**6. Plant growth-promoting rhizobacteria as a vital tool under abiotic stress**

Micro-organisms are an integral part of all ecosystems. They perform varied functions that are not only vital to their own survival but are also beneficial to different other life forms, including plants. Be it the free-living or symbiotic N2 fixing bacteria, or the mycorrrhizal associations of fungi and plants, micro-organisms are famous for their service to the most fundamental trophic level of all food chains, i.e. plants, and consequently to the entire planet as well. For several decades now, the environmental significance of bacteria has been a matter of utmost interest, and as such scientists from all over the world have kept a close watch on the developments in this field. Bacteria are exploited to amend environmental problems arising from both natural and anthropogenic challenges, and this includes environmental pollution with toxic heavy metals or pesticides, and abiotic stress factors such as salinity or drought. Bacteria are also utilised to assist the sessile plant beings in overcoming such environmental challenges, and one of the versatile number of tools with which they help their plant hosts is collectively termed as 'plant growth promoting properties'. Some essential plant growth-promoting properties of bacteria include N2 fixation, phosphate or potassium or other mineral solubilisation (through organic acid production), siderophore production, auxin (indole acetic acid) production and ACC deaminase activity [47–49]. If these bacteria can colonise in the rhizosphere of the plant, they form a group called 'plant growth-promoting rhizobacteria' (PGPR in short). The PGPR can also assist the plants in enhancing their antioxidant defence machinery and adapt better to the growing stress [50, 51]. In recent times, the role of PGPR in assisting plants under various abiotic stresses is being extensively explored. For instance, the efficient role of PGPR of *Bacillus* genus in salt tolerance in tall fescue, alleviating drought stress in maize and wheat through PGPR of genera *Bacillus* and *Enterobacter*, using PGPR *Bacillus subtilis* to mitigate drought stress in potatoes by suppressing oxidative stress in the plant and enhancing antioxidative enzymes, improvement in essential oil production by the medicinally important rosemary plant under salinity stress through treatment with PGPR *Pseudomonas fluorescens*, and many more [52–55]. Moreover, given the fact that bacterial species have high potential for adaptation and plasticity in metabolism, their applicability can be dynamic. The utility of PGPR thus covers a wide range of plants, from forest trees, grassland, agricultural crops to medicinally as well as aesthetically important plants, making them well applicable for cultivating more stress-resilient plants in the current backdrop of climate change.
