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).
22.5 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 (PérezMontaño 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).
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
22 Beneficial Role of Plant Growth-Promoting Rhizobacteria in Bioremediation of. . .
471
this decreased fitness may be due to overburden of metabolic load by the expression
of foreign genes (Glick 2020).
22.5 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 (PérezMontaño 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).
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
22 Beneficial Role of Plant Growth-Promoting Rhizobacteria in Bioremediation of. . .
471