Cadmium, Chromium, Cobalt, Copper, Lead, Manganese, Mercury, Nickel and
Zinc) on crops and the HM-resistant PGPR discovered since 20 years. In addition,
a general account of fundamental principles behind bacterial heavy metal resistance
has been elaborated. Hence, this chapter will be of great interest especially to
environmental microbiologists.
22.1 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-aminocyclopropane1-carboxylic acid (ACC) deaminase activity, phosphate solubilisation, indole-3acetic 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
442
K. Pramanik et al.
Zinc) on crops and the HM-resistant PGPR discovered since 20 years. In addition,
a general account of fundamental principles behind bacterial heavy metal resistance
has been elaborated. Hence, this chapter will be of great interest especially to
environmental microbiologists.
22.1 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-aminocyclopropane1-carboxylic acid (ACC) deaminase activity, phosphate solubilisation, indole-3acetic 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
442
K. Pramanik et al.