Learning from evolution: Thellungiella generates new knowledge on essential and critical components of abiotic stress tolerance in plants

Recent Advances in Adaptation Genomics in Fumarole Fields: An Overlooked Extreme Environment

Extreme environments and plants thriving in them, known as extremophytes, offer promising platforms for studying the diverse adaptive mechanisms that have evolved in plants. However, research on adaptation to extreme environments is still limited to those environments where model species or their relative can survive. Fumarole fields, an extreme environment often overlooked, are characterized by multi-hazardous abiotic stressors, including atmospheric contamination (high concentration of H2S, SO2 and CO2), high soil temperature (∼60°C) and strong soil acidification (pH = 2-3). These conditions make fumarole fields a rich source for studying stress tolerance mechanisms in plants. In this review, we highlight the recent ecological, physiological and genomic advances involved in fumarole field adaptation and discuss the forward avenues. The studies outlined in this paper demonstrate that the extreme levels of abiotic stressors found in fumarole fields make them unparalleled field laboratories for studying the unknown stress tolerance mechanisms, warranting further genomic assessments. Some studies succeeded in identifying genes associated with fumarole field adaptation and shedding light on evolutionary implications; however, they have also encountered challenges such as limited genome resources and high genetic differentiation from related species and/or neighboring populations. To overcome such difficulties, we propose integrating ecophysiological and genomic approaches, drawing from the recent studies in other extreme environments. We expect that further studies in the fumarole fields will contribute to broadening our general knowledge of the limits of life.

Overexpression of auxin early response gene LcSAUR1 (Leymus chinensis) increases sensitivity to alkali and drought stresses in Arabidopsis and rice

Alkali and drought stresses are two common abiotic factors affecting plants growth and development. Auxin signal also regulates plant responses to abiotic stresses. Especially, auxin early response genes can quickly respond after sensing auxin signal. However, auxin early response genes related to alkali and drought stresses are rarely reported in Leymus chinensis. In this study, LcSAUR1 (small auxin-up RNA) was isolated from the difference expression analysis of the transcriptome data in Leymus chinensis under alkali and drought stresses. And LcSAUR1 exhibited inhibitory expression under alkali and drought stresses. Further research showed that LcSAUR1 was localized in the nucleus, cell membrane, and chloroplast, suggesting that it might has special biofunction. Overexpression of LcSAUR1 led to shorter root lengths in LcSAUR1-transgenic Arabidopsis and rice. Under alkali and drought stresses, the OE-LcSAUR1-Col lines showed delayed germination and larger stomatal aperture, and the OE-LcSAUR1-NIP lines had lower survival rates. The determination of physiological indicators including hydrogen peroxide (H2O2), malondialdehyde (MDA), catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), the contents of proline (PRO), and the staining of nitro-blue tetrazolium (NBT) and Diaminobenzidine (DAB) indicated that the overexpressed LcSAUR1-transgenic Arabidopsis and rice produced more reactive oxygen species (ROS). In addition, for the genes related to abiotic stresses, the expression of AtSnRK2.6, AtNCEB3, AtCAT2, and AtAPX1 in the OE-LcSAUR1-Col lines, and OsLEA3-2, OsABF1, OsCAT2, and OsAPX2 in the OE-LcSAUR1-NIP lines were all lower than their WT under alkali and drought stresses, suggesting that LcSAUR1 regulates alkali and drought tolerances might through those abiotic-related genes. The study suggests that the LcSAUR1 negatively regulates alkali and drought stresses, providing a novel insight into auxin signal and abiotic stresses.

Birch (Betula platyphylla) BES/BZR transcription factor BpBZR1-6 improves salt tolerance in transgenic Arabidopsis thaliana

BACKGROUND

Salt stress is one of the major environmental factors affecting plant growth and productivity. BRI1-EMS suppressor 1/brassinazole-resistant 1 ((BES1/BZR1) plays an important role in responding to abiotic stress in plants. Although the impacts of BES1/BZR1 on plant growth and resistance have been documented, the potential mechanisms are not fully elucidated in Betula platyphylla. This work contributes to the understanding of how BES1/BZR1 promotes stress tolerance in woody plants.

RESULTS

Six BES1/BZR1 family members were identified from Betula platyphylla. Cis-element analysis showed that the promoters of six genes were rich in ABA-responsive element (ABRE), MYB and MBS cis-acting elements, which are reported to be involved in abiotic stress responses. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that BpBZR1-6 (BPChr10G06000) could be induced by salt stress, ABA and BRs. BpBZR1-6 was localized in the nucleus and had transactivation activity. Ectopic expression of BpBZR1-6 enhanced Arabidopsis tolerance and decreased abscisic acid (ABA) sensitivity under salt treatment. Specifically, the seed germination rate, root length, fresh weight and chlorophyll content were significantly higher in BpBZR1-6-overexpressing (OE) transgenic plants than in wild-type (WT) plants after salt stress (P < 0.05). Additionally, BpBZR1-6 overexpression showed enhanced the reactive oxygen species (ROS) scavenging capability under salt stress, including increasing the activities of antioxidant enzyme, resulting in a decrease in O2- and H2O2 accumulation, and reducing malondialdehyde (MDA) content. Meanwhile, the expression levels of six antioxidant enzyme genes were higher in OE plants than in WT plants after stress.

CONCLUSION

BpBZR1-6 overexpression enhanced the salt tolerance of transgenic plants by modulating antioxidant enzyme gene expression and ROS scavenging, which may provide underlying strategy for breeding of salt-tolerant plants.

Cakile maritima: A Halophyte Model to Study Salt Tolerance Mechanisms and Potential Useful Crop for Sustainable Saline Agriculture in the Context of Climate Change

Salinity is an increasing problem for agriculture. Most plant species tolerate low or, at best, moderate soil salinities. However, a small (<1%) proportion of species, termed halophytes, can survive and complete their life cycle in natural habitats with salinities equivalent to 200 mM NaCl or more. Cakile maritima is a succulent annual halophyte belonging to the Brassicaceae family; it is dispersed worldwide and mainly grows in foreshores. Cakile maritima growth is optimal under slight (i.e., 100 mM NaCl) saline conditions, measured by biomass and seed production. Higher salt concentrations, up to 500 mM NaCl, significantly impact its growth but do not compromise its survival. Cakile maritima alleviates sodium toxicity through different strategies, including anatomical and morphological adaptations, ion transport regulation, biosynthesis of osmolytes, and activation of antioxidative mechanisms. The species is potentially useful as a cash crop for the so-called biosaline agriculture due to its production of secondary metabolites of medical and nutritional interest and the high oil accumulation in its seeds. In this review, we highlight the relevance of this species as a model for studying the basic mechanisms of salt tolerance and for sustainable biosaline agriculture in the context of soil salination and climate change.

The Small Auxin-Up RNA 50 (SAUR50) Gene from Ammopiptanthus nanus Negatively Regulates Drought Tolerance

Drought stress is a primary abiotic stress that causes significant losses to forestry and agricultural production. Therefore, exploring drought-responsive genes and their regulatory mechanism is crucial for plant molecular breeding for forestry and agriculture production safety. Small auxin-up RNA (SAUR) proteins are essential in plant growth and development but show functional diversity in stress response. In this study, the transcriptome sequencing data of Ammopiptanthus nanus seedlings revealed that the expression of AnSAUR50 was continuously downregulated under drought stress. Hence, the AnSAUR50 gene was cloned and functionally analyzed in drought response. The results showed that the coding sequence of AnSAUR50 was 315 bp in length and encoded 104 amino acids. The AnSAUR50 protein showed high conservation, possessed a SAUR-specific domain, and localized in the nucleus and cell membrane. The heterologous expression of the AnSAUR50 gene enhanced the drought sensitivity of the transgenic Arabidopsis with a lower survival rate, biomass, and higher malondialdehyde content and relative electrolyte leakage. Moreover, transgenic plants showed shorter root lengths and bigger stomatal apertures, resulting in facilitating water loss under drought stress. The study indicates that AnSAUR50 negatively regulates drought tolerance by inhibiting root growth and stomatal closure, which provides insights into the underlying function and regulatory mechanism of SAURs in plant stress response.

Understanding the salinity resilience and productivity of halophytes in saline environments

The escalating salinity levels in cultivable soil pose a significant threat to agricultural productivity and, consequently, human sustenance. This problem is being exacerbated by natural processes and human activities, coinciding with a period of rapid population growth. Developing halophytic crops is needed to ensure food security is not impaired and land resources can be used sustainably. Evolution has created many close halophyte relatives of our major glycophytic crops, such as Puccinellia tenuiflora (relative of barley and wheat), Oryza coarctata (relative of rice) and Glycine soja (relative of soybean). There are also some halophytes have been subjected to semi-domestication and are considered as minor crops, such as Chenopodium quinoa. In this paper, we examine the prevailing comprehension of robust salinity resilience in halophytes. We summarize the existing strategies and technologies that equip researchers with the means to enhance the salt tolerance capabilities of primary crops and investigate the genetic makeup of halophytes.

PHT1;5 Repressed by ANT Mediates Pi Acquisition and Distribution under Low Pi and Salinity in Salt Cress

Salinity and phosphate (Pi) starvation are the most common abiotic stresses that threaten crop productivity. Salt cress (Eutrema salsugineum) displays good tolerance to both salinity and Pi limitation. Previously, we found several Phosphate Transporter (PHT) genes in salt cress upregulated under salinity. Here, EsPHT1;5 induced by both low Pi (LP) and salinity was further characterized. Overexpression of EsPHT1;5 in salt cress enhanced plant tolerance to LP and salinity, while the knock-down lines exhibited growth retardation. The analysis of phosphorus (P) content and shoot/root ratio of total P in EsPHT1;5-overexpressing salt cress seedlings and the knock-down lines as well as arsenate uptake assays suggested the role of EsPHT1;5 in Pi acquisition and root-shoot translocation under Pi limitation. In addition, overexpression of EsPHT1;5 driven by the native promoter in salt cress enhanced Pi mobilization from rosettes to siliques upon a long-term salt treatment. Particularly, the promoter of EsPHT1;5 outperformed that of AtPHT1;5 in driving gene expression under salinity. We further identified a transcription factor EsANT, which negatively regulated EsPHT1;5 expression and plant tolerance to LP and salinity. Taken together, EsPHT1;5 plays an integral role in Pi acquisition and distribution in plant response to LP and salt stress. Further, EsANT may be involved in the cross-talk between Pi starvation and salinity signaling pathways. This work provides further insight into the mechanism underlying high P use efficiency in salt cress in its natural habitat, and evidence for a link between Pi and salt signaling.

Genome-wide identification of PYL/PYR-PP2C (A)-SnRK2 genes in Eutrema and their co-expression analysis in response to ABA and abiotic stresses

ABA signaling core components PYR/PYL, group A PP2C and SnRK2 play important roles in various environmental stress responses of plants. This study identified 14 PYR/PYL, 9 PP2C (A), and 10 SnRK2 genes from halophytic Eutrema. Phylogenetic analysis showed 4 EsPYR/PYL, 4 EsPP2C (A) and 3 EsSnRK2 subfamilies characterized, which was supported by their gene structures and protein motifs. Large-scale segmental duplication event was demonstrated to be a major contributor to expansion of the EsPYL-PP2C (A)-SnRK2 gene families. Synteny relationship analysis revealed more orthologous PYL-PP2C (A)-SnRK2 gene pairs located in collinear blocks between Eutrema and Brassica than that between Eutrema and Arabidopsis. RNA-seq and qRT-PCR revealed EsABI1, EsABI2 and EsHAL2 showed a significantly up-regulated expression in leaves and roots in response to ABA, NaCl or cold stress. Three markedly co-expression modules of ABA/R-brown, NaCl/L-lightsteelblue1 and Cold/R-lightgreen were uncovered to contain EsPYL-PP2C (A)-SnRK2 genes by WGCNA analysis. GO and KEGG analysis indicated that the genes of ABA/R-brown module containing EsHAB1, EsHAI2 and EsSnRK2.6 were enriched in proteasome pathway. Further, EsHAI2-OE transgenic Arabidopsis lines showed significantly enhanced seeds germination and seedlings growth. This work provides a new insight for elucidating potential molecular functions of PYL-PP2C (A)-SnRK2 responding to ABA and abiotic stresses.

Analysis of Raffinose Synthase Gene Family in Bread Wheat and Identification of Drought Resistance and Salt Tolerance Function of TaRS15-3B

Raffinose synthase (RS) plays a crucial role in plant growth and development, as well as in responses to biotic stresses and abiotic stresses, yet few studies have been conducted on its role in bread wheat. Therefore, in this study we screened and identified a family of bread wheat raffinose synthase genes based on bread wheat genome information and analyzed their physicochemical properties, phylogenetic evolutionary relationships, conserved structural domains, promoter cis-acting elements, and expression patterns. The BSMV-induced silencing of TaRS15-3B resulted in the bread wheat seedlings being susceptible to drought and salt stress and reduced the expression levels of stress-related and ROS-scavenging genes in bread wheat plants. This further affected the ability of bread wheat to cope with drought and salt stress. In conclusion, this study revealed that the RS gene family in bread wheat plays an important role in plant response to abiotic stresses and that the TaRS15-3B gene can improve the tolerance of transgenic bread wheat to drought and salt stresses, provide directions for the study of other RS gene families in bread wheat, and supply candidate genes for use in molecular breeding of bread wheat for stress resistance.

A high-quality chromosome-level Eutrema salsugineum genome, an extremophile plant model

BACKGROUND

Eutrema salsugineum (2n = 14), a halophyte in the family Brassicaceae, is an attractive model to study abiotic stress tolerance in plants. Two versions of E. salsugineum genomes that previously reported were based on relatively short reads; thus, the repetitive regions were difficult to characterize.

RESULTS

We report the sequencing and assembly of the E. salsugineum (Shandong accession) genome using long-read sequencing and chromosome conformation capture data. We generated Oxford Nanopore long reads at high depth (> 60X) of genome coverage with additional short reads for error correction. The new assembly has a total size of 295.5 Mb with 52.8% repetitive sequences, and the karyotype of E. salsugineum is consistent with the ancestral translocation Proto-Calepineae Karyotype structure in both order and orientation. Compared with previous assemblies, this assembly has higher contiguity, especially in the centromere region. Based on this new assembly, we predicted 25,399 protein-coding genes and identified the positively selected genes associated with salt and drought stress responses.

CONCLUSION

The new genome assembly will provide a valuable resource for future genomic studies and facilitate comparative genomic analysis with other plants.

Positive selection and heat-response transcriptomes reveal adaptive features of the Brassicaceae desert model, Anastatica hierochuntica

Plant adaptation to a desert environment and its endemic heat stress is poorly understood at the molecular level. The naturally heat-tolerant Brassicaceae species Anastatica hierochuntica is an ideal extremophyte model to identify genetic adaptations that have evolved to allow plants to tolerate heat stress and thrive in deserts. We generated an A. hierochuntica reference transcriptome and identified extremophyte adaptations by comparing Arabidopsis thaliana and A. hierochuntica transcriptome responses to heat, and detecting positively selected genes in A. hierochuntica. The two species exhibit similar transcriptome adjustment in response to heat and the A. hierochuntica transcriptome does not exist in a constitutive heat 'stress-ready' state. Furthermore, the A. hierochuntica global transcriptome as well as heat-responsive orthologs, display a lower basal and higher heat-induced expression than in A. thaliana. Genes positively selected in multiple extremophytes are associated with stomatal opening, nutrient acquisition, and UV-B induced DNA repair while those unique to A. hierochuntica are consistent with its photoperiod-insensitive, early-flowering phenotype. We suggest that evolution of a flexible transcriptome confers the ability to quickly react to extreme diurnal temperature fluctuations characteristic of a desert environment while positive selection of genes involved in stress tolerance and early flowering could facilitate an opportunistic desert lifestyle.

Adaptative Mechanisms of Halophytic Eutrema salsugineum Encountering Saline Environment

Salt cress (Eutrema salsugineum), an Arabidopsis-related halophyte, can naturally adapt to various harsh climates and soil conditions; thus, it is considered a desirable model plant for deciphering mechanisms of salt and other abiotic stresses. Accumulating evidence has revealed that compared with Arabidopsis, salt cress possesses stomata that close more tightly and more succulent leaves during extreme salt stress, a noticeably higher level of proline, inositols, sugars, and organic acids, as well as stress-associated transcripts in unstressed plants, and they are induced rapidly under stress. In this review, we systematically summarize the research on the morphology, physiology, genome, gene expression and regulation, and protein and metabolite profile of salt cress under salt stress. We emphasize the latest advances in research on the genome adaptive evolution encountering saline environments, and epigenetic regulation, and discuss the mechanisms underlying salt tolerance in salt cress. Finally, we discuss the existing questions and opportunities for future research in halophytic Eutrema. Together, the review fosters a better understanding of the mechanism of plant salt tolerance and provides a reference for the research and utilization of Eutrema as a model extremophile in the future. Furthermore, the prospects for salt cress applied to explore the mechanism of salt tolerance provide a theoretical basis to develop new strategies for agricultural biotechnology.

Dissecting the Molecular Regulation of Natural Variation in Growth and Senescence of Two Eutrema salsugineum Ecotypes

Salt cress (Eutrema salsugineum, aka Thellungiella salsuginea) is an extremophile and a close relative of Arabidopsis thaliana. To understand the mechanism of selection of complex traits under natural variation, we analyzed the physiological and proteomic differences between Shandong (SD) and Xinjiang (XJ) ecotypes. The SD ecotype has dark green leaves, short and flat leaves, and more conspicuous taproots, and the XJ ecotype had greater biomass and showed clear signs of senescence or leaf shedding with age. After 2-DE separation and ESI-MS/MS identification, between 25 and 28 differentially expressed protein spots were identified in shoots and roots, respectively. The proteins identified in shoots are mainly involved in cellular metabolic processes, stress responses, responses to abiotic stimuli, and aging responses, while those identified in roots are mainly involved in small-molecule metabolic processes, oxidation-reduction processes, and responses to abiotic stimuli. Our data revealed the evolutionary differences at the protein level between these two ecotypes. Namely, in the evolution of salt tolerance, the SD ecotype highly expressed some stress-related proteins to structurally adapt to the high salt environment in the Yellow River Delta, whereas the XJ ecotype utilizes the specialized energy metabolism to support this evolution of the short-lived xerophytes in the Xinjiang region.

Maize ZmBES1/BZR1-3 and -9 Transcription Factors Negatively Regulate Drought Tolerance in Transgenic Arabidopsis

The BRI1-EMS suppressor 1 (BES1)/brassinazole-resistant 1(BZR1) transcription factors play crucial roles in plant growth, development, and stress response. However, little is known about the function of maize’s BES1/BZR1s. In this study, the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes were cloned from maize’s inbred line, B73, and they were functionally evaluated by analyzing their expression pattern, subcellular localization, transcriptional activation activity, as well as their heterologous expression in Arabidopsis, respectively. The results of the qRT-PCR showed that the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes were predominantly expressed in the root, and their expression was significantly down-regulated by drought stress. The ZmBES1/BZR1-3 and ZmBES1/BZR1-9 proteins localized in the nucleus but showed no transcriptional activation activity as a monomer. Subsequently, it was found that the heterologous expression of the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes in Arabidopsis decreased drought tolerance, respectively. The transgenic lines showed a more serious wilting phenotype, shorter root length, lower fresh weight, and higher relative electrolyte leakage (REL) and malondialdehyde (MDA) content compared to the control under drought stress. The RNA-sequencing data showed that the 70.67% and 93.27% differentially expressed genes (DEGs) were significantly down-regulated in ZmBES1/BZR1-3 and ZmBES1/BZR1-9 transgenic Arabidopsis, respectively. The DEGs of ZmBES1/BZR1-3 gene’s expressing lines were mainly associated with oxidative stress response and amino acid metabolic process and enriched in phenylpropanoid biosynthesis and protein processing in the endoplasmic reticulum. But the DEGs of the ZmBES1/BZR1-9 gene’s expressing lines were predominantly annotated with water deprivation, extracellular stimuli, and jasmonic acid and enriched in phenylpropanoid biosynthesis and plant hormone signal transduction. Moreover, ZmBES1/BZR1-9 increased stomatal aperture in transgenic Arabidopsis under drought stress. This study indicates that ZmBES1/BZR1-3 and ZmBES1/BZR1-9 negatively regulate drought tolerance via different pathways in transgenic Arabidopsis, and it provides insights into the underlying the function of BES1/BZR1s in crops.

Constitutive and Adaptive Traits of Environmental Stress Tolerance in the Threatened Halophyte Limonium angustebracteatum Erben (Plumbaginaceae)

Limonium angustebracteatum is a halophyte endemic to the E and SE Iberian Peninsula with interest in conservation. Salt glands represent an important adaptive trait in recretohalophytes like this and other Limonium species, as they allow the excretion of excess salts, reducing the concentration of toxic ions in foliar tissues. This study included the analysis of the salt gland structure, composed of 12 cells, 4 secretory and 8 accessory. Several anatomical, physiological and biochemical responses to stress were also analysed in adult plants subjected to one month of water stress, complete lack of irrigation, and salt stress, by watering with aqueous solutions of 200, 400, 600 and 800 mM NaCl. Plant growth was inhibited by the severe water deficit and, to a lesser extent, by high NaCl concentrations. A variation in the anatomical structure of the leaves was detected under conditions of salt and water stress; plants from the salt stress treatment showed salt glands sunken between epidermal cells, bordered by very large epidermal cells, whereas in those from the water stress treatment, the epidermal cells were heterogeneous in shape and size. In both, the palisade structure of the leaves was altered. Salt excretion is usually accompanied by the accumulation of salts in the foliar tissue. This was also found in L. angustebracteatum, in which the concentration of all ions analysed was higher in the leaves than in the roots. The increase of K⁺ in the roots of plants subjected to water stress was also remarkable. The multivariate analysis indicated differences in water and salt stress responses, such as the accumulation of Na and Cl, or proline, but K⁺ homeostasis played a relevant role in the mechanism of tolerance to both stressful conditions.

Genetic background and cis-organization regulate ALDH7B4 gene expression in Eutrema salsugineum: a promoter analysis case study

ALDH7B4 promoter analysis in A. thaliana and E. salsugineum reveals that both genetic background and promoter architecture contribute to gene expression in response to stress in different species. Many genes are differentially regulated in a comparison of salinity-sensitive and salinity-tolerant plant species. The aldehyde dehydrogenase 7B4 (ALDH7B4) gene is turgor-responsive in A. thaliana and encodes a highly conserved detoxification enzyme in plants. This study compared the ALDH7B4 gene in A. thaliana (salinity-sensitive) and in the salinity-tolerant close relative Eutrema salsugineum. EsALDH7B4 in E. salsugineum is the ortholog of AtALDH7B4 and the expression is also salinity, drought, and wound responsive. However, E. salsugineum requires higher salinity stress to induce the EsALDH7B4 transcriptional response. The GUS expression driven either by the promoter AtALDH7B4 or EsALDH7B4 was induced under 300 mM NaCl treatment in A. thaliana while 600 mM NaCl treatment was required in E. salsugineum, suggesting that the genetic background plays a crucial role in regulation of gene expression. Promoter sequences of ALDH7B4 are less conserved than the protein coding region. A series of EsALDH7B4 promoter deletion fragments were fused to the GUS reporter gene and promoter activity was determined in A. thaliana. The promoter region that contains two conserved ACGT-containing motifs was identified to be essential for stress induction. Furthermore, a 38 bp "TC" rich motif in the EsALDH7B4 promoter, absent from the AtALDH7B4 promoter, negatively affects EsALDH7B4 expression. A MYB-like transcription factor was identified to bind the "TC" motif and to repress the EsALDH7B4 promoter activity. This study reveals that genetic background and cis-acting elements coordinately regulate gene expression.

Rewilding staple crops for the lost halophytism: Toward sustainability and profitability of agricultural production systems

Abiotic stress tolerance has been weakened during the domestication of all major staple crops. Soil salinity is a major environmental constraint that impacts over half of the world population; however, given the increasing reliance on irrigation and the lack of available freshwater, agriculture in the 21st century will increasingly become saline. Therefore, global food security is critically dependent on the ability of plant breeders to create high-yielding staple crop varieties that will incorporate salinity tolerance traits and account for future climate scenarios. Previously, we have argued that the current agricultural practices and reliance on crops that exclude salt from uptake is counterproductive and environmentally unsustainable, and thus called for a need for a major shift in a breeding paradigm to incorporate some halophytic traits that were present in wild relatives but were lost in modern crops during domestication. In this review, we provide a comprehensive physiological and molecular analysis of the key traits conferring crop halophytism, such as vacuolar Na⁺ sequestration, ROS desensitization, succulence, metabolic photosynthetic switch, and salt deposition in trichomes, and discuss the strategies for incorporating them into elite germplasm, to address a pressing issue of boosting plant salinity tolerance.

Transcriptome Profiling of the Salt Stress Response in the Leaves and Roots of Halophytic Eutrema salsugineum

Eutrema salsugineum can grow in natural harsh environments; however, the underlying mechanisms for salt tolerance of Eutrema need to be further understood. Herein, the transcriptome profiling of Eutrema leaves and roots exposed to 300 mM NaCl is investigated, and the result emphasized the role of genes involved in lignin biosynthesis, autophagy, peroxisome, and sugar metabolism upon salt stress. Furthermore, the expression of the lignin biosynthesis and autophagy-related genes, as well as 16 random selected genes, was validated by qRT-PCR. Notably, the transcript abundance of a large number of lignin biosynthesis genes such as CCoAOMT, C4H, CCR, CAD, POD, and C3′H in leaves was markedly elevated by salt shock. And the examined lignin content in leaves and roots demonstrated salt stress led to lignin accumulation, which indicated the enhanced lignin level could be an important mechanism for Eutrema responding to salt stress. Additionally, the differentially expressed genes (DEGs) assigned in the autophagy pathway including Vac8, Atg8, and Atg4, as well as DEGs enriched in the peroxisome pathway such as EsPEX7, EsCAT, and EsSOD2, were markedly induced in leaves and/or roots. In sugar metabolism pathways, the transcript levels of most DEGs associated with the synthesis of sucrose, trehalose, raffinose, and xylose were significantly enhanced. Furthermore, the expression of various stress-related transcription factor genes including WRKY, AP2/ERF-ERF, NAC, bZIP, MYB, C2H2, and HSF was strikingly improved. Collectively, the increased expression of biosynthesis genes of lignin and soluble sugars, as well as the genes in the autophagy and peroxisome pathways, suggested that Eutrema encountering salt shock possibly possess a higher capacity to adjust osmotically and facilitate water transport and scavenge reactive oxidative species and oxidative proteins to cope with the salt environment. Thus, this study provides a new insight for exploring the salt tolerance mechanism of halophytic Eutrema and discovering new gene targets for the genetic improvement of crops.

Variation of PHT families adapts salt cress to phosphate limitation under salinity

Salt cress (Eutrema salsugineum) presents relatively high phosphate (Pi) use efficiency cy in its natural habitat. Phosphate Transporters (PHTs) play critical roles in Pi acquisition and homeostasis. Here, a comparative study of PHT families between salt cress and Arabidopsis was performed. A total of 27 putative PHT genes were identified in E. salsugineum genome. Notably, seven tandem genes encoding PHT1;3 were found, and function analysis in Arabidopsis indicated at least six EsPHT1;3s participated in Pi uptake. Meanwhile, different expression profiles of PHT genes between the two species under Pi limitation and salt stress were documented. Most PHT1 genes were down-regulated in Arabidopsis while up-regulated in salt cress under salinity, among which EsPHT1;9 was further characterized. EsPHT1;9 was involved in root-to-shoot Pi translocation. Particularly, the promoter of EsPHT1;9 outperformed that of AtPHT1;9 in promoting Pi translocation, K⁺ /Na⁺ ratio, thereby salt tolerance. Through cis-element analysis, we identified a bZIP transcription factor EsABF5 negatively regulating EsPHT1;9 and plant tolerance to low-Pi and salt stress. Altogether, more copies and divergent transcriptional regulation of PHT genes contribute to salt cress adaptation to the co-occurrence of salinity and Pi limitation, which add our knowledge on the evolutionary and molecular component of multistress- tolerance of this species.

HKT sodium and potassium transporters in Arabidopsis thaliana and related halophyte species

High salinity induces osmotic stress and often leads to sodium ion-specific toxicity, with inhibitory effects on physiological, biochemical and developmental pathways. To cope with increased Na⁺ in soil water, plants restrict influx, compartmentalize ions into vacuoles, export excess Na⁺ from the cell, and distribute ions between the aerial and root organs. In this review, we discuss our current understanding of how high-affinity K⁺ transporters (HKT) contribute to salinity tolerance, focusing on HKT1-like family members primarily involved in long-distance transport, and in the recent research in the model plant Arabidopsis and its halophytic counterparts of the Eutrema genus. Functional characterization of the salt overly sensitive (SOS) pathway and HKT1-type transporters in these species indicate that they utilize similar approaches to deal with salinity, regardless of their tolerance.

Identification and Functional Characterization of Plant MiRNA Under Salt Stress Shed Light on Salinity Resistance Improvement Through MiRNA Manipulation in Crops

Salinity, as a major environmental stressor, limits plant growth, development, and crop yield remarkably. However, plants evolve their own defense systems in response to salt stress. Recently, microRNA (miRNA) has been broadly studied and considered to be an important regulator of the plant salt-stress response at the post-transcription level. In this review, we have summarized the recent research progress on the identification, functional characterization, and regulatory mechanism of miRNA involved in salt stress, have discussed the emerging manipulation of miRNA to improve crop salt resistance, and have provided future direction for plant miRNA study under salt stress, suggesting that the salinity resistance of crops could be improved by the manipulation of microRNA.

Insights Into Plant Surgery: An Overview of the Multiple Grafting Techniques for Arabidopsis thaliana

Plant grafting, the ancient practice of cutting and joining different plants, is gaining popularity as an elegant way to generate chimeras that combine desirable traits. Grafting was originally developed in woody species, but the technique has evolved over the past century to now encompass a large number of herbaceous species. The use of plant grafting in science is accelerating in part due to the innovative techniques developed for the model plant Arabidopsis thaliana. Here, we review these developments and discuss the advantages and limitations associated with grafting various Arabidopsis tissues at diverse developmental stages.

Function and application of the Eutrema salsugineum PHT1;1 gene in phosphate deficiency stress

Low availability of inorganic phosphate (Pi) in soil is often a limiting factor for plant growth and productivity. The Pi transporter plays an important role in the absorption and utilization of phosphorus in plants. Eutrema salsugineum shows strong tolerance under Pi stress conditions, but the function of the E. salsugineum PHT1 genes has not yet been systematically studied. This study isolated a phosphate transporter gene (EsPHT1;1) from the halophyte E. salsugineum and functionally characterized it in the herbaceous model plant, Arabidopsis thaliana, and in an important oil crop species, soybean (Glycine max (L.) Merr.). Under Pi deficient conditions, transgenic Arabidopsis and transgenic soybean grew better and exhibited significant improvement in root growth, biomass accumulation and seed yield compared with wild-type (WT) plants. These phenotypic enhancements were more apparent under inadequate Pi conditions than under sufficient or no Pi conditions, which is in agreement with the observation that the transgenic plants accumulated higher amounts of Pi and total P in shoots and roots than WT plants only when inadequate Pi was supplied. The results of the present study indicate that overexpression of EsPHT1;1 can efficiently enhance the growth and reproductive performance of both Arabidopsis and soybean plants challenged by low P stress, which results confirm the important role of PHT1;1 in dealing with Pi deficiency.

Overexpression of a Eutrema salsugineum phosphate transporter gene EsPHT1;4 enhances tolerance to low phosphorus stress in soybean

OBJECTIVE

To enhance Pi absorption and utilization efficiency of soybean, a member of PHT1 gene family was isolated and characterized from E. salsugineum, which was a homologous gene of AtPHT1;4 and consequently designated as EsPHT1;4.

RESULTS

Quantitative real-time PCR (qRT-PCR) analysis showed that the transcript level of EsPHT1;4 significantly increased both in roots and leaves of E. salsugineum under Pi deficient conditions. Furthermore, EsPHT1;4 was transferred to soybean cultivar "YD22" using an Agrobacterium-mediated cotyledonary-node transformation method. Overexpression of EsPHT1;4 in soybean not only promoted the increase of plant biomass and yield of transgenic plants upon low P stress, but also increased the accumulation and transportation of Pi from roots to leaves in the transgenic soybean lines.

CONCLUSION

EsPHT1;4 was critical for controlling the accumulation and translocation of Pi in plants, and can be subsequently used as an effective foreign gene for the improvement of P use efficiency of crops by genetic manipulation.

The genome of Ectocarpus subulatus - A highly stress-tolerant brown alga

Brown algae are multicellular photosynthetic stramenopiles that colonize marine rocky shores worldwide. Ectocarpus sp. Ec32 has been established as a genomic model for brown algae. Here we present the genome and metabolic network of the closely related species, Ectocarpus subulatus Kützing, which is characterized by high abiotic stress tolerance. Since their separation, both strains show new traces of viral sequences and the activity of large retrotransposons, which may also be related to the expansion of a family of chlorophyll-binding proteins. Further features suspected to contribute to stress tolerance include an expanded family of heat shock proteins, the reduction of genes involved in the production of halogenated defence compounds, and the presence of fewer cell wall polysaccharide-modifying enzymes. Overall, E. subulatus has mainly lost members of gene families down-regulated in low salinities, and conserved those that were up-regulated in the same condition. However, 96% of genes that differed between the two examined Ectocarpus species, as well as all genes under positive selection, were found to encode proteins of unknown function. This underlines the uniqueness of brown algal stress tolerance mechanisms as well as the significance of establishing E. subulatus as a comparative model for future functional studies.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Maize ZmBES1/BZR1-5 Decreases ABA Sensitivity and Confers Tolerance to Osmotic Stress in Transgenic Arabidopsis

The BRI1-EMS suppressor 1 (BES1)/brassinazole-resistant 1 (BZR1) transcription factors, key components in the brassinosteroid signaling pathway, play pivotal roles in plant growth and development. However, the function of BES1/BZR1 in crops during stress response remains poorly understood. In the present study, we characterized ZmBES1/BZR1-5 from maize, which was localized to the nucleus and was responsive to abscisic acid (ABA), salt and drought stresses. Heterologous expression of ZmBES1/BZR1-5 in transgenic Arabidopsis resulted in decreased ABA sensitivity, facilitated shoot growth and root development, and enhanced salt and drought tolerance with lower malondialdehyde (MDA) content and relative electrolyte leakage (REL) under osmotic stress. The RNA sequencing (RNA-seq) analysis revealed that 84 common differentially expressed genes (DEGs) were regulated by ZmBES1/BZR1-5 in transgenic Arabidopsis. Subsequently, gene ontology and KEGG pathway enrichment analyses showed that the DEGs were enriched in response to stress, secondary metabolism and metabolic pathways. Furthermore, 30 DEGs were assigned to stress response and possessed 2-15 E-box elements in their promoters, which could be potentially recognized and bound by ZmBES1/BZR1-5. Taken together, our results reveal that the ZmBES1/BZR1-5 transcription factor positively regulates salt and drought tolerance by binding to E-box to induce the expression of downstream stress-related genes. Therefore, our study contributes to the better understanding of BES1/BZR1 function in the stress response of plants.

The relationship between cuticular lipids and associated gene expression in above ground organs of Thellungiella salsugineum (Pall.) Al-Shehbaz & Warwick

The cuticle plays a critical role as barrier between plant and environment. Here, cuticular wax morphology, cuticular wax and cutin monomer composition, and expression of associated genes in five above ground organs were examined in model extremophyte Thellungiella salsugineum. Alkanes, ketones, and 2-alcohols were the predominant wax constitutes in rosette leaves, inflorescence stem leaves, stems, and siliques, whereas alkanes and acids were the predominant cuticular lipids in whole flowers. Unsubstituted acids were the most abundant cutin monomers in vegetative organs, especially C18:2 dioic acids, which reached the highest levels in stems. Hydroxy fatty acids were the predominant cutin monomers in flowers, especially 16-OH C16:0 and diOH C16:0. High-throughput RNA-Seq analysis using the Hiseq4000 platform was performed on these five above organs of T. salsugineum, and the differentially expressed lipid-associated genes and their associated metabolic pathways were identified. Expression of genes associated in previous reports to cuticle production, including those having roles in cuticle lipid biosynthesis, transport, and regulation were examined. The association of cuticle lipid composition and gene expression within different organs of T. salsugineum, and potential relationships between T. salsugineum's extreme cuticle and its adaptation to extreme environments is discussed.

Aquaporins and their function in root water transport under salt stress conditions in Eutrema salsugineum

Eutrema salsugineum is considered as extremophile model species. To gain insights into the root hydraulic conductivity and the role played by aquaporins in E. salsugineum, we investigated the aquaporin family profiles, plant water status and root hydraulic conductivity under standard (salt-free) and salt stress conditions. We found that there was no variation in the relative electric conductivity of the leaves when the salt concentration was less than 200 mM NaCl, and the transpiration rate dropped to 60.6% at 100 mM NaCl for 14 days compared to that at standard conditions. The pressure chamber techniques indicated that the root hydraulic conductivity of E. salsugineum was repressed by salt stress. However, propionic acid, usually used as an aquaporin inhibitor, unexpectedly enhanced the root hydraulic conductivity of E. salsugineum. The aquaporin family in E. salsugineum was profiled and the PIP aquaporin expression was investigated at the transcriptional and translational levels. Finally, two EsPIPs were identified to play a role in salt stress. The overall study provides evidence on how halophytes maintain their water status and aquaporin regulation pattern under salt stress conditions.

Chemical and Transcriptomic Analysis of Cuticle Lipids under Cold Stress in Thellungiella salsuginea

Plant cuticle lipids form outer protective layers to resist environmental stresses; however, the relationship between cuticle properties and cold tolerance is unclear. Here, the extremophyte Thellungiella salsuginea was stressed under cold conditions (4 °C) and the cuticle of rosette leaves was examined in terms of epicuticular wax crystal morphology, chemical composition, and cuticle-associated gene expression. The results show that cold induced formation of distinct lamellas within the cuticle ultrastructure. Cold stress caused 14.58% and 12.04% increases in the amount of total waxes and cutin monomer per unit of leaf area, respectively, probably associated with the increase in total fatty acids. The transcriptomic analysis was performed on rosette leaves of Thellungiella exposed to cold for 24 h. We analyzed the expression of 72 genes putatively involved in cuticle lipid metabolism, some of which were validated by qRT-PCR (quantitative reverse transcription PCR) after both 24 h and one week of cold exposure. Most cuticle-associated genes exhibited higher expression levels under cold conditions, and some key genes increased more dramatically over the one week than after just 24 h, which could be associated with increased amounts of some cuticle components. These results demonstrate that the cuticle provides some aspects of cold adaptation in T. salsuginea.

Global transcriptome analysis provides new insights in Thellungiella salsuginea stress response

Thellungiella salsuginea is highly tolerant to abiotic stress, while its a close relative Arabidopsis thaliana is sensitive to stress. This characteristic makes T. salsuginea an excellent model for uncovering the mechanisms of abiotic stress tolerance. Abscisic acid (ABA) plays essential roles in plant abiotic and biotic stress tolerance. To test the changes in gene expression of T. salsuginea under ABA treatment, in this study, the transcriptomes of T. salsuginea roots and leaves were compared in response to exogenously application of ABA. The results showed that ABA treatment caused different expression of 2,200 and 3,305 genes in leaves and roots, respectively, compared with the untreated control. In particular, genes encoding transcription factors such as WRKY, MYB, NAC, GATA, ethylene-responsive factors (ERFs), heat stress transcription factors, basic helix-loop-helix, PLATZ and B3 domain-containing family members were enriched. In addition, 49 and 114 differentially expressed genes were identified as ABA-regulated genes, separately in leaves and roots, respectively, which were related to biotic and abiotic stresses. The expression levels of some genes were validated by qRT-PCR. Different responses of genes to ABA treatment were discovered in T. salsuginea and A. thaliana. This transcriptome analysis expands our understanding of the role of ABA in stress tolerance in T. salsuginea. Our study provides a wealth of information for improving stress tolerance in crop plants.

Molecular Evolution and Functional Analysis of Rubredoxin-Like Proteins in Plants

Rubredoxins are a class of iron-containing proteins that play an important role in the reduction of superoxide in some anaerobic bacteria and also act as electron carriers in many biochemical processes. Unlike the more widely studied about rubredoxin proteins in anaerobic bacteria, very few researches about the function of rubredoxins have been proceeded in plants. Previous studies indicated that rubredoxins in A. thaliana may play a critical role in responding to oxidative stress. In order to identify more rubredoxins in plants that maybe have similar functions as the rubredoxin-like protein of A. thaliana, we identified and analyzed plant rubredoxin proteins using bioinformatics-based methods. Totally, 66 candidate rubredoxin proteins were identified based on public databases, exhibiting lengths of 187-360 amino acids with molecular weights of 19.856-37.117 kDa. The results of subcellular localization showed that these candidate rubredoxins were localized to the chloroplast, which might be consistent with the fact that rubredoxins were predominantly expressed in leaves. Analyses of conserved motifs indicated that these candidate rubredoxins contained rubredoxin and PDZ domains. The expression patterns of rubredoxins in glycophyte and halophytic plant under salt/drought stress revealed that rubredoxin is one of the important stress response proteins. Finally, the coexpression network of rubredoxin in Arabidopsis thaliana under abiotic was extracted from ATTED-II to explore the function and regulation relationship of rubredoxin in Arabidopsis thaliana. Our results showed that putative rubredoxin proteins containing PDZ and rubredoxin domains, localized to the chloroplast, may act with other proteins in chloroplast to responses to abiotic stress in higher plants. These findings might provide value inference to promote the development of plant tolerance to some abiotic stresses and other economically important crops.

Drought resistance is mediated by divergent strategies in closely related Brassicaceae

Droughts cause severe crop losses worldwide and climate change is projected to increase their prevalence in the future. Similar to the situation for many crops, the reference plant Arabidopsis thaliana (Ath) is considered drought-sensitive, whereas, as we demonstrate, its close relatives Arabidopsis lyrata (Aly) and Eutrema salsugineum (Esa) are drought-resistant. To understand the molecular basis for this plasticity we conducted a deep phenotypic, biochemical and transcriptomic comparison using developmentally matched plants. We demonstrate that Aly responds most sensitively to decreasing water availability with early growth reduction, metabolic adaptations and signaling network rewiring. By contrast, Esa is in a constantly prepared mode as evidenced by high basal proline levels, ABA signaling transcripts and late growth responses. The stress-sensitive Ath responds later than Aly and earlier than Esa, although its responses tend to be more extreme. All species detect water scarcity with similar sensitivity; response differences are encoded in downstream signaling and response networks. Moreover, several signaling genes expressed at higher basal levels in both Aly and Esa have been shown to increase water-use efficiency and drought resistance when overexpressed in Ath. Our data demonstrate contrasting strategies of closely related Brassicaceae to achieve drought resistance.

Landscape of gene transposition-duplication within the Brassicaceae family

We developed the CLfinder-OrthNet pipeline that detects co-linearity among multiple closely related genomes, finds orthologous gene groups, and encodes the evolutionary history of each orthologue group into a representative network (OrthNet). Using a search based on network topology, we identified 1,394 OrthNets that included gene transposition-duplication (tr-d) events, out of 17,432 identified in six Brassicaceae genomes. Occurrences of tr-d shared by subsets of Brassicaceae genomes mirrored the divergence times between the genomes and their repeat contents. The majority of tr-d events resulted in truncated open reading frames (ORFs) in the duplicated loci. However, the duplicates with complete ORFs were significantly more frequent than expected from random events. These were derived from older tr-d events and had a higher chance of being expressed. We also found an enrichment of tr-d events with complete loss of intergenic sequence conservation between the original and duplicated loci. Finally, we identified tr-d events uniquely found in two extremophytes among the six Brassicaceae genomes, including tr-d of SALT TOLERANCE 32 and ZINC TRANSPORTER 3 that relate to their adaptive evolution. CLfinder-OrthNet provides a flexible toolkit to compare gene order, visualize evolutionary paths among orthologues as networks, and identify gene loci that share an evolutionary history.

Linking Duplication of a Calcium Sensor to Salt Tolerance in Eutrema salsugineum

The SALT-OVERLY-SENSITIVE (SOS) pathway in Arabidopsis (Arabidopsis thaliana) functions to prevent the toxic accumulation of sodium in the cytosol when plants are grown in salt-affected soils. In this pathway, the CALCINEURIN B-LIKE10 (AtCBL10) calcium sensor interacts with the AtSOS2 kinase to activate the AtSOS1 plasma membrane sodium/proton exchanger. CBL10 has been duplicated in Eutrema (Eutrema salsugineum), a salt-tolerant relative of Arabidopsis. Because Eutrema maintains growth in salt-affected soils that kill most crop plants, the duplication of CBL10 provides a unique opportunity to functionally test the outcome of gene duplication and its link to plant salt tolerance. In Eutrema, individual down-regulation of the duplicated CBL10 genes (EsCBL10a and EsCBL10b) decreased growth in the presence of salt and, in combination, led to an even greater decrease, suggesting that both genes function in response to salt and have distinct functions. Cross-species complementation assays demonstrated that EsCBL10b has an enhanced ability to activate the SOS pathway while EsCBL10a has a function not performed by AtCBL10 or EsCBL10b Chimeric EsCBL10a/EsCBL10b proteins revealed that the specific functions of the EsCBL10 proteins resulted from changes in the amino terminus. The duplication of CBL10 increased calcium-mediated signaling capacity in Eutrema and conferred increased salt tolerance to salt-sensitive Arabidopsis.

Compositional and transcriptomic analysis associated with cuticle lipid production on rosette and inflorescence stem leaves in the extremophyte Thellungiella salsuginea

The plant cuticle is a complex structure composed primarily of wax and cutin, but also contains cutan, glycerolipids, phenolics, polysaccharides and proteins. The cuticle plays an important protective role as barrier between plants and their environment. In this paper, 4-week-old leaves produced either on the rosette or on the inflorescence stem of the model extremophyte Thellungiella salsuginea were examined using scanning electron microscopy, cuticle permeability assays and chemical composition analysis. Results showed that stem leaves (SL) had more abundant cuticle lipids and lower cuticle permeability than rosette leaves (RL). SL were dominated by alkanes, especially the C29 and C31 homologs, whereas in RL the most abundant wax class was free very long-chain acids. The major cutin monomers for both leaf types were C18:2 dioic acids and 18-OH C18:2 acids. We performed Illumina high-throughput sequencing for SL and RL, and 3577 differentially expressed genes were identified. Sixty-five genes possibly involved in cuticular lipid biosynthesis, transport, or regulation was selected for further analysis. Many cuticle-associated genes exhibited differential expression levels that could be associated with compositional differences between these two leaf types. Furthermore, transcription factors and other regulatory proteins previously associated with cuticle production were expressed at higher levels in SL than in RL. The associations between gene expression and characteristics of this extremophile's leaf cuticles sheds new light on cuticle as an adaptive trait in extreme environments, and contributes new information that may guide efforts to modify crop cuticles for improved stress tolerance.

Comparative Transcriptome Profiling in a Segregating Peach Population with Contrasting Juiciness Phenotypes

Cold storage of fruit is one of the methods most commonly employed to extend the postharvest lifespan of peaches ( Prunus persica (L.) Batsch). However, fruit quality in this species is affected negatively by mealiness, a physiological disorder triggered by chilling injury after long periods of exposure to low temperatures during storage and manifested mainly as a lack of juiciness, which ultimately modifies the organoleptic properties of peach fruit. The aim of this study was to identify molecular components and metabolic processes underlying mealiness in susceptible and nonsusceptible segregants. Transcriptome and qRT-PCR profiling were applied to individuals with contrasting juiciness phenotypes in a segregating F₂ population. Our results suggest that mealiness is a multiscale phenomenon, because juicy and mealy fruit display distinctive reprogramming processes affecting translational machinery and lipid, sugar, and oxidative metabolism. The candidate genes identified may be useful tools for further crop improvement.

Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Developing naturally stress-resistant crops for a sustainable agriculture

A major problem facing humanity is that our numbers are growing but the availability of land and fresh water for agriculture is not. This problem is being exacerbated by climate change-induced increases in drought, and other abiotic stresses. Stress-resistant crops are needed to ensure yield stability under stress conditions and to minimize the environmental impacts of crop production. Evolution has created thousands of species of naturally stress-resistant plants (NSRPs), some of which have already been subjected to human domestication and are considered minor crops. Broader cultivation of these minor crops will diversify plant agriculture and the human diet, and will therefore help improve global food security and human health. More research should be directed toward understanding and utilizing NSRPs. Technologies are now available that will enable researchers to rapidly improve the genetics of NSRPs, with the goal of increasing NSRP productivity while retaining NSRP stress resistance and nutritional value.

It's Hard to Avoid Avoidance: Uncoupling the Evolutionary Connection between Plant Growth, Productivity and Stress "Tolerance"

In the last 100 years, agricultural developments have favoured selection for highly productive crops, a fact that has been commonly associated with loss of key traits for environmental stress tolerance. We argue here that this is not exactly the case. We reason that high yield under near optimal environments came along with hypersensitization of plant stress perception and consequently early activation of stress avoidance mechanisms, such as slow growth, which were originally needed for survival over long evolutionary time periods. Therefore, mechanisms employed by plants to cope with a stressful environment during evolution were overwhelmingly geared to avoid detrimental effects so as to ensure survival and that plant stress "tolerance" is fundamentally and evolutionarily based on "avoidance" of injury and death which may be referred to as evolutionary avoidance (EVOL-Avoidance). As a consequence, slow growth results from being exposed to stress because genes and genetic programs to adjust growth rates to external circumstances have evolved as a survival but not productivity strategy that has allowed extant plants to avoid extinction. To improve productivity under moderate stressful conditions, the evolution-oriented plant stress response circuits must be changed from a survival mode to a continued productivity mode or to avoid the evolutionary avoidance response, as it were. This may be referred to as Agricultural (AGRI-Avoidance). Clearly, highly productive crops have kept the slow, reduced growth response to stress that they evolved to ensure survival. Breeding programs and genetic engineering have not succeeded to genetically remove these responses because they are polygenic and redundantly programmed. From the beginning of modern plant breeding, we have not fully appreciated that our crop plants react overly-cautiously to stress conditions. They over-reduce growth to be able to survive stresses for a period of time much longer than a cropping season. If we are able to remove this polygenic redundant survival safety net we may improve yield in moderately stressful environments, yet we will face the requirement to replace it with either an emergency slow or no growth (dormancy) response to extreme stress or use resource management to rescue crops under extreme stress (or both).

Effect of salinity on osmotic adjustment, proline accumulation and possible role of ornithine-δ-aminotransferase in proline biosynthesis in Cakile maritima

The short time response to salt stress was studied in Cakile maritima. Plants were exposed to different salt concentrations (0, 100, 200 and 400 mM NaCl) and harvested after 4, 24, 72 and 168 h of treatment. Before harvesting plants, tissue hydration, osmotic potential, inorganic and organic solute contents, and ornithine-δ-aminotransferase activity were measured. Plants of C. maritima maintained turgor and tissue hydration at low osmotic potential mainly at 400 mM NaCl. The results showed that, in leaves and stems, Na⁺ content increased significantly after the first 4 h of treatment. However, in roots, the increase of Na⁺ content remained relatively unchanged with increasing salt. The K⁺ content decreased sharply at 200 and 400 mM NaCl with treatment duration. This decrease was more pronounced in roots. The content of proline and amino acids increased with increasing salinity and treatment duration. These results indicated that the accumulation of inorganic and organic compounds was a central adaptive mechanism by which C. maritima maintained intracellular ionic balance under saline conditions. However, their percentage contribution to total osmotic adjustment varies from organ to organ; for example, Na⁺ accumulation mainly contributes in osmotic adjustment of stem tissue (60%). Proline contribution to osmotic adjustment reached 36% in roots. In all organs, proline as well as δ-OAT activity increased with salt concentration and treatment duration. Under normal growth conditions, δ-OAT is mainly involved in the mobilization of nitrogen required for plant growth. However, the highly significant positive correlation between proline and δ-OAT activity under salt-stress conditions suggests that ornithine pathway contributed to proline synthesis.

Halophytism: What Have We Learnt From Arabidopsis thaliana Relative Model Systems?

Halophytes are able to thrive in salt concentrations that would kill 99% of other plant species, and identifying their salt-adaptive mechanisms has great potential for improving the tolerance of crop plants to salinized soils. Much research has focused on the physiological basis of halophyte salt tolerance, whereas the elucidation of molecular mechanisms has traditionally lagged behind due to the absence of a model halophyte system. However, over the last decade and a half, two Arabidopsis (Arabidopsis thaliana) relatives, Eutrema salsugineum and Schrenkiella parvula, have been established as transformation-competent models with various genetic resources including high-quality genome assemblies. These models have facilitated powerful comparative analyses with salt-sensitive Arabidopsis to unravel the genetic adaptations that enable a halophytic lifestyle. The aim of this review is to explore what has been learned about halophytism using E. salsugineum and S. parvula We consider evidence from physiological and molecular studies suggesting that differences in salt tolerance between related halophytes and salt-sensitive plants are associated with alterations in the regulation of basic physiological, biochemical, and molecular processes. Furthermore, we discuss how salt tolerance mechanisms of the halophytic models are reflected at the level of their genomes, where evolutionary processes such as subfunctionalization and/or neofunctionalization have altered the expression and/or functions of genes to facilitate adaptation to saline conditions. Lastly, we summarize the many areas of research still to be addressed with E. salsugineum and S. parvula as well as obstacles hindering further progress in understanding halophytism.

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

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

The Tropical Invasive Seagrass, Halophila stipulacea, Has a Superior Ability to Tolerate Dynamic Changes in Salinity Levels Compared to Its Freshwater Relative, Vallisneria americana

The tropical seagrass species, Halophila stipulacea, originated from the Indian Ocean and the Red Sea, subsequently invading the Mediterranean and has recently established itself in the Caribbean Sea. Due to its invasive nature, there is growing interest in understanding this species' capacity to adapt to new conditions. One approach to understanding the natural tolerance of a plant is to compare the tolerant species with a closely related non-tolerant species. We compared the physiological responses of H. stipulacea exposed to different salinities, with that of its nearest freshwater relative, Vallisneria americana. To achieve this goal, H. stipulacea and V. americana plants were grown in dedicated microcosms, and exposed to the following salt regimes: (i) H. stipulacea: control (40 PSU, practical salinity units), hyposalinity (25 PSU) and hypersalinity (60 PSU) for 3 weeks followed by a 4-week recovery phase (back to 40 PSU); (ii) V. americana: control (1 PSU), and hypersalinity (12 PSU) for 3 weeks, followed by a 4-week recovery phase (back to 1 PSU). In H. stipulacea, leaf number and chlorophyll content showed no significant differences between control plants and plants under hypo and hypersalinities, but a significant decrease in leaf area under hypersalinity was observed. In addition, compared with control plants, H. stipulacea plants exposed to hypo and hypersalinity were found to have reduced below-ground biomass and C/N ratios, suggesting changes in the allocation of resources in response to both stresses. There was no significant effect of hypo/hypersalinity on dark-adapted quantum yield of photosystem II (Fv/Fm) suggesting that H. stipulacea photochemistry is resilient to hypo/hypersalinity stress. In contrast to the seagrass, V. americana exposed to hypersalinity displayed significant decreases in above-ground biomass, shoot number, leaf number, blade length and Fv/Fm, followed by significant recoveries of all these parameters upon return of the plants to non-saline control conditions. These data suggest that H. stipulacea shows remarkable tolerance to both hypo and hypersalinity. Resilience to a relatively wide range of salinities may be one of the traits explaining the invasive nature of this species in the Mediterranean and Caribbean Seas.

Strengthening desert plant biotechnology research in the United Arab Emirates: a viewpoint

The biotechnology of desert plants is a vast subject. The main applications in this broad field of study comprises of plant tissue culture, genetic engineering, molecular markers and others. Biotechnology applications have the potential to address biodiversity conservation as well as agricultural, medicinal, and environmental issues. There is a need to increase our knowledge of the genetic diversity through the use of molecular genetics and biotechnological approaches in desert plants in the Arabian Gulf region including those in the United Arab Emirates (UAE). This article provides a prospective research for the study of UAE desert plant diversity through DNA fingerprinting as well as understanding the mechanisms of both abiotic stress resistance (including salinity, drought and heat stresses) and biotic stress resistance (including disease and insect resistance). Special attention is given to the desert halophytes and their utilization to alleviate the salinity stress, which is one of the major challenges in agriculture. In addition, symbioses with microorganisms are thought to be hypothesized as important components of desert plant survival under stressful environmental conditions. Thus, factors shaping the diversity and functionality of plant microbiomes in desert ecosystems are also emphasized in this article. It is important to establish a critical mass for biotechnology research and applications while strengthening the channels for collaboration among research/academic institutions in the area of desert plant biotechnology.

Coping With Metal Toxicity - Cues From Halophytes

Being the native flora of saline soil, halophytes are well studied for their salt tolerance and adaptation mechanism at the physiological, biochemical, molecular and metabolomic levels. However, these saline habitats are getting contaminated due to various anthropogenic activities like urban waste, agricultural runoff, mining, industrial waste that are rich in toxic metals and metalloids. These toxic metals impose detrimental effects on growth and development of most plant species. Halophytes by virtue of their tolerance to salinity also show high tolerance to heavy metals which is attributed to the enhanced root to shoot metal translocation and bioavailability. Halophytes rapidly uptake toxic ions from the root and transport them toward aerial parts by using different transporters which are involved in metal tolerance and homeostasis. A number of defense related physiological and biochemical strategies are known to be crucial for metal detoxification in halophytes however; there is paucity of information on the molecular regulators. Understanding of the phenomenon of cross-tolerance of salinity with other abiotic stresses in halophytes could very well boost their potential use in phytoremediation. In this article, we present an overview of heavy metal tolerance in case of halophytes, associated mechanisms and cross-tolerance of salinity with other abiotic stresses.