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-metalcontaminated 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 growthpromoting 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 growthpromoting 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 growthpromoting 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.
22 Beneficial Role of Plant Growth-Promoting Rhizobacteria in Bioremediation of. . .
443
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-metalcontaminated 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 growthpromoting 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 growthpromoting 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 growthpromoting 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.
22 Beneficial Role of Plant Growth-Promoting Rhizobacteria in Bioremediation of. . .
443