Insights into the Status of Heavy Metal Resistant Rhizobacterial …
23
have reported that PGPR act as potential elicitors for heavy metal tolerance as well
as abiotic stress tolerance (Dary et al. 2010; Tiwari et al. 2016, 2017). These PGPR
bind with the bioavailablemetals by forming complexes with siderophores such as
Desferroxamines, Dihydroxybenzoic acids and Rhizoferrins (Dimpka et al. 2016),
particular metabolites like heme in Bacillus japonicum by ferrochelatases, metallothionein cation-binding proteins (Chandrangsu 2017) and bacterial heavy metal
transporters such as Pb(II)/Cd(II)/Zn(II)-transporting ATPase in E. coli (Rajkumar
et al. 2010; Ahemad 2012). The agriculturally important microorganisms evolved
various mechanisms to overcome heavy metal stress which includes (a) transport
of metals across cytoplasmic membrane; (b) biosorption and bioaccumulation to the
cell walls; (c) metal entrapment in the extracellular capsules; (d) heavy metals precipitation; and (e) metal detoxification via oxidation–reduction reactions (Zubair et al.
2016). The harmful effects of heavy metals are reduced through various microbes
of Heavy-metal-tolerant PGPR including Bacillus, Pseudomonas, Streptomyces and
Methylobacterium which has the potentials to improve the growth and yield of the
crops (Sessitsch et al. 2013).
Plant growth promoting bacteria are involved in the biosorption of heavy metals
in which siderophores and IAA are accountable for metal uptake with which indirect
defence mechanisms are activated (Spaepen and Vanderleyden 2011). Siderophores
reduce the abiotic stresses forced on plants by making stable complexes with toxic
heavy metals of environmental concern such as Cd, Cu, Cr, Pb and Zn (Rajkumar
et al. 2010).
4.1 Role of Microbes in Detoxification of Heavy Metal
In order to survive at high concentration of heavy metals, bacteria need to develop
different mechanisms to confer resistances to these heavy metals. There is no general
mechanism for resistance in bacteria towards all heavy metal ions. Though it is well
known that both living and dead cells are capable of metal accumulation but there
are differences in the mechanism involved.
There are four possible known mechanisms postulated in bacterial heavy metal
resistances. They are as follows:
• The first mechanism is by keeping the toxic ion out of the cell by altering a
membrane transport system involved in initial cellular accumulation.
• The second mechanism is the intracellular or extracellular sequestration by
specific metal-ion binding components (analogous to the phytochelatins in plants
and the metallothioneins of eukaryotes, but generally binding occurs at the level
of the cell wall in bacteria). Extracellular accumulation/precipitation may be
facilitated by using viable microorganisms. However, cell surface sorption or
complexation can occur within alive or dead microorganisms, while intracellular
accumulation requires microbial activity (Macek et al. 2011).
23
have reported that PGPR act as potential elicitors for heavy metal tolerance as well
as abiotic stress tolerance (Dary et al. 2010; Tiwari et al. 2016, 2017). These PGPR
bind with the bioavailablemetals by forming complexes with siderophores such as
Desferroxamines, Dihydroxybenzoic acids and Rhizoferrins (Dimpka et al. 2016),
particular metabolites like heme in Bacillus japonicum by ferrochelatases, metallothionein cation-binding proteins (Chandrangsu 2017) and bacterial heavy metal
transporters such as Pb(II)/Cd(II)/Zn(II)-transporting ATPase in E. coli (Rajkumar
et al. 2010; Ahemad 2012). The agriculturally important microorganisms evolved
various mechanisms to overcome heavy metal stress which includes (a) transport
of metals across cytoplasmic membrane; (b) biosorption and bioaccumulation to the
cell walls; (c) metal entrapment in the extracellular capsules; (d) heavy metals precipitation; and (e) metal detoxification via oxidation–reduction reactions (Zubair et al.
2016). The harmful effects of heavy metals are reduced through various microbes
of Heavy-metal-tolerant PGPR including Bacillus, Pseudomonas, Streptomyces and
Methylobacterium which has the potentials to improve the growth and yield of the
crops (Sessitsch et al. 2013).
Plant growth promoting bacteria are involved in the biosorption of heavy metals
in which siderophores and IAA are accountable for metal uptake with which indirect
defence mechanisms are activated (Spaepen and Vanderleyden 2011). Siderophores
reduce the abiotic stresses forced on plants by making stable complexes with toxic
heavy metals of environmental concern such as Cd, Cu, Cr, Pb and Zn (Rajkumar
et al. 2010).
4.1 Role of Microbes in Detoxification of Heavy Metal
In order to survive at high concentration of heavy metals, bacteria need to develop
different mechanisms to confer resistances to these heavy metals. There is no general
mechanism for resistance in bacteria towards all heavy metal ions. Though it is well
known that both living and dead cells are capable of metal accumulation but there
are differences in the mechanism involved.
There are four possible known mechanisms postulated in bacterial heavy metal
resistances. They are as follows:
• The first mechanism is by keeping the toxic ion out of the cell by altering a
membrane transport system involved in initial cellular accumulation.
• The second mechanism is the intracellular or extracellular sequestration by
specific metal-ion binding components (analogous to the phytochelatins in plants
and the metallothioneins of eukaryotes, but generally binding occurs at the level
of the cell wall in bacteria). Extracellular accumulation/precipitation may be
facilitated by using viable microorganisms. However, cell surface sorption or
complexation can occur within alive or dead microorganisms, while intracellular
accumulation requires microbial activity (Macek et al. 2011).