Identification of rice cornichon as a possible cargo receptor for the Golgi-localized sodium transporter OsHKT1;3

The Role of Cornichons in the Biogenesis and Functioning of Monovalent-Cation Transport Systems

Monovalent-cation homeostasis, crucial for all living cells, is ensured by the activity of various types of ion transport systems located either in the plasma membrane or in the membranes of organelles. A key prerequisite for the functioning of ion-transporting proteins is their proper trafficking to the target membrane. The cornichon family of COPII cargo receptors is highly conserved in eukaryotic cells. By simultaneously binding their cargoes and a COPII-coat subunit, cornichons promote the incorporation of cargo proteins into the COPII vesicles and, consequently, the efficient trafficking of cargoes via the secretory pathway. In this review, we summarize current knowledge about cornichon proteins (CNIH/Erv14), with an emphasis on yeast and mammalian cornichons and their role in monovalent-cation homeostasis. Saccharomyces cerevisiae cornichon Erv14 serves as a cargo receptor of a large portion of plasma-membrane proteins, including several monovalent-cation transporters. By promoting the proper targeting of at least three housekeeping ion transport systems, Na+, K+/H+ antiporter Nha1, K+ importer Trk1 and K+ channel Tok1, Erv14 appears to play a complex role in the maintenance of alkali-metal-cation homeostasis. Despite their connection to serious human diseases, the repertoire of identified cargoes of mammalian cornichons is much more limited. The majority of current information is about the structure and functioning of CNIH2 and CNIH3 as auxiliary subunits of AMPAR multi-protein complexes. Based on their unique properties and easy genetic manipulation, we propose yeast cells to be a useful tool for uncovering a broader spectrum of human cornichons´ cargoes.

The rice LATE ELONGATED HYPOCOTYL enhances salt tolerance by regulating Na+/K+ homeostasis and ABA signalling

The circadian clock plays multiple functions in the regulation of plant growth, development and response to various abiotic stress. Here, we showed that the core oscillator component late elongated hypocotyl (LHY) was involved in rice response to salt stress. The mutations of OsLHY gene led to reduced salt tolerance in rice. Transcriptomic analyses revealed that the OsLHY gene regulates the expression of genes related to ion homeostasis and the abscisic acid (ABA) signalling pathway, including genes encoded High-affinity K+ transporters (OsHKTs) and the stress-activated protein kinases (OsSAPKs). We demonstrated that OsLHY directly binds the promoters of OsHKT1;1, OsHKT1;4 and OsSAPK9 to regulate their expression. Moreover, the ossapk9 mutants exhibited salt tolerance under salt stress. Taken together, our findings revealed that OsLHY integrates ion homeostasis and the ABA pathway to regulate salt tolerance in rice, providing insights into our understanding of how the circadian clock controls rice response to salt stress.

Overexpression of a C3HC4-type E3-ubiquitin ligase contributes to salinity tolerance by modulating Na+ homeostasis in rice

Soil salinity has a negative effect on crop yield. Therefore, plants have evolved many strategies to overcome decreases in yield under saline conditions. Among these, E3-ubiquitin ligase regulates salt tolerance. We characterized Oryza sativa Really Interesting New Gene (RING) Finger C3HC4-type E3 ligase (OsRFPHC-4), which plays a positive role in improving salt tolerance. The expression of OsRFPHC-4 was downregulated by high NaCl concentrations and induced by abscisic acid (ABA) treatment. GFP-fused OsRFPHC-4 was localized to the plasma membrane of rice protoplasts. OsRFPHC-4 encodes a cellular protein with a C3HC4-RING domain with E3 ligase activity. However, its variant OsRFPHC-4C161A does not possess this activity. OsRFPHC-4-overexpressing plants showed enhanced salt tolerance due to low accumulation of Na+ in both roots and leaves, low Na+ transport in the xylem sap, high accumulation of proline and soluble sugars, high activity of reactive oxygen species (ROS) scavenging enzymes, and differential regulation of Na+ /K+ transporter expression compared to wild-type (WT) and osrfphc-4 plants. In addition, OsRFPHC-4-overexpressing plants showed higher ABA sensitivity under exogenous ABA treatment than WT and osrfphc-4 plants. Overall, these results suggest that OsRFPHC-4 contributes to the improvement of salt tolerance and Na+ /K+ homeostasis via the regulation of changes in Na+ /K+ transporters.

CmCNIH1 improves salt tolerance by influencing the trafficking of CmHKT1;1 in pumpkin

Pumpkin is often used as a rootstock for other Cucurbitaceae crops due to its resistance to soil-borne diseases and abiotic stress. Pumpkin rootstocks use a sodium transporter (CmHKT1;1) to promote the transport of Na⁺ from the shoot to the root effectively and improve the salt tolerance of the scion. However, the molecular regulatory mechanisms that influence the activity of CmHKT1;1 during salt stress response remain unknown. In this study, CmCNIH1, a cornichon homolog, was identified as a potential cargo receptor for CmHKT1;1. Yeast two-hybrid, biomolecular fluorescence complementation and luciferase complementary assays demonstrated that CmCNIH1 and CmHKT1;1 could interact. CmCNIH1 was a key component of the cellular vesicle transport machinery located in the endoplasmic reticulum (ER), ER export site and Golgi apparatus. A CmCNIH1 knockout mutant was more sensitive to salt stress than the wild-type (WT). In addition, ion homeostasis was disrupted in cmcnih1 mutants, which had higher Na⁺ and lower K⁺ content in shoots and roots than the WT. Two-electrode voltage-clamp experiment displayed that CmCNIH1 could not influence the Na⁺ current that passed through the plasma membrane (PM) in CmHKT1;1-expressing Xenopus laevis oocytes. Data from co-localization assays indicated that intact CmCNIH1 protein could alter the subcellular localization of CmHKT1;1 in tobacco leaf, pumpkin root and yeast. In summary, CmCNIH1 may function as a cargo receptor that regulates the localization of CmHKT1;1 to the PM to improve salt tolerance in pumpkin.

Nitrate-responsive OsMADS27 promotes salt tolerance in rice

Salt stress is a major constraint on plant growth and yield. Nitrogen (N) fertilizers are known to alleviate salt stress. However, the underlying molecular mechanisms remain unclear. Here, we show that nitrate-dependent salt tolerance is mediated by OsMADS27 in rice. The expression of OsMADS27 is specifically induced by nitrate. The salt-inducible expression of OsMADS27 is also nitrate dependent. OsMADS27 knockout mutants are more sensitive to salt stress than the wild type, whereas OsMADS27 overexpression lines are more tolerant. Transcriptomic analyses revealed that OsMADS27 upregulates the expression of a number of known stress-responsive genes as well as those involved in ion homeostasis and antioxidation. We demonstrate that OsMADS27 directly binds to the promoters of OsHKT1.1 and OsSPL7 to regulate their expression. Notably, OsMADS27-mediated salt tolerance is nitrate dependent and positively correlated with nitrate concentration. Our results reveal the role of nitrate-responsive OsMADS27 and its downstream target genes in salt tolerance, providing a molecular mechanism for the enhancement of salt tolerance by nitrogen fertilizers in rice. OsMADS27 overexpression increased grain yield under salt stress in the presence of sufficient nitrate, suggesting that OsMADS27 is a promising candidate for the improvement of salt tolerance in rice.

Involvement of the Membrane Nanodomain Protein, AtFlot1, in Vesicular Transport of Plasma Membrane H+-ATPase in Arabidopsis thaliana under Salt Stress

The aim of this study was to elucidate whether the membrane nanodomain protein AtFlot1 is involved in vesicular transport pathways and regulation of the P-type H⁺-ATPase content in plasma membrane of A. thaliana under salt stress. Transmission electron microscopy revealed changes in the endosomal system of A. thaliana root cells due to knockout mutation SALK_205125C (Atflot1ko). Immunoblotting of the plasma membrane-enriched fractions isolated from plant organs with an antibody to the H⁺-ATPase demonstrated changes in the H⁺-ATPase content in plasma membrane in response to the Atflot1ko mutation and salt shock. Expression levels of the main H⁺-ATPase isoforms, PMA1 and PMA2, as well as endocytosis activity of root cells determined by endocytic probe FM4-64 uptake assay, were unchanged in the Atflot1ko mutant. We have shown that AtFlot1 participates in regulation of the H⁺-ATPase content in the plasma membrane. We hypothesized that AtFlot1 is involved in both exocytosis and endocytosis, and, thus, contributes to the maintenance of cell ion homeostasis under salt stress. The lack of a pronounced Atflot1ko phenotype under salt stress conditions may be due to the assumed ability of Atflot1ko to switch vesicular transport to alternative pathways. Functional redundancy of AtFlot proteins may play a role in the functioning of these alternative pathways.

Ice plant root plasma membrane aquaporins are regulated by clathrin-coated vesicles in response to salt stress

The regulation of root Plasma membrane (PM) Intrinsic Protein (PIP)-type aquaporins (AQPs) is potentially important for salinity tolerance. However, the molecular and cellular details underlying this process in halophytes remain unclear. Using free-flow electrophoresis and label-free proteomics, we report that the increased abundance of PIPs at the PM of the halophyte ice plant (Mesembryanthemum crystallinum L.) roots under salinity conditions is regulated by clathrin-coated vesicles (CCV). To understand this regulation, we analyzed several components of the M. crystallinum CCV complexes: clathrin light chain (McCLC) and subunits μ1 and μ2 of the adaptor protein (AP) complex (McAP1μ and McAP2μ). Co-localization analyses revealed the association between McPIP1;4 and McAP2μ and between McPIP2;1 and McAP1μ, observations corroborated by mbSUS assays, suggesting that AQP abundance at the PM is under the control of CCV. The ability of McPIP1;4 and McPIP2;1 to form homo- and hetero-oligomers was tested and confirmed, as well as their activity as water channels. Also, we found increased phosphorylation of McPIP2;1 only at the PM in response to salt stress. Our results indicate root PIPs from halophytes might be regulated through CCV trafficking and phosphorylation, impacting their localization, transport activity, and abundance under salinity conditions.

Variety-Specific Transcriptional and Alternative Splicing Regulations Modulate Salt Tolerance in Rice from Early Stage of Stress

Salt stress poses physiological drought, ionic toxicity and oxidative stress to plants, which causes premature senescence and death of the leaves if the stress sustained. Salt tolerance varied between different rice varieties, but how different rice varieties respond at the early stage of salt stress has been seldom studied comprehensively. By employing third generation sequencing technology, we compared gene expressional changes in leaves of three rice varieties that varied in their level of tolerance after salt stress treatment for 6 h. Commonly up-regulated genes in all rice varieties were related to water shortage response and carbon and amino acids metabolism at the early stage of salt stress, while reactive oxygen species cleavage genes were induced more in salt-tolerant rice. Unexpectedly, genes involved in chloroplast development and photosynthesis were more significantly down-regulated in the two salt tolerant rice varieties 'C34' and 'Nona Bokra'. At the same time, genes coding ribosomal protein were suppressed to a more severe extent in the salt-sensitive rice variety 'IR29'. Interestingly, not only variety-specific gene transcriptional regulation, but also variety-specific mRNA alternative splicing, on both coding and long-noncoding genes, were found at the early stage of salt stress. In summary, differential regulation in gene expression at both transcriptional and post-transcriptional levels, determine and fine-tune the observed response in level of damage in leaves of specific rice genotypes at early stage of salt stress.

CRISPR/Cas9 Mediated Knockout of the OsbHLH024 Transcription Factor Improves Salt Stress Resistance in Rice (Oryza sativa L.)

Salinity stress is one of the most prominent abiotic stresses that negatively affect crop production. Transcription factors (TFs) are involved in the absorption, transport, or compartmentation of sodium (Na⁺) or potassium (K⁺) to resist salt stress. The basic helix-loop-helix (bHLH) is a TF gene family critical for plant growth and stress responses, including salinity. Herein, we used the CRISPR/Cas9 strategy to generate the gene editing mutant to investigate the role of OsbHLH024 in rice under salt stress. The A nucleotide base deletion was identified in the osbhlh024 mutant (A91). Exposure of the A91 under salt stress resulted in a significant increase in the shoot weight, the total chlorophyll content, and the chlorophyll fluorescence. Moreover, high antioxidant activities coincided with less reactive oxygen species (ROS) and stabilized levels of MDA in the A91. This better control of oxidative stress was accompanied by fewer Na⁺ but more K⁺, and a balanced level of Ca²⁺, Zn²⁺, and Mg²⁺ in the shoot and root of the A91, allowing it to withstand salt stress. Furthermore, the A91 also presented a significantly up-regulated expression of the ion transporter genes (OsHKT1;3, OsHAK7, and OsSOS1) in the shoot when exposed to salt stress. These findings imply that the OsbHLH024 might play the role of a negative regulator of salt stress, which will help to understand better the molecular basis of rice production improvement under salt stress.

Mechanism of high affinity potassium transporter (HKT) towards improved crop productivity in saline agricultural lands

Glycophytic plants are susceptible to salinity and their growth is hampered in more than 40 mM of salt. Salinity not only affects crop yield but also limits available land for farming by decreasing its fertility. Presence of distinct traits in response to environmental conditions might result in evolutionary adaptations. A better understanding of salinity tolerance through a comprehensive study of how Na⁺ is transported will help in the development of plants with improved salinity tolerance and might lead to increased yield of crops growing in strenuous environment. Ion transporters play pivotal role in salt homeostasis and maintain low cytotoxic effect in the cell. High-affinity potassium transporters are the critical class of integral membrane proteins found in plants. It mainly functions to remove excess Na⁺ from the transpiration stream to prevent sodium toxicity in the salt-sensitive shoot and leaf tissues. However, there are large number of HKT proteins expressed in plants, and it is possible that these members perform in a wide range of functions. Understanding their mechanism and functions will aid in further manipulation and genetic transformation of different crops. This review focuses on current knowledge of ion selectivity and molecular mechanisms controlling HKT gene expression. The current review highlights the mechanism of different HKT transporters from different plant sources and how this knowledge could prove as a valuable tool to improve crop productivity.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

To exclude or to accumulate? Revealing the role of the sodium HKT1;5 transporter in plant adaptive responses to varying soil salinity

Arid/semi-arid and coastal agricultural areas of the world are especially vulnerable to climate change-driven soil salinity. Salinity tolerance in plants is a complex trait, with salinity negatively affecting crop yield. Plants adopt a range of mechanisms to combat salinity, with many transporter genes being implicated in Na⁺-partitioning processes. Within these, the high-affinity K⁺ (HKT) family of transporters play a critical role in K⁺ and Na⁺ homeostasis in plants. Among HKT transporters, Type I transporters are Na⁺-specific. While Arabidopsis has only one Na ⁺ -specific HKT (AtHKT1;1), cereal crops have a multiplicity of Type I and II HKT transporters. AtHKT1; 1 (Arabidopsis thaliana) and HKT1; 5 (cereal crops) 'exclude' Na⁺ from the xylem into xylem parenchyma in the root, reducing shoot Na⁺ and hence, confer sodium tolerance. However, more recent data from Arabidopsis and crop species show that AtHKT1;1/HKT1;5 alleles have a strong genetic association with 'shoot sodium accumulation' and concomitant salt tolerance. The review tries to resolve these two seemingly contradictory effects of AtHKT1;1/HKT1;5 operation (shoot exclusion vs shoot accumulation), both conferring salinity tolerance and suggests that contrasting phenotypes are attributable to either hyper-functional or weak AtHKT1;1/HKT1;5 alleles/haplotypes and are under strong selection by soil salinity levels. It also suggests that opposite balancing mechanisms involving xylem ion loading in these contrasting phenotypes exist that require transporters such as SOS1 and CCC. While HKT1; 5 is a crucial but not sole determinant of salinity tolerance, investigation of the adaptive benefit(s) conferred by naturally occurring intermediate HKT1;5 alleles will be important under a climate change scenario.

A differential subcellular localization of two copper transporters from the COPT family suggests distinct roles in copper homeostasis in Physcomitrium patens

The moss Physcomitrium (Physcomitrella) patens is a bryophyte that provides genetic information about the adaptation to the life on land by early Embryophytes and is a reference organism for comparative evolutionary studies in plants. Copper is an essential micronutrient for every living organism, its transport across the plasma membrane is achieved by the copper transport protein family COPT/CTR. Two genes related to the COPT family were identified in Physcomitrella patens, PpaCOPT1 and PpaCOPT2. Homology modelling of both proteins showed the presence of three putative transmembrane domains (TMD) and the Mx₃M motif, constituting a potential Cu ⁺ selectivity filter present in other members of this family. Functional characterization of PpaCOPT1 and PpaCOPT2 in the yeast mutant ctr1Δctr3Δ restored its growth on medium with non-fermentable carbon sources at micromolar Cu concentrations, providing support that these two moss proteins function as high affinity Cu ⁺ transporters. Localization of PpaCOPT1 and PpaCOPT2 in yeast cells was observed at the tonoplast and plasma membrane, respectively. The heterologous expression of PpaCOPT2 in tobacco epidermal cells co-localized with the plasma membrane marker. Finally, only PpaCOPT1 was expressed in seven-day old protonema and was influenced by extracellular copper levels. This evidence suggests different roles of PpaCOPT1 and PpaCOPT2 in copper homeostasis in Physcomitrella patens.

Identification and Characterization of Rice OsHKT1;3 Variants

In rice, the high-affinity K⁺ transporter, OsHKT1;3, functions as a Na⁺-selective transporter. mRNA variants of OsHKT1;3 have been reported previously, but their functions remain unknown. In this study, five OsHKT1;3 variants (V1-V5) were identified from japonica rice (Nipponbare) in addition to OsHKT1;3_FL. Absolute quantification qPCR analyses revealed that the transcript level of OsHKT1;3_FL was significantly higher than other variants in both the roots and shoots. Expression levels of OsHKT1;3_FL, and some variants, increased after 24 h of salt stress. Two electrode voltage clamp experiments in a heterologous expression system using Xenopus laevis oocytes revealed that oocytes expressing OsHKT1;3_FL and all of its variants exhibited smaller Na⁺ currents. The presented data, together with previous data, provide insights to understanding how OsHKT family members are involved in the mechanisms of ion homeostasis and salt tolerance in rice.

Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation

HKT channels are a plant protein family involved in sodium (Na⁺) and potassium (K⁺) uptake and Na⁺-K⁺ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K⁺ shortage by allowing increased Na⁺ uptake under K⁺ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na⁺/K⁺ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.

Identification of Putative Interactors of Arabidopsis Sugar Transporters

Hexoses and disaccharides are the key carbon sources for essentially all physiological processes across kingdoms. In plants, sucrose, and in some cases raffinose and stachyose, are transported from the site of synthesis in leaves, the sources, to all other organs that depend on import, the sinks. Sugars also play key roles in interactions with beneficial and pathogenic microbes. Sugar transport is mediated by transport proteins that fall into super-families. Sugar transporter (ST) activity is tuned at different levels, including transcriptional and posttranslational levels. Understanding the ST interactome has a great potential to uncover important players in biologically and physiologically relevant processes, including, but not limited to Arabidopsis thaliana. Here, we combined ST interactions and coexpression studies to identify potentially relevant interaction networks.

C5 conserved region of hydrophilic C-terminal part of Saccharomyces cerevisiae Nha1 antiporter determines its requirement of Erv14 COPII cargo receptor for plasma-membrane targeting

Erv14, a conserved cargo receptor of COPII vesicles, helps the proper trafficking of many but not all transporters to the yeast plasma membrane, for example, three out of five alkali-metal-cation transporters in Saccharomyces cerevisiae. Among them, the Nha1 cation/proton antiporter, which participates in cell cation and pH homeostasis, is a large membrane protein (985 aa) possessing a long hydrophilic C-terminus (552 aa) containing six conserved regions (C1-C6) with unknown function. A short Nha1 version, lacking almost the entire C-terminus, still binds to Erv14 but does not need it to be targeted to the plasma membrane. Comparing the localization and function of ScNha1 variants shortened at its C-terminus in cells with or without Erv14 reveals that only ScNha1 versions possessing the complete C5 region are dependent on Erv14. In addition, our broad evolutionary conservation analysis of fungal Na⁺ /H⁺ antiporters identified new conserved regions in their C-termini, and our experiments newly show C5 and other, so far unknown, regions of the C-terminus, to be involved in the functionality and substrate specificity of ScNha1. Taken together, our results reveal that also relatively small hydrophilic parts of some yeast membrane proteins underlie their need to interact with the Erv14 cargo receptor.

The trihelix transcription factor OsGTγ-2 is involved adaption to salt stress in rice

OsGTγ-2, a trihelix transcription factor, is a positive regulator of rice responses to salt stress by regulating the expression of ion transporters. Salinity stress seriously restricts rice growth and yield. Trihelix transcription factors (GT factors) specifically bind to GT elements and play a diverse role in plant morphological development and responses to abiotic stresses. In our previous study, we found that the GT-1 element (GAAAAA) is a key element in the salinity-induced OsRAV2 promoter. Here, we identified a rice OsGTγ family member, OsGTγ-2, which directly interacted with the GT-1 element in the OsRAV2 promoter. OsGTγ-2 specifically targeted the nucleus, was mainly expressed in roots, sheathes, stems and seeds, and was induced by salinity, osmotic and oxidative stresses and abscisic acid (ABA). The seed germination rate, seedling growth and survival rate under salinity stress was improved in OsGTγ-2 overexpressing lines (P_ZmUbi::OsGTγ-2). In contrast, CRISPR/Cas9-mediated OsGTγ-2 knockout lines (osgtγ-2) showed salt-hypersensitive phenotypes. In response to salt stress, different Na⁺ and K⁺ acclamation patterns were observed in P_ZmUbi::OsGTγ-2 lines and osgtγ-2 plants were observed. The molecular mechanism of OsGTγ-2 in rice salt adaptation was also investigated. Several major genes responsible for ion transporting, such as the OsHKT2; 1, OsHKT1; 3 and OsNHX1 were transcriptionally regulated by OsGTγ-2. A subsequent yeast one-hybrid assay and EMSA indicated that OsGTγ-2 directly interacted with the promoters of OsHKT2; 1, OsNHX1 and OsHKT1; 3. Taken together, these results suggest that OsGTγ-2 is an important positive regulator involved in rice responses to salt stress and suggest a potential role for OsGTγ-2 in regulating salinity adaptation in rice.

The grapevine NaE sodium exclusion locus encodes sodium transporters with diverse transport properties and localisation

Grapevine (Vitis vinifera L.) is a valuable crop for human consumption and wine production, and is prone to suffering from salinity stress in arid regions or when exposed to low quality irrigation water. A previous study identified a quantitative trait locus (QTL) NaE, containing six High-affinity Potassium Transporter 1 genes, that was associated with shoot Na⁺ exclusion in grapevine. While HKT1;1 was predicted to be the most likely gene within this QTL to encode for this important salinity tolerance sub-trait, four other HKTs within the QTL remained uncharacterised; VviHKT1;2 encodes a truncated transcript unlikely to form a functional transporter. In this study, two allelic variants for each of VviHKT1;6, VviHKT1;7 and VviHKT1;8 from the heterozygous grapevine variety Cabernet Sauvignon were functionally characterised. Using the Xenopus laevis oocyte heterologous expression system, as well as transient expression in tobacco leaves, we found that the VviHKT1;6 and VviHKT1;7 alleles encoded plasma membrane localised proteins that facilitated significant non-rectifying Na⁺ transport. Conversely, proteins encoded by the VviHKT1;8 alleles were inwardly-rectifying, weak Na⁺ transporters that localised to intracellular organelles. Mining of previous RNA-seq gene expression data suggested that VviHKT1;6-8 are weakly expressed in grapevine roots, flower buds, and seeds under normal conditions and different nutrient regimes. We propose that VviHKT1;6 and VviHKT1;7 are likely to have a less significant role in grapevine leaf Na⁺ exclusion than VviHKT1;1, and that VviHKT1;8 is involved in endomembrane Na⁺ transport.

Expression and Ion Transport Activity of Rice OsHKT1;1 Variants

OsHKT1;1 in rice, belongs to the high-affinity K⁺ Transporter family, has been found to be involved in salt tolerance. OsHKT1;1 in japonica rice (Nipponbare) produces mRNA variants, but their functions remain elusive. In salt tolerant rice, Pokkali, eight OsHKT1;1 variants (V1-V8) were identified in addition to the full-length OsHKT1;1 (FL) cDNA. Absolute quantification by qPCR revealed that accumulation of OsHKT1;1-FL mRNA is minor in contrast to that of OsHKT1;1-V1, -V2, -V4, and -V7 mRNAs, all of which are predominant in shoots, while only V1 and V7 mRNAs are predominant in roots. Two electrode voltage clamp (TEVC) experiments using Xenopus laevis oocytes revealed that oocytes-expressing OsHKT1;1-FL from Pokkali exhibited inward-rectified currents in the presence of 96 mM Na⁺ as reported previously. Further TEVC analyses indicated that six of eight OsHKT1;1 variants elicited currents in a Na⁺ or a K⁺ bath solution. OsHKT1;1-V6 exhibited a similar inward rectification to the FL protein. Contrastingly, however, the rests mediated bidirectional currents in both Na⁺ and K⁺ bath solutions. These data suggest possibilities that novel mechanisms regulating the transport activity of OsHKT1;1 might exist, and that OsHKT1;1 variants might also carry out distinct physiological roles either independently or in combination with OsHKT1;1-FL.

Broad-Spectrum Amino Acid Transporters ClAAP3 and ClAAP6 Expressed in Watermelon Fruits

Watermelon fruit contains a high percentage of amino acid citrulline (Cit) and arginine (Arg). Cit and Arg accumulation in watermelon fruit are most likely mediated by both de novo synthesis from other amino acids within fruits and direct import from source tissues (leaves) through the phloem. The amino acid transporters involved in the import of Cit, Arg, and their precursors into developing fruits of watermelon have not been reported. In this study, we have compiled the list of putative amino acid transporters in watermelon and characterized transporters that are expressed in the early stage of fruit development. Using the yeast complementation study, we characterized ClAAP3 (Cla023187) and ClAAP6 (Cla023090) as functional amino acid transporters belonging to the family of amino acid permease (AAP) genes. The yeast growth and uptake assays of radiolabeled amino acid suggested that ClAAP3 and ClAAP6 can transport a broad spectrum of amino acids. Expression of translational fusion proteins with a GFP reporter in Nicotiana benthamiana leaves confirmed the ER- and plasma membrane-specific localization, suggesting the role of ClAAP proteins in the cellular import of amino acids. Based on the gene expression profiles and functional characterization, ClAAP3 and ClAAP6 are expected to play a major role in regulation of amino acid import into developing watermelon fruits.

OsC2DP, a Novel C2 Domain-Containing Protein Is Required for Salt Tolerance in Rice

Salt stress is one of the major factors limiting crop production globally, including rice (Oryza sativa). Although a number of genes involved in salt tolerance have been functionally identified, the mechanism underlying salt tolerance in rice is still poorly understood. Here, we reported a novel C2 domain-containing protein, OsC2DP required for salt tolerance in rice. OsC2DP was predominately expressed in the roots and its expression was repressed by salt stress. Transient expression of OsC2DP in rice protoplast cells showed that it was localized in the cytosol. Immunostaining further showed that OsC2DP was able to translocate from the cytosol to plasma membrane under salt conditions. Knockout of OsC2DP did not affect Na+ concentration in the roots, but increased shoot Na+ concentration, resulting in a significant sensitivity of rice to salt stress. Furthermore, the quantitative Real-time PCR and transcriptomic analysis showed that the expression level of some genes related to salt tolerance were indirectly regulated by OsC2DP, especially OsSOS1 and OsNHX4. These results indicate that OsC2DP has an important role in salt tolerance and these findings provide new insights into the regulation of OsC2DP gene for rice breeding with high salt tolerance.

Saccharomyces cerevisiae as a Tool to Investigate Plant Potassium and Sodium Transporters

Sodium and potassium are two alkali cations abundant in the biosphere. Potassium is essential for plants and its concentration must be maintained at approximately 150 mM in the plant cell cytoplasm including under circumstances where its concentration is much lower in soil. On the other hand, sodium must be extruded from the plant or accumulated either in the vacuole or in specific plant structures. Maintaining a high intracellular K+/Na+ ratio under adverse environmental conditions or in the presence of salt is essential to maintain cellular homeostasis and to avoid toxicity. The baker's yeast, Saccharomyces cerevisiae, has been used to identify and characterize participants in potassium and sodium homeostasis in plants for many years. Its utility resides in the fact that the electric gradient across the membrane and the vacuoles is similar to plants. Most plant proteins can be expressed in yeast and are functional in this unicellular model system, which allows for productive structure-function studies for ion transporting proteins. Moreover, yeast can also be used as a high-throughput platform for the identification of genes that confer stress tolerance and for the study of protein-protein interactions. In this review, we summarize advances regarding potassium and sodium transport that have been discovered using the yeast model system, the state-of-the-art of the available techniques and the future directions and opportunities in this field.

Meta-Analysis of Quantitative Trait Loci Associated with Seedling-Stage Salt Tolerance in Rice (Oryza sativa L.)

Soil and water salinity is one of the major abiotic stresses that reduce growth and productivity in major food crops including rice. The lack of congruence of salt tolerance quantitative trait loci (QTLs) in multiple genetic backgrounds and multiple environments is a major hindrance for undertaking marker-assisted selection (MAS). A genome-wide meta-analysis of QTLs controlling seedling-stage salt tolerance was conducted in rice using QTL information from 12 studies. Using a consensus map, 11 meta-QTLs for three traits with smaller confidence intervals were localized on chromosomes 1 and 2. The phenotypic variance of 3 meta-QTLs was ≥20%. Based on phenotyping of 56 diverse genotypes and breeding lines, six salt-tolerant genotypes (Bharathy, I Kung Ban 4-2 Mutant, Langmanbi, Fatehpur 3, CT-329, and IARI 5823) were identified. The perusal of the meta-QTL regions revealed several candidate genes associated with salt-tolerance attributes. The lack of association between meta-QTL linked markers and the level of salt tolerance could be due to the low resolution of meta-QTL regions and the genetic complexity of salt tolerance. The meta-QTLs identified in this study will be useful not only for MAS and pyramiding, but will also accelerate the fine mapping and cloning of candidate genes associated with salt-tolerance mechanisms in rice.

Arabidopsis UMAMIT24 and 25 are amino acid exporters involved in seed loading

Phloem-derived amino acids are the major source of nitrogen supplied to developing seeds. Amino acid transfer from the maternal to the filial tissue requires at least one cellular export step from the maternal tissue prior to the import into the symplasmically isolated embryo. Some members of UMAMIT (usually multiple acids move in an out transporter) family (UMAMIT11, 14, 18, 28, and 29) have previously been implicated in this process. Here we show that additional members of the UMAMIT family, UMAMIT24 and UMAMIT25, also function in amino acid transfer in developing seeds. Using a recently published yeast-based assay allowing detection of amino acid secretion, we showed that UMAMIT24 and UMAMIT25 promote export of a broad range of amino acids in yeast. In plants, UMAMIT24 and UMAMIT25 are expressed in distinct tissues within developing seeds; UMAMIT24 is mainly expressed in the chalazal seed coat and localized on the tonoplast, whereas the plasma membrane-localized UMAMIT25 is expressed in endosperm cells. Seed amino acid contents of umamit24 and umamit25 knockout lines were both decreased during embryogenesis compared with the wild type, but recovered in the mature seeds without any deleterious effect on yield. The results suggest that UMAMIT24 and 25 play different roles in amino acid translocation from the maternal to filial tissue; UMAMIT24 could have a role in temporary storage of amino acids in the chalaza, while UMAMIT25 would mediate amino acid export from the endosperm, the last step before amino acids are taken up by the developing embryo.

CORNICHON sorting and regulation of GLR channels underlie pollen tube Ca2+ homeostasis

Compared to animals, evolution of plant calcium (Ca²⁺) physiology has led to a loss of proteins for influx and small ligand-operated control of cytosolic Ca²⁺, leaving many Ca²⁺ mechanisms unaccounted for. Here, we show a mechanism for sorting and activation of glutamate receptor-like channels (GLRs) by CORNICHON HOMOLOG (CNIH) proteins. Single mutants of pollen-expressed Arabidopsis thaliana GLRs (AtGLRs) showed growth and Ca²⁺ flux phenotypes expected for plasma membrane Ca²⁺ channels. However, higher-order mutants of AtGLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular AtGLR localization, and we explored the implication of AtCNIHs in this sorting. We found that AtGLRs interact with AtCNIH pairs, yielding specific intracellular localizations. AtCNIHs further trigger AtGLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca²⁺ homeostasis by sorting and activation of AtGLRs by AtCNIHs.

Genome-wide association studies to identify rice salt-tolerance markers

Salinity is an ever increasing menace that affects agriculture worldwide. Crops such as rice are salt sensitive, but its degree of susceptibility varies widely between cultivars pointing to extensive genetic diversity that can be exploited to identify genes and proteins that are relevant in the response of rice to salt stress. We used a diversity panel of 306 rice accessions and collected phenotypic data after short (6 h), medium (7 d) and long (30 d) salinity treatment (50 mm NaCl). A genome-wide association study (GWAS) was subsequently performed, which identified around 1200 candidate genes from many functional categories, but this was treatment period dependent. Further analysis showed the presence of cation transporters and transcription factors with a known role in salinity tolerance and those that hitherto were not known to be involved in salt stress. Localization analysis of single nucleotide polymorphisms (SNPs) showed the presence of several hundred non-synonymous SNPs (nsSNPs) in coding regions and earmarked specific genomic regions with increased numbers of nsSNPs. It points to components of the ubiquitination pathway as important sources of genetic diversity that could underpin phenotypic variation in stress tolerance.

Plant SAM-Domain Proteins Start to Reveal Their Roles

Proteins often act in complexes assembled via protein-protein interaction domains. The sterile alpha motif (SAM) domain is one of the most prominent interaction domains in animals and is present in proteins of diverse functions. This domain allows head-to-tail closed oligomerisation or polymer formation resulting in homo- and/or heterocomplexes that have been shown to be important for proper protein localisation and function. In plants this domain is also present but has been poorly studied except for recent studies on the LEAFY floral regulator and the tRNA import component (TRIC)1/2 proteins. Here we catalogue SAM domain-containing proteins from arabidopsis (Arabidopsis thaliana), compare plant and other eukaryotic SAM domains, and perform homology modelling to probe plant SAM domain interaction capabilities.

Transport from the endoplasmic reticulum to the Golgi in plants: Where are we now?

The biogenesis of about one third of the cellular proteome is initiated in the endoplasmic reticulum (ER), which exports proteins to the Golgi apparatus for sorting to their final destination. Notwithstanding the close proximity of the ER with other secretory membranes (e.g., endosomes, plasma membrane), the ER is also important for the homeostasis of non-secretory organelles such as mitochondria, peroxisomes, and chloroplasts. While how the plant ER interacts with most of the non-secretory membranes is largely unknown, the knowledge on the mechanisms for ER-to-Golgi transport is relatively more advanced. Indeed, over the last fifteen years or so, a large number of exciting results have contributed to draw parallels with non-plant species but also to highlight the complexity of the plant ER-Golgi interface, which bears unique features. This review reports and discusses results on plant ER-to-Golgi traffic, focusing mainly on research on COPII-mediated transport in the model species Arabidopsis thaliana.

A Magnesium Transporter OsMGT1 Plays a Critical Role in Salt Tolerance in Rice

Salt stress is one of the major factors limiting rice (Oryza sativa) production globally. Although several transporters involved in salt tolerance have been identified in rice, the mechanisms regulating their transport activity are still poorly understood. Here, we show evidence that a rice Mg transporter OsMGT1 is required for salt tolerance probably by regulating transport activity of OsHKT1;5, a key transporter for the removal of Na⁺ from the xylem sap at the root mature zone. Knockout of OsMGT1 did not affect total Na uptake, but increased Na concentration in the shoots and xylem sap, resulting in a significant increase in salt sensitivity at low external Mg²⁺ concentration (20-200 μm). However, such differences were abolished at a higher Mg²⁺ concentration (2 mm), although the total Na uptake was not altered. OsMGT1 was expressed in both the roots and shoots, but only that in the roots was moderately up-regulated by salt stress. Spatial expression analysis revealed that OsMGT1 was expressed in all root cells of the root tips but was highly expressed in the pericycle of root mature zone. OsMGT1 was also expressed in the phloem region of basal node, leaf blade, and sheath. When expressed in Xenopus laevis oocytes, the transport activity of OsHKT1;5 was enhanced by elevating external Mg²⁺ concentration. Furthermore, knockout of OsHKT1;5 in osmgt1 mutant background did not further increase its salt sensitivity. Taken together, our results suggest that Mg²⁺ transported by OsMGT1 in the root mature zone is required for enhancing OsHKT1;5 activity, thereby restricting Na accumulation to the shoots.

De novo and reference transcriptome assembly of transcripts expressed during flowering provide insight into seed setting in tetraploid red clover

Red clover (Trifolium pratense L.) is one of the most important legume forage species in temperate livestock agriculture. Tetraploid red clover cultivars are generally producing less seed than diploid cultivars. Improving the seed setting potential of tetraploid cultivars is necessary to utilize the high forage quality and environmentally sustainable nitrogen fixation ability of red clover. In the current study, our aim was to identify candidate genes involved in seed setting. Two genotypes, ‘Tripo’ with weak seed setting and ‘Lasang’ with strong seed setting were selected for transcriptome analysis. De novo and reference based analyses of transcriptome assemblies were conducted to study the global transcriptome changes from early to late developmental stages of flower development of the two contrasting red clover genotypes. Transcript profiles, gene ontology enrichment and KEGG pathway analysis indicate that genes related to flower development, pollen pistil interactions, photosynthesis and embryo development are differentially expressed between these two genotypes. A significant number of genes related to pollination were overrepresented in ‘Lasang’, which might be a reason for its good seed setting ability. The candidate genes detected in this study might be used to develop molecular tools for breeding tetraploid red clover varieties with improved seed yield potentials.

An unrecognized function for COPII components in recruiting the viral replication protein BMV 1a to the perinuclear ER

Positive-strand RNA viruses invariably assemble their viral replication complexes (VRCs) by remodeling host intracellular membranes. How viral replication proteins are targeted to specific organelle membranes to initiate VRC assembly remains elusive. Brome mosaic virus (BMV), whose replication can be recapitulated in Saccharomyces cerevisiae, assembles its VRCs by invaginating the outer perinuclear endoplasmic reticulum (ER) membrane. Remarkably, BMV replication protein 1a (BMV 1a) is the only viral protein required for such membrane remodeling. We show that ER-vesicle protein of 14 kD (Erv14), a cargo receptor of coat protein complex II (COPII), interacts with BMV 1a. Moreover, the perinuclear ER localization of BMV 1a is disrupted in cells lacking ERV14 or expressing dysfunctional COPII coat components (Sec13, Sec24 or Sec31). The requirement of Erv14 for the localization of BMV 1a is bypassed by addition of a Sec24-recognizable sorting signal to BMV 1a or by overexpressing Sec24, suggesting a coordinated effort by both Erv14 and Sec24 for the proper localization of BMV 1a. The COPII pathway is well known for being involved in protein secretion; our data suggest that a subset of COPII coat proteins have an unrecognized role in targeting proteins to the perinuclear ER membrane.

The plant secretory pathway for the trafficking of cell wall polysaccharides and glycoproteins

Plant endomembranes are required for the biosynthesis and secretion of complex cell wall matrix polysaccharides, glycoproteins and proteoglycans. To define the biochemical roadmap that guides the synthesis and deposition of these cell wall components it is first necessary to outline the localization of the biosynthetic and modifying enzymes involved, as well as the distribution of the intermediate and final constituents of the cell wall. Thus far, a comprehensive understanding of cell wall matrix components has been hampered by the multiplicity of trafficking routes in the secretory pathway, and the diverse biosynthetic roles of the endomembrane organelles, which may exhibit tissue and development specific features. However, the recent identification of protein complexes producing matrix polysaccharides, and those supporting the synthesis and distribution of a grass-specific hemicellulose are advancing our understanding of the functional contribution of the plant secretory pathway in cell wall biosynthesis. In this review, we provide an overview of the plant membrane trafficking routes and report on recent exciting accomplishments in the understanding of the mechanisms underlying secretion with focus on cell wall synthesis in plants.

OsHKT1;4-mediated Na(+) transport in stems contributes to Na(+) exclusion from leaf blades of rice at the reproductive growth stage upon salt stress

BACKGROUND

Na(+) exclusion from leaf blades is one of the key mechanisms for glycophytes to cope with salinity stress. Certain class I transporters of the high-affinity K(+) transporter (HKT) family have been demonstrated to mediate leaf blade-Na(+) exclusion upon salinity stress via Na(+)-selective transport. Multiple HKT1 transporters are known to function in rice (Oryza sativa). However, the ion transport function of OsHKT1;4 and its contribution to the Na(+) exclusion mechanism in rice remain to be elucidated.

RESULTS

Here, we report results of the functional characterization of the OsHKT1;4 transporter in rice. OsHKT1;4 mediated robust Na(+) transport in Saccharomyces cerevisiae and Xenopus laevis oocytes. Electrophysiological experiments demonstrated that OsHKT1;4 shows strong Na(+) selectivity among cations tested, including Li(+), Na(+), K(+), Rb(+), Cs(+), and NH4 (+), in oocytes. A chimeric protein, EGFP-OsHKT1;4, was found to be functional in oocytes and targeted to the plasma membrane of rice protoplasts. The level of OsHKT1;4 transcripts was prominent in leaf sheaths throughout the growth stages. Unexpectedly however, we demonstrate here accumulation of OsHKT1;4 transcripts in the stem including internode II and peduncle in the reproductive growth stage. Moreover, phenotypic analysis of OsHKT1;4 RNAi plants in the vegetative growth stage revealed no profound influence on the growth and ion accumulation in comparison with WT plants upon salinity stress. However, imposition of salinity stress on the RNAi plants in the reproductive growth stage caused significant Na(+) overaccumulation in aerial organs, in particular, leaf blades and sheaths. In addition, (22)Na(+) tracer experiments using peduncles of RNAi and WT plants suggested xylem Na(+) unloading by OsHKT1;4.

CONCLUSIONS

Taken together, our results indicate a newly recognized function of OsHKT1;4 in Na(+) exclusion in stems together with leaf sheaths, thus excluding Na(+) from leaf blades of a japonica rice cultivar in the reproductive growth stage, but the contribution is low when the plants are in the vegetative growth stage.

Physiological Basis and Transcriptional Profiling of Three Salt-Tolerant Mutant Lines of Rice

Salinity is a complex trait that affects growth and productivity in many crops, including rice. Mutation induction, a useful tool to generate salt tolerant plants, enables the analysis of plants with similar genetic background, facilitating the understanding of the salt tolerance mechanisms. In this work, we generated three salt tolerant mutant lines by irradiation of a salt-sensitive cultivar plants and screened M2 plants at seedling stage in the presence of high salinity. These three lines, SaT20, SaS62, and SaT58, showed different responses to salinity, but exhibited similar phenotype to wild type plants, except SaT20 that displayed shorter height when grown in the absence of salt. Under salt conditions, all three mutants and the parental line showed similar reduction in yield, although relevant differences in other physiological parameters, such as Na⁺ accumulation in healthy leaves of SaT20, were registered. Microarray analyses of gene expression profiles in roots revealed the occurrence of common and specific responses in the mutants. The three mutants showed up-regulation of responsive genes, the activation of oxido-reduction process and the inhibition of ion transport. The participation of jasmonate in the plant response to salt was evident by down-regulation of a gene coding for a jasmonate O-methyltransferase. Genes dealing with lipid transport and metabolism were, in general, up-regulated except in SaS62, that also exhibited down-regulation of genes involved in ion transport and Ca²⁺ signal transduction. The two most tolerant varieties, SaS62 and SaT20, displayed lower levels of transcripts involved in K⁺ uptake. The physiological study and the description of the expression analysis evidenced that the three lines showed different responses to salt: SaT20 showed a high Na⁺ content in leaves, SaS62 presented an inhibition of lipid metabolism and ion transport and SaT58 differs in both features in the response to salinity. The analysis of these salt tolerant mutants illustrates the complexity of this trait evidencing the breadth of the plant responses to salinity including simultaneous cooperation of alternative or complementary mechanisms.