Salt tolerance

Macro- and microplastics leachates: Characterization and impact on seed germination

Although plastic mulch enhances crop yield, its removal and disposal present significant challenges, contributing to macro- and microplastic pollution in agricultural soils. The adverse effects of this pollution on soil and plant health are not fully understood but may stem from the plastic particles or the toxicity of leached chemical additives. This study assessed the impact of macro- and microplastics from nondegradable LDPE-based (LDPEb) and biodegradable PBAT-based (PBATb) mulch films, along with their leachates, on the germination of three plant species. After seven days of incubation, PBAT mulch leached compounds that significantly inhibited Arabidopsis germination, while cotton and tomato exhibited notable tolerance. Notably, PBATb mulch released a higher concentration of compounds, whereas LDPEb mulch exhibited a greater diversity of leached chemicals. Microplastic particles alone did not hinder seed germination, indicating that plastic toxicity primarily arises from the leachates. Many of these leached compounds lack global regulation and hazard information, underscoring the urgent need for further investigation into their environmental impacts and the development of appropriate regulatory frameworks to mitigate the potential toxicity of chemicals from conventional and biodegradable mulches.

Reactive oxygen species (ROS) modulate nitrogen signaling using temporal transcriptome analysis in foxtail millet

Reactive oxygen species (ROS) is a chemically reactive chemical substance containing oxygen and a natural by-product of normal oxygen metabolism. Excessive ROS affect the growth process of crops, which will lead to the decrease of yield. Nitrogen, as a critical nutrient element in plants and plays a vital role in plant growth and crop production. Nitrate is the primary nitrogen source available to plants in agricultural soil and various natural environments. However, the molecular mechanism of ROS-nitrate crosstalk is still unclear. In this study, we used the foxtail millet (Setaria italica L.) as the material to figure it out. Here, we show that excessive NaCl inhibits nitrate-promoted plant growth and nitrogen use efficiency (NUE). NaCl induces ROS accumulation in roots, and ROS inhibits nitrate-induced gene expression in a short time. Surprisingly, low concentration ROS slight promotes and high concentration of ROS inhibits foxtail millet growth under long-term H2O2 treatment. These results may open a new perspective for further exploration of ROS-nitrate signaling pathway in plants.

Mechanisms of plant saline-alkaline tolerance

Saline-alkaline soil affects crop growth and development, thereby suppressing the yields. Human activities and climate changes are putting arable land under the threat of saline-alkalization. To feed a growing global population in limited arable land, it is of great urgence to breed saline-alkaline tolerant crops to cope with food security. Plant salt-tolerance mechanisms have already been explored for decades. However, to date, the molecular mechanisms underlying plants responses to saline-alkaline stress have remained largely elusive. Here, we summarize recent advances in plant response to saline-alkaline stress and propose some points deserving of further exploration.

Physiological and Biochemical Parameters of Salinity Resistance of Three Durum Wheat Genotypes

The area of farming lands affected by increasing soil salinity is growing significantly worldwide. For this reason, breeding works are conducted to improve the salinity tolerance of important crop species. The goal of the present study was to indicate physiological or biochemical parameters characterizing three durum wheat accessions with various tolerance to salinity. The study was carried out on germinating seeds and mature plants of a Polish SMH87 line, an Australian cultivar 'Tamaroi' (salt-sensitive), and the BC5Nax2 line (salt-tolerant) exposed to 0-150 mM NaCl. Germination parameters, electrolyte leakage (EL), and salt susceptibility index were determined in the germinating caryopses, whereas photosynthetic parameters, carbohydrate and phenolic content, antioxidant activity as well as yield were measured in fully developed plants. The parameters that most differentiated the examined accessions in the germination phase were the percentage of germinating seeds (PGS) and germination vigor (Vi). In the fully developed plants, parameters included whether the plants had the maximum efficiency of the water-splitting reaction on the donor side of photosystem II (PSII)-Fv/F0, energy dissipation from PSII-DIo/CSm, and the content of photosynthetic pigments and hydrogen peroxide, which differentiated studied genotypes in terms of salinity tolerance degree. Salinity has a negative impact on grain yield by reducing the number of seeds per spike and the mass of one thousand seeds (MTS), which can be used as the most suitable parameter for determining tolerance to salinity stress. The most salt-tolerant BC5Nax2 line was characterized by the highest PGS, and Vi for NaCl concentration of 100-150 mM, content of chlorophyll a, b, carotenoids, and also MTS at all applied salt concentrations as compared with the other accessions. The most salt-sensitive cv. 'Tamaroi' demonstrated higher H2O2 concentration which proves considerable oxidative damage caused by salinity stress. Mentioned parameters can be helpful for breeders in the selection of genotypes the most resistant to this stress.

Hydrogen sulphide signalling in plant response to abiotic stress

Throughout their whole life cycle, higher plants are often exposed to diverse environmental stresses, such as drought, salinity, heavy metals and extreme temperatures. In response to such stress, plant cells initiate signalling transduction, resulting in downstream responses, such as specific gene transcription and protein expression. Accumulating evidence has revealed that hydrogen sulphide (H₂ S) serves as a signalling molecule in plant acclimation to stressful conditions. More important, H₂ S interacts with other signalling molecules and phytohormones, contributing to transcriptional regulation and post-translational modification. Overall, the H₂ S-mediated signalling pathway and its interaction with other signals remains elusive. Here, we describe the role of the H₂ S signalling network in regulating physiological and molecular processes under various abiotic stresses.

Structure, Function, and Regulation of the Plasma Membrane Na+/H+ Antiporter Salt Overly Sensitive 1 in Plants

Physiological studies have confirmed that export of Na⁺ to improve salt tolerance in plants is regulated by the combined activities of a complex transport system. In the Na⁺ transport system, the Na⁺/H⁺ antiporter salt overly sensitive 1 (SOS1) is the main protein that functions to excrete Na⁺ out of plant cells. In this paper, we review the structure and function of the Na⁺/H⁺ antiporter and the physiological process of Na⁺ transport in SOS signaling pathway, and discuss the regulation of SOS1 during phosphorylation activation by protein kinase and the balance mechanism of inhibiting SOS1 antiporter at molecular and protein levels. In addition, we carried out phylogenetic tree analysis of SOS1 proteins reported so far in plants, which implied the specificity of salt tolerance mechanism from model plants to higher crops under salt stress. Finally, the high complexity of the regulatory network of adaptation to salt tolerance, and the feasibility of coping strategies in the process of genetic improvement of salt tolerance quality of higher crops were reviewed.

Comparative proteomic analysis for revealing the advantage mechanisms of salt-tolerant tomato (Solanum lycoperscium)

Salt stress causes the quality change and significant yield loss of tomato. However, the resources of salt-resistant tomato were still deficient and the mechanisms of tomato resistance to salt stress were still unclear. In this study, the proteomic profiles of two salt-tolerant and salt-sensitive tomato cultivars were investigated to decipher the salt-resistance mechanism of tomato and provide novel resources for tomato breeding. We found high abundance proteins related to nitrate and amino acids metabolismsin the salt-tolerant cultivars. The significant increase in abundance of proteins involved in Brassinolides and GABA biosynthesis were verified in salt-tolerant cultivars, strengthening the salt resistance of tomato. Meanwhile, salt-tolerant cultivars with higher abundance and activity of antioxidant-related proteins have more advantages in dealing with reactive oxygen species caused by salt stress. Moreover, the salt-tolerant cultivars had higher photosynthetic activity based on overexpression of proteins functioned in chloroplast, guaranteeing the sufficient nutrient for plant growth under salt stress. Furthermore, three key proteins were identified as important salt-resistant resources for breeding salt-tolerant cultivars, including sterol side chain reductase, gamma aminobutyrate transaminase and starch synthase. Our results provided series valuable strategies for salt-tolerant cultivars which can be used in future.

Differential Salt Tolerance Strategies in Three Halophytes from the Same Ecological Habitat: Augmentation of Antioxidant Enzymes and Compounds

Understanding the salt tolerance mechanism in obligate halophytes provides valuable information for conservation and re-habitation of saline areas. Here, we investigated the responses of three obligate halophytes namely Arthrocnemum macrostachyum, Sarcocornia fruticosa and Salicornia europaea to salt stress (0, 100, 200, 400 and 600 mM NaCl) during their vegetative growth with regard to biomass, ions contents (Na⁺, K⁺ and Ca⁺²), chlorophyll contents, carotenoids, phenolic compounds, flavonoids, and superoxide dismutase, peroxidase and esterase activities. S. europaea showed the lowest biomass, root K⁺ content, Chl a/b ratio, and carotenoids under salinity. This reduction of biomass is concomitant with the increase in proline contents and peroxidase activity. On the other hand, the promotion of growth under low salinity and maintenance under high salinity (200 and 400 Mm NaCl) in A. Macrostachyum and S. fruticosa are accompanied by an increase in Chl a/b ratio, carotenoids, phenolics contents, and esterase activity. Proline content was decreased under high salinity (400 and 600 mM NaCl) in both species compared to S. europaea, while peroxidase showed the lowest activity in both plants under all salt levels except under 600 mM NaCl in Arthrocnemum macrostachyum compared to S. europaea. These results suggest two differential strategies; (1) the salt tolerance is due to activation of antioxidant enzymes and biosynthesis of proline in S. europaea, (2) the salt tolerance in A. macrostachyum, S. fruticosa are due to rearrangement of chlorophyll ratio and biosynthesis of antioxidant compounds (carotenoids, phenolics and flavonoids) which their cost seem to need less energy than activation of antioxidant enzymes. The differential behavior in halophytes of the same habitat confirms that the tolerance mechanism in halophytes is species-specific which provides new insight about the restoration strategy of saline areas.

Gene expression patterns in shoots of Camelina sativa with enhanced salinity tolerance provided by plant growth promoting bacteria producing 1-aminocyclopropane-1-carboxylate deaminase or expression of the corresponding acdS gene

Growth of plants in soil inoculated with plant growth promoting bacteria (PGPB) producing 1-aminocyclopropane-1-carboxylate (ACC) deaminase or expression of the corresponding acdS gene in transgenic lines reduces the decline in shoot length, shoot weight and photosynthetic capacity triggered by salt stress in Camelina sativa. Reducing the levels of ethylene attenuated the salt stress response as inferred from decreases in the expression of genes involved in development, senescence, chlorosis and leaf abscission that are highly induced by salt to levels that may otherwise have a negative effect on plant growth and productivity. Growing plants in soil treated with Pseudomonas migulae 8R6 negatively affected ethylene signaling, auxin and JA biosynthesis and signalling, but had a positive effect on the regulation of genes involved in GA signaling. In plants expressing acdS, the expression of the genes involved in auxin signalling was positively affected, while the expression of genes involved in cytokinin degradation and ethylene biosynthesis were negatively affected. Moreover, fine-tuning of ABA signaling appears to result from the application of ACC deaminase in response to salt treatment. Moderate expression of acdS under the control of the root specific rolD promoter or growing plants in soil treated with P. migulae 8R6 were more effective in reducing the expression of the genes involved in ethylene production and/or signaling than expression of acdS under the more active Cauliflower Mosaic Virus 35S promoter.

Ectopic expression of apple hexose transporter MdHT2.2 reduced the salt tolerance of tomato seedlings with decreased ROS-scavenging ability

Salt is one of the main stresses that limit plant growth, especially at the seedling stage, reducing crop production and severely impacting food security. However, the relationship between salt stress and sugar content regulated by sugar transporters remains unknown. Here, we investigated the salt tolerance of transgenic tomato seedlings ectopically expressing MdHT2.2, which is a fructose and glucose/H⁺ symporter located on the plasma membrane in apple. Although the contents of fructose, glucose and sucrose in the leaves of seedlings ectopically expressing MdHT2.2 obviously increased compared with those of WT seedlings, the transgenic seedlings were significantly less tolerance to salt stress. Under salt stress, the SlSOS1/2 and SlNHX1 genes were highly expressed, and the accumulation of Na⁺ was lower in the transgenic seedlings than in WT, however, ROS accumulated to a greater degree in the former, and the ROS-scavenging-related enzyme activities and AsA content were lower in the transgenic seedlings than WT. Taken together, these results indicated that the relatively low salt tolerance of the MdHT2.2 transgenic seedlings was related with the accumulation of ROS, which was caused by reduced ROS-scavenging ability. Our results offer proof that changes in sugar content caused by sugar transporters are related to salt tolerance, and provide new insight into the regulation of sugar content, quality improvement and stress tolerance.

Bacterial endophyte mediated plant tolerance to salinity: growth responses and mechanisms of action

Salinity stress is one of the key constraints for sustainable crop production. It has gained immense importance in the backdrop of climate change induced imbalanced terrestrial water budgets. The traditional agronomic approaches and breeding salt-tolerant genotypes have often proved insufficient to alleviate salinity stress. Newer approaches like the use of bacterial endophytes associated with agricultural crops have occupied center place recently, owing to their advantageous role in improving crop growth, health and yield. Research evidences have revealed that bacterial endophytes can promote plant growth by accelerating availability of mineral nutrients, helping in production of phytohormones, siderophores, and enzymes, and also by activating systemic resistance against insect pest and pathogens in plants. These research developments have opened an innovative boulevard in agriculture for capitalizing bacterial endophytes, single species or consortium, to enhance plant salt tolerance capabilities, and ultimately lead to translational refinement of crop-production business under salty environments. This article reviews the latest research progress on the identification and functional characterization of salt tolerant endophytic bacteria and illustrates various mechanisms triggered by them for plant growth promotion under saline environment.

Over-expression of an R2R3 MYB Gene, GhMYB73, increases tolerance to salt stress in transgenic Arabidopsis

MYB family genes act as important regulators modulating the response to abiotic stress in plants. However, much less is known about MYB proteins in cotton. Here, we found that a cotton MYB gene, GhMYB73, was induced by NaCl and abscisic acid (ABA). Silencing GhMYB73 expression in cotton increased sensitivity to salt stress. The cotyledon greening rate of Arabidopsis thaliana over-expressing GhMYB73 under NaCl or mannitol treatment was significantly enhanced during the seedling germination stage. What's more, several osmotic stress-induced genes, such as AtNHX1, AtSOS3 and AtP5CS1, were more highly induced in the over-expression lines than in wild type under salt treatment, supporting the hypothesis that GhMYB73 contributes to salinity tolerance by improving osmotic stress resistance. Arabidopsis lines over-expressing GhMYB73 had superior germination and cotyledon greening under ABA treatment, and some abiotic stress-induced genes involved in ABA pathways (AtPYL8, AtABF3, AtRD29B and AtABI5), had increased transcription levels under salt-stress conditions in these lines. Furthermore, we found that GhMYB73 physically interacts with GhPYL8 and AtPYL8, suggesting that GhMYB73 regulates ABA signaling during salinity stress response. Taken together, over-expression of GhMYB73 significantly increases tolerance to salt and ABA stress, indicating that it can potentially be used in transgenic technology approaches to improve cotton salt tolerance.

CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 6: A Key Regulator of Na+ Homeostasis during Germination

Salinity impairs seed germination and seedling establishment. We investigated the role of Arabidopsis (Arabidopsis thaliana) CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 6 (CAMTA6) in salinity stress responses during early germination. Compared with the wild type, the camta6-4 and camta6-5 mutants were more tolerant to NaCl and abscisic acid (ABA) and accumulated less Na⁺ In contrast, 4- to 11-d-old camta6 seedlings were more sensitive to NaCl. In camta6, expression of HIGH-AFFINITY K⁺ TRANSPORTER1 (AtHKT1;1), encoding an Na⁺/K⁺ transporter, was restricted to the radicles and was not enhanced by NaCl or ABA. During germination, the camta6 hkt1 double mutant was as sensitive as the wild type and hkt1 to NaCl, suggesting that HKT1;1 is crucial for the salt tolerance of camta6 An ABA response element in the HKT1;1 promoter was found to be indispensable for the enhanced expression of the gene in response to NaCl and to ABA. Transcriptome analysis of the wild type and camta6-5 with and without salt treatment revealed 1,020 up-regulated and 1,467 down-regulated salt-responsive genes in the wild type. Among these, 638 up-regulated and 1,242 down-regulated genes were classified as CAMTA6-dependent. Expression of several known salt stress-associated genes, including SALT OVERLY SENSITIVE1 and Na⁺/H⁺ ANTIPORTER, was impaired in camta6 mutants. Bioinformatics analysis of the 5' upstream sequences of the salt-responsive CAMTA6-dependent up-regulated genes revealed the CACGTGTC motif as the most prominent element, representing an ABA response element and a potential CAMTA-binding site. We suggest that CAMTA6 regulates, directly or indirectly, the expression of most of the salt-responsive genes in germinating seeds, including genes that are crucial for Na⁺ homeostasis and salt stress tolerance.

The effect of salt stress on resveratrol and piceid accumulation in two Vitis vinifera L. cultivars

Salinity is one of the most important abiotic stresses, especially in arid regions. Such devastating constraint is converted mainly to oxidative burst. Thus, plants have to develop strategies to scavenge salt-related regenerated oxidant molecules. In the present work, fully aged plants derived from two Vitis vinifera L. cultivars, the Tunisian autochthonous tolerant genotype Razegui and the salt sensitive Syrah, were analyzed regarding their short term response to 100 mM NaCl, in hydroponic cultures. The ratio [ASA/ASA + DHA] was calculated on the basis of the oxidation of ascorbic acid (ASA) into dehydroascorbic acid (DHA) in leaves. Results proved that oxidative stress was generated. This led to the accumulation of malondialdehyde which referred to a lipid peroxidation mainly in the sensitive Syrah. In order to cope with these oxidative disturbances, trans-resveratrol as well as its glucosides trans-piceid and cis-piceid have been de novo synthesized in the sensitive variety. Razegui stilbene concentrations were presented here for the first time and unexpectedly did not show a very important variation during the salt elicitation.

The Arabidopsis Ca2+-Dependent Protein Kinase CPK12 Is Involved in Plant Response to Salt Stress

CDPKs (Ca²⁺-Dependent Protein Kinases) are very important regulators in plant response to abiotic stress. The molecular regulatory mechanism of CDPKs involved in salt stress tolerance remains unclear, although some CDPKs have been identified in salt-stress signaling. Here, we investigated the function of an Arabidopsis CDPK, CPK12, in salt-stress signaling. The CPK12-RNA interference (RNAi) mutant was much more sensitive to salt stress than the wild-type plant GL1 in terms of seedling growth. Under NaCl treatment, Na⁺ levels in the roots of CPK12-RNAi plants increased and were higher than levels in GL1 plants. In addition, the level of salt-elicited H₂O₂ production was higher in CPK12-RNAi mutants than in wild-type GL1 plants after NaCl treatment. Collectively, our results suggest that CPK12 is required for plant adaptation to salt stress.

Comparison of the responses to NaCl stress of three tomato introgression lines

We aimed to examine the response of three tomato introgression lines (IL925.3, IL925.5 and IL925.6) to NaCl stress. These lines originated from a cross between M82 (Solanum lycopersicum) and the wild salttolerant tomato Solanum pennellii, each line containing a different fragment of the S.pennellii genome. Salt-sensitive phenotypes related to plant growth and physiology, and the response of antioxidants, pigments and antioxidant enzymes were measured. In general, salt stress decreased the fresh weight of leaves, leaf area and leaf number and an increase of Na⁺ accumulation in aerial parts was observed, which caused a reduction in the absorption of K⁺ and Ca²⁺. Salt stress also induced a decrease in chlorophyll, carotenoids and lipid peroxidation (MDA) and an increase in anthocyanins and reduced ascorbate, although some differences were seen between the lines, for example for carotenoid levels. Guaiacol peroxidase, catalase and glutathione reductase activity enhanced in aerial parts of the lines, but again some differences were seen between the three lines. It is concluded that IL925.5 might be the most sensitive line to salt stress as its dry weight loss was the greatest in response to salt and this line showed the highest Na⁺ ion accumulation in leaves.

Do volatile compounds produced by Fusarium oxysporum and Verticillium dahliae affect stress tolerance in plants?

Volatile compounds (VCs) produced by diverse microbes seem to affect plant growth, development and/or stress tolerance. We investigated how VCs released by soilborne fungi Fusarium oxysporum and Verticillium dahliae affect Arabidopsis thaliana responses to abiotic and biotic stresses. Under salt stress, VCs from both fungi helped its growth and increased chlorophyll content. However, in contrast to wild-type A. thaliana (Col-0), V. dahliae VCs failed to increase leaf surface area in auxin signalling mutants aux1-7, tir1-1 and axr1-3. Compared to wild-type Col-0, the degree of lateral root density enhanced by V. dahliae VCs in these mutants was also reduced. Consistent with the involvement of auxin signalling in fungal VC-mediated salt torelance, A. thaliana line carrying DR5::GUS displayed increased auxin accumulation in root apex upon exposure to V. dahliae VCs, and 1-naphthylphthalamic acid, an auxin transport inhibitor, adversely affected V. dahliae VC-mediated salt tolerance. F. oxysporum VCs induced the expression of PR1 but not PDF1.2 in A. thaliana lines containing PR1::GUS and PFD1.2::GUS. When challenged with Pseudomonas syringae after the exposure to F. oxysporum VCs, A. thaliana showed reduced disease symptoms. However, the number of bacterial cells in F. oxysporum VC-treated plants was not significantly different from that in control plants.

Elemental bioimaging by means of LA-ICP-OES: investigation of the calcium, sodium and potassium distribution in tobacco plant stems and leaf petioles

Laser ablation-inductively coupled plasma-optical emission spectroscopy (LA-ICP-OES) is presented as a valuable tool for elemental bioimaging of alkali and earth alkali elements in plants. Whereas LA-ICP-OES is commonly used for micro analysis of solid samples, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has advanced to the gold standard for bioimaging. However, especially for easily excitable and ubiquitous elements such as alkali and earth alkali elements, LA-ICP-OES holds some advantages regarding simultaneous detection, costs, contamination, and user-friendliness. This is demonstrated by determining the calcium, sodium and potassium distribution in tobacco plant stem and leaf petiole tissues. A quantification of the calcium contents in a concentration range up to 1000 μg g-1 using matrix-matched standards is presented as well. The method is directly compared to a LA-ICP-MS approach by analyzing parallel slices of the same samples.

Comprehensive study of excess phosphate response reveals ethylene mediated signaling that negatively regulates plant growth and development

Excess Phosphorus (P) in agriculture is causing serious environmental problems like eutrophication of lakes and rivers. Unlike the enormous information available for phosphate starvation response (P₀), very few information is available for the effect of excess phosphate P_i on plants. Characterization of Excess Phosphate Response (EP_iR) is essential for designing strategies to increase phosphate accumulation and tolerance. We show a significant modulation in the root developmental plasticity under the increasing supply of excess P_i. An excess supply of 20 mM P_i (P₂₀) produces a shallow root system architecture (RSA), reduces primary root growth, root apical meristem size, and meristematic activity in Arabidopsis. The inhibition of primary root growth and development is indeterminate in nature and caused by the decrease in number of meristematic cortical cells due to EP_iR. Significant changes occurred in metal nutrients level due to excess P_i supply. A comparative microarray investigation of the EP_iR response reveals a modulation in ethylene biosynthesis and signaling, metal ions deficiency response, and root development related genes. We used ethylene-insensitive or sensitive mutants to provide more evidence for ethylene-mediated signaling. A new role of EP_iR in regulating the developmental responses of plants mediated by ethylene has been demonstrated.

Moderate salt treatment alleviates ultraviolet-B radiation caused impairment in poplar plants

The effects of moderate salinity on the responses of woody plants to UV-B radiation were investigated using two Populus species (Populus alba and Populus russkii). Under UV-B radiation, moderate salinity reduced the oxidation pressure in both species, as indicated by lower levels of cellular H2O2 and membrane peroxidation, and weakened the inhibition of photochemical efficiency expressed by O-J-I-P changes. UV-B-induced DNA lesions in chloroplast and nucleus were alleviated by salinity, which could be explained by the higher expression levels of DNA repair system genes under UV-B&salt condition, such as the PHR, DDB2, and MutSα genes. The salt-induced increase in organic osmolytes proline and glycine betaine, afforded more efficient protection against UV-B radiation. Therefore moderate salinity induced cross-tolerance to UV-B stress in poplar plants. It is thus suggested that woody plants growing in moderate salted condition would be less affected by enhanced UV-B radiation than plants growing in the absence of salt. Our results also showed that UV-B signal genes in poplar plants PaCOP1, PaSTO and PaSTH2 were quickly responding to UV-B radiation, but not to salt. The transcripts of PaHY5 and its downstream pathway genes (PaCHS1, PaCHS4, PaFLS1 and PaFLS2) were differently up-regulated by these treatments, but the flavonoid compounds were not involved in the cross-tolerance since their concentration increased to the same extent in both UV-B and combined stresses.

Interaction Between ABA Signaling and Copper Homeostasis in Arabidopsis thaliana

ABA is involved in plant responses to non-optimal environmental conditions, including nutrient availability. Since copper (Cu) is a very important micronutrient, unraveling how ABA affects Cu uptake and distribution is relevant to ensure adequate Cu nutrition in plants subjected to stress conditions. Inversely, knowledge about how the plant nutritional status can interfere with ABA biosynthesis and signaling mechanisms is necessary to optimize stress tolerance in horticultural crops. Here the reciprocal influence between ABA and Cu content was addressed by using knockout mutants and overexpressing transgenic plants of high affinity plasma membrane Cu transporters (pmCOPT) with altered Cu uptake. Exogenous ABA inhibited pmCOPT expression and drastically modified COPT2-driven localization in roots. ABA regulated SPL7, the main transcription factor responsive for Cu deficiency responses, and subsequently affected expression of its targets. ABA biosynthesis (aba2) and signaling (hab1-1 abi1-2) mutants differentially responded to ABA according to Cu levels. Alteration of Cu homeostasis in the pmCOPT mutants affected ABA biosynthesis, transport and signaling as genes such as NCED3, WRKY40, HY5 and ABI5 were differentially modulated by Cu status, and also in the pmCOPT and ABA mutants. Altered Cu uptake resulted in modified plant sensitivity to salt-mediated increases in endogenous ABA. The overall results provide evidence for reciprocal cross-talk between Cu status and ABA metabolism and signaling.

Transcriptional regulation of receptor-like protein genes by environmental stresses and hormones and their overexpression activities in Arabidopsis thaliana

Receptor-like proteins (RLPs) have been implicated in multiple biological processes, including plant development and immunity to microbial infection. Fifty-seven AtRLP genes have been identified in Arabidopsis, whereas only a few have been functionally characterized. This is due to the lack of suitable physiological screening conditions and the high degree of functional redundancy among AtRLP genes. To overcome the functional redundancy and further understand the role of AtRLP genes, we studied the evolution of AtRLP genes and compiled a comprehensive profile of the transcriptional regulation of AtRLP genes upon exposure to a range of environmental stresses and different hormones. These results indicate that the majority of AtRLP genes are differentially expressed under various conditions that were tested, an observation that will help to select certain AtRLP genes involved in a specific biological process for further experimental studies to eventually dissect their function. A large number of AtRLP genes were found to respond to more than one treatment, suggesting that one single AtRLP gene may be involved in multiple physiological processes. In addition, we performed a genome-wide cloning of the AtRLP genes, and generated and characterized transgenic Arabidopsis plants overexpressing the individual AtRLP genes, presenting new insight into the roles of AtRLP genes, as exemplified by AtRLP3, AtRLP11 and AtRLP28 Our study provides an overview of biological processes in which AtRLP genes may be involved, and presents valuable resources for future investigations into the function of these genes.

In vitro functional characterization of the Na+/H+ antiporters in Corynebacterium glutamicum

Corynebacterium glutamicum, typically used as industrial workhorse for amino acid production, is a moderately salt-alkali-tolerant microorganism with optimal growth at pH 7-9. However, little is known about the mechanisms of salt-alkali tolerance in C. glutamicum. Here, the catalytic capacity of three putative Na(+)/H(+) antiporters from C. glutamicum (designated as Cg-Mrp1, Cg-Mrp2 and Cg-NhaP) were characterized in an antiporter-deficient Escherichia coli KNabc strain. Only Cg-Mrp1 was able to effectively complement the Na(+)-sensitive of E. coli KNabc. Cg-Mrp1 exhibited obvious Na(+)(Li(+))/H(+) antiport activities with low apparent Km values of 1.08 mM and 1.41 mM for Na(+) and Li(+), respectively. The Na(+)/H(+) antiport activity of Cg-Mrp1 was optimal in the alkaline pH range. All three antiporters showed detectable K(+)/H(+) antiport activitiy. Cg-NhaP also exhibited Na(+)(Li(+),Rb(+))/H(+) antiport activities but at lower levels of activity. Interestingly, overexpression of Cg-Mrp2 exhibited clear Na(+)(K(+))/H(+) antiport activities. These results suggest that C. glutamicum Na(+)(K(+))/H(+) antiporters may have overlapping roles in coping with salt-alkali and perhaps high-osmolarity stress.

Natural variation of root traits: from development to nutrient uptake

The root system has a crucial role for plant growth and productivity. Due to the challenges of heterogeneous soil environments, diverse environmental signals are integrated into root developmental decisions. While root growth and growth responses are genetically determined, there is substantial natural variation for these traits. Studying the genetic basis of the natural variation of root growth traits can not only shed light on their evolution and ecological relevance but also can be used to map the genes and their alleles responsible for the regulation of these traits. Analysis of root phenotypes has revealed growth strategies and root growth responses to a variety of environmental stimuli, as well as the extent of natural variation of a variety of root traits including ion content, cellular properties, and root system architectures. Linkage and association mapping approaches have uncovered causal genes underlying the variation of these traits.

Global analysis of gene expression profiles in physic nut (Jatropha curcas L.) seedlings exposed to salt stress

BACKGROUND

Salt stress interferes with plant growth and production. Plants have evolved a series of molecular and morphological adaptations to cope with this abiotic stress, and overexpression of salt response genes reportedly enhances the productivity of various crops. However, little is known about the salt responsive genes in the energy plant physic nut (Jatropha curcas L.). Thus, excavate salt responsive genes in this plant are informative in uncovering the molecular mechanisms for the salt response in physic nut.

METHODOLOGY/PRINCIPAL FINDINGS

We applied next-generation Illumina sequencing technology to analyze global gene expression profiles of physic nut plants (roots and leaves) 2 hours, 2 days and 7 days after the onset of salt stress. A total of 1,504 and 1,115 genes were significantly up and down-regulated in roots and leaves, respectively, under salt stress condition. Gene ontology (GO) analysis of physiological process revealed that, in the physic nut, many "biological processes" were affected by salt stress, particular those categories belong to "metabolic process", such as "primary metabolism process", "cellular metabolism process" and "macromolecule metabolism process". The gene expression profiles indicated that the associated genes were responsible for ABA and ethylene signaling, osmotic regulation, the reactive oxygen species scavenging system and the cell structure in physic nut.

CONCLUSIONS/SIGNIFICANCE

The major regulated genes detected in this transcriptomic data were related to trehalose synthesis and cell wall structure modification in roots, while related to raffinose synthesis and reactive oxygen scavenger in leaves. The current study shows a comprehensive gene expression profile of physic nut under salt stress. The differential expression genes detected in this study allows the underling the salt responsive mechanism in physic nut with the aim of improving its salt resistance in the future.

Role of Arabidopsis UV RESISTANCE LOCUS 8 in plant growth reduction under osmotic stress and low levels of UV-B

In high-light environments, plants are exposed to different types of stresses, such as an excess of UV-B, but also drought stress which triggers a common morphogenic adaptive response resulting in a general reduction of plant growth. Here, we report that the Arabidopsis thaliana UV RESISTANCE LOCUS 8 (UVR8) gene, a known regulator of the UV-B morphogenic response, was able to complement a Saccharomyces cerevisiae osmo-sensitive mutant and its expression was induced after osmotic or salt stress in Arabidopsis plants. Under low levels of UV-B, plants overexpressing UVR8 are dwarfed with a reduced root development and accumulate more flavonoids compared to control plants. The growth defects are mainly due to the inhibition of cell expansion. The growth inhibition triggered by UVR8 overexpression in plants under low levels of UV-B was exacerbated by mannitol-induced osmotic stress, but it was not significantly affected by ionic stress. In contrast, uvr8-6 mutant plants do not differ from wild-type plants under standard conditions, but they show an increased shoot growth under high-salt stress. Our data suggest that UVR8-mediated accumulation of flavonoid and possibly changes in auxin homeostasis are the underlying mechanism of the observed growth phenotypes and that UVR8 might have an important role for integrating plant growth and stress signals.

A wheat allene oxide cyclase gene enhances salinity tolerance via jasmonate signaling

One of the two branches of the α-linolenic acid metabolism pathway is catalyzed by 12-oxo-phytodienoic acid reductase I, and the other is involved in jasmonic acid (JA) synthesis. The former is known to be active in the response to salinity tolerance in wheat (Triticum aestivum), but the participation of the latter in this response has not been established as yet. Here, the salinity-responsive bread wheat gene TaAOC1, which encodes an allene oxide cyclase involved in the α-linolenic acid metabolism pathway, was constitutively expressed in both bread wheat and Arabidopsis (Arabidopsis thaliana). In both species, transgenic lines exhibited an enhanced level of tolerance to salinity. The transgenic plants accumulated a higher content of JA and developed shorter roots. Both the shortened roots and the salinity tolerance were abolished in a background lacking a functional AtMYC2, a key component of the JA and abscisic acid signaling pathway, but were still expressed in a background deficient with respect to abscisic acid synthesis. We provide the first evidence, to our knowledge, suggesting that JA is also involved in the plant salinity response and that the α-linolenic acid metabolism pathway has a regulatory role over this response.

CYP709B3, a cytochrome P450 monooxygenase gene involved in salt tolerance in Arabidopsis thaliana

BACKGROUND

Within the Arabidopsis genome, there are 272 cytochrome P450 monooxygenase (P450) genes. However, the biological functions of the majority of these P450s remain unknown. The CYP709B family of P450s includes three gene members, CYP709B1, CYP709B2 and CYP709B3, which have high amino acid sequence similarity and lack reports elucidating biological functions.

RESULTS

We identified T-DNA insertion-based null mutants of the CYP709B subfamily of genes. No obvious morphological phenotypes were exhibited under normal growth conditions. When the responses to ABA and salt stress were studied in these mutants, only the cyp709b3 mutant showed sensitivity to ABA and salt during germination. Under moderate salt treatment (150 mM NaCl), cyp709b3 showed a higher percentage of damaged seedlings, indicating a lower tolerance to salt stress. CYP709B3 was highly expressed in all analyzed tissues and especially high in seedlings and leaves. In contrast, CYP709B1 and CYP709B2 were highly expressed in siliques, but were at very low levels in other tissues. Under salt stress condition, CYP709B3 gene expression was induced after 24 hr and remained at high expression level. Expression of the wild type CYP709B3 gene in the cyp709b3 mutant fully complemented the salt intolerant phenotype. Furthermore, metabolite profiling analysis revealed some differences between wild type and cyp709b3 mutant plants, supporting the salt intolerance phenotype of the cyp709b3 mutant.

CONCLUSIONS

These results suggest that CYP709B3 plays a role in ABA and salt stress response and provides evidence to support the functions of cytochrome P450 enzymes in plant stress response.

A spatio-temporal understanding of growth regulation during the salt stress response in Arabidopsis

Plant environmental responses involve dynamic changes in growth and signaling, yet little is understood as to how progress through these events is regulated. Here, we explored the phenotypic and transcriptional events involved in the acclimation of the Arabidopsis thaliana seedling root to a rapid change in salinity. Using live-imaging analysis, we show that growth is dynamically regulated with a period of quiescence followed by recovery then homeostasis. Through the use of a new high-resolution spatio-temporal transcriptional map, we identify the key hormone signaling pathways that regulate specific transcriptional programs, predict their spatial domain of action, and link the activity of these pathways to the regulation of specific phases of growth. We use tissue-specific approaches to suppress the abscisic acid (ABA) signaling pathway and demonstrate that ABA likely acts in select tissue layers to regulate spatially localized transcriptional programs and promote growth recovery. Finally, we show that salt also regulates many tissue-specific and time point-specific transcriptional responses that are expected to modify water transport, Casparian strip formation, and protein translation. Together, our data reveal a sophisticated assortment of regulatory programs acting together to coordinate spatially patterned biological changes involved in the immediate and long-term response to a stressful shift in environment.

Arabidopsis SOS3 plays an important role in salt tolerance by mediating calcium-dependent microfilament reorganization

KEY MESSAGE : SOS3 mediates calcium dependent actin filament reorganization that plays important roles in plant responses to salt stress. Arabidopsis salt overly sensitive 3 (SOS3) plays an important role in plant salt tolerance by regulation of Na(+)/K(+) homeostasis. Plants lacking SOS3 are hypersensitive to salt stress and this phenomenon can be partially rescued by the addition of calcium. However the mechanism underlying remains elusive. We here report that the organization of actin filaments in sos3 mutant differs from that in wild-type plant. Under salt stress abnormal actin assembly and arrangement in sos3 are more pronounced, which can be partially complemented by addition of external calcium or low concentration of latrunculin A, an actin monomer-sequestering agent. The effects of calcium and Lat A on actin filament organization of sos3 mutant are accordant with their effects on sos3 salt sensitivity under salt stress. These findings indicate that the salt-hypersensitivity of sos3 mutant partially results from its disordered actin filaments, and SOS3 mediated actin filament reorganization plays important roles in plant responses to salt stress.

Toxic effect of NaCl on ion metabolism, antioxidative enzymes and gene expression of perennial ryegrass

Two-month old seedlings of perennial ryegrass (Lolium perenne L.) were subjected to four different levels of salinity for 7 days. The NaCl treatments reduced turf quality and normalized transpiration rates. Both chlorophyll (Chl) a and Chl b contents decreased in the grass exposed to 255 mM relative to the control. An increase in the lipid peroxidationin was observed. The activity of leaf superoxide dismutase increased while, peroxidase and catalase activities decreased in response to NaCl treatments. The expression of Chl Cu/ZnSOD, Cyt Cu/ZnSOD, FeSOD, CAT, POD, GPX and GR was up-regulated for NaCl-treated grass. Salt stress increased accumulation of Na(+) and decreased K(+)/Na(+) ratio, Mg(2+) and P content in both shoots and roots of perennial ryegrass. The findings of this study suggest that salt stress may cause toxicity to perennial ryegrass through oxidative injury and damage to Chl and cell membrane integrity.

Overexpression of AtMYB52 confers ABA hypersensitivity and drought tolerance

We carried out activation tagging screen to isolate genes regulating abscisic acid (ABA) response. From the screen of approximately 10,000 plants, we isolated ca 100 ABA response mutants. We characterized one of the mutants, designated ahs1, in this study. The mutant is ABA-hypersensitive, and AtMYB52 was found to be activated in the mutant. Overexpression analysis to recapitulate the mutant phenotypes demonstrated that ATMYB confers ABA-hypersensitivity during postgermination growth. Additionally, AtMYB52 overexpression lines were drought-tolerant and their seedlings were salt-sensitive. Changes in the expression levels of a few genes involved in ABA response or cell wall biosynthesis were also observed. Together, our data suggest that AtMYB52 is involved in ABA response. Others previously demonstrated that AtMYB52 regulates cell wall biosynthesis; thus, our results imply a possible connection between ABA response and cell wall biosynthesis.

Probing the roles of LRR RLK genes in Arabidopsis thaliana roots using a custom T-DNA insertion set

Leucine-rich repeat receptor-like protein kinases (LRR RLKs) represent the largest group of Arabidopsis RLKs with approximately 235 members. A minority of these LRR RLKs have been assigned to diverse roles in development, pathogen resistance and hormone perception. Using a reverse genetics approach, a collection of homozygous T-DNA insertion lines for 69 root expressed LRR RLK genes was screened for root developmental defects and altered response after exposure to environmental, hormonal/chemical and abiotic stress. The obtained data demonstrate that LRR RLKs play a role in a wide variety of signal transduction pathways related to hormone and abiotic stress responses. The described collection of T-DNA insertion mutants provides a valuable tool for future research into the function of LRR RLK genes.

Functional-genomics-based identification of genes that regulate Arabidopsis responses to multiple abiotic stresses

Abiotic stresses are a primary cause of crop loss worldwide. The convergence of stress signalling pathways to a common set of transcription factors suggests the existence of upstream regulatory genes that control plant responses to multiple abiotic stresses. To identify such genes, data from published Arabidopsis thaliana abiotic stress microarray analyses were combined with our presented global analysis of early heat stress-responsive gene expression, in a relational database. A set of Multiple Stress (MST) genes was identified by scoring each gene for the number of abiotic stresses affecting expression of that gene. ErmineJ over-representation analysis of the MST gene set identified significantly enriched gene ontology biological processes for multiple abiotic stresses and regulatory genes, particularly transcription factors. A subset of MST genes including only regulatory genes that were designated 'Multiple Stress Regulatory' (MSTR) genes, was identified. To validate this strategy for identifying MSTR genes, mutants of the highest-scoring MSTR gene encoding the circadian clock protein CCA1, were tested for altered sensitivity to stress. A double mutant of CCA1 and its structural and functional homolog, LATE ELONGLATED HYPOCOTYL, exhibited greater sensitivity to salt, osmotic and heat stress than wild-type plants. This work provides a reference data set for further study of MSTR genes.

Calcium: just another regulator in the machinery of life?

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BACKGROUND

Current hypotheses imply that stimulus-response systems in plants are networks of signal transduction pathways. It is usually assumed that these pathways connect receptors with effectors via chains of molecular events. Diverse intermediate signalling components (transducers) participate in these processes. However, many cellular transducers respond to several stimuli. Hence, there are no discrete chains but rather pathways that interconnect network-modules of different command structure. In particular, the cytosolic free Ca2+ concentration ([Ca2+](cyt)) is thought to perform many different tasks in a wide range of cellular events. However, this range of putative functions is so wide that it is often questioned how Ca2+ can comply with the definition of a second messenger. *THE Ca2+ SIGNATURE HYPOTHESIS: Some authors have suggested the concept of a specific signature of the ([Ca2+](cyt)) response. This implies that characteristics of the time course of changes in ([Ca2+](cyt)) and their localized sites of appearance in cells are used by the plant to identify the type and intensity of the stimulus. This hypothesis has triggered many investigations, which have yielded contradictory results. * THE CURRENT PICTURE: Much evidence suggests that the functions of calcium can be grouped into three classes: Ca2+ as a protective agent, Ca2+ as a chemical switch and Ca2+ as a 'digital' information carrier. Examples of the first two classes are presented here. The third is more controversial; while some investigations seem to support this idea, others call the Ca2+ signature hypothesis into question. Further investigations are needed to shed more light on Ca(2+)-driven signalling cascades.

Analysis of natural allelic variation of Arabidopsis seed germination and seed longevity traits between the accessions Landsberg erecta and Shakdara, using a new recombinant inbred line population

Quantitative trait loci (QTL) mapping was used to identify loci controlling various aspects of seed longevity during storage and germination. Similar locations for QTLs controlling different traits might be an indication for a common genetic control of such traits. For this analysis we used a new recombinant inbred line population derived from a cross between the accessions Landsberg erecta (Ler) and Shakdara (Sha). A set of 114 F9 recombinant inbred lines was genotyped with 65 polymerase chain reaction-based markers and the phenotypic marker erecta. The traits analyzed were dormancy, speed of germination, seed sugar content, seed germination after a controlled deterioration test, hydrogen peroxide (H2O2) treatment, and on abscisic acid. Furthermore, the effects of heat stress, salt (NaCl) stress, osmotic (mannitol) stress, and natural aging were analyzed. For all traits one or more QTLs were identified, with some QTLs for different traits colocating. The relevance of colocation for mechanisms underlying the various traits is discussed.

Salt stress-induced chloride flux: a study using transgenic Arabidopsis expressing a fluorescent anion probe

Salt stress leads to massive accumulation of toxic levels of Na(+) and Cl(-) ions in plants. By using the recombinant fluorescent probe CLOMELEON, we demonstrate passive anion flux under salt stress. Chloride influx is restricted in the presence of divalent cations like Mg(2+) and Ca(2+), and completely blocked by La(3+). The amount but not the rate of the reported chloride uptake is independent from the kind of corresponding permeable cation (K(+) versus Na(+)), external pH and magnitude of osmotic stress. Cl(-) efflux however seems to involve stretch-activated transport. From the influence of Ca(2+) on reported changes of cytosolic anion concentrations, we speculate that transport mechanisms of Cl(-) and Na(+) might be thermodynamically coupled under saline conditions.

Self-reporting Arabidopsis expressing pH and [Ca2+] indicators unveil ion dynamics in the cytoplasm and in the apoplast under abiotic stress

For noninvasive in vivo measurements of intra- and extracellular ion concentrations, we produced transgenic Arabidopsis expressing pH and calcium indicators in the cytoplasm and in the apoplast. Ratiometric pH-sensitive derivatives of the green fluorescent protein (At-pHluorins) were used as pH indicators. For measurements of calcium ([Ca(2+)]), luminescent aequorin variants were expressed in fusion with pHluorins. An Arabidopsis chitinase signal sequence was used to deliver the indicator complex to the apoplast. Responses of pH and [Ca(2+)] in the apoplast and in the cytoplasm were studied under salt and "drought" (mannitol) stress. Results are discussed in the frame of ion flux, regulation, and signaling. They suggest that osmotic stress and salt stress are differently sensed, compiled, and processed in plant cells.

Overexpression of SOD2 increases salt tolerance of Arabidopsis

The yeast (Schizosaccharomyces pombe) SOD2 (Sodium2) gene was introduced into Arabidopsis under the control of the cauliflower mosaic virus 35S promoter. Transformants were selected for their ability to grow on medium containing kanamycin. Southern- and northern-blot analyses confirmed that SOD2 was transferred into the Arabidopsis genome. There were no obvious morphological or developmental differences between the transgenic and wild-type (wt) plants. Several transgenic homozygous lines and wt plants (control) were evaluated for salt tolerance and gene expression. Overexpression of SOD2 in Arabidopsis improved seed germination and seedling salt tolerance. Analysis of Na+ and K+ contents of the symplast and apoplast in the parenchyma cells of the root cortex and mesophyll cells in the spongy tissue of the leaf showed that transgenic lines accumulated less Na+ and more K+ in the symplast than the wt plants did. The photosynthetic rate and the fresh weight of the transgenic lines were distinctly higher than that of wt plants after NaCl treatment. Results from different tests indicated that the expression of the SOD2 gene promoted a higher level of salt tolerance in vivo in transgenic Arabidopsis plants.