Salinity Stress-Mediated Suppression of Expression of Salt Overly Sensitive Signaling Pathway Genes Suggests Negative Regulation by AtbZIP62 Transcription Factor in Arabidopsis thaliana

Unraveling the Contribution of MulSOS2 in Conferring Salinity Tolerance in Mulberry (Morus atropurpurea Roxb)

Salinity is one of the most serious threats to sustainable agriculture. The Salt Overly Sensitive (SOS) signaling pathway plays an important role in salinity tolerance in plants, and the SOS2 gene plays a critical role in this pathway. Mulberry not only has important economic value but also is an important ecological tree species; however, the roles of the SOS2 gene associated with salt stress have not been reported in mulberry. To gain insight into the response of mulberry to salt stress, SOS2 (designated MulSOS2) was cloned from mulberry (Morus atropurpurea Roxb), and sequence analysis of the amino acids of MulSOS2 showed that it shares some conserved domains with its homologs from other plant species. Our data showed that the MulSOS2 gene was expressed at different levels in different tissues of mulberry, and its expression was induced substantially not only by NaCl but also by ABA. In addition, MulSOS2 was exogenously expressed in Arabidopsis, and the results showed that under salt stress, transgenic MulSOS2 plants accumulated more proline and less malondialdehyde than the wild-type plants and exhibited increased tolerance to salt stress. Moreover, the MulSOS2 gene was transiently overexpressed in mulberry leaves and stably overexpressed in the hairy roots, and similar results were obtained for resistance to salt stress in transgenic mulberry plants. Taken together, the results of this study are helpful to further explore the function of the MulSOS2 gene, which provides a valuable gene for the genetic breeding of salt tolerance in mulberry.

Comparative assessment of metabolic, ionic and molecular responsiveness of four facultative halophytes to habitat salinization in the southwest of Jeddah Governorate, Saudi Arabia

This study explores the influence of salinity on some physiological and biochemical pathways of four facultative halophytes (Abutilon pannosum, Indigofera oblongifolia, Senna italica, and Tetraena coccinea) along the southwest coast of Jeddah Governorate. Through a comparative analysis of these plants in both saline and non-saline environments, the study investigates chlorophyll levels, ion concentrations within the plants, the correlation with the SOS1 gene, and the impact of salinity on metabolic compounds. The overarching goal is to gain insights into the adaptive mechanisms of these specific plants to salt stress, providing valuable information for addressing global agricultural challenges associated with salinity. Throughout the study, metabolic, ionic, and molecular responses of these plants were scrutinized in both environments. The findings revealed elevated levels of Na+, K+, Ca2+, and Mg2+ in saline habitats, except for Na+ in I. oblongifolia. Despite increased concentrations of Chl b, variations were noted in Chl a and carotenoids in plants exposed to salt. Osmoregulatory patterns in A. pannosum and I. oblongifolia exhibited reversible changes, including heightened protein and proline levels in A. pannosum and decreased levels in I. oblongifolia, accompanied by alterations in amino acids and soluble carbohydrates. Senna italica displayed higher levels of osmolytes, excluding proline, compared to salinized environments, while T. coccinea exhibited lower levels of amino acids. The accumulation of Na+ emerged as the primary mechanism for ionic homeostasis in these plants, with non-significant decreases observed in K+, Mg2+, and Ca2+. Notably, an overexpression of the SOS1 gene (plasma membrane Na+/H+ antiporter) was observed as a response to maintaining ionic balance. Understanding these halophytes will be critical in addressing salinity challenges and enhancing crop tolerance to salinity.

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.

Algal Bio-Stimulants Enhance Salt Tolerance in Common Bean: Dissecting Morphological, Physiological, and Genetic Mechanisms for Stress Adaptation

Salinity adversely affects the plant's morphological characteristics, but the utilization of aqueous algal extracts (AE) ameliorates this negative impact. In this study, the application of AE derived from Chlorella vulgaris and Dunaliella salina strains effectively reversed the decline in biomass allocation and water relations, both in normal and salt-stressed conditions. The simultaneous application of both extracts in salt-affected soil notably enhanced key parameters, such as chlorophyll content (15%), carotene content (1%), photosynthesis (25%), stomatal conductance (7%), and transpiration rate (23%), surpassing those observed in the application of both AE in salt-affected as compared to salinity stress control. Moreover, the AE treatments effectively mitigated lipid peroxidation and electrolyte leakage induced by salinity stress. The application of AE led to an increase in GB (6%) and the total concentration of free amino acids (47%) by comparing with salt-affected control. Additionally, salinity stress resulted in an elevation of antioxidant enzyme activities, including superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase. Notably, the AE treatments significantly boosted the activity of these antioxidant enzymes under salinity conditions. Furthermore, salinity reduced mineral contents, but the application of AE effectively counteracted this decline, leading to increased mineral levels. In conclusion, the application of aqueous algal extracts, specifically those obtained from Chlorella vulgaris and Dunaliella salina strains, demonstrated significant efficacy in alleviating salinity-induced stress in Phaseolus vulgaris plants.

Fungal Elicitation Enhances Vincristine and Vinblastine Yield in the Embryogenic Tissues of Catharanthus roseus

Fungal elicitation could improve the secondary metabolite contents of in vitro cultures. Herein, we report the effect of Fusarium oxysporum on vinblastine and vincristine alkaloid yields in Catharanthus roseus embryos. The study revealed increased yields of vinblastine and vincristine in Catharanthus tissues. Different concentrations, i.e., 0.05% (T1), 0.15% (T2), 0.25% (T3), and 0.35% (T4), of an F. oxysporum extract were applied to a solid MS medium in addition to a control (T0). Embryogenic calli were formed from the hypocotyl explants of germinating seedlings, and the tissues were exposed to Fusarium extract elicitation. The administration of the F. oxysporum extract improved the growth of the callus biomass, which later differentiated into embryos, and the maximum induction of somatic embryos was noted T2 concentration (102.69/callus mass). A biochemical analysis revealed extra accumulations of sugar, protein, and proline in the fungus-elicitated cultivating tissues. The somatic embryos germinated into plantlets on full-strength MS medium supplemented with 2.24 µM of BA. The germination rate of the embryos and the shoot and root lengths of the embryos were high at low doses of the Fusarium treatment. The yields of vinblastine and vincristine were measured in different treated tissues via high-pressure thin-layer chromatography (HPTLC). The yield of vinblastine was high in mature (45-day old) embryos (1.229 µg g-1 dry weight), which were further enriched (1.267 µg g-1 dry weight) via the F. oxysporum-elicitated treatment, especially at the T2 concentration. Compared to vinblastine, the vincristine content was low, with a maximum of 0.307 µg g-1 dry weight following the addition of the F. oxysporum treatment. The highest and increased yields of vinblastine and vincristine, 7.88 and 15.50%, were noted in F. oxysporum-amended tissues. The maturated and germinating somatic embryos had high levels of SOD activity, and upon the addition of the fungal extracts, the enzyme's activity was further elevated, indicating that the tissues experienced cellular stress which yielded increased levels of vinblastine and vincristine following the T2/T1 treatments. The improvement in the yields of these alkaloids could augment cancer healthcare treatments, making them easy, accessible, and inexpensive.

Genome-wide characterization of SOS1 gene family in potato (Solanum tuberosum) and expression analyses under salt and hormone stress

Salt Overly Sensitive 1 (SOS1) is one of the members of the Salt Overly Sensitive (SOS) signaling pathway and plays critical salt tolerance determinant in plants, while the characterization of the SOS1 family in potato (Solanum tuberosum) is lacking. In this study, 37 StSOS1s were identified and found to be unevenly distributed across 10 chromosomes, with most of them located on the plasma membrane. Promoter analysis revealed that the majority of these StSOS1 genes contain abundant cis-elements involved in various abiotic stress responses. Tissue specific expression showed that 21 of the 37 StSOS1s were widely expressed in various tissues or organs of the potato. Molecular interaction network analysis suggests that 25 StSOS1s may interact with other proteins involved in potassium ion transmembrane transport, response to salt stress, and cellular processes. In addition, collinearity analysis showed that 17, 8, 1 and 5 of orthologous StSOS1 genes were paired with those in tomato, pepper, tobacco, and Arabidopsis, respectively. Furthermore, RT-qPCR results revealed that the expression of StSOS1s were significant modulated by various abiotic stresses, in particular salt and abscisic acid stress. Furthermore, subcellular localization in Nicotiana benthamiana suggested that StSOS1-13 was located on the plasma membrane. These results extend the comprehensive overview of the StSOS1 gene family and set the stage for further analysis of the function of genes in SOS and hormone signaling pathways.

CIPK-B is essential for salt stress signalling in Marchantia polymorpha

Calcium signalling is central to many plant processes, with families of calcium decoder proteins having expanded across the green lineage and redundancy existing between decoders. The liverwort Marchantia polymorpha has fast become a new model plant, but the calcium decoders that exist in this species remain unclear. We performed phylogenetic analyses to identify the calcineurin B-like (CBL) and CBL-interacting protein kinase (CIPK) network of M. polymorpha. We analysed CBL-CIPK expression during salt stress, and determined protein-protein interactions using yeast two-hybrid and bimolecular fluorescence complementation. We also created genetic knockouts using CRISPR/Cas9. We confirm that M. polymorpha has two CIPKs and three CBLs. Both CIPKs and one CBL show pronounced salt-responsive transcriptional changes. All M. polymorpha CBL-CIPKs interact with each other in planta. Knocking out CIPK-B causes increased sensitivity to salt, suggesting that this CIPK is involved in salt signalling. We have identified CBL-CIPKs that form part of a salt tolerance pathway in M. polymorpha. Phylogeny and interaction studies imply that these CBL-CIPKs form an evolutionarily conserved salt overly sensitive pathway. Hence, salt responses may be some of the early functions of CBL-CIPK networks and increased abiotic stress tolerance required for land plant emergence.

Grapevine (Vitis vinifera) responses to salt stress and alkali stress: transcriptional and metabolic profiling

BACKGROUND

Soil salinization and alkalization are widespread environmental problems that limit grapevine (Vitis vinifera L.) growth and yield. However, little is known about the response of grapevine to alkali stress. This study investigated the differences in physiological characteristics, chloroplast structure, transcriptome, and metabolome in grapevine plants under salt stress and alkali stress.

RESULTS

We found that grapevine plants under salt stress and alkali stress showed leaf chlorosis, a decline in photosynthetic capacity, a decrease in chlorophyll content and Rubisco activity, an imbalance of Na⁺ and K⁺, and damaged chloroplast ultrastructure. Fv/Fm decreased under salt stress and alkali stress. NPQ increased under salt stress whereas decreased under alkali stress. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed the differentially expressed genes (DEGs) induced by salt stress and alkali stress were involved in different biological processes and have varied molecular functions. The expression of stress genes involved in the ABA and MAPK signaling pathways was markedly altered by salt stress and alkali stress. The genes encoding ion transporter (AKT1, HKT1, NHX1, NHX2, TPC1A, TPC1B) were up-regulated under salt stress and alkali stress. Down-regulation in the expression of numerous genes in the 'Porphyrin and chlorophyll metabolism', 'Photosynthesis-antenna proteins', and 'Photosynthesis' pathways were observed under alkali stress. Many genes in the 'Carbon fixation in photosynthetic organisms' pathway in salt stress and alkali stress were down-regulated. Metabolome showed that 431 and 378 differentially accumulated metabolites (DAMs) were identified in salt stress and alkali stress, respectively. L-Glutamic acid and 5-Aminolevulinate involved in chlorophyll synthesis decreased under salt stress and alkali stress. The abundance of 19 DAMs under salt stress related to photosynthesis decreased. The abundance of 16 organic acids in salt stress and 22 in alkali stress increased respectively.

CONCLUSIONS

Our findings suggested that alkali stress had more adverse effects on grapevine leaves, chloroplast structure, ion balance, and photosynthesis than salt stress. Transcriptional and metabolic profiling showed that there were significant differences in the effects of salt stress and alkali stress on the expression of key genes and the abundance of pivotal metabolites in grapevine plants.

AtbZIP62 Acts as a Transcription Repressor to Positively Regulate ABA Responses in Arabidopsis

The basic region/leucine zipper (bZIP) transcription factor AtbZIP62 is involved in the regulation of plant responses to abiotic stresses, including drought and salinity stresses, NO₃ transport, and basal defense in Arabidopsis. It is unclear if it plays a role in regulating plant responses to abscisic acid (ABA), a phytohormone that can regulate plant abiotic stress responses via regulating downstream ABA-responsive genes. Using RT-PCR analysis, we found that the expression level of AtbZIP62 was increased in response to exogenously applied ABA. Protoplast transfection assays show that AtbZIP62 is predominantly localized in the nucleus and functions as a transcription repressor. To examine the roles of AtbZIP62 in regulating ABA responses, we generated transgenic Arabidopsis plants overexpressing AtbZIP62 and created gene-edited atbzip62 mutants using CRISPR/Cas9. We found that in both ABA-regulated seed germination and cotyledon greening assays, the 35S:AtbZIP62 transgenic plants were hypersensitive, whereas atbzip62 mutants were hyposensitive to ABA. To examine the functional mechanisms of AtbZIP62 in regulating ABA responses, we generated Arabidopsis transgenic plants overexpressing 35S:AtbZIP62-GR, and performed transcriptome analysis to identify differentially expressed genes (DEGs) in the presence and absence of DEX, and found that DEGs are highly enriched in processes including response to abiotic stresses and response to ABA. Quantitative RT-PCR results further show that AtbZIP62 may regulate the expression of several ABA-responsive genes, including USP, ABF2, and SnRK2.7. In summary, our results show that AtbZIP62 is an ABA-responsive gene, and AtbZIP62 acts as a transcription repressor to positively regulate ABA responses in Arabidopsis.

Genome-Wide Identification and Salt Stress Response Analysis of the bZIP Transcription Factor Family in Sugar Beet

As one of the largest transcription factor families in plants, bZIP transcription factors play important regulatory roles in different biological processes, especially in the process of stress response. Salt stress inhibits the growth and yield of sugar beet. However, bZIP-related studies in sugar beet (Beta vulgaris L.) have not been reported. This study aimed to identify the bZIP transcription factors in sugar beet and analyze their biological functions and response patterns to salt stress. Using bioinformatics, 48 BvbZIP genes were identified in the genome of sugar beet, encoding 77 proteins with large structural differences. Collinearity analysis showed that three pairs of BvbZIP genes were fragment replication genes. The BvbZIP genes were grouped according to the phylogenetic tree topology and conserved structures, and the results are consistent with those reported in Arabidopsis. Under salt stress, the expression levels of most BvbZIP genes were decreased, and only eight genes were up-regulated. GO analysis showed that the BvbZIP genes were mainly negatively regulated in stress response. Protein interaction prediction showed that the BvbZIP genes were mainly involved in light signaling and ABA signal transduction, and also played a certain role in stress responses. In this study, the structures and biological functions of the BvbZIP genes were analyzed to provide foundational data for further mechanistic studies and for facilitating the efforts toward the molecular breeding of stress-resilient sugar beet.

Wheat ocs-Element Binding Factor 1 Enhances Thermotolerance by Modulating the Heat Stress Response Pathway

The basic leucine zipper family (bZIP) represents one of the largest families of transcription factors that play an important role in plant responses to abiotic stresses. However, their role in contributing to thermotolerance in plants is not well explored. In this article, two homoeologs of wheat ocs-element binding factor 1 (TaOBF1-5B and TaOBF1-5D) were found to be heat-responsive TabZIP members. Their expression analysis in Indian wheat cultivars revealed their differential expression pattern and TaOBF1-5B was found to be more receptive to heat stress. Consistent with this, the heterologous overexpression of TaOBF1-5B in Arabidopsis thaliana and Oryza sativa promoted the expression of stress-responsive genes, which contributed to thermotolerance in transgenic plants. TaOBF1-5B was seen to interact with TaHSP90 in the nucleus and TaSTI in the nucleolus and the ER. Thus, the results suggest that TaOBF1-5B might play an important regulatory role in the heat stress response and is a major factor governing thermotolerance in plants.

Comparative Ionomics and Metabolic Responses and Adaptive Strategies of Cotton to Salt and Alkali Stress

Soil salinization and alkalization severely inhibit agriculture. However, the response mechanisms of cotton to salt stress or alkali stress are unclear. Ionomics and metabolomics were used to investigate salt and alkali stresses in cotton roots and leaves. Compared with the control, salt-treated and alkali-treated cotton plants showed 51.8 and 53.0% decreases in biomass, respectively. Under salt stress, the concentration of N decreased in roots but increased in leaves, and the concentrations of P and K increased in roots but decreased in leaves. Salt stress inhibited Ca, B, N, and Fe uptake and Mg, K, P, S, and Cu transport, but promoted Mo, Mn, Zn, Mg, K, P, S, and Cu uptake and Mo, Mn, Zn, B, N, and Fe transport. Under alkali stress, the concentrations of N and P in roots and leaves decreased, while the concentrations of K in roots and leaves increased. Alkali stress inhibited P, Ca, S, N, Fe, and Zn uptake and N, P, Mg and B transport, but promoted K, Mn, Cu, Mo, Mg, and B uptake and K, Mn, Cu, Mo, Fe, and Zn transport. Under salt stress in the leaves, 93 metabolites increased, mainly organic acids, amino acids, and sugars, increased in abundance, while 6 decreased. In the roots, 72 metabolites increased, mainly amino acids, organic acids, and sugars, while 18 decreased. Under alkali stress, in the leaves, 96 metabolites increased, including organic acids, amino acids, and sugars, 83 metabolites decreased, including organic acids, amino acids, and sugars; In the roots, 108 metabolites increased, including organic acids, amino acids, and sugars. 83 metabolites decreased, including organic acids and amino acids. Under salt stress, cotton adapts to osmotic stress through the accumulation of organic acids, amino acids and sugars, while under alkali stress, osmoregulation was achieved via inorganic ion accumulation. Under salt stress, significant metabolic pathways in the leaves and roots were associated with amino acid and organic acid metabolism, sugar metabolism was mainly used as a source of energy, while under alkali stress, the pathways in the leaves were related to amino acid and linoleic acid metabolism, β-Oxidation, TCA cycle, and glycolysis were enhanced to provide the energy needed for life activities. Enhancing organic acid accumulation and metabolism in the roots is the key response mechanism of cotton to alkalinity.

OsbZIP18, a Positive Regulator of Serotonin Biosynthesis, Negatively Controls the UV-B Tolerance in Rice

Serotonin (5-hydroxytryptamine) plays an important role in many developmental processes and biotic/abiotic stress responses in plants. Although serotonin biosynthetic pathways in plants have been uncovered, knowledge of the mechanisms of serotonin accumulation is still limited, and no regulators have been identified to date. Here, we identified the basic leucine zipper transcription factor OsbZIP18 as a positive regulator of serotonin biosynthesis in rice. Overexpression of OsbZIP18 strongly induced the levels of serotonin and its early precursors (tryptophan and tryptamine), resulting in stunted growth and dark-brown phenotypes. A function analysis showed that OsbZIP18 activated serotonin biosynthesis genes (including tryptophan decarboxylase 1 (OsTDC1), tryptophan decarboxylase 3 (OsTDC3), and tryptamine 5-hydroxylase (OsT5H)) by directly binding to the ACE-containing or G-box cis-elements in their promoters. Furthermore, we demonstrated that OsbZIP18 is induced by UV-B stress, and experiments using UV-B radiation showed that transgenic plants overexpressing OsbZIP18 exhibited UV-B stress-sensitive phenotypes. Besides, exogenous serotonin significantly exacerbates UV-B stress of OsbZIP18_OE plants, suggesting that the excessive accumulation of serotonin may be responsible for the sensitivity of OsbZIP18_OE plants to UV-B stress. Overall, we identified a positive regulator of serotonin biosynthesis and demonstrated that UV-B-stress induced serotonin accumulation, partly in an OsbZIP18-dependent manner.

Overexpression of the Salix matsudana SmAP2-17 gene improves Arabidopsis salinity tolerance by enhancing the expression of SOS3 and ABI5

BACKGROUND

Salix matsudana (Koidz.) is a widely planted ornamental allotetraploid tree species. Genetic engineering can be used to enhance the tolerance of this species to soil salinization, endowing varieties with the ability to grow along coastlines, thereby mitigating afforestation and protecting the environment. The AP2/ERF family of transcription factors (TFs) plays multidimensional roles in plant biotic/abiotic stress tolerance and plant development. In this study, we cloned the SmAP2-17 gene and performed functional analysis of its role in salt tolerance. This study aims to identify key genes for future breeding of stress-resistant varieties of Salix matsudana.

RESULTS

SmAP2-17 was predicted to be a homolog of AP2-like ethylene-responsive transcription factor ANT isoform X2 from Arabidopsis, with a predicted ORF of 2058 bp encoding an estimated protein of 685 amino acids containing two conserved AP2 domains (PF00847.20). SmAP2-17 had a constitutive expression pattern and was localized to the nucleus. The overexpression of the native SmAP2-17 CDS sequence in Arabidopsis did not increase salt tolerance because of the reduced expression level of ectopic SmAP2-17, potentially caused by salt-induced RNAi. Transgenic lines with high expression of optimized SmAP2-17 CDS under salt stress showed enhanced tolerance to salt. Moreover, the expression of general stress marker genes and important salt stress signaling genes, including RD29A, ABI5, SOS3, AtHKT1, and RBohF, were upregulated in SmAP2-17-overexpressed lines, with expression levels consistent with that of SmAP2-17 or optimized SmAP2-17. Promoter activity analysis using dual luciferase analysis showed that SmAP2-17 could bind the promoters of SOS3 and ABI5 to activate their expression, which plays a key role in regulating salt tolerance.

CONCLUSIONS

The SmAP2-17 gene isolated from Salix matsudana (Koidz.) is a positive regulator that improves the resistance of transgenic plants to salt stress by upregulating SOS3 and ABI5 genes. This study provides a potential functional gene resource for future generation of salt-resistant Salix lines by genetic engineering.

Exploring the correlation between salt tolerance and yield: research advances and perspectives for salt-tolerant forage sorghum selection and genetic improvement

Some salt stress response mechanisms can translate into sorghum forage yield and thus act as targets for genetic improvement. Sorghum is a drought-tolerant cereal that is widely grown in the vast Africa's arid and semi-arid areas. Apart from drought, salinity is a major abiotic factor that, in addition to natural causes, has been exacerbated by increased poor anthropological activities. The importance of sorghum as a forage crop in saline areas has yet to be fully realized. Despite intraspecific variation in salt tolerance, sorghum is generally moderately salt-tolerant, and its productivity in saline soils can be remarkably limited. This is due to the difficulty of replicating optimal field saline conditions due to the great heterogeneity of salt distribution in the soil. As a promising fodder crop for saline areas, classic phenotype-based selection methods can be integrated with modern -omics in breeding programs to simultaneously address salt tolerance and production. To enable future manipulation, selection, and genetic improvement of sorghum with high yield and salt tolerance, here, we explore the potential positive correlations between the reliable indices of sorghum performance under salt stress at the phenotypic and genotypic level. We then explore the potential role of modern selection and genetic improvement programs in incorporating these linked salt tolerance and yield traits and propose a mechanism for future studies.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

Transcript profiling of stress-responsive genes and metabolic changes during salinity in indica and japonica rice exhibit distinct varietal difference

In the present study, we carried out comprehensive transcript profiling of diverse genes under salinity (200 mM NaCl) at different time points, accompanied by certain biochemical alterations of the indica (IR-64 and Pokkali) and japonica (Nipponbare and M-202) rice. The higher susceptibility of Nipponbare and IR-64 was reflected by lower relative water content, chlorophyll loss, higher malondialdehyde content, and accumulation of H₂ O₂ , and reduced nitrate reductase activity, compared to M-202 and Pokkali, where such changes were less pronounced. Enhanced levels of anthocyanins and reduced glutathione, together with elevated phenylalanine ammonia lyase activity, mainly conferred protection to Nipponbare and IR-64, while metabolites like phenolics, flavonoids, proline, and polyamines were more induced in M-202 and Pokkali. Varietal differences in the expression pattern of diverse groups of genes during different durations (6, 24, and 48 h) of stress were striking. A gene showing early induction for a particular variety exhibited a delayed induction in another variety or a gradually decreased expression with treatment time. Pokkali was clearly identified as the salt-tolerant genotype among the examined varieties based on increased antioxidant potential and enhanced expression of genes encoding for PAL, CHS, and membrane transporters like SOS3, NHX-1, and HKT-1. The results presented in this work provide insight into the complex varying regulation patterns for different genes across the investigated rice varieties in providing salt tolerance and highlights distinct differences in expression patterns between susceptible and tolerant indica and japonica rice.

Genetic, Epigenetic, Genomic and Microbial Approaches to Enhance Salt Tolerance of Plants: A Comprehensive Review

Simple Summary

Globally, soil salinity, which refers to salt-affected soils, is increasing due to various environmental factors and human activities. Soil salinity poses one of the most serious challenges in the field of agriculture as it significantly reduces the growth and yield of crop plants, both quantitatively and qualitatively. Over the last few decades, several studies have been carried out to understand plant biology in response to soil salinity stress with a major emphasis on genetic and other hereditary components. Based on the outcome of these studies, several approaches are being followed to enhance plants’ ability to tolerate salt stress while still maintaining reasonable levels of crop yields. In this manuscript, we comprehensively list and discuss various biological approaches being followed and, based on the recent advances in the field of molecular biology, we propose some new approaches to improve salinity tolerance of crop plants. The global scientific community can make use of this information for the betterment of crop plants. This review also highlights the importance of maintaining global soil health to prevent several crop plant losses.

Abstract

Globally, soil salinity has been on the rise owing to various factors that are both human and environmental. The abiotic stress caused by soil salinity has become one of the most damaging abiotic stresses faced by crop plants, resulting in significant yield losses. Salt stress induces physiological and morphological modifications in plants as a result of significant changes in gene expression patterns and signal transduction cascades. In this comprehensive review, with a major focus on recent advances in the field of plant molecular biology, we discuss several approaches to enhance salinity tolerance in plants comprising various classical and advanced genetic and genetic engineering approaches, genomics and genome editing technologies, and plant growth-promoting rhizobacteria (PGPR)-based approaches. Furthermore, based on recent advances in the field of epigenetics, we propose novel approaches to create and exploit heritable genome-wide epigenetic variation in crop plants to enhance salinity tolerance. Specifically, we describe the concepts and the underlying principles of epigenetic recombinant inbred lines (epiRILs) and other epigenetic variants and methods to generate them. The proposed epigenetic approaches also have the potential to create additional genetic variation by modulating meiotic crossover frequency.

Analysis of Salinity Tolerance in Tomato Introgression Lines Based on Morpho-Physiological and Molecular Traits

The development of salt-tolerant tomato genotypes is a basic requirement to overcome the challenges of tomato production under salinity in the field or soil-free farming. Two groups of eight tomato introgression lines (ILs) each, were evaluated for salinity tolerance. Group-I and the group-II resulted from the following crosses respectively: Solanum lycopersicum cv-6203 × Solanum habrochaites and Solanum lycopersicum M82 × Solanum pennellii. Salt tolerance level was assessed based on a germination percentage under NaCl (0, 75, 100 mM) and in the vegetative stage using a hydroponic growing system (0, 120 mM NaCl). One line from group I (TA1648) and three lines from group II (IL2-1, IL2-3, and IL8-3) were shown to be salt-tolerant since their germination percentages were significantly higher at 75 and 100 mM NaCl than that of their respective cultivated parents cvE6203 and cvM82. Using the hydroponic system, IL TA1648 and IL 2-3 showed the highest value of plant growth traits and chlorophyll concentration. The expression level of eight salt-responsive genes in the leaves and roots of salt-tolerant ILs (TA1648 and IL 2-3) was estimated. Interestingly, SlSOS1, SlNHX2, SlNHX4, and SlERF4 genes were upregulated in leaves of both TA1648 and IL 2-3 genotypes under NaCl stress. While SlHKT1.1, SlNHX2, SlNHX4, and SlERF4 genes were upregulated under salt stress in the roots of both TA1648 and IL 2-3 genotypes. Furthermore, SlSOS2 and SlSOS3 genes were upregulated in TA1648 root and downregulated in IL 2-3. On the contrary, SlSOS1 and SlHKT1.2 genes were upregulated in the IL 2-3 root and downregulated in the TA1648 root. Monitoring of ILs revealed that some of them have inherited salt tolerance from S. habrochaites and S. pennellii genetic background. These ILs can be used in tomato breeding programs to develop salt-tolerant tomatoes or as rootstocks in grafting techniques under saline irrigation conditions.

Positive Salt Tolerance Modulation via Vermicompost Regulation of SOS1 Gene Expression and Antioxidant Homeostasis in Viciafaba Plant

Strategic implementation of vermicompost as safe biofertilizer besides defensing saline soils offer dual function solving problems in developing countries. The current study aims to utilize vermicompost (VC) for amelioration of 200mM NaCl in Vicia faba Aspani cultivar and investigate the molecular role of salt overly sensitive pathway (SOS1). The experiment was conducted following a completely randomized design with three replicates. Treatments include 0; 2.5; 5; 10; 15% dried VC intermingled with soil mixture (clay: sand; 1:2) and/or 200 mM NaCl. The results show that salinity stress decreased broad bean fresh and dry weight; and K⁺/Na⁺. However, malonedialdehyde and H₂O₂ contents; increased. Application of 10% VC and salinity stress increases Ca²⁺ (41% and 50%), K⁺/Na⁺ (125% and 89%), Mg²⁺ (25% and 36%), N (8% and 11%), indole acetic acid (70% and 152%) and proteins (9% and 13%) for root and shoot, respectively, in comparison to salt treated pots. Moreover, all examined enzymatic antioxidants and their substrates increased, except glutathione reductase. A parallel decrease in abscisic acid (75% and 29%) and proline (59% and 58%) was also recorded for roots and leaves, respectively. Interestingly, the highly significant increase in gene expression of SOS1 (45-fold) could drive defense machinery of broad bean to counteract 200 mM NaCl.

Enhanced Resistance of atbzip62 against Pseudomonas syringae pv. tomato Suggests Negative Regulation of Plant Basal Defense and Systemic Acquired Resistance by AtbZIP62 Transcription Factor

The intrinsic defense mechanisms of plants toward pathogenic bacteria have been widely investigated for years and are still at the center of interest in plant biosciences research. This study investigated the role of the AtbZIP62 gene encoding a transcription factor (TF) in the basal defense and systemic acquired resistance in Arabidopsis using the reverse genetics approach. To achieve that, the atbzip62 mutant line (lacking the AtbZIP62 gene) was challenged with Pseudomonas syringae pv. tomato (Pst DC3000) inoculated by infiltration into Arabidopsis leaves at the rosette stage. The results indicated that atbzip62 plants showed an enhanced resistance phenotype toward Pst DC3000 vir over time compared to Col-0 and the susceptible disease controls, atgsnor1-3 and atsid2. In addition, the transcript accumulation of pathogenesis-related genes, AtPR1 and AtPR2, increased significantly in atbzip62 over time (0–72 h post-inoculation, hpi) compared to that of atgsnor1-3 and atsid2 (susceptible lines), with AtPR1 prevailing over AtPR2. When coupled with the recorded pathogen growth (expressed as a colony-forming unit, CFU mL⁻¹), the induction of PR genes, associated with the salicylic acid (SA) defense signaling, in part explained the observed enhanced resistance of atbzip62 mutant plants in response to Pst DC3000 vir. Furthermore, when Pst DC3000 avrB was inoculated, the expression of AtPR1 was upregulated in the systemic leaves of Col-0, while that of AtPR2 remained at a basal level in Col-0. Moreover, the expression of AtAZI (a systemic acquired resistance -related) gene was significantly upregulated at all time points (0–24 h post-inoculation, hpi) in atbzip62 compared to Col-0 and atgsnor1-3 and atsid2. Under the same conditions, AtG3DPH exhibited a high transcript accumulation level 48 hpi in the atbzip62 background. Therefore, all data put together suggest that AtPR1 and AtPR2 coupled with AtAZI and AtG3DPH, with AtAZI prevailing over AtG3DPH, would contribute to the recorded enhanced resistance phenotype of the atbzip62 mutant line against Pst DC3000. Thus, the AtbZIP62 TF is proposed as a negative regulator of basal defense and systemic acquired resistance in plants under Pst DC3000 infection.

Regulation of Nitrate (NO3) Transporters and Glutamate Synthase-Encoding Genes under Drought Stress in Arabidopsis: The Regulatory Role of AtbZIP62 Transcription Factor

Nitrogen (N) is an essential macronutrient, which contributes substantially to the growth and development of plants. In the soil, nitrate (NO₃) is the predominant form of N available to the plant and its acquisition by the plant involves several NO₃ transporters; however, the mechanism underlying their involvement in the adaptive response under abiotic stress is poorly understood. Initially, we performed an in silico analysis to identify potential binding sites for the basic leucine zipper 62 transcription factor (AtbZIP62 TF) in the promoter of the target genes, and constructed their protein–protein interaction networks. Rather than AtbZIP62, results revealed the presence of cis-regulatory elements specific to two other bZIP TFs, AtbZIP18 and 69. A recent report showed that AtbZIP62 TF negatively regulated AtbZIP18 and AtbZIP69. Therefore, we investigated the transcriptional regulation of AtNPF6.2/NRT1.4 (low-affinity NO₃ transporter), AtNPF6.3/NRT1.1 (dual-affinity NO₃ transporter), AtNRT2.1 and AtNRT2.2 (high-affinity NO₃ transporters), and AtGLU1 and AtGLU2 (both encoding glutamate synthase) in response to drought stress in Col-0. From the perspective of exploring the transcriptional interplay of the target genes with AtbZIP62 TF, we measured their expression by qPCR in the atbzip62 (lacking the AtbZIP62 gene) under the same conditions. Our recent study revealed that AtbZIP62 TF positively regulates the expression of AtPYD1 (Pyrimidine 1, a key gene of the de novo pyrimidine biosynthesis pathway know to share a common substrate with the N metabolic pathway). For this reason, we included the atpyd1-2 mutant in the study. Our findings revealed that the expression of AtNPF6.2/NRT1.4, AtNPF6.3/NRT1.1 and AtNRT2.2 was similarly regulated in atzbip62 and atpyd1-2 but differentially regulated between the mutant lines and Col-0. Meanwhile, the expression pattern of AtNRT2.1 in atbzip62 was similar to that observed in Col-0 but was suppressed in atpyd1-2. The breakthrough is that AtNRT2.2 had the highest expression level in Col-0, while being suppressed in atbzip62 and atpyd1-2. Furthermore, the transcript accumulation of AtGLU1 and AtGLU2 showed differential regulation patterns between Col-0 and atbzip62, and atpyd1-2. Therefore, results suggest that of all tested NO₃ transporters, AtNRT2.2 is thought to play a preponderant role in contributing to NO₃ transport events under the regulatory influence of AtbZIP62 TF in response to drought stress.

RNA-Seq and Electrical Penetration Graph Revealed the Role of Grh1-Mediated Activation of Defense Mechanisms towards Green Rice Leafhopper (Nephotettix cincticeps Uhler) Resistance in Rice (Oryza sativa L.)

The green rice leafhopper (GRH, Nephotettix cincticeps Uhler) is one of the most important insect pests causing serious damage to rice production and yield loss in East Asia. Prior to performing RNA-Seq analysis, we conducted an electrical penetration graph (EPG) test to investigate the feeding behavior of GRH on Ilpum (recurrent parent, GRH-susceptible cultivar), a near-isogenic line (NIL carrying Grh1) compared to the Grh1 donor parent (Shingwang). Then, we conducted a transcriptome-wide analysis of GRH-responsive genes in Ilpum and NIL, which was followed by the validation of RNA-Seq data by qPCR. On the one hand, EPG results showed differential feeding behaviors of GRH between Ilpum and NIL. The phloem-like feeding pattern was detected in Ilpum, whereas the EPG test indicated a xylem-like feeding habit of GRH on NIL. In addition, we observed a high death rate of GRH on NIL (92%) compared to Ilpum (28%) 72 h post infestation, attributed to GRH failure to suck the phloem sap of NIL. On the other hand, RNA-Seq data revealed that Ilpum and NIL GRH-treated plants generated 1,766,347 and 3,676,765 counts per million mapped (CPM) reads, respectively. The alignment of reads indicated that more than 75% of reads were mapped to the reference genome, and 8859 genes and 15,815,400 transcripts were obtained. Of this number, 3424 differentially expressed genes (DEGs, 1605 upregulated in Ilpum and downregulated in NIL; 1819 genes upregulated in NIL and downregulated in Ilpum) were identified. According to the quantile normalization of the fragments per kilobase of transcript per million mapped reads (FPKM) values, followed by the Student’s t-test (p < 0.05), we identified 3283 DEGs in Ilpum (1935 upregulated and 1348 downregulated) and 2599 DEGs in NIL (1621 upregulated and 978 downregulated) with at least a log₂ (logarithm base 2) twofold change (Log₂FC ≥2) in the expression level upon GRH infestation. Upregulated genes in NIL exceeded by 13.3% those recorded in Ilpum. The majority of genes associated with the metabolism of carbohydrates, amino acids, lipids, nucleotides, the activity of coenzymes, the action of phytohormones, protein modification, homeostasis, the transport of solutes, and the uptake of nutrients, among others, were abundantly upregulated in NIL (carrying Grh1). However, a high number of upregulated genes involved in photosynthesis, cellular respiration, secondary metabolism, redox homeostasis, protein biosynthesis, protein translocation, and external stimuli response related genes were found in Ilpum. Therefore, all data suggest that Grh1-mediated resistance against GRH in rice would involve a transcriptome-wide reprogramming, resulting in the activation of bZIP, MYB, NAC, bHLH, WRKY, and GRAS transcription factors, coupled with the induction of the pathogen-pattern triggered immunity (PTI), systemic acquired resistance (SAR), symbiotic signaling pathway, and the activation of genes associated with the response mechanisms against viruses. This comprehensive transcriptome profile of GRH-responsive genes gives new insights into the molecular response mechanisms underlying GRH (insect pest)–rice (plant) interaction.

Ion homeostasis and Na+ transport-related gene expression in two cotton (Gossypium hirsutum L.) varieties under saline, alkaline and saline-alkaline stresses

The sensitivity of cotton to salt stress depends on the genotypes and salt types. Understanding the mechanism of ion homeostasis under different salt stresses is necessary to improve cotton performance under saline conditions. A pot experiment using three salt stresses saline stress (NaCl+Na2SO4), alkaline stress (Na2CO3+NaHCO3), and saline-alkaline stress (NaCl+Na2SO4+Na2CO3+NaHCO3) and two cotton varieties (salt-tolerant variety L24 and salt-sensitive variety G1) was conducted. The growth, ion concentrations, and Na+ transport-related gene expression in the cotton varieties were determined. The inhibitory effects of saline-alkaline stress on cotton growth were greater than that of either saline stress or alkaline stress alone. The root/shoot ratio under alkaline stress was significantly lower than that under saline stress. The salt-tolerant cotton variety had lower Na and higher K concentrations in the leaves, stems and roots than the salt-sensitive variety under different salt stresses. For the salt-sensitive cotton variety, saline stress significantly inhibited the absorption of P and the transport of P, K, and Mg, while alkaline stress and saline-alkaline stress significantly inhibited the uptake and transport of P, K, Ca, Mg, and Zn. Most of the elements in the salt-tolerant variety accumulated in the leaves and stems under different salt stresses. This indicated that the salt-tolerant variety had a stronger ion transport capacity than the salt-sensitive variety under saline conditions. Under alkaline stress and salt-alkaline stress, the relative expression levels of the genes GhSOS1, GhNHX1 and GhAKT1 in the salt-tolerant variety were significantly higher than that in the salt-sensitive variety. These results suggest that this salt-tolerant variety of cotton has an internal mechanism to maintain ionic homeostasis.

Identification and Analysis of bZIP Family Genes in Potato and Their Potential Roles in Stress Responses

The bZIP proteins comprise one of the largest transcription factor families and play important roles in plant growth and development, senescence, metabolic reactions, and stress responses. In this study, 49 bZIP transcription factor-encoding genes (StbZIP genes) on the potato genome were identified and analyzed. The 49 StbZIP genes, which are located on 12 chromosomes of the potato genome, were divided into 11 subgroups together with their Arabidopsis homologs based on the results of phylogenetic analysis. Gene structure and protein motif analysis revealed that members from the same subgroup often possessed similar exon/intron structures and motif organizations, further supporting the results of the phylogenetic analysis. Syntenic analysis indicated the existence of gene duplication events, which might play an important role in the expansion of the bZIP gene family in potato. Expressions of the StbZIP genes were analyzed in a variety of tissues via RNA-Seq data, suggesting functional diversity. Several StbZIP genes were found to be induced by different stress conditions. For example, the expression of StbZIP25, the close homolog of AtbZIP36/ABF2, was significantly upregulated by salt stress treatments. The StbZIP25 protein was found to be located in the nucleus and function as a transcriptional activator. Overexpression of StbZIP25 enhanced salt tolerance in Arabidopsis. The results from this study imply potential roles of the bZIP family genes in the stress response of potato.

Genome-wide characterization of bZIP transcription factors and their expression patterns in response to drought and salinity stress in Jatropha curcas

The basic leucine zipper (bZIP) family is one of the largest families of transcription factors (TFs) in plants and is responsible for various functions, including regulating development and responses to abiotic/biotic stresses. However, the roles of bZIPs in the regulation of responses to drought stress and salinity stress remain poorly understood in Jatropha curcas L., a biodiesel crop. In the present study, 50 JcbZIP genes were identified and classified into ten groups. Cis-element analysis indicated that JcbZIP genes are associated with abiotic stress. Gene expression patterns and quantitative real-time PCR (qRT-PCR) showed that four JcbZIP genes (JcbZIPs 34, 36, 49 and 50) are key resistance-related genes under both drought and salinity stress conditions. On the basis of the results of cis-element and phylogenetic analyses, JcbZIP49 and JcbZIP50 are likely involved in responses to drought and salinity stress; moreover, JcbZIP34 and JcbZIP36 might also play important roles in seed development and response to abiotic stress. These findings advance our understanding of the comprehensive characteristics of JcbZIP genes and provide new insights for functional validation in the further.

New Insights into the Transcriptional Regulation of Genes Involved in the Nitrogen Use Efficiency under Potassium Chlorate in Rice (Oryza sativa L.)

Potassium chlorate (KClO3) has been widely used to evaluate the divergence in nitrogen use efficiency (NUE) between indica and japonica rice subspecies. This study investigated the transcriptional regulation of major genes involved in the NUE in rice treated with KClO3, which acts as an inhibitor of the reducing activity of nitrate reductase (NR) in higher plants. A set of two KClO3 sensitive nitrate reductase (NR) and two nitrate transporter (NRT) introgression rice lines (BC2F7), carrying the indica alleles of NR or NRT, derived from a cross between Saeilmi (japonica, P1) and Milyang23 (indica, P2), were exposed to KClO3 at the seedling stage. The phenotypic responses were recorded 7 days after treatment, and samples for gene expression, physiological, and biochemical analyses were collected at 0 h (control) and 3 h after KClO3 application. The results revealed that Saeilmi (P1, japonica) and Milyang23 (P2, indica) showed distinctive phenotypic responses. In addition, the expression of OsNR2 was differentially regulated between the roots, stem, and leaf tissues, and between introgression lines. When expressed in the roots, OsNR2 was downregulated in all introgression lines. However, in the stem and leaves, OsNR2 was upregulated in the NR introgression lines, but downregulation in the NRT introgression lines. In the same way, the expression patterns of OsNIA1 and OsNIA2 in the roots, stem, and leaves indicated a differential transcriptional regulation by KClO3, with OsNIA2 prevailing over OsNIA1 in the roots. Under the same conditions, the activity of NR was inhibited in the roots and differentially regulated in the stem and leaf tissues. Furthermore, the transcriptional divergence of OsAMT1.3 and OsAMT2.3, OsGLU1 and OsGLU2, between NR and NRT, coupled with the NR activity pattern in the roots, would indicate the prevalence of nitrate (NO3¯) transport over ammonium (NH4+) transport. Moreover, the induction of catalase (CAT) and polyphenol oxidase (PPO) enzyme activities in Saeilmi (P1, KClO3 resistant), and the decrease in Milyang23 (P2, KClO3 sensitive), coupled with the malondialdehyde (MDA) content, indicated the extent of the oxidative stress, and the induction of the adaptive response mechanism, tending to maintain a balanced reduction-oxidation state in response to KClO3. The changes in the chloroplast pigments and proline content propose these compounds as emerging biomarkers for assessing the overall plant health status. These results suggest that the inhibitory potential of KClO3 on the reduction activity of the nitrate reductase (NR), as well as that of the genes encoding the nitrate and ammonium transporters, and glutamate synthase are tissue-specific, which may differentially affect the transport and assimilation of nitrate or ammonium in rice.

Insights into the Transcriptional Regulation of Branching Hormonal Signaling Pathways Genes under Drought Stress in Arabidopsis

A large number of hormonal biosynthetic or signaling pathways genes controlling shoot branching are widely known for their roles in regulating plant growth and development, operating in synergetic or antagonistic manner. However, their involvement in abiotic stress response mechanism remains unexplored. Initially, we performed an in silico analysis to identify potential transcription binding sites for the basic leucine zipper 62 transcription factor (bZIP62 TF) in the target branching related genes. The results revealed the presence of cis-regulatory elements specific to two bZIP TFs, AtbZIP18 and AtbZIP69, rather than AtbZIP62. Interestingly, these bZIP TFs were previously proposed to be negatively regulated by the AtbZIP62 TF under salinity in Arabidopsis. Therefore, we investigated the transcriptional regulation of more axillary branching (MAX, strigolactone), PIN-FORMED (PINs, auxin carriers), gibberellic acid (GA)-biosynthetic genes as well as isopentenyltransferase (IPT, cytokinin biosynthesis pathway) genes in response to drought stress in Arabidopsis Col-0 wild type. In addition, in the perspective of exploring the transcriptional interplay of the selected genes with the AtbZIP62, we measured their expression by qPCR in the atbzip62 (lacking the AtbZIP62 gene) background under the same conditions. Our findings revealed that the expression of AtMAX2, AtMAX3, and AtMAX4 was differentially regulated by drought stress between the atbzip62 and Col-0 wild type, but not AtMAX1. Similarly, the transcripts accumulation of AtPIN3 and AtPIN7 (known as auxin efflux carriers), and that of the AtAXR1 showed similar regulation patterns in atbzip62. However, AtPIN1 expression was downregulated in Col-0, but no change was observed in atbzip62. Furthermore, AtIPT5 and AtIPT7 exhibited a differential transcripts accumulation pattern in atbzip62 and Col-0 wild type (WT). In the same way, the expression of the GA biosynthetic genes AtGA2ox1 and AtGA20ox2, and that of AtRGA1 were differentially regulated in atbzip62 compared to the Col-0. Meanwhile, AtGA2ox1 showed a similar expression pattern with Col-0. Therefore, all results suggest PIN, MAX, IPT, and GA-biosynthetic genes, which are differentially regulated by AtbZIP62 transcription factor, as emerging candidate genes that could be involved in drought stress response mechanism in Arabidopsis.

Monitoring of Salinity, Temperature, and Drought Stress in Grafted Watermelon Seedlings Using Chlorophyll Fluorescence

Chlorophyll fluorescence (CF) is used to measure the physiological status of plants affected by biotic and abiotic stresses. Therefore, we aimed to identify the changes in CF parameters in grafted watermelon seedlings exposed to salt, drought, and high and low temperatures. Grafted watermelon seedlings at the true three-leaf stage were subjected to salinity levels (0, 50, 100, 150, and 200 mM) and temperature [low (8°C), moderate (24°C), and high (40°C)] stresses for 12 days under controlled environmental conditions independently. Eight CF parameters were measured at 2-day intervals using the FluorCam machine quenching protocol of the FluorCam machine. The seedlings were also exposed to drought stress for 3 days independent of salinity and temperature stress; CF parameters were measured at 1-day intervals. In addition, growth parameters, proline, and chlorophyll content were evaluated in all three experiments. The CF parameters were differentially influenced depending on the type and extent of the stress conditions. The results showed a notable effect of salinity levels on CF parameters, predominantly in maximum quantum yield (Fv/Fm), non-photochemical quenching (NPQ), the ratio of the fluorescence decrease (Rfd), and quantum yield of non-regulated energy dissipation in PSII [Y(NO)]. High temperature had significant effects on Rfd and NPQ, whereas low temperature showed significant results in most CF parameters: Fv/Fm, Y(NO), NPQ, Rfd, the efficiency of excitation capture of open photosystem II (PSII) center (Fv'/Fm'), and effective quantum yield of photochemical energy conversion in PSII [Y(PSII)]. Only NPQ and Rfd were significantly influenced by severe drought stress. Approximately, all the growth parameters were significantly influenced by the stress level. Proline content increased with an increase in stress levels in all three experiments, whereas the chlorophyll (a and b) content either decreased or increased depending upon the stressor. The results provided here may be useful for understanding the effect of abiotic stresses on CF parameters and the selection of index CF parameters to detect abiotic stresses in grafted watermelon seedlings.

Drought-induced AtbZIP62 transcription factor regulates drought stress response in Arabidopsis

We investigated the role of AtbZIP62, an uncharacterized Arabidopsis bZIP TF, in oxidative, nitro-oxidative and drought stress conditions using reverse genetics approach. We further monitored the expression of AtPYD1 gene (orthologous to rice OsDHODH1 involved in the pyrimidine biosynthesis) in atbzip62 knock-out (KO) plants in order to investigate the transcriptional interplay of AtbZIP62 and AtPYD1. The atbzip62 KO plants showed significant increase in shoot length under oxidative stress, while no significant difference was recorded for root length compared to WT. However, under nitro-oxidative stress conditions, atbzip62 showed differential response to both NO-donors. Further characterization of AtbZIP62 under drought conditions showed that both atbzip62 and atpyd1-2 showed a sensitive phenotype to drought stress, and could not recover after re-watering. Transcript accumulation of AtbZIP62 and AtPYD1 showed that both were highly up-regulated by drought stress in wild type (WT) plants. Interestingly, AtPYD1 transcriptional level significantly decreased in atbzip62 exposed to drought stress. However, AtbZIP62 expression was highly induced in atpyd1-2 under the same conditions. Both AtbZIP62 and AtPYD1 were up-regulated in atnced3 and atcat2 while showing a contrasting expression pattern in atgsnor1-3. The recorded increase in CAT, POD, and PPO-like activities, the accumulation of chlorophylls and total carotenoids, and the enhanced proline and malondialdehyde levels would explain the sensitivity level of atbzip62 towards drought stress. All results collectively suggest that AtbZIP62 could be involved in AtPYD1 transcriptional regulation while modulating cellular redox state and photosynthetic processes. In addition, AtbZIP62 is suggested to positively regulate drought stress response in Arabidopsis.