Ion homeostasis in a salt-secreting halophytic grass

Aeluropus lagopoides: an important halophyte with key physiological and molecular mechanisms for salinity tolerance and a unique genetic resource for developing climate resilient crops

Aeluropus lagopoides is salt secreting halophytic perennial grass that commonly grows in coastal regions. Under excessive saline conditions, A. lagopoides is able to thrive and completes its life cycle. It has developed various adaptive mechanisms to tolerate harsh environmental conditions. Aeluropus follow the novel mechanism of salt secretion by excreting Na+ from the leaf sheath and stem of the plant in the form of salt crystals. Various salt responsive genes and transcription factors have been studied under salinity stress in A. lagopoides. Economically important phytochemicals are also present in this plant, thus, making it industrially important. Utilization of salt stress responsive genes and transcription factors in developing salt tolerant transgenics crops can also provide significant benefits, and potentially boost the agricultural industry for sustainable growth and production.

ROS scavenging and ion homeostasis is required for the adaptation of halophyte Karelinia caspia to high salinity

The halophyte Karelinia caspia has not only fodder and medical value but also can remediate saline-alkali soils. Our previous study showed that salt-secreting by salt glands is one of main adaptive strategies of K. caspia under high salinity. However, ROS scavenging, ion homeostasis, and photosynthetic characteristics responses to high salinity remain unclear in K. caspia. Here, physio-biochemical responses and gene expression associated with ROS scavenging and ions transport were tested in K. caspia subjected to 100-400 mM NaCl for 7 days. Results showed that both antioxidant enzymes (SOD, APX) activities and non-enzymatic antioxidants (chlorogenic acid, α-tocopherol, flavonoids, polyamines) contents were significantly enhanced, accompanied by up-regulating the related enzyme and non-enzymatic antioxidant synthesis gene (KcCu/Zn-SOD, KcAPX6, KcHCT, KcHPT1, Kcγ-TMT, KcF3H, KcSAMS and KcSMS) expression with increasing concentrations of NaCl. These responses are beneficial for removing excess ROS to maintain a stable level of H2O2 and O2 - without lipid peroxidation in the K. caspia response to high salt. Meanwhile, up-regulating expression of KcSOS1/2/3, KcNHX1, and KcAVP was linked to Na+ compartmentalization into vacuoles or excretion through salt glands in K. caspia. Notably, salt can improve the function of PSII that facilitate net photosynthetic rates, which is helpful to growing normally in high saline. Overall, the findings suggested that ROS scavenging systems and Na+/K+ transport synergistically contributed to redox equilibrium, ion homeostasis, and the enhancement of PSII function, thereby conferring high salt tolerance.

Exogenous hydrogen sulfide mediates Na+ and K+ fluxes of salt gland in salt-secreting mangrove plant Avicennia marina

Hydrogen sulfide (H2S), is a crucial biological player in plants. Here, we primarily explored the interaction between sodium hydrosulfide (NaHS, a H2S donor) and the fluxes of Na+ and K+ from the salt glands of mangrove species Avicennia marina (Forsk.) Vierh. with non-invasive micro-test technology (NMT) and quantitative real-time PCR (qRT-PCR) approaches under salinity treatments. The results showed that under 400-mM NaCl treatment, the addition of 200-μM NaHS markedly increased the quantity of salt crystals in the adaxial epidermis of A. marina leaves, accompanied by an increase in the K+/Na+ ratio. Meanwhile, the endogenous content of H2S was dramatically elevated in this process. The NMT result revealed that the Na+ efflux was increased from salt glands, whereas K+ efflux was decreased with NaHS application. On the contrary, the effects of NaHS were reversed by H2S scavenger hypotaurine (HT), and DL-propargylglycine (PAG), an inhibitor of cystathionine-γ-lyase (CES, a H2S synthase). Moreover, enzymic assay revealed that NaHS increased the activities of plasma membrane and tonoplast H+-ATPase. qRT-PCR analysis revealed that NaHS significantly increased the genes transcript levels of tonoplast Na+/H+ antiporter (NHX1), plasma membrane Na+/H+ antiporter (SOS1), plasma membrane H+-ATPase (AHA1) and tonoplast H+-ATPase subunit c (VHA-c1), while suppressed above-mentioned gene expressions by the application of HT and PAG. Overall, H2S promotes Na+ secretion from the salt glands of A. marina by up-regulating the plasma membrane and tonoplast Na+/H+ antiporter and H+-ATPase.

The Plant V-ATPase

V-ATPase is the dominant proton pump in plant cells. It contributes to cytosolic pH homeostasis and energizes transport processes across endomembranes of the secretory pathway. Its localization in the trans Golgi network/early endosomes is essential for vesicle transport, for instance for the delivery of cell wall components. Furthermore, it is crucial for response to abiotic and biotic stresses. The V-ATPase's rather complex structure and multiple subunit isoforms enable high structural flexibility with respect to requirements for different organs, developmental stages, and organelles. This complexity further demands a sophisticated assembly machinery and transport routes in cells, a process that is still not fully understood. Regulation of V-ATPase is a target of phosphorylation and redox-modifications but also involves interactions with regulatory proteins like 14-3-3 proteins and the lipid environment. Regulation by reversible assembly, as reported for yeast and the mammalian enzyme, has not be proven in plants but seems to be absent in autotrophic cells. Addressing the regulation of V-ATPase is a promising approach to adjust its activity for improved stress resistance or higher crop yield.

LED spectral quality and NaCl salinity interact to affect growth, photosynthesis and phytochemical production of Mesembryanthemum crystallinum

The edible halophyte Mesembryanthemum crystallinum L. was grown at different NaCl salinities under different combined red and blue light-emitting diode (LED) light treatments. High salinity (500 mM NaCl) decreased biomass, leaf growth, and leaf water content. Interactions between LED ratio and salinity were detected for shoot biomass and leaf growth. All plants had F v /F m ratios close to 0.8 in dark-adapted leaves, suggesting that they were all healthy with similar maximal efficiency of PSII photochemistry. However, measured under the actinic light near or above the growth light, the electron transport rate (ETR) and photochemical quenching (qP) of M. crystallinum grown at 100 and 250 mM NaCl were higher than at 500 mM NaCl. Grown under red/blue LED ratios of 0.9, M. crystallinum had higher ETR and qP across all salinities indicating higher light energy utilisation. Crassulacean acid metabolism (CAM) was induced in M. crystallinum grown at 500 mM NaCl. CAM-induced leaves had much higher non-photochemical quenching (NPQ), suggesting that NPQ can be used to estimate CAM induction. M. crystallinum grown at 250 and 500 mM NaCl had higher total chlorophyll and carotenoids contents than at 100 mM NaCl. Proline, total soluble sugar, ascorbic acid, and total phenolic compounds were higher in plants at 250 and 500 mM NaCl compared with those at 100 mM NaCl. An interaction between LED ratio and salinity was detected for proline content. Findings of this study suggest that both salinity and light quality affect productivity, photosynthetic light use efficiency, and proline accumulation of M. crystallinum .

AlRab7 from Aeluropus lagopoides ameliorates ion toxicity in transgenic tobacco by regulating hormone signaling and reactive oxygen species homeostasis

The plants endomembrane system of the cellular compartments with its complex membrane trafficking network facilitates transport of macromolecules. The endomembrane dynamics are essential for maintaining basic and specific cellular functions including adaptation to the extracellular environment. The plant vacuole serves as a reservoir for nutrients and toxic metabolites and performs detoxification processes to maintain cellular homeostasis. The overexpression of AlRab7, a vesicle trafficking gene from Aeluropus lagopoides, improved germination and growth and reduced ionic and oxidative stress in transgenics. Moreover, the root and shoot of transgenic tobacco showed differential accumulation of phytohormone ABA and IAA with different ionic stresses. The improved growth (root and shoot length) can be co-related with higher IAA accumulation with NaCl stress. The low Na+ /K+ ratio with different NaCl stress treatments indicates better ion homeostasis in transgenics. Furthermore, the increased stomatal density and higher number of open stomata on both leaf surfaces in transgenics during NaCl stress suggest better gaseous exchange/functioning of guard cells. The maintained or increased superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase antioxidative enzyme activities suggest that an extensive reactive oxygen species (ROS) scavenging system was triggered to detoxify cellular ROS, which remained at low levels in transgenics during the different stress treatments. Our results suggest that the AlRab7 transgenic tobacco ameliorates ionic stress by facilitating differential and selective ion transport at vacuolar membrane regulating hormone signaling, ROS homeostasis, stomatal development, and movement.

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.

Regulation of salt-stressed sunflower (Helianthus annuus) seedling's water status by the coordinated action of Na+/K+ accumulation, nitric oxide, and aquaporin expression

Among abiotic stresses, salt stress is a major threat to crop production all over the world. Present work demonstrates the profuse accumulation of Na+ in 2-day-old, dark-grown sunflower (Helianthus annuus L.) seedlings roots in response to salt stress (NaCl). The pattern of K+ accumulation in response to salt stress is similar to that of Na+ but on relatively lower scale. Application of nitric oxide (NO) donor (DETA) scales down Na+ accumulation in salt-stressed seedlings. The impact of NO donor on K+ accumulation is, however, different in control and salt-stressed seedling roots. In control seedlings, it enhances K+ accumulation, whereas, it gets reduced in salt-stressed seedlings. Specialised channels called 'aquaporins' (AQPs) play a major role maintaining the water status and transport across plant parts under salt-stress. Thus, accumulation of plasma-membrane intrinsic proteins (PIPs) and tonoplast-intrinsic proteins (TIPs), localised on plasma-membrane and vacuolar-membrane, respectively was undertaken in 2-day-old, dark-grown seedling roots. Salt stress increased the abundance of these isoforms, whereas, NO application resulted in decreased accumulation of PIP2 and TIP1. PIP1 and TIP2 isoforms remained undetectable. Present work thus, puts forward a correlation between AQP expression and ions (Na+ and K+) homeostasis in response to salt stress and NO.

Overexpression of an Apocynum venetum flavonols synthetase gene confers salinity stress tolerance to transgenic tobacco plants

Soil salinity is a major limiting factor for agricultural production, threatening food security worldwide. A thorough understanding of the mechanisms underlying plant responses is required to effectively counter its deleterious effects on crop productivity. Total flavonoid accumulation reportedly improves salinity tolerance in many crops. Therefore, we isolated the full-length cDNA of a flavonol synthetase (FLS) gene from Apocynum venetum (AvFLS). The gene contained a 1008-bp open reading frame encoding a protein composed of 335 amino acid residues. Multiple sequence alignment showed that the AvFLS protein was highly homologous to FLSs from other plants. AvFLS was expressed in leaves, stems, roots, flowers, and germinated seeds. Expression pattern analysis revealed that AvFLS was significantly induced by salinity stress. AvFLS overexpression in tobacco positively affected the development and growth of transgenic plants under salinity stress: root and seedling growth were inhibited to a lesser extent, while seed germination rate increased. Additionally, the overexpression of AvFLS under salinity stress resulted in an increase in total flavonoid content (1.63 mg g-1 in wild-type samples and 4.63 mg g-1 on average in transgenic samples), which accompanied the increase in the activity of antioxidant enzymes and inhibited the production of reactive oxygen species. Further, AvFLS-overexpressing transgenic tobacco plants absorbed more K+ than wild type plants, leading to an increased K+/Na+ ratio, which in turn contributed to the maintenance of Na+/K+ homeostasis. These findings suggest that an AvFLS-induced increase in total flavonoid content enhanced plant salinity tolerance, implying the importance of AvFLS gene responses to salinity stress.

Current Understanding of Role of Vesicular Transport in Salt Secretion by Salt Glands in Recretohalophytes

Soil salinization is a serious and growing problem around the world. Some plants, recognized as the recretohalophytes, can normally grow on saline-alkali soil without adverse effects by secreting excessive salt out of the body. The elucidation of the salt secretion process is of great significance for understanding the salt tolerance mechanism adopted by the recretohalophytes. Between the 1950s and the 1970s, three hypotheses, including the osmotic potential hypothesis, the transfer system similar to liquid flow in animals, and vesicle-mediated exocytosis, were proposed to explain the salt secretion process of plant salt glands. More recently, increasing evidence has indicated that vesicular transport plays vital roles in salt secretion of recretohalophytes. Here, we summarize recent findings, especially regarding the molecular evidence on the functional roles of vesicular trafficking in the salt secretion process of plant salt glands. A model of salt secretion in salt gland is also proposed.

Early effects of salt stress on the physiological and oxidative status of the halophyte Lobularia maritima

Soil salinity is an abiotic stress that reduces agricultural productivity. For decades, halophytes have been studied to elucidate the physiological and biochemical processes involved in alleviating cellular ionic imbalance and conferring salt tolerance. Recently, several interesting genes with proven influence on salt tolerance were isolated from the Mediterranean halophyte Lobularia maritima (L.) Desv. A better understanding of salt response in this species is needed to exploit its potential as a source of stress-related genes. We report the characterisation of L. maritima's response to increasing NaCl concentrations (100-400 mM) at the physiological, biochemical and molecular levels. L. maritima growth was unaffected by salinity up to 100 mM NaCl and it was able to survive at 400 mM NaCl without exhibiting visual symptoms of damage. Lobularia maritima showed a Na+ and K+ accumulation pattern typical of a salt-includer halophyte, with higher contents of Na+ in the leaves and K+ in the roots of salt-treated plants. The expression profiles of NHX1, SOS1, HKT1, KT1 and VHA-E1 in salt-treated plants matched this Na+ and K+ accumulation pattern, suggesting an important role for these transporters in the regulation of ion homeostasis in leaves and roots of L. maritima. A concomitant stimulation in phenolic biosynthesis and antioxidant enzyme activity was observed under moderate salinity, suggesting a potential link between the production of polyphenolic antioxidants and protection against salt stress in L. maritima. Our findings indicate that the halophyte L. maritima can rapidly develop physiological and antioxidant mechanisms to adapt to salt and manage oxidative stress.

De novo RNA sequencing analysis of Aeluropus littoralis halophyte plant under salinity stress

The study of salt tolerance mechanisms in halophyte plants can provide valuable information for crop breeding and plant engineering programs. The aim of the present study was to investigate whole transcriptome analysis of Aeluropus littoralis in response to salinity stress (200 and 400 mM NaCl) by de novo RNA-sequencing. To assemble the transcriptome, Trinity v2.4.0 and Bridger tools, were comparatively used with two k-mer sizes (25 and 32 bp). The de novo assembled transcriptome by Bridger (k-mer 32) was chosen as final assembly for subsequent analysis. In general, 103290 transcripts were obtained. The differential expression analysis (log₂^FC > 1 and FDR < 0.01) showed that 1861 transcripts expressed differentially, including169 up and 316 down-regulated transcripts in 200 mM NaCl treatment and 1035 up and 430 down-regulated transcripts in 400 mM NaCl treatment compared to control. In addition, 89 transcripts were common in both treatments. The most important over-represented terms in the GO analysis of differentially expressed genes (FDR < 0.05) were chitin response, response to abscisic acid, and regulation of jasmonic acid mediated signaling pathway under 400 mM NaCl treatment and cell cycle, cell division, and mitotic cell cycle process under 200 mM treatment. In addition, the phosphatidylcholine biosynthetic process term was common in both salt treatments. Interestingly, under 400 mM salt treatment, the PRC1 complex that contributes to chromatin remodeling was also enriched along with vacuole as a general salinity stress responsive cell component. Among enriched pathways, the MAPK signaling pathway (ko04016) and phytohormone signal transduction (ko04075) were significantly enriched in 400 mM NaCl treatment, whereas DNA replication (ko03032) was the only pathway that significantly enriched in 200 mM NaCl treatment. Finally, our findings indicate the salt-concentration depended responses of A. littoralis, which well-known salinity stress-related pathways are induced in 400 mM NaCl, while less considered pathways, e.g. cell cycle and DNA replication, are highlighted under 200 mM NaCl treatment.

The AlRabring7 E3-Ub-ligase mediates AlRab7 ubiquitination and improves ionic and oxidative stress tolerance in Saccharomyces cerevisiae

The maintenance of ROS homeostasis, membrane biogenesis and recycling of molecules are common stress responses involving specific and complex regulatory network. Ubiquitination is an important and common mechanism which facilitates environmental adaptation in eukaryotes. In the present study we have cloned the AlRabring7, an E3-Ub-ligase, previously identified as AlRab7 interacting partner. The role of AlRabring7 for ubiquitinating AlRab7 and facilitating stress tolerance is analysed. The AlRabring7, with an open-reading frame of 702 bp encodes a protein of 233 amino acids, with RING-HC domain of 40 amino acids. In silico analysis shows that AlRabring7 is a C3HC4-type RING E3 Ub ligase. The protein - protein docking show interaction dynamics between AlRab7-AlRabring7-Ubiquitin proteins. The AlRab7 and AlRabring7 transcript showed up-regulation in response to different salts i.e: NaCl, KCl, CaCl₂, NaCl + KCl, NaCl + CaCl₂, imposing ionic as well as hyperosmotic stress, and also with oxidative stress by H₂O₂ treatment. Interestingly, the AlRabring7 showed early transcript expression with maximum expression in shoots on combinatorial stresses. The AlRab7 showed delayed and maximum expression with NaCl + CaCl₂ stress treatment. The AlRab7 complements yeast ypt7Δ mutants and restored the fragmented vacuole. The in vitro ubiquitination assay revealed that AlRabring7 function as E3 ubiquitin ligase and mediates AlRab7 ubiquitination. Overexpression of AlRab7 and AlRabring7 independently and when co-transformed enhanced the growth of yeast cells during stress conditions. Further, the bimolecular fluorescence complementation assay shows the in planta interaction of the two proteins. Our results suggest that AlRab7 and AlRabring7 confers enhanced stress tolerance in yeast.

Microscopic Characteristic and Chemical Composition Analysis of Three Medicinal Plants and Surface Frosts

The accumulation of chemical constituents of some medicinal plants, such as Paeonia ostii T. Hong et J. X. Zhang, Houpoëa officinalis (Rehder and E. H. Wilson) N. H. Xia and C. Y. Wu. and Atractylodes lancea (Thunb.) DC, can precipitate on the surface and form frosts after natural or artificial intervention. The characteristics of these three medicinal plants and their frosts were analyzed by light microscope, polarizing microscope, stereomicroscope, and metalloscope. The results of ordinary Raman of P. ostii and H. officinalis showed that the frosts of P. ostii matched paeonol, while that of H. officinalis matched magnolol and honokiol. In P. ostii and its frost, 19 peaks were identified by UPLC-Q/TOF-MS, and the main component was paeonol. Eleven components were identified in H. officinalis and its frosts, and the main components were magnolol and honokiol. A. lancea and its frosts were analyzed by gas chromatography-mass spectrometry (GC-MS), 21 were identified, and its main components were hinesol and β-eudesmol. These three medicinal plants accumulate compounds and precipitate frosts on the surface. The results show that the components of the frosts provide a basis for quality evaluation and research on similar medicinal plants, and reveals the scientific connotation of "taking the medicinal materials' precipitated frosts as the best" of P. ostii, H. officinalis, and A. lancea, to some extent.

Temporal and spatial changes in ion homeostasis, antioxidant defense and accumulation of flavonoids and glycolipid in a halophyte Sesuvium portulacastrum (L.) L

Salinity is an important environmental constraint limiting plant productivity. Understanding adaptive responses of halophytes to high saline environments may offer clues to manage and improve salt stress in crop plants. We have studied physiological, biochemical and metabolic changes in a perennial, fast growing halophyte, Sesuvium portulacastrum under 0 mM (control), 150 mM (low salt, LS) and 500 mM (high salt, HS) NaCl treatments. The changes in growth, relative water content, cation, osmolyte accumulation, H2O2 and antioxidant enzyme activity (SOD, CAT and APX) were observed under different treatment conditions. A positive correlation was revealed for sodium ion accumulation with malondialdehyde (r2 = 0.77), proline (r2 = 0.88) and chlorophyll content (r2 = 0.82) under salt treatment while a negative correlation was observed with relative tissue water content (r2 = -0.73). The roots and leaves showed contrasting accumulation of potassium and sodium ions under LS treatment. Temporal and spatial study of sodium and potassium ion content indicated differential accumulation pattern in roots and leaves, and, high potassium levels in root. Higher H2O2 content was recorded in roots than leaves and the antioxidant enzyme activities also showed significant induction under salt treatment conditions. Gene expression profiling of sodium transporters, Sodium proton exchanger (NHX3), Vacuolar ATPase (vATPase) and Salt overly sensitive1 (SOS1) showed up regulation under salt stress after 6-24 hr of NaCl treatment. Metabolite changes in the salt stressed leaves showed increased accumulation of flavonoids (3,5-dihydroxy-6,4'-dimethoxy-flavone-7-O-[α-L-rhamnopyranosyl-(1→6)-β-D-glucopyranoside], and3,5-dihydroxy-6,3',4'-trimethoxy-flavone-7-O-[α-L-rhamnopyranosyl-(1→6)-β-D-glucopyranoside] in both LS and HS treatments, while a glycolipid, 1-O-linolenyl-2-O-(palmitoyl)-3-O-galactopyranosyl glycerol, accumulated more in LS over HS treatments and control. The results suggest that differential spatial and temporal cation levels in roots and leaves, and accumulation of flavanoid and glycolipid could be responsible for salt adaptation of S. portulacastrum.

Ectopic Expression of JcWRKY Confers Enhanced Resistance in Transgenic Tobacco Against Macrophomina phaseolina

Plants possess an innate immune system comprising of a complex network of closely regulated defense responses involving differential gene expression mediated by transcription factors (TFs). The WRKYs comprise of an important plant-specific TF family, which is involved in regulation of biotic and abiotic defenses. The overexpression of JcWRKY resulted in improved resistance in transgenic tobacco against Macrophomina phaseolina. The production of reactive oxygen species (ROS) and its detoxification through antioxidative system in the transgenics facilitates defense against Macrophomina. The enhanced catalase activity on Macrophomina infection limits the spread of infection. The transcript expression of antioxidative enzymes gene (CAT and SOD) and salicylic acid (SA) biosynthetic gene ICS1 showed upregulation during Macrophomina infection and combinatorial stress. The enhanced transcript of pathogenesis-related genes PR-1 indicates the accumulation of SA during different stresses. The PR-2 and PR-5 highlight the activation of defense responses comprising of activation of hydrolytic cleavage of glucanases and thaumatin-like proteins causing disruption of fungal cells. The ROS homeostasis in coordination with signaling molecules regulate the defense responses and inhibit fungal growth.

Role of sodium ion transporters and osmotic adjustments in stress alleviation of Cynodon dactylon under NaCl treatment: a parallel investigation with rice

Comparative analyses of the responses to NaCl in Cynodon dactylon and a sensitive crop species like rice could effectively unravel the salt tolerance mechanism in the former. C. dactylon, a wild perennial chloridoid grass having a wide range of ecological distribution is generally adaptable to varying degrees of salinity stress. The role of salt exclusion mechanism present exclusively in the wild grass was one of the major factors contributing to its tolerance. Salt exclusion was found to be induced at 4 days when the plants were treated with a minimum conc. of 200 mM NaCl. The structural peculiarities of the salt exuding glands were elucidated by the SEM and TEM studies, which clearly revealed the presence of a bicellular salt gland actively functioning under NaCl stress to remove the excess amount of Na⁺ ion from the mesophyll tissues. Moreover, the intracellular effect of NaCl on the photosynthetic apparatus was found to be lower in C. dactylon in comparison to rice; at the same time, the vacuolization process increased in the former. Accumulation of osmolytes like proline and glycine betaine also increased significantly in C. dactylon with a concurrent check on the H₂O₂ levels, electrolyte leakage and membrane lipid peroxidation. This accounted for the proper functioning of the Na⁺ ion transporters in the salt glands and also in the vacuoles for the exudation and loading of excess salts, respectively, to maintain the osmotic balance of the protoplasm. In real-time PCR analyses, CdSOS1 expression was found to increase by 2.5- and 5-fold, respectively, and CdNHX expression increased by 1.5- and 2-fold, respectively, in plants subjected to 100 and 200 mM NaCl treatment for 72 h. Thus, the comparative analyses of the expression pattern of the plasma membrane and tonoplast Na⁺ ion transporters, SOS1 and NHX in both the plants revealed the significant role of these two ion transporters in conferring salinity tolerance in Cynodon.

AlNAC4 Transcription Factor From Halophyte Aeluropus lagopoides Mitigates Oxidative Stress by Maintaining ROS Homeostasis in Transgenic Tobacco

NAC proteins are a large family of plant-specific transcription factors which regulate both ABA-dependent and -independent gene expression. These transcription factors participate in biotic and abiotic stress-response through intricate regulation at transcriptional, post-transcriptional and post-translational levels. In the present study, AlNAC4 transcription factor was isolated from a salt excreting halophyte Aeluropus lagopoides. The AlNAC4 has an open reading frame of 936 bp, encoding a protein of 312 amino acid, with an estimated molecular mass of 34.9 kDa. The AlNAC4 showed close homology to monocot NACs in the phylogenetic tree. In silico analysis revealed that AlNAC4 possess the characteristic A-E subdomains within the NAC domain. The AlNAC4 showed sixteen post-translational phosphorylation sites. The AlNAC4 transcript was significantly upregulated with dehydration and H₂O₂ treatments, showing its role in osmotic and oxidative stress, respectively. The recombinant protein showed binding to mono as well as tandem repeats of NAC recognition sequence (NACRS) of the erd1 promoter. This is the first report mentioning that overexpression of AlNAC4 improved oxidative stress tolerance in tobacco transgenics. The transgenics maintained ROS homeostasis during H₂O₂ treatment. The transgenics showed regulation of stress-responsive genes including CAT, SOD, LEA5, PLC3, ERD10B, THT1 and transcription factors like AP2, ZFP during oxidative stress. Key Message: The AlNAC4 transcription factor from recretohalophyte Aeluropus showed regulation with abiotic stresses and binding to NACRS elements of erd1 promoter. The AlNAC4 tobacco transgenics showed improved growth with oxidative stress.

Relative contribution of Na+/K+ homeostasis, photochemical efficiency and antioxidant defense system to differential salt tolerance in cotton (Gossypium hirsutum L.) cultivars

In this study, the role of specific components of different coping strategies to salt load were identified. A pot experiment was conducted with four cotton (Gossypium hirsutum L.) cultivars (differing in salt-sensitivity) under salinity stress. Based on observed responses in growth performance and physiological characteristics, CZ91 was the most tolerant of the four cultivars, followed by cultivars CCRI44 and CCRI49, with Z571 being much more sensitive to salt stress. To perform this tolerant response, they implement different adaptative mechanisms to cope with salt-stress. The superior salt tolerance of CZ91 was conferred by at least three complementary physiological mechanisms: its ability to regulate K⁺ and Na⁺ transport more effectively, its higher photochemical efficiency and better antioxidant defense capacity. However, only one or a few specific components of these defense systems play crucial roles in moderately salt tolerant CCRI44 and CCRI49. Lower ROS load in CCRI44 may be attributed to simultaneous induction of antioxidant defenses by maintaining an unusually high level of SOD, and higher activities of CAT, APX, and POD during salt stress. CCRI49 could reduce the excess generation of ROS not only by maintaining a higher selective absorption of K⁺ over Na⁺ in roots across the membranes through SOS1, AKT1, and HAK5, but also by displaying higher excess-energy dissipation (e.g., higher ETR, P_R and qN) during salt stress. Overall, our data provide a mechanistic explanation for differential salt stress tolerance among these cultivars and shed light on the different strategies employed by cotton cultivars to minimize the ill effects of stress.

Chemical Derivatization of Metabolite Mass Profiling of the Recretohalophyte Aeluropus lagopoides Revealing Salt Stress Tolerance Mechanism

Plants are the primary producers of food for human being. Their intracellular environment alternation is influenced by abiotic stress factors such as drought, heat and soil salinity. Aeluropus lagopoides is a strong halophyte that grows with ease under high saline muddy banks of creeks of Gujarat, India. To study the response of salinity on metabolite changes in Aeluropus, three treatments, i.e. control, salinity and recovery, were selected for both shoot and root tissue. The cytosolic metabolite state was analysed by molecular chemical derivatization gas chromatography mass profiling. During saline treatment, significant increase of compatible solutes in shoot and root tissue was observed as compared to control. Subsequently, metabolic concentration decreased under recovery conditions. The metabolites like amino acids, organic acids and polyols were significantly detected in both shoot and root of Aeluropus under salinity. The metabolites like proline, aspartic acid, glycine, succinic acid and glycolic acid were significantly upregulated under stress. The salicylic acid was found to play a role in maintaining the polyols level by its down-regulation during salinity. The principle component analysis of all detected metabolites in both shoot and root showed that metabolites expressed under salinity (component 1) were highly variable, while metabolites expressed under recovery (component 2) were comparatively less variable as compared to control. The evolved intracellular compartmentalization of amino acids, organic acids and polyols in A. lagopoides can be a hallmark to sustaining at high salinity stress.

Natural variation in primary root growth and K+ retention in roots of habanero pepper (Capsicum chinense) under salt stress

In this work, we analysed the natural variation in mechanisms for protection against salt stress in pepper varieties (Capsicum chinense Jacq. cv. Rex, Chichen-Itza and Naranja and Capsicum annuum L. cv. Padron), considering primary root growth and viability of the post-stressed seedlings. NaCl-induced K+ and H+ efflux in roots was also studied by ion-selective microelectrodes under application of pharmacological agents. In these pepper varieties, the magnitude of the K+ leakage in the roots positively correlated with growth inhibition of the primary root in the presence of NaCl, with Rex variety showing a higher level of tolerance than Chichen-Itza. The K+ leakage and the activity of the H+ pump in the roots were dependent on the NaCl concentration. Pharmacological analysis indicated that the NaCl-induced K+ leakage was mediated by TEA+-sensitive KOR channels but not by NSCC channels. In addition, we present evidence for the possible participation of proline, and a Na+-insensitive HAK K+ transporter expressed in habanero pepper roots for maintaining K+ homeostasis under salt stress conditions.

Oxidative defense metabolites induced by salinity stress in roots of Salicornia herbacea

High salinity is a major abiotic stress that affects the growth and development of plants. This type of stress can influence flowering, the production of crops, defense mechanisms and other physiological processes. Previous studies have attempted to elucidate salt-tolerance mechanisms to improve plant growth and productivity in the presence of sodium chloride. One such plant that has been studied in detail is Salicornia, a well-known halophyte, which has adapted to grow in the presence of high salt. To further the understanding of how Salicornia grows and develops under high saline conditions, Salicornia herbacea (S. herbacea) was grown under varying saline concentrations (0, 50, 100, 200, 300, and 400mM), and the resulting phenotype, ion levels, and metabolites were investigated. The optimal condition for the growth of S. herbacea was determined to be 100mM NaCl, and increased salt concentrations directly decreased the internal concentrations of other inorganic ions including Ca²⁺, K⁺, and Mg²⁺. Metabolomics were performed on the roots of the plant as a systematic metabolomics study has not yet been reported for Salicornia roots. Using ethylacetate and methanol extraction followed by high resolution ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS), 1793 metabolites were identified at different NaCl levels. Structural and functional analyses demonstrated that the concentration of 53 metabolites increased as the concentration of NaCl increased. These metabolites have been linked to stress responses, primarily oxidative stress responses, which increase under saline stress. Most metabolites can be classified as polyols, alkaloids, and steroids. Functional studies of these metabolites show that shikimic acid, vitamin K1, and indole-3-carboxylic acid are generated as a result of defense mechanisms, including the shikimate pathway, to protect against reactive oxygen species (ROS) generated by salt stress. This metabolite profiling provides valuable information on the salt-tolerance mechanisms of S. herbacea and may be applied to bioengineer plants with improved salt tolerance.

Progress in Studying Salt Secretion from the Salt Glands in Recretohalophytes: How Do Plants Secrete Salt?

To survive in a saline environment, halophytes have evolved many strategies to resist salt stress. The salt glands of recretohalophytes are exceptional features for directly secreting salt out of a plant. Knowledge of the pathway(s) of salt secretion in relation to the function of salt glands may help us to change the salt-tolerance of crops and to cultivate the extensive saline lands that are available. Recently, ultrastructural studies of salt glands and the mechanism of salt secretion, particularly the candidate genes involved in salt secretion, have been illustrated in detail. In this review, we summarize current researches on salt gland structure, salt secretion mechanism and candidate genes involved, and provide an overview of the salt secretion pathway and the asymmetric ion transport of the salt gland. A new model recretohalophyte is also proposed.

Proteomics, metabolomics, and ionomics perspectives of salinity tolerance in halophytes

Halophytes are plants which naturally survive in saline environment. They account for ∼1% of the total flora of the world. They include both dicots and monocots and are distributed mainly in arid, semi-arid inlands and saline wet lands along the tropical and sub-tropical coasts. Salinity tolerance in halophytes depends on a set of ecological and physiological characteristics that allow them to grow and flourish in high saline conditions. The ability of halophytes to tolerate high salt is determined by the effective coordination between various physiological processes, metabolic pathways and protein or gene networks responsible for delivering salinity tolerance. The salinity responsive proteins belong to diverse functional classes such as photosynthesis, redox homeostasis; stress/defense, carbohydrate and energy metabolism, protein metabolism, signal transduction and membrane transport. The important metabolites which are involved in salt tolerance of halophytes are proline and proline analog (4-hydroxy-N-methyl proline), glycine betaine, pinitol, myo-inositol, mannitol, sorbitol, O-methylmucoinositol, and polyamines. In halophytes, the synthesis of specific proteins and osmotically active metabolites control ion and water flux and support scavenging of oxygen radicals under salt stress condition. The present review summarizes the salt tolerance mechanisms of halophytes by elucidating the recent studies that have focused on proteomic, metabolomic, and ionomic aspects of various halophytes in response to salinity. By integrating the information from halophytes and its comparison with glycophytes could give an overview of salt tolerance mechanisms in halophytes, thus laying down the pavement for development of salt tolerant crop plants through genetic modification and effective breeding strategies.