Genetic Dissection of Alkalinity Tolerance at the Seedling Stage in Rice (Oryza sativa) Using a High-Resolution Linkage Map

Although both salinity and alkalinity result from accumulation of soluble salts in soil, high pH and ionic imbalance make alkaline stress more harmful to plants. This study aimed to provide molecular insights into the alkalinity tolerance using a recombinant inbred line (RIL) population developed from a cross between Cocodrie and Dular with contrasting response to alkalinity stress. Forty-six additive QTLs for nine morpho-physiological traits were mapped on to a linkage map of 4679 SNPs under alkalinity stress at the seedling stage and seven major-effect QTLs were for alkalinity tolerance scoring, Na⁺ and K⁺ concentrations and Na⁺:K⁺ ratio. The candidate genes were identified based on the comparison of the impacts of variants of genes present in five QTL intervals using the whole genome sequences of both parents. Differential expression of no apical meristem protein, cysteine protease precursor, retrotransposon protein, OsWAK28, MYB transcription factor, protein kinase, ubiquitin-carboxyl protein, and NAD binding protein genes in parents indicated their role in response to alkali stress. Our study suggests that the genetic basis of tolerance to alkalinity stress is most likely different from that of salinity stress. Introgression and validation of the QTLs and genes can be useful for improving alkalinity tolerance in rice at the seedling stage and advancing understanding of the molecular genetic basis of alkalinity stress adaptation.

Enhancing growth and salinity stress tolerance of date palm using Piriformospora indica

Endophytic fungi are known to enhance plant growth and performance under salt stress. The current study investigated the growth, as well as biochemical and molecular properties of Phoenix dactylifera colonized with the mutualistic fungus Piriformospora indica, under control and salinity stress. Our findings indicated an increase in the plant biomass, lateral root density, and chlorophyll content of P. indica-colonized plants under both normal and salt stress conditions. Furthermore, there was a decline in the inoculated plants leaf and root Na⁺/K⁺ ratio. The colonization enhanced the levels of antioxidant enzymes such as catalase, superoxide dismutase, and peroxidase in plants. Increased ionic content of Zn and P were also found in salt-stressed date palm. The fungus colonization was also associated with altered expression levels of essential Na⁺ and K⁺ ion channels in roots like HKT1;5 and SOS1 genes. This alteration improved plant growth due to their preservation of Na⁺ and K⁺ ions balanced homeostasis under salinity stress. Moreover, it was confirmed that RSA1 and LEA2 genes were highly expressed in salt-stressed and colonized plant roots and leaves, respectively. The current study exploited P. indica as an effective natural salt stress modulator to ameliorate salinity tolerance in plants.

Ecophysiological Responses of Tall Wheatgrass Germplasm to Drought and Salinity

Tall wheatgrass (Thinopyrum ponticum (Podp.) Barkworth and D.R. Dewey) is an important, highly salt-tolerant C3 forage grass. The objective of this work was to learn about the ecophysiological responses of accessions from different environmental origins under drought and salinity conditions, to provide information for selecting superior germplasm under combined stress in tall wheatgrass. Four accessions (P3, P4, P5, P9) were irrigated using combinations of three salinity levels (0, 0.1, 0.3 M NaCl) and three drought levels (100%, 50%, 30% water capacity) over 90 days in a greenhouse. The control treatment showed the highest total biomass, but water-use efficiency (WUE), δ¹³C, proline, N concentration, leaf length, and tiller density were higher under moderate drought or/and salinity stress than under control conditions. In tall wheatgrass, K⁺ functions as an osmoregulator under drought, attenuated by salinity, and Na⁺ and Cl^- function as osmoregulators under salinity and drought, while proline is an osmoprotector under both stresses. P3 and P9, from environments with mild/moderate stress, prioritized reproductive development, with high evapotranspiration and the lowest WUE and δ¹³C values. P4 and P5, from more stressful environments, prioritized vegetative development through tillering, showing the lowest evapotranspiration, the highest δ¹³C values, and different mechanisms for limiting transpiration. The δ¹³C value, leaf biomass, tiller density, and leaf length had high broad-sense heritability (H²), while the Na⁺/K⁺ ratio had medium H². In conclusion, the combined use of the δ¹³C value, Na⁺/K⁺ ratio, and canopy structural variables can help identify accessions that are well-adapted to drought and salinity, also considering the desirable plant characteristics. Tall wheatgrass stress tolerance could be used to expand forage production under a changing climate.

Nanoceria seed priming enhanced salt tolerance in rapeseed through modulating ROS homeostasis and α-amylase activities

BACKGROUND

Salinity is a big threat to agriculture by limiting crop production. Nanopriming (seed priming with nanomaterials) is an emerged approach to improve plant stress tolerance; however, our knowledge about the underlying mechanisms is limited.

RESULTS

Herein, we used cerium oxide nanoparticles (nanoceria) to prime rapeseeds and investigated the possible mechanisms behind nanoceria improved rapeseed salt tolerance. We synthesized and characterized polyacrylic acid coated nanoceria (PNC, 8.5 ± 0.2 nm, -43.3 ± 6.3 mV) and monitored its distribution in different tissues of the seed during the imbibition period (1, 3, 8 h priming). Our results showed that compared with the no nanoparticle control, PNC nanopriming improved germination rate (12%) and biomass (41%) in rapeseeds (Brassica napus) under salt stress (200 mM NaCl). During the priming hours, PNC were located mostly in the seed coat, nevertheless the intensity of PNC in cotyledon and radicle was increased alongside with the increase of priming hours. During the priming hours, the amount of the absorbed water (52%, 14%, 12% increase at 1, 3, 8 h priming, respectively) and the activities of α-amylase were significantly higher (175%, 309%, 295% increase at 1, 3, 8 h priming, respectively) in PNC treatment than the control. PNC primed rapeseeds showed significantly lower content of MDA, H2O2, and •O2- in both shoot and root than the control under salt stress. Also, under salt stress, PNC nanopriming enabled significantly higher K+ retention (29%) and significantly lower Na+ accumulation (18.5%) and Na+/K+ ratio (37%) than the control.

CONCLUSIONS

Our results suggested that besides the more absorbed water and higher α-amylase activities, PNC nanopriming improves salt tolerance in rapeseeds through alleviating oxidative damage and maintaining Na+/K+ ratio. It adds more knowledge regarding the mechanisms underlying nanopriming improved plant salt tolerance.

Evaluation of the Diuretic Activity of the Aqueous and 80% Methanol Extracts of Ficus sur Forssk (Moraceae) Leaves in Saline-loaded Rats

Background

Although investigations on different pharmacological activities of the experimental plant, Ficus sur have been conducted, its folklore use for diuresis has not yet been validated. The current study, therefore, focused on the diuretic activity of aqueous and 80% methanol extracts of F. sur Forssk. leaves in rats.

Methods

Rats were randomly assigned into eight groups each consisting of six rats. Test groups received either 100 mg/kg, 200 mg/kg, or 400 mg/kg of aqueous or 80% methanol leaves extract. The negative control group and positive control were treated with 2 mL/100 g of distilled water and furosemide (10 mg/kg), respectively. Thereafter urine volume was recorded every hour until the end of the fifth hour, and cumulative urine volume of each rat was measured. Then, diuretic activity, diuretic index, saliuretic index, natriuretic index and carbonic anhydrase inhibition index in each group were calculated, and results were compared among the groups.

Results

The middle (200 mg/kg) and the highest (400 mg/kg) doses of both extracts significantly increased diuresis at the fifth hour (p<0.001) compared to the negative control, although the diuretic activity was less than that of the positive control. Regarding electrolyte excretion, all dose levels of both extracts showed significant natriuresis (p<0.001) and chloriuresis (p<0.01) compared to the negative control. Aqueous extract displayed more significant diuretic effect than 80% methanol extract. The aqueous and 80% methanol extracts produced alkaline urine.

Conclusion

The crude leaves extracts of F. sur increased urinary excretion and concentration of urinary electrolytes in a dose-dependent manner. These findings are in agreement with the traditional claim for use of F. sur as diuretic agent.

HKT1;5 Transporter Gene Expression and Association of Amino Acid Substitutions With Salt Tolerance Across Rice Genotypes

Plants need to maintain a low Na+/K+ ratio for their survival and growth when there is high sodium concentration in soil. Under these circumstances, the high affinity K+ transporter (HKT) and its homologs are known to perform a critical role with HKT1;5 as a major player in maintaining Na+ concentration. Preferential expression of HKT1;5 in roots compared to shoots was observed in rice and rice-like genotypes from real time PCR, microarray, and RNAseq experiments and data. Its expression trend was generally higher under increasing salt stress in sensitive IR29, tolerant Pokkali, both glycophytes; as well as the distant wild rice halophyte, Porteresia coarctata, indicative of its importance during salt stress. These results were supported by a low Na+/K+ ratio in Pokkali, but a much lower one in P. coarctata. HKT1;5 has functional variability among salt sensitive and tolerant varieties and multiple sequence alignment of sequences of HKT1;5 from Oryza species and P. coarctata showed 4 major amino acid substitutions (140 P/A/T/I, 184 H/R, D332H, V395L), with similarity amongst the tolerant genotypes and the halophyte but in variance with sensitive ones. The best predicted 3D structure of HKT1;5 was generated using Ktrab potassium transporter as template. Among the four substitutions, conserved presence of aspartate (332) and valine (395) in opposite faces of the membrane along the Na+/K+ channel was observed only for the tolerant and halophytic genotypes. A model based on above, as well as molecular dynamics simulation study showed that valine is unable to generate strong hydrophobic network with its surroundings in comparison to leucine due to reduced side chain length. The resultant alteration in pore rigidity increases the likelihood of Na+ transport from xylem sap to parenchyma and further to soil. The model also proposes that the presence of aspartate at the 332 position possibly leads to frequent polar interactions with the extracellular loop polar residues which may shift the loop away from the opening of the constriction at the pore and therefore permit easy efflux of the Na+. These two substitutions of the HKT1;5 transporter probably help tolerant varieties maintain better Na+/K+ ratio for survival under salt stress.

TaPUB1, a Putative E3 Ligase Gene from Wheat, Enhances Salt Stress Tolerance in Transgenic Nicotiana benthamiana

High salinity is one of the most severe environmental stresses and limits the growth and yield of diverse crop plants. We isolated a gene named TaPUB1 from wheat (Triticum aestivum L. cv HF9703) that encodes a novel protein containing a U-box domain, the precursor RNA processing 19p (Prp19) superfamily and WD-40 repeats. Real-time reverse transcription-PCR analysis showed that TaPUB1 transcript accumulation was up-regulated by high salinity, drought and phytohormones, suggesting that it plays a role in the abiotic-related defense response. We overexpressed TaPUB1 in Nicotiana benthamiana to evaluate the function of TaPUB1 in the regulation of the salt stress response. Transgenic N. benthamiana plants (OE) with constitutively overexpressed TaPUB1 under the control of the Cauliflower mosaic virus 35S (CaMV 35S) promoter exhibited a higher germination rate, less growth inhibition, less Chl loss and higher photosynthetic capacity than wild-type (WT) plants under salt stress conditions. These results demonstrated the increased tolerance of OE plants to salt stress compared with the WT. The OE plants had lower osmotic potential (OP), reduced Na+ toxicity and less reactive oxygen species accumulation compared with the WT, which may be related to their higher level of osmolytes, lower Na+/K+ ratio and higher antioxidant enzyme activities under salt stress conditions. Consistent with these results, the up-regulated expression of osmic- and antioxidant-related genes in OE plants indicated a role for TaPUB1 in plant salt tolerance.

Diuretic activity of the aqueous crude extract and solvent fractions of the leaves of Thymus serrulatus in mice

The present study was undertaken to investigate the diuretic activity and acute toxicity profile of the crude aqueous extract and solvent fraction of the leaves of Thymus serrulatus in saline-loaded Swiss albino mice. Mice of either sex were divided into six groups (five animals in each group). The control group received normal saline (25 mL/kg), while the reference group received hydrochlorothiazide (10 mg/kg). Group III to Group VI received the test substances at dose levels of 125, 250, 500, and 1,000 mg/kg orally, respectively. At the end of the fifth hour, urine was collected, and total volume of urine excreted by each animal was recorded. Concentrations of urinary Na⁺ and K⁺ were determined, and the Na⁺/K⁺ ratio was calculated to make comparison among the groups. The acute toxicity of the most active fraction was also evaluated. The findings demonstrated that the crude aqueous extract of T. serrulatus leaves showed significant diuretic (P<0.01), natriuretic (P<0.01), and kaliuretic (P<0.01) effects. At the dose of 1,000 mg/kg, the n-butanol fraction demonstrated the highest diuretic activity comparable to the reference drug. It also showed a good natriuretic activity. The dichloromethane fraction, however, did not have significant diuretic activity. Both the crude aqueous extract of the leaves of T. serrulatus and its n-butanol fraction have diuretic activity with high concentration of urinary electrolytes in mice. Further studies, however, need to be pursued on the possible mechanism(s) of diuretic action.

Physio-biochemical and morphological characters of halophyte legume shrub, Acacia ampliceps seedlings in response to salt stress under greenhouse

Acacia ampliceps (salt wattle), a leguminous shrub, has been introduced in salt-affected areas in the northeast of Thailand for the remediation of saline soils. However, the defense mechanisms underlying salt tolerance A. ampliceps are unknown. We investigated various physio-biochemical and morphological attributes of A. ampliceps in response to varying levels of salt treatment (200-600 mM NaCl). Seedlings of A. ampliceps (25 ± 2 cm in plant height) raised from seeds were treated with 200 mM (mild stress), 400 and 600 mM (extreme stress) of salt treatment (NaCl) under greenhouse conditions. Na(+) and Ca(2+) contents in the leaf tissues increased significantly under salt treatment, whereas K(+) content declined in salt-stressed plants. Free proline and soluble sugar contents in plants grown under extreme salt stress (600 mM NaCl) for 9 days significantly increased by 28.7 (53.33 μmol g(-1) FW) and 3.2 (42.11 mg g(-1) DW) folds, respectively over the control, thereby playing a major role as osmotic adjustment. Na(+) enrichment in the phyllode tissues of salt-stressed seedlings positively related to total chlorophyll (TC) degradation (R (2) = 0.72). Photosynthetic pigments and chlorophyll fluorescence in salt-stressed plants increased under mild salt stress (200 mM NaCl). However, these declined under high levels of salinity (400-600 mM NaCl), consequently resulting in a reduced net photosynthetic rate (R (2) = 0.81) and plant dry weight (R (2) = 0.91). The study concludes that A. ampliceps has an osmotic adjustment and Na(+) compartmentation as effective salt defense mechanisms, and thus it could be an excellent species to grow in salt-affected soils.

Genetic variation in Southern USA rice genotypes for seedling salinity tolerance

The success of a rice breeding program in developing salt tolerant varieties depends on genetic variation and the salt stress response of adapted and donor rice germplasm. In this study, we used a combination of morphological and physiological traits in multivariate analyses to elucidate the phenotypic and genetic variation in salinity tolerance of 30 Southern USA rice genotypes, along with 19 donor genotypes with varying degree of tolerance. Significant genotypic variation and correlations were found among the salt injury score (SIS), ion leakage, chlorophyll reduction, shoot length reduction, shoot K(+) concentration, and shoot Na(+)/K(+) ratio. Using these parameters, the combined methods of cluster analysis and discriminant analysis validated the salinity response of known genotypes and classified most of the USA varieties into sensitive groups, except for three and seven varieties placed in the tolerant and moderately tolerant groups, respectively. Discriminant function and MANOVA delineated the differences in tolerance and suggested no differences between sensitive and highly sensitive (HS) groups. DNA profiling using simple sequence repeat markers showed narrow genetic diversity among USA genotypes. However, the overall genetic clustering was mostly due to subspecies and grain type differentiation and not by varietal grouping based on salinity tolerance. Among the donor genotypes, Nona Bokra, Pokkali, and its derived breeding lines remained the donors of choice for improving salinity tolerance during the seedling stage. However, due to undesirable agronomic attributes and photosensitivity of these donors, alternative genotypes such as TCCP266, Geumgangbyeo, and R609 are recommended as useful and novel sources of salinity tolerance for USA rice breeding programs.

SbHKT1;4, a member of the high-affinity potassium transporter gene family from Sorghum bicolor, functions to maintain optimal Na⁺ /K⁺ balance under Na⁺ stress

In halophytic plants, the high-affinity potassium transporter HKT gene family can selectively uptake K⁺ in the presence of toxic concentrations of Na⁺. This has so far not been well examined in glycophytic crops. Here, we report the characterization of SbHKT1;4, a member of the HKT gene family from Sorghum bicolor. Upon Na⁺ stress, SbHKT1;4 expression was more strongly upregulated in salt-tolerant sorghum accession, correlating with a better balanced Na⁺ /K⁺ ratio and enhanced plant growth. Heterogeneous expression analyses in mutants of Saccharomyces cerevisiae and Arabidopsis thaliana indicated that overexpressing SbHKT1;4 resulted in hypersensitivity to Na⁺ stress, and such hypersensitivity could be alleviated with the supply of elevated levels of K⁺, implicating that SbHKT1;4 may mediate K⁺ uptake in the presence of excessive Na⁺. Further electrophysiological evidence demonstrated that SbHKT1;4 could transport Na⁺ and K⁺ when expressed in Xenopus laevis oocytes. The relevance of the finding that SbHKT1;4 functions to maintain optimal Na⁺ /K⁺ balance under Na⁺ stress to the breeding of salt-tolerant glycophytic crops is discussed.

Role of HKT1 in Thellungiella salsuginea, a model extremophile plant

Maintenance of the cytosolic Na(+)/K(+) ratio under saline conditions is crucial for plants. HKT-type Na(+) transporters play a key role in keeping low cytosolic Na(+) concentrations thus retaining a low Na(+)/K(+) ratio, that reduces Na(+) toxicity and causing high salinity stress tolerance. Two HKT-type transporters, AtHKT1 from Arabidopsis and TsHKT1;2 from Thellungiella salsuginea, that share high DNA and protein sequence identities, are distinguished by fundamentally different ion selection and salinity stress behavior. On the level of transcription, TsHKT1;2 is dramatically induced upon salt stress, whereas AtHKT1 is downregulated. TsHKT1;2-RNAi lines show severe potassium deficiency and are also sensitive to high [Na(+)]. We have validated the ability of the TsHKT1;2 protein to act as an efficient K(+) transporter in the presence of high [Na(+)] by expression in yeast cells. K(+) specificity is based on amino acid differences in the pore of the transporter protein relative to AtHKT1.