Salt-Tolerant Crops: Time to Deliver.

Salt stress is a globally increasing environmental detriment to crop growth and productivity. Exposure to salt stress evokes a complex medley of cellular signals, which rapidly reprogram transcriptional and metabolic networks to shape plant phenotype. To date, genetic engineering approaches were used with success to enhance salt tolerance; however, their performance is yet to be evaluated under realistic field conditions. Regulatory short non-coding RNAs (rsRNAs) are emerging as next-generation candidates for engineering salt tolerance in crops. In view of this, the present review provides a comprehensive analysis of a decade's worth of functional studies on non-coding RNAs involved in salt tolerance. Further, we have integrated this knowledge of rsRNA-mediated regulation with the current paradigm of salt tolerance to highlight two regulatory complexes (RCs) for regulating salt tolerance in plants. Finally, a knowledge-driven roadmap is proposed to judiciously utilize RC component(s) for enhancing salt tolerance in crops.

Overexpression of the Suaeda salsa SsNHX1 gene confers enhanced salt and drought tolerance to transgenic Zea mays

Salinity is the most severe abiotic stress perceived by plants and affects about 800 million hectares of land worldwide, including 20% of the world’s highly productive irrigated land. Significant crop yield losses are observed due to salinity. Salinization is increasing because of poor irrigation management and climate change. Improving salt tolerance in crops is for these reasons an important target for plant breeding in the near future. However, salinity tolerance in plants is not easy to breed for due to its interaction with many physiological processes controlled by many genes and their interaction with the environment. Barley is a good model crop to study different mechanisms conferring salt tolerance in cereals. A traditional QTL mapping approach in combination with a new association mapping method allowed us to efficiently explore the genetics and genetic diversity of salt tolerance in barley. Improvements of the association mapping technology highly increased detection power and mapping accuracy. The traits and QTLs identified in this thesis point out both osmotic and ionic stress tolerant genes as important targets for salt tolerance breeding. This thesis provides tools to plant breeders for the application of marker-assisted introgression breeding of salt tolerance genes in their breeding programs. Some QTLs were found to be syntenic with the important QTLs/genes for salt tolerance found in wheat and rice such as Na+ and K+ transporter gene families. Other QTLs were new and suggest the presence of novel genes that play an important role in plant ion homeostasis, transportation of Cl- and Ca2+ and osmotic tolerance. We demonstrated that association mapping can be a powerful approach to dissect the complexity of salt tolerance in barley. The newly available high-density SNP map of barley and the barley genome sequence in the near future further increases the accuracy of mapping studieswith the association panel and will greatly facilitate the cloning of the genes underlying salt tolerance in barley. This thesis thus contributes to better a understanding of the physiological and genetic basis of salt tolerance and improved breeding strategies for the development of salt tolerant varieties.

Soil salinization is a major environmental stressor that reduces the growth and yield of crops. Maintaining the balance of ions under salinity is vital for plant salt tolerance; however, little is known about the correlation between the salt tolerance of crops and the ion contents of their roots and shoots. Here, we investigated the poorly understood salt-tolerance mechanisms, particularly regarding ion contents (particularly Na+), in Brassica napus subsp. napus L., an agriculturally important species. Twenty B. napus inbred lines were randomly chosen from five salt-tolerance categories and treated with increasing concentrations of NaCl (0–200 mmol) for this work. We found that the root Na+ content is the most correlated limiting factor for the salt tolerance of B. napus; the higher the salt tolerance, the lower the root Na+ content. Correspondingly, the Ca2+/Na+ and K+/Na+ ratios of the roots were highly correlated with B. napus salt tolerance, indicating that the selective absorption ability of these ions by the roots and their translocation to the shoots play a pivotal role in this trait. These data provide a foundation for the further study of the molecular mechanisms underlying salt tolerance and for breeding salt-tolerant B. napus cultivars.

Water deficit imposed by either drought or salinity brings about severe growth retardation and yield loss of crops. Since Brassica crops are important contributors to total oilseed production, it is urgently needed to develop tolerant cultivars to ensure yields under such adverse conditions. There are various physiochemical mechanisms for dealing with drought and salinity in plants at different developmental stages. Accordingly, different indicators of tolerance to drought or salinity at the germination, seedling, flowering and mature stages have been developed and used for germplasm screening and selection in breeding practices. Classical genetic and modern genomic approaches coupled with precise phenotyping have boosted the unravelling of genes and metabolic pathways conferring drought or salt tolerance in crops. QTL mapping of drought and salt tolerance has provided several dozen target QTLs in Brassica and the closely related Arabidopsis. Many drought- or salt-tolerant genes have also been isolated, some of which have been confirmed to have great potential for genetic improvement of plant tolerance. It has been suggested that molecular breeding approaches, such as marker-assisted selection and gene transformation, that will enhance oil product security under a changing climate be integrated in the development of drought- and salt-tolerant Brassica crops.

A thorough understanding of ionic detoxification and homeostasis is imperative for improvement of salt tolerance in crops. However, the homeostasis of elements and their relationship to metabolites under salt stress have not been fully elucidated in plants. In this study, Tibetan wild barley accessions, XZ16 and XZ169, differing in salt tolerance, and a salt-tolerant cultivar CM72 were used to investigate ionomic profile changes in tissues in response to 150 and 300 mM NaCl at the germination and seedling stages. At the germination stage, the contents of Ca and Fe significantly decreased in roots, while K and S contents increased, and Ca and Mg contents decreased in shoots, after 10 d of treatment. At the seedling stage, the contents of K, Mg, P and Mn in roots and of K, Ca, Mg and S in shoots decreased significantly after 21 d of treatment. Moreover, Na had a significant negative correlation with metabolites involved in glycolysis, α-ketoglutaric acid, maleic acid and alanine in roots, and metabolites associated with the tricarboxylic acid (TCA) cycle, sucrose, polyols and aspartate in leaves. The salt-tolerant genotypes XZ16 and CM72 showed a lower Na content in tissues, and less reduction in Zn and Cu in roots, of Ca, Mg and S in leaves, and shoot DW than the sensitive genotype XZ169, when exposed to a higher salt level. The results indicated that restriction of Na accumulation and rearrangement of nutrient elements and metabolites in barley tissues are possibly attributable to development of salt tolerance.

Salt stress triggers the overdose accumulation of reactive oxygen species (ROS) in crop plants, leading to severe oxidative damage to living tissues. MicroRNAs (miRNAs) act as master regulators orchestrating the stress responsive regulatory networks as well as salt tolerance. However, the fundamental roles of miRNAs in modulating salt tolerance in cereal crops, especially in salt-triggered ROS scavenging remain largely unknown. Through small RNA sequencing, a salt-responsive miRNA, miR172 was identified in rice. Further, by generating the miR172-overexpression or MIR172 gene loss-of-function mutant lines, the biological significance of miR172 and its downstream signaling pathways related to salt tolerance were defined. We demonstrated that miR172 is a positive regulator of salt tolerance in both rice and wheat. More interestingly, miR172a and miR172b, but not miR172c or miR172d are involved in salt stress response, emphasizing the functional differentiation within miR172 family members. Further evidence uncovers a novel miR172/IDS1 regulatory module that functions as a crucial molecular rheostat in maintaining ROS homeostasis during salt stress, mainly through balancing the expression of a group of ROS-scavenging genes. Our findings establish a direct molecular link between miRNAs and detoxification response in cereal crops for improving salt tolerance.

Simple SummarySoil salinization is a serious global issue that limits plant growth, reduces crop yields, and threatens agricultural productivity. As a result, plants have developed sophisticated adaptive strategies to improve salt tolerance for sustainable agricultural development. The MYB transcription factor family is one of the largest in plants and is integral to the regulation of physiological and biochemical processes under abiotic and biotic stresses. This study presents the first isolation and characterization of a new tomato R2R3-MYB factor, SlMYB306-like, which is shown to enhance salt tolerance by modulating both plant growth and physiological parameters such as photosynthetic efficiency, osmotic adjustment substance levels, and antioxidant enzyme activities. Mechanistically, SlMYB306-like mediated salt tolerance by upregulating key genes involved in ROS scavenging and salt stress responses. Overall, the results reveal SlMYB306-like as a critical regulator of salt tolerance, providing a valuable genetic resource for developing salt-tolerant crops and boosting yields in high-salinity soils, in turn supporting more stable food production.Soil salinization significantly limits plant growth and agricultural productivity, with MYB transcription factors playing crucial roles in mediating plant responses to salt stress. In this study, a novel R2R3-MYB transcription factor gene, SlMYB306-like, was isolated from tomato. Phylogenetic comparison indicated that SlMYB306-like shared the highest sequence homology with potato StMYB306-like. Subcellular localization assays demonstrated nuclear localization of SlMYB306-like protein, while yeast transactivation assays confirmed its function as a transcriptional activator. Expression profiling showed that SlMYB306-like was inducible by NaCl and abscisic acid (ABA) treatments. In addition, functional characterization via the overexpression of SlMYB306-like in Arabidopsis thaliana revealed enhanced salt tolerance, evidenced by an increased maximum quantum efficiency of photosystem II (Fv/Fm) and proline levels alongside decreased accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA) content under salt stress conditions. Furthermore, the overexpression of SlMYB306-like upregulated the expression of several stress-responsive genes, including AtSOD1, AtCAT1, AtEGY3, AtP5CS2, and AtRD29A. Collectively, these findings suggest that SlMYB306-like enhances salt tolerance by modulating ROS scavenging, osmotic adjustment, and ABA signaling pathways, thereby representing a promising candidate gene for the development of salt-tolerant crops.

Plants are more and more affected by environmental stresses, especially by the devastating consequences of desertification and water scarcity which can be seen and felt all over the world. About 3.6 billion of the world’s 5.2 billion hectares of dryland used for agriculture have already suffered erosion, soil degradation, and salinization. Desertification can hinder efforts for sustainable development and introduces new threats to human health, ecosystems, and national economies. This problem is catalyzed by global climate change which exacerbates desertification and salinization. Therefore, solutions are desperately needed, such as the improvement of drought and salinity tolerance of crops, which in turn requires a detailed knowledge about tolerance mechanisms in plants. These mechanisms comprise a wide range of responses on molecular, cellular, and whole plant levels, which include amongst others the synthesis of compatible solutes/osmolytes and radical scavenging mechanisms. Regarding global change, elevated atmospheric CO2 concentrations can enhance salt and drought tolerance because oxidative stress is alleviated and more energy can be provided for energy-dependent tolerance mechanisms such as the synthesis of compatible solutes and antioxidants, thus increasing the suitability of plants as crops in future. A detailed knowledge of the physiological and biochemical basis of drought and salt tolerance and its interaction with elevated CO2 concentration can provide a basis for the cultivation of suitable plants in regions threatened by desertification and water scarcity under sustainable culture conditions. Even the drylands could offer tangible economic and ecological opportunities.

CeO2 nanoparticles downregulate the expression of AhRbohB6 and AhRbohC18 to improve salt tolerance in peanut.

Salinity is one of the most alarming abiotic stress affecting crop productivity and significant crop loss globally. It impedes plant performance by inducing deleterious effects on germination and plant vigor. Moreover, 6% of total land area and 30% in irrigated lands are affected by salinity (Fahad et al., 2014). This leads to massive loss in terms of productivity and arable land, as most of the economically important particularly horticultural crops are sensitive to salinity. Chilli is an important cash crop due to its high economical and nutritional value of its fruit (Howard et al., 2000). It is generally categorized as salt sensitive crop. Salt stress reduces chilli production (Navarro et al., 2002). Salinity stress cause changes in physiological processes that cause reduction in plant growth and productivity. Salt stress, increased osmotic potential, generates water deficit conditions in the soil (Navarro et al., 2003), ionic toxicity, because of high concentration of toxic ions (Na and Cl) that accumulate within the plant, ionic imbalance due to the accumulation of ions in the plant cells, which cause the deficiency of the others nutrients (Lycoskoufis et al., 2005). Under salinity stress increased level of reactive oxygen species (ROS) in cells causes K efflux from the cells. Increased ROS concentrations weakens the defense mechanism which leads to oxidative stress. ROS are very destructive to plants at higher concentrations. Under salinity stress ROS increases and can pose a threat to plant cells by causing lipids peroxidation, proteins oxidation, impairment of nucleic acids, inhibit activation of enzymes, activates programmed cell death and ultimately leading to death of the cells (Sharma et al., 2012). There are different approaches used to combat salt stress like conventional breeding, genetic manipulation and foliar application of osmoprotectants. The complexity and polygenic nature of salt tolerance in crops are important factors contributing to the difficulties in breeding salt tolerant crop varieties. Breeding efforts have been hampered by a lack of understanding to salt tolerance mechanisms as well as a lack of field and laboratory screening tests (Zhu, 2000). Genetic screening technique includes diverse plant genetic resources that provides opportunity to develop improved and resistant cultivars with desirable features, that are beneficial for farmers (yield potential and large seed) as well as for breeders (abiotic stress tolerance potential, disease and pest resistance and photosynthesis) (Govindaraj et al., 2015). Proline which is usually considered as an osmoprotection agent is also known to be involved in reducing the oxidative damage by scavenging the free radicals. Proline may also play a role as protein compatible hydrotrope (Ashraf and Foolad, 2007). It supports cytoplasmic acidosis and maintain Pak. J. Agri. Sci., Vol. 53(1), 43-49; 2016 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/16.4623 http://www.pakjas.com.pk

Salinity affects crop growth and productivity, and this stress can be increased along with drought or high temperature stresses and poor irrigation management. Cultivation of salt-tolerant crops plays a critical role in enhancing crop yield under salt stress. In the past few decades, the mechanisms of plant adaptation to salt stress have been described, especially relying on ionic homeostasis, reactive oxygen species (ROS) scavenging, and phytohormone signaling. The studies of these molecular mechanisms have provided a basis for breeding new salt-tolerant crop germplasm and have facilitated the entry into the era of molecular breeding of salt-tolerant crops. In this review, we outline the recent progress in the molecular regulations underlying crop salt tolerance, focusing on the double-edged sword effect of ROS, the regulatory role of phytohormones, and the trade-off effects of ROS and phytohormones between crop yield and salt tolerance. A future challenge is to identify superior alleles of key salt-tolerant genes that will accelerate the breeding of high-yield and salt-tolerant varieties.

Domestication has resulted in reduced salt tolerance in tomato. To identify the genetic components causing this deficiency, we performed a genome-wide association study (GWAS) for root Na+ /K+ ratio in a population consisting of 369 tomato accessions with large natural variations. The most significant variations associated with root Na+ /K+ ratio were identified within the gene SlHAK20 encoding a member of the clade IV HAK/KUP/KT transporters. We further found that SlHAK20 transports Na+ and K+ and regulates Na+ and K+ homeostasis under salt stress conditions. A variation in the coding sequence of SlHAK20 was found to be the causative variant associated with Na+ /K+ ratio and confer salt tolerance in tomato. Knockout mutations in tomato SlHAK20 and the rice homologous genes resulted in hypersensitivity to salt stress. Together, our study uncovered a previously unknown molecular mechanism of salt tolerance responsible for the deficiency in salt tolerance in cultivated tomato varieties. Our findings provide critical information for molecular breeding to improve salt tolerance in tomato and other crops.

Salinity continues to be a major abiotic stress limiting crop productivity. As rice is staple food for nearly half of the world population, improvement in its salt tolerance will have a major impact on global food security. Compared to rice and other field crops, halophytes have evolved special physiological mechanisms to withstand high salinity. The overall goal of this study was to characterize plasma membrane protein 3 genes, SaPMP3-2 and SaPMP3-1, from a halophyte, Spartina alterniflora L., and evaluate their potential in single gene as well as pyramided transgenic plants in combination with the vacuolar ATPase subunit c1 (SaVHAc1) gene in improving salt tolerance in cv. Cocodrie background. Both genes, SaPMP3-2 and SaPMP3-1, enhanced the ability of E. coli to survive at 600 mM NaCl. Genetic complementation of the mutant yeast strain and enhanced salt tolerance in wild type yeast strain by SaPMP3-2 indicated its conserved functional role in salt tolerance. Subsequently, enhanced salt tolerance in transgenic rice plants was demonstrated through overexpression of SaPMP3-2 and SaPMP3-1 independently as well as the combination of SaPMP3-1 and SaVHAc1. Chlorophyll retention and relative water content were higher in transgenic plants compared to Cocodrie under salt stress during the vegetative stage. The transgenic plants survived wilting and drying symptoms with enhanced growth and higher K+/Na+ ratio at 100 mM NaCl stress during early seedling stage in hydroponic conditions. Salt stress screening during reproductive stage revealed that the single gene and the pyramided transgenic plants had better grain filling whereas only the pyramided plants showed significantly higher grain yield per plant and higher test weight compared to Cocodrie. The improvement in salt tolerance in transgenic rice plants could be due to the role played by SaPMP3-2 and SaPMP3-1 through maintenance of ion homoestasis by restricting uptake of salts. The impact of SaPMP3 gene was further amplified when combined with SaVHAc1 in pyramided transgenic plants, which showed better growth, vigor, and enhanced salt tolerance at all stages of crop growth compared with Cocodrie. Our study provided evidence that S. alterniflora could be a potential source for mining genes to enhance salt tolerance in rice and other cereal crops.

Saline soils are a major challenge in agriculture, and salinization is increasing worldwide due to climate change and destructive agricultural practices. Excessive amounts of salt in soils cause imbalances in ion distribution, physiological dehydration, and oxidative stress in plants. Breeding and genetic engineering methods to improve plant salt tolerance and the better use of saline soils are being explored; however, these approaches can take decades to accomplish. A shorter-term approach to improve plant salt tolerance is to be inoculated with bacteria with high salt tolerance or adjusting the balance of bacteria in the rhizosphere, including endosymbiotic bacteria (living in roots or forming a symbiont) and exosymbiotic bacteria (living on roots). Rhizosphere bacteria promote plant growth and alleviate salt stress by providing minerals (such as nitrogen, phosphate, and potassium) and hormones (including auxin, cytokinin, and abscisic acid) or by reducing ethylene production. Plant growth-promoting rhizosphere bacteria are a promising tool to restore agricultural lands and improve plant growth in saline soils. In this review, we summarize the mechanisms of plant growth-promoting bacteria under salt stress and their applications for improving plant salt tolerance to provide a theoretical basis for further use in agricultural systems.