Rhizobacteriome: Promising Candidate for Conferring Drought Tolerance in Crops

Drought is a global water shortage problem which poses challenge to crop productivity. Novel strategies are being tried to find out solution to sustain agriculture under drought conditions. Rhizobacteriome is an exclusive genetic material of bacteria resident to rhizosphere plays critical role to health and yield of plant. The interaction of rhizobacteriome with plant provides basis for protecting plants from various abiotic and biotic stresses. Plant growth promoting rhizobacteria (PGPR) are root-colonizing bacteria which produce array of enzymes and metabolites that assist plants to withstand harsh environmental conditions. Various formulations of rhizobacteria are being applied to enhance the tolerance or endurance to drought in crops which in turn increase crop productivity. This could be a one of the promising methods with wide potentiality to improve the growth and yield of crops under limited water resources and changing climatic conditions to ensure food security of the globe. In this review, we summarize (1) existing knowledge and understanding about the rhizobacteria, (2) their role in imparting tolerance to crops in drought conditions and (3) discuss future line of work in this frontier research area.


INTRODUCTION
D r o u g h t s t r e s s h a s i n c r e a s e d tremendously in last few years affecting food security at global level. The drought stress duration is ranged as short, severe, extremely severe and prolonged that adversely affects the agricultural productivity 1 . Drought is the most destructive abiotic stress which may affect crops of 50% of the arable lands by 2050 2 . It is a serious issue in the context of agricultural sector as it reduces crop yield in regions with scanty rainfall in various parts of the world 3 . Presently, various effective practices like efficient water irrigation techniques, conventional and modern plant breeding methods, and production of drought-tolerant transgenic plants through genetic engineering can be adopted to address the problem of sustainable crop production in drought situations. However, such techniques or procedures or methods need sophisticated technical knowhow and are costly and labor intensive as they are arduous to implement. An alternative method for promoting plant growth under drought conditions is to manipulate plant growth promoting rhizobacteria (PGPR) that are found in the rhizosphere and endorhizosphere in plant root systems. PGPR induces plant growth by various direct or indirect mechanisms under normal, biotic or abiotic stress conditions 4

.
Rhizosphere is the area where, interaction among soil, plants and microorganisms take place. The microorganisms present in the rhizosphere, compete for their survival. This competition is for the need of nutrients, water and space to develop their association with plant. The plantmicrobes interactions lead to the improvement in growth and development of plants 5 . Diverse bacterial genera form the important component of soils facilitating various biotic activities like recycling nutrient of the soil ecosystem which is essential for sustainable crop yield 6,7 . PGPR mobilize different nutritive components in soil, produce plant growth regulators and inhibit phytopathogens 8 . They also improve quality of soil by bioremediation of the pollutants by facilitating uptake of heavy toxic metal and degradation of xenobiotic compounds including pesticides 9,10 . Agronomists and environmentalists adapting various biological methods for integrated plant nutrient management system 11 . Rigorous research has been undertaken globally on exploring rhizobacteria possessing novel characteristics like ability to detoxify heavy metals 12 , salinity tolerance 13 , biological control of phytopathogens and insects 14 along with the plant growth promoting properties like, phytohormones production 15,16 phosphate solubilization 17 ,1-aminocyclopropane-1-carboxylate 18 , hydrogen cyanide (HCN), and ammonia production 19 nitrogenase activity 20 , siderophore 21 production. Hence, diverse groups of symbiotic bacteria like Bradyrhizobium, Rhizobium, Mesorhizobium and non-symbiotic like Bacillus, Klebsiella, P s e u d o m o n a s , A zo to b a c te r, A zo m o n a s , Azospirillum have been used worldwide as biofertilizer for promoting growth and development of plants under abiotic stress 22,7 . Although no single mechanism of rhizobacteria -mediated plant growth promotion is completely understood, however PGPR show significant contribution to the improvement in crop production 23 .The potential of inoculated bacteria to survive, multiply to outnumber the native bacteria and other microflora, and colonize the rhizosphere is crucial for its successful application 22 specifically in drought-affected soils. The bacteria that are not adapted to drought conditions will die out under these unfavorable growth conditions 24,25 . But, the drought-tolerant rhizobacteria are capable of thriving in new drought stressed soil in sufficient number to show plant growth promoting manifestations on plants 26,27 . The present review highlights past and current status of role of rhizobacteriome on plant growth promotion under drought conditions. Further, it will also emphasize mechanisms associated with in conferring drought tolerance in crops on application of rhizobacteria.

Rhizosphere and rhizobacteriome
The term "rhizosphere" was first used by Hiltner 27 . Rhizosphere is multidimensional and dynamic region around root where significant plant-microbe interactions occur 28 . The root exudates alter the physicochemical properties of soil, which directly effects the multiplication of soil microorganisms 29 . These root exudates have ability to attract or repel microorganisms and promote symbiotic interactions which help in growth and development of plant 30 . PGPR are characterized by their capability to colonize the plant root surface, multiply, compete and survive to promote plant  24 . The entire set of genetic material of the root associated bacteria is called "rhizobacteriome".
The rhizosphere is hot spot for number of organisms which represent most complicated and dynamic ecosystems on the Earth 32,33 . Rhizosphere organisms consist of arthropods, archaea, viruses, algae, protozoa, nematodes, oomycetes, fungi and bacteria 34 . The rhizosphere examplifies complicated food web which utilise various nutrients produced by plants. Rhizosphere is identified by presence of exudates, border cells, mucilage called as rhizodeposits. Rhizodeposits represent diverse microbial community and microbial activity on plant roots 35 . However, the organisms of rhizosphere are analysed for their beneficial impact on growth and development of plants including nitrogen fixing bacteria, protozoa, mycoparasitic fungi, biocontrol microorganisms, fungi and plant growth promoting bacteria (PGPR)/ rhizobacteria. Some of organisms present in rhizosphere like nematodes, bacteria, oomycetes and pathogenic fungi, have adverse effects on growth of plants. Some human pathogens are also found in the rhizosphere 36 . Abiotic stresses have various impacts on rhizospheric bacteria. Total bacterial biomass decline under drought situations 37 resource limitation but stable biomass has been observed in certain cases of soil bacteria in drought condition 31 as repeated drought exposures make; bacteria to learn to survive 38 .
D ro u g ht fo rc e s s h i f t m i c ro b i a l composition in drought affected soil 39 . An increased ratio of Gram-positive to Gramnegative bacteria has been observed during water stressed conditions 40 . Drought affected soil decreases members of Gram-negative phyla like Proteobacteria, Verrucomicrobia, and Bacteroidetes and increases members of Gram-positive phyla like Actinobacteria and Firmicutes 41,42 . Also, the total numbers of genes of microbes present in the drought striken rhizosphere are exceeding the numbers of genes in plant in that area. Variation in metatranscriptome and metagenomics profiling of microbial genes related to metabolism, signal transduction and other vital activities of dry and well aerated soil suggests that microbial genes might contribute to plant survival and drought tolerance 43 . Some important stress in crops by production of phytohormones, producing volatile compounds, ACC deaminase, osmolyte and exopolysaccharides, and triggering antioxidant activities.

Role of rhizobacterial phytohormones in drought stress tolerance
In drought stress, there is reduced production of phytohormones which inhibit normal plant growth. PGPR are capable for producing phytohormones that help to sustain growth and division of plant cell under abiotic environmental stress 45 . Phytohormones like indole -3-acetic acid (IAA), gibberellin (GA), cytokinin, abscisic acid and ethylene produced by rhizobacteriome become significant for promoting growth and development and helping plants to escape abiotic stress 46,47 . These pose as important targets for engineering metabolic products for conferring drought tolerance to crop plants 48 .
Inoculation with various IAA producing bacteria enhanced lateral roots and roots hairs formation along with overall root growth, thus effecting increased water and nutrient uptake in drought conditions 49,50 . For example, IAA produced by Azospirillum increased plant ability to tolerate drought stress in maize and wheat 51 , and by nitric oxide production in tomato 52 . The simultaneous production of siderophores and auxins by Streptomyces increases the plant growth-promoting effects of auxins, which in turn enhances the phytoremediation potential of plants 53 . A. brasilense Sp245 applied in wheat (Triticum aestivum) improved crop yield, micronutrients content, water content, water potential thus increased drought tolerance in plants 54 . A.brasilense also triggers nitric oxide signaling in IAA pathway and thereby improved growth of lateral root and root hair in tomato under drought stress 52 . B.thuringiensis improved nutritive value, physiological activities and metabolic activities of Lavandula dentate through IAA produced by the bacteria 54,55 . IAA signaling by consortium of Rhizobium leguminosarum (LR-30), Mesorhizobium ciceri (CR-30 and CR-39), and Rhizobium phaseoli (MR-2) inoculated in wheat improved crop 56 . Inoculation of Pseudomonas putida, Pseudomonas sp. and Bacillus megaterium increased water content and shoot / root biomass in Trifolium repens under water stressed conditions 57 (Table 1). Bacillus subtilis, B. cereus, Enterobacter cloacae, Pseudomonas koreensis, and P. fluorescens promoted seed germination by IAA production and phosphate solubilization under drought like condition induced by different concentrations of polyethylene glycol (PEG 6000) 58 .
The capability of gibberellin producing bacteria to stimulate plant growth has also been well documented as it plays prominent role in various physiological processes. For example gibberellin produced by bacterial strains B. macroides CJ-29, B. cereus MJ-1, and B. pumilus  59 . Similarly, gibberrelin producing P. putida H-2-3, a increased growth of soybean plants in drought 60 ( Table 2). Azospirillum lipoferum supported in mitigating activity of stress created by drought in plants of maize via yielding of ABA and gibberellin 50 .
Under water deficit situation, biosynthesis of stress hormone i.e. ABA is triggered by dehydration conditions 61 . The involvement of ABA has been observed in regulating water loss through controlling the closing of stomata and transduction pathways of stress signals 62 . Arabidopsis plants showed elevated levels of ABA when inoculated with A. brasilense sp245 50 . Phyllobacterium brassicacearum strain STM196 isolated from the rhizosphere of Brassica napus, elevated ABA content leading to decreased leaf transpiration and enhanced osmotic stress tolerance in Arabidopsis plants 63 . Cytokinin producing Bacillus subtilis enhanced ABA in shoots and increased the stomatal conductance conferring drought stress resistance in Platycladus orientalis seedlings 64 (Table 3).
Cytokinin producing bacterial strains like Pseudomonas E2, Bacillus licheniformis Am2 and Bacillus subtilis BC1 reported to enhance cotyledon growth in cucumber 65 . Inoculation of lettuce with cytokinin producing bacteria increased shoot cytokinins and also promoted the accumulation of shoot mass and shortened roots 66 . Cytokinin producing B. subtilis strain IB-21 stimulate rhizodeposition for rhizobacterial colonization in the wheat rhizosphere 67,68 (Table 4).

ACC deaminase production by rhizobacteria
Ethylene, a ubiquitous hormone in plants, plays role in seed germination, leaf abscission, ripening of fruits, senescence of leaf, initiation and elongation of roots, rhizobia nodule formation etc. 69,70 . In drought stress, synthesis of ethylene increase by conversion of S-adenosylmethionine (SAM) into 1-aminocylcopropene-1-carboxylase (ACC), the precursor of ethylene, in presence of ACC synthase 71 . PGPR act as sink of ACC by controlling ethylene formation using the ACC (1-aminocyclopropane-1-carboxylate) deaminase enzyme. These PGPR hydrolyse the ACC into ammonia and α-ketobutyrate, and thereby stimulate the expulsion of ACC from the roots to the soil 72 . Decreased ACC concentration in root further decreases the formation of endogenous ethylene preventing retardation in plant growth. Reducing ethylene-mediated inhibitory effects on plant growth and facilitate enhanced plant resistance to drought. Achromobacter picchaudii ARU8 secretes ACC deaminase that degrades ACC to ammonia for nitrogen and energy supply and thus decreases ethylene production under water deficit condition 73,74 . Pseudomonas fluorescens, Enterobacter hormaechei, and Pseudomonas migulae are three ACC and EPS producing microbes which when inoculated in foxtail millet could promote seedling germination in drought condition 75   Various VOCs produced by different species of microorganisms in soil include 11-decyldocosane, dotriacontane, 2,6,10-trimethyl, tetradecane, 1-chlorooctadecane, dodecane, benzene(1methylnonadecyl),1-(N-phenylcarbamyl)-2-morpholinocyclohexene, decane, methyl, benzene, 2-(benzyloxy) ethanamine and cyclohexane 87 .
Gram-positive Bacillus spp. (GB03 and IN937a) and Gram-negative E. cloacae strain JM22 elicited growth promotion of Arabidopsis seedlings through VOCs production 88 . Inoculated with P. chlororaphis O6 or exposed to 2,3-butanediol increased process of stomata closure and hence reduced loss of water in Arabidopsis plants thereby enhanced drought tolerance 89 . High rate of photosynthesis correlated with reduced VOCs production, enhanced survival under drought stress in plants primed with Bacillus thuringiensis AZP2. This proved that inoculation with bacterial improved drought stress tolerance 90 (Table 6).

Exopolysaccarides (EPS) producing rhizobacteria and drought tolerance
Many bacteria like Pseudomonas are capable of surviving in drought conditions due to development of exopolysaccharides (EPS). Pseudomonas sp. P45 produces EPS and protects sunflower plant from stress created by drought condition 91 . EPS consist of high molecular weight polymer of monosaccharide residues and their derivatives. These are biodegradable polymers biosynthesized by various algae, plants and bacteria 91 . Microbes produce EPS in capsular form and release it into the soil, the clay surface absorbs the EPS by Van der Waals force, hydrogen bonding, cation bridges or anionic absorption 92 . This protective capsule provides soil, the capacity of holding water and drying water more slowly under drought condition 93 and nutrients uptake by increasing the water potential around roots. Inoculating with EPS and catalase producing  (Table 7).

Role of osmolytes on drought tolerance in plants
Under water deficit condition, plants secrete different forms of osmolytes such as sugar, betaine, proline, polyhydric alcohol or other amino acids or dehydrin (drought stress protein) 100 . PGPR also release osmolytes in drought stress  (Table 8). These osmolytes interact with those produced by plants and enhance growth of plants 101 . These secreted solutes trap water molecules which help in decreasing the hydric potential of cells. This kind of regulation is known as osmoregulation. These accumulated solutes increase membrane integrity and protein stability to counteract cellular damage. Bacillus spp. effects osmoregulation by preventing electrolyte leakage and enhancing proline synthesis, sugars, free amino acids accumulation 102 . The function of the osmolytes is to prevent water molecules loss by reducing the cell water potential during drought period. Also, osmolytes help in protecting cellular damage by maintaining the integrity and stability of membranes and proteins in water scarce condition. PGPR consortia lessened the effect of drought stress in rice crop by accumulation of proline which improved the plant growth 103 . Inoculation of B. thuringiensis (Bt) in L. dentate showed increased shoot proline content in water shortage conditions 55 105 showed that priming cultivars of rice with consortia containing Pseudomonas jessenii R62, Pseudomonas synxantha R81 and Arthrobacter nitroguajacolicus strain YB3 and YB5 increased plant growth in drought area. This consortium enhanced proline accumulation in plants by up regulating its biosynthetic pathway hence preserving cell water potential, stabilizing the cell membrane and protein during drought stress 105 . It has been reported that enhanced concentration of osmolytes like proline, betaine, glutamate, glycine and trehalose stimulated by Azospirillum help plants to overcome osmotic stress 106 .
Similarly, A. lipoferum metabolic activities lead to accumulation of free amino acids and soluble sugars thus improving maize growth in drought 107 . Pseudomonas putida GAP-P45 enhance plant biomass, relative water content and leaf water potential by stimulating accumulation of proline in maize plants in drought conditions 97 . Azospirillium spp. z19 made maize seedling to tolerate drought stress to a higher level as compared to uninoculated plants due to higher proline levels 108 . Evidences of increased biosynthesis and accumulation of choline, a precursor of gibberellin (GB), showed increased biosynthesis in maize when inoculated with Klebsiella variicola F2, P. fluorescens YX2 and Raoultella planticola YL2.This resulted in upgraded level GB thereby bettering leaf relative water content (RWC) and dry matter weight (DMW) 109,110 . Inoculating plants with PGPR increases existing concentrations of proline in maize plants by P. fluorescens under drought stress 111 . Phaseolus vulgaris plants inoculated with Rhizobium showed improved metabolism of carbon and nitrogen and upregulation of trehalose-6-phosphate synthase gene 112,113 . Pseudomonas putida GAP-P45 showed upgraded expression of polyamine biosynthetic genes (ADC, AIH, CPA, SPDS, SPMS and SAMDC) and polyamine levels in Arabidopsis thaliana during drought stress 114,98 .

Role of rhizobacteria on antioxidant defense system for induction of drought tolerance
During normal growth of plant, ROS is produced at low level. Stress condition results into overproduction of ROS which causes oxidative damage. ROS affects signalling, transport, metabolism and biosynthesis of auxin. It also interacts with phytohormones production process, for example, H 2 O 2 causes ethylene production. In response to the stress condition, antioxidant  defense system is used by plants, in which plants produce various enzymatic and nonenzymatic antioxidants 115 . It has been observed that enzymatic activities lead to reduction of oxidative damage but at very high level of ROS, it can results into deleterious effects 116 . Thus, it is important to maintain balance between ROS production and annihilation of free radicals produced 117 . This can be done by using PGPR and their inoculation to plants shows higher survival rate by preventing oxidative damage than those which were not inoculated with PGPR.
Pseudomonas sp. is reported to improve catalase activity in drought stress condition in basil plants (Ocimum basilicum L.). Similarly, Pseudomonas sp., Bacillus lentus and A. brasilense consortium induce high activity of glutathione peroxidase and ascorbate peroxidase in Ocimum basilicum L. 118 . Consortium of PGPR containing P. jessenii R62, P. synxantha R81 and A. nitroguajacolicus strainYB3 and YB5 improved growth of plant along with inducing superoxide dismutase, catalase (CAT), peroxidase (PX), ascorbate peroxidase (APX) and lowering H 2 O 2 , malondialdehyde (MDA) in Sahbhagi (drought tolerance) and IR-64 (drought sensitive) rice crop 103 . Pseudomonas spp. namely P. entomophila, P. stutzeri, P. putida, P. syringae and P. montelli are responsible for reducing action of antioxidant enzymes significantly in maize under drought stress 97 . Bacillus species have also shown protection against drought stress by decreasing antioxidant enzymes APX and glutathione peroxidase (GPX) 96 . B. thuringiensis (Bt) improved growth via drought avoidance and reduction of glutathione reductase (GR) and ascorbate peroxidase (APX) activity in Lavandula dentata and Salvia officinalis in drought conditions 55 . Streptomyces pactum Act12 treatment in wheat increased osmoregulation and antioxidant efficiency of plants. Bacillus pumilus DH-11 and B. firmus 40 induced ROSscavenging enzymes like ascorbate peroxidase and catalase in tomato plants. A remarkable increase in antioxidant enzymes like APX, SOD, and CAT was evident under drought stress in PGPR treated plants compared with non-treated plants 119,120 . Increased activity of CAT in green gram plants inoculated with Pseudomonas fluorescens Pf1 and Bacillus subtilis EPB was reported by Saravanakumar et al. (2011) 121 . Similarly, increased level of CAT production and drought tolerance has also been correlated in cucumber 122 and maize 96,98,123 . Up-regulation of expression of drought resistance-related genes like EXPA2, EXPA6, P5CS, SAMSI HSP17.8 and SnRK2 and accumulation of abscisic acid mitigated drought stress impact in wheat 124,119 . These experimental evidences proves that PGPR have significant role in increasing plant tolerance towards drought (Table 9).

Molecular mechanism of drought stress tolerance induced by rhizobacteria
In water deficit conditions, gene induction forms two different types of proteins: functional proteins and regulatory proteins. Functional proteins include mRNA binding proteins, LEA proteins, water channel proteins, enzymes for osmolytes biosynthesis, proteases etc 125 . They function directly in abiotic stresses. On the other hand, regulatory proteins include protein kinase, calmodulin binding protein, phosphatase and other transcription factors. These are involved in stress responsive genes expression and signal transduction 126 . Hsps are heat shock proteins which inhibit misfolding of protein and are classified according to their molecular weight 127 . LEA proteins are the proteins which accumulate during late embryonic phase in response to abiotic stress. Plants inoculated with PGPR helps in up regulation of stress tolerance inducing genes. Various molecular strategies have established the mechanism of microbes induced gene expression modulation for abiotic stress tolerance. The differential expression of multiple genes such as COX1 (regulates energy and carbohydrate metabolism), ERD15 (Early response to dehydration 15), PKDP (protein kinase), AP2-EREBP (stress responsive pathway), Hsp20, bZIP1 and COC1(chaperones in ABA signalling) in Pseudomonas fluorescens treated rice was established. Similarly RAB18 (ABA-responsive gene), LbKT1, LbSKOR (encoding potassium channels) in Lycium barbarum, jasmonate MYC2 gene in chickpea, ADC, AIH, CPA, SPDS,SPMS and SAMDC (polyamine biosynthesis), ACO, ACS (ethylene biosynthesis), PR1 (SA regulated gene), pdf1.2 (JA marker genes) and VSP1 (ethylene-response gene) in Pseudomonas treated Arabidopsis plants were established for drought tolerance 125,128,129 . Molecular networks of signal transduction genes are also involved in drought stress responses 130,131 .
There are different molecular techniques which give huge amount of information about induced genes expressions and pathways during plant and rhizobacteria interactions. The techniques include high throughput whole genome gene expression such as microarrays, proteomics, RNA sequencing, 2D-PAGE, differential display 132,133 . This helps in exploring physiological functions of such genes and tolerance induced by PGPR 134 . Upregulation of EARLY RESPONSE TO DEHYDRATION 15 (ERD15) in Arabidopsis thaliana was seen when inoculated with Paenibacillus polymyxa B2 as investigated at transcriptional level 135 . Pepper plants when inoculated with Bacillus showed more than 1.5-folds increase in Cadhn, VA, sHSP and CaPR-1084. Inoculation of Bacillus amyloliquefaciens 5113 and A. brasilense NO40 alleviating the deleterious impact of drought stress in leaves of wheat by upregulation of stress response genes APX1, SAMS1, and HSP17.8. These upregulated genes enhanced plant ascorbate-glutathione redox cycle help in alleviating drought stress 124 . Bacterial priming of Gluconacetobacter diazotrophicus PAL5 stimulated the ABA-dependent signalling genes which confer tolerance to drought in sugarcane cv. SP70-1143 as studied by Illumina sequencing (HiSeq 2000 system) 135,136 (Table 10). In Pseudomonas chlororaphis colonized Arabidopsis thaliana plants, upregulated but differential expression of jasmonic acid-marker genes, VSP1 and pdf-1.2, salicylic acid regulated gene, PR-1 and the ethylene-response gene, was observed 137 .
In the past several decades, researchers have been able develop many resistant varieties of plant species, but they have gained a very little success in development of drought tolerant crops using genetic engineering 138 . Monsanto introduced  139,140 . In bacteria, cold shock proteins help in preserving normal cellular functions by stabilizing cellular RNA and enhancing gene expression under abiotic stress 141 . Similarly, the translation of CSPB have been reported to enhance tolerance to abiotic stress in Arabidopsis and rice 142 . Another important gene OsNLI-IF overexpressed by cold, heat, salt and drought stresses improved drought tolerance in transgenic tobacco plants 143 . Argentina developed genetically modified soybean contains a gene from a naturally drought-resistant sunflower adapted to drought. Rhizospheric microbes not only support the growth of plants in limited water conditions but also reduce use of chemical fertilisers.
The rhizosphere research field is flooded with metagenomics and metabolomics data, establishing genes identity and their functional taxonomic relationships. Scientists are putting their research efforts on developing consortia of microbes and metabolites of microbial origin in the formulations that best suited for individual crops in stressed environment 144 .

CONCLUSION
In this review, we have attempted to highlights the existing knowledge of plant-bacterial interactions in maintaining plant growth under drought stress. To overcome drought conditions, plants adapt various morphological, biochemical and physiological changes. Now, it has been established that members of the rhizospheric bacteria can alleviate abiotic stress of drought in plants. This can be a promising alternative to tedious and costly genetic engineering and plant breeding methods. This review establishes that various PGPR play significant role in inducing tolerance to drought stress in plants employing different mechanisms. The rhizobacterial induced drought stress tolerance in the plant is over and above the drought resistance genes either present or absent in the plant (Fig. 1).

Future Perspectives
Future research should be undertaken to increase crop yield, soil fertility and shelf life of products of PGPRs. Drought stress is a severe environmental factor that limits agricultural productivity. Rhizobacteriome offer plethora of PGPR in imparting adaptation and tolerance to drought stresses and prove to be promising strategy to improve productivity in drought areas. The plant and rhizobacteria interaction changes plant as well as soil properties in drought conditions. Rhizobacterial stimulation of osmotic responses and induction of novel genes expression play a significant role in ensuring plant survival under drought stress conditions. The development of drought tolerant crop varieties through genetic engineering and plant breeding approaches is good option but it is a labor intensive, lengthy and costly affair. Alternately, rhizobacteria inoculation to mitigate drought stresses in plants is environment friendly and safe option for agriculture drought affected areas. Future research must focus on (1) identification and characterization of the novel abiotic stress-tolerant bacteria from unexplored niches, (2) discover novel bacteria with novel molecule or mechanism, (3) better formulation with appropriate delivery system and (4) perform rigorous field trial in order to select potential rhizobacterial candidate to combat drought stress.

DATA AVAILABILITY
All datasets generated or analyzed during this study are included in the manuscript.

ETHICS STATEMENT
This article does not contain any studies with human participants or animals performed by any of the authors.