Salt stress reveals differential physiological, biochemical and molecular responses in T. monococcum and T. durum wheat genotypes

Defense Pathways of Wheat Plants Inoculated with Zymoseptoria tritici under NaCl Stress Conditions: An Overview

Due to being sessile, plants develop a broad range of defense pathways when they face abiotic or biotic stress factors. Although plants are subjected to more than one type of stress at a time in nature, the combined effects of either multiple stresses of one kind (abiotic or biotic) or more kinds (abiotic and biotic) have now been realized in agricultural lands due to increases in global warming and environmental pollution, along with population increases. Soil-borne pathogens, or pathogens infecting aerial parts, can have devastating effects on plants when combined with other stressors. Obtaining yields or crops from sensitive or moderately resistant plants could be impossible, and it could be very difficult from resistant plants. The mechanisms of combined stress in many plants have previously been studied and elucidated. Recent studies proposed new defense pathways and mechanisms through signaling cascades. In light of these mechanisms, it is now time to develop appropriate strategies for crop protection under multiple stress conditions. This may involve using disease-resistant or stress-tolerant plant varieties, implementing proper irrigation and drainage practices, and improving soil quality. However, generation of both stress-tolerant and disease-resistant crop plants is of crucial importance. The establishment of a database and understanding of the defense mechanisms under combined stress conditions would be meaningful for the development of resistant and tolerant plants. It is clear that leaf pathogens show great tolerance to salinity stress and result in pathogenicity in crop plants. We noticed that regulation of the stomata through biochemical applications and some effort with the upregulation of the minor gene expressions indirectly involved with the defense mechanisms could be a great way to increase the defense metabolites without interfering with quality parameters. In this review, we selected wheat as a model plant and Zymoseptoria tritici as a model leaf pathogen to evaluate the defense mechanisms under saline conditions through physiological, biochemical, and molecular pathways and suggested various ways to generate tolerant and resistant cereal plants.

Coping with salinity stress: segmental group 7 chromosome introgressions from halophytic Thinopyrum species greatly enhance tolerance of recipient durum wheat

Increased soil salinization, tightly related to global warming and drought and exacerbated by intensified irrigation supply, implies highly detrimental effects on staple food crops such as wheat. The situation is particularly alarming for durum wheat (DW), better adapted to arid/semi-arid environments yet more sensitive to salt stress than bread wheat (BW). To enhance DW salinity tolerance, we resorted to chromosomally engineered materials with introgressions from allied halophytic Thinopyrum species. "Primary" recombinant lines (RLs), having portions of their 7AL arms distally replaced by 7el1L Th. ponticum segments, and "secondary" RLs, harboring Th. elongatum 7EL insertions "nested" into 7el1L segments, in addition to near-isogenic lines lacking any alien segment (CLs), cv. Om Rabia (OR) as salt tolerant control, and BW introgression lines with either most of 7el1 or the complete 7E chromosome substitution as additional CLs, were subjected to moderate (100 mM) and intense (200 mM) salt (NaCl) stress at early growth stages. The applied stress altered cell cycle progression, determining a general increase of cells in G1 and a reduction in S phase. Assessment of morpho-physiological and biochemical traits overall showed that the presence of Thinopyrum spp. segments was associated with considerably increased salinity tolerance versus its absence. For relative water content, Na+ accumulation and K+ retention in roots and leaves, oxidative stress indicators (malondialdehyde and hydrogen peroxide) and antioxidant enzyme activities, the observed differences between stressed and unstressed RLs versus CLs was of similar magnitude in "primary" and "secondary" types, suggesting that tolerance factors might reside in defined 7el1L shared portion(s). Nonetheless, the incremental contribution of 7EL segments emerged in various instances, greatly mitigating the effects of salt stress on root and leaf growth and on the quantity of photosynthetic pigments, boosting accumulation of compatible solutes and minimizing the decrease of a powerful antioxidant like ascorbate. The seemingly synergistic effect of 7el1L + 7EL segments/genes made "secondary" RLs able to often exceed cv. OR and equal or better perform than BW lines. Thus, transfer of a suite of genes from halophytic germplasm by use of fine chromosome engineering strategies may well be the way forward to enhance salinity tolerance of glycophytes, even the sensitive DW.

Triticum aestivum: antioxidant gene profiling and morpho-physiological studies under salt stress

BACKGROUND

Soil salinity drastically reduced wheat growth and production in Pakistan. It is a need of an hour to identify the best suitable salt tolerance or resistant wheat varieties which shows good growth under salinity affected areas. In presented study, two wheat varieties Johar (salt tolerant) and Sarsabaz (salt sensitive) were examined under NaCl stress conditions.

METHODS

Antioxidant enzyme activities were investigated in 10-days old wheat seedlings under 200 mM NaCl stress in hydroponic conditions. To investigate the various growth parameters, antioxidant enzyme activities such as superoxide dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6) and ascorbate peroxidase (APX: EC 1.11.1.11) were monitored and studied. Besides this various growth parameters such as length of the roots, shoots, as well as Physiological parameters likes lipid peroxidation by malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and proline contents and antioxidant enzyme activities were estimated. The effect of salinity was also observed on gene transcription level and eventually expression level.

RESULTS

Shoot and root length were decreased in Sarsabaz variety while it showed opposite trend in johar at 200 mM salt concentration. The concentration of proline showed a noticeable rise in salt dependency. Higher concentrations of Proline in Johar were observed as compared to Sarsabaz. SOD showed the increase in activity for antioxidant enzymes. Significant increase of SOD levels were observed in shoot tissues as compared to root tissues. The results indicated that the shoots were more susceptible to salt stress. Activity of APX showed similar affects in both varieties. The production of CAT enzyme in the shoot and root tissues of both varieties showed substantial growth under increased salt stress. Furthermore, NaCl stress has increased the expression of certain genes coding for antioxidant enzymes such as catalase, superoxide dismutase, and peroxidase. Maximum expression of all the antioxidant enzyme coding genes were observed in Johar (tolerant) at 48 h exposure to salt. In contrast the expression of the all mentioned genes in Sarsabaz variety were found maximum at early hours (24 h) and gradually decreased at 48 h.

CONCLUSION

The study showed that the selected salt tolerant wheat variety Johar is significantly resistant to 200 mM NaCl salt level as compared to Sarsabaz.

No escape: The influence of substrate sodium on plant growth and tissue sodium responses

As an essential micronutrient for many organisms, sodium plays an important role in ecological and evolutionary dynamics. Although plants mediate trophic fluxes of sodium, from substrates to higher trophic levels, relatively little comparative research has been published about plant growth and sodium accumulation in response to variation in substrate sodium. Accordingly, we carried out a systematic review of plants' responses to variation in substrate sodium concentrations.We compared biomass and tissue-sodium accumulation among 107 cultivars or populations (67 species in 20 plant families), broadly expanding beyond the agricultural and model taxa for which several generalizations previously had been made. We hypothesized a priori response models for each population's growth and sodium accumulation as a function of increasing substrate NaCl and used Bayesian Information Criterion to choose the best model. Additionally, using a phylogenetic signal analysis, we tested for phylogenetic patterning of responses across taxa.The influence of substrate sodium on growth differed across taxa, with most populations experiencing detrimental effects at high concentrations. Irrespective of growth responses, tissue sodium concentrations for most taxa increased as sodium concentration in the substrate increased. We found no strong associations between the type of growth response and the type of sodium accumulation response across taxa. Although experiments often fail to test plants across a sufficiently broad range of substrate salinities, non-crop species tended toward higher sodium tolerance than domesticated species. Moreover, some phylogenetic conservatism was apparent, in that evolutionary history helped predict the distribution of total-plant growth responses across the phylogeny, but not sodium accumulation responses.Our study reveals that saltier plants in saltier soils proves to be a broadly general pattern for sodium across plant taxa. Regardless of growth responses, sodium accumulation mostly followed an increasing trend as substrate sodium levels increased.

Functional analysis of TmHKT1;4-A2 promoter through deletion analysis provides new insight into the regulatory mechanism underlying abiotic stress adaptation

Bioinformatic, molecular, and biochemical analysis were performed to get more insight into the regulatory mechanism by which TmHKT1;4-A2 is regulated. HKT transporters from different plant species have been shown to play important role in plant response to salt. In previous work, TmHKT1;4-A2 gene from Triticum monococcum has been characterized as a major gene for Nax1 QTL (Tounsi et al. Plant Cell Physiol 57:2047-2057, 2016). So far, little is known about its regulatory mechanism. In this study, the promoter region of TmHKT1;4-A2 (1400 bp) was isolated and considered as the full-length promoter (PA2-1400). In silico analysis revealed the presence of important cis-acting elements related to abiotic stresses and phytohormones. Interestingly, our real-time RT-PCR analysis provided evidence that TmHKT1;4-A2 is regulated not only by salt stress but also by osmotic, heavy metal, oxidative, and hormones stresses. In transgenic Arabidopsis plants, TmHKT1;4-A2 is strongly active in vascular tissues of roots and leaves. Through 5'-end deletion analysis, we showed that PA2-1400 promoter is able to drive strong GUS activity under normal conditions and in response to different stresses compared to PA2-824 and PA2-366 promoters. These findings provide new information on the regulatory mechanism of TmHKT1;4-A2 and shed more light on its role under different stresses.

Salt-Induced Differences During the Gene Expression of Telomerase Enzyme in Sunflower

Background:

Salinity is one of the most important environmental stresses which reduces the nutrient uptake, growth and yield of crops including sunflower.

Objectives:

The aim of this study was evaluating the expression pattern of telomerase gene, TERT, in sunflower plants under salinity stress.

Materials and Methods:

Sunflower plants of both sensitive and resistant lines were grown in greenhouse and treated with different levels of NaCl (2, 5 and 8 dSm^-1). The expression pattern of TERT gene was evaluated at 8^th leaf stage 6, 12 and 24 hours post salt treatment using real time-PCR, since the effects of salt stress are eventually manifested in the leaves.

Results:

In both lines, salt-subjected plants showed reduced size and dried leaves, due to breakthrough of the growth. Compared to the control group, treated groups tended to indicate downregulated pattern of TERT gene expression.

Conclusions:

This study offers TERT as a new gene affected by salt stress when growth is arrested.

Morphological and physiological responses of two willow species from different habitats to salt stress

Plant salt tolerance is a complex mechanism, and different plant species have different strategies for surviving salt stress. In the present study, we analyzed and compared the morphological and physiological responses of two willow species (Salix linearistipularis and Salix matsudana) from different habitats to salt stress. S. linearistipularis exhibited higher seed germination rates and seedling root Na⁺ efflux than S. matsudana under salt stress. After salt treatment, S. linearistipularis leaves exhibited less Na⁺ accumulation, loss of water and chlorophyll, reduction in photosynthetic capacity, and damage to leaf cell structure than leaves of S. matsudana. Scanning electron microscopy combined with gas chromatography mass spectrometry showed that S. linearistipularis leaves had higher cuticular wax loads than S. matsudana leaves. Overall, our results showed that S. linearistipularis had higher salt tolerance than S. matsudana, which was associated with different morphological and physiological responses to salt stress. Furthermore, our study suggested that S. linearistipularis could be a promising tree species for saline-alkali land greening and improvement.

Varietal differences in physiological and biochemical responses to salinity stress in six finger millet plants

Finger millet is an important cereal that is grown in semi-arid and arid regions of East-Africa. Salinity stress is a major environmental impediment for the crop growth and production. This study aimed to understand the physiological and biochemical responses to salinity stress of six Kenyan finger millet varieties (GBK043137, GBK043128, GBK043124, GBK043122, GBK043094, GBK043050) grown across different agroecological zones under NaCl-induced salinity stress (100, 200 and 300 mM NaCl). Seeds were germinated on the sterile soil and treated using various concentrations of NaCl for 2 weeks. Early-seedling stage of germinated plants were irrigated with the same salt concentrations for 60 days. The results indicated depression in germination percentage, shoot and root growth rate, leaf relative water content, chlorophyll content, leaf K⁺ concentration, and leaf K⁺/Na⁺ ratios with increased salt levels and the degree of increment differed among the varieties. On the contrary, the content of proline, malonaldehyde, leaf total proteins, and reduced sugar increased with increasing salinity. At the same time, the leaf Na⁺ and Cl^- amounts of all plants increased substantially with increasing stress levels. Clustering analysis placed GBK043094 and GBK043137 together and these varieties were identified as salt-tolerant based on their performance. Taken together, our findings indicated a significant varietal variability for most of the parameters analysed. The superior varieties identified could be used as promising genetic resources in future breeding programmes directed towards development of salt-tolerant finger millet hybrids. Further analysis at genomic level needs to be undertaken to better understand the genetic factors that promote salinity tolerance in finger millet.

Seed priming with sorghum extracts and benzyl aminopurine improves the tolerance against salt stress in wheat (Triticum aestivum L.)

Salt stress impedes the productivity of wheat (Triticum aestivum L.) in many parts of the world. This study evaluated the potential role of benzyl aminopurine (BAP) and sorghum water extract (SWE) in improving the wheat performance under saline conditions. Seeds were primed with BAP (5 mg L^-1), SWE (5% v/v), BAP + SWE, and distilled water (hydropriming). Soil filled pots maintained at the soil salinity levels of 4 and 10 dS m^-1 were used for the sowing of primed and non-primed seeds. Salt stress suppressed the wheat growth; seed priming treatments significantly improved the wheat growth under optimal and suboptimal conditions. Total phenolics, total soluble sugars and proteins, α-amylase activity, chlorophyll contents, and tissue potassium ion (K⁺) contents were increased by seed priming under salt stress; while, tissue sodium ion (Na⁺) contents were decreased. Seed priming with SWE + BAP was the most effective in this regard. Under salt stress, the tissue Na⁺ contents were reduced by 5.78, 28.3, 32.2, 36.7% by hydropriming, seed priming with SWE, seed priming with BAP, and seed priming with SWE + BAP, respectively over the non-primed control. Effectiveness of seed priming techniques followed the order SWE + BAP > BAP > SWE > Hydropriming. In conclusion, seed priming with SWE + BAP may be opted to improve the tolerance against salt stress in wheat.

Molecular characterization and expression analysis of the Na+/H+ exchanger gene family in Medicago truncatula

One important mechanism plants use to cope with salinity is keeping the cytosolic Na⁺ concentration low by sequestering Na⁺ in vacuoles, a process facilitated by Na⁺/H⁺ exchangers (NHX). There are eight NHX genes (NHX1 through NHX8) identified and characterized in Arabidopsis thaliana. Bioinformatics analyses of the known Arabidopsis genes enabled us to identify six Medicago truncatula NHX genes (MtNHX1, MtNHX2, MtNHX3, MtNHX4, MtNHX6, and MtNHX7). Twelve transmembrane domains and an amiloride binding site were conserved in five out of six MtNHX proteins. Phylogenetic analysis involving A. thaliana, Glycine max, Phaseolus vulgaris, and M. truncatula revealed that each individual MtNHX class (class I: MtNHX1 through 4; class II: MtNHX6; class III: MtNHX7) falls under a separate clade. In a salinity-stress experiment, M. truncatula exhibited ~ 20% reduction in biomass. In the salinity treatment, sodium contents increased by 178 and 75% in leaves and roots, respectively, and Cl^- contents increased by 152 and 162%, respectively. Na⁺ exclusion may be responsible for the relatively smaller increase in Na⁺ concentration in roots under salt stress as compared to Cl^-. Decline in tissue K⁺ concentration under salinity was not surprising as some antiporters play an important role in transporting both Na⁺ and K ⁺ . MtNHX1, MtNHX6, and MtNHX7 display high expression in roots and leaves. MtNHX3, MtNHX6, and MtNHX7 were induced in roots under salinity stress. Expression analysis results indicate that sequestering Na⁺ into vacuoles may not be the principal component trait of the salt tolerance mechanism in M. truncatula and other component traits may be pivotal.