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Introduction
Drought-induced water stress is known to limit crop growth and productivity (Nakashima and Yamaguchi-Shinozaki 2013), and the most important environmental stress affecting agriculture. Prolonged water stress decreases the water potential of leaves, their size and opening of the stomata, stops root growth, reduces the number of seeds, their size and viability, delays flowering and fruiting and limits the plant growth and its productivity (Osakabe et al. 2014 and Xu et al. 2016).
The excessive presence of salts in the soil is another of the major factors responsible for the reduction of plant growth and the productivity of crops throughout the planet. Salinity creates an osmotic stress, which can be considered as a physiological drought; however, higher salt accumulation can cause ionic toxicity, which induces leaves senescence (Munns and Tester 2008). Some of the effects of water stress caused by drought in plants can also be found under salt stress conditions.
The problem of saline soils is frequent in arid and semi-arid zones, due to the irrational use of chemical fertilizers and the inappropriate use of irrigation systems (Bharti et al. 2013), so both types of stress are highly correlated. . This type of environmental stress has turned agronomically useful land into unproductive land, and reaches an impact of 20 % in the world (Liu et al. 2020).
According to Saikia et al. (2018), in addition to the classical or transgenic hybridization approaches in plant species, the application of plant growth promoting rhizobacteria (PGPR) is an alternative strategy to improve plant health under stressful environmental conditions. Glick (2016) stated that PGPRs not only directly promote plant growth, but also protect plants against a wide range of abiotic stresses, including drought and salinity.
In this material, a set of conceptual elements will be analyzed first. Then, some research results about the beneficial effect of PGPR on crops, under stress conditions due to drought and salinity, will be deal with.
The objective of this review is to synthesize knowledge about the effect of drought and salinity on crops. Research results on this subject are showed, with emphasis on cereals, legumes and grasses, as well as on the use of PGPR. Some achievements in this line of research in the international arena and in Cuba are also showed.
Conceptual elements: effect of drought on plants
Drought stress is considered the most damaging abiotic stress for crop productivity (Mir et al. 2012). Water stress caused by drought increases the production of reactive oxygen species (ROS), which can cause damage to cellular structures, as well as oxidative stress. Oxidative molecules initially damage chloroplasts and cause deleterious effects, including chlorophyll destruction, lipid peroxidation, and protein loss (Zhang and Kirkham 1994). The generation of ROS, such as hydrogen peroxide (H2O2), is one of the earliest biochemical responses to stress and helps trigger subsequent defense reactions in plants (Apel and Hirt 2004). According to Noctor et al. (2014), maintaining the balance of ROS production and extraction is crucial for drought tolerance. Enzymatic and non-enzymatic defense systems reduce the damaging effects of these compounds. The enzyme defense system includes superoxide dismutase, catalase, guaiacol peroxidase, ascorbate peroxidase, glutathione reductase, monodihydroascorbate reductase, and dihydroascorbate reductase (Munns and Tester 2008).
One of the mechanisms that plants develop to counteract the effect of drought stress is the accumulation of osmolytes or osmoprotectors (Anjum et al. 2017 and Tarveer et al. 2019). A variety of osmotically active molecules (sugars, proline, glycine betaine, and organic acids) are accumulated to balance water relations during drought stress. The amino acid proline is the key osmolyte, which acts as a protector of enzymes and the cell membrane. This well-known osmoprotector promotes plant protection against drought, salinity and other types of stress (Peng et al. 2008). According to Szabados and Savoure (2010), the increase in proline levels can be attributed to the increase in synthesis and decrease in degradation under conditions of saline or water stress. Rezayian et al. (2018) observed that the content of H2O2 and proline is increased in rapeseed plants subjected to water stress, compared to the control. The increase of this amino acid under water stress conditions helps the plant through osmotic fits.
Abscisic acid (ABA) is important in many physiological processes in plants. This hormone is necessary for the regulation of various events during the last stage of seminal development, and it is crucial for the response to environmental stress (dryness, salinity and cold). Likewise, it controls plant growth and inhibits root elongation (Pilet and Chanson 1981), which means that there is a negative correlation between growth and endogenous ABA content in plants (Pilet and Saugy 1987). It also plays a central role in cellular signaling and that between the roots and the aerial part of the plant during drought stress; in addition to participating in the regulation of growth and stomatal conductance (Davies et al. 2005).
According to Villagra et al. (2011), the meadow grasses of the central mount of Argentina, in the dry season, regulate water loss through stomatal closure, and later through changes in leaf architecture, by folding the leaves over the central vein or folding the leaf blade in half, as is the case of Pappophorum caespitosum and Trichloris crinita, respectively.
Cenchrus ciliaris, another meadow grass, has been classified as resistant to water stress (Ruiz and Terenti 2012). This species is cultivated extensively in arid and semi-arid ecosystems in several countries, and is used to stabilize soils and increase the productivity of grasslands that have experienced the effect of drought combined with overgrazing (Lyons et al. 2013).
The induction of volatile oils takes place when plants are exposed to various types of stress (Loreto and Schnitzler 2010). These stress-induced volatile oils are useful as signals to develop primary and systemic responses in the plant and its surroundings (Niinemets 2010). According to Timmusk et al. (2014), volatile oils are promising candidates in a non-invasive technique to assess drought stress and its mitigation during stress development.
In plants, through the ethylene biosynthetic pathway, the amino acid methionine is converted to S-adenosyl methionine (S-AdoMet, or SAM) by the enzyme S-adenosyl-L-methionine synthetase (SAM synthetase). The S-AdoMet, in turn, is transformed by 1-aminocyclopropane-1-carboxylate synthetase (ACS) into 1-aminocyclopropane-1-carboxylate (ACC), an immediate precursor of ethylene (Vurukonda et al. 2015).
Ethylene is a plant hormone related to the regulation of various physiological processes in plants, but its production in plants, due to climate change, inflicts a significant reduction on plant growth and development, and if not properly controlled, can lead to in plant death (Iqbal et al. 2017 and Dubois et al. 2018). Therefore, the increase in ethylene production in a large number of plants (figure 1) is an indicator of susceptibility to various types of environmental stress, among which drought and salinity stress can be mentioned (Glick 2014, Müller and Munné-Bosch 2015, Liu et al. 2015 and Abiri et al. 2017).
Figure 1 Ethylene plant hormone affects a great number of different processes growth and development of the plant
Effect of salinity on plants
Saline soils are high in electrical conductivity, low in water potential and have an excess of ionic salts, which makes it difficult for plants and other life forms to survive (Mishra et al. 2018 and Egamberdieva et al. 2019).
The central problem faced by plants subjected to high concentration of salt (NaCl) is the osmotic retention of water and specific ionic effects of toxicity on the proteins of the cytoplasm and membranes. Water is osmotically retained in saline solutions, in such a way that as the salt concentration increases, the water becomes less and less available to the plant (Benavides 2002). According to Glick (2014), it is important to highlight that many of the early effects of salt stress are attributed to water stress caused by salt in plants. Due to the increased concentration of osmolytes in cells subjected to osmotic stress and water stress, the osmotic potential becomes negative and causes water endosmosis, which maintains turgor pressure and cell integrity (Sharma et al. 2019).
Like water stress, salinity increase an ionic imbalance in plants, which causes nutritional deficiency, disturbances in carbon (C) and nitrogen (N), assimilatory pathways, reduced photosynthetic generation rate, generation of ROS, osmotic and oxidative stress, which retards crop growth and yield (Hashem et al. 2016 and Pan et al. 2019). The K+/Na+ ratio is very important for plants and salt stress causes alterations in the balance between these ions, which reduces this ratio and decreases the availability of nutrients (Reich et al. 2017).
Proline accumulation is one of the response mechanisms of many plants during various types of stress (Anjum et al. 2016, 2017), including saline. The formation of this amino acid in plants occurs mainly from glutamate (Khan et al. 2015).
Plants tolerate salinity by accumulating low molecular weight osmolytes such as proline, glycine betaine (GB), and polyamines, which help maintain membrane stability. These osmoprotectors improve the germination rate, growth and development of the plant, which induces tolerance towards saline stress (Sudhakar et al. 2001) and also towards water stress (Kubis et al. 2014). The first step in the formation of ethylene is the formation of S-adenosylmethionine (SAM) from methionine (figure 2). The SAM, which is formed during ethylene synthesis, is also a precursor for GB biosynthesis (Sharma et al. 2019). Polyamines are related to the biosynthesis of ethylene, since its precursor (SAM) is common to both compounds (Petruzzelli et al. 2000).
Figure 2 Role of ethylene, glycine betaine (GB), and polyamines that form under salinity stress. ACC: 1-amino-cyclo-propane-1- carboxylic acid; SAM: S-adenosyl methionine; dcSAM: enzyme SAM decarboxylase (Petruzzelli et al. 2000, Sudhakar et al. 2001 and Sharma et al. 2019).
The high concentration of salt in plants not only increases the excessive production of ethylene, but also induces ionic toxicity and oxidative stress, in addition to affecting the osmotic potential of plants. All physiological processes, such as respiration, photosynthesis and nitrogen fixation, among others, are affected by soil salinity, which leads to a decrease in crop productivity (Paul and Lade 2014 and Acosta-Motos et al. 2017).
As explained in the previous section, the increase in ethylene production in plants is an indicator of susceptibility, not only to drought stress, but also to salinity stress (Glick 2014, Müller and Munné-Bosch 2015, Liu et al. 2015 and Abiri et al. 2017).
Importance of pgpr against environmental stress
Microbial interactions with plant crops are fundamental for the adaptation and survival of microorganisms as well as plants, in any abiotic environment. Induced systemic tolerance (IST) is the term used to define the induction of responses to abiotic stress by microorganisms (Meena et al. 2017). The function of microorganisms to relieve abiotic stress in plants has been an area of great interest for a few decades (de Zelicourt et al. 2013, Nadeem et al. 2014 and Souza et al. 2015). According to Gopalakrishnan et al. (2015), microorganisms, with their intrinsic metabolic and genetic capacities, contribute to alleviating the effect of abiotic stress on plants.
This section explains the role played by PGPR, as inducers of tolerance to water stress due to drought and saline stress in plants. As both can cause similar responses in plants, PGPR can induce tolerance to water and salt stress, indistinctly, through similar metabolic mechanisms.
Importance of pgpr against water stress
The PGPR are highly efficient in promoting plant growth through direct and indirect mechanisms (Hassan et al. 2015). The direct effects are related to the synthesis of phytohormones by PGPR (auxins, gibberellins and cytokinins), either in the rhizosphere or in plant tissues. These phytohormones stimulate higher root development, which facilitates the absorption of nutrients in plants and provides protection against different types of environmental stress (figure 2) (Kumari et al. 2009 and García-Fraile et al. 2015). Ahmad et al. (2008) found that 80% of the dinitrogen-fixing bacteria produce indoleacetic acid, a growth substance that leads to an increase in total phenols, calcium content and activity of the polyphenol oxidase enzyme, which protects the plant against pathogens and improves its growth through removal of ROS (Chowdhury 2003).
The production of the enzyme ACC-deaminase by bacteria (figure 3), by inhibiting the production of ethylene in plants (Yang et al. 2008) by dividing ACC ethylene into α-ketobutyrate and ammonium, allows the root system develops without the inhibition of this compound, which favors greater absorption of nutrients. There are many reports about the enhancement of plant development by inoculating bacterial strains that are positive for ACC-deaminase production during drought states (Sarma and Saikia 2014), hypersalinity (Nadeem et al. 2007) and other types of stress.
Figure 3 IST caused by PGPR against drought and salinity stress. Block arrows show plant compounds that originate from environmental stress, dashed arrows show bioactive compounds secreted by PGPRs, solid arrows show plant compounds that are affected by bacterial components. IST (Induced Systemic Tolerance). PGPR (Plant Growth Promoting Rhizobacteria). ABA (Absicic Acid). ROS (Reactive Organic Species). ACC (1-aminocyclopropane-1-carboxylase). HKT1: high-affinity K+ transporter. IAA: Indoleacetic Acid
Various authors also refer to the activity of cytokinins and catalase, which act as antioxidants, such as catalase (ROS degradation factor), or which prevent the presence of other compounds that hinder the normal development of the plant subjected to water stress, as is the case of cytokinins (figure 3), which counteract the negative effect of ABA in leaves, produced by the plant against this type of stress (Yang et al. 2008).
The formation of bacterial thin layer or extracellular matrix (figure 3) is another of the mechanisms that PGPR can use in favor of plants (Dimkpa et al. 2009 and Timmusk and Nevo 2011). In particular, an extracellular matrix, formed by a bacterial thin layer, can provide an almost infinite range of beneficial macromolecules for plant development and it growth. Thin layer contain sugars, as well as oligo and polysaccharides, which can play different roles in bacteria-plant interactions, such as improving water availability in roots. The water-holding capacity of some polysaccharides can exceed up to seven times their mass (Timmusk and Nevo 2011).
Cho et al. (2008) observed that root colonization of Arabidopsis thaliana with Pseudomonas chlororaphis O6 prevents water loss caused by stomatal closure, due to the effect of 2R, 3R-butanediol, a volatile metabolite produced by P. chlororaphis O6. Meanwhile, bacteria deficient in the production of 2R, 3R-butanediol, did not show induction of drought tolerance. According to the cited authors, the increase in free salicylic acid (SA) in plants colonized by P. chlororaphis O6, under water stress conditions, after a treatment with 2R, 3R-butanediol, suggests the primary function of the signals of the SA in the induction of drought tolerance, which coincides with the criteria of Hussain et al. (2020) about the beneficial effect of SA on Cicer arietinum L.
Importance of pgpr against saline stress
According to Kumar Arora et al. (2020), it is probable that the mitigation of salt stress by PGPRs, which are halo tolerant, involves an action intertwined at three levels, such as the survival of the bacteria by itself in a hyperosmotic environment, the induction of tolerant mechanisms to salt in plants and the improvement of soil quality through various mechanisms.
Plants regulate the synthesis of phytohormones, but bacteria are also capable of producing phytohormones and releasing them outside the cell, either in the rhizosphere (rhizobacteria) (figure 3) or inside the plant tissues (endophytes). The excretion of these molecules by bacteria positively affects the performance of plants under salt stress, since, in some situations, plants do not generate enough quantities to achieve optimal development (Egamberdieva et al. 2017).
Etesami and Maheswari (2018) assure that the main aspect of tolerance to salt stress in plants through halotolerant PGPRs (PGPR-HT) involves the generation of receptive machinery, which eliminates toxicity and establishes a state of osmotic balance to avoid desiccation and flaccidity in plant cells. These authors assure that PGPR-HT limit the acquisition of Na+ by changing the composition of the cell wall/membrane. Likewise, the PGPR-HT can promote plant growth and indirectly develop tolerance against salt stress, by altering the selectivity of Na+, K+ and Ca2+ to support a higher K+/Na+ ratio, thus regulating the levels of various antioxidant enzymes in the cells. These enzymes not only detoxify harmful substances, but also reduce undesirable physiological changes caused by stress (Sukweenadhi et al. 2018).
Volatile organic compounds (VOCs), such as N-acylhomoserine lactone and cyclodipeptides, that are produced by PGPR-HT (figure 3) can also increase the induction of the high-affinity K+ (HKT1) transporter in the branches and the reduction of HKT1 in the roots, which limits the entry of Na+ into the roots and facilitates the recirculation of Na+ between branches and roots (Qin et al. 2016, Schikora et al. 2016, Rosier et al. 2018 and Hartmann et al. 2019).
The PGPR-HTs are also known to stimulate antioxidant defense machinery in plants that are involved in the synthesis of antioxidant enzymes (figure 3) against oxidative stress caused by ROS during salt stress (Islam et al. 2016). These include superoxide dismutase, peroxidase, catalase, nitrate reductase, glutathione reductase, polyphenol oxidase, guaiacol peroxidase, monohydrate dehydrogenase and dihydroascorbate reductase.
According to Belimov et al. (2014) and with Maksimov et al. (2015), the ability to synthesize ABA, particularly under stressful conditions, such as salinity, and to affect the level of ABA in plants, occurs in PGPR of the genera Azospirillum, Bacillus, Pseudomonas, Brevibacterium and Lysinibacillus, so this characteristic of these rhizobacteria is useful to confer tolerance to plants in against environmental stress.
Different biopolymers are secreted by microbial cells (polysaccharides, polyesters, polyamides) into the surrounding environment. Biopolymers play an irreplaceable role in plant-microorganism relations, especially in alleviating salt stress in plants (Etesami and Maheshwari 2018 and Gupta et al. 2019), as they excrete out of cells and join cations such as Na+ in decreasing bioavailable concentrations. They also serve as signal molecules for the defensive response to infection. These polysaccharides help PGPR to survive in saline environments, and thus most halotolerant bacteria have the ability to excrete this type of compound (Etesami and Glick 2020). According to Vaishnav et al. (2016), extracellular polysaccharides, as thin layer, function as a physical barrier around the roots, which helps plant growth under saline stress conditions (figure 2).
When studying the response to salt stress in wheat by inoculation of Enterobacter cloacae SBP-8, Singh et al. (2017) reported an increase in the level of proteins related to plant defense, photosynthesis, and proteins associated with ion transport. The cited study also showed that bacterial inoculation regulate the expression of proteins involved in cell wall strengthening and membrane integrity to prevent cell damage and lateral diffusion of molecules into the endodermis. A similar mechanism of tolerance was reported in hard wheat when inoculated with PGPR-HT under drought stress and heat stress, resulting in better plant adaptation (Tenhaken 2015). These results suggest that there is a correlation between salinity and drought through common mechanisms at the molecular level, with the purpose of combating these types of stress with the help of PGPR-HT.
Some results of the application of pgpr in plants, under water or saline stress conditions
Khan et al. (2019) reported that two genotypes of Cicer arietinum, treated with PGPR consortia under drought stress conditions, had better results in the relative content of water in leaves, higher biomass in leaves and stems, as well as higher accumulation of protein, sugars and phenolic compounds. In grain legumes, Vigna mungo L. and Pisum sativum L., Saikia et al. (2018) found that proline levels significantly increased when plants were inoculated with a collection made up of different PGPRs. Also Garcia et al. (2017) noted that the inoculation of corn with the Az19 strain of Azospirillum spp., under water stress, increased the proline content in the plants.
Jochum et al. (2019) showed the superior effect of two PGPR strains, when applied to wheat and corn, under simulated water stress conditions, by significantly increasing variables related to root architecture and leaves and stems elongation, compared to the control not inoculated. Timmusk et al. (2014) observed that in wheat subjected to water stress, the inoculation of Bacillus thuringiensis AZP2 led to higher plant biomass and five times higher survival to drought, due to the significant reduction in the emission of volatile oils and higher photosynthesis.
Curá et al. (2017) found that the inoculation of corn plants with the strains Azospirillum brasilense, SP-7 or Herbaspirillum seropedicae, Z-152, resulted in a decrease in ABA and ethylene content, as well as a reduction in proline content. This was associated with a lower perception of water stress by the plant, which resulted in lower lipid peroxidation and a higher content of carbon, nitrogen, chlorophylls and relative water content at the foliar level; in addition to a higher biomass production.
The inoculation with a mixture of Bacillus sp. and Pseudomonassp. proved to be the most efficient treatment to improve tolerance to water deficit in two wheat genotypes, by increasing plant biomass and other morphological and physiological parameters (Mutumba et al. 2018).
Timmusk et al. (2014) found through electron microscopy the formation of thin layers in root hairs of wheat seedlings, under simulated water stress conditions. In general, it was determined that the efficiency of water use in plants inoculated with B. thuringiensis, strain AZP2, increased by 63 % compared to control plants.
Zhang et al. (2020), when isolating different rhizobacteria from the rhizosphere of Ziziphus jujuba, observed that Pseudomonas lini and Serratia plymuthica, under simulated drought stress conditions, increased the height of Z. jujuba, as well as its radical dry mass, aerial dry mass and the relative water content. Also, ABA levels decreased. In addition, the antioxidant enzyme activity increased, especially with mixed inoculation.
Agronomic studies by Bécquer et al. (2016, 2017a) in triticale and corn, respectively, show the stimulating effect of Bradyrhizobium sp., alone or in combination with Trichoderma harzianum, on the growth and development of these species, cultivated under agricultural drought conditions. Also Becquer et al. 2017b, (2018, 2019b obtained promising results with the application of Bradyrhizobium sp., combined with beneficial fungi, in different meadow grasses (Cenchrus ciliaris, Cynodon dactylon and Brachiaria hibrido, respectively) under drought stress conditions. From this it is inferred that there has been positive microbial interaction between Bradyrhizobium sp. and Trichoderma harzianum, as well as between Bradyrhizobium sp. and Glomus cubense (figures 4 and 5). Likewise, Becquer et al. (2019a), when inoculating C. ciliaris with Bradyrhizobium sp., Funneliformis mosseae and a biostimulant under drought stress conditions, they obtained highly promising results.
Figure. 4 Effect of Bradyrhizobium sp. Ho13 and Trichoderma harzianum A-34, on the aerial biomass of Cenchrus ciliaris L. (Buffel Formidable), under drought stress conditions.
Figure. 5 Effect of Bradyrhizobium sp. Ho13 and Trichoderma harzianum A-34, on the aerial biomass of Cynodon dactylon Tifton 85, under drought stress conditions.
The isolates of Bradyrhizobium sp. used in these studies come from livestock ecosystems affected by drought and other stressful environmental factors (Bécquer et al. 2000, 2001, 2002, 2016, 2017c), which suggests the influence of these factors on the positive effect of this PGPR in crops affected by environmental stress.
Abril et al. (2017) confirmed that the co-inoculation of Bacillus and Azotobacter contributed to promote the growth of Megathyrsus maximus under drought conditions.
Mapelli et al. (2013) observed that the bacterial microbiome of Salicornia plants, cultivated in hypersaline ecosystems in Tunisia, showed resistance to a wide range of abiotic stress and do different plant growth promotion activities, as well as greater root colonization. This suggests that PGPR-HT that inhabits arid and saline ecosystems have the potential to promote plant growth in plants that are under hydric or saline stress.
Mayak et al. (2004) reported highly satisfactory results of inoculation with PGPR Achromobacter piechaudii ARV8 to stimulate the growth of plants subjected to salt stress. These authors consider that ACC-deaminase, produced by A. piechaudii, may be the cause of protection against salt stress in plants, by inhibiting the ethylene precursor, the ACC.
In studies by Zhang et al. (2008) it was showed that PGPR-HT Bacillus subtilis reduced Na+ uptake in Arabidopsis thaliana roots by inhibiting HKT1 under salt-affected conditions. Likewise, Yasmin et al. (2020) reported that the inoculation of soybean plants under salt stress with Pseudomonas pseudoalcaligenes increased the synthesis of main defense enzymes, which reduced the concentration of Na+ in roots and foliage. At the same time, cell condition was balanced by increasing intracellular K+ levels.
Atouei et al. (2019) reported that the PGPR-HT Bacillus subtilis subsp. inaquosorum, which produces exopolysaccharides, as well as Marinobacter lipolyticus SM19, reduced the adverse effects of salinity and drought on wheat.
Ullah and Bano (2015) inoculated strains of Bacillus sp. and Arthrobacter pascens, able to solubilizing phosphorus and producing siderophores in corn plants induced by salinity stress, and observed that co-inoculation significantly improved biomass production, increased proline accumulation and increased the response of antioxidant enzymes, such as superoxide dismutase, catalase and ascorbate peroxidase.
Rajput et al. (2018) found that a collection of halotolerant strains of Aeromonas spp., with indoleacetic acid (IAA) production, ACC-deaminase activity and ability to solubilize P-/Zn, when applied to wheat with a reduced dose of NPK, resulted in superior to non-halotolerant strains of Azospirillum sp. and Pseudomonas sp., in terms of plant growth, biomass production and grain yield. This collection showed significant potential in promoting wheat growth at germination, vegetative stage, and maturity under normal conditions, induced salinity, and natural salinity.
Lopez et al. (2011), when inoculating the forage legume Clitoria ternatea with a strain of Rhizobium sp., previously isolated in soils affected by salinity, observed that the effectiveness index of the inoculation based on aerial dry mass was higher, when they were compared with commercial strains.
Gupka and Pandey (2019), when inoculating beans (Phaseolus vulgaris L.) with strains of Aneurinibacillus aneurinilyticus and Paenibacillus sp., highly producers of ACC-deaminase, confirmed its effect significantly superior to the control under simulated salinity conditions.
In a research carried out by Hidri et al. (2019) with the legume Sulla carnosa, which is an important forage resource for animals feeding in areas affected by salinity, the plants received a positive effect from the inoculation with Bacillus subtilis, simply as combined with the actinomycorrhizal fungus Rhizophagus intraradices. These authors suggest that the efficiency of B. subtilis was due to the high production of AIA, under salinity stress conditions.
Use of pgpr to combat water stress and salt stress in the future
Agriculture is considered the most vulnerable sector to climate change. By exploiting the benefits of plant-microbe interaction, a relevant approach is made to increase food production for a growing population in the current scenario of climate change. According to Kaushal and Wani (2015), future research should seek an efficient microbial formulation to stimulate the yield of plants subjected to drought stress, in a way that substantially reduces the use of chemical fertilizers and pesticides.
The isolation of PGPR in areas that are subject to environmental stress is crucial for the formulation of bioinoculants that can be applied to crops that grow in stressful ecosystems. The results of Marasco et al. (2013), Mapelli et al. (2013), Becquer et al. (2017b, 2018, 2019a) 2019b), among other authors, corroborate this hypothesis.
Barea (2015) assures that many of the mechanisms underlying plant-microbe interactions in the rhizosphere are still poorly explained. According to this author, the difficulties are mainly in describing the wide range of processes involved in microbial communities. The understanding of this exchange of signals is essential to optimize the adaptation mechanisms of plants and to improve the ability of soil microorganisms, so that stress in crops can be relieved.
Although still in an incipient phase, current research shows that the application of PGPR-HT represents an effective and sustainable solution for the rescue of saline soils. With the progress of methodologies and techniques, a wide range of PGPR metabolites and genes, which respond to salt stress, have been identified. However, new knowledges into the metabolomics of PGPR-HT during their interaction with plants in normal and stress environments are needed (Meena et al. 2017). According to Kumar Arora et al. (2020), novel bioformulations can be developed by using various PGPR-HT or its metabolites to improve the productivity and quality of saline soils.
Conclusions
Water stress, caused by drought, as well as stress due to excess salts, limit the growth and productivity of plants. However, there are various ways to combat these types of environmental stress, and among the most innovative is the use of microbial inoculants.
There may be several mechanisms of action of bacteria to induce tolerance to stress, among which is the action of the enzyme ACC, the activity of catalase, the production of AIA, gibberellins and cytokinins, as well as other useful substances. There are proven results with the application of bioinoculants based on rhizobacteria and other beneficial microorganisms, which have showed their usefulness in increasing the productivity of different crops under environmental stress conditions. Many of the PGPRs that have been effective in transferring ISTs to plants have been isolated in ecosystems that are affected by different types of stress.
Future researches are needed to develop and apply novel bioinoculants in agriculture that neutralize the threats of drought and salinity. This objective can be achieved through the applied study of plant-microorganism interactions, under environmental stress conditions. It should not be ignored that several of defense mechanisms that exist in PGPR, and to transfer stress tolerance to plants, are indistinctly common to counteract the consequences of drought or salinity.
