Therefore, bioremediation is a cost-effective tool for destruction of contaminants
with the help of biological activity of microbes (Kamaludeen et al. 2003). This
activity can be enhanced by the supplementation of nutrients (P and N) and other
substrates like phenol, methane, and toluene (Baldwin et al. 2008; Akhtar et al.
2013). According to Weyens et al. (2009), plant-microbe interaction has a crucial
role in phytoremediation by plant growth promotion and sequestration of pollutants,
its detoxification, and degradation. These microbes have certain metabolic abilities
and degradation pathways which results in degradation of organic pollutants and
evapotranspiration of volatile organic contaminants in more effective way (Weyens
et al. 2009). Some root endophytes are equipped by metal sequestration/resistance
and can enhance the accumulation of these toxic metals in plant tissue even if they
are present as trace element in soil (Rajkumar et al. 2012). Therefore, it is expected
that microbes present in phyllosphere of the plant can resist the stress due to
particulate matter contamination and promote the phytoremediation ability of
plant. Growing indoor plants can increase humidity level in the air, and
allelochemicals released into the environment through them can inhibit airborne
harmful microbes (Berg et al. 2014; Wolverton 2008).
4.1 Role of Plant Growth-Promoting Rhizobacteria
Plant growth-promoting rhizobacteria have a very significant role in growth promotion of plants resulting in sustainable agricultural development. They aid in growth
promotion of plant by two mechanisms, viz., direct and indirect mechanism. Fixation of atmospheric nitrogen, solubilization of phosphorus, synthesis of siderophore
for iron chelation, and supplying siderophore-iron complex to plant so that plant may
synthesize various phytohormones like gibberellins, cytokinins, and auxins come
under direct mechanism, whereas indirect mechanism is brought about by control of
disease-causing phytopathogens by producing antibiotics, depletion of iron in the
soil, and ultimately stimulation of plant growth. On the basis of interaction with host
plant, PGPR are categorized into two groups: (1) symbiotic rhizobacteria, which
invade and infest the interior of the plant cell to survive (known as intracellular
PGPR, e.g., bacteria forming nodule), and (2) free-living rhizobacteria that reside
outside the plant and are also known as extracellular PGPR, e.g., Azotobacter,
Pseudomonas, Burkholderia, and Bacillus (Babalola and Akindolire 2011; Khan
2005). Microorganisms like plant growth-promoting rhizobacteria can enhance the
nutrients availability in rhizosphere (Choudhary et al. 2011). For example, nitrogen
is the most limiting factor in plant growth as it is not easily available for plant, but
Azospirillum present in cereal ecosystem can fix the free nitrogen and improve the
crop yield (Tejera et al. 2005). Additionally, phosphate is also solubilized by PGPR
(Wani et al. 2007), which is further taken up by plants. Vejan et al. (2016) decribe
that Lavakush et al. (2014) conducted a study on PGPR strain like Pseudomonas
putida and Pseudomonas fluorescens for their effect on nutrient uptake in rice.
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A. Mishra et al.
with the help of biological activity of microbes (Kamaludeen et al. 2003). This
activity can be enhanced by the supplementation of nutrients (P and N) and other
substrates like phenol, methane, and toluene (Baldwin et al. 2008; Akhtar et al.
2013). According to Weyens et al. (2009), plant-microbe interaction has a crucial
role in phytoremediation by plant growth promotion and sequestration of pollutants,
its detoxification, and degradation. These microbes have certain metabolic abilities
and degradation pathways which results in degradation of organic pollutants and
evapotranspiration of volatile organic contaminants in more effective way (Weyens
et al. 2009). Some root endophytes are equipped by metal sequestration/resistance
and can enhance the accumulation of these toxic metals in plant tissue even if they
are present as trace element in soil (Rajkumar et al. 2012). Therefore, it is expected
that microbes present in phyllosphere of the plant can resist the stress due to
particulate matter contamination and promote the phytoremediation ability of
plant. Growing indoor plants can increase humidity level in the air, and
allelochemicals released into the environment through them can inhibit airborne
harmful microbes (Berg et al. 2014; Wolverton 2008).
4.1 Role of Plant Growth-Promoting Rhizobacteria
Plant growth-promoting rhizobacteria have a very significant role in growth promotion of plants resulting in sustainable agricultural development. They aid in growth
promotion of plant by two mechanisms, viz., direct and indirect mechanism. Fixation of atmospheric nitrogen, solubilization of phosphorus, synthesis of siderophore
for iron chelation, and supplying siderophore-iron complex to plant so that plant may
synthesize various phytohormones like gibberellins, cytokinins, and auxins come
under direct mechanism, whereas indirect mechanism is brought about by control of
disease-causing phytopathogens by producing antibiotics, depletion of iron in the
soil, and ultimately stimulation of plant growth. On the basis of interaction with host
plant, PGPR are categorized into two groups: (1) symbiotic rhizobacteria, which
invade and infest the interior of the plant cell to survive (known as intracellular
PGPR, e.g., bacteria forming nodule), and (2) free-living rhizobacteria that reside
outside the plant and are also known as extracellular PGPR, e.g., Azotobacter,
Pseudomonas, Burkholderia, and Bacillus (Babalola and Akindolire 2011; Khan
2005). Microorganisms like plant growth-promoting rhizobacteria can enhance the
nutrients availability in rhizosphere (Choudhary et al. 2011). For example, nitrogen
is the most limiting factor in plant growth as it is not easily available for plant, but
Azospirillum present in cereal ecosystem can fix the free nitrogen and improve the
crop yield (Tejera et al. 2005). Additionally, phosphate is also solubilized by PGPR
(Wani et al. 2007), which is further taken up by plants. Vejan et al. (2016) decribe
that Lavakush et al. (2014) conducted a study on PGPR strain like Pseudomonas
putida and Pseudomonas fluorescens for their effect on nutrient uptake in rice.
428
A. Mishra et al.