Arabidopsis Qc-SNARE gene AtSFT12 is involved in salt and osmotic stress responses and Na(+) accumulation in vacuoles

Differential gene expression of salt-tolerant alfalfa in response to salinity and inoculation by Ensifer meliloti

BACKGROUND

Alfalfa (Medicago sativa L.) experiences many negative effects under salinity stress, which may be mediated by recurrent selection. Salt-tolerant alfalfa may display unique adaptations in association with rhizobium under salt stress.

RESULTS

To elucidate inoculation effects on salt-tolerant alfalfa under salt stress, this study leveraged a salt-tolerant alfalfa population selected through two cycles of recurrent selection under high salt stress. After experiencing 120-day salt stress, mRNA was extracted from 8 random genotypes either grown in 0 or 8 dS/m salt stress with or without inoculation by Ensifer meliloti. Results showed 320 and 176 differentially expressed genes (DEGs) modulated in response to salinity stress or inoculation x salinity stress, respectively. Notable results in plants under 8 dS/m stress included upregulation of a key gene involved in the Target of Rapamycin (TOR) signaling pathway with a concomitant decrease in expression of the SNrK pathway. Inoculation of salt-stressed plants stimulated increased transcription of a sulfate-uptake gene as well as upregulation of the Lysine-27-trimethyltransferase (EZH2), Histone 3 (H3), and argonaute (AGO, a component of miRISC silencing complexes) genes related to epigenetic and post-transcriptional gene control.

CONCLUSIONS

Salt-tolerant alfalfa may benefit from improved activity of TOR and decreased activity of SNrK1 in salt stress, while inoculation by rhizobiumstimulates production of sulfate uptake- and other unique genes.

Vesicle formation-related protein CaSec16 and its ankyrin protein partner CaANK2B jointly enhance salt tolerance in pepper

Vesicle transport plays important roles in plant tolerance against abiotic stresses. However, the contribution of a vesicle formation related protein CaSec16 (COPII coat assembly protein Sec16-like) in pepper tolerance to salt stress remains unclear. In this study, we report that the expression of CaSec16 was upregulated by salt stress. Compared to the control, the salt tolerance of pepper with CaSec16-silenced was compromised, which was shown by the corresponding phenotypes and physiological indexes, such as the death of growing point, the aggravated leaf wilting, the higher increment of relative electric leakage (REL), the lower content of total chlorophyll, the higher accumulation of dead cells, H2O2, malonaldehyde (MDA), and proline (Pro), and the inhibited induction of marker genes for salt-tolerance and vesicle transport. In contrast, the salt tolerance of pepper was enhanced by the transient overexpression of CaSec16. In addition, heterogeneously induced CaSec16 protein did not enhance the salt tolerance of Escherichia coli, an organism lacking the vesicle transport system. By yeast two-hybrid method, an ankyrin protein, CaANK2B, was identified as the interacting protein of CaSec16. The expression of CaANK2B showed a downward trend during the process of salt stress. Compared with the control, pepper plants with transient-overexpression of CaANK2B displayed increased salt tolerance, whereas those with CaANK2B-silenced exhibited reduced salt tolerance. Taken together, both the vesicle formation related protein CaSec16 and its interaction partner CaANK2B can improve the pepper tolerance to salt stress.

A SNARE-like protein from Solanum lycopersicum increases salt tolerance by modulating vesicular trafficking in tomato

Intracellular vesicular trafficking ensures the exchange of lipids and proteins between endomembrane compartments. This is relevant under high salinity conditions, since both the removal of transporters and ion channels from the plasma membrane and the compartmentalization of toxic ions require the formation of vesicles, which can be maintained as multivesicular bodies or be fused to the central vacuole. SNARE proteins (Soluble N-ethylmaleimide-sensitive factor attachment receptor) participate in the vesicle fusion process and give specificity to their destination. Plant genome studies have revealed a superfamily of genes that encode for proteins called SNARE-like. These proteins appear to be participating in vesicular trafficking with similar functions to those of SNARE proteins. A SNARE-like, named SlSLSP6, in Solanum lycopersicum plants has been shown to be induced under high salinity conditions. A phylogenetic relationship of SlSLSP6 with SNARE-like proteins of salinity-tolerant plants, including Salicornia brachiata, Zostera marina and Solanum pennelli, was determined. Considering its amino acid sequence, a putative clathrin adapter complex domain and palmitoylation site was predicted. Subcellular localization analysis evidenced that SlSLSP6 is mostly localized in the plasma membrane. Using transgenic tomato plants, we identified that overexpression of SlSLSP6 increased tolerance to salt stress. This tolerance was evident when we quantified an improvement in physiological and biochemical parameters, such as higher chlorophyll content, performance index, efficiency of photosystem II and relative water content, and lower malondialdehyde content, compared to control plants. At the subcellular level, the overexpression of SlSLSP6 reduced the presence of H2O2 in roots and increased the compartmentalization of sodium in vacuoles during salt stress. These effects appear to be associated with the higher endocytic rate of FM4-64, determined in the plant root cells. Taken together, these results indicate that SlSLSP6 increases tolerance to salt stress by modulating vesicular trafficking through over-induction of the endocytic pathway. This work contributes to understanding the role of this type of SNARE-like protein during salt stress and could be a potential candidate in breeding programs for tolerance to salt stress in tomato plants.

The Bro1-like domain-containing protein, AtBro1, modulates growth and abiotic stress responses in Arabidopsis

Abscisic acid (ABA) affects plant physiology by altering gene expression, enabling plants to adapt to a wide range of environments. Plants have evolved protective mechanisms to allow seed germination in harsh conditions. Here, we explore a subset of these mechanisms involving the AtBro1 gene, which encodes one of a small family of poorly characterised Bro1-like domain-containing proteins, in Arabidopsis thaliana plants subjected to multiple abiotic stresses. AtBro1 transcripts were upregulated by salt, ABA and mannitol stress, while AtBro1-overexpression lines demonstrated robust tolerance to drought and salt stress. Furthermore, we found that ABA elicits stress-resistance responses in loss-of-function bro1-1 mutant plants and AtBro1 regulates drought resistance in Arabidopsis. When the AtBro1 promoter was fused to the β-glucuronidase (GUS) gene and introduced into plants, GUS was expressed mainly in rosette leaves and floral clusters, especially in anthers. Using a construct expressing an AtBro1-GFP fusion protein, AtBro1 was found to be localized in the plasma membrane in Arabidopsis protoplasts. A broad RNA-sequencing analysis revealed specific quantitative differences in the early transcriptional responses to ABA treatment between wild-type and loss-of-function bro1-1 mutant plants, suggesting that ABA stimulates stress-resistance responses via AtBro1. Additionally, transcripts levels of MOP9.5, MRD1, HEI10, and MIOX4 were altered in bro1-1 plants exposed to different stress conditions. Collectively, our results show that AtBro1 plays a significant role in the regulation of the plant transcriptional response to ABA and the induction of resistance responses to abiotic stress.

MAG2 and MAL Regulate Vesicle Trafficking and Auxin Homeostasis With Functional Redundancy

Auxin is a central phytohormone and controls almost all aspects of plant development and stress response. Auxin homeostasis is coordinately regulated by biosynthesis, catabolism, transport, conjugation, and deposition. Endoplasmic reticulum (ER)-localized MAIGO2 (MAG2) complex mediates tethering of arriving vesicles to the ER membrane, and it is crucial for ER export trafficking. Despite important regulatory roles of MAG2 in vesicle trafficking, the mag2 mutant had mild developmental abnormalities. MAG2 has one homolog protein, MAG2-Like (MAL), and the mal-1 mutant also had slight developmental phenotypes. In order to investigate MAG2 and MAL regulatory function in plant development, we generated the mag2-1 mal-1 double mutant. As expected, the double mutant exhibited serious developmental defects and more alteration in stress response compared with single mutants and wild type. Proteomic analysis revealed that signaling, metabolism, and stress response in mag2-1 mal-1 were affected, especially membrane trafficking and auxin biosynthesis, signaling, and transport. Biochemical and cell biological analysis indicated that the mag2-1 mal-1 double mutant had more serious defects in vesicle transport than the mag2-1 and mal-1 single mutants. The auxin distribution and abundance of auxin transporters were altered significantly in the mag2-1 and mal-1 single mutants and mag2-1 mal-1 double mutant. Our findings suggest that MAG2 and MAL regulate plant development and auxin homeostasis by controlling membrane trafficking, with functional redundancy.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.

The sweetpotato GIGANTEA gene promoter is co-regulated by phytohormones and abiotic stresses in Arabidopsis thaliana

GIGANTEA (GI) is known to play significant roles in various molecular pathways. Nevertheless, the underlying mechanism of the transcriptional regulation of GI remains obscure in sweetpotato. In the present study, a 1518-bp promoter sequence was obtained from the Ipomoea batatas GIGANTEA (IbGI) gene, and several potential cis-elements responsive to light, phytohormones and abiotic stresses were identified by in silico analysis. In order to functionally validate the IbGI promoter, the 5' deletion analysis of the promoter was performed by cloning the full-length promoter (D0) and its four deletion fragments, D1 (1235 bp), D2 (896 bp), D3 (549 bp) and D4 (286 bp), upstream of the β-glucuronidase (GUS) reporter gene. Then, these were stably transformed in Arabidopsis plants. All transgenic seedlings exhibited stable GUS activity in the condition of control, but with decreased activity in the condition of most treatments. Interestingly, merely D1 seedlings that contained an abscisic acid responsive cis-element (ABRE-element) had an extremely powerful GUS activity under the treatment of ABA, which implies that fragment spanning nucleotides of -1235 to -896 bp might be a crucial component for the responses of ABA. Eight different types of potential transcriptional regulators of IbGI were isolated by Y1H, including TGA2.2, SPLT1 and GADPH, suggesting the complex interaction mode of protein-DNA on the IbGI promoter. Taken together, these present results help to better understand the transcriptional regulation mechanism of the IbGI gene, and provides an insight into the IbGI promoter, which can be considered as an alternation for breeding transgenic plants.

The Exocytosis Associated SNAP25-Type Protein, SlSNAP33, Increases Salt Stress Tolerance by Modulating Endocytosis in Tomato

In plants, vesicular trafficking is crucial for the response and survival to environmental challenges. The active trafficking of vesicles is essential to maintain cell homeostasis during salt stress. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are regulatory proteins of vesicular trafficking. They mediate membrane fusion and guarantee cargo delivery to the correct cellular compartments. SNAREs from the Qbc subfamily are the best-characterized plasma membrane SNAREs, where they control exocytosis during cell division and defense response. The Solanum lycopersicum gene SlSNAP33.2 encodes a Qbc-SNARE protein and is induced under salt stress conditions. SlSNAP33.2 localizes on the plasma membrane of root cells of Arabidopsis thaliana. In order to study its role in endocytosis and salt stress response, we overexpressed the SlSNAP33.2 cDNA in a tomato cultivar. Constitutive overexpression promoted endocytosis along with the accumulation of sodium (Na⁺) in the vacuoles. It also protected the plant from cell damage by decreasing the accumulation of hydrogen peroxide (H₂O₂) in the cytoplasm of stressed root cells. Subsequently, the higher level of SlSNAP33.2 conferred tolerance to salt stress in tomato plants. The analysis of physiological and biochemical parameters such as relative water content, the efficiency of the photosystem II, performance index, chlorophyll, and MDA contents showed that tomato plants overexpressing SlSNAP33.2 displayed a better performance under salt stress than wild type plants. These results reveal a role for SlSNAP33.2 in the endocytosis pathway involved in plant response to salt stress. This research shows that SlSNAP33.2 can be an effective tool for the genetic improvement of crop plants.

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.

Vesicle Transport in Plants: A Revised Phylogeny of SNARE Proteins

Communication systems within and between plant cells involve the transfer of ions and molecules between compartments, and are essential for development and responses to biotic and abiotic stresses. This in turn requires the regulated movement and fusion of membrane systems with their associated cargo. Recent advances in genomics has provided new resources with which to investigate the evolutionary relationships between membrane proteins across plant species. Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are known to play important roles in vesicle trafficking across plant, animal and microbial species. Using recent public expression and transcriptomic data from 9 representative green plants, we investigated the evolution of the SNARE classes and linked protein changes to functional specialization (expression patterns). We identified an additional 3 putative SNARE genes in the model plant Arabidopsis. We found that all SNARE classes have expanded in number to a greater or lesser degree alongside the evolution of multicellularity, and that within-species expansions are also common. These gene expansions appear to be associated with the accumulation of amino acid changes and with sub-functionalization of SNARE family members to different tissues. These results provide an insight into SNARE protein evolution and functional specialization. The work provides a platform for hypothesis-building and future research into the precise functions of these proteins in plant development and responses to the environment.

Identification and transcriptional analysis of SNARE vesicle fusion regulators in tomato (Solanum lycopersicum) during plant development and a comparative analysis of the response to salt stress with wild relatives

Intracellular vesicular trafficking ensures the exchange of lipids and proteins between the membranous compartments. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) play a central role in membrane fusion and they are key factors for vesicular trafficking in plants, including crops economically important such as tomato (Solanum lycopersicum). Taking advantage of the complete genome sequence available of S. lycopersicum, we identified 63 genes that encode putative SNARE proteins. Then, phylogenetic analysis allowed the classification of SNAREs in five main groups and recognizing their possible functions. A structure analysis of the genes, their syntenic relationships and their location in the chromosomes were also carried out for their characterization. In addition, the expression profiles of SNARE genes in different tissues were investigated using microarray-based analysis. The results indicated that specific SNAREs had a higher induction in leaf, root, flower and mature green fruit. S. lycopersicum is characterized for being a crop sensitive to saline stress unlike its wild relatives, such as Solanum pennellii, Solanum pimpinellifolium, Solanum habrochaites or Solanum chilense, which are tolerant. In this context, we analyzed different microarrays and evaluated and validated the transcript levels through qRT-PCR experiments. The results showed that SlGOS12.2, SlVAMP727 and SlSYP51.2 could have a positive relationship with salt stress and probably an important role in their tolerance. All these data increase our knowledge and can also be utilized to identify potential molecular targets for conferring tolerance to various stresses in tomato.

Transcriptomic profiling and analysis of differentially expressed genes in asparagus bean (Vigna unguiculata ssp. sesquipedalis) under salt stress

Asparagus bean (Vigna unguiculata ssp. sesquipedalis) is a warm season legume which is widely distributed over subtropical regions and semiarid areas. It is mainly grown as a significant protein source in developing countries. Salinity, as one of the main abiotic stress factors, constrains the normal growth and yield of asparagus bean. This study used two cultivars (a salt-sensitive genotype and a salt-tolerant genotype) under salt stress vs. control to identify salt-stress-induced genes in asparagus bean using RNA sequencing. A total of 692,086,838 high-quality clean reads, assigned to 121,138 unigenes, were obtained from control and salt-treated libraries. Then, 216 root-derived DEGs (differentially expressed genes) and 127 leaf-derived DEGs were identified under salt stress between the two cultivars. Of these DEGs, thirteen were assigned to six transcription factors (TFs), including AP2/EREBP, CCHC(Zn), C2H2, WRKY, WD40-like and LIM. GO analysis indicated four DEGs might take effects on the "oxidation reduction", "transport" and "signal transduction" process. Moreover, expression of nine randomly-chosen DEGs was verified by quantitative real-time-PCR (qRT-PCR) analysis. Predicted function of the nine tested DEGs was mainly involved in the KEGG pathway of cation transport, response to osmotic stress, and phosphorelay signal transduction system. A salt-stress-related pathway of "SNARE interactions in vesicular transport" was concerned. As byproducts, 15, 321 microsatellite markers were found in all the unigenes, and 17 SNP linked to six salt-stress induced DEGs were revealed. These candidate genes provide novel insights for understanding the salt tolerance mechanism of asparagus bean in the future.

Genome-wide association analysis of salinity responsive traits in Medicago truncatula

Salinity stress is an important cause of crop yield loss in many parts of the world. Here, we performed genome-wide association studies of salinity-stress responsive traits in 132 HapMap genotypes of the model legume Medicago truncatula. Plants grown in soil were subjected to a step-wise increase in NaCl concentration, from 0 through 0.5% and 1.0% to 1.5%, and the following traits were measured: vigor, shoot biomass, shoot water content, leaf chlorophyll content, leaf size, and leaf and root concentrations of proline and major ions (Na+ , Cl- , K+ , Ca2+ , etc.). Genome-wide association studies were carried out using 2.5 million single nucleotide polymorphisms, and 12 genomic regions associated with at least four traits each were identified. Transcript-level analysis of the top eight candidate genes in five extreme genotypes revealed association between salinity tolerance and transcript-level changes for seven of the genes, encoding a vacuolar H+ -ATPase, two transcription factors, two proteins involved in vesicle trafficking, one peroxidase, and a protein of unknown function. Earlier functional studies on putative orthologues of two of the top eight genes (a vacuolar H+ -ATPase and a peroxidase) demonstrated their involvement in plant salinity tolerance.

Overexpression of the DEAD-Box RNA Helicase Gene AtRH17 Confers Tolerance to Salt Stress in Arabidopsis

Plants adapt to abiotic stresses by complex mechanisms involving various stress-responsive genes. Here, we identified a DEAD-box RNA helicase (RH) gene, AtRH17, in Arabidopsis, involved in salt-stress responses using activation tagging, a useful technique for isolating novel stress-responsive genes. AT895, an activation tagging line, was more tolerant than wild type (WT) under NaCl treatment during germination and seedling development, and AtRH17 was activated in AT895. AtRH17 possesses nine well-conserved motifs of DEAD-box RHs, consisting of motifs Q, I, Ia, Ib, and II-VI. Although at least 12 orthologs of AtRH17 have been found in various plant species, no paralog occurs in Arabidopsis. AtRH17 protein is subcellularily localized in the nucleus. AtRH17-overexpressing transgenic plants (OXs) were more tolerant to high concentrations of NaCl and LiCl compared with WT, but no differences from WT were detected among seedlings exposed to mannitol and freezing treatments. Moreover, in the mature plant stage, AtRH17 OXs were also more tolerant to NaCl than WT, but not to drought, suggesting that AtRH17 is involved specifically in the salt-stress response. Notably, transcriptions of well-known abscisic acid (ABA)-dependent and ABA-independent stress-response genes were similar or lower in AtRH17 OXs than WT under salt-stress treatments. Taken together, our findings suggest that AtRH17, a nuclear DEAD-box RH protein, is involved in salt-stress tolerance, and that its overexpression confers salt-stress tolerance via a pathway other than the well-known ABA-dependent and ABA-independent pathways.

Functional Identification of Salt-Stress-Related Genes Using the FOX Hunting System from Ipomoea pes-caprae

Ipomoea pes-caprae is a seashore halophytic plant and is therefore a good model for studying the molecular mechanisms underlying salt and stress tolerance in plant research. Here, we performed Full-length cDNA Over-eXpressor (FOX) gene hunting with a functional screening of a cDNA library using a salt-sensitive yeast mutant strain to isolate the salt-stress-related genes of I. pes-caprae (IpSR genes). The library was screened for genes that complemented the salt defect of yeast mutant AXT3 and could grow in the presence of 75 mM NaCl. We obtained 38 candidate salt-stress-related full-length cDNA clones from the I. pes-caprae cDNA library. The genes are predicted to encode proteins involved in water deficit, reactive oxygen species (ROS) scavenging, cellular vesicle trafficking, metabolic enzymes, and signal transduction factors. When combined with the quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analyses, several potential functional salt-tolerance-related genes were emphasized. This approach provides a rapid assay system for the large-scale screening of I. pes-caprae genes involved in the salt stress response and supports the identification of genes responsible for the molecular mechanisms of salt tolerance.

GsSNAP33, a novel Glycine soja SNAP25-type protein gene: Improvement of plant salt and drought tolerances in transgenic Arabidopsis thaliana

The N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) superfamily, specifically the SNAP25-type proteins and t-SNAREs, have been proposed to regulate cellular processes and plant resistance mechanisms. However, little is known about the role of SNAP25-type proteins in combating abiotic stresses, specifically in wild soybean. In the current study, the isolation and functional characterization of the putative synaptosomal-associated SNAP25-type protein gene GsSNAP33 from wild soybean (Glycine soja) were performed. GsSNAP33 has a molecular weight of 33,311 Da and comprises 300 amino acid residues along with Qb-Qc SNARE domains. Multiple sequence alignment revealed the highest similarity of the GsSNAP33 protein to GmSNAP33 (91%), VrSNAP33 (89%), PvSNAP33 (86%) and AtSNAP33 (63%). Phylogenetic studies revealed the abundance of SNAP33 proteins mostly in dicotyledons. Quantitative real-time PCR assays confirmed that GsSNAP33 expression can be induced by salt, alkali, ABA and PEG treatments and that GsSNAP33 is highly expressed in the pods, seeds and roots of Glycine soja. Furthermore, the overexpression of the GsSNAP33 gene in WT Arabidopsis thaliana resulted in increased germination rates, greater root lengths, improved photosynthesis, lower electrolyte leakage, higher biomass production and up-regulated expression levels of various stress-responsive marker genes, including KINI, COR15A, P5Cs, RAB18, RD29A and COR47 in transgenic lines compared with those in WT lines. Subcellular localization studies revealed that the GsSNAP33-eGFP fusion protein was localized to the plasma membrane, while eGFP was distributed throughout whole cytoplasm of onion epidermal cells. Collectively, our findings suggest that GsSNAP33, a novel plasma membrane protein gene of Glycine soja, might be involved in improving plant responses to salt and drought stresses in Arabidopsis.

Involvement of SchRabGDI1 from Solanum chilense in endocytic trafficking and tolerance to salt stress

Physiological responses of plants to salinity stress requires the coordinated activation of many genes. A salt-induced gene was isolated from roots of the wild tomato species Solanum chilense and named SchRabGDI1 because it encodes a protein with high identity to GDP dissociation inhibitors of plants. These proteins are regulators of the RabGTPase cycle that play key roles in intracellular vesicular trafficking. The expression pattern of SchRabGDI1 showed an early up-regulation in roots and leaves under salt stress. Functional activity of SchRabGDI1 was shown by restoring the defective phenotype of the yeast sec19-1 mutant and the capacity of SchRabGDI1 to interact with RabGTPase was demonstrated through BiFC assays. Expression of SchRabGDI1 in Arabidopsis thaliana plants resulted in increased salt tolerance. Also, the root cells of transgenic plants showed higher rate of endocytosis under normal growth conditions and higher accumulation of sodium in vacuoles and small vesicular structures under salt stress than wild type. Our results suggest that in salt tolerant species such as S. chilense, bulk endocytosis is one of the early mechanisms to avoid salt stress, which requires the concerted expression of regulatory genes involved in vesicular trafficking of the endocytic pathway.

Identification of CkSNAP33, a gene encoding synaptosomal-associated protein from Cynanchum komarovii, that enhances Arabidopsis resistance to Verticillium dahliae

SNARE proteins are essential to vesicle trafficking and membrane fusion in eukaryotic cells. In addition, the SNARE-mediated secretory pathway can deliver diverse defense products to infection sites during exocytosis-associated immune responses in plants. In this study, a novel gene (CkSNAP33) encoding a synaptosomal-associated protein was isolated from Cynanchum komarovii and characterized. CkSNAP33 contains Qb- and Qc-SNARE domains in the N- and C-terminal regions, respectively, and shares high sequence identity with AtSNAP33 from Arabidopsis. CkSNAP33 expression was induced by H₂O₂, salicylic acid (SA), Verticillium dahliae, and wounding. Arabidopsis lines overexpressing CkSNAP33 had longer primary roots and larger seedlings than the wild type (WT). Transgenic Arabidopsis lines showed significantly enhanced resistance to V. dahliae, and displayed reductions in disease index and fungal biomass, and also showed elevated expression of PR1 and PR5. The leaves of transgenic plants infected with V. dahliae showed strong callose deposition and cell death that hindered the penetration and spread of the fungus at the infection site. Taken together, these results suggest that CkSNAP33 is involved in the defense response against V. dahliae and enhanced disease resistance in Arabidopsis.

AhDGR2, an amaranth abiotic stress-induced DUF642 protein gene, modifies cell wall structure and composition and causes salt and ABA hyper-sensibility in transgenic Arabidopsis

An amaranth DGR gene, induced under abiotic stress, modifies cell wall structure and causes hypersensitivity to ABA and salt when overexpressed in Arabidopsis. DUF642 is a highly conserved plant-specific family of unknown cell wall-associated proteins. The AhDGR2 gene, coding for a DUF642 protein, was significantly induced in grain amaranth (Amaranthus hypochondriacus) plants subjected to water-deficit and salinity stress, thereby suggesting its participation in abiotic stress tolerance in this plant. A role in development was also inferred from the higher AhDGR2 expression rates detected in young tissues. Subsequent overexpression of AhDGR2 in transgenic Arabidopsis plants (OE-AhDGR2) supported its possible role in development processes. Thus, OE-AhDGR2 plants generated significantly longer roots when grown in normal MS medium. However, they showed a hypersensitivity to increasing concentrations of abscisic acid or NaCl in the medium, as manifested by shorter root length, smaller and slightly chlorotic rosettes, as well as highly reduced germination rates. Contrary to expectations, OE-AhDGR2 plants were intolerant to abiotic stress. Moreover, cell walls in transgenic plants were thinner, in leaves, and more disorganized, in roots, and had significantly modified pectin levels. Lower pectin methylesterase activity detected in leaves of OE-AhDGR2 plants, but not in roots, was contrary to previous reports associating DUF642 proteins and decreased pectin esterification levels in cell walls. Nonetheless, microarray data identified candidate genes whose expression levels explained the phenotypes observed in leaves of OE-AhDGR2 plants, including several involved in cell wall integrity and extension, growth and development, and resistance to abiotic stress. These results support the role of DUF642 proteins in cell wall-related processes and offer novel insights into their possible role(s) in plants.

Arabidopsis AtNAP functions as a negative regulator via repression of AREB1 in salt stress response

AtNAP , an Arabidopsis NAC transcription factor family gene, functions as a negative regulator via transcriptional repression of AREB1 in salt stress response. AtNAP is an NAC family transcription factor in Arabidopsis and is known to be a positive regulator of senescence. However, its exact function and underlying molecular mechanism in stress responses are not well known. Here, we investigated functional roles of AtNAP in salt stress response. AtNAP expression significantly increased at the seedling stage, with higher expression in both shoots and roots under NaCl, mannitol, and ABA treatments. T-DNA insertional loss-of-function mutants of AtNAP were more tolerant to salt stress than wild type (WT), whereas AtNAP-overexpressing transgenic plants (OXs) were more sensitive to salt stress than WT during germination, seedling development, and mature plant stage. Transcript levels of stress-responsive genes in the ABA-dependent pathway, such as AREB1, RD20, and RD29B, were significantly higher and lower in atnap mutants and AtNAP OXs, respectively, than in WT under salt stress conditions, suggesting that AtNAP might negatively regulate the expression of those genes under salt stress conditions. Indeed, AtNAP repressed the promoter activity of AREB1 under normal and salt stress conditions. These results indicate that AtNAP functions as a negative regulator in the salt stress response. Our results, together with previous studies, suggest that AtNAP functions as a negative regulator in osmotic stress responses, whereas it functions as a positive regulator in senescence.

The Promoter of AtUSP Is Co-regulated by Phytohormones and Abiotic Stresses in Arabidopsis thaliana

Universal stress proteins (USPs) are known to be expressed in response to various abiotic stresses in a wide variety of organisms, such as bacteria, archaebacteria, protists, algae, fungi, plants, and animals. However, in plants, biological function of most of the USPs still remains obscure. In the present study, Arabidopsis USP gene (AtUSP) showed induction in response to abscisic acid (ABA) and various abiotic stresses viz. heat, dehydration, salt, osmotic, and cold stresses. Additionally, in silico analysis of AtUSP promoter identified several cis-elements responsive to phytohormones and abiotic stresses such as ABRE, ERE, DRE, and HSE, etc. To functionally validate the AtUSP promoter, the 1115 bp region of promoter was characterized under phytohormone and abiotic stress treatments. Deletion analysis of promoter was carried out by cloning the full length promoter (D0) and its three 5' deletion derivatives, D1 (964 bp), D2 (660 bp), and D3 (503 bp) upstream of the β-glucuronidase (GUS) reporter gene, which were then stably transformed in Arabidopsis plants. The AtUSP promoter (D0) showed minimal activity under non-stress conditions which was enhanced in response to phytohormone treatments (ABA and ACC) and abiotic stresses such as dehydration, heat, cold, salt, and osmotic stresses. The seedlings harboring D1 and D2 deletion fragments showed constitutive GUS expression even under control condition with increased activity almost under all the treatments. However, D3 seedlings exhibited complete loss of activity under control condition with induction under ACC treatment, dehydration, heat, oxidative, salt, and osmotic stresses. Thus, present study clearly showed that AtUSP promoter is highly inducible by phytohormones and multiple abiotic stresses and it can be exploited as stress inducible promoter to generate multi-stress tolerant crops with minimal effects on their other important traits.

Expression partitioning of homeologs and tandem duplications contribute to salt tolerance in wheat (Triticum aestivum L.)

Salt stress dramatically reduces crop yield and quality, but the molecular mechanisms underlying salt tolerance remain largely unknown. To explore the wheat transcriptional response to salt stress, we performed high-throughput transcriptome sequencing of 10-day old wheat roots under normal condition and 6, 12, 24 and 48 h after salt stress (HASS) in both a salt-tolerant cultivar and salt-sensitive cultivar. The results demonstrated global gene expression reprogramming with 36,804 genes that were up- or down-regulated in wheat roots under at least one stress condition compared with the controls and revealed the specificity and complexity of the functional pathways between the two cultivars. Further analysis showed that substantial expression partitioning of homeologous wheat genes occurs when the plants are subjected to salt stress, accounting for approximately 63.9% (2,537) and 66.1% (2,624) of the homeologous genes in ‘Chinese Spring’ (CS) and ‘Qing Mai 6’ (QM). Interestingly, 143 salt-responsive genes have been duplicated and tandemly arrayed on chromosomes during wheat evolution and polyploidization events, and the expression patterns of 122 (122/143, 85.3%) tandem duplications diverged dynamically over the time-course of salinity exposure. In addition, constitutive expression or silencing of target genes in Arabidopsis and wheat further confirmed our high-confidence salt stress-responsive candidates.

Functional Identification and Characterization of Genes Cloned from Halophyte Seashore Paspalum Conferring Salinity and Cadmium Tolerance

Salinity-affected and heavy metal-contaminated soils limit the growth of glycophytic plants. Identifying genes responsible for superior tolerance to salinity and heavy metals in halophytes has great potential for use in developing salinity- and Cd-tolerant glycophytes. The objective of this study was to identify salinity- and Cd-tolerance related genes in seashore paspalum (Paspalum vaginatum), a halophytic perennial grass species, using yeast cDNA expression library screening method. Based on the Gateway-compatible vector system, a high-quality entry library was constructed, which contained 9.9 × 10(6) clones with an average inserted fragment length of 1.48 kb representing a 100% full-length rate. The yeast expression libraries were screened in a salinity-sensitive and a Cd-sensitive yeast mutant. The screening yielded 32 salinity-tolerant clones harboring 18 salinity-tolerance genes and 20 Cd-tolerant clones, including five Cd-tolerance genes. qPCR analysis confirmed that most of the 18 salinity-tolerance and five Cd-tolerance genes were up-regulated at the transcript level in response to salinity or Cd stress in seashore paspalum. Functional analysis indicated that salinity-tolerance genes from seashore paspalum could be involved mainly in photosynthetic metabolism, antioxidant systems, protein modification, iron transport, vesicle traffic, and phospholipid biosynthesis. Cd-tolerance genes could be associated with regulating pathways that are involved in phytochelatin synthesis, HSFA4-related stress protection, CYP450 complex, and sugar metabolism. The 18 salinity-tolerance genes and five Cd-tolerance genes could be potentially used as candidate genes for genetic modification of glycophytic grass species to improve salinity and Cd tolerance and for further analysis of molecular mechanisms regulating salinity and Cd tolerance.

A SNARE-Like Superfamily Protein SbSLSP from the Halophyte Salicornia brachiata Confers Salt and Drought Tolerance by Maintaining Membrane Stability, K(+)/Na(+) Ratio, and Antioxidant Machinery

About 1000 salt-responsive ESTs were identified from an extreme halophyte Salicornia brachiata. Among these, a novel salt-inducible gene SbSLSP (Salicornia brachiata SNARE-like superfamily protein), showed up-regulation upon salinity and dehydration stress. The presence of cis-regulatory motifs related to abiotic stress in the putative promoter region supports our finding that SbSLSP gene is inducible by abiotic stress. The SbSLSP protein showed a high sequence identity to hypothetical/uncharacterized proteins from Beta vulgaris, Spinacia oleracea, Eucalyptus grandis, and Prunus persica and with SNARE-like superfamily proteins from Zostera marina and Arabidopsis thaliana. Bioinformatics analysis predicted a clathrin adaptor complex small-chain domain and N-myristoylation site in the SbSLSP protein. Subcellular localization studies indicated that the SbSLSP protein is mainly localized in the plasma membrane. Using transgenic tobacco lines, we establish that overexpression of SbSLSP resulted in elevated tolerance to salt and drought stress. The improved tolerance was confirmed by alterations in a range of physiological parameters, including high germination and survival rate, higher leaf chlorophyll contents, and reduced accumulation of Na(+) ion and reactive oxygen species (ROS). Furthermore, overexpressing lines also showed lower water loss, higher cell membrane stability, and increased accumulation of proline and ROS-scavenging enzymes. Overexpression of SbSLSP also enhanced the transcript levels of ROS-scavenging and signaling enzyme genes. This study is the first investigation of the function of the SbSLSP gene as a novel determinant of salinity/drought tolerance. The results suggest that SbSLSP could be a potential candidate to increase salinity and drought tolerance in crop plants for sustainable agriculture in semi-arid saline soil.

Vesicular trafficking and salinity responses in plants

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