Oxidative Stress (OS) in Plants, Beneficial Interactions with Their Microbiome and Practical Implications for Agricultural Biotechnology

Abstract

Plants are aerobic, sessile, and autotrophic organisms that face a wide variety of climatic adversities and pathogen attacks. They have evolved to deal with such challenges, that is, the case of the antioxidant defense to avoid oxidative stress (OS) caused by the overproduction of reactive oxygen and nitrogen species (ROS/RONS). ROS/RONS can be by-products of many physiological functions and biochemical pathways, but particularly from the fundamental electronic transfer processes: photosynthesis and respiration. Photosynthesis is crucial for plant nutrition, trophic webs and maintenance of O₂/CO₂ balance in biosphere. Respiration is a source of energy for organisms. Both processes generate ROS and its overproduction can lead OS, modifying essential biomolecules and altering fundamental biochemical pathways and plant development. Antioxidant defense prevents such harmful accumulation of ROS. Plants interact with microbiota, a well-structured microbial community conferring adaptive and defense tools in both abiotic and biotic stressing conditions. We present the beneficial influence of the plant microbiome promotes the adaptability, resistance, and defense of plants using our results obtained in plants confronted drought. Microbiota can be used in agriculture in different ways, including adaptation to soil of micro-propagated plants, bioproducts for plant growth and pest control and processing and preservation of agriculture products.

1. Introduction

About 4.5 billion years ago our planet was formed from condensed cosmic debris, with primitive oceans and atmosphere, created by early volcanic activity, composed of such materials as gas methane (CH₄), ammonia (NH₃), and water vapor (H₂O) among others. Those hostile conditions, followed by the period of gradual cooling, climatic moderation and reducing conditions, generated new more complex organic molecules, including organic molecules, and made possible the first forms of anaerobic life [1]. In the atmosphere, oxygen (O₂) appeared as a result of water (H₂O) disruption by solar photolysis, shocking anaerobic life with a massive extinction. Survivors must adapt to using oxygen and cope with the potential dangers of oxygen and its metabolic by-products, ROS and RONS. Many of them are free radicals (hydroxyl radical (HO∙), peroxyl (ROO∙)) others are superoxide anions ((O₂⁻) hydrogen peroxide (H₂O₂), reactive aldehydes (ROCH) and nitric oxide (NO)). About 1–2% of total oxygen used in aerobic cells is diverted into ROS primarily in the mitochondria, chloroplasts, and peroxisomes [2, 3]. Survivors undergo evolutionary developed antioxidant defense capable of maintaining oxidative balance and maintaining ROS within certain values. The antioxidant defense includes antioxidant molecules (free amino acids, vitamins, proteins, polysaccharides, ions, and cell wall compounds), and antioxidant enzymes (including superoxide dismutase (SOD), glutathione peroxidase (GPO) and catalases (CAT) [4, 5, 6]. Plants are exposed to environmental conditions, including those that trigger and cause OS [7]. The complexity of plant biology and their living mode demands a fast adaptive response to environmental stressors for maintaining cellular homeostasis and essential physiological processes. Important processes like the disruption of seed dormancy, seed and spore germination, glyoxalase cycle, photosynthesis, respiration, and oxidation of amines, among others, are intrinsically linked with ROS generations [8, 9]. At low ROS levels, they function as fast-acting and signaling mediators, stimulating adaptive responses to abiotic and biotic stresses, keeping cellular homeostasis and plant development [10]. Increases in ROS trigger the activation of different adaptive and defense mechanisms against external abiotic (climate, salinity, extreme temperature, drought, and toxic soil components like heavy metals, agrochemicals, among others) and biotic stressors (phytopathogens, weeds) [9, 11, 12, 13, 14, 15]. Only 1–2% of total O₂ consumption in plant tissues becomes ROS [16, 17]. The antioxidant defense in plants displays a variety of antioxidant compounds and enzymes, with specific subcellular localization and particularly are presented in the most important ROS-generating sources. Climate change increases the presence of external stressors, representing a great challenge for agriculture and the conservation of natural ecosystems [18, 19]. In this chapter, we present an approach to OS in plants induced by biotic and abiotic stressors, their relationship and synergistic effect, as well as the importance of the microbiota in preventing and mitigating their adverse effects. We use some previously reported practical research as well as some data of our own experimental results, where the plant-microbe interactions are used to promote the adaptive response in plants to external stressors [19, 20, 21, 22]. Such results encourage the development of microbial consortiums to improve plant development and induce adaptive responses in different wild and cropped plants [20, 21, 22, 23].

2. Oxidative stress in plants

2.1 Brief overview of oxidative stress in plants

Plants are exposed to abiotic and biotic environmental stressors, that result in the overproduction of high ROS and RONS exceeding the capacity of the antioxidant defense and inducing OS. Specifically, ROS reacts and damage cellular structures, including cellular membranes, and biomolecules (proteins, lipids, carbohydrates and DNA), affecting homeostasis and important physiological functions.

Under normal conditions, ROS mainly derived from metabolic functions of chloroplasts, mitochondria, and peroxisomes [24, 25, 26, 27, 28, 29, 30]. Plant photosynthesis occurs in chloroplasts, this organelle displays a well-structured thylakoid membrane housing chlorophyll and the other components of photosynthetic systems (PS). Water is the final acceptor of electrons passing through the PS forming O₂⁻. The balance between the production and the scavenging of ROS may be affected by various abiotic and biotic stressors. Those include higher levels of salinity, UV radiation, drought, heavy metals, temperature extremes, nutrient deficiency, air pollution, herbicides and pathogen attacks, lead adverse effects in plant cells, and activating the antioxidant defense to detoxify and prevent the damages [27, 28]. The most common ROS generated in plant cells are the free radicals ^•OH, 1O₂, alkoxyl radical (RO^•), and peroxyl radical (ROO^•), while no radical species like H₂O₂, O₂^•−, carbonate (CO₃^•−), excited carbonyl group (RO*), hypochlorous acid (HOCl), hypoiodous acid (HOI); hypobromous acid (HOBr) were found too [27, 28]. Peroxyl radical (LOO^•), alkoxyl radical (LO^•), hydroperoxyl radical (HO₂^•) peroxynitrite (HNO₃⁻), ozone (O₃), and trichloromethyl peroxyl radical (C_l3COO^•) could also be found in plant cells. The biological effects of ROS concentration vary up to the chemical reactivity, origin (pathway, subcellular compartment, or organelle), and diffusion rates [29, 30, 31].

Within certain concentration values, ROS act as specific signals activating antioxidant defense and promoting adaptive response to environmental conditions [32, 33, 34]. In chloroplast, ROS generation depends on the interaction of chlorophyll (chl) and light and the subsequent oxidation of chlI leading the transfer of an electron through ETC of PSI and PSII [34, 35, 36]. In PSI, increases in H₂O₂ activate photoreduction and the production of superoxide radicals (Mehler reaction), here oxygen can be reduced by ferredoxin to form superoxide (O₂^•−), followed by reduction by SOD to H₂O₂, forming water and oxidized ascorbate by APO [37, 38]. In the presence of metal ions, such as Fe²⁺, more highly reactive short-lived HO^• are formed from O₂^•− and H₂O₂ [38]. This reaction prevents over-reduction of the electron carriers and helps to keep a redox balance to protect the different components of the photosynthetic ETC, dissipating the excess of photons and the non-photochemical quenching-formation, and improving ATP/NADPH balance during photosynthesis [38]. Peroxisomal GOX plays a major role in ROS generation [39]. Here O₂^•− can be produced by XOD and NADPH oxidase in peroxisomal membrane [39]. The formed O₂^•− is dismutated into H₂O₂ by metalloenzymes SODs, (Cu-Zn-SOD and Mn-SOD). Other processes producing H₂O₂ in peroxisomes are: photorespiration GOX reaction, O₂^•− disproportionation, β-oxidation of fatty acids, flavin oxidase polyamine oxidase, sulfite oxidase, copper amine oxidase, and sarcosine oxidase activity [40].

Respiration occurs in mitochondria where ETC holds electrons generating a great amount of free energy, from which 0.2–2% of all transferred electrons interact with O₂ to form ROS [34, 35, 36, 37, 38, 39, 40, 41, 42, 43]. In stressed plant, the complex I and III of mitochondrial ETC generate excessive ROS, particularly O₂^•− due to electron leakage, that later is catalyzed to H₂O₂ by Mn-SOD and Cu-Zn-SOD [40, 41, 42]. Mitochondria are the major ROS producers in the non-photosynthetic plant tissues but they are also produced in other plant cell compartments [42, 43, 44]. In plant cell wall ROS generation provoked a rigidity and growth reduction because of the polymerization of glycoproteins and phenolic compounds. NADH-dependent peroxidases produce H₂O₂ in the presence of NADH provided by malate dehydrogenase [42]. ROS can also be increased by the reduction of polyamines to quinine in the cell wall and the hydroperoxidation of polyunsaturated fatty acid driven by lipoxygenase [45]. Plasma membrane NADPH oxidase and quinine reductase mediate the O₂^•− generation when electron transfer from cytoplasmic NADPH and formed O₂^•− is transformed again to H₂O_2. In the endoplasmic reticulum O₂^•− is a by-product during reaction with an organic substrate to form an intermediate (Cyt P450-ROO⁻), driven by Cyt P450 where NADPH acts as an electron donor [45]. In glyoxysome the fatty acid oxidation by GOX and urate oxidase generate both O₂^•− and H₂O₂ [46] as well as the XOD and AO are involved in ROS production in the cytosol [47].

2.2 Oxidative stress damage in plant cells

The antioxidant defense maintains the homeostasis in plant cells required levels. Daily and seasonal changes induce differences in the oxidative status of the plant are kept within harmless limits. External stressors can induce ROS overproduction disrupting the oxidative balance and provoke. Within specific values ROS are important intracellular and intercellular signaling during abiotic and biotic stress, sensing the integration of different environmental signals, activation of stress response, antioxidant defense and adaptive mechanisms of plant resilience [13]. OS can result IN extensive damage and cell death [28]. Excessive ROS cause lipid peroxidation (LPO) and the most susceptible target is the carbon atoms and the ester bond between fatty acid and glycerol. LPO affects polyunsaturated fatty acids of plasmatic membrane, as linoleic and linolenic acid, by the attack of 1O₂ and OH^• in a multistep reaction, forming lipid dimmers, lipids-derived radicals with disruption of membranal functions, including PSI [28, 48]. PSI includes proteins, pigments, and co-factors joined in a complex responsible for the primary energy conversion during photosynthesis. During OS, this complex is the main target of 1O₂ during OS, producing LOOH, lowering photosynthetic efficiency. Other harm to membranes includes oxidation of structural, receptors, and enzymatic complexes, causing loss of integrity and barrier functions, leading to cellular death [28, 48, 49]. Some LPO-resulted compounds, such as α, β-unsaturated hydroxyalkenal, and malondialdehyde (MDA), can react with proteins and majorly modify specific amino acids: arginine, lysine, proline, threonine and tryptophan. Those modified amino acids made these proteins more susceptible to proteolysis, and the loss of biological functions. That is the case of damaged D1 and D2 reaction center proteins via aminoacids oxidation of PhII resulted I in photoinhibition. Other of the most frequent protein targets are Rubisco and SOD promoting disulfide bond formation, oxidation, nitrosylation, glutathionylation, and sulfhydration [50, 51]. MDA is the most frequently used biomarker of oxidative stress in cells. ROS can oxidize deoxyribose sugar of DNA by ^•OH attacks by taking H⁺ from C∙H bonds of 2-deoxyribose and methyl group, forming hydroxyl methyl urea, thymine glycol and lead to break locally the double-stranded DNA into single-strands [52]. Modifications of nucleotides and DNA-protein cross-linking can form some products, like 8-hydroxyquinine and dehydro-2′-deoxyguanosine [53]. Those damages, if not repaired, denaturation and DNA unfolding made DNA processing impossible [53]. DNA damages alter protein synthesis, photosynthesis, transcription, correct signal transduction, induce replicative errors, and generate genomic instability [31, 53].

The enzymes of glycolysis, Kreps’ cycle (TCA) and pentose phosphate (PP) pathways are severely attacked by ROS, some of them, like glyceraldehyde 3-phosphate dehydrogenase and fructose-1,6-bisphosphate aldolase, can be completed inhibited, other can suffer the reduction in their activities. To compensate this situation, cells react by increasing the carbon flux to produce the necessary NADPH and the generation of in Ribose 5-P, and ribulose 5-P intermediates [53, 54]. In OS conditions, stressed plants reduce glycolysis and TCA activity and the abundance of citrate, isocitrate, fumarate, malate, succinate and 2-oxoglutarate have been observed [55].

2.3 Abiotic and biotic stresses in plants

The plant’s adaptive response to climate change stands out as one of the most important issues to face by agriculture today. Those issues include both biotic and abiotic stress affecting the wealth and health of wild ecosystems and resulting in significant productive losses in cropped plants [54, 56]. Abiotic stress is closely related to affectation caused by climate change and anthropogenic activities, particularly economic projects without eco-friendly and sustainable technologies. Such factors as salinity, extreme temperature, drought, and toxic soil components like heavy metals, climatic factors agrochemicals and anthropogenic activities are listed as abiotic stressors. Texture, carbon, and organic matter content (C) content, essential plant nutrients, soil compaction, aeration, and moisture levels among others are considered as physicochemical factors. Biotic stressors are living organisms such as phytopathogens (virus, bacteria, fungi and nematodes, causing diseases), weeds (wild flora that competes for nutrients with cropped plants), and parasitic plants (plants take nutrients from cropped and timber plants). It is important to elucidate the plant adaptive response and the mechanisms of ROS production and OS to develop plant breeding programs and practical methods for the mitigation of adverse factors in agriculture and natural ecosystems.

2.3.1 Abiotic stress

Generation of ROS is one of the most common plant responses to different stresses, and it is the starting point at which various signaling pathways come together to modulate the plant response to environmental external changes. Integral Redox regulation is carried by interconnected pathway proteins that provides a rapid antioxidant response to protect cellular integrity. Its different components and mechanisms keep plant development and defense pathways. Each plant cellular compartment is equipped with its own ROS homeostasis control. ROS signaling is altered depending on the cell type, stage of development, and level of stress. Under different abiotic stress conditions production of ROS molecules in cells are recognized by various ROS sensors to produce stress-specific action for the development of acclimation and adaptive response. ROS signals are decoded through various redox reactions which induce specific gene expression modulating specific transcription factors that directly interact with the DNA.

Drought stress is one of the major limitations to global agricultural production. Drought induces stomal closure in leaves, and other photosynthetic tissues, reduces transpiration, CO₂ capture and fixation, and lowers photosynthesis rates. The imbalance between light capturing and its photosynthetic use causes ROS and lead to OS [22, 28, 46]. The over-exposition of chloroplasts to the light induces excess of photorespiration increasing the formation of H₂O₂ [28]. The reduction of ferredoxin and problems with the acceptance of electrons alter ETC and reduce regeneration of NADP⁺ [57, 58, 59]. Drought increased the amount of H₂O₂ and MDA and other ROS [59, 60]. Those results were obtained when osmotic stress was induced by the addition of PEG 6000 (15–20%) in different plant species (Brassica napus L., Oryza sativa L., Amaranthus, Triticum aestivum, Zea mays L.) [61, 62]. In all case, tolerant cultivars showed reduced ROS production with reduced oxidative damage and better adaptive response, compared to the sensitive cultivar [61, 62]. In rice (O. sativa L.) subsp. japonica. cv. Nipponbare, drought stress (20% PEG) increased O₂^•− by 23%, enhanced H₂O₂ and increased MDA contents by 16%, compared to controls [63].

Extreme temperature. Temperature and water are the two most critical environmental factors that influence in plant development, from seed germination, and through all stages of its life cycle [60, 63]. Plants, as well as all the living organisms, have specific ranges of temperature represented as a minimum, maximum, and optimum surrounding temperature. The mechanisms of adaptation to high and low temperatures share some common responses, but distinct signaling components in high- and low-temperature response have differences, including the up- and downregulation of specific transcription factors (TFs) such as heat shock factor (HSF) family. Temperature changes induce the activation of complex signaling networks resulting in decrease membrane thermostability; higher malondialdehyde (MDA) accumulation; and the generation of ROS such as hydrogen peroxide (H₂O₂), singlet oxygen (1O₂), superoxide (O₂⁻), or hydroxyl radical [11, 52]. High temperatures can provoke oxidative stress, which harms plant cells and impairs their growth by releasing reactive oxygen species (ROS); and increased transpiration rates due to high temperatures and resulting also in water stress. High temperatures affect cell division, plant development and growth, pollen viability, fertilization, and fruit formation [60, 63, 64]. At low temperatures significant changes in the metabolism of ROS imply an in increase tyrosine nitration (NO₂) and LPO, ascorbate, glutathione, and the NADPH-generating dehydrogenases. Low temperature inhibits the activity of metabolic enzymes delaying many biochemical processes and plant growth and promoting the expression of cold-induced genes. Low-temperature stresses negatively impact plant growth and development by inhibiting the activity of metabolic enzymes [60, 63, 64].

Salinity. Salt excess is frequently accompanied by drought, both are caused by climatic effects combined with poor agricultural practices, such as inadequate irrigation and drainage, excessive water extraction from wells in coastal areas, extraction of peat, and poor drainage practices. The excess of salts causes osmotic stress, ion toxicity, genotoxicity, nutritional deficiency, and induces ROS overproduction, including MDA, and H₂O₂, in roots and mature leaves [62]. Direct DNA damage by ROS was observed in mung bean grown under severe salt stress by Comet Assay [52]. In salt-tolerant plants increases in H₂O₂ are correlated with lower cellular damage compared with the sensitive ones evidencing the signaling roles of ROS to stimulate the adaptive response [65, 66].

Soil composition. Soil is a complex and dynamic mixture composed of inorganic and organic substances, air, water, and living organisms interacting with plants and constitutes a dynamic, ecological and trophic network. Soil characteristics joined with climatic conditions determine their edaphological properties, ecology and productivity. The most common compounds present in soil contain elements like phosphorus, potassium and nitrogen, whereas the rest are presented in a smaller proportion. Some soils have an excessive content of heavy metalloids toxic to the majority of plants, causing morpho-physiological dysfunctions and OS [52, 67]. Increases in ROS production, LPO, MDA, and H₂O₂ were observed in Pisum sativum L. seedlings in substrate containing Ni [52, 67, 68] and similar results were obtained in other species as A. thaliana L [68], V. radiata L. [69], B. napus L. [70], B. juncea L. [71], and Cucumis sativus L. [72], evidencing that present toxicity induces OS [73, 74].

UV radiation. Another important stressor is UV (200–400 nm). About 95% of the UV reaching the planet is UV-A (315–400 nm) and the other 5% is UV-B (280–315 nm) and the majority of plants is sensible to type UV-B [75]. The UV excess directly attacks nucleic acids, nucleotides and proteins and also induces OS, damaging all cellular membranes and reducing all metabolic functions [76]. UV radiation affects photosynthesis, stomatal conductance, CO₂ assimilation, and electron transport chain. In mitochondria radiation interferes with electro transport chain (ETC) too, leading to increase production of H₂O₂, O₂^•−, and MDA in a dose-dependent tendency [77, 78, 79]. Plants mitigate UV-B-induced damage by synthesizing more compounds like flavanol, anthocyanins, and proanthocyanidins [79].

Ozone. O₃ is a highly reactive molecule, an important pollutant and an external stressor for plants, whose atmospheric accumulation increases due the effects of climate change and anthropogenic activities. Ozone penetrates to the aqueous phase substomatal cavity, interfering with surface tissues and reaching mesophyll cells, where it is degraded to ROS in the apoplastic space of mesophyll cells, causing phytotoxic damages [80]. Plants adapted to higher O₃ stress can induce programmed cell death (PCD) by interaction with the synthesis of PCD mediators during stress conditions, including a cysteine proteinase vacuolar processing enzyme (VPE) [81]. The reaction of O₃ with membrane fatty acids promotes peroxidation (LPO) and showed increases in H₂O₂, O₂^•−, OH^• and MDA levels [82].

Soil pH. Soil pH exerts great influence on the mobilization and uptake of different compounds and nutrients, cellular membrane functions, and internalization of phytotoxic compounds. The plasmatic membrane proton pumps try to combat the extreme pH by influx and efflux of H⁺, generating ROS overaccumulation conducing to OS [83]. In some genotypes of wheat (Triticum aestivum L.) and alfalfa (Medicago sativa L.) under extreme acid conditions H₂O₂, LPO and LOX activities and membrane damage increased [84, 85, 86, 87]. In alfalfa (Medicago sativa L.) under extreme alkaline conditions severe membrane damage was observed [84]. Wheat, in extreme pH conditions, increases generation of H₂O₂, MDA and LOX [85]. Acid pH facilitates the mobilization and absorption of toxic metals/metalloids (Fe, Cu, Mn, Zn, and Al) and restricts the assimilation of essential macronutrients (P, Mg, Ca, K, and Na) limiting plant nutrition and productivity in acid soils [86, 87]. In basic soils are common some deficiencies (P, Fe, Zn, Mn, Cu, and Mo), while in acid soils were found deficiencies in N, P, K, Ca, Mg, and S and excesses Fe, Mn, B, Zn, and Cu were found. [87]. The optimum pH for plant cells is considered around the cytoplasmic pH values (pH 7.0–7.5) [88, 89, 90].

Other ROS sources of ROS in plants are the toxicity of agrochemicals: herbicides, pesticides, fungicides, and vermicides, which affect plant cell membranes, lipids, photosynthetic pigments, and enzyme activities in a dose-effect manner tendency too [91, 92].

2.3.2 Biotic stress

Plant pathogens (viruses, bacteria, fungi, nematodes, and parasitic plants) are a cause of significant losses in crop productivity and ecosystem wealth. The infestation process follows a wide spectrum of physiological and biochemical reactions and symptoms accomplished by a decrease in photosynthesis, alterations in enzymatic patterns, changes in gene expression patterns, and the production and accumulation of ROS.

Pathogen attack. Plant–pathogen recognition is a process that includes different factors integrating in an overall result that determines the success of infestation [93]. When a potential plant host is unable to coordinate effective defense responses to prevent and reject pathogens from colonizing and proliferating within the host; it is considered that interaction is compatible [94]. In case of the incompatible interaction, plant defense is strong enough to prevent colonization and spread of the pathogen within host tissue [94, 95]. The plant immunity system is ROS-mediated induced and the plants generate first a primary ROS wave in response to preliminary infective contact by potential pathogens. In general, ROS synthesis occurs in two phases: I and II. Both phases differ in location, goal, magnitude, permanence in time, and magnitude. Phase I is a low-amplitude and transient action, like an alert signal. Phase II is more time prolonged and stronger with a typical increase of ROS production [95]. If a pathogen escapes recognition by the host, it fails to induce the second phase, and plants can not arm its integral defense system against the pathogen, suggesting a functional link between the ROS generation and the display of an effective immune response [96].

Plant pathogen recognition starts in the apoplast with the earliest oxidative reaction and the increment in ROS production O₂^•−, and dismutated products of H₂O₂ [97]. The plasma membrane-localized pattern recognition receptors (PRRs) recognize the intruder and induce transient oxidative wave, mediated by respiratory burst oxidases (RBOH) and apoplastic peroxidases (aPO) [98]. RBOHs are signaling nodes in the ROS gene network of plants integrating a signal transduction pathway with ROS signaling. Plasma-membrane-localized PRRs are the main components of multiprotein complexes having additional transmembrane and cytosolic kinases. The pattern recognition receptors (PRRs) are a plasma membrane-localized class of proteins involved in the innate immune response of plants and located on the surface of plant cells, recognizing pathogen-associated molecular patterns (PAMPs). PRRs are considered the first line of defense against invading pathogens. In this process, some apoplastic enzymes are activated, as plasma membrane NADPH oxidases and cell wall peroxidases [13, 99]. Although the first oxidative wave is located in the apoplast, apoplastic ROS acts as mediator in cell signaling to induce ROS production in other cellular organelles and compartments (mitochondria and chloroplast) where ETC are placed. The ROS wave is followed by the accumulation of salicylic acid (SA) in its reduced form in the cytoplasm, accumulation of hormones and the deposition of callose at the cell wall and plasmodesmata, to prevent pathogen spread. Increases of ROS and salicylic acid after the pathogen recognition, induce redox-regulated transcriptional response mediated by NPR1, which is the central regulator of pathogens’ attack response through a transcriptional gene expression cascade and mediated by salicylic acid and TGA transcription factors [100]. This process includes gene expression driven by redox signaling, while TGA transcription factors are involved in various cellular processes, connecting different hormonal and biochemical pathways, and regulatory intracellular and systemic elements. Usually, NPR1 is localized to the cytoplasm in oligomeric form with monomeric units bound by intermolecular bonds between Cys 82 and Cys 216. Monomeric NPR1 can be translocated into the nucleus for interacting with transcriptional factor TGA1 and subsequent regulatory defensive response. Translocation on monomeric NPR1 is antagonized by the presence of jasmonic acid, a plant hormone that promotes the S-nitrosylation of NPR1 on Cys156 leading oligomerization of monomeric NPR1, preventing its translocation to the cell nucleus. The nuclear localization of NPR1 is essential for the expression of PR genes [101]. Those transcriptional factors are essential to drive the gene expression including hormonal pathways, activation of genes encoded for pathogenesis related (PR) proteins, and transcriptional factors such as WRKY, a plant signaling system, modulating diverse biological functions, plant disease resistance and stress responses [24, 102, 103, 104]. Antioxidant enzymes allow the cells to resist the penetration of Aphanomyces euteiches and Sclerotinia sclerotiorum in plant tissues of lentil (Lens culinaris) [104]. In Arabidopsis thaliana L plants inoculated with the incompatible fungal pathogen Alternaria brassicicola, increments ROS generation and changes in the expression patterns of 35 different genes were observed from those of which 10 were down-regulated, and 25 were up-regulated. Among the genes that actived its expression (up-regulated) were those that code for the enzymes that drive the fatty acid ß-oxidation pathway [94]. The study of dynamic changes in enzymatic activities must be fundamental for understanding their role in resistance reactions and can be a useful tool for the preliminary selection of resistant cultivars. Enzymes like polyphenol oxidase (PPO) and pyranose peroxidase (POX) are important in the metabolism of ROS [105]. Experimental data show the role of oxidative enzymes in activating defense, like in the case of F. oxysporum in melon (Cucumis melo) [106] and in wheat cultivars assayed to a fungal pathogen, Septoria tritici, where resistant cultivar (cv. Stakado) generates higher magnitude ROS wave and susceptible cultivar (cv. Sevin) fails to generate the second Phase II of the ROS response [10, 107]. The face of Septoria tritici with wheat leaves infiltrated with catalase increases fungal penetration, colonization, and overall fungal biomass, suggesting that H₂O₂ is a critical factor in plant defense [107]. These results are consistent with those obtained in barley (Hordeum vulgare) versus two necrotrophic fungi, Rhynchosporium secalis and Pyrenophora teres [108]. In transgenic potato (Solanum tuberosum) transformed with a fungal gene encoding, glucose oxidase–conferred was observed resistance to bacterial soft rot disease and potato late blight [109]. In addition, H₂O₂ is also associated with the early events leading to the biosynthesis of phytoalexin [110]. ROS may be critical for establishing the hypersensitivity response (HR) of plants following infection and pathogen recognition to limit pathogen spread by initiating cell death at infected sites [111]. Low molecular weight elicitins proteins secreted by Phytophthora [112] induce an HR in many plant species [113].

Respiratory burst oxidase homolog (Rboh) genes have an important role in the pathogen-induced oxidative and hypersensitivity response (HR) [114]. In tobacco (Nicotiana tabacum), after elicitation induced with cryptogein, a 10-kD protein secreted by the oomycete Phytophthora cryptogea, tobacco cells transformed with antisense constructs of NtrbohD showed the same extracellular alkalinization as control plants, but they no longer produced ROS [10, 115]. In the tobacco benthi (Nicotiana benthamiana), the silencing of two Rboh cDNAs, NtrbohA and NtrbohB, lead to lower ROS levels and lower resistance to Phytophthora infestans showed that both genes are critical for H₂O₂ accumulation and for induction of resistance to this fungus [116]. In tomato (Lycopersicon esculentum M.), infection by Botrytis cinerea alters the action of the peroxisomal antioxidant system, and subsequently causes damage and enhances the pathogen-induced tissue senescence [13]. The capacity of plants to stand against sequentially or synergically stressors with harmless interference in their physiological functions depends on many aspects including nature, type and intensity of the stress and form of stress combination [24]. It was recently found that the increasing number and complexity of stressing combinations impact the plant, affecting the plant growth, declining survival dramatically [19, 117].

2.3.3 Antioxidant defense in plants

Plants display an antioxidant defense based on nonenzymatic and enzymatic components. Antioxidant defenses are adapted and evolved according to the nature of plant cells. Some antioxidant components have been mentioned previously in this chapter when the oxidative damages were described. The antioxidant defense in plants is a complex, coordinated system displaying specificities in each organ, cellular organelles, and compartments, adapted to the plant living conditions. In steady stage conditions the antioxidant defense is able to maintain appropriate homeostasis and oxidative balance [13, 33, 45, 54]. The nature of the components in antioxidant defense has nonenzymatic or enzymatic natures. Nonenzymatic components include ascorbate (AsA), the major cellular redox buffers mostly located in cytoplasm, glutathione, (γ-glutamyl-cysteinyl-glycine, GSH), tocopherol, carotenoids, phenolic compounds, free aminoacids, vitamins, etc. Ascorbate (AsA) is a well-known and efficient antioxidant. It serves as a cofactor in large number of dioxygenases and other variety of biochemical reactions, scavenging primary ROS. It is mostly provided by the so-called Smirnoff-Wheeler pathway and from uronic acid intermediates [118, 119]. Part of AsA is translocated to the apoplast, where it acts as the first line of antioxidant defense [120]. Glutathione (GSH: γ-glutamyl cysteinyl-glycine) is one a crucial antioxidant located in cytosol, chloroplasts, endoplasmic reticulum, mitochondria, and other cell compartments. Although it is synthesized in the cytosol and chloroplast, where it can react with O₂^•−, ^•OH, H₂O₂ and also has the capacity to detoxify cell from free radicals, protecting biopolymers from ROS and organic radicals [120]. The level of reduced form of GSH is an indicator of the oxidative status of the plant cells facing external stressors. Initially, external stressor stimulates GSH oxidation and increases in GSSH (glutathione oxidized form), followed by increment in GSH (glutathione reduced form), but sustained stressor strengths lead to OS [120].

Tocopherols (α, β, γ, and δ) are lipophilic molecules synthesized in photosynthetic tissues, the main form, α-tocopherol, is located in leaf chloroplasts, in the envelope, thylakoid membranes, and plastoglobuli. It scavenges ROS, usually 1O₂ and OH from photosynthesis and protects lipids and thylakoid membranes from LPO, inducing tolerance to chilling, water deficit, and salinity in different plant species [121, 122]. Increases of alpha-tocopherol can help to mitigate plant stress tolerance, while low level of tocopherols indicates OS [122]. Carotenoids is another lipophilic antioxidant to 1O₂ during photosynthesis. It is a pigment that absorbs light energy for further transfer to chlorophyll I (400 and 550 nm) harboring double bonds isoprene residues, allowing energic capture, caloric dissipation, and better adaptation of plants under drought/salinity stress [123, 124]. They are “de novo” synthesized in plant plastids of photosynthetic and sink organs. Carotenoids are important components of PhI and play a key role in photoprotection and adaptive response to stressors through cell signaling and phytohormone synthesis, as abscisic acid and strigolactone [125]. Other phenolic and polyphenolic compounds, such as flavonoids, tannins, hydroxycinnamate esters, and lignin, also have antioxidant properties [125].

The antioxidant defense also displays antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), enzymes of ascorbate-glutathione (AsA-GSH), cycle ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR). Those enzymes are present in all aerobic and evolve together with organisms to respond properly against OS coordinately in different subcellular organelles and compartments [126]. Plant SOD belongs to the group of metalloenzymes catalyzing the dismutation of O₂^•− to O₂ and H₂O₂, and are present in three different isoenzymes differing in the associated metal they have: copper/zinc SOD (Cu/Zn-SOD), manganese SOD (Mn-SOD), and iron SOD (Fe-SOD) [126]. All genes encoding SOD isoforms are nuclear, and contain transit peptides for its translocation to the mitochondria (MnSOD, to chloroplasts (FeSOD), and to cytosol, chloroplast, peroxisome, and mitochondria (Cu/Zn-SOD isoforms). SOD overexpression is correlated with increased tolerance of the plant against environmental stresses and it could be used as an indirect selection criterion for screening drought-resistant plant materials [126, 127].

Other antioxidant enzymes are Catalases (CATs), which are highly active to H₂O₂ but much weaker against organic peroxides (R▬O▬O▬R′) [127]. The major reservoirs of CAT in plant cells are the peroxisomes. Other enzymatic systems, like Xanthine Oxidase (XOD) are coupled to SOD [127, 128]. XOD are located in cytosol, chloroplast, and mitochondria in three isoforms. The guaiacol peroxidase (GPX) oxidizes aromatic electron found in compounds like guaiacol and pyragallo in vacuoles, cytosol and cell wall. Synthesis of ethylene and wound healing GPX increases its activity under the metal, chemical toxicity, and salinity stresses. In Safflower (Carthamus tinctorius), cultivars are tolerant to salinity by increasing of the GPX activity [128]. Ascorbate-glutathione cycle (AsA-GSH cycle) is the key enzyme in Halliwell-Asada pathway for the control of intracellular ROS levels [129]. The cycle involves ascorbate, glutathione, NADPH, and the related enzymes: ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) [129]. AsA and glutathione are linked through the ascorbate–glutathione cycle. In basal conditions, AsA is mostly found in its reduced form but under stress, ascorbic exists in oxidized form: dehydroascorbic acid (DHA). Alteration in the ratios AsA/DHA and GSH/GSSG are censoring parameters of cell oxidative status and of the correct work of antioxidant defense [28, 46]. In stressed conditions, ratio AsA/DHA decreases. The concentration of GSH is increased under stressed conditions showing the ability to deploy antioxidation activity so the ratio GSH/GSSG increases. Ascorbate peroxidase (APX) is another enzymatic component of antioxidant defense. It has different isoforms and it is reported to be an efficient antioxidant in response to abiotic stresses, such as drought, salinity, chilling, metal toxicity, and UV irradiation [46].

Antioxidant defense is a multifactorial interrelated system where whole antioxidant tools rather than a single detoxifying mechanism are necessary to keep oxidative balance. Consequently, the overexpression of one of its components probably will not change the antioxidant capacity of the whole pathway [130]. However, the overexpression in plants of one or various antioxidant enzymes can help to study their synergistic effect on stress tolerance [60, 67, 70].|

3. Oxidative stress and plant microbiota

Plants live in a close relationship with soil microbiota. Microbiota is composed by microorganisms, such as bacteria, yeasts, fungi, protists and nematodes that form a complex, well-structured, interdependent trophic network. Symbiotic association are present in more than 70% of the vascular plants [131]. Microbiota displays multiple beneficial functions for plant development, soil productivity, biomass production, and conservation of the whole ecosystem, mitigating the negative effects of environmental stressors and OS damages [132]. It is crucial to make the nutrients accessible for plants and to improve water uptake. Plants and its microbiota produce ROS that coordinate the wealth and diversity of microbial populations, plant immune, and adaptive responses, allowing integrated regulation of ROS levels [8, 10, 133]. Microbiota is also known for their production of plant growth hormones, soil-borne, and systemic pathogen control [134]. The microbiota is distributed in different locations: surface (epiphytic), cavities, and vascular systems (endophytic), and plant vicinity, associated with the roots (rhizospheric) [131]. Some microorganisms can be endophytic and rhizospheric at the same time wild others are in free form in soils, but interact with rhizospheric microorganisms [22]. The endophytic bacteria, Sphingomonas sp. LK11, isolated from leaves of Tephrosia apollinea, mitigate salinity stress in wild-type tomato (Solanum lycopersicum) and tomato cultivar Got-3 by promoting root growth and expression of peroxiredoxin-glutathione S-transferase-, and glutaredoxin-related genes [133, 134]. Rice plants inoculated with the root endophytic fungus Piriformospora indica, showed improvement in roots and plant development due the proline accumulation [135]. The application of endophyte fungus Metarhizium brunneum strain CB15 in potato mitigated the nutrient deficiency by enhancing the photon capture capacity of PSII, better water uptakes and of N, P, and other nutrients [136]. Maize plants faced drought stress and inoculated with the endophytic fungus, Piriformospora indica, increase biomass, proline content, and enhanced up-regulation of antioxidants [6, 135, 136, 137]. In maize cultivars, tolerant to drought higher expression in drought-related genes were observed in plants inoculated with Piriformospora indica [137]. Wheat plants inoculated with six endophytic fungi enhanced the efficiency of photosynthesis increased plants development, grain weight, and germination rates under heat and drought [138].

Better adaptive response were obtained in japonica rice related to Paecilomyces formosus and faced heat stress. In this case, results evidenced the induction of downregulation of the stress-related signaling molecules, including ABA and jasmonic acid (JA) [139]. This fact can be explained by higher phytohormonal synthesis and expression of genes encoding for indole-3-acetamide hydrolase, aldehyde dehydrogenase for indoleacetic acid, and geranylgeranyl-diphosphate synthase, kaurene oxidase (P450-4), C13-oxidase (P450-3) for gibberellins synthesis [140]. Under drought conditions, chili pepper plants (Capsicum annuum) exhibited a significant increase in plant growth and higher activity in antioxidant enzymes: peroxidase, catalase and polyphenol oxidase, phenylalanine ammonia-lyase and Capsaicin synthase genes. Those activities induce phenylpropanoid biosynthesis, under the influence of endophytic fungus Penicillium resedanum LK6 [134]. The same results have been reported in wheat, maize, tomato, and sweet basil (Ocimum basilicum) plants inoculated with Glomus fasciculatum, Cyclamen persicum, Rhizophagus irregularis/Variovorax paradoxus, and Glomus deserticola [140, 141, 142, 143, 144, 145, 146, 147].

Plant growth promotion bacteria (PGPR) synthesize phytohormones, including IAA, gibberellins, ethylene, ABA and cytokinins, promoting plants to become more tolerant of various abiotic environmental stresses, with increased plant dry biomass, flowering, and fruit production [148]. Strains of soil bacteria, such as Azospirillum ssp., Bacillus ssp., Burkholderia ssp., Erwinia ssp., Pseudomonas ssp., Rhizobium ssp., or Serratia ssp. among others, have been reported to be phosphate solubilizing, improver of nitrogen fixation, uptake of Fe, K, and Zn and other nutrients and production of plant hormones like IAA, an auxin involved in differentiation of vascular tissue, adventitious and lateral root, cell division, and shoot growth. [149]. They also stimulate the production of other enzymes like ACC-deaminase, chitinase, and glucanase, useful to control ethylene production and some phytopathogenic fungi [148]. Maize, sorghum, and soybean plants inoculated with PGPB better confront drought and salinity stresses by enhancing the concentration of ABA, regulating transcription of drought-related genes and increasing the production of salicylic acid, polyphenol, flavonoids, SOD, and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) scavenging activity [22, 133, 134, 135, 136, 137, 138, 139]. Secretion of ACC deaminase helps to improve tolerance to drought, keep homeostasis, reduction of ethylene production, and photosynthesis. That is the cases of Bacillus aryabhattai H19-1 and Bacillus mesonae H20-5 in tomato plants and Variovorax paradoxus in wheat, result in increasing proline, ABA, and improved antioxidant enzyme activities [150]. The absence of plant NADPH oxidase RBOHD (activator of ROS production) demonstrated that bacterial pathogen Xanthomonas sp. can alter phyllospheric and endospheric composition in microbiota [150]. Loss of RBOHD directly impacts the growth of individual bacteria promoting reconfiguration of microbe–microbe interactions, but the mechanism of this action remains unclear [150, 151, 152, 153]. The dynamic of ROS-mediated plant microbiome interaction suggests that ROS also modulates plant immunity and microbiota homeostasis [153] and induces systemic tolerance (IST) [154]. Both plant and microbial communities produce ROS to coordinate the diversity and abundance of microbial populations of ROS gradients within plant organs (roots and shoots) and plant cell compartments (apoplast, plasma membrane, organelles) [155, 156]. The selection for ROS scavenging strains could explain compositional trends within plant microbiomes [155, 156, 157].

4. Characterization of beneficial bacterial strains to mitigate drought stress in maize and sorghum plants, a practical example

Microbiota plays an important role in plant development and the whole ecosystem [157]. We evaluate the plant growth and water stress resistance in maize and sorghum plants inoculated with of three selected bacterial isolates [22]. The selected strains were isolated from a natural ecosystem and preliminarily evaluated in activities considered important for deploying important biochemical activities to plant growth, nutrition and pest prevention, individually or in complementarity with each other, allowing them to be a part of a multifunctional microbial consortium. Those characteristics were: Gram-staining, IAA production, ACC deaminase activity in no stressed and stressed conditions, siderophores production, cellulase activity, chitinase activities, phosphate’s solubilization, polyhydroxybutyrate (PHB) production, H₂S production, and phospholipase activity. Each bacterium was classified by comparative analysis of 16 S amplicon sequences in each case and the results were compared to the data bank available and were classified as: Pseudomonas ssp. XiU1297, Luteibacter ssp. 1292, Acinetobacter ssp. XiU12138. Mutual compatibility was also assayed with positive results. Those three strains were subjected to several rounds of mutagenesis by culturing them in a minimal medium supplemented with guanidium thiocyanate and those capable of chelating heavy metals such as Al, Zn, and a mixture of Cu, Fe, and Ni were selected. Metal chelating putative mutants were individualized and analyzed to select those that, in addition to acquiring the ability to chelate metals, retained the original characteristics that we consider important for plants. Selected mutants of each bacterial species were used to form a bacterial consortium and designed as: Pseudomonas ssp. XiU1297-Nmut, Luteibacter ssp. XiU1292-Nmut, Acinetobacter ssp. XiU12138-Nmut (Table 1). The consortium contains about 5 × 10⁷ colony-forming units of each strain and in drought-induced experiments in maize and sorghum plants in greenhouse conditions according with standard methodology during 6 week-period of drought stress [22].

N°	Evaluated characteristics	Bacterial strain
		Pseudomonas ssp. XiU1297-Nmut	Luteibacter ssp. XiU1292-Nmut	Acinetobacter ssp. XiU12138-Nmut
1	Gram staining	Gram Positive	Gram Positive	Gram Positive
2	Qualitative IAA	High	Medium	Medium
3	Quantitative IAA (μg/ml)	25.34 ± 0.25	22.62 ± 0.19	23.09 ± 0.14
4	ACC deaminase (no drought) (μM/mg α-ketobutyrate)	5.35 ± 0.22	4.97 ± 0.31	5.15 ± 017
5	ACC deaminase (drought) (μM/mg α-ketobutyrate)	3.23 ± 0.16	3.18 ± 0.21	3.27 ± 0.13
6	Siderophores production	Medium	Low	Medium
7	Cellulase activity	High	Medium	High
8	Chitinase activities	Medium	High	Medium
9	Solubilization of phosphates	High	High	High
10	Polyhydroxybutyrate (PHB) production	Medium	High	High
11	Production of H2S	Low	High	Medium
12	Phospholipase activity	High	High	High
13	Chelating Al	High	—	Medium
14	Chelating Zn	Medium	High	Low
15	Chelating Cu, Ni, Co	High	Medium	Medium
16	Intercompatibility	Compatible	Compatible	Compatible
17	Localization in host plants	Rhizospheric and endophytic	Rhizospheric and endophytic	Rhizospheric and endophytic

Table 1.

Some characteristic and qualitative properties of individual bacterial strains were used as criteria for selection of Pseudomonas ssp. XiU1297-Nmut, and Luteibacter ssp. XiU1292-Nmut, Acinetobacter ssp. XiU12138-Mnut (De la Riva G.A. Unpublished data, [22]).

Nmut: Strain chemically mutagenized

Maize and sorghum plants were analyzed for the following parameters: root fresh weigh, root dry weight, leave-stem fresh weight, leave-stem dry weight. In case of maize, the best growth promotion was achieved by Pseudomonas ssp. XiU1297-Nmut, followed by Luteibacter ssp. XiU1292-NMut and Acinetobacter ssp. XiU12138-Nmut (Figure 1). For sorghum plants, the highest growth-promotions were obtained by Luteibacter ssp. XiU1292-Nmut followed by Pseudomonas ssp. XiU1297-NMut, and Acinetobacter ssp. XiU12138-Nmut (Figure 2).

Figure 1.

Effect of bacterial strains on maize´s roots (A), stems-leaves (B). The values correspond to fresh and dry weight. Treatments: T1-No bacterial treatment, T2-Luteibacter ssp. XiU1292-Nmut, T3-Pseudomonas ssp. XiU1297-Nmut, T4-Acinetobacter ssp. XiU12138-Nmut, T5-Daily irrigated plant without bacterial treatment. Water irrigation was discontinued at the end of week 3 and restarted at the end of week 4. Best results in maize were obtained by inoculation with Pseudomonas ssp. XiU1297-Nmut (T3). Nmut: Strain chemically mutagenized and selected [22].

Figure 2.

Effect of bacterial strains on sorghum’s roots (A), stems-leaves (B). The values correspond to fresh and dry weight. Treatments: T1-No bacterial treatment, T2-Luteibacter ssp. XiU1292-Nmut, T3- Pseudomonas ssp. XiU1297-Nmut, T4- Acinetobacter ssp. XiU12138-Nut, T5- Daily irrigated plant without bacterial treatment. Water irrigation was discontinued at the end of week 3 and restarted at the end of week 4. Best results were obtained by Luteibacter ssp. XiU1292-Nmut (T2) [22]. Nmut: Strain chemically mutagenized and selected.

We also explore some oxidative parameters after six weeks of hydric stress including: total antioxidant capacity, LPO, hydrogen peroxide assay, ascorbic acid assay, glutathione, assay, and MDA. We observed that treatments with selected bacterial strains promote better oxidative balance and better resistance to drought conditions in both maize and sorghum compared with negative control (no inoculated plants) and plants maintained in a normal irrigation regimen (Table 2). Relatively high levels of exogenous ROS production are common features of pathogenic microbes, whereas ROS scavenging (and/or low net ROS production) has been continually linked to beneficial microbes [14, 15, 148].

N	Bacterial strain treatment	Total antioxidant capacity (mmol of TE/g fresh weight)	Lipid peroxidation (nM/g fresh weight)	Hydrogen peroxide assay (μM/g fresh weigh)	Ascorbic acid assay (mg/g)	Glutathione assay (μg/g fresh weight)	MDA (mM/g fresh weight)	OC
Maize week 6 drought stress
1	Pseudomonas ssp. XiU1297-Nmut	8.56 ± 0.14	3.52 ± 0.14	3.61 ± 0.16	3.86 ± 0.14	3.56 ± 0.14	3.56 ± 0.14	C
2	Luteibacter ssp. XiU1292-Nmut	7.75 ± 0.12	5.45 ± 0.12	3.85 ± 0.12	3.55 ± 0.12	3.75 ± 0.12	3.75 ± 0.12	C
3	Acinetobacter ssp. XiU12138-Nmut	7.23 ± 0.15	4.23 ± 0.15	3.63 ± 0.15	3.83 ± 0.15	3.23 ± 0.15	3.23 ± 0.15	C
4	No bacterial treatment drought	3.75 ± 0.14	8.75 ± 0.14	6.75 ± 0.14	6.55 ± 0.19	6.75 ± 0.14	6.75 ± 0.14	OS
5	No bacterial treatment/water	6.67 ± 0.14	4.08 ± 0.12	3.87 ± 0.15	3.45 ± 0.14	3.21 ± 0.14	3.21 ± 0.17	C
Sorghum week 6 drought stress
1	Pseudomonas ssp. XiU1297-Nmut	6.56 ± 0.12	10.56 ± 0.14	3.56 ± 0.14	38.56 ± 0.14	13.56 ± 0.14	3.56 ± 0.13	C
2	Luteibacter ssp. XiU1292- Nmut	7.75 ± 0.16	13.75 ± 0.12	3.75 ± 0.12	39.75 ± 0.12	14.75 ± 0.12	3.75 ± 0.12	C
3	Acinetobacter ssp. XiU12138-Mmut	8.23 ± 0.13	12.23 ± 0.15	3.23 ± 0.15	45.23 ± 0.15	13.23 ± 0.15	3.23 ± 0.10	C
4	No bacterial treatment drougt	4.75 ± 0.19	26.75 ± 0.14	6.75 ± 0.14	16.75 ± 0.14	6.03 ± 0.14	6.75 ± 0.11	OS
5	Not bacterial treatment/water	6.55 ± 0.11	4.42 ± 0.14	3.96 ± 0.19	3.25 ± 0.24	3.29 ± 0.12	3.33 ± 0.19	C

Table 2.

Effect of plant beneficial bacteria on plant antioxidant capacity in maize and sorghum leaves under drought conditions at week 6 of hydric stress. Treatments with Pseudomonas ssp. XiU1297-Nmut, and Luteibacter ssp. XiU1292-Nmut, Acinetobacter ssp. XiU12138-Mnut were evaluated and resulted in better antioxidant defense in maize and sorghum plants (De la Riva G.A. Unpublished data, [22]).

Nmut: strain chemically mutagenized and selected; OC: oxidative condition; C: controlled oxidative status; OS: oxidative stress.

The short living period and nature of ROS (particularly O₂^•− and ^•OH) present serious methodological challenges for their detection and trajectory within complex systems, but established links and dynamic in microbiota, its wealth, structure, diversity, and concentration, seems to be in a function of these ROS gradients. The state-of the-arts in the knowledge on plant-microorganism interrelationships calls us to be cautelous when attempting to generalize microbial functions across varied environmental contexts, including the mechanisms of action [8, 9, 157]. The relation between ROS activity, antioxidant defense status, and the ability to mitigate drought stress is evidenced. The complementation in activities displayed by selected bacterial strains and the established intercompatibility between them open. They open up the possibility of reconstitute a microbial consortium suitable for application according to the environmental conditions and crop’s specificities [158].

5. Conclusions

Oxidative stress (OS) is closely related to plant adaptive response to biotic and abiotic stressors. Adverse environmental abiotic stress is a very limiting factor for crop productivity and wild ecosystem wealth and includes drought, salinity, heavy metals, nutrient deficiency, extreme temperatures, high radiation, and even stress caused by agrochemicals. Biotic stress is caused by plant pathogens, fungi, virus, bacteria, nematodes, and weeds. In environmental conditions, both abiotic and biotic stressors can act in sequential or synergically combination enhancing its adverse effects in affected plants. The ROS production within certain limiting doses is essential as a subcellular and cellular signaling system and inductor plant antioxidant defense, plant immunity and adaptive response. ROS overproduction can escape the antioxidant defense causing OS, damaging cellular structures and affecting important physiological functions. Plant microbiota interacts with plants in a complex relation and interdependence. Although many aspects of plant-microbiota functioning are still under research, it is evident its capacity to mitigate the biotic and abiotic stresses. The beneficial microbes such as plant-growth-promoting rhizobacteria (PGPRs) and arbuscular mycorrhizal fungi (AMFs) among other microorganisms can help improve stress tolerance by enhancing plant growth, stimulating the production of phytohormones, siderophores, and solubilizing phosphates, lowering ethylene levels, secreting enzymes, and metabolites with antipathogenic activities and upregulating the expression of antioxidant genes [159].

The plant-microbe interaction leads to the modulation of complex mechanisms in the plant cellular system. Moreover, the residing microbial flora also inhibits the phytopathogens, therefore, it becomes part of plants’ innate defense system and this represents an important opportunity to develop suitable strategies to improve ecosystem wealth and crop productivity facing the challenges of climatic changes and growing needs to use more eco-friendly agricultural technologies and safer products for pest control and plant nutrition.

Acknowledgments

This work was supported by CORFO Chile (Corporación de Fomento de la Producción) through the research project entitled “Research and development of novel endemic microbial consortiums and their validations as high impact biostimulators against challenging conditions provoked by climate change affecting strategic crops (cherry, blueberries, hazelnut)” Grant 23CV12-251461.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

OS	oxidative stress
ROS	reactive oxygen species
RONS	reactive oxygen and nitrogen species
SOD	superoxide dismutase
GPO	glutathione peroxidase
CAT	catalases
ETC	electronic transport chain
PSI	photosynthetic system I
PSII	photosynthetic system II
APO	ascorbate peroxidase
GOX	glycolate oxidase
AO	aldehyde oxidase
XOD	xanthine oxidase
MDA	malondialdehyde
LPO	lipid peroxidation
UV	ultraviolet radiation
PCD	programmed cell death
RBOH	respiratory burst oxidases
PRRs	pattern recognition receptors
NPR1	non-expresser of pathogenesis related genes 1
PR	pathogenesis related genes/proteins
IAA	indole acetic acid