60

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

siderophore production and anti-pathogenic metabolites production (Zhang

et al., 2019; Verma et al., 2021).

Among different microorganisms colonize in the plant roots, rhizo­

spheric bacteria are the most common organisms that promote plant growth

and exhibited antagonistic properties to different pathogens. Beneficial

bacteria living in the plant root includes both symbiotic and free-living

bacteria. This growth promoting free-living bacteria are known as plant

growth promoting rhizobacteria or PGPR (Majeed et al., 2015). Benefi­

cial bacteria belonging from different genera like Bacillus, Azotobacter,

Pseudomonas, Azospirillum, Klebsiella, and Enterobacter are important

PGPR that enhance plant growth and development under stress condition

(Glick et al., 2007). Salinity is one of the severe threats to plant growth

as it deteriorates the soil fertility and also induce osmotic stress. It was

found that wheat species inoculated with a halophilic bacterium Serratia

sp. improved salt tolerance in a Salt Lake (Singh & Jha, 2016). Beside

this, PGPR also inhibit the disease cycle of several plant pathogens by

exhibiting antagonistic mechanisms. There are some mechanisms through

which PGPR enhance stress tolerance in plant which include production of

enzymes like ACC-deaminase and chitinase, phytohormones, siderophore,

inhibition of ethylene and induction of systemic resistance (Berg et al.,

2009; Hyat et al., 2010).

Besides bacteria, fungal microorganisms occupied a great portion of the

soil rhizosphere that supports plant growth and development. AMF found

to form symbiotic relationship with 90% of plant and currently it is very

difficult to find plants without having mycorrhizal colonization. Interaction

between AMF and plants is considered as the most primitive symbiotic

relationship that was first discovered (Diagne et al., 2020). AMF help in

the uptake of different nutrients like phosphorus and nitrogen under stress

condition and improved plant growth. AMF increases the root surface area

that enables excess absorption of water and nutrients. Moreover, fungal

hyphae can penetrate small pores of the soil, which contributes to more

nutrient uptake and water absorption. Mycorrhizal fungi also help in the

mobilization of those ions in the plant which are generally not found during

abiotic stress condition (Allen, 2011; Faber et al., 1991). AMF not only

promote plant growth but also it could induce tolerance against drought,

salinity, temperature, and heavy metal stresses. An overall mechanism of

how these symbiotic microorganisms provides tolerance to plants under

stress condition is presented in Figure 3.1.