Towards a Sustainable Agriculture: Strategies Involving Phytoprotectants against Salt Stress

: Salinity is one of the main constraints for agriculture productivity worldwide. This important abiotic stress has worsened in the last 20 years due to the increase in water demands in arid and semi-arid areas. In this context, increasing tolerance of crop plants to salt stress is needed to guarantee future food supply to a growing population. This review compiles knowledge on the use of phytoprotectants of microbial origin (arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria), osmoprotectants, melatonin, phytohormones and antioxidant metabolism-related compounds as alleviators of salt stress in numerous plant species. Phytoprotectants are discussed in detail, including their nature, applicability, and role in the plant in terms of physiological and phenotype effects. As a result, increased crop yield and crop quality can be achieved, which in turn positively impact food security. Herein, efforts from academic and industrial sectors should focus on defining the treatment conditions and plant-phytoprotectant associations providing higher benefits.


Introduction
Plants are increasingly subjected to a variety of environmental stresses that adversely impacts plant growth and productivity [1]. In a recent report, the food and agricultural organization (FAO) highlighted the necessity of devising strategies to face the impacts of climate change on agriculture and food security worldwide [2]. Among environmental factors, salinity has a major impact, as most crop plants are salt stress-sensitive [3,4].
Soil salinity includes saline, sodic and alkaline soils, characterized by high salt content, high sodium cation (Na + ) content, and high pH, respectively [5]. Soil salinity affects approximately 800 [50,58,59]. In addition to high levels of Na + and Cl -, and depending on its source and treatment, reclaimed wastewater may contain high levels of beneficial elements, such as P, K + , Ca 2+ and Mg 2+ , which can alter AMF function [58]. Gómez-Bellot et al. [59] found that the nutrient content of saline reclaimed water improved the nutritional status of euonymus plants, but no additive effect was observed when AMF was added to the same plants. In fact, the beneficial mycorrhizal effect was clearer in mycorrhizal plants irrigated with conventional water. These results are not in accordance with those found in lettuce [58], where irrigation with similar reclaimed wastewater (4 dS m −1 ) and applying the same AMF colonization had a synergetic effect on promoting plant growth. Nevertheless, the inoculation of the same AMF using reclaimed wastewater with higher salinity (6 dS m −1 ), improved the physiological behavior of laurustinus plants growing in soil, by ameliorating the negative effects of toxic ions [50]. Moreover, results suggested that plant mycorrhizal inoculation was more effective when the saline water for irrigation displayed a higher concentration of toxic ions, but lower content of beneficial nutrients [58,59].
At the molecular level, the elucidation of the mechanisms implicated in AMF symbiosis under salt stress has advanced over the past decade [60][61][62][63][64]. The stress perception is channelized inside the nucleus, resulting in the expression of multiple stress-responsive genes, related to cellular protection against the stress (e.g., antioxidants, late embryogenesis abundant proteins and chaperons) or implicated on the perception, transmission and transcription of the signal [38, 60,61]. Recent studies revealed the molecular mechanisms of high K + /Na + ratio in mycorrhizal plants, which are based on the induced expression of genes involved in K + acquisition and Na + extrusion to the soil [61][62][63][64]. For example, in Oryza sativa L., up-regulation of OsNHX3 (sodium/hydrogen exchanger) by AMF allows the compartmentalization of Na + into the vacuole [63]. In black locust, AMF induced the expression of genes encoding for membrane transport proteins and aquaporins (SOS1, HKT1, NHX1, and SKOR), which contributes to maintaining ion homeostasis [64] The current trend towards the use of biological products, such as mycorrhizal inoculation is helping to increase the efficiency of intensive agriculture [65,66]. Nowadays, the combination of AMF inoculation and controlled deficit irrigation in drought or salinity conditions are increasingly applied in arid and semi-arid areas [67][68][69][70][71] However, in order to be an effective and sustainable tool for agriculture, numerous factors have to be taken into consideration. Thereby, the fungal colonization in open-field experiments is more productive in terms of root biomass experiments, which is associated with the growth constrains of containerized roots in a greenhouse. Contrarily, shoot biomass, plant nutrition and yield seem to be independent on the location factor [72]. Another relevant factor determining the applicability of the AMF-plant association is the source of inoculant. In this sense, native AMF obtained from saline soils may cope with salt stress more efficiently compared to other AMF [73,74]. Therefore, the type of stress may determine the type AMF. Moreover, aspects, such as AMF evaluation under multiple environmental stressors, and the identification of specific AMF strain/plant species combinations, should be taken into account to obtain the most effective responses. Herein, it was observed that single species inoculation experiments use to confer higher benefits to plants in greenhouse trials, whereas, in open-field experiments where multiple stresses are acting, a more complex fungal community may provide a better output [75].

Plant Growth-Promoting Rhizobacteria (PGPR)
Plant growth-promoting rhizobacteria (PGPR) [76] referred to free-living soil bacteria that establish beneficial associations with plants. Around 1 to 2% of bacteria are classified as PGPR [77]. Among the diverse genera that compose the group, Pseudomonas and Bacillus spp are prevalent [78]. Independently on the strategy to promote plant growth, PGPR colonize either the root surface, the radicular tissues, or the rhizosphere [79]. Their beneficial action relies on indirect and direct mechanisms. The indirect stimulation of plant growth occurs by competing against plant pathogen, by means of antagonistic compounds produced by the PGPR or by induced resistance to pathogens in the plants [80][81][82][83][84][85]. This may include the production of antibiotics [86,87], lysing enzymes as proteases, cellulases, glucanases, chitinases, lactonases [88][89][90], siderophores to chelate iron [91,92] or other toxic compounds as cyanide [93,94], as well as volatiles compounds, such as dimethyl disulfide and 2-methylpentanoate [95,96]. On the other hand, the direct growth promotion occurs by increased nutrient availability (by nitrogen-fixing or solubilization of P and K + , among other nutrients), altered phytohormone and enzymatic production [97][98][99], and modulated osmotic balance [100,101]. PGPR has also been associated with modulation of ethylene level, increased nutrient solubilization and root absorption and production of phytohormones [102]. In addition, Mohamed and Gomaa [103] found that PGPR enhanced total chlorophyll pigments in radish plants grown under salinity, due to enhanced levels of proline and free total amino acids, which slowed down the chlorophyll degradation.
In agreement with the mechanisms described above, numerous studies have reported improved vegetative development and increased yield of different plant species by the utilization of PGPR [104][105][106][107]. Likewise, PGPR have also been applied to reduce the use of chemical fertilizers and pesticides, as a consequence of the more efficient nutrient uptake and the plant protection conferred by PGPR [108,109]. This results in improved economic viability, soil fertility, and environmental sustainability [110][111][112]. In this sense, several PGPR formulations are currently available as commercial products for agricultural production [113,114]. The ability to develop plant responses to certain PGPR may rely on the specificity of the interaction between the bacteria and the plant, and on the soil conditions [115,116]. Hence, efforts are focused worldwide on evaluating a wide range of rhizobacteria conferring resilience against environmental stressors, such as salt stress [117][118][119][120].
Overall, physiological and chemical responses in the modulation of abiotic stress in PGPR-inoculated plants have been termed as "induced systemic tolerance" (IST) [121]. Recently, a meta-analysis study has reported that PGPR inoculation induces more ostensible plant responses in non-optimal conditions [106]. In addition, PGPR effects mitigating salt stress have also been noticed in early plant stages of canola, lettuce, rice, chickpea and mung bean [122][123][124], wheat seedlings [125], and potato [126].
PGPR-inoculated plants grown under salinity conditions generally display an improved nutrient and water balance and enhanced rooting and plant growth [127]. Na + reduction in leaves has been observed by PGPR inoculation of wheat [128]. Likewise, an altered selectivity of Na + , K + and Ca 2+ have been reported, resulting in an increased K + /Na + ratio, and N, P, and K + uptake in several crops, such wheat and maize [128,129]. In other cases, inoculated plants (soybean, lettuce and pepper) maintained their osmotic homeostasis by higher proline levels [130][131][132]. Moreover, an overall increase in the water use efficiency was observed in PGRP-inoculated plants under salt stress [117]. Furthermore, enhancement in the activities of antioxidant enzymes to cope with salinity-induced oxidative stress has been reported in lettuce [131], wheat plants [133], and Phaseolus vulgaris L. [134]. In addition, several studies showed that certain PGPR diminish the biosynthesis of salt-induced ethylene in several crops, such as tomato, canola and maize treated plants [117,135,136].
To date, our knowledge on beneficial microbes diversity, mechanism of action, application, and also physiological and biochemical inoculated plant responses, has facilitated the emergence of numerous PGPR commercial products. In the last decades, a large number of studies have demonstrated beneficial PGPR effects in both laboratory and greenhouse studies. However, fewer studies conducted field trials, and more comprehensive knowledge related to PGPR and plant interaction in different agro-ecosystems is required. The beneficial effects of PGPR are modulated by diverse growth habitats, native microbial communities and environmental conditions. For example, weather fluctuations influence the effectiveness of PGPR [137], which has to be considered in field experiments and in PGPR applications in agriculture. Regarding the molecular aspects of PGPR action, the expression of novel plant genes, such as SOS1, which codifies for a plasma membrane Na+/H+ antiporter [138], was found to be promoted in PGPR-applied wheat shoots and roots, resulting in improved salt stress resistance [139]. Moreover, the use of genes from halo-tolerant PGPR may confer salt tolerance to transgenic plants; for example, expression of Bacillus subtilis proline biosynthesis (ProBA) gene conferred salt tolerance in transgenic Arabidopsis through increased proline production [140]. Likewise, halotolerant properties of PGPR, such as Enterobacter ludwigii and other enterobacters rely on the role of Na+/H+ antiporters, such as NhaA. In conclusion, it is notable that PGPRs are postulated as an eco-friendly and versatile tool to stimulate crops to mitigate adverse conditions [89,127,141,142]. As our knowledge expands, more commercial formulations may be designed, accomplishing more sustainable agrarian systems.

Osmoprotectants: Proline, Glycine Betaine and Polyamines
The accumulation of osmolytes, i.e., low molecular weight water-soluble compounds, is a common strategy of many plants to cope with water deficit and salt stress as they confer stress tolerance without affecting cellular machinery [143]. Among them, the most common osmolytes are glycine betaine (GB), proline (Pro) and polyamines (PA) [143][144][145][146]. The accumulation of these molecules in the cytosol and organelles decreases plant internal osmotic potential, in response to the decrease in external soil osmotic potential, due to excessive accumulation of phytotoxic ions in the substrate. Consequently, the cell membrane and macromolecules integrity are maintained.
Pro biosynthesis from glutamate involves Δ1-pyrroline-5-carboxylate synthetase and glutamate dehydrogenase enzyme reactions. Pro accumulation depends on the balance between their biosynthesis and degradation rates, the latter catalyzed by the mitochondrial enzyme proline dehydrogenase (PDH) [147]. Studies on Pro mode of action have focused on its capacity to mediate osmotic adjustment, stabilize subcellular structures and ROS scavenging during salt stress. Likewise, increased Pro in response to stress contributes to maintaining the NAD(P) + /NAD(P)H ratio [148]. Yang et al. [147] reported Pro accumulation as a result of increased biosynthesis and inhibition of its degradation by H2O2.
Pro supplements were proven to confer salt tolerance and enhance plant growth by improving the antioxidative enzyme system, photosynthetic activity, water status, and consequently, plant performance in olive tree [149]. Moreover, Pro treated plants displayed lower soluble sugars content, which shows the role of Pro as osmoprotectant [149]. The application of 50-100 mM Pro in the irrigation water improved germination, seedling growth and chlorophyll contents of sorghum and wheat [150,151]. In another study, the application of seaweed extracts ameliorated the negative impacts of salt stress by increasing aminoacids, such as serine, threonine and Pro in roots of chickpea plant [152]. In soybean, 25 mM Pro applied as foliar spray increased nitrogen fixation and specific nodule activity [130]. Moreover, seed spraying of P. vulgaris at low Pro concentration (5 mM) alleviated oxidative stress and enhanced plant growth, increased carotenoids and ascorbate contents, and the endogenous levels of proline [153]. However, although the beneficial effects of Pro applied exogenously, toxic effects have also been reported. For example, Pro applied at high concentration in tomato resulted in ion imbalance and poor plant growth [154]. In rice, 30 mM Pro in the irrigation water ameliorated the negative effects of salt stress, whereas, higher concentrations (40-50 mM Pro) resulted in deleterious for plant growth [155]. Such toxic effect of exogenous Pro has been explained by the fact that high concentrations may mimic the effects of Cd, as reported in rice seedlings [156] GB is a quaternary ammonium compound that is a highly effective osmoprotectant in many plant species in response to salt stress [143][144][145][146]. The metabolic pathway for GB biosynthesis in plants involves two enzymes, i.e., betaine aldehyde dehydrogenase (BADH) and choline monooxygenase (CMO), which are located in the chloroplasts [157]. This pathway takes place in some crops, like cotton and sugar beet, as well as in many drought-tolerant and highly salt-tolerant wild plants, including halophytes [158]. Evidence indicates that plants are able to translocate foliar-applied GB from their leaves to other organs mainly via phloem, where it may ameliorate stress tolerance by maintaining membrane integrity, adjusting cell osmolarity, and protecting the photosynthetic complex, especially photosystem II, among other mechanisms [159]. Furthermore, GB also acts in maintaining Rubisco activity and enhancing stomatal conductance [159]. GB accumulation has long been a target for engineering stress resistance. Remarkably, it has been found that transgenic carrot overexpressing BADH gene accumulated high amounts of GB, which in turn resulted in high salt tolerance at high salt concentrations (up to 400 mM) [160]. Likewise, the transgenic rice plants converted high levels of exogenously applied betaine aldehyde (up to 10 mol m -3 ) to glycinebetaine more efficiently than did wild-type plants acquiring greater resistance to salinity [161]. Conversely, transgenic rice plants overexpressing CMO gene were not effective for accumulation of GB and improvement of productivity, although displayed enhanced tolerance to salt stress in the seedling stage [162].
The use of GB applied exogenously to plants that accumulate little or none of this compound may alleviate the harmful effects of environmental stress [157]. There are several reports displaying the beneficial effects of exogenous application of GB on plant growth and final crop yield under salt stress or drought. Examples include those in rice, sunflower, maize, canola, eggplant, pepper, ryegrass, lettuce, safflower, pigeon pea, soybean, wheat and tomato [163][164][165][166][167][168][169][170][171][172][173][174][175]. Some reports have also shown that application of GB facilitated the accumulation of other osmotic substances, such as free Pro and soluble sugars in plant organs under salt stress conditions [176,177]. This has been attributed to a protective role of GB on key enzymes that catalyze the biosynthesis of soluble free proline [176].
PA are polybasic aliphatic amines extensively disseminated in plants and implicated in numerous cellular functions [178][179][180]. The most common PA present in halophytes are: Cadaverine (Cad), putrescine (Put), spermidine (Spd) and spermine (Spm) [181,182]. These compounds can be located in all plant organs, being implicated in a plethora of metabolic processes, such as leaf senescence, morphogenesis, protection of photosynthetic tissues, growth and embryogenesis, serving as bioindicators of stress-tolerant genotypes [183][184][185][186][187][188][189]. Moreover, PA regulates the antioxidant machinery acting as free-radical scavengers [190], and contribute to abiotic stress signaling [191]. In these processes, PA have been proven to activate the expression of genes associated with stress response and establish the interaction with other metabolic pathways [191,192]. PA bind covalently to biomacromolecules to form bound PA, which usually exerts stronger physiological activity than free PA [193].
The use of exogenous PA and transgenic plants have demonstrated an important role of these compounds in a variety of adaptive responses to salt stress [186,194,195]. Exogenous application of Spm, Spd, Put and Cad resulted in improved plant growth by means of improved photosynthesis and activation of antioxidative metabolism, which ameliorate the negative effects of salt stress [196][197][198]. Ikbal et al. [185] reported that enhanced S-adenosylmethionine decarboxylase (SAMDC) activity would maintain high PA levels in salt-stressed micropropagated grapevine (Vitis vinifera L.) plants. The application of exogenous Spd (0.1 and 1 mM) enhanced salt tolerance (150 mM NaCl) of Panax ginseng C.A. Mey. plants by enhancing the antioxidant defenses and thus, alleviating salt-induced oxidative stress. As a result, Spd application increased plant growth, shoots and roots relative water content, and prevented chlorophyll degradation under salinity conditions [186]. More recently, Nahar et al. [188] observed that the pre-treatment of mung bean seedlings with PA-0.2 mM of either Put, Spd or Spm-improved the response to salt stress (200 mM NaCl) by reducing cellular Na + accumulation, maintaining ion homeostasis, and by increasing the levels of antioxidant defenses, including ascorbic acid (AsA) and glutathione (GSH), as well as catalase (CAT), dehydroascorbate reductase (DHAR), glutathione peroxidase (GPX), and glutathione reductase (GR) activities [188]. Similarly, the up-regulation of SAMDC expression induced accumulation of Spd and Spm, resulting in an attenuation of salt stress in cucumber and grasses [199,200]. Under salt stress, the growth of transgenic rice seedlings overexpressing SAMDC was improved, in comparison to non-transformed rice plants, which was correlated with a three to four-fold increase in Spd and Spm [201].
In summary, exogenous application of osmoprotectants can be a suitable means to increase salt tolerance in different crop species by regulating osmotic potential, reducing the accumulation of phytotoxic ions and protecting essential macromolecules, such as proteins from the photosynthetic complex.

Melatonin
Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous molecule derived from the aminoacid tryptophan which is present in almost all living organisms, including plants [202]. Melatonin was named because of its ability to pigment the skin of animals, such as fish, reptiles, and amphibians [203]. Numerous studies have demonstrated its role in seed germination, plant growth, organogenesis, and leaf and fruit senescence. All these functions might be mediated by the effect of melatonin up-regulated genes related to a multitude of cellular functions, including primary metabolism, redox processes, lipid and carbohydrate metabolism, cell cycle, DNA replication, photosynthesis, and senescence-associated genes, among others [204,205]. It also enhances the secondary metabolism and protecting the photosynthetic machinery. Moreover, it establishes crosstalk with plant hormones and polyamines, among other molecules, and regulates its own biosynthesis [206][207][208][209][210][211]. In addition, it is well established that melatonin helps to protect plants against different abiotic stresses, including salt stress [204,205,212,213].
Melatonin is synthesized by at least four different alternative biosynthetic routes, all implying serotonin as an intermediary, indicating that this molecule is a hub intermediate for this purpose in plants [214]. Each of the four known alternative routes for the biosynthesis of melatonin takes place in different organelles, where the production of the intermediates and melatonin occurred. Its chemical structure confers melatonin amphiphilic properties, which favors its movement through the plasma membranes and entry into different cellular compartments, especially chloroplasts, endoplasmic reticulum and cytoplasm [214].
In relation to salt stress, the functions of melatonin can be classified as follows: Activation of antioxidant defenses to protect plants against loss of water and physiological injuries associated with salinity, enhancement of photosynthesis, boost of ion homeostasis, regulation of plant hormone metabolism, and stimulation of polyamine metabolism pathway [213,[215][216][217][218][219]. Under salt stress, the activities of ascorbate peroxidase (APX), peroxidase (POD), superoxide dismutase (SOD), CAT, GR, and GPX in maize seedlings treated with melatonin increased significantly compared to the levels on non-treated seedlings [216,220]. Similarly, the exogenous application of melatonin in Malus hupehensis Pamp. seedlings reduced hydrogen peroxide (H2O2) and superoxide radicals (O2 .− ) levels by activating the same antioxidant enzymes [221]. In addition, the contents of non-enzymatic antioxidants, such as GSH and AsA in cucumber subjected to salinity conditions were higher when ROS interacted with melatonin [217]. This evidence implies that exogenous application of melatonin could improve the antioxidant defenses (non-enzymatic and enzymatic) of plants mainly by removing the ROS generated by salt stress.
Globally, the application of exogenous melatonin helps plants to better perform photosynthesis by avoiding chlorophyll degradation and stomatal closure, due to salt stress [222][223][224]. The application of exogenous melatonin (50-150 μM) in cucumber significantly improved plant responses to salt stress by increasing maximum quantum efficiency of PSII, chlorophyll levels and leaf net photosynthetic rate [217]. Similarly, in watermelon, pretreatment with melatonin (50-500 μM) clearly favored salt tolerance by improving leaf net photosynthetic rate and stomata opening [225]. Melatonin significantly upregulated MdNHX1 and MdAKT1 transcript levels in M. hupehensis seedlings subjected to salt stress, providing a decrease in Na + concentrations + and an increase in K + that incremented K + /Na + ratio [221]. In a similar way, melatonin provoked a remarkable increase in K + /Na + ratio in shoots of maize seedlings when grew under salt stress [216]. Other genes, such as AKT, NHX, and SOS seem to be correlated with better ion homeostasis coherent with high K + levels, and consequently, with a greater K + /Na + ratio [226]. Melatonin also up-or down-regulates the expression of genes related to hormonal homeostasis, which encode for enzymes of the biosynthesis or degradation of plant hormones, such as indole-3-acetic acid, gibberellins (GA), abscisic acid (ABA) cytokinins, ethylene, salycilic acid (SA) and jasmonic acid (JA) [227][228][229][230].
In conclusion, melatonin participates in a variety of molecular stress mechanisms in plants related to the response to biotic and abiotic stresses. In general, increased melatonin levels-as a consequence of either a stressful situation or exogenous application-results in an improved stress tolerance mediated by changes in the hormone profile, antioxidative metabolism and expression of specific stress-responsive genes [209,211,228,[231][232][233][234][235]. Recently, the novel characterization of a melatonin receptor in Arabidopsis opens up melatonin as a new phytohormone [236].

Antioxidative Metabolism
Many studies have associated pre-treatments with different ROS, especially H2O2, reactive nitrogen species (RNS, especially nitric oxide (NO)), ascorbic acid (AsA), and plant hormones, such as gibberellins (GA) and salicylic acid (SA), with an improved resilience of plants against different abiotic stresses, such as salinity. Despite their harmful nature, H2O2 has a dual role in plant stress response; at high concentration, H2O2 is deleterious for the cell function, whereas, at low concentration it may act as a signal molecule in a variety of biotic and abiotic stresses [237,238]. Consequently, ROS production is tightly controlled in plant cells by enzymatic and non-enzymatic antioxidants. It has been reported that H2O2-priming seeds can induce salt stress tolerance by modulating the antioxidant defenses [239]. The pre-treatment of barley seedlings with low H2O2 concentrations (1 or 5 μM) for two days induced tolerance upon 150 mM NaCl treatment. This response was parallel with the induction of three SOD isoenzymes and the decrease in Na + and Cluptake [240]. The treatment of wheat seedlings with very low H2O2 levels (0.05 μM) improved its response to 150 mM NaCl in terms of plant growth (plant height, biomass and root length) coupled with a reduced O2 − generation and lipid peroxidation levels and the increase of different antioxidant defenses [241]. More recently, [242] indicated that exogenous H2O2 could enhance salt tolerance in Vigna radiata L. plants by regulating the AsA and GSH metabolism.
AsA or vitamin C, as one of the most important antioxidants occurring in plants [243], plays an important role in ROS detoxification, providing protection and modulating other fundamental functions in plants under stress conditions [244][245][246]. AsA is also involved in key processes in plants, such as regulating photosynthetic capacity [247], flowering and senescence [248], and also as a co-factor for the biosynthesis and the signaling of several plant hormones (such as ethylene, JA, SA, ABA, and GA) under stressful environments [249]. AsA content and environmental conditions are linked in plants. As with other antioxidants, the variability of AsA content has been often considered relevant for explaining the different phenotype-dependent sensitivity against several stresses. It has been suggested that tolerance to stress is correlated with the capability of increasing AsA biosynthesis [244,250,251]. In this context, exogenous AsA is proposed to counteract the adverse effects of abiotic stress, when endogenous levels are not sufficient [252][253][254]. Thus, endogenous AsA can be increased by external treatments of AsA during different phases of development, applied on different organs (on roots, as a foliar spray or as seed priming), and in different types of crops, including vegetables, indeciduous fruits, oil-seeds, cereals and ornamental plants, among others. Exogenous application of AsA has been reported to enhance yield, growth and fruit quality [255][256][257][258], and more recently, it has been detected its potential effects on physiological responses of plants grown under abiotic stresses, such as salinity [259,260], and counteracting the adverse effects of NaCl when wheat grains were soaked with AsA [261]. Effectiveness of AsA in a wide concentration range (from 0.1 to 34 mM) has been studied. For instance, the addition of 0.5 mM AsA in tomato seedlings root medium attenuated the negative effects of salinity [262]. Particularly, 100 mg/L AsA applied on the root counteracted the adverse effects of salt stress on the growth of wheat under 150 mM NaCl [259], while 50-100 mg/l AsA applied as foliar spray protected the photosynthetic machinery from the damaging effects of salt stress in halophytes [263].
In addition to increased ROS production, salinity produces nitrosative stress in plants mediated by the overproduction of some RNS, such as NO and peroxynitrite (ONOO − ) [264,265], to which peroxisomes are the main source [264]. However, the pre-treatment with a NO-donor (such as sodium nitroprusside, SNP) can improve the response of plants to NaCl-induced damage [266][267][268]. SNP is a small bioactive signaling molecule which plays multiple roles in plant growth and development under normal or stressful conditions [269,270]. SNP ameliorates salt stress acting as a ROS scavenger throughout the formation of ONOO -, of less toxicity than NO [271] Moreover, SNP application stimulates proton-pump activity in the tonoplast, which in turn increases the K + /Na + ratio [272]. Moreover, SNP regulates post-translational modifications of proteins involved in the cellular division [273]. The pre-treatment of Citrus aurantium L. plants with 100 μM SNP or 10 mM H2O2 for 48 h alleviated the physiological effects produced by 150 mM NaCl [266]. This response was linked to the regulation of protein carbonylation and S-nitrosylation of specific proteins [266]. Among the S-nitrosylated proteins there are involved in redox signaling and ROS scavenging, glycolysis, Calvin cycle, amino acids metabolism, photorespiration, and stress-related proteins. Some of the S-nitrosylated proteins were also subject of protein oxidation, including antioxidant proteins [(GST (glutathione S-transferase), SOD, peroxiredoxin), cytoskeleton proteins (tubulin, actin)], heat shock proteins (HSPs), translation and elongation factors, and metabolic proteins related to photosynthesis, glycolysis and the Krebs cycle [266]. The authors suggested the existence of a link between the different oxidative modifications events of proteins resulting from protein carbonylation or S-nitrosylation. They also pointed out that S-nitrosylation can prevent protein carbonylation events, thus, avoiding loss of function of proteins. In addition, S-nitrosylation of specific proteins can induce changes in protein function and/or structure. All these proteome changes, induced by H2O2 or NO treatments, can prepare the plants to cope with salt stress [266]. In soybean plants, the foliar application of 150 μM SNP increased relative water content, chlorophyll and ABA levels, and improved stomatal control under stress conditions (NaCl 200 mM) in soybean plants [267]. Moreover, at short-term (6 and 12 h), NO induced GST gene expression and GST enzyme activity under salt stress conditions. In addition, the pre-treatment of sunflower seeds with 250 μM SNP demonstrated that NO can modulate the GSH content and its redox state via GR activity in sunflower seedlings subjected to 120 mM NaCl [268]. Thus, NO can act as a regulator of GSH metabolism, modulating the activity of GSH-dependent enzymes [267,268,274]

Plant. Hormones (GA, SA)
SA is a phenolic plant hormone widely distributed in plants, playing an important role in the regulation of different physiological processes, such as seed germination, vegetative growth, photosynthesis, respiration, thermogenesis, flower formation, fruit yield, seed production and senescence [275]. It is widely documented that exogenous SA treatments improved plant growth and seed germination under saline stress in different plant species [276][277][278][279][280][281][282][283][284]. In general, SA-treated plants showed lower NaCl-induced symptoms and higher growth rate, chlorophyll contents, and photosynthetic capacity, as well as enhanced antioxidant capacity in relation to non-treated plants [276][277][278][279][280][281][282][283][284]. Hence, SA induces genes encoding antioxidants, chaperones and secondary metabolites, among others. However, evidence linking exogenous SA and resistance against salt stress remains controversial. In this sense, [285] observed that SA negatively affects the response of pea plants to NaCl stress, although low SA levels led to the induction of a pathogenesis-related protein (PR-1b) gene in leaves from NaCl-stressed pea plants. The induction of PR-1b gene could be an adaptive response in order to prevent an opportunistic fungal or bacterial infection in a weakness situation [285]. In this sense, the precise mechanisms of SA in the response of plants to salinity have to be unraveled, since SA effect seems to be dependent on the concentration, the plant species, the application mode, the physiological state of the plant during the application, as well as the level of salinity and the exposure time to NaCl. In Arabidopsis, SA has been reported to play a dual role: At low concentration, it induced antioxidant defenses, but excess application induced programmed cell death [286].
GA are a group of plant hormones involved in many developmental processes, such as seed germination, plant growth, flowering and fruit development [287]. Different studies showed that exogenous GA treatments can reverse the negative effects of salt stress. Kaur et al. [288] reported that low concentrations of exogenous GA3 (6 μM) promoted seed germination and seedling growth of chickpeas under NaCl stress (75 mM). These authors also observed that GA3 treatment increased α-amylase activity, which enhanced the starch mobilization, thus, leading to a better seedling growth [288]. Exogenous-applied GA3 alleviated the effects of salt stress in tomato plants through a partial stomatal closure that reduced the upload of toxic ions to the shoots, and as a result, the appearance of toxicity symptoms [289]. Application of GA3 also enabled the maintenance of photosynthetic capacity under salt stress by mitigating the salt-induced reduction of chlorophyll content [290]. Several studies have shown the positive effect of GA3 in alleviating the toxic effects of NaCl on rice growth [291][292][293][294]. Exogenous application of 50 μM GA3 improved the seed germination and early seedling growth of Arabidopsis in the presence of 100 mM NaCl. These positive effects of GA3 was accompanied by an increase of endogenous SA levels, suggesting that the improved response of GA-treated plants under salinity conditions can be mediated by the modulation of SA levels [295].

Conclusions
Upon salt stress, typical effects in the plant are reduction of fresh weight, the appearance of chlorosis and necrosis and altered metabolic processes, such as photosynthesis, phytohormone regulation, respiration, nutrient balance, ion toxicity, and protein biosynthesis, which are generally mediated by oxidative stress. In recent years, the application of phytoprotectants, such as those of microbial origin (arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria), osmoprotectants, melatonin, phytohormones and antioxidant metabolism-related compounds has proven to be a sustainable agronomic practice to obtain plants more resistant to salinity avoiding losses in crop yields. Figure 1 shows an overview of factors involved in phytoprotectants application, their role in the plant and the output obtained from an agro-economical point of view. Beyond the phytoprotectant nature, other factors, such as plant or phenotype sensitivity towards a specific phytoprotectant, application form and targeted plant organ determine their applicability. Herein, efforts from academic and industrial sectors should focus on defining the conditions and associations providing higher benefits. Overall, phytoprotectants mode of action converge into the management of water and nutrient balance, increased photosynthesis efficiency and activation of enzymatic and non-enzymatic antioxidant defenses that leads to an alleviation of the oxidative stress produced in response to salinity. As a result, increased crop yield and crop quality are achieved, which in turn positively impact food security and consumer acceptance of the product. In this sense, improved tolerance of the plant to salt stress avoids growth abnormalities and provides greener leafy vegetables, which determine visual consumer preferences. Table 1 shows the literature collected in this review concerning the exogenous application of phytoprotectants, which resulted in beneficial effects in the plant. The type of phytoprotectant, application mode and dose, plant species, and major effects in the plant in terms of physiological changes and phenotype are listed. Herein, finding a suitable interaction between phyoprotectant and plant species or phenotype is key. The amelioration of salt stress impact in the plant by phytoprotectants is commonly achieved through management of water and nutrient balance, increased photosynthesis efficiency and activation of enzymatic and non-enzymatic antioxidant defenses. Consequently, increased crop yield, resilience and quality are achieved, which in turn report economic benefits through higher food security-improved productivity and land use-and consumer preferences on a higher quality product. AsA: ascorbic acid; GA: gibberellic acid; RNS: reactive nitrogen species; SA: salicylic acid. Increased photosynthetic efficiency, proline accumulation, and activity of ROS-scavenging enzymes [126] Bacillus subtilis T. aestivum Increased plant yield and N, P and Ca 2+ leaf content, and reduced leaf Na + content [128] Azospirillum spp Z. mays Restricted Na + uptake and enhanced uptake of K + and Ca 2+ ; increased protein concentration [129] B. subtilis or Serratia sp Glycine max Increased antioxidant activity, the concentration of proline and specific nodule activity [130] Pseudomonas mendocina L. sativa Higher shoot biomass, leaf ratio of K + /Na + and leaf proline content; enhanced antioxidant enzyme activities [131] Bacillus sp.

Capsicum annuum
Higher plant biomass, leaf proline content, and enhanced antioxidant enzyme activities [132]

Proline 25 or 50 mM in Hoagland's solution
Olea europaea Improved plant growth and ameliorated photosynthetic activity, antioxidative enzyme activities and leaf water status [149] 50 mM or 100 mM in irrigation water Sorghum bicolor Improved germination, seedling growth and chlorophyll contents [150] 50 or 100mM as foliar spray to the seedlings T. aestivum Improved germination, plant growth and chlorophyll contents [151] 5 mM as seed spray P. vulgaris Alleviated oxidative stress and enhanced plant growth, increased antioxidant enzyme activities, carotenoids concentration, ascorbate content, and endogenous proline [153] 30 mM / L in irrigation water O. sativa Higher K + /Na + ratio and growth in seedlings [155] 25 mM as foliar spray G. max Increased nitrogen fixation and specific nodule activity [173] Glycine betaine 15 mM in nutrient solution O. sativa Increased relative water content, antioxidant enzyme activities, and reduced lipid peroxidation [163] 50 mM as foliar spray

Helianthus annuus
Increased plant growth and improved photosynthetic rate [164] 50 or 100 mM as foliar spray Z. mays Improved growth and water status, especially when applied at vegetative stage [165] 1 or 5 mM during imbibition and seedling growth B. napus Improved fresh and dry weight of plant [166] 50 mM as foliar spray Solanum melongena Improved growth, photosynthetic rate, transpiration, stomatal conductance, and yield of plant and fruit [167] Imbibition in 10 mM

Piper nigrum
Improved germination and radicle emergence, reduced lipid peroxidation and elevated superoxide dismutase enzyme activity [168] 20 mM in Hoagland's nutrient solution Lolium perenne Reduced electrolyte leakage and lipid peroxidation, suppressed Na + accumulation and increased shoot K + /Na + ratio [169] 25 mM as foliar spray L. sativa Increased dry matter, antioxidant enzyme activities and total phenolic contents, and altered contents of organic acids and amino acids [170] Imbibition in 30 or 60 mM

Carthamus tinctorius
Increased germination rate and decreased lipid peroxidation [171] 25 mM as foliar spray G. max Increased nitrogen fixation and specific nodule activity [173] Polyamines 0.5 mM Put and 0.5 mM Hordeum vulgare Restored root tonoplast function in Spd seedlings by increasing phospholipids, endogenous polyamines and activities of H + -ATPase, H + -PPase and vacuolar Na + /H + antiport [183] 1mM Spd O. sativa Partial restore of H + -ATPase activity and increased K + /Na + ratio [184] 0.01, 0.1, or 1mM Spd in nutrient solution Panax ginseng Improved seedling growth by preventing chlorophyll degradation, increasing endogenous polyamines levels, and antioxidant enzyme activities [186] 0.2 mM Put, Spm or Spd Vigna radiata Improved plant growth and nutrient homeostasis, decreased ROS accumulation and increased enzymatic and non-enzymatic antioxidant defenses [188] 8 mM Put Cucumis sativus Improved plant growth and stimulated glycolysis and Krebs cycle pathways of leaves [189] 100 mg / L Spd, 150 mg / L Spm or 150 mg / L Put as foliar spray

G. max
Enhanced seedling length and root growth, and peroxidase and catalase activities [196] Combination of 1 mM Put, 1 mM Spd and 1 mM Spm as foliar spray

Arachis hypogaea
Improved plant height, branching, dry weight, chlorophyll contents, and antioxidant enzyme activities, and reduced relative electrolytic leakage and lipid peroxidation [197] 1 mM Spd as foliar spray Cucumis sativus Enhanced tolerance to salinity by regulation of the metabolic status of polyamines [199] Melatonin Seed coating in 50 or 100 μM G. max Promoted soybean growth (leaf size and plant height), production and fatty acid content [204] 10 or 20 μM in solid medium at seedling stage

O. sativa
Reduced chlorophyll degradation, suppressed the transcripts of senescenceassociated genes, delayed leaf senescence and cell death; enhanced antioxidant protection by melatonin role as a potent free radical scavenger [205] 1 μM in Hoagland's nutrient solution

Citrus aurantium
Reduced toxicity symptoms in leaves (chlorosis, dehydration, necrosis, twisted leaves) lipid peroxidation, and electrolyte leakage; increased photosynthetic pigments, Claccumulation in leaves and roots, glutathione levels and superoxide dismutase activity in roots [212] 1 μM in Hoagland's nutrient solution at the seedling stage

Z. mays
Increased plant dry matter, K + /Na + ratio, net photosynthetic rate, chlorophyll contents, antioxidant enzyme activities, and decreased electrolyte leakage and lipid peroxidation [216] 50 or 150 μM in the Hoagland's solution C. sativus Increased net photosynthetic rate, maximum quantum efficiency of photosystem II, total chlorophyll content, and enhanced enzymatic and non-enzymatic antioxidant system [217] 1 μM at the seedling stage T. aestivum Improved plant growth, shoot dry weight, leaf photosynthesis rate, maximum photochemistry efficiency of photosystem II, and chlorophyll content; reduced accumulation of H2O2 [218] Factorial combinations of melatonin in nutrient solution (0.1, 0.5, and 1 μM) and foliar spray (50, 100, and 200 μM) before salt stress
Further investigation should be conducted for understanding the physiological and molecular mechanisms involved during the action of phytoprotectants on plants and their effectiveness. In addition, aspects, such as phytoprotectant evaluation under multiple environmental stressors, or innovative approaches, such as the combination of phytoprotectants and intercropping systems should be taken into account. As our knowledge expands, more commercial formulations may be designed, accomplishing more sustainable agrarian systems.