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Role of Endophytes, Plant Growth Promoting Rhizobacteria

ACC deaminase enzyme (Glick et al., 2007). Inoculation of Enterobacter sp.

with endophytes improved drought resistance in maize, wheat, sugarcane,

and sunflower plant by enhanced photosynthetic efficiency and water uptake

(Kasim et al., 2013).

3.5.1.2 PGPR-MEDIATED SALT TOLERANCE IN PLANT

Increasing amount of salinity in the soil atmosphere greatly reduce crop

productivity. Salinity is currently one of the major challenges in the sustain­

able agriculture system. Almost all the major crops grown part of the earth

is facing more or less saline condition affecting plant growth. Production

of salt tolerant crops can be achieved by genetic engineering but till date

very few salts tolerant crops has been introduced. Thus, PGPR is used as an

alternative strategy to induce salt tolerance in plants. Microorganism living

in the root vicinity promotes growth by regulating source-sink interaction,

biochemical metabolism, leaf senescence and biomass production. PGPR

regulate signaling cascade in the root and shoot through which enhanced

the production of stress responsive phytohormones under salinity stress

(Ghanem et al., 2011). Under extreme saline condition plant faces osmotic

inhibition of water uptake. Accumulation of excess Na⁺ ions under saline

environment decrease the amount of Ca²⁺ and K⁺ ions which creates ionic

toxicity. In a study, Azospirillum brasilense improved shoot elongation in

wheat seedlings under saline environment (Creus et al., 1997). Under saline

condition, PGPR inhibits ROS and ethylene production via antioxidant

enzymes and ACC deaminase enzymes.

3.5.1.3 PGPR-MEDIATED HEAVY METAL TOLERANCE IN PLANT

Heavy metal contamination in the soil affects plant growth by developing

impaired metabolism system. Cd, Zn, Pb, and Ni are some of the principal

heavy metals that are released by different industrial waste materials

contributing to the heavy metal toxicity in the soil. PGPR has several key

functions which help to detoxify the impact of heavy metals in plants. PGPR

improves water uptake and mineral solubilization in the plant under heavy

metal contamination promoting plant growth (Majeed et al., 2015). PGPR

promotes IAA and cytokinin production which helps plant to expand their

roots in the soil to uptake minerals. Moreover, rhizosphere bacteria secrete

some organic acids which increase the amount of solubilized phosphorus.