water potential stress and drought. Previous analysis indicated that AFL1 may have functions related to endocytosis and regulation of actin filament organization, processes for which the effects of low water potential are little known. We found that low water potential led to a decrease in endocytosis, as measured by uptake of the membrane-impermeable dye FM4-64. Ectopic expression of AFL1 reversed the decrease in FM4-64 uptake seen in wild type, while reduced AFL1 expression led to further inhibition of FM4-64 uptake. Increased AFL1 also made FM4-64 uptake less sensitive to the actin filament disruptor Latrunculin B (LatB). LatB decreased AFL1-Clathrin Light Chain colocalization, further indicating that effects of AFL1 on endocytosis may be related to actin filament organization or stability. Consistent with this hypothesis, ectopic AFL1 expression made actin filaments less sensitive to disruption by LatB or Cytochalasin D and led to increased actin filament skewness and decreased occupancy, indicative of more bundled actin filaments. This latter effect could be partially mimicked by the actin filament stabilizer Jasplakinolide (JASP). However, AFL1 did not substantially inhibit actin filament dynamics, indicating that AFL1 acts via a different mechanism than JASP-induced stabilization. AFL1 partially colocalized with actin filaments but not with microtubules, further indicating actin-filament-related function of AFL1. These data provide insight into endocytosis and actin filament responses to low water potential stress and demonstrate an involvement of AFL1 in these key cellular processes.

Quantitative phosphoproteomics of lectin receptor-like kinase VI.4 dependent abscisic acid response in Arabidopsis thaliana

Lectin receptor-like kinases (LecRKs) play important roles in the responses to adverse environment stress. Abscisic acid (ABA) is a plant hormone involved in plant growth, development and adverse environmental stress responses. Although some studies of ABA response LecRK genes have been reported, the molecular mechanisms of LecRKs regulation of downstream pathways under ABA induction are not well understood. The present study showed that LecRK-VI.4 responded to ABA and negatively regulated stomatal closure. Here, a quantitative phosphoproteomics approach based on mass spectrometry was employed to study the roles of LecRK-VI.4 in the ABA signaling pathway. Metal oxide affinity beads and C18 chromatography were used for phosphopeptide enrichment and separation. The isobaric tags for relative and absolute quantitation were used for profiling the phosphoproteome of mutant lecrk-vi.4-1 and wild-type Col-0 Arabidopsis under normal growth conditions or ABA treatments. In total, 475 unique phosphopeptides were quantified, including 81 phosphopeptides related to LecRK-VI.4 regulation. Gene ontology, protein-protein interaction and motif analysis were performed. The bioinformatics data showed that phosphorylated proteins regulated by LecRK-VI.4 had close relations with factors of stomatal function, which included aquaporin activity, H⁺ pump activity and the Ca²⁺ concentration in the cytoplasm. These data have expanded our understanding of how LecRK-VI.4 regulates ABA-mediated stomatal movements.

Ectopic expression of a bacterial mercury transporter MerC in root epidermis for efficient mercury accumulation in shoots of Arabidopsis plants

For mercury phytoextraction, we previously demonstrated in Arabidopsis thaliana that a constitutive and ubiquitous promoter-driven expression of a bacterial mercury transporter MerC fused with SYP121, a plant SNARE for plasma membrane protein trafficking increases plant mercury accumulation. To advance regulation of ectopic expression of the bacterial transporter in the plant system, the present study examined whether merC-SYP121 expression driven by a root epidermis specific promoter (pEpi) is sufficient to enhance mercury accumulation in plant tissues. We generated five independent transgenic Arabidopsis plant lines (hereafter pEpi lines) expressing a transgene encoding MerC-SYP121 N-terminally tagged with a fluorescent protein mTRQ2 under the control of pEpi, a root epidermal promoter. Confocal microscopy analysis of the pEpi lines showed that mTRQ2-MerC-SYP121 was preferentially expressed in lateral root cap in the root meristematic zone and epidermal cells in the elongation zone of the roots. Mercury accumulation in shoots of the pEpi lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the over-expressing line. The results suggest that cell-type specific expression of the bacterial transporter MerC in plant roots sufficiently enhances mercury accumulation in shoots, which could be a useful phenotype for improving efficiency of mercury phytoremediation.

A Hybrid Approach Enabling Large-Scale Glycomic Analysis of Post-Golgi Vesicles Reveals a Transport Route for Polysaccharides

The plant endomembrane system facilitates the transport of polysaccharides, associated enzymes, and glycoproteins through its dynamic pathways. Although enzymes involved in cell wall biosynthesis have been identified, little is known about the endomembrane-based transport of glycan components. This is partially attributed to technical challenges in biochemically determining polysaccharide cargo in specific vesicles. Here, we introduce a hybrid approach addressing this limitation. By combining vesicle isolation with a large-scale carbohydrate antibody arraying technique, we charted an initial large-scale map describing the glycome profile of the SYNTAXIN OF PLANTS61 (SYP61) trans-Golgi network compartment in Arabidopsis (Arabidopsis thaliana). A library of antibodies recognizing specific noncellulosic carbohydrate epitopes allowed us to identify a range of diverse glycans, including pectins, xyloglucans (XyGs), and arabinogalactan proteins in isolated vesicles. Changes in XyG- and pectin-specific epitopes in the cell wall of an Arabidopsis SYP61 mutant corroborate our findings. Our data provide evidence that SYP61 vesicles are involved in the transport and deposition of structural polysaccharides and glycoproteins. Adaptation of our methodology can enable studies characterizing the glycome profiles of various vesicle populations in plant and animal systems and their respective roles in glycan transport defined by subcellular markers, developmental stages, or environmental stimuli.

AQUA1 is a mercury sensitive poplar aquaporin regulated at transcriptional and post-translational levels by Zn stress

Aquaporins are water channel proteins that regulate plant development, growth, and response to environmental stresses. Populus trichocarpa is one of the plants with the highest number of aquaporins in its genome, but only few of them have been characterized at the whole plant functional level. Here we analyzed a putative aquaporin gene, aqua1, a gene that encodes for a protein of 257 amino acid with the typical NPA (Asp-Pro-Ala) signature motif of the aquaporin gene family. aqua1 was down-regulated of ∼10 fold under excess Zn in both leaves and roots, and conferred Zn tolerance when expressed in yeast Zn hypersensitive strain. In vivo localization of AQUA1-GFP in Arabidopsis protoplast showed a heterogeneous distribution of this protein on different membranes destined to form aggregates related to autophagic multivesicular bodies. Zn-dependent AQUA1-GFP re-localization was perturbed by phosphatases' and kinases' inhibitors that could affect both intracellular trafficking and aquaporins' activity. Exposed to high concentration of Zn, AQUA1 also co-localized with AtTIP1;1, a well-known Arabidopsis vacuolar marker, probably in pro-vacuolar multivesicular bodies. These findings suggest that high concentration of Zn down-regulates aqua1 and causes its re-localization in new forming pro-vacuoles. This Zn-dependent re-localization appears to be mediated by mechanisms regulating intracellular trafficking and aquaporins' post-translational modifications. This functional characterization of a poplar aquaporin in response to excess Zn will be a useful reference for understanding aquaporins' roles and regulation in response to high concentration of Zn in poplar.

Transmembrane Helices 2 and 3 Determine the Localization of Plasma Membrane Intrinsic Proteins in Eukaryotic Cells

In plants, plasma membrane intrinsic protein (PIP) PIP1s and PIP2s mediate the transport of disparate substrates across plasma membranes (PMs), with a prerequisite that the proteins correctly localize to the PMs. While PIP2s can take correct localization by themselves in plant cells, PIP1s cannot unless aided by a specific PIP2. Here, we analyzed the localization of the Arabidopsis aquaporins, AtPIP1s, AtPIP2;4, and their mutants in yeast, Xenopus oocytes, and protoplasts of Arabidopsis. Most of AtPIP2;4 localized in the PM when expressed alone, whereas AtPIP1;1 failed to realize it in yeast and Xenopus oocytes. Switch of the transmembrane helix 2 (TM2) or TM3 from AtPIP1;1 to AtPIP2;4 disabled the latter's PM targeting activity. Surprisingly, a replacement of TM2 and TM3 of AtPIP1;1 with those of AtPIP2;4 created a PM-localized AtPIP1;1 mutant, 1;1Δ(TM2+TM3)/2;4(TM2+TM3), which could act as a water and hydrogen peroxide channel just like AtPIP2;4. A localization and function analysis on mutants of AtPIP1;2, AtPIP1;3, AtPIP1;4, and AtPIP1;5, with the same replaced TM2 and TM3 from AtPIP2;4, showed that these AtPIP1 variants could also localize in the PM spontaneously, thus playing an inherent role in transporting solutes. Sequential and structural analysis suggested that a hydrophilic residue and a defective LxxxA motif are modulators of PM localization of AtPIP1s. These results indicate that TM2 and TM3 are necessary and, more importantly, sufficient in AtPIP2 for its PM localization.

Heterologous Expression of Serine Hydroxymethyltransferase-3 From Rice Confers Tolerance to Salinity Stress in E. coli and Arabidopsis

Among abiotic stresses, salt stress adversely affects growth and development in rice. Contrasting salt tolerant (CSR27), and salt sensitive (MI48) rice varieties provided information on an array of genes that may contribute for salt tolerance of rice. Earlier studies on transcriptome and proteome profiling led to the identification of salt stress-induced serine hydroxymethyltransferase-3 (SHMT3) gene. In the present study, the SHMT3 gene was isolated from salt-tolerant (CSR27) rice. OsSHMT3 exhibited salinity-stress induced accentuated and differential expression levels in different tissues of rice. OsSHMT3 was overexpressed in Escherichia coli and assayed for enzymatic activity and modeling protein structure. Further, Arabidopsis transgenic plants overexpressing OsSHMT3 exhibited tolerance toward salt stress. Comparative analyses of OsSHMT3 vis a vis wild type by ionomic, transcriptomic, and metabolic profiling, protein expression and analysis of various traits revealed a pivotal role of OsSHMT3 in conferring tolerance toward salt stress. The gene can further be used in developing gene-based markers for salt stress to be employed in marker assisted breeding programs.

HIGHLIGHTS

- The study provides information on mechanistic details of serine hydroxymethyl transferase gene for its salt tolerance in rice.

Plant TGN in the stress response: a compartmentalized overview

The cellular responses to abiotic and biotic stress rely on the regulation of vesicle trafficking to ensure the correct localization of proteins specialized in sensing stress stimuli and effecting the response. Several studies have implicated the plant trans-Golgi network (TGN)-mediated trafficking in different types of biotic and abiotic stress responses; however, the underlying molecular mechanisms are poorly understood. Further, the identity, specialization and stress-relevant cargo transported by the TGN subcompartments involved in stress responses await more in depth characterization. This review presents TGN trafficking players implicated in stress and discusses potential avenues to understand the role of this dynamic network under such extreme circumstances.

The Expanding Role of Vesicles Containing Aquaporins

In animals and plants, membrane vesicles containing proteins have been defined as key for biological systems involving different processes such as trafficking or intercellular communication. Docking and fusion of vesicles to the plasma membrane occur in living cells in response to different stimuli, such as environmental changes or hormones, and therefore play an important role in cell homeostasis as vehicles for certain proteins or other substances. Because aquaporins enhance the water permeability of membranes, their role as proteins immersed in vesicles formed of natural membranes is a recent topic of study. They regulate numerous physiological processes and could hence serve new biotechnological purposes. Thus, in this review, we have explored the physiological implications of the trafficking of aquaporins, the mechanisms that control their transit, and the proteins that coregulate the migration. In addition, the importance of exosomes containing aquaporins in the cell-to-cell communication processes in animals and plants have been analyzed, together with their potential uses in biomedicine or biotechnology. The properties of aquaporins make them suitable for use as biomarkers of different aquaporin-related diseases when they are included in exosomes. Finally, the fact that these proteins could be immersed in biomimetic membranes opens future perspectives for new biotechnological applications.

A GPI Signal Peptide-Anchored Split-Ubiquitin (GPS) System for Detecting Soluble Bait Protein Interactions at the Membrane

Bait fusion proteins with a glycosyl-phosphatidylinositol signal sequence anchor enable effective split ubiquitin screening for interactions with otherwise soluble membrane proteins.

Plant and Mammal Aquaporins: Same but Different

Aquaporins (AQPs) constitute an ancient and diverse protein family present in all living organisms, indicating a common ancient ancestor. However, during evolution, these organisms appear and evolve differently, leading to different cell organizations and physiological processes. Amongst the eukaryotes, an important distinction between plants and animals is evident, the most conspicuous difference being that plants are sessile organisms facing ever-changing environmental conditions. In addition, plants are mostly autotrophic, being able to synthesize carbohydrates molecules from the carbon dioxide in the air during the process of photosynthesis, using sunlight as an energy source. It is therefore interesting to analyze how, in these different contexts specific to both kingdoms of life, AQP function and regulation evolved. This review aims at highlighting similarities and differences between plant and mammal AQPs. Emphasis is given to the comparison of isoform numbers, their substrate selectivity, the regulation of the subcellular localization, and the channel activity.

Signal motif-dependent ER export of the Qc-SNARE BET12 interacts with MEMB12 and affects PR1 trafficking in Arabidopsis

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) are well-known for their role in controlling membrane fusion, the final, but crucial step, in vesicular transport in eukaryotes. SNARE proteins contribute to various biological processes including pathogen defense and channel activity regulation, as well as plant growth and development. Precise targeting of SNARE proteins to destined compartments is a prerequisite for their proper functioning. However, the underlying mechanism(s) for SNARE targeting in plants remains obscure. Here, we investigate the targeting mechanism of the Arabidopsis thaliana Qc-SNARE BET12, which is involved in protein trafficking in the early secretory pathway. Two distinct signal motifs that are required for efficient BET12 ER export were identified. Pulldown assays and in vivo imaging implicated that both the COPI and COPII pathways were required for BET12 targeting. Further studies using an ER-export-defective form of BET12 revealed that the Golgi-localized Qb-SNARE MEMB12, a negative regulator of pathogenesis-related protein 1 (PR1; At2g14610) secretion, was its interacting partner. Ectopic expression of BET12 caused no inhibition in the general ER-Golgi anterograde transport but caused intracellular accumulation of PR1, suggesting that BET12 has a regulatory role in PR1 trafficking in A. thaliana.

Post-Golgi Trafficking and Transport of Cell Wall Components

The cell wall, a complex macromolecular composite structure surrounding and protecting plant cells, is essential for development, signal transduction, and disease resistance. This structure is also integral to cell expansion, as its tensile resistance is the primary balancing mechanism against internal turgor pressure. Throughout these processes, the biosynthesis, transport, deposition, and assembly of cell wall polymers are tightly regulated. The plant endomembrane system facilitates transport of polysaccharides, polysaccharide biosynthetic and modifying enzymes and glycoproteins through vesicle trafficking pathways. Although a number of enzymes involved in cell wall biosynthesis have been identified, comparatively little is known about the transport of cell wall polysaccharides and glycoproteins by the endomembrane system. This review summarizes our current understanding of trafficking of cell wall components during cell growth and cell division. Emerging technologies, such as vesicle glycomics, are also discussed as promising avenues to gain insights into the trafficking of structural polysaccharides to the apoplast.

Variation in Membrane Trafficking Linked to SNARE AtSYP51 Interaction With Aquaporin NIP1;1

SYP51 and 52 are the two members of the SYP5 Qc-SNARE gene family in Arabidopsis thaliana. These two proteins, besides their high level of sequence identity (85%), have shown to have differential functional specificity and possess a different interactome. Here we describe a unique and specific interaction of SYP51 with an ER aquaporin, AtNIP1;1 (also known as NLM1) indicated to be able to transport arsenite [As(III)] and previously localized on PM. In the present work we investigate in detail such localization in vivo and characterize the interaction with SYP51. We suggest that this interaction may reveal a new mechanism regulating tonoplast invagination and recycling. We propose this interaction to be part of a regulatory mechanism associated with direct membrane transport from ER to tonoplast and Golgi mediated vesicle trafficking. We also demonstrate that NIP1;1 is important for plant tolerance to arsenite but does not alter its uptake or translocation. To explain such phenomenon the hypothesis that SYP51/NIP1;1 interaction modifies ER and vacuole ability to accumulate arsenite is discussed.

Mapping of Plasma Membrane Proteins Interacting With Arabidopsis thaliana Flotillin 2

Arabidopsis flotillin 2 (At5g25260) belongs to the group of plant flotillins, which are not well characterized. In contrast, metazoan flotillins are well known as plasma membrane proteins associated with membrane microdomains that act as a signaling hub. The similarity of plant and metazoan flotillins, whose functions most likely consist of affecting other proteins via protein-protein interactions, determines the necessity of detecting their interacting partners in plants. Nevertheless, identifying the proteins that form complexes on the plasma membrane is a challenging task due to their low abundance and hydrophobic character. Here we present an approach for mapping Arabidopsis thaliana flotillin 2 plasma membrane interactors, based on the immunoaffinity purification of crosslinked and enriched plasma membrane proteins with mass spectrometry detection. Using this approach, 61 proteins were enriched in the AtFlot-GFP plasma membrane fraction, and 19 of them were proposed to be flotillin 2 interaction partners. Among our proposed partners of Flot2, proteins playing a role in the plant response to various biotic and abiotic stresses were detected. Additionally, the use of the split-ubiquitin yeast system helped us to confirm that plasma-membrane ATPase 1, early-responsive to dehydration stress protein 4, syntaxin-71, harpin-induced protein-like 3, hypersensitive-induced response protein 2 and two aquaporin isoforms interact with flotillin 2 directly. Based on the results of our study and the reported properties of Flot2 interactors, we propose that Flot2 complexes may be involved in plant-pathogen interactions, water transport and intracellular trafficking.

GhSNAP33, a t-SNARE Protein From Gossypium hirsutum, Mediates Resistance to Verticillium dahliae Infection and Tolerance to Drought Stress

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins mediate membrane fusion and deliver cargo to specific cellular locations through vesicle trafficking. Synaptosome-associated protein of 25 kDa (SNAP25) is a target membrane SNARE that drives exocytosis by fusing plasma and vesicular membranes. In this study, we isolated GhSNAP33, a gene from cotton (Gossypium hirsutum), encoding a SNAP25-type protein containing glutamine (Q)b- and Qc-SNARE motifs connected by a linker. GhSNAP33 expression was induced by H₂O₂, salicylic acid, abscisic acid, and polyethylene glycol 6000 treatment and Verticillium dahliae inoculation. Ectopic expression of GhSNAP33 enhanced the tolerance of yeast cells to oxidative and osmotic stresses. Virus-induced gene silencing of GhSNAP33 induced spontaneous cell death and reactive oxygen species accumulation in true leaves at a later stage of cotton development. GhSNAP33-deficient cotton was susceptible to V. dahliae infection, which resulted in severe wilt on leaves, an elevated disease index, enhanced vascular browning and thylose accumulation. Conversely, Arabidopsis plants overexpressing GhSNAP33 showed significant resistance to V. dahliae, with reduced disease index and fungal biomass and elevated expression of PR1 and PR5. Leaves from GhSNAP33-transgenic plants showed increased callose deposition and reduced mycelia growth. Moreover, GhSNAP33 overexpression enhanced drought tolerance in Arabidopsis, accompanied with reduced water loss rate and enhanced expression of DERB2A and RD29A during dehydration. Thus, GhSNAP33 positively mediates plant defense against stress conditions and V. dahliae infection, rendering it a candidate for the generation of stress-resistant engineered cotton.

Toward understanding of the high number of plant aquaporin isoforms and multiple regulation mechanisms

Since the discovery of the first plant aquaporin (AQP) in 1993, our conception of the way plants control cell water homeostasis as well as their global water balance has been revisited. Plant AQPs constitute a large family of evolutionarily related channels that, in addition to water, can also facilitate the membrane diffusion of a number of small solutes, such as urea, CO₂, H₂O₂, ammonia, metalloids, and even ions, indicating a wide range of cellular functions. At the cellular level, AQPs are subject to various regulation mechanisms leading to active/inactive channels in their target membranes. In this review, we discuss several specific questions that need to be addressed in future research. Why are so many different AQPs simultaneously expressed in specific cellular types? How is their selectivity to different solutes controlled (in particular in the case of multiple permeation properties)? What does the molecular interaction between AQPs and other molecules tell us about their regulation and their involvement in specific cellular and physiological processes? Resolving these questions will definitely help us better understand the physiological advantages that plants have to express and regulate so many AQP isoforms.

Plant and animal aquaporins crosstalk: what can be revealed from distinct perspectives

Aquaporins (AQPs) can be revisited from a distinct and complementary perspective: the outcome from analyzing them from both plant and animal studies. (1) The approach in the study. Diversity found in both kingdoms contrasts with the limited number of crystal structures determined within each group. While the structure of almost half of mammal AQPs was resolved, only a few were resolved in plants. Strikingly, the animal structures resolved are mainly derived from the AQP2-lineage, due to their important roles in water homeostasis regulation in humans. The difference could be attributed to the approach: relevance in animal research is emphasized on pathology and in consequence drug screening that can lead to potential inhibitors, enhancers and/or regulators. By contrast, studies on plants have been mainly focused on the physiological role that AQPs play in growth, development and stress tolerance. (2) The transport capacity. Besides the well-described AQPs with high water transport capacity, large amount of evidence confirms that certain plant AQPs can carry a large list of small solutes. So far, animal AQP list is more restricted. In both kingdoms, there is a great amount of evidence on gas transport, although there is still an unsolved controversy around gas translocation as well as the role of the central pore of the tetramer. (3) More roles than expected. We found it remarkable that the view of AQPs as specific channels has evolved first toward simple transporters to molecules that can experience conformational changes triggered by biochemical and/or mechanical signals, turning them also into signaling components and/or behave as osmosensor molecules.