Expression of tomato SlTIP2;2 enhances the tolerance to salt stress in the transgenic Arabidopsis and interacts with target proteins

The differential expressions of aquaporins underline the diverse strategies of cucumber and tomato against salinity and zinc stress

Salinity and excess zinc are two main problems that have limited agriculture in recent years. Aquaporins are crucial in regulating the passage of water and solutes through cells and may be essential for mitigating abiotic stresses. In the present study, the adaptive response to moderate salinity (60 mM NaCl) and excess Zn (1 mM ZnSO4 ) were compared alone and in combination in Cucumis sativus L. and Solanum lycopersicum L. Water relations, gas exchange and the differential expression of all aquaporins were analysed. The results showed that cucumber plants under salinity maintained the internal movement of water through osmotic adjustment and the overexpression of specific PIPs aquaporins, following a "conservation strategy". As tomato has a high tolerance to salinity, the physiological parameters and the expression of most aquaporins remained unchanged. ZnSO4 was shown to be stressful for both plant species. While cucumber upregulated 7 aquaporin isoforms, the expression of aquaporins increased in a generalized manner in tomato. Despite the differences, water relations and transpiration were adjusted in both plants, allowing the RWC in the shoot to be maintained. The aquaporin regulation in cucumber plants facing NaCl+ZnSO4 stress was similar in the two treatments containing NaCl, evidencing the predominance of salt in stress. However, in tomato, the induced expression of specific isoforms to deal with the combined stress differed from independent stresses. The results clarify the key role of aquaporin regulation in facing abiotic stresses and their possible use as markers of tolerance to salinity and heavy metals in plants.

Smart reprograming of plants against salinity stress using modern biotechnological tools

Climate change gives rise to numerous environmental stresses, including soil salinity. Salinity/salt stress is the second biggest abiotic factor affecting agricultural productivity worldwide by damaging numerous physiological, biochemical, and molecular processes. In particular, salinity affects plant growth, development, and productivity. Salinity responses include modulation of ion homeostasis, antioxidant defense system induction, and biosynthesis of numerous phytohormones and osmoprotectants to protect plants from osmotic stress by decreasing ion toxicity and augmented reactive oxygen species scavenging. As most crop plants are sensitive to salinity, improving salt tolerance is crucial in sustaining global agricultural productivity. In response to salinity, plants trigger stress-related genes, proteins, and the accumulation of metabolites to cope with the adverse consequence of salinity. Therefore, this review presents an overview of salinity stress in crop plants. We highlight advances in modern biotechnological tools, such as omics (genomics, transcriptomics, proteomics, and metabolomics) approaches and different genome editing tools (ZFN, TALEN, and CRISPR/Cas system) for improving salinity tolerance in plants and accomplish the goal of "zero hunger," a worldwide sustainable development goal proposed by the FAO.

Interplay between membrane proteins and membrane protein-lipid pertaining to plant salinity stress

High salinity in agricultural lands is one of the predominant issues limiting agricultural yields. Plants have developed several mechanisms to withstand salinity stress, but the mechanisms are not effective enough for most crops to prevent and persist the salinity stress. Plant salt tolerance pathways involve membrane proteins that have a crucial role in sensing and mitigating salinity stress. Due to a strategic location interfacing two distinct cellular environments, membrane proteins can be considered checkpoints to the salt tolerance pathways in plants. Related membrane proteins functions include ion homeostasis, osmosensing or ion sensing, signal transduction, redox homeostasis, and small molecule transport. Therefore, modulating plant membrane proteins' function, expression, and distribution can improve plant salt tolerance. This review discusses the membrane protein-protein and protein-lipid interactions related to plant salinity stress. It will also highlight the finding of membrane protein-lipid interactions from the context of recent structural evidence. Finally, the importance of membrane protein-protein and protein-lipid interaction is discussed, and a future perspective on studying the membrane protein-protein and protein-lipid interactions to develop strategies for improving salinity tolerance is proposed.

Physiological and transcriptome analyses demonstrate the silver nanoparticles mediated alleviation of salt stress in pearl millet (Pennisetum glaucum L)

Pearl millet (Pennisetum glaucum L.) is a highly nutritive-value summer-annual forage crop used for hay, silage, grazing, and green chop. However, abiotic stresses including salinity negatively affect its growth and productivity. Furthermore, the nanotechnology is attaining greater consideration to reduce the impact of environmental stresses in plants. In the present study, transcriptome responses of silver nanoparticles (AgNPs) in pearl millet under salinity were investigated. The treatments were given as Control, NaCl (250 mM), AgNPs (20 mg/L), and NaCl + AgNPs to pearl millet seedlings after thirteen days of seed sowing. After 1 h of given treatments, leaf samples were collected and subjected to physio-chemical examination and transcriptome analyses. Salt stress increased the hydrogen peroxide (H₂O₂), malondialdehyde (MDA) content, and proline as compared to other treatments. In addition, the combined applications of NaCl + AgNPs ameliorated the oxidative damage by increasing antioxidant enzymes activities including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Furthermore, RNA sequencing data showed 6016 commonly annotated Differentially Expressed Transcripts (DETs) among various treated combinations. Among them, 427 transcripts were upregulated, and 136 transcripts were downregulated at nanoparticles vs control, 1469 upregulated and 1182 downregulated at salt vs control, 494 upregulated and 231 downregulated at salt + nanoparticles vs control, 783 upregulated and 523 downregulated at nanoparticles vs salt. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that Mitogen-activated protein kinase (MAPK) signaling pathway, biosynthesis of secondary metabolites, and plant hormonal signal transduction pathway were the enriched among all identified pathways. In addition, Reverse transcription quantitative real-time polymerase chain reaction (qRT-PCR) showed that salinity up regulated the relative expression of DETs in pearl millet while, AgNPs optimized their expression that are associated with various molecular and metabolic functions. Overall, AgNPs treatments effectively improved the morphology, physiology, biochemistry, and gene expression pattern under salinity which could be attributed to positive impacts of AgNPs on pearl millet.

Arbuscular mycorrhizal fungi inoculation impacts expression of aquaporins and salt overly sensitive genes and enhances tolerance of salt stress in tomato

Arbuscular mycorrhizal fungi (AMF) that establish reciprocal symbiosis with plant roots can enhance resistance to various stresses, including salt stress, but relevant mechanisms, especially at the molecular level, are scarce. The objective of this study was to analyze the effect of an arbuscular mycorrhizal fungus Paraglomus occultum on plant growth, leaf gas exchange, and expression of plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs) and salt overly sensitive (SOS) genes in tomato under salt (150 mmol/L NaCl) and non-salt stress. Salt stress for 4 weeks inhibited root colonization rate of P. occultum and soil hyphal length by 0.21- and 0.57-fold, respectively. Salt stress also inhibited plant growth performance and leaf gas exchange, while inoculation with P. occultum significantly enhanced them under salt and non-salt stress conditions. AMF showed diverse regulation of root SlPIPs and SlTIPs expression, among which under salt stress, SlPIP1;2, SlPIP1;5, SlPIP2;1, SlPIP2;6, SlPIP2;9, SlPIP2;10, SlTIP2;2, SlTIP3;2, and SlTIP5;1 were up-regulated by AMF colonization, and SlPIP1;7, SlPIP2;5, SlPIP2;8, SlPIP2;11, SlPIP2;12, SlTIP2;3, and SlTIP3;1 were down-regulated, accompanied by no change in SlPIP1;1, SlPIP1;3, SlPIP2;4, SlTIP1;1, SlTIP1;2, SlTIP1;3, SlTIP2;1, and SlTIP2;5. Interestingly, salt stress inhibited the expression of SlSOS1 and SlSOS2 in non-mycorrhizal plants, while it increased the expression of SlSOS1 and SlSOS2 in mycorrhizal plants. AMF colonization down-regulated expression of SlSOS1 and SlSOS2 under non-salt stress while up-regulated expression of SlSOS1 and SlSOS2 under salt stress. It was concluded that AMF inoculation impacted the expression of stress-responsive genes, especially SOS1 and SOS2, and enhanced salt resistance of tomato.

Graphical Abstract

Promoter of Vegetable Soybean GmTIP1;6 Responds to Diverse Abiotic Stresses and Hormone Signals in Transgenic Arabidopsis

Tonoplast intrinsic proteins (TIPs), a sub-family of aquaporins (AQPs), are known to play important roles in plant abiotic stress responses. However, evidence for the promoters of TIPs involvement in abiotic stress processes remains scarce. In this study, the promoter of the vegetable soybean GmTIP1;6 gene, which had the highest similarity to TIP1-type AQPs from other plants, was cloned. Expression pattern analyses indicated that the GmTIP1;6 gene was dramatically induced by drought, salt, abscisic acid (ABA), and methyl jasmonate (MeJA) stimuli. Promoter analyses revealed that the GmTIP1;6 promoter contained drought, ABA, and MeJA cis-acting elements. Histochemical staining of the GmTIP1;6 promoter in transgenic Arabidopsis corroborated that it was strongly expressed in the vascular bundles of leaves, stems, and roots. Beta-glucuronidase (GUS) activity assays showed that the activities of the GmTIP1;6 promoter were enhanced by different concentrations of polyethylene glycol 6000 (PEG 6000), NaCl, ABA, and MEJA treatments. Integrating these results revealed that the GmTIP1;6 promoter could be applied for improving the tolerance to abiotic stresses of the transgenic plants by promoting the expression of vegetable soybean AQPs.

microRNA172 targets APETALA2 to regulate flavonoid biosynthesis in apple (Malus domestica)

MicroRNA172 (miR172) plays a role in regulating a diverse range of plant developmental processes, including flowering, fruit development and nodulation. However, its role in regulating flavonoid biosynthesis is unclear. In this study, we show that transgenic apple plants over-expressing miR172 show a reduction in red coloration and anthocyanin accumulation in various tissue types. This reduction was consistent with decreased expression of APETALA2 homolog MdAP2_1a (a miR172 target gene), MdMYB10, and targets of MdMYB10, as demonstrated by both RNA-seq and qRT-PCR analyses. The positive role of MdAP2_1a in regulating anthocyanin biosynthesis was supported by the enhanced petal anthocyanin accumulation in transgenic tobacco plants overexpressing MdAP2_1a, and by the reduction in anthocyanin accumulation in apple and cherry fruits transfected with an MdAP2_1a virus-induced-gene-silencing construct. We demonstrated that MdAP2_1a could bind directly to the promoter and protein sequences of MdMYB10 in yeast and tobacco, and enhance MdMYB10 promotor activity. In Arabidopsis, over-expression of miR172 reduced flavonoid (including anthocyanins and flavonols) concentration and RNA transcript abundance of flavonoid genes in plantlets cultured on medium containing 7% sucrose. The anthocyanin content and RNA abundance of anthocyanin genes could be partially restored by using a synonymous mutant of MdAP2_1a, which had lost the miR172 target sequences at mRNA level, but not restored by using a WT MdAP2_1a. These results indicate that miR172 inhibits flavonoid biosynthesis through suppressing the expression of an AP2 transcription factor that positively regulates MdMYB10.

A bimolecular fluorescence complementation flow cytometry screen for membrane protein interactions

Interactions between membrane proteins within a cellular environment are crucial for all living cells. Robust methods to screen and analyse membrane protein complexes are essential to shed light on the molecular mechanism of membrane protein interactions. Most methods for detecting protein:protein interactions (PPIs) have been developed to target the interactions of soluble proteins. Bimolecular fluorescence complementation (BiFC) assays allow the formation of complexes involving PPI partners to be visualized in vivo, irrespective of whether or not these interactions are between soluble or membrane proteins. In this study, we report the development of a screening approach which utilizes BiFC and applies flow cytometry to characterize membrane protein interaction partners in the host Saccharomyces cerevisiae. These data allow constructive complexes to be discriminated with statistical confidence from random interactions and potentially allows an efficient screen for PPIs in vivo within a high-throughput setup.

Salt tolerance of Calotropis procera begins with immediate regulation of aquaporin activity in the root system

The ability to respond quickly to salt stress can determine the tolerance level of a species. Here, we test how rapidly the roots of Calotropis procera react to high salinity conditions. In the first 24 h after saline exposure, the plants reduced stomatal conductance, increased CO₂ assimilation, and water use efficiency. Thus, the root tissue showed an immediate increase in soluble sugars, free amino acid, and soluble protein contents. Twelve aquaporins showed differential gene expression in the roots of C. procera under salinity. Transcriptional upregulation was observed only after 2 h, with greater induction of CpTIP1.4 (fourfold). Transcriptional downregulation, in turn, occurred mainly after 8 h, with the largest associated with CpPIP1.2 (fourfold). C. procera plants responded quickly to high saline levels. Our results showed a strong stomatal control associated with high free amino acid and soluble sugar contents, regulated aquaporin expression in roots, and supported the high performance of the root system of C. procera under salinity. Moreover, this species was able to maintain a lower Na⁺/K⁺ ratio in the leaves compared to that of the roots of stressed plants. The first response of the root system, after immediate contact with saline solution, present an interesting scenario to discuss.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00957-9.

The Major Intrinsic Protein Family and Their Function Under Salt-Stress in Peanut

Peanut (Arachis hypogaea) is an important oil crop cultivated across the world. Abiotic stresses are the major constraint factors that defect its yield, especially in the rainfed peanut cultivation areas. Aquaporins are proteins that form a large family of more than 30 members in higher plants and play key roles in plant water balance under abiotic stress conditions. To comprehensively understand the functions of aquaporins in peanut, we identified their family genome-wide and characterized the phylogenetics, gene structure, and the conserved motif of the selective filter. In total, 64 aquaporin isoforms were identified, the NIPs were firstly categorized into NIP1s and NIP2s groups based on the phylogenetic analysis and the selective filter structure classification system. Further, we analyzed the gene expression pattern under the salt-stress conditions and found that a TIP3 member is strongly induced by salt stress, which in turn contributed to improved seed germination under salt stress when expressed in Arabidopsis. Our study thus provides comprehensive profiles on the MIP superfamily and their expression and function under salt-stress conditions. We believe that our findings will facilitate the better understanding of the roles of aquaporins in peanuts under salt salt-stress conditions.

Understanding salt tolerance mechanism using transcriptome profiling and de novo assembly of wild tomato Solanum chilense

Soil salinity affects the plant growth and productivity detrimentally, but Solanum chilense, a wild relative of cultivated tomato (Solanum lycopersicum L.), is known to have exceptional salt tolerance. It has precise adaptations against direct exposure to salt stress conditions. Hence, a better understanding of the mechanism to salinity stress tolerance by S. chilense can be accomplished by comprehensive gene expression studies. In this study 1-month-old seedlings of S. chilense and S. lycopersicum were subjected to salinity stress through application of sodium chloride (NaCl) solution. Through RNA-sequencing here we have studied the differences in the gene expression patterns. A total of 386 million clean reads were obtained through RNAseq analysis using the Illumina HiSeq 2000 platform. Clean reads were further assembled de novo into a transcriptome dataset comprising of 514,747 unigenes with N50 length of 578 bp and were further aligned to the public databases. Genebank non-redundant (Nr), Viridiplantae, Gene Ontology (GO), KOG, and KEGG databases classification suggested enrichment of these unigenes in 30 GO categories, 26 KOG, and 127 pathways, respectively. Out of 265,158 genes that were differentially expressed in response to salt treatment, 134,566 and 130,592 genes were significantly up and down-regulated, respectively. Upon placing all the differentially expressed genes (DEG) in known signaling pathways, it was evident that most of the DEGs involved in cytokinin, ethylene, auxin, abscisic acid, gibberellin, and Ca²⁺ mediated signaling pathways were up-regulated. Furthermore, GO enrichment analysis was performed using REVIGO and up-regulation of multiple genes involved in various biological processes in chilense under salinity were identified. Through pathway analysis of DEGs, “Wnt signaling pathway” was identified as a novel pathway for the response to the salinity stress. Moreover, key genes for salinity tolerance, such as genes encoding proline and arginine metabolism, ROS scavenging system, transporters, osmotic regulation, defense and stress response, homeostasis and transcription factors were not only salt-induced but also showed higher expression in S. chilense as compared to S. lycopersicum. Thus indicating that these genes may have an important role in salinity tolerance in S. chilense. Overall, the results of this study improve our understanding on possible molecular mechanisms underlying salt tolerance in plants in general and tomato in particular.

EST-SNP Study of Olea europaea L. Uncovers Functional Polymorphisms between Cultivated and Wild Olives

BACKGROUND

The species Olea europaea includes cultivated varieties (subsp. europaea var. europaea), wild plants (subsp. europaea var. sylvestris), and five other subspecies spread over almost all continents. Single nucleotide polymorphisms in the expressed sequence tag able to underline intra-species differentiation are not yet identified, beyond a few plastidial markers.

METHODS

In the present work, more than 1000 transcript-specific SNP markers obtained by the genotyping of 260 individuals were studied. These genotypes included cultivated, oleasters, and samples of subspecies guanchica, and were analyzed in silico, in order to identify polymorphisms on key genes distinguishing different Olea europaea forms.

RESULTS

Phylogeny inference and principal coordinate analysis allowed to detect two distinct clusters, clearly separating wilds and guanchica samples from cultivated olives, meanwhile the structure analysis made possible to differentiate these three groups. Sequences carrying the polymorphisms that distinguished wild and cultivated olives were analyzed and annotated, allowing to identify 124 candidate genes that have a functional role in flower development, stress response, or involvement in important metabolic pathways. Signatures of selection that occurred during olive domestication, were detected and reported.

CONCLUSION

This deep EST-SNP analysis provided important information on the genetic and genomic diversity of the olive complex, opening new opportunities to detect gene polymorphisms with potential functional and evolutionary roles, and to apply them in genomics-assisted breeding, highlighting the importance of olive germplasm conservation.

Overexpression of wheat α-mannosidase gene TaMP impairs salt tolerance in transgenic Brachypodium distachyon

The TaMP gene from wheat encodes an α-mannosidase induced by salt stress that functions as negative regulator of salt tolerance in plants. Salt stress significantly affects growth and yield of crop plants. The α-mannosidases function in protein folding, trafficking, and endoplasmic reticulum-associated degradation in eukaryotic cells, and they are involved in abiotic stress tolerance in plants. Previously, we identified the α-mannosidase gene TaMP in wheat (Triticum aestivum). In this study, we investigated the function of TaMP in salt stress tolerance. TaMP expression was induced in wheat leaves by salt, drought, abscisic acid, and H₂O₂ treatments. Overexpressing TaMP in Brachypodium distachyon was associated with a salt-sensitive phenotype. Under salt stress, the overexpressing plants had reduced height, delayed growth status, low photosynthetic rate, decreased survival rate, and diminished yield. Moreover, the overexpression of TaMP aggravated the tendency for ions to become toxic under salt stress by significantly affecting the Na⁺ and K⁺ contents in cells. In addition, TaMP could negatively regulate salt tolerance by affecting the antioxidant enzyme system capacity and increasing the reactive oxygen species accumulation. Our study was helpful to understand the underlying physiological and molecular mechanisms of salt stress tolerance in plants.

The involvement of wheat U-box E3 ubiquitin ligase TaPUB1 in salt stress tolerance

U-box E3 ubiquitin ligases play important roles in the ubiquitin/26S proteasome machinery and in abiotic stress responses. TaPUB1-overexpressing wheat (Triticum aestivum L.) were generated to evaluate its function in salt tolerance. These plants had more salt stress tolerance during seedling and flowering stages, whereas the TaPUB1-RNA interference (RNAi)-mediated knock-down transgenic wheat showed more salt stress sensitivity than the wild type (WT). TaPUB1 overexpression upregulated the expression of genes related to ion channels and increased the net root Na⁺ efflux, but decreased the net K⁺ efflux and H⁺ influx, thereby maintaining a low cytosolic Na⁺ /K⁺ ratio, compared with the WT. However, RNAi-mediated knock-down plants showed the opposite response to salt stress. TaPUB1 could induce the expression of some genes that improved the antioxidant capacity of plants under salt stress. TaPUB1 also interacted with TaMP (Triticum aestivum α-mannosidase protein), a regulator playing an important role in salt response in yeast and in plants. Thus, low cytosolic Na⁺ /K⁺ ratios and better antioxidant enzyme activities could be maintained in wheat with overexpression of TaPUB1 under salt stress. Therefore, we conclude that the U-box E3 ubiquitin ligase TaPUB1 positively regulates salt stress tolerance in wheat.

Photosynthetic Regulation Under Salt Stress and Salt-Tolerance Mechanism of Sweet Sorghum

Sweet sorghum is a C4 crop with the characteristic of fast-growth and high-yields. It is a good source for food, feed, fiber, and fuel. On saline land, sweet sorghum can not only survive, but increase its sugar content. Therefore, it is regarded as a potential source for identifying salt-related genes. Here, we review the physiological and biochemical responses of sweet sorghum to salt stress, such as photosynthesis, sucrose synthesis, hormonal regulation, and ion homeostasis, as well as their potential salt-resistance mechanisms. The major advantages of salt-tolerant sweet sorghum include: 1) improving the Na+ exclusion ability to maintain ion homeostasis in roots under salt-stress conditions, which ensures a relatively low Na+ concentration in shoots; 2) maintaining a high sugar content in shoots under salt-stress conditions, by protecting the structures of photosystems, enhancing photosynthetic performance and sucrose synthetase activity, as well as inhibiting sucrose degradation. To study the regulatory mechanism of such genes will provide opportunities for increasing the salt tolerance of sweet sorghum by breeding and genetic engineering.

An overview on improvement of crop productivity in saline soils by halotolerant and halophilic PGPRs

Salinity of water and soil are of the most important factors limiting the production of crops. Moreover, with the increasing population of the planet and saline fields worldwide there is no choice but to use saline soil and water in the near future. Therefore, to increase plant growth under saline stress condition, provision of sustainable and environmentally friendly management for the use of saline water and soil resources is necessary. The development of saline resistant plants is a potent approach to solve this problem. Generally, soil salinity negatively affects the plant growth through ion toxicity, oxidative stress, osmotic stress and ethylene generation. In recent years, scientists through genetic engineering techniques, which are based on molecular and physiological characteristics of plants, have made salt tolerance plants. However, the validation of the present technique is restricted to laboratory condition and it is not easily applied in the agronomy research under field environment. Another option would be to isolate and utilize salinity resistant microorganisms from the rhizosphere of halophyte plants, namely plant growth-promoting rhizobacteria (PGPR). The mechanisms of these bacteria includes; ACC-deaminase and exopolysachared production, osmolite accumulation, antioxidant system activation, ion hemostasis and etc. In this review, we will discuss mechanisms of PGPR in producing tolerate plants under salt stress and how to improve the plant-microbe interactions in future for increasing agricultural productivity to feed all of the world's people.

SpPKE1, a Multiple Stress-Responsive Gene Confers Salt Tolerance in Tomato and Tobacco

Understanding the mechanism of abiotic-tolerance and producing germplasm of abiotic tolerance are important in plant research. Wild species often show more tolerance of environmental stress factors than their cultivated counterparts. Genes from wild species show potential abilities to improve abiotic resistance in cultivated species. Here, a tomato proline-, lysine-, and glutamic-rich type gene SpPKE1 was isolated from abiotic-resistant species (Solanum pennellii LA0716) for over-expression in tomato and tobacco for salt tolerance. The protein encoded by SpPKE1 was predominantly localized in the cytoplasm in tobacco. SpPKE1 and SlPKE1 (from cultivated species S. lycopersicum cv. M82) shared 89.7% similarity in amino acid sequences and their transcripts abundance in flowers and fruits was reduced by the imposition of drought or oxidative stress and the exogenous supply of abscisic acid. The DNA of the PKE1 promoter was highly methylated in fruit and leaf, and the methylation of the coding sequence in leaf was significantly higher than that in fruit at different development stages. The over-expression of SpPKE1 under the control of a CaMV (Cauliflower Mosaic Virus) 35S promoter in transgenic tomato and tobacco plants enhanced their tolerance to salt stress. PKE1 was downregulated by abiotic stresses but enhanced the plant's salt stress tolerance. Therefore, this gene may be involved in post-transcriptional regulation and may be an important candidate for molecular breeding of salt-tolerant plants.

Ectopic Expression of a Thellungiella salsuginea Aquaporin Gene, TsPIP1;1, Increased the Salt Tolerance of Rice

Aquaporins play important regulatory roles in the transport of water and small molecules in plants. In this study, a Thellungiella salsuginea TsPIP1;1 aquaporin was transformed into Kitaake rice, and three transgenic lines were evaluated by profiling the changes of the physiological metabolism, osmotic potential, and differentially expressed genes under salt stress. The TsPIP1;1 protein contains six transmembrane domains and is localized in the cytoplasm membrane. Overexpression of the TsPIP1;1 gene not only increased the accumulation of prolines, soluble sugars and chlorophyll, but also lowered the osmotic potential and malondialdehyde content in rice under salt stress, and alleviated the amount of salt damage done to rice organs by regulating the distribution of Na/K ions, thereby promoting photosynthetic rates. Transcriptome sequencing confirmed that the differentially expressed genes that are up-regulated in rice positively respond to salt stimulus, the photosynthetic metabolic process, and the accumulation profiles of small molecules and Na/K ions. The co-expressed Rubisco and LHCA4 genes in rice were remarkably up-regulated under salt stress. This data suggests that overexpression of the TsPIP1;1 gene is involved in the regulation of water transport, the accumulation of Na/K ions, and the translocation of photosynthetic metabolites, thus conferring enhanced salt tolerance to rice.

ThPP1 gene, encodes an inorganic pyrophosphatase in Thellungiella halophila, enhanced the tolerance of the transgenic rice to alkali stress

An inorganic pyrophosphorylase gene, ThPP1 , modulated the accumulations of phosphate and osmolytes by up-regulating the differentially expression genes, thus enhancing the tolerance of the transgenic rice to alkali stress (AS). Inorganic pyrophosphorylase is essential in catalyzing the hydrolysis of pyrophosphate to inorganic phosphate during plant growth. Here, we report the changes of physiological osmolytes and differentially expression genes in the transgenic rice overexpressing a soluble inorganic pyrophosphatase gene ThPP1 of Thellungiella halophila in response to AS. Analyses showed that the ThPP1 gene was a PPase family I member which is located to the cytoplasm. Data showed that the transgenic lines revealed an enhanced tolerance to AS compared to the wild type, and effectively increased the accumulations of inorganic phosphate and organic small molecules starch, sucrose, proline and chlorophyll, and maintained the balance of osmotic potential by modulating the ratio of Na⁺/K⁺ in plant cells. Under AS, total 379 of differentially expression genes were up-regulated in the leaves of the transgenic line compared with control, and the enhanced tolerance of the transgenic rice to the AS seemed to be associated with the up-regulations of the osmotic stress-related genes such as the L-type lectin-domain containing receptor kinase (L-type LecRK), the cation/H⁺ antiporter gene and the vacuolar cation/proton exchanger 1 gene (CAX1), which conferred the involvements in the biosynthesis and metabolic pathways. Protein interaction showed that the ThPP1 protein specifically interacted with a 16# target partner of the photosystem II light-harvesting-Chl-binding protein. This study suggested that the ThPP1 gene plays an important regulatory role in conferring the tolerance of the transgenic rice to AS, and is an effective candidate in molecular breeding for crop cultivation of the alkali tolerance.

Heterologous Expression of Panax ginseng PgTIP1 Confers Enhanced Salt Tolerance of Soybean Cotyledon Hairy Roots, Composite, and Whole Plants

The Panax ginseng TIP gene PgTIP1 was previously demonstrated to have high water channel activity by its heterologous expression in Xenopus laevis oocytes and in yeast; it also plays a significant role in growth of PgTIP1-transgenic Arabidopsis plants under favorable conditions and has enhanced tolerance toward salt and drought treatment. In this work, we first investigated the physiological effects of heterologous PgTIP1 expression in soybean cotyledon hairy roots or composite plants mediated by Agrobacterium rhizogenes toward enhanced salt tolerance. The PgTIP1-transgenic soybean plants mediated by the pollen tube pathway, represented by the lines N and J11, were analyzed at the physiological and molecular levels for enhanced salt tolerance. The results showed that in terms of root-specific heterologous expression, the PgTIP1-transformed soybean cotyledon hairy roots or composite plants displayed superior salt tolerance compared to the empty vector-transformed ones according to the mitigatory effects of hairy root growth reduction, drop in leaf RWC, and rise in REL under salt stress. Additionally, declines in K+ content, increases in Na+ content and Na+/K+ ratios in the hairy roots, stems, or leaves were effectively alleviated by PgTIP1-transformation, particularly the stems and leaves of composite soybean plants. At the whole plant level, PgTIP1-trasgenic soybean lines were found to possess stronger root vigor, reduced root and leaf cell membrane damage, increased SOD, POD, CAT, and APX activities, steadily increased leaf Tr, RWC, and Pn values, and smaller declines in chlorophyll and carotenoid content when exposed to salt stress compared to wild type. Moreover, the distribution patterns of Na+, K+, and Cl- in the roots, stems, and leaves of salt-stressed transgenic plants were readjusted, in that the absorbed Na+ and Cl- were mainly restricted to the roots to reduce their transport to the shoots, and the transport of root-absorbed K+ to the shoots was simultaneously promoted. PgTIP1 transformation into soybean plants enhanced the expression of some stress-related genes (GmPOD, GmAPX1, GmSOS1, and GmCLC1) in the roots and leaves under salt treatment. This indicates that the causes of enhanced salt tolerance of heterologous PgTIP1-transformed soybean are associated with the positive regulation on water relations, ion homeostasis, and ROS scavenging under salt stress both at root-specific and whole plant levels.

Mutual Interactions between Aquaporins and Membrane Components

In recent years, a number of studies have been focused on the structural evaluation of protein complexes in order to get mechanistic insights into how proteins communicate at the molecular level within the cell. Specific sites of protein-aquaporin interaction have been evaluated and new forms of regulation of aquaporins described, based on these associations. Heterotetramerizations of aquaporin isoforms are considered as novel regulatory mechanisms for plasma membrane (PIPs) and tonoplast (TIPs) proteins, influencing their intrinsic permeability and trafficking dynamics in the adaptive response to changing environmental conditions. However, protein-protein interaction is an extensive theme that is difficult to tackle and new methodologies are being used to study the physical interactions involved. Bimolecular fluorescence complementation and the identification of cross-linked peptides based on tandem mass spectra, that are complementary to other methodologies such as heterologous expression, co-precipitation assays or confocal fluorescence microscopy, are discussed in this review. The chemical composition and the physical characteristics of the lipid bilayer also influence many aspects of membrane aquaporins, including their functionality. The molecular driving forces stabilizing the positions of the lipids around aquaporins could define their activity, thereby altering the conformational properties. Therefore, an integrative approach to the relevance of the membrane-aquaporin interaction to different processes related to plant cell physiology is provided. Finally, it is described how the interactions between aquaporins and copolymer matrixes or biological compounds offer an opportunity for the functional incorporation of aquaporins into new biotechnological advances.

Soybean TIP Gene Family Analysis and Characterization of GmTIP1;5 and GmTIP2;5 Water Transport Activity

Soybean, one of the most important crops worldwide, is severely affected by abiotic stress. Drought and flooding are the major abiotic stresses impacting soybean yield. In this regard, understanding water uptake by plants, its utilization and transport has great importance. In plants, water transport is mainly governed by channel forming aquaporin proteins (AQPs). Tonoplast intrinsic proteins (TIPs) belong to the plant-specific AQP subfamily and are known to have a role in abiotic stress tolerance. In this study, 23 soybean TIP genes were identified based on the latest soybean genome annotation. TIPs were characterized based on conserved structural features and phylogenetic distribution. Expression analysis of soybean TIP genes in various tissues and under abiotic stress conditions demonstrated tissue/stress-response specific differential expression. The natural variations for TIP genes were analyzed using whole genome re-sequencing data available for a set of 106 diverse soybean genotypes including wild types, landraces and elite lines. Results revealed 81 single-nucleotide polymorphisms (SNPs) and several large insertions/deletions in the coding region of TIPs. Among these, non-synonymous SNPs are most likely to have a greater impact on protein function and are candidates for molecular studies as well as for the development of functional markers to assist breeding. The solute transport function of two TIPs was further validated by expression in Xenopus laevis oocytes. GmTIP1;5 was shown to facilitate the rapid movement of water across the oocyte membrane, while GmTIP2;5 facilitated the movement of water and boric acid. The present study provides an initial insight into the possible roles of soybean TIP genes under abiotic stress conditions. Our results will facilitate elucidation of their precise functions during abiotic stress responses and plant development, and will provide potential breeding targets for modifying water movement in soybean.

Molecular mechanisms of foliar water uptake in a desert tree

Water deficits severely affect growth, particularly for the plants in arid and semiarid regions of the world. In addition to precipitation, other subsidiary water, such as dew, fog, clouds and small rain showers, may also be absorbed by leaves in a process known as foliar water uptake. With the severe scarcity of water in desert regions, this process is increasingly becoming a necessity. Studies have reported on physical and physiological processes of foliar water uptake. However, the molecular mechanisms remain less understood. As major channels for water regulation and transport, aquaporins (AQPs) are involved in this process. However, due to the regulatory complexity and functional diversity of AQPs, their molecular mechanism for foliar water uptake remains unclear. In this study, Tamarix ramosissima, a tree species widely distributed in desert regions, was investigated for gene expression patterns of AQPs and for sap flow velocity. Our results suggest that the foliar water uptake of T. ramosissima occurs in natural fields at night when the humidity is over a threshold of 85 %. The diurnal gene expression pattern of AQPs suggests that most AQP gene expressions display a circadian rhythm, and this could affect both photosynthesis and transpiration. At night, the PIP2-1 gene is also upregulated with increased relative air humidity. This gene expression pattern may allow desert plants to regulate foliar water uptake to adapt to extreme drought. This study suggests a molecular basis of foliar water uptake in desert plants.

Variations in DREB1A and VP1.1 Genes Show Association with Salt Tolerance Traits in Wild Tomato (Solanum pimpinellifolium)

Association analysis was conducted in a core collection of 94 genotypes of Solanum pimpinellifolium to identify variations linked to salt tolerance traits (physiological and yield traits under salt stress) in four candidate genes viz., DREB1A, VP1.1, NHX1, and TIP. The candidate gene analysis covered a concatenated length of 4594 bp per individual and identified five SNP/Indels in DREB1A and VP1.1 genes explaining 17.0% to 25.8% phenotypic variation for various salt tolerance traits. Out of these five alleles, one at 297 bp in DREB1A had in-frame deletion of 6 bp (CTGCAT) or 12 bp (CTGCATCTGCAT), resulting in two alleles, viz., SpDREB1A_297_6 and SpDREB1A_297_12. These alleles individually or as haplotypes accounted for maximum phenotypic variance of about 25% for various salt tolerance traits. Design of markers for selection of the favorable alleles/haplotypes will hasten marker-assisted introgression of salt tolerance from S. pimpinellifolium into cultivated tomato.

A Novel Soybean Intrinsic Protein Gene, GmTIP2;3, Involved in Responding to Osmotic Stress

Water is essential for plant growth and development. Water deficiency leads to loss of yield and decreased crop quality. To understand water transport mechanisms in plants, we cloned and characterized a novel tonoplast intrinsic protein (TIP) gene from soybean with the highest similarity to TIP2-type from other plants, and thus designated GmTIP2;3. The protein sequence contains two conserved NPA motifs and six transmembrane domains. The expression analysis indicated that this gene was constitutively expressed in all detected tissues, with higher levels in the root, stem and pod, and the accumulation of GmTIP2;3 transcript showed a significant response to osmotic stresses, including 20% PEG6000 (polyethylene glycol) and 100 μM ABA (abscisic acid) treatments. The promoter-GUS (glucuronidase) activity analysis suggested that GmTIP2;3 was also expressed in the root, stem, and leaf, and preferentially expressed in the stele of root and stem, and the core promoter region was 1000 bp in length, located upstream of the ATG start codon. The GUS tissue and induced expression observations were consistent with the findings in soybean. In addition, subcellular localization showed that GmTIP2;3 was a plasma membrane-localized protein. Yeast heterologous expression revealed that GmTIP2;3 could improve tolerance to osmotic stress in yeast cells. Integrating these results, GmTIP2;3 might play an important role in response to osmotic stress in plants.