18

Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

(Mishra et al., 2016). Nannipieri et al. (2007) identified higher interactions

among rhizospheric microbes in metal contaminated soil (Nannipieri et al.,

2007). Several reports are available on the potential of biochar (a porous

carbonaceous material produced by pyrolysis of crop residues) in heavy

metal remediation (Guimarães et al., 2021; He et al., 2019). The use of PGPR

and biochar along with a hyperaccumulator plant significantly increased the

Cd content and bioaccumulation factor of accumulator, about 412% and

403% higher than that of control, respectively (Wu et al., 2019). This system

also increased the fresh and dry biomass of the metal accumulator plant by

227.27% and 178.33%, respectively. Khan & Bano (2016) identified cata­

lase and oxidase enzymes secreted from the PGPR isolated from municipal

wastewater that solubilize insoluble bound phosphate and exhibit antifungal

and antibacterial activities. The use of Ag-nanoparticles with these PGPR

enhanced the levels of abscisic acid (ABA) by 34%, indole acetic acid

(IAA) by 55%, gibberellin by 82%, and proline synthesis by 70%, as well

as alleviating oxidative stress and enhancing the bioremediation capacity of

PGPR for Pb, Cd, and Ni. Table 1.3 summarizes the function of PGPR in

the remediation of heavy metal-polluted soil. Biofilms have been used to

remove heavy metals, according to studies (Yin et al., 2019). Biofilms are

characterized as bacterial communities attached to a surface or to each other

inside a matrix (Poulsen, 1999; Moussa & Algamal, 2017). Harekrushna &

Kumar (2012) investigated bacterial biofilm zinc sorption and the role of

extracellular polymeric molecules (EPS).

The microbial EPS can bind with heavy metals and protect the cells from

the hostile environment. Forming biofilms by these strains is considered a

natural strategy to maintain a favorable niche in stressful environments with

increased metals concentrations. Costley & Wallis (1999) reported the effi­

ciency of biofilm in removing the heavy metals in order of Cu>Zn>Cd with

the removing efficiency of 73%, 42%, and 33% respectively. Workentine

et al. (2008) reported that metal toxicity might be reduced by biofilm by

altering their physiology for protection of sensitive chemical targets against

the reactive metal species.

1.5 ENDOPHYTIC MICROBES ASSISTED BIOREMEDIATION

Natural remediation techniques provide more environment friendly oppor­

tunity for cleanup of the polluted soils and water. Phytoremediation due to

its cost-effectiveness has caught attention worldwide. It uses the natural