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

ability of plants to purify soil and water bodies of potentially harmful toxic

elements (organic and inorganic). This method provides a huge advantage

over conventionally used physical or chemical technologies in terms of

efficiency, environmental sustainability, cost-effectiveness, and ecology

enhancement. However, several limitations such as phytotoxicity, slow

degradation, and limited uptake of contaminant, evapotranspiration of vola­

tile contaminants and disposal of plant residue constrain the application of

this technology (Khan & Doty, 2011). Majority of these limitations can be

overcome by plant-associated microbes (bacteria, fungi, and actinomycetes)

or endophytes which can improve the amelioration of some pollutants.

Endophytes reside in the living plants sharing a mutualistic relationship

without causing apparent negative symptoms of infection (Huo et al., 2012).

Heinrich Friedrich Link, a German botanist, was the first to characterize

endophytes as a unique category of partially parasitic fungus that live in plants

in 1809 (Kumar & Saxena, 2020). Enterobacteriaceae, Pseudomonadaceae,

and Burkholderiaceae are some of the most common endophyte genera.

Endophytic bacteria have a variety of advantages over rhizospheric bacteria

in phytoremediation. Rhizospheric bacterial populations are difficult to

regulate, and competition between rhizospheric bacterial strains frequently

lowers the number of the desired strain. Quantitative gene expression of

xenobiotic catabolic genes and genetic modification of the catabolic pathway

may aid in improving the efficacy of plant-based bioremediation.

1.5.1 ENDOPHYTE AIDED PHYTOREMEDIATION OF INORGANIC

POLLUTANTS

Extracellular precipitation, intracellular accumulation and sequestration,

biotransformation of hazardous metal ions to less or non-toxic forms, and

adsorption/desorption of metal ions are all methods by which endophytic

bacteria might lower metal phytotoxicity (Ma et al., 2016). The genes that

code for metal or antibiotic resistance proteins can help to relieve or remove

abiotic or biotic stress. Sun et al. (2010) discovered that endophytic bacteria

could modulate the activity of plant antioxidant enzymes (such as POS, CAT,

SOD, glutathione (GSH) peroxidase, and ascorbate peroxidase (APX)) as

well as lipid peroxidation, which confronted plant defense mechanisms, and

that this could help plants resist heavy metal-induced oxidative stress.

Furthermore, methylation can be used by certain endophytic bacteria as a

metal resistance or detoxifying process. Some mercury-resistant endophytic