Antioxidant Properties of Lippia alba Essential Oil: A Potential Treatment for Oxidative Stress-Related Conditions in Plants and Cancer Cells.

Lippia alba (Mill.) N.E.Br. ex Britton and P. Wilson is used in folk medicine of Central and South America for its biological activities: i.e., antifungal, antibacterial, antiviral, and anti-inflammatory. Based on ethnopharmacological information and the increasing interest in this species, this work aimed to test a possible wide use of its essential oil (EO) in pharmaceutical and horticultural applications. Therefore, we focused the attention on the antioxidant activity of the oil as a possible tool to overcome the oxidative stress in both applications. For this purpose, we have chosen three aggressive breast cancer cell lines and two horticultural species (Solanum lycopersicum L. and Phaseolus acutifolius L.) that are very sensitive to salt stress. We determined the antioxidant activity of L. alba EO through the quantification of phenols and flavonoids. Regarding tomato and bean plants under salt stress, L. alba EO was used for the first time as a seed priming agent to enhance plant salt tolerance. In this case, the seed treatment enhanced the content of phenolic compounds, reduced power and scavenger activity, and decreased membrane lipid peroxidation, thus mitigating the oxidative stress induced by salt. While in breast cancer cells the EO treatment showed different responses according to the cell lines, i.e., in SUM149 and MDA-MB-231 the EO decreased proliferation and increased antioxidant activity and lipid peroxidation, showing high cytotoxic effects associated with the release of lactate dehydrogenase, vice versa no effect was observed in MDA-MB-468. Such antioxidant activity opens a new perspective about this essential oil as a possible tool to counteract proliferation in some cancer cell lines and in horticulture as a seed priming agent to protect from oxidative damage in crops sensitive to salinity.

Role and Functional Differences of HKT1-Type Transporters in Plants under Salt Stress.

Abiotic stresses generally cause a series of morphological, biochemical and molecular changes that unfavorably affect plant growth and productivity. Among these stresses, soil salinity is a major threat that can seriously impair crop yield. To cope with the effects of high salinity on plants, it is important to understand the mechanisms that plants use to deal with it, including those activated in response to disturbed Na⁺ and K⁺ homeostasis at cellular and molecular levels. HKT1-type transporters are key determinants of Na⁺ and K⁺ homeostasis under salt stress and they contribute to reduce Na⁺-specific toxicity in plants. In this review, we provide a brief overview of the function of HKT1-type transporters and their importance in different plant species under salt stress. Comparison between HKT1 homologs in different plant species will shed light on different approaches plants may use to cope with salinity.

Elucidating the molecular mechanisms mediating plant salt-stress responses.

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Can Environmental Stressors Determine the Condition of Ecological Plant Groups?

There is still a need to investigate the relationships between glycophytes and halophytes and the many biotic and abiotic factors in their natural environments. Therefore, we study the effects of the type of environment on the ecophysiological responses and condition of the glycophyte Elder Sambucus nigra L., the macrophyte Common Reed Phragmites australis (Cav.) Trin. ex Steud., the facultative halophyte Weeping Alkaligrass Puccinellia distans (Jacq.) Parl, and the obligate halophyte Common Glasswort Salicornia europaea L. in a saline-disturbed anthropogenic region of central Poland. We analyzed the effects of salinity, acidity, and soil organic matter on shoot length, lipoperoxidation, and proline in roots and green parts, and evaluated plant responses to environmental disturbance, which allowed for the comparison of adaptation strategies. The studies were carried out in (1) "sodium production" (near sodium factories), (2) "anthropogenic environments" (waste dumps, agroecosystems, calcium deposits, post-production tanks), (3) "wetland environments" (near river channels and riparian areas), and (4) "control" (natural, unpolluted environments). Green parts of plants are better suited to indicate environmental stress than roots. Their higher structural MDA membrane damage is related to the transport of toxic ions to the shoots by a rapid transpiration stream in the xylem. We found high salinity to be the main factor inducing growth and found it to be correlated with the high pH effect on proline increase in glycophytes (Elder, Reed) and Weeping Alkaligrass, in contrast to Common Glasswort. We suggest that proline accumulation allows osmotic adjustment in the green parts of reeds and alkaligrasses, but may have another function (in Elder). Common Glasswort accumulates large amounts of Na+, which is energetically more effective than proline accumulation for osmotic adjustment. Organic matter affects plant growth and proline levels, but soil salinity and pH alter nutrient availability. Plant distribution along the salinity gradient indicates that Elder is the most salt-sensitive species compared to Reed, Alkaligrass, and Glasswort. Salinity and the lack of control of thick reeds, which compete with other plant groups, affect the distribution of halophytes in saline environments.

Wild Halophytes: Tools for Understanding Salt Tolerance Mechanisms of Plants and for Adapting Agriculture to Climate Change.

Halophytes, wild plants adapted to highly saline natural environments, represent extremely useful-and, at present, underutilised-experimental systems with which to investigate the mechanisms of salt tolerance in plants at the anatomical, physiological, biochemical and molecular levels. They can also provide biotechnological tools for the genetic improvement of salt tolerance in our conventional crops, such as salt tolerance genes or salt-induced promoters. Furthermore, halophytes may constitute the basis of sustainable 'saline agriculture' through commercial cultivation after some breeding to improve agronomic traits. All these issues are relevant in the present context of climate emergency, as soil salinity is-together with drought-the most critical environmental factor in reducing crop yield worldwide. In fact, climate change represents the most serious challenge for agricultural production and food security in the near future. Several of the topics mentioned above-mainly referring to basic studies on salt tolerance mechanisms-are addressed in the articles published within this Special Issue.

An Insight into Abiotic Stress and Influx Tolerance Mechanisms in Plants to Cope in Saline Environments.

Salinity is significant abiotic stress that affects the majority of agricultural, irrigated, and cultivated land. It is an issue of global importance, causing many socio-economic problems. Salt stress mainly occurs due to two factors: (1) soil type and (2) irrigation water. It is a major environmental constraint, limiting crop growth, plant productivity, and agricultural yield. Soil salinity is a major problem that considerably distorts ecological habitats in arid and semi-arid regions. Excess salts in the soil affect plant nutrient uptake and osmotic balance, leading to osmotic and ionic stress. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, the production of enzymes, compatible solutes, metabolites, and molecular or genetic networks. Different plant species have different salt overly sensitive pathways and high-affinity K+ channel transporters that maintain ion homeostasis. However, little progress has been made in developing salt-tolerant crop varieties using different breeding approaches. This review highlights the interlinking of plant morpho-physiological, molecular, biochemical, and genetic approaches to produce salt-tolerant plant species. Most of the research emphasizes the significance of plant growth-promoting rhizobacteria in protecting plants from biotic and abiotic stressors. Plant growth, survival, and yield can be stabilized by utilizing this knowledge using different breeding and agronomical techniques. This information marks existing research areas and future gaps that require more attention to reveal new salt tolerance determinants in plants-in the future, creating genetically modified plants could help increase crop growth and the toleration of saline environments.

Halophytes and other molecular strategies for the generation of salt-tolerant crops.

The current increase in salinity can intensify the disparity between potential and actual crop yields, thus affecting economies and food security. One of the mitigating alternatives is plant breeding via biotechnology, where advances achieved so far are significant. Considering certain aspects when developing studies related to plant breeding can determine the success and accuracy of experimental design. Besides this strategy, halophytes with intrinsic and efficient abilities against salinity can be used as models for improving the response of crops to salinity stress. As crops are mostly glycophytes, it is crucial to point out the molecular differences between these two groups of plants, which may be the key to guiding and optimizing the transformation of glycophytes with halophytic tolerance genes. Therefore, this can broaden perspectives in the trajectory of research towards the cultivation, commercialization, and consumption of salt-tolerant crops on a large scale.

Growth and Element Uptake by Salt-Sensitive Crops under Combined NaCl and Cd Stresses.

To test an assumption that organic soil can ameliorate nutritional disorders associated with metal and salinity stresses, we exposed salt-sensitive strawberry and lettuce to four salinity (0-60 mM NaCl) and three contamination (0.3-5 mg Cd/kg) rates in peat (pHH2O = 5.5). The results showed that, even at 20 mM NaCl, salinity stress exerted a dominant effect on rhizosphere biogeochemistry and physiological processes, inducing leaf-edge burns, chlorosis/necrosis, reducing vegetative growth in crops; at ≥40 mM, NaCl mortality was induced in strawberry. Signifiacntly decreased K/Na, Ca/Na and Mg/Na concentration ratios with raising salinity were confirmed in all tissues. The combined CdxNaCl stresses (vs. control) increased leaf Cd accumulation (up to 42-fold in lettuce and 23-fold in strawberry), whereas NaCl salinity increased the accumulation of Zn (>1.5-fold) and Cu (up to 1.2-fold) in leaves. Lettuce accumulated the toxic Cd concentration (up to 12.6 mg/kg) in leaves, suggesting the strong root-to-shoot transport of Cd. In strawberry Cd, concentration was similar (and sub-toxic) in fruits and leaves, 2.28 and 1.86 mg/kg, respectively, suggesting lower Cd root-to-shoot translocation, and similar Cd mobility in the xylem and phloem. Additionally, the accumulation of Cd in strawberry fruits was exacerbated at high NaCl exposure (60 mM) compared with lower NaCl concentrations. Thus, in salinized, slightly acidic and organically rich rhizosphere, pronounced organo- and/or chloro-complexation likely shifted metal biogeochemistry toward increased mobility and phytoavailability (with metal adsorption restricted due to Na+ oversaturation of the caton exchange complex in the substrate), confirming the importance of quality water and soils in avoiding abiotic stresses and producing non-contaminated food.

Do halophytes and glycophytes differ in their interactions with arbuscular mycorrhizal fungi under salt stress? A meta-analysis.

BACKGROUND: Halophytes are better than glycophytes at employing mechanisms to avoid salt injury, but both types of plants can undergo damage due to high soil salinity. Arbuscular mycorrhizal fungi (AMF) can mitigate the damage from salt stress in both halophytes and glycophytes by enhancing salt tolerance and improving energy efficiency. However, variations in mycorrhizal symbiotic efficiency between halophytes and glycophytes were still poorly understood. Therefore, we evaluated the magnitude of AMF effects on plant growth and determined the mechanisms that regulate the growth response of halophytes and glycophytes by performing a meta-analysis of 916 studies (from 182 publications). RESULTS: Arbuscular mycorrhizal fungi significantly enhance biomass accumulation, osmolytes synthesis (soluble sugar and soluble protein), nutrients acquisition (nitrogen, phosphorus, and potassium ion), antioxidant enzyme activities (superoxide dismutase and catalase), and photosynthetic capacity (chlorophyll and carotenoid contents, photosynthetic rate, stomatal conductance, and transpiration rate). AMF also substantially decreased sodium ion acquisition and malondialdehyde levels in both halophytes and glycophytes under salt stress conditions. Mycorrhizal halophytes deploy inorganic ions (potassium and calcium ions) and limited organic osmolytes (proline and soluble sugar) to achieve energy-efficient osmotic adjustment and further promote biomass accumulation. Mycorrhizal glycophytes depend on the combined actions of soluble sugar accumulation, nutrients acquisition, sodium ion exclusion, superoxide dismutase elevation, and chlorophyll synthesis to achieve biomass accumulation. CONCLUSIONS: Arbuscular mycorrhizal fungi inoculation is complementary to plant function under salt stress conditions, not only facilitating energy acquisition but also redistributing energy from stress defence to growth. Glycophytes are more dependent on AMF symbiosis than halophytes under salt stress conditions.

Regulation of Ammonium Cellular Levels is An Important Adaptive Trait for the Euhalophytic Behavior of Salicornia europaea.

Salinization of agricultural land is a devastating phenomenon which will affect future food security. Understanding how plants survive and thrive in response to salinity is therefore critical to potentiate tolerance traits in crop species. The halophyte Salicornia europaea has been used as model system for this purpose. High salinity causes NH4+ accumulation in plant tissues and consequent toxicity symptoms that may further exacerbate those caused by NaCl. In this experiment we exposed Salicornia plants to five concentrations of NaCl (0, 1, 10, 50 and 200 mM) in combination with two concentrations of NH4Cl (1 and 50 mM). We confirmed the euhalophytic behavior of Salicornia that grew better at 200 vs. 0 mM NaCl in terms of both fresh (+34%) and dry (+46%) weights. Addition of 50 mM NH4Cl to the growth medium caused a general growth reduction, which was likely caused by NH4+ accumulation and toxicity in roots and shoots. When plants were exposed to high NH4Cl, high salinity reduced roots NH4+ concentration (-50%) compared to 0 mM NaCl. This correlates with the activation of the NH4+ assimilation enzymes, glutamine synthetase and glutamate dehydrogenase, and the growth inhibition was partially recovered. We argue that NH4+ detoxification is an important trait under high salinity that may differentiate halophytes from glycophytes and we present a possible model for NH4+ detoxification in response to salinity.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes.

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.