Keywords

1 Introduction

Environmental change with rapidly increasing population throughout the world is becoming a big challenge to feed all the people. Within 2020, the population of the world will be about 8 billion (Glick 2012) and it would be 9 billion in 2050 (Vejan et al. 2016). About 50% of grain yield is required to be increased in most important crops like rice, wheat, and maize to feed all the people in 2050 (Shrivastava and Kumar 2015). Rapid changes to the environment mainly due to excessive use of chemical fertilizers and hazardous material in the field are decreasing crop productivity in one hand and increasing pollution on the other hand (Chakraborty et al. 2014; Roychowdhury 2014). Plant–microbes interactions draw a lot of attention to many scientists from time to time throughout the world. Deep relationship and interactions between plants, soil, soil microfauna, and microorganisms take place at rhizosphere (Antoun and Prevost 2005). Rhizospheric microorganisms that live in the rhizosphere of plants may or may not invade the plant root for shelter and make a symbiotic association with plants by providing some essential elements as well as protection. Rhizobacteria are involved in the promotion of plant growth and development, known as plant growth-promoting rhizobacteria, simply, PGPR. A bacterium can be called as PGPR when it shows three important characters or at least two characters as stimulation, fast colonization, and beneficial activity to plants on growth (Bhattacharyya and Jha 2012). Despite huge numbers of microbes present in the rhizosphere, only 7–15% microbial cells occupy the root surface (Gray and Smith 2005), and only 1–2% bacteria are responsible for the beneficial activity to the plants as PGPR (Beneduzi et al. 2012). There are two basic types of PGPR: intracellular PGPR (iPGPR), which makes root nodule and resides in it; another one is extracellular PGPR (ePGPR), which cannot make nodule and resides outside of the root. iPGPR can fix nitrogen symbiotically in the root nodule of the host; on the other hand, ePGPR helps by providing protection to plant, forming siderophores, increasing phytohormone production, enhancing the resistant potential to plants, etc. (Gray and Smith 2005). Basically, Rhizobia and Frankia are not called as PGPR (Antoun and Prevost 2005), but in this study, we will also focus on the Rhizobia as they have a direct effect on growth of the plants by nitrogen fixation.

The concept of nitrogen cycle was first inaugurated by Reyset in 1856 by describing the release of nitrogen from organic matter. Berthelot in 1885 was able to demonstrate chemical nitrogen fixing by lightning strike. Although biological nitrogen fixation by the microorganisms was first introduced by Jodinin in 1862. At the end of nineteenth to early twentieth century nitrogen-fixing microbes got much attention, and interest is increasing day to day (Elmerich 2007).

Being sessile, plant has to deal with different types of environmental hazards like biotic (pathogenic fungi, bacteria, virus, nematodes, etc.) as well as abiotic (extreme temperature, salt stress, flood, drought, high wind, etc.). Plants overcome these situations by modulating their mode of gene expression (Yang et al. 2009; Roychowdhury et al. 2014; Hasanuzzaman et al. 2015; Anumalla et al. 2016).

Acceleration of nutrient availability, assimilation of nutrients, suppression of disease-causing microorganisms, and enhancing growth and metabolisms are beneficial activities for plant which are commonly performed by PGPR (Perez-Montano et al. 2014). For example, PGPR and other plant beneficial microorganisms help plants to overcome the stress conditions by exhorting induced systemic tolerance (IST) (Yang et al. 2009).

In stress conditions, PGPR induces many stress-tolerating genes, proteins, enzymes, etc. In drought stress, transcription of ERD15 gene takes place in Arabidopsis thaliana by the activity of PGPR Paenibacillus polymyxa (Timmusk and Wagner 1999). Achromobacter piechaudii ARV8 is another example of PGPR, which produces 1-aminocyclopropane-1-carboxylate (ACC) deaminase in drought stress condition, inhibits the function of ethylene (responsible for the reduction of root and shoot length) in pepper (Capsicum annuum L.) and tomato (Solanum lycopersicum) plants (Mayak et al. 2004). HIGH-AFFINITY K+ TRANSPORTER 1 (HKT1) is a transporter protein expressed in the Arabidopsis responsible for the Na+ import to the root system; in presence of PGPR, its expression is decreased in salt stress condition (Yang et al. 2009).

According to the experiment of Guo and Chi 2014, presence of PGPR influenced Cd accumulation in root and it is balanced in rhizospheric region of Lolium multiflorum Lam. but in case of Glycine max L. Cd accumulation showed significantly decreases in both root as well as shoot.

However, in this review, we point out recent knowledge on PGPR, a very brief history, and its impact on the plant growth and development in stress conditions, like salt, drought, and heavy metal.

2 Impact of Environmental Stresses on Crop Productivity

2.1 Adaptation of Defense System Under Stress Conditions

Some plants are not able to take action against the pathogenic microorganisms. Usually, some physical and chemical materials are the basic weapons for the plant defense. Pathogenic microorganisms induce systemic acquired resistance (SAR) in plants. SAR is associated with the pathogen-related (PR) protein and salicylic acid. On the other hand, induced systemic resistance (ISR) is induced by PGPR. ISR is associated with ethylene and jasmonic acid production. PGPR induce the production of oxidative enzymes like peroxidase, superoxide dismutase, etc. which give protection to plant from different pathogens like bacteria, fungi, virus, etc. (Kumar and Verma 2019). Wheat plants treated with PGPR Dietzianatrono limnaea STR1, supplemented with 150 mM salt concentration, showed better dry weight and length against control condition due to the overexpression of ABA-responsive gene (ABARE) and TaOPR1 gene in root and shoot system (Bharti et al. 2016). Some Rhizobacteria produce exopolysaccharides that accumulate Na+ ions and relieve plants from salt stress (Arora et al. 2013). Physiological, biochemical, and morphological adaptations with different beneficial activities of PGPR help to induce the defense system in plants to overcome other abiotic stresses.

2.2 Nitrogen Fixation Under Stress Conditions

The eukaryotic organisms are not able to fix molecular nitrogen into their cells. Plants used to uptake nitrogenous compounds such as nitrate, ammonia, etc. from the environment through their root system. Here, the role of microorganisms is noticeable; many beneficial free-living and symbiotic microbes are well documented as nitrogen fixers in plants. But in stress condition, nitrogen fixation is also hampered dramatically. So, the basic metabolisms in plants get partially or fully arrested because of the low level of nitrogen-containing compounds resulting reduction of growth and development. Nitrogen fixation is a very energy-consuming procedure. PGPR is not too good as nitrogen fixers (Martínez-Viveros et al. 2010), but it helps plants directly or indirectly to overcome the stressed condition. Symbiotic association by Rhizobium sp. with nodule formation is restricted in the legume plants only, so nonleguminous plants are dependent on rhizospheric bacteria (Martínez-Viveros et al. 2010). The well-known nitrogen-fixing nonsymbiotic bacteria are as Azoarcus sp. (Reinhold-Hurek et al. 1993), Burkholderia sp. (Santos et al. 2001); Azospirillum sp. (Bashan and de-Bashan 2010), etc. nif gene is responsible for the biological nitrogen fixation , and this gene is present also in the PGPR (Gupta et al. 2015). Pseudomonas stutzeri A1501 is an ACC deaminase-producing strain containing acdS gene; besides ACC deaminase production in salt stress, it also regulates the function of nitrogenase, an important nitrogen-fixing enzyme, and increases crop yield in rice plant (Han et al. 2015).

3 Plant Growth-Promoting Rhizobacteria (PGPR)

3.1 History of PGPR

The term PGPR was first used by Joseph W. Kloepper in late 1970s (Vessey 2003) and defined by Kloepper and Schroth (Kloepper 1978). From the last decade of the nineteenth century, nitrogen-fixing bacteria act as a PGPR, and its molecular mechanism (Bhattacharyya and Jha 2012) has become an interesting topic to the scientists. At the very beginning, PGPR studies were restricted on beneficial activity regarding biological control of plant diseases only (Antoun and Prevost 2005).

The common plants which make symbiotic associations with the rhizobia are soybean (Glycine max), alfalfa (Medicago sativa), bean (Phaseolus vulgaris), pea (Pisum sativum), clover (Trifolium sp.), peanut (Arachis hypogaea), acacia (Acacia sp.), lentil (Lens culinaris), vetch (Coronilla sp.), birdsfoot trefoil (Lotus corniculatus), chickpea (Cicer arietinum), etc. (Gray and Smith 2005). Rhizobial species that can associate with these plants are as Bradyrhizobium japonicum E109 (Cassan et al. 2009), Bradyrhizobium japonicum 532C, etc. Many bacterial strains like Aeromonas hydrophila P73, Pseudomonas fluorescens 31-12, Serratia liquefaciens2-68, Pseudomonas putida G11-32, etc. (Zhang et al. 1996), Rhizobium sp. (Nyoki and Ndakidemi 2018), and Sphingomonas sp. LK11 (Bilal et al. 2018) make relationships with soybean (Glycine max). Ensifer meliloti (Cedeño-García et al. 2018; Kang et al. 2018), Rhizobium radiobacter, Rhizobium rosettiformans (Kang et al. 2018); Sinorhizobium meliloti, Achromobacter spanium, Serratia plymuthica (Aroua et al. 2018), Sinorhizobium meliloti with Paenibacillus mucilaginosus (Ju et al. 2019), etc. can make symbiotic relationship with alfalfa (Medicago sativa) plants. Bean (Phaseolus vulgaris) plants are associated with the common rhizobacteria as Azospirillum brasilense (Malinich and Bauer 2018) and Rhizobium tropici (Nogales et al. 2002) along with Rhizobium sp. (Ormeño-Orrillo et al. 2012), Rhizobium tropici, R. etli, R. gallicum, R. leguminosarum bv. phaseoli, R. giardinii (Fernandez-Aunión et al. 2010), etc. One of the common legume plant peas (Pisum sativum) is associated with rhizobacteria as Streptomyces lydicus WYEC108 (Tokala et al. 2002), Bacillus thuringeinsis-KR1, along with R. leguminosarum (Mishra et al. 2009), etc. Co-inoculation of PGPR with the Rhizobium is also extensively studied in case of the pea plant.

Many species of Bacillus and Pseudomonas belong to free-living PGPR, i.e., ePGPR (Beneduzi et al. 2012). Other bacterial species such as Enterobacter, Klebsiella, Azotobacter, Variovorax, Azospirillum, Serratia, Burkholderia (Nadeem et al. 2014; Vejan et al. 2016), etc. are also reported as ePGPR. These bacteria are involved directly or indirectly with the plant growth and development.

3.2 PGPR to Mitigating Stress

Different types of abiotic stresses like salinity, drought, heavy metal, water logging, temperature, water contamination, air pollutant, etc. and biotic stresses like pathogenicity, weeds, parasites, etc. are present in the environment (Saleem et al. 2007). By interactions with plants, PGPR help them to mitigate both abiotic and biotic stresses. Some examples of PGPR that mitigate stresses like salt by Achromobacter piechaudii, (Mayak et al. 1999) and Variovorax paradoxus5C-2 against drought on pea plants (Dodd et al. 2004); Pseudomonas putida UW4 to tomato plants (Grichko and Glick 2001); Burkholderia phytofirmans relieves potato plants in temperature (Bensalim et al. 1998); Pseudomonas fluorescens can reduce pathogenicity stress over Chamaecytisus proliferus plant (Donate Correa et al. 2005); Kluyvera ascorbata SUD165 has an effect on Brassica napus in heavy metal stress (Burd et al. 1998). ACC deaminase is the very common enzyme, produced by PGPR, which helps plant to mitigate all abovementioned stresses. Here, in this review, we just discuss the most important plant biotic and abiotic stresses (salt, drought, and heavy metal) and their mitigation strategies (Fig. 17.1).

Fig. 17.1
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Activity of PGPR to mitigate different stress conditions on plants

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3.3 Potential Synergistic As Well As Antagonistic Effects of PGPR

There are many biotic factors like bacteria, fungi, nematodes, parasites, etc. that directly and indirectly interact with plants. These organisms may involve in beneficial activities with plants in one hand, while in another, they can produce a detrimental effect on normal metabolism.

PGPR have the ability to interact with these factors and reduce their pathological activities by the production of antibiotic, siderophore, HCN, etc. (Gupta et al. 2015). For example, antibiotics are produced as amphisin, tropolone, oomycin A, phenazine, pyrrolnitrin, tensin, 2, 4-diacetylphloroglucinol (DAPG), pyoluteorin, and cyclic lipopeptides by different species of Pseudomonas (Loper and Gross 2007). From the study of Srivastava et al. 2010 it is revealed that Trichoderma, fluorescent Pseudomonas, and Glomus have a synergistic effect on the Fusarium wilting disease of potato. Alizadeh et al. in 2013 reported synergistic effects of Pseudomonas sp. Ps14 and fungus TrichodermaharzianumTr6 on Cucumis sativus to express some defense-related genes. In studies on cotton, Gossypium hirsutum, two different bacterial strains, Azospirillum sp. AZ204 and Pseudomonas fluorescens Pf1, showed growth promotion against normal conditions (Marimuthu et al. 2013). Also Pseudomonas aeruginosa PHU094, Trichoderma harzianum THU0816, and Mesorhizobium sp. RL091 have the capability to activate the phenylpropanoid pathway (Singh et al. 2014).

Some microorganisms have the ability to inhibit the growth of other microorganisms by secretion of some toxic chemicals like antibiotics. PGPR also produce some chemicals and destroy the growth of many pathogenic microbes (Siddiqui and Singh 2005) like synthesis of hydrolytic enzymes (protease, lipase, glucanase, etc.), competition for nutrient, regulation of ethylene production and siderophore and antibiotic production, etc. (Beneduzi et al. 2012). Siderophores or iron-chelating chemicals are a good weapon for the rhizospheric microbes. More than 100 types of siderophores produced by microbes are discovered until now. PGPR can produce siderophores, attract iron ions, and accumulate iron for their metabolic activity. So, pathogenic bacteria are deprived of iron and ultimately die. For example, siderophore pseudobactin is produced by Pseudomonas putida B10 that can inhibit the growth of Fusarium oxysporum (Kloepper et al. 1980). Bacteriocins are the chemicals produced by bacteria that are antagonistic to the same group of bacteria. E. coli, a gram-negative bacterium, produces bacteriocin and colicin, which is antagonistic to many gram-negative bacteria (Beneduzi et al. 2012). Chitinase and beta-glucanase are two important enzymes produced by PGPR that can inhibit the growth of fungi (Vejan et al. 2016). Induced systemic resistance (ISR) and systemic acquired resistance (SAR) synergistically affect against the biotic stress in presence of PGPR. Species of Pseudomonas are mainly responsible for the stimulation of these responses (Fig. 17.2).

Fig. 17.2
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PGPR-induced defense system against pathogenic attack on plants

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4 Rhizoremediation to Mitigate Stress-Induced Adverse Effects on Crop Productivity

4.1 Effects of PGPR on Salty Crops

About 20% agricultural lands and 50% crop (about 5.2 billion hectares of fertile land, Numan et al. 2018) are under salt stress in the world (Paul and Lade 2014). When electrical conductivity of a saturated paste soil extract is ECe ≥ 4dS/m, it is known as saline soil (Forni et al. 2017). There are five different classes of soil salinity, such as nonsaline, slightly saline, moderately saline, strongly saline, and very strongly saline (Paul and Lade 2014). Among all, less number of salt-tolerable plants can grow in very strongly saline class. Soil salinity can inhibit many process in plant including protein synthesis, lipid metabolism, photosynthesis, etc.

Usually, salts induce ROS production, such as superoxide radicals (O2−), hydroxyl radicals (OH), and hydrogen peroxide (H2O2), are responsible for DNA damage, protein degradation, and lipid peroxidations of membranes. It also hampers seed set and crop yield and reduces flowering in different plants like wheat, barley, rice, cotton, etc. (Numan et al. 2018). Accumulation of sodium and chlorine ions in the soil can reduce the availability of other important essential elements; cause high osmotic potential; affect ion transport, DNA damage, cell viability (Jha and Subramanian 2014), plant morphology, and root and shoot growth; etc.

To overcome the salt stress, plants upregulate different enzyme production such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and proline catalase (CAT), essential to scavenge and detoxify the effects of ROS (Noreen et al. 2010). Na+/H+ anti-transporter plays a crucial role in sodium accumulation inside the vacuole of plant cell. Sometimes Na+ is transported and accumulated inside the older cells of plant, and ultimately these cells sacrifice themselves. SOS response genes (SOS1, SOS2, and SOS3) are expressed under tight regulations of salt stress in plants (Numan et al. 2018). In salinity stress, plants upregulate ABA production in root and shoot system (Cramer and Quarrie 2002; Kang et al. 2014a, b). Nitric oxide regulates the synthesis of H+ ATPase actively and forces to Na+/H+ exchange by H+ gradient formation which leads to the homeostasis of Na+ and K+ ultimately (Zhang et al. 2008a, b). Genes are expressed by plant to mitigate salt stress as P5CS mod in tobacco (Hong et al. 2000), BADH1 (betaine aldehyde dehydrogenase) in tomato (Zhang et al. 2001), DcHsp17.7 in carrot (Song and Ahn 2011), SOS1 in Brassica (Chakraborty et al. 2012), etc.

In the presence of PGPR, root length, shoot length, and dry weight of the rice are increased in both salty and normal conditions (Jha and Subramanian 2014). Studies have shown that different hormones, siderophores, HCN productions, phosphate solubilizations, etc. (Sarkar et al. 2018) have been accelerated in plants in the presence of PGPR. Auxins like indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), etc. and its precursor may be produced by the bacteria through its metabolism. IAA induces to produce a huge number of lateral roots, increase the length of the hypocotyl (Zhao et al. 2001) and shoot to root ratio, and reduce root elongation (Loper and Schroth 1986). Cytokinin and gibberellins, produced by the bacteria, are also directly involved to mitigate salt stress in plants by promoting its growth. GA1, GA19, GA20, and GA44 gibberellins are produced by different bacteria (Numan et al. 2018). Volatile organic compounds (VOCs) are produced by PGPR for plant stress management. Ryu et al. in 2004 reported that 2, 3-butanediol and acetoin are two VOCs produced by bacteria able to promote growth in Arabidopsis thaliana. Nonsymbiotic nitrogen fixation and organic and inorganic phosphate solubilizations are also done by these specific types of bacteria in salt stress. Downregulating the expression of K+ ion transporter (HKT1) and upregulating the same gene in shoot may provide protection to plants from salt stress causing less accumulation of Na+ (Zhang et al. 2008a, b). Activities of caspase-like protease, superoxide dismutase, lipid peroxidation, etc. are reduced in the presence of PGPR in rice plants (Jha and Subramanian 2014). Transcription factors such as TaMYB and TaWRKY are modulated by PGPR Dietzianatrono limnaea STR1 to activate the genes of wheat plants which are actively involved in salt elimination from the cytosol by proper expressions of transporter genes. TaST, a salt stress-induced gene; TaNHX1, TaHAK, and TaHKT1, ion transporter genes; and APX, MnSOD, CAT, POD, GPXb, and GR antioxidant proteins are also expressed on wheat plant in presence of PGPR D. natronolimnaea STR1 (Bharti et al. 2016) (Fig. 17.3).

Fig. 17.3
figure 3

Rhizoremediation to mitigate salt-induced adverse effects on plant cell

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Sulla carsona is a species of Leguminosae used as cattle food in salty regions of the world. Presence of PGPR-like Acinetobacter sp. Br3, Pseudomonas putida Br18, and Curtobacterium sp. Br20 along with Sulla carcona shows increases biomass, more chlorophyll content and antioxidant property. (Hmaeid et al. 2019). Other plant growth-regulating rhizobacteria and their impacts on plant against salt stress are discussed in Table 17.1.

Table 17.1 PGPR activity against salt stress
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4.2 Effects of PGPR on Thirsty Crops

In drought stress , plant faces very detrimental effect on the crop production (Vinocur and Altman 2005). Damage of photosynthetic apparatus and change of chlorophyll content in the plants (Ortiz et al. 2015) are major issues in this condition. Due to less amount of water, concentrations and viscosity of the cells increased dramatically; proteins or enzymes which are mixed in the cytoplasm may easily come near to each other and may deteriorate (Hoekstra et al. 2001).

According to Farooq et al. (2009), dry and fresh weight of the root and shoot is remarkably reduced and root length is increased in drought stress. Nutrition uptake and transpiration in plant are also hampered in water scarcity. Ions like SO24 −, NO3−, etc. are not assimilated due to the unavailability of energy (Grossman and Takahashi 2001) in drought condition which negatively affects plant growth and development.

PGPR produce phytohormone (abscisic acid, gibberellin, indole acetic acid, cytokinin); important enzymes like ACC deaminase, which is responsible for reduction of ethylene in root system; exopolysaccharide, which increases systemic tolerance in plant; etc. (Yang et al. 2009; Dimkpa et al. 2009; Timmusk and Nevo 2011; Kim et al. 2012; Timmusk et al. 2014). In 2007, Arkhipova et al. reported the effect of Bacillus sp. on lettuce plant and concluded the effect of bacterial cytokinin on plant growth in drying soil. ACC deaminase-producing bacteria Variovorax paradoxus showed better growth and development of pea plant in the drying soil by deactivating ethylene production (Belimov et al. 2009). ACC deaminase metabolizes ACC into ammonia and α-ketobutyrate (Saleem et al. 2007). In pepper plants, Bacillus licheniformis K11 can upregulate several genes like Cadhn, VA, sHSP, and CaPR-10 and their respective proteins such as dehydrin-like protein, vacuolar H+-ATPase, small heat shock protein, pathogenesis-related protein 10, etc. (Lim and Kim 2013). Pseudomonas putida MTCC5279 showed regulation of several important genes like DREB1A, NAC, LEA, DHN, etc. (Tiwari et al. 2016).

The activity of phosphatase and accumulation of the proline in roots and leaves, respectively, increased in lettuce plants due to the interaction of Pseudomonas mendocina; also activities of peroxidase and catalase are enhanced, and superoxidase dismutase is reduced (Kohler et al. 2008). Some examples of PGPR and their potentiality against drought stress are shown in Table 17.2.

Table 17.2 PGPR activity against drought stress
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4.3 Effects of PGPR on Crops Under Heavy Metal Stress

The environment is becoming polluted beyond our expectation because of progressive industrialization and urbanization. Besides other factors, excessive accumulation of heavy metals in soil, water, and air causes loss of soil fertility, which affects metabolic pathways of plants, ecosystem functioning, and health issues to humans and animals.

Heavy metals (>5 g/cm3) are classified into three types, namely, precious metals, radionuclides, and toxic metals. Precious metals [palladium (Pd), platinum (Pt), silver (Ag), gold (Au), ruthenium (Ru), etc.] are less reactive with high economic value; radionuclides [uranium (U), thorium (Th), radium (Ra), americium (Am), etc.] contain unstable nucleus and emit harmful rays like alpha, beta, and gamma; and toxic metals [mercury (Hg), chromium (Cr), lead (Pb), zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), arsenic (As), cobalt (Co), tin (Sn), selenium (Se), etc.] are known for its potential toxicity mainly in environmental contexts. The nature and concentration of elements determine the toxicity of heavy metals. A little amount of heavy metals (copper, cobalt, molybdenum, etc.) are required for the metabolic pathways of organisms, but if the amount is high, then it could be harmful to the organisms (Roychowdhury and Tah 2011; Basu et al. 2012; Roychowdhury et al. 2018, 2019).

Heavy metals are nonbiodegradable in nature and they are hard to remove. There are many processes to mitigate heavy metals from the environment like ultrafiltration, immobilization, coagulation, electrodialysis, soil washing, chemical precipitation, stabilization, ion exchange, etc., but these processes are too expensive because it requires many chemical reagents and high energy sources (Gupta et al. 2016; Selatnia et al. 2004). The most efficient methods of removing heavy metals from environment are use of microbes, which have the ability to degrade heavy metals by means of its intrinsic properties or to convert it into toxic to nontoxic form (Gupta et al. 2016; Ledin 2000).

PGPR developed many mechanisms and play an important role in extraction process of heavy metal. These mechanisms include

  1. (i)

    Biotransformation or mineralization – alteration of highly toxic metals into low or nontoxic forms (Gupta and Diwan 2017).

  2. (ii)

    Metals bind with metal-binding proteins and peptides – metal-binding proteins like metallothioneins and phytochelatins (Mejare and Bulow 2001) and peptides composed of metal-binding amino acids (mainly cysteine and histidine residues).

  3. (iii)

    Methylation, volatilization, and demethylation processes mediated by microorganisms to remove toxic metals (Ullah et al. 2015).

  4. (iv)

    Extrusion metals that are extruded out from the bacterial cells through plasmid or chromosomal mediated methods (Tak et al. 2013).

  5. (v)

    Exclusion metal ions change in the position of target sites (Tak et al. 2013).

Overexpression of GSH synthetase in the cytosol of Indian mustard (Brassica juncea) by E. coli gshII gene enhanced accumulation and tolerance of Cd (Mosa et al. 2016). Mercuric ion reductase (encoded by merA gene) and organomercurial lyase (encoded by merB gene) present on bacterial cell help to convert the toxic form of mercury into less toxic forms (Meagher 2000; Dhankher et al. 2012) (Fig. 17.4).

Fig. 17.4
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Rhizoremediation to mitigate heavy metal-induced adverse effects on plant cell

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Bacillus sp. SC2b isolated from Sedum plumbizincicola tolerate high concentration of Cd, Zn, and Pb (Ma et al. 2015). Microbacterium oxydans AY509223 isolated from Alyssum murale mobilized high concentration of Ni present in Ni-contaminated soil (Abou-Shanab et al. 2006). Some Cr-resistant PGPR play an important role to harbor the tolerance capacity of heavy metals like Cu, Pb, Zn, and Cd (Ma et al. 2015). A detailed account on heavy metal tolerance is given in Table 17.3.

Table 17.3 PGPR activity against heavy metal stress
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5 Genetic Engineering Approaches of PGPR

As bacteria thrive in always changing environments, genetic material of microbes changes in many ways to overcome different types of stresses. Rhizospheric bacteria , Pseudomonas putida VTw33, shows ars operon with arsH gene (Chang et al. 2018); the same operon with ars R gene (Ramanathan et al. 1998) can bioremediate the arsenic metalloid stress. Mercury causes many severe diseases in human and is toxic to plants. Mercury-tolerating gene, mercury reductase (merA) , is present in some PGPR. This gene was incorporated in plants by genetic engineering and showed that transgenic plants were able to phytoremediate mercury (Karenlampi et al. 2000). The report says that many bacterial strains as Pseudomonas putida, Ralstonia eutropha, E. coli, Mycobacterium marianum, etc. are genetically modified and applied on the heavy metal-contaminated field (Sarma and Prasad 2019). Not only that, genetically modified plant species may be applied with the PGPR, and its synergistic activity is also very promising approach. Genetically engineered Arabidopsis thaliana can remove cadmium and lead when inoculated with Rhizobacteria (Bhattacharyya and Jha 2012).

6 Conclusion and Future Perspective

Food scarcity along with population burst throughout the world is becoming a very common problem, and we need to develop sustainable agriculture to feed all the people. Due to climate change and anthropogenic activities, plants are facing different biotic and abiotic stresses throughout their life. Besides this, we are using a tremendous amount of chemical fertilizers, chemical pesticides, and herbicides for increased crop production, leads to loss of agronomic fields and productivity from day to day. Stresses are different types; among them, drought stress, salt stress, and heavy metal stress are more detrimental and cause the main loss in agriculture. Plants’ own defense system is not enough to overcome these detrimental stressed conditions. Plant growth-promoting bacteria (PGPR) play a central role to mitigate stress by physiological, biochemical, and molecular modification of plant responses on stress. Synergistic effects of PGPR are also extensively studiedand it revealed that more than one microorganism gave the better result against the individual one. Bacteria with multiple functions against separate stresses may be a very useful tool for trace management and improvement of the crop. Among different bacterial strains, Pseudomonas sp. is much common in rhizosphere and potent bacteria for the member of synergistic activity. PGPR help in nitrogen fixation by their nif genes, which produce siderophore and antibiotics to inhibit the growth of other microorganisms, and through ACC deaminase activity ethylene content is reduced which leads to continuous growth and development of plants under stress conditions. Heavy metal-accumulating bacteria can accumulate different heavy metals like cadmium, lead, mercury, copper, arsenic, etc. Not only that, genetic engineering approaches help to insert desired microbial genes to the microorganisms and plants that can express microbial proteins which help to bioremediate heavy metal from the environment. The activity of eukaryotic organisms, i.e., fungi, in association with the prokaryotic bacteria to mitigate several stresses in plants is an interesting topic, and getting much more attention to fight against the common use of chemical fertilizers.