Keywords

1 Introduction

The global food crisis is one of the discernible situations that necessitate substantial attention. Due to high population growth (especially in China and India, the top two populated countries in the world) with a proportionate decrease in cultivable land, this catastrophe is becoming more acute daily. Apart from natural sources, several unplanned anthropogenic activities are known to generate an additional burden that jeopardises the environment and its ecosystem, contaminating its different components including soil and groundwater (Sharma and Archana 2016; Liu and Ma 2020). Heavy metal(loid)s (HMs) are one of the recalcitrant contaminants in agricultural fields that degrade the soil quality affecting the growth and crop yield, causing severe to chronic phytotoxicity. This might be due in part to the selection pressure that HMs impose on the soil-dwelling microbiome involved in phytostimulation and maintaining soil-biogeochemical cycling. However, certain microorganisms with their unequivocal properties combat HMs, developing an array of active or passive resistance mechanisms to survive in such a harsh environment (Chen et al. 2016; Tiwari and Lata 2018; Kotoky et al. 2019). There are successful candidates among them that have been found to colonise the soil area around the rhizosphere and rhizoplane (root surface) in response to enriched soil nutrients including the attractants released as root exudates from host plants. Host root exudates provide nutrients and act as signaling molecules to the colonisers to establish effective plant-microbe interactions. These exudates take the foremost part in controlling the diversity and composition of plant-associated soil microbial communities (Steinauer et al. 2016).

Plant growth-promoting rhizobacteria (PGPR) are group of free-living rhizobacterial communities that competitively colonise around the root surfaces stimulating plant growth by secreting a variety of phytostimulating substances and preventing some causes of host’s diseases in a sustainable manner (Kloepper 1978). Rhizobacterial plant growth-promoting (PGP) traits include 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, phosphate solubilisation, indole-3- acetic acid (IAA) production, nitrogen fixation, siderophore production and many more. PGPR also protect plants from invading phytopathogens by secreting antibiotics, antifungal compounds, hydrocyanic acid (HCN), chitinase, etc. The PGPR strains with remarkable HM-withstanding property assist their immobile host to develop HM-tolerance for their combined survival in their contaminated habitat. These microbes are known as HM-resistant PGPR (HMR-PGPR). For several years, these PGPR strains have been isolated from the metal-contaminated rhizosphere of different crops including vegetables (Mitra et al. 2018a, b; Pramanik et al. 2017, 2018a, b; Khanna et al. 2019).

So, to ensure food security, the development of environmental cleanup methods is urgently needed to accomplish the reclamation of contaminated agricultural lands. Unlike the issue of organic pollutants, which sometimes seemed easier to resolve, mitigation of heavy metal contamination has been proving to be one of the more difficult tasks ever undertaken. Organic contaminants can be degraded. The metal pollutants are instead non-degradable in nature, and these contaminants can only be transformed into less toxic forms or removed by means that include accumulation and adsorption. Most of the conventional methods for remediation of heavy-metal-contaminated soil are physicochemical in nature which is expensive, ineffective, creates secondary pollutants and unsuitable for large areas (Quartacci et al. 2006). In this context, HM-resistant PGPR-induced bioremediation is one such approach which is inexpensive, effective, sustainable and ecofriendly. Unlike some non-PGPR microbial strains (Hu et al. 2007; Rehman et al. 2008; Muneer et al. 2009; Shakya et al. 2012; Liu et al. 2013; Davolos and Pietrangeli 2013) isolated from contaminated soil and groundwater, HM-resistant PGPR play a dual role in heavy metal bioremediation as well as plant growth promotion. Some of the non-PGPR strains have also been proven promising as potent bioremediators.

This chapter encompasses heavy metal and metalloid resistant plant growth-promoting rhizobacteria (HMR-PGPR), which are a functionally defined group of microorganisms, discovered during the last two decades that have been found to improve the growth of different crops across the world under different levels of HMs contamination. It covers latest information on diverse HMR-PGPR that exhibited various degrees of HM-resistance, different levels of release of plant growth-promoting substances and different capacities to accelerate plant growth by reducing HM stress-induced morpho-biochemical changes in the affected plants. A brief account of how biotic stress tolerance is facilitated by plant growth-promoting bacteria (PGPB), general HM resistant mechanisms, signaling cascades and genetically modified PGPR are also presented and discussed. Furthermore, we will provide some conclusions about the major obstacles to the application in HMR-PGPR in the field and future prospects of these strains. We will also discuss the times and places where non-HM resistant PGPR, metal-resistant plant growth-promoting bacteria (PGPB) and rhizobia have been advocated. Overall, this chapter is a substantial collection of information on heterogeneous microbial communities (especially HMR-PGPR) interacting with diverse hosts working in different soil types for crop improvement in a sustainable manner.

2 Heavy Metal(loid)-Induced Phytotoxicity in Crop Plants

The incessant spread and increasing levels of HMs in agricultural soils have caused severe impairment of crops which not only results in reduced yield but also a serious toxic threat to the crop consumers. Plants, being immobile, are unable to escape from this stressful environment and uptake bioavailable non-essential HM cations into their plant cells along with essential soil nutrients. These HMs, upon surpassing certain threshold levels, impose severe cellular damages with various unusual morphological manifestations. The threshold level of HMs to induce phytotoxicity highly depends on plant species or even a particular cultivar. The uptake, translocation and cellular compartmentalisation of heavy metals may be governed by perhaps only one or just a few genes (Ernst 1996). Moreover, this also depends on the cationic forms of HMs. The observable external changes include reduction of seed germination, changes in root-shoot length and changes in root-shoot fresh and dry weight that ultimately decrease plant biomass (Table 22.1). As the root is directly exposed to the soil HMs, the root is the first organ encountered by toxic HMs, and the toxic effects follow into the shoots and other aerial parts of the plants. Affected root growth results in the poor acquisition of essential nutrients, and thereby an insufficient supply of nutrients to the photosynthetic cells in the aerial parts. To date, the members of Poaceae are the most studied crops on which the phytotoxic effects of different HMs have been investigated (Fig. 22.1). The phytotoxic consequences of all the ten HMs (viz. arsenic, cadmium, chromium, cobalt, copper, lead, manganese, mercury, nickel and zinc) discussed here have been studied on Poaceae (Fig. 22.1). After Poaceae, the HM-phytotoxicity studies have focused mainly on members of Fabaceae, Solanaceae and Brassicaceae, as predominant crops (Fig. 22.1). The less-studied families in the context with HM phytotoxicity are Amaryllidaceae, Euphorbiaceae, Amaranthaceae, Rosaceae, Linaceae, Malvaceae, Asteraceae and Cucurbitaceae (Fig. 22.1).

Table 22.1 Heavy metal(loid)-induced phytotoxicity in different crops
Full size table
Fig. 22.1
figure 1

Families of studied agricultural crops affected by heavy metal(loid)s

Full size image

Among HMs, arsenic (As) is considered as an analog of phosphate (P) that competes with P-transporters in the root plasma membrane (Meharg and Macnair 1992). Although As-tolerance has been identified in a number of plant species (Meharg and Macnair 1992), elevated As-level has been found to negatively affect rice, maize, black gram, soybean, mung bean, cucumber, sorghum, barley, mustard, broccoli, pea and Chinese cabbage (Table 22.1). Biochemical changes identified in these crops include a reduction in photosynthetic pigments (chlorophyll, carotenoids), increased accumulation of reactive oxygen species (ROS), membrane lipid peroxidation, inhibition of ATP formation, enhanced proline and protein content and increased abscisic acid (ABA) synthesis (Table 22.1). Furthermore, altered activities of various cellular enzymes including RuBisCO, amylase, protease, catalase, peroxidase and other antioxidant enzymes are evident (Stoeva et al. 2005; Srivastava et al. 2017; Ghosh et al. 2018; Dong et al. 2020; Chauhan et al. 2020). Besides, As-mediated induction of cell death in root tips, proteomic alteration and disruption of normal cellular function have also been identified (Requejo and Tena 2006; Armendariz et al. 2016).

Likewise, phytotoxicity of other HMs reported almost parallel kinds of morpho-biochemical dysfunctions (Table 22.1). Studies of cadmium (Cd)-induced phytotoxicity have focused mainly on rice, wheat, tomato, potato, cucumber, pea, lettuce and mung bean (Table 22.1). An upsurge of ethylene content in rice seedlings has been noticed in response to Cd stress (Mitra et al. 2018a; Pramanik et al. 2018a) that is linked to increased accumulation of H2O2, leading to cell apoptosis (Chmielewska-Bak et al. 2014). Cobalt (Co), one of the naturally occurring HMs in the earth’s crust, spreads through human activities as well, and that element is taken up by plants from the contaminated soil. However, information on Co-phytotoxicity is less available in the literature compared to As and Cd. Wheat, barley, oilseed rape, tomato and cauliflower have been studied so far to elucidate Co-induced phytotoxicity (Chatterjee and Chatterjee 2000, 2003; Li et al. 2009; Ozfidan-Konakci et al. 2020). Co was found to decrease plant growth, photosynthetic rate, water content, osmotic potential, stomatal conductance, transpiration rate and cause chlorosis that ultimately manifested as decreased plant biomass (Table 22.1). An exogenous application of CoCl2 was shown to decrease plant ethylene levels compared to controls (Pramanik et al. 2017, 2018a). The number of phytotoxicity studies on chromium (Cr), copper (Cu), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) on the more common crop plants is also impressive, with reporting of various morpho-biochemical malfunctions in plants.

3 Role of Heavy Metal(loid) Resistant Plant Growth-Promoting Rhizobacteria in Crop Improvement

Soil, being the sink of nutrients for plants, is also the chief source of contaminants. The information summarised in Table 22.1 provides an idea of observed intensification of heavy metal contamination and consequences of the major HM contaminants on some common crops. Plants have developed their own natural mechanisms to regulate the uptake, translocation and accumulation of HMs, which is known as natural phytoremediation. In reality, plants are not the only warriors that are exposed to and affected by soil HMs, and indeed there similarly exist some close neighbors like the rhizospheric microbial community that also have direct or indirect influences on plant growth. Phytoremediation is one of the safest, eco-friendly technologies and is often triggered by plant growth-promoting bacteria (PGPB) as a response to accelerated HM uptake and accumulation in the plant cells (Ullah et al. 2015). This concept of designing and promoting bacteria-assisted phytoremediation technology is not intended to be applied only in the case of agricultural crops that are consumed by humans, cattle or other animals to reduce the high chances of HM toxicity in the food chain (Fig. 22.2). Rather, the preferred usage of PGPR-mediated bioremediation would be in such cases where some specific group of PGPR reduce both the HM-induced phytotoxic effects and HM-uptake as well (Fig. 22.2). PGPR fall under a special group of fast-growing microorganisms which are a good instance of phytostimulating biological agents of natural occurrence. Since many years, soil microbiologists and environmentalists have been devoting their tireless efforts to isolate PGPR strains with greater efficiency of bioremediation and plant growth promotion, and to apply their discoveries about HM-contaminated soil for the benefit of sustainable agriculture (Table 22.2). Here, in this review, we will largely examine HM-resistant PGPR (involved in PGPR-mediated bioremediation) publications from the last two decades and present their results in brief (Table 22.2). We have considered only those HM-resistant PGPR strains which were tested for their plant growth-promoting activities on selected crops, with those microbes having been applied as bioinoculants either in laboratory conditions or in the field. It is evident from Table 22.2 that the phytotoxic effects mentioned in Table 22.1 have been significantly reduced by the use of HM-resistant PGPR.

Fig. 22.2
figure 2

Bacteria-assisted phytoremediation and PGPR-mediated bioremediation of heavy metal(loid)s

Full size image
Table 22.2 Heavy metal(loid)-resistant PGPR including rhizobia discovered in the last two decades and their applications
Full size table

One of the most vital and key representations of this chapter is the documentation of culture media for the isolation of HM-resistant PGPR. Proteobacteria seem to have been the most commonly isolated group from all the stated culture media. Yeast extract mannitol (YEM) medium has been the most preferable isolation medium, followed by Davis Mingioli (DM) medium with Cd (Fig. 22.3). From a critical analysis of the information presented in Table 22.2, we find that the diversity of the HM-resistant PGPR community covers only three bacterial groups, i.e. proteobacteria, firmicutes and actinobacteria, and it is prominantly dominated by proteobacteria (Fig. 22.4). Furthermore, proteobacteria is the most abundant PGPR member responsible for resistance to all the studied heavy metal(oid)s. Actinobacteria exhibit their remediational property only against Cd. The firmicutes are a set of PGPR sensitive to As, Hg and Zn (Fig. 22.5). Additionally, among the PGPR members, all the documented phenomenal PGP traits are mainly portrayed by the proteobacterial representatives, and actinobacterial agents are accountable only for their IAA and ACC deaminase producing capabilities (Fig. 22.6). Moreover, in case of firmicutes, they are the silent member in case of N2 fixation, potassium solubilisation, ammonia and HCN production. However, the firmicutes have exhibited ACC activity, P-solubilisation, siderophore activity and IAA production (Fig. 22.6).

Fig. 22.3
figure 3

Medium used for isolation of heavy metal(loid)-resistant PGPR. (CDM Chemically defined medium, KBM King’s B medium, TM+HM T-medium with HM, YEM+CD Yeast extract mannitol with Cd, NA Nutrient agar, TCS Tryptone casein soya, TYE Tryptone yeast extract, LB+MM Luria–Bertani minmal media, DM+CD Davis Mingioli with Cd, AM Ashby’s mannitol, YEM Yeast extract mannitol, LB+CD Luria–Bertani with Cd, DFN+CR Dworkin and Foster nutrient with Cr, USA Urease screening agar)

Full size image
Fig. 22.4
figure 4

Diversity and distribution of heavy metal(loid)-resistant PGPR

Full size image
Fig. 22.5
figure 5

Diversity and abundance of heavy metal(loid)-resistant PGPR

Full size image
Fig. 22.6
figure 6

Plant growth-promoting traits in heavy metal(loid)-resistant PGPR

Full size image

4 Genetically Modified Plant Growth-Promoting Rhizobacteria for Crop Enhancement

Natural components like the PGPR play an indispensable role in the advancement of sustainable agriculture and also serve as an imperishable treasure box for the environment. Considering the limitations of these natural bio-agents, the idea of using genetic modification approaches has attracted the attention of scientists with the goal of attaining greater desired efficiency. With the improvements achieved by genetically engineering PGPR, the heavy metal accumulating gene and the biocontrolling genes can be assembled to conduct enhanced bioremediation and potentially achieve biocontrol in the rhizospheric soil. In this context, for superior cadmium (Cd2+) bioaccumulation purpose, the phytochelatin synthase gene (PCSAT) from Arabidopsis thaliana was introduced into Mesorhizobium huakuii strain B3 and then set up as a symbiosis with M. huakuii strain B3 and Astragalus sinicus, whereupon a desired activity was noted accordingly (Sriprang et al. 2003). It was possible to carry out that project because the peptides like phytochelatins (PC) and metallothioneins (MT) exhibit high affinity towards a variety of heavy metals (Chaudhary and Shukla 2019). Furthermore, genetically transformed rhizobacterial strains demonstrated significant biocontrol potentiality over fungal phytopathogens (Sattiraju et al. 2019). In such cases, incorporation of a mini-Tn5 vector containing the complete operon for the biosynthesis of an antifungal metabolite phenazine- 1- carboxylic acid (PCA), within Pseudomonas fluorescens has been documented to accelerate the suppression of fungal diseases by the genetically engineered bacterial strain in comparison to the natural bacterial strain (Timms-Wilson et al. 2000). Similar kinds of approaches were reported from several studies where genetically engineered PGPR strains showed enhanced PGP traits as well as biocontrol efficiency (Bloemberg and Lugtenberg 2001) and can be exemplified by the integration of Cry-toxin-encoding cry1Ac7 gene from Bacillus thuringiensis, chitinase-encoding chiA gene from Serratia marcescens and ACC deaminase-producing gene from Enterobacter cloacae into rhizobacterial strains like Pseudomonas sp. (Sattiraju et al. 2019). The relocation of sss gene from biocontrol strain P. fluorescens WCS365 to other P. fluorescens rhizobacterial strains was found to improve the competitive root colonising efficiency (Dekkers et al. 2000). Apart from the genetically modified PGPR, transgenic plants also display greater PGP traits, especially higher ACC deaminase activity and heavy metal accumulation (Zhuang et al. 2007; Stearns et al. 2005; Nie et al. 2002). However, genetically modified PGPB are considered less effective in terms of their survival and proliferation as compared to non-transformed versions of the same organisms; and this decreased fitness may be due to overburden of metabolic load by the expression of foreign genes (Glick 2020).

5 Plant Growth-Promoting Rhizobacteria in Biotic Stress Tolerance

The rhizosphere is a phenomenal environment where the plant-beneficial microbes especially the bacteria renowned as rhizobacteria, colonise and steadily perform several plant growth-promoting activities by means of facilitating nutrient availability and assimilation, and help conquer over disease-instigating microbes (Pérez-Montaño et al. 2014). The plant growth-promoting activities of these beneficial rhizobacteria include nitrogen fixation, solubilisation of minerals like phosphorus, production of ACC-deaminase and other plant growth regulators like auxins, gibberellins and cytokinins. Biocontrol properties are one of the key characteristic features of these PGPR (Kloepper 1978). Their antagonistic potentiality against phytopathogens is mainly categorised according to activities like the production of siderophores, lytic enzymes, antibiotics, bacteriocins, volatile organic compounds (VOC), hydrogen cyanide (HCN) and their ability to obstruct bacterial quorum sensing (Aloo et al. 2019; Pérez-Montaño et al. 2014; Kumar and Dubey 2012). Apart from these capabilities, PGPR also induce systemic resistance (ISR) proficiency which can help suppress pathogenicity that other microbes exhibit against host plants, and PGPR do as well improve the sustainability of agricultural systems (Beneduzi et al. 2012). Among the reported PGPR genera, Pseudomonas sp., Bacillus sp. and Streptomyces sp. are the warhorses in the avenue of biocontrol of phytopathogens (Table 22.3; Arrebola et al. 2019). Moreover, the rhizobacterial phyla involved in this job are dominated by proteobacteria, firmicutes and actinobacteria (Fig. 22.7). The bio-protecting efficiency of PGPR are not only restricted to countering the pathogenic microbial members of the rhizosphere community like fungi and bacteria, but are also promising as agents against metazoan phytopathogens like insects and nematodes (Table 22.3; Fig. 22.8).

Table 22.3 Biocontrol activities of different PGPR
Full size table
Fig. 22.7
figure 7

Diversity and abundance of PGPR with biocontrol potentiality

Full size image
Fig. 22.8
figure 8

Biocontrol proficiency of various PGPR against different phytopathogens

Full size image

The biological control of phytopathogens by the PGPR group of organisms does in many ways strengthen both plant and soil health. Rhizobacterial secretion of siderophores is among the mechanisms exhibited by the PGPR members that are antagonistic against other microoganisms. The actions of siderophores are based upon their chelation of iron which inhibits iron-dependent nutritional or energetic processes in those other microbes (Chaiharn et al. 2009). In iron-limiting soil environments, the binding of iron by siderophore-producing rhizobacteria can also boost up the availability of iron to those plants that are able to accumulate siderophore-bound iron (Tank et al. 2012). Apart from iron chelation, siderophores can bind with other heavy metals like Cd, Cu, Pb, Al and Zn which in turn diminishes the stress to plants that may be imposed by those other heavy metals (Ahemad and Kibret 2014). PGPR additionally produce various defensive lytic enzymes such as chitinase, glucanase, cellulase, protease, chitosanase, peroxidase, catalase, phenolic lyase, superoxide dismutase, etc. (Aloo et al. 2019) which can act to protect plants from the pathogens. Pathogens responsible for several plant diseases are directly liable for plant growth inhibition and these are mainly fungi and insects (Banerjee and Mandal 2019). The lytic enzymes like chitinase, chitosanase, glucanase and cellulases produced by PGPR act in a straight line biocontrol mechanism against the chitin and glucan cell wall components of those fungi and insects. Disease control management by the PGPR is additionally accomplished not only by means of antibiotics produced like zwittermicin, mycosubtilin, gramicidin S, polymyxin B, bacilysin, rhizocticins, etc. but also by bacteriocins (Saraf et al. 2014; Haggag 2008; Leclere et al. 2005; Chin-A-Woeng et al. 2003). Enhancement of plant defense mechanisms by a combination of ISR plus biocontrol ability was also validated by studies of several PGPR that produce VOCs (Shafi et al. 2017; Cao et al. 2011). The occurrence of such dual potentiality can be exemplified by VOCs like 2, 3-butanediol, isoprene and acetoin that are produced by different PGPR (Lee et al. 2015; Ryu et al. 2004). Plant pathogens can also be controlled by many PGPR via HCN production, a recognised VOC which disrupts the electron transport system that leads to blocking the energy supply of the pathogens (Patel and Minocheherhomji 2018).

In recent years, biocontrol has become an emerging and promising technological approach in developing sustainability in agriculture with optimism both for its comprehensive potentiality against various types of plant pathogens as well as its being an efficient alternative resource over chemical fungicides and pesticides. In addition, several PGPR have been documented for their ability to remediate heavy metals in agricultural fields. There are indeed many published reports on heavy metal remediation by the PGPR (Table 22.2); although reporting on the combinational effect of HM bioremediation cum biocontrol activity by PGPR is very scarce. Two such examples of combined activity by PGPR are Alcaligenes sp. and Pseudomonas aeruginosa, where nickel and manganese bioremediations were testified along with aptitude for biocontrol of phytopathogens like Aspergillus niger, A. flavus, Fusarium oxysporum, Alternaria alternata, Cercospora arachichola and Metarhizium anisopliae (Sayyed and Patel 2011). There is some justifiable optimism that the application of this kind of heavy metal remediating cum biocontrolling PGPR in agricultural fields will replace the usage of chemical pesticides and fertilisers, which in turn will decrease the bioaccumulation of hazardous chemicals into agronomic plants and passage of these contaminants further up the biological chain, leading to a more environmentally safe and affordable agriculture in terms of human welfare. However, the effective biocontrol property of PGPR against invading phytopathogens is subject to the considerations of soil type, host plant species and influential holobiont microbial community in the rhizosphere (Subrahmanyam et al. 2020).

6 Mechanism of Heavy Metal(loid) Resistance by Plant Growth-Promoting Rhizobacteria

Plant-associated HM-resistant PGPR are more profoundly present in heavy-metal-contaminated soil, as evidenced by many earlier publications (Pandey et al. 2010; Chen et al. 2016; Treesubsuntorn et al. 2018; Pramanik et al. 2017, 2018a, b; Mitra et al. 2018a, b). Such PGPR strains are known to develop resistance mechanisms in adaptation to the different HM ions present in their habitats (Table 22.4). The various known survival strategies which metal tolerant species have used to combat HMs are summarised in Table 22.4. These include active transport of metal ions (efflux/influx) by the presence of a group of specific membrane bound, cytoplasmic or periplasmic metal transporters (Nies 2003; Yang et al. 2019), production of biodegradable metal chelators like siderophores (Sinha and Mukherjee 2008; Dimkpa et al. 2008), intracellular bioaccumulation and biosorption (Chen et al. 2016; Treesubsuntorn et al. 2018; Pramanik et al. 2017, 2018a, b; Mitra et al. 2018a, b; Pal and Sengupta 2019), enzymatic oxidation and reduction metal transformations (Chatterjee et al. 2009; Pramanik et al. 2016; Ghosh et al. 2018; Kamaruzzaman et al. 2019), extracellular complexation by the secretion of extracellular polysaccharides (EPSs) (Gupta and Diwan 2017), etc. (Table 22.4). The genetic determinants of metal resistance can be localised either in chromosomal or extrachromosomal genetic elements.

Table 22.4 General mechanism of heavy metal(loid)-resistant PGPR including rhizobia
Full size table

Heavy metals most commonly exist in the form of cations which can form many unspecific complexes. Among all these, a few HM cations are important biological trace elements (such as Mn2+, Zn2+, Cu2+, Ni2+, Mo2+, Co2+) used in regulating several important biochemical reactions. The intracellular passage of different HMs is, in fact, governed by two opposite types of uptake systems. The first of these systems is constitutively expressed, fast, unspecific and uses a variety of substrates, while the second system is inducible, slow and highly specific for substrates (Nies 1999). The main driving force for the first system is an electrochemical gradient across the plasma membrane, and for the second system it is the energy generated by ATP hydrolysis (Nies and Silver 1995). The constitutive and unspecific nature of the first kind of system causes most of the HM-toxicity in bacteria as it continuously accumulates a heavy metal even if the cell already contains a high concentration of that same HM (Nies and Silver 1995). After a metal has been accumulated beyond threshold levels, HMs impart several toxic effects such as inhibition of enzyme actions due to the binding of Hg2+, Cd2+ and Ag2+ to -SH groups, generation of oxidative stress and inhibition of the activity of sulphate and phosphate compounds by structurally related chromate and arsenate, respectively. Briefly, there are six widely known heavy metal resistance mechanisms in bacteria, they are: (1) exclusion of HMs by permeability barriers, (2) extracellular sequestration, (3) intracellular sequestration, (4) enzymatic detoxification of HMs, (5) active transport or efflux system of HMs and (6) reduction in HM sensitivity of cellular targets.

However, the details of many heavy metal resistance mechanisms used by PGPR are still to be fully explored, and we will have to unravel the genetic mysteries behind metal-PGPR interactions to effectively apply them for HM-bioremediation.

7 Constraints in the Application of Plant Growth-Promoting Rhizobacteria

Although the PGPR strains far discovered have proven promising in controlled laboratory conditions, their efficacy in reality is contingent on how they act in field conditions. During the last few decades, a number of PGPR strains have been discovered around the world but few reached the ultimate goal of having utility for farmers. In contrast to the laboratory, the reality of field work is one of non-optimal conditions that may or may not be favouarbale for the survival and proliferation of the PGPR strains (Glick 2020). The existence and growth of field-applied PGPR strains indeed depends on a vast range of adverse environmental factors that need to be overcome so that the microbes take part in assisting plant growth-promotion activities in contaminated soil (Fig. 22.9). It is not an easy task to achieve successful application of such PGPR strains even if they hold a bunch of potentially beneficial traits for the crop plants. Apart from following government-enforced guidelines, one of the major constraints in field application is soil type and it directly influences the survival and growth of the microbial communities (Fig. 22.9). To introduce a genetically engineered orgainsm, we need to give special attention to the fact that government legislation varies from country-to-country. Soil parameters such as compaction, oxygen content, pH and temperature are also crucial in this respect because they can affect the functioning of the microbes. In contrast to wild type indigenous strains, the genetically modified organisms are often less adaptive perhaps as a consequence of burdensome metabolic demands due to the expression and perhaps overexpression of foreign DNA (Glick 2020). In addition, PGPR strains often do not have equal abilities to compete with soil-borne phytopathogens and other antagonistic soil microbial communities, the PGPR strains sometimes do not have the capacities to tolerate a wide range of soil contaminants, and habituation to growing in nutrient-rich media under laboratory conditions may have resulted in functional loss of active genes that previously made the microbes suitable in contaminated rhizopshere environments (Glick 2020; Fig. 22.9).

Fig. 22.9
figure 9

Factors affecting survival and proliferation of PGPR

Full size image

8 Conclusion

Heavy metal(loid)-affected agricultural crops have benefitted for many years from the application of indigenous HM-resistant PGPR. Although there are a lot of constraints associated with the application of these microorganisms, their great diversity and natural abundance in contaminated soil offers a ray of hope as we explore their potential role in agriculture. Recent advancements in bioremediation strategies have given us cause for optimism. But, before field application, these PGPR should be verified for their degree of metal resistance, their level of plant growth-promoting traits, and obviously their ability to reduce HM-content in plant parts under controlled conditions. Henceforth, these PGPR are naturally dwelling microflora that should be isolated, enriched and applied for sustainable agriculture in HM-contaminated fields.

figure a

Contributing authors of this book chapter