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).
22.6 Mechanism of Heavy Metal(loid) Resistance by Plant
Growth-Promoting Rhizobacteria
Plant-associated HM-resistant PGPR are more profoundly present in heavy-metalcontaminated 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
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K. Pramanik et al.
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).
22.6 Mechanism of Heavy Metal(loid) Resistance by Plant
Growth-Promoting Rhizobacteria
Plant-associated HM-resistant PGPR are more profoundly present in heavy-metalcontaminated 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
476
K. Pramanik et al.