ISSN: 0973-7510

E-ISSN: 2581-690X

Review Article | Open Access
Vinod Kumar Yadav1,2, Meenu Raghav2, Sushil K. Sharma1,3 and Neeta Bhagat2
1ICAR-National Bureau of Agriculturally Important Microorganisms (ICAR-NBAIM), Kushmaur, Maunath Bhanjan – 275 103, Uttar Pradesh, India.
2Amity Institute of Biotechnology, Amity University, Sector 125, Noida – 201 301, Uttar Pradesh, India.
3ICAR-National Institute of Biotic Stress Management (ICAR-NIBSM), Baronda, Raipur – 493 225, Chhattisgarh, India.
J. Pure Appl. Microbiol., 2020, 14 (1): 73-92 | Article Number: 5778 | © The Author(s). 2020
Received: 14/08/2019 | Accepted: 14/01/2020 | Published: 15/02/2020

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.


Rhizobacteriome, bacteria- plant interactions, rhizosphere, drought stress, ACC deaminase, rhizobacteria.


Drought stress has increased tremendously in last few years affecting world’s food security at global level. The drought stress duration is ranged as short, severe, extremely severe and prolonged that adversely affects the agricultural productivity1. Drought is the most destructive abiotic stress which may affect crops of 50% of the arable lands by 20502. It is a serious issue in context of agricultural sector as it reduces crop yield in regions with scanty rainfall in various parts of the world3. 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 conditions4. 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 for the need of nutrients, water and space develop their association with plant. The plant-microbes interactions lead to the improvement in growth and development of plants5. 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 yield6,7. PGPR mobilize different nutritive components in soil, produce plant growth regulators and inhibit phytopathogens8. They also improve quality of soil by bioremediation of the pollutants by facilitating uptake of heavy toxic metal and degradation of xenobiotic compounds including pesticides9,10. Agronomists and environmentalists adapting various biological methods for integrated plant nutrient management system11. Rigorous research has been undertaken globally on exploring rhizobacteria possessing novel characteristics like ability to detoxify heavy metals12, salinity tolerance13, biological control of phytopathogens and insects14 along with the plant growth promoting properties like, phytohormones production15,16 phosphate solubilization17,1-aminocyclopropane-1-carboxylate18, hydrogen cyanide (HCN), and ammonia production19 nitrogenase activity20, siderophore21 production. Hence, diverse groups of symbiotic bacteria like Bradyrhizobium, Rhizobium, Mesorhizobium and non-symbiotic like Bacillus, Klebsiella, Pseudomonas, Azotobacter,Azomonas, Azospirillum have been used worldwide as biofertilizer for promoting growth and development of plants under abiotic stress22,7. Although no single mechanism of rhizobacteria –mediated plant growth promotion is completely understood, however PGPR show significant contribution to the improvement in crop production23.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 application22 specifically in drought-affected soils. The bacteria that are not adapted to drought conditions will die out under these unfavorable growth conditions24,25. But, the drought-tolerant rhizobacteria are capable of thriving in new drought stressed soil in sufficient number to show plant growth promoting manifestations on plants26,27. 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 Hiltner27. Rhizosphere is multidimensional and dynamic region around root where significant plant-microbe interactions occur28. The root exudates alter the physicochemical properties of soil, which directly effects the multiplication of soil microorganisms29. These root exudates have ability to attract or repel microorganisms and promote symbiotic interactions which help in growth and development of plant30. PGPR are characterized by their capability to colonize the plant root surface, multiply, compete and survive to promote plant growth31. PGPR are broadly categorized into two classes: 1) ePGPR (extracellular) which grows in the rhizospheric area or in between cells of root cortex, examples include Agrobacterium, Azotobacter, Erwinia, Serratia, Bacillus etc. 2) iPGPR (intracellular) which grows inside root cells, examples include Azorhizobium, Mesorhizobium, Allorhizobium etc24. 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 Earth32,33. Rhizosphere organisms consist of arthropods, archaea, viruses, algae, protozoa, nematodes, oomycetes, fungi and bacteria34. 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 roots35. 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 rhizosphere36. Abiotic Stresses have various impacts on rhizospheric bacteria. Total bacterial biomass decline under drought situations37 resource limitation but stable biomass has been observed in certain cases of soil bacteria in drought condition31 as repeated drought exposures make, bacteria to learn to survive38.

Drought forces shift microbial composition in drought affected soil39. An increased ratio of Gram-positive to Gram-negative bacteria has been observed during water stressed conditions40. Drought affected soil decrease in members of Gram-negative phyla like Proteobacteria, Verrucomicrobia, and Bacteroidetes and increases in members of Gram-positive phyla like Actinobacteria and Firmicutes41,42. 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 tolerance43. Some important members of rhizobacteriome are Acinetobacter, Achromobacter, Agrobacterium, Alcaligenes, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Bradyrhizobium, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Gluconacetobacter, Herbaspirillum, Klebsiella, Leclercia, Micrococcus, Paenibacillus, Phyllobacterium, Proteus, Pseudomonas, Raoultella, Rhizobium, Rhodococcus, Serratia, Variovorax and Xanthomonas24. These rhizospheric bacteria show profound impact on germination of seed, plant growth, seedling vigor, development, diseases, nutrition and productivity44.

PGPR and their drought tolerance mechanisms
PGPR induce tolerance to drought 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 to producing phytohormones that help to sustain growth and division of plant cell under abiotic environmental stress45. 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 stress46,47. These pose as important targets for engineering metabolic products for conferring drought tolerance to crop plants48.

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 conditions49,50. For example, IAA produced by Azospirillum, increased plant ability to tolerate drought stress in maize and wheat51, and by nitric oxide production in tomato52. 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 plants53. A. brasilense Sp245 applied  in wheat (Triticum aestivum) improved crop yield, micronutrients content, water content, water potential thus increased drought tolerance in plants54. A.brasilense also triggers nitric oxide signaling in IAA pathway and thereby improved growth of lateral root and root hair in tomato,  in drought stress52. B.thuringiensis improved nutritive value, physiological activities, and metabolic activities of Lavandula dentate through IAA produced by the bacteria54,55. 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 crop56. Inoculation of Pseudomonas putida, Pseudomonas sp. and Bacillus megaterium increased water content and shoot / root biomass in Trifolium repens under water stressed conditions57 (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.

Table (1):
Role of bacterial IAA on plants for drought stress mitigation.

S.No. PGPR Crop plant Impact on plant Reference Year
1. Azospirillum brasilense Tomato Nitric oxide a signaling molecules and IAA pathway for induction of lateral and root hair growth Molina-Favero et al. 52 2008
2. Azospirillum sp. Wheat Enhanced  lateral roots, root growth, increased water and nutrient uptake Azareesh et al.51 2011
3.  Pseudomonas putida,
Bacillus megaterium
Trifolium repens Increased shoot and root mass Marulanda et al57 2009
4. B. thuringiensis Lavandula dentate Increased levels of K-and proline, Decresed glutathione reductase (GR) and ascorbate peroxidase (APX) Armada et al.55 2014
5 Rhizobium phaseoli (MR-2) Mesorhizobium ciceri (CR-30 and CR-39) and Rhizobium phaseoli (MR-2) Wheat IAA from consortia improved  growth, biomass and drought tolerance index Hussain et al.56 2014

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 CJ enhanced the growth of red pepper plants59. Similarly, gibberrelin producing P. putida H-2–3, a increased growth of soybean plants in drought60 (Table 2). Azospirillum lipoferum supported in mitigating activity of stress created by drought in plants of maize via yielding of ABA and gibberellin50.

Table (2):
Role of rhizobacterial bacterial gibberellins on plant growth under drought stress conditions.

S.No PGPR Plant Impact on plant Reference Year
1 P. putida H-2-3 Soybean Improved plant growth using  gibberellins Sang-SM et al.60 2014b
2. Azospirillum lipoferum Maize Gibberellins increased ABA levels and alleviated drought stress Cohen et al.50 2009
3. B. cereus MJ-1, B.macroides CJ-29,and B. pumilus CJ- 69 Pepper Increased GA Joo et al.59 2005

Under water deficit situation, biosynthesis of stress hormone i.e. ABA is triggered by dehydration conditions61. The involvement of ABA has been observed in regulating water loss through controlling the closing of stomata and transduction pathways of stress signals62. Arabidopsis plants showed elevated levels of ABA when inoculated with A. brasilense sp24550. 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 plants63. Cytokinin producing Bacillus subtilis enhanced ABA in shoots and increased the stomatal conductance conferring drought stress resistance in Platycladus orientalis seedlings64 (Table 3).

Table (3):
Role of rhizobacterial bacterial abscisic acid on q22plants growth under drought stress.

S.No. PGPR Plant Impact on plant Reference Year
1 Bacillus subtilis Platycladus orientalis Increased shoot ABA levels and increased the stomatal conductance Liu et al64 2013
2 Phyllobacterium brassicacearum STM 196


Reduced leaf transpiration due to increase level of ABA Arzanesh et al51 2013
3 Azospirillum lipoferum Maize Increased Gibberellins and ABA levels Cohen et al.50 2009

Cytokinin producing bacterial strains like Pseudomonas E2, Bacillus licheniformis Am2 and Bacillus subtilis BC1 reported to enhance cotyledon growth in cucumber65. Inoculation of lettuce with cytokinin producing bacteria increased shoot cytokinins and also promoted the accumulation of shoot mass and shortened roots66. Cytokinin producing B. subtilis strain IB-21 stimulate rhizodeposition for rhizobacterial colonization in the wheat rhizosphere67,68(Table 4).

Table (4):
Role of cytokinin producing rhizobacteria bacteria on plants growth under drought stress.

S.No. PGPR Plant Impact on plant Reference Year
1 Bacillus Subtilis  IB-21 Wheat Stimulate rhizodeposition Kudoyarova et al.67 2014
2 Micrococcus luteus Zea mays Growth promotion Raza and Faisal 68 2013
3 Bacillus Subtilis Platycladus orientalis Stomatal conductance Liu et al.64 2013
4 Bacillus Lettuce Increased growth of plant Arkhipova et al.66 2007
5. PseudomonasBacillus and Azospirillum Maize Increased spike length, tiller number and seeds weight Hussain et al.65 2011

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 synthase71. 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 soil72. 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 condition73,74. 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 condition75. PGPR possessing ACC deaminase activity reduce toxicity of heavy metals, drought stress and other abiotic stresses like extreme temperature, salinity and soil pH, besides, antagonism against phytopathogens76. Dodd et al. (2005)77 studied effect of ACC deaminase producing Variovorax paradoxus 5C-2 on pea plant physiological (Pisum sativum L.) in water conditions. Consortium of Ochrobactrum. pseudogrignonense RJ12, Pseudomonas sp. RJ15, and B. subtilis RJ46 showed mitigation of drought stress in garden pea and black gram plants73. Leclercia adecarboxylata and Agrobacterium fabrum, Bacillus amyloliquifaciens with higher ACC-deaminase and IAA production traits elevated nutrients uptake and high chlorophyll contents78,79. Pseudomonas fluorescens DR7 having high ACC deaminase- and EPS-producing ability increased moisture content in soil and enhanced the root adhering soil and root growth in foxtail millet80. Pot trials experiment showed that inoculation with ACC deaminase-producing bacterial strains of Pseudomonas (DPB13, DPB15, and DPB16) conferred vital improvement in growth of wheat plant in drought-stressed conditions81,82. Similarly,  Bacillus  lecheniformis K11 protect pepper and Bacillus, Psuedomonas and Mesorhizobium ciceri protected chickpea in drought stress83,84,85  (Table 5).

Table (5):
Role of ACC deaminase producing rhizobacteria on plants growth under drought stress.

S.No. PGPR Plant Impact on plant Reference Year
1. Agrobacterium Fabrum, Bacillus amyloliquifaciens Wheat Increased grain yield and biomass Zafar  et al79 2019
2. Leclercia decarboxylata and A. fabrum Wheat Elevated nutrients uptake and high chlorophyll contents Danish et al78 2019
3. O. pseudogrignonense RJ12, Pseudomonas sp. RJ15, and B. subtilis RJ46 Pea Decreased ACC Accumulation Saika et al73 2018
4. Pseudomonas fluorescens, Enterobacter hormaecheiPseudomonas migulae Foxtail millet Improved seed germination and seedling growth Niu et al75 2017
5. Psuedomonas flourescens DPB15 and P.palleroniana DPB16 Wheat Enhanced length of root, shoot and biomass Chandra  et al.81 2018
6. Variovorax paradoxus Pea Reduction in ethylene production, increased growth, yield and efficiency of water use Belimov et al82 2009
7. Pseudomonas fluorescens Pea Enhanced water uptake and induced longer roots Zahir et al.83 2008
8. Variovorax paradoxus Pea Increased yield, nitrogen content and number of seed Dodd et al77 2005
9. Achromobacter piechaudii Tomato and Pepper Increased fresh and dry weight Mayak et al.74 2004
10. B. licheniformis Pepper Increased expression of stress genes Lim and Kim84 2013
11. Bacillus and Pseudomonas with Mesorhizobium ciceris Chickpea Increased concentration of proline, improved root and shoot, length, seed germination Sharma et al85 2013

Volatile organic compounds (VOCs) producing rhizobacteria and drought stress tolerance
Under stress condition, plants produce volatiles which act as signal for development of systemic response or for priming within the plant or in neighboring plants. VOCs that are produced by diverse group of bacteria Pseudomonas, Bacillus, Arthrobacter, Stenotrophomonas, and Serratia increase growth of plants, inhibit fungal and bacterial pathogens and nematodes along with inducing systematic resistance in plants towards phytopathogens86. Various VOCs produced by different species of microorganisms in soil include 11-decyldocosane, dotriacontane, 2,6,10-trimethyl, tetradecane, 1-chlorooctadecane, dodecane, benzene(1-methylnonadecyl),1-(N-phenylcarbamyl)-2-morpholinocyclohexene, decane, methyl, benzene, 2-(benzyloxy) ethanamine and cyclohexane87.

Gram-positive Bacillus spp. (GB03 and IN937a) and Gram-negative E. cloacae strain JM22 elicited growth promotion of Arabidopsis seedlings through VOCs production88. 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 tolerance89. 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 tolerance90 (Table 6).

Table (6):
Role of VOCs on plants growth under drought stress.

S.No. PGPR Plant Impact on plant Reference Year
1 Bacillus thuringiensis Wheat Increased rate of photosynthesis and reduction in emission of volatiles Timmusk et al90 2014
2 Pseudomonas chlororaphis Arabidopsis thaliana Prevent loss of water by stomatal closure Cho et al89 2008
3. Bacillus spp. (GB03) and (IN937a), E. cloacae strain JM22 Arabidopsis thaliana Phenotypic improvement Zhang et al88 2010

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 condition91. EPS consist of high molecular weight polymer of monosaccharide residues and their derivatives. These are biodegradable polymers biosynthesized by various algae, plants and bacteria91. 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 absorption92. This protective capsule provides soil, the capacity of holding water and drying water more slowly under drought condition93 and nutrients uptake by increasing the water potential around roots. Inoculating with EPS and catalase producing Mesorhizobium ciceri (CR-30 and CR-39), Rhizobium leguminosarum (LR-30), and Rhizobium phaseoli (MR-2) increased root length, biomass and drought tolerant index in seedlings of wheat in presence of polyethylene glycol (PEG) 6000 induced drought94. Priming of maize seeds with EPS- producing strains like Alcaligenes faecalis AF3, Proteus penneri Pp1 and Pseudomonas aeruginosa Pa2 increased root and shoot length, biomass of plants, and moisture content in soil94. Under dehydrated conditions, sunflower showed increase in root tissue when inoculated with EPS-producing bacterial strain YAAF3495. EPS play a pivotal role to maintain water potential, make sure obligate connection among rhizobacteria and roots in stress condition created by drought96. Pseudomonas sp. strain P45 improved soil structure through EPS formation to protect sunflower seedlings from dehydration97,91. Ghosh et al., (2019)98 observed four drought tolerant bacterial strains namely Pseudomonas aeruginosa PM389, P. aeruginosa ZNP1, Bacillus endophyticus J13 and B. tequilensis J12 were able to alleviate the deterimental effects of osmotic-stress induced in Arabidopsis thaliana by adding 25% PEG in agar medium. Rhizobium sp., Xanthomonas sp., Agrobacterium sp., Enterobacter cloacae, Bacillus drentensis, Azotobacter vinelandii and Rhizobium leguminosarum play significant function in improving fertility of soil thus sustain agriculture99 (Table 7).

Table (7):
Effect of EPS on plants growth under drought stress.

S.No. PGPR Plant Impact on plant Reference Year
1 Pseudomonas aeruginosa PM389, Pseudomonas aeruginosaZNP1, Bacillus endophyticus J13 and B. tequilensis J12 Arabidopsis thaliana Increased in IAA,  cytokinin, gibberellins, and EPS secretion Ghosh et al99 2019
2 Proteus perneri Pseudomonas aeruginosa Alcaligenes faecalis Maize Enhanced protein, proline, sugar and relative water content Naseem and Bano93 2014
3 R. leguminosarum Mesorhizobium ciceri R. phaseoli Wheat Promoted growth of plant, drought tolerance index and biomass Hussain et al56 2014
4 B. thuringenesis Wheat Production of alginate resulted into drought tolerance Timmusk et al90 2014
5 Pseudomonas sp. Sunflower Enhanced plant biomass, RAS/RT ratio Sandhya et al91 2009
6 P. putida Maize Improved physiological response Vardharajul et al 96 2009
7 Rhizobium sp. YAS34 Sunflower Enhanced ratio of RAS/RT (Root adhering soil per Root tissue) Alami et al95 2000

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 condition (Table 8). These osmolytes interact with those produced by plants and enhance growth of plants101. 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 accumulation102. 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 growth103.

Inoculation of B. thuringiensis (Bt) in L. dentate showed increased shoot proline content in water shortage conditions55. Similarly, phosphate solubilizing bacteria Bacillus polymyxa secreted excess proline in tomato plants to induce drought tolerance104. Sandhaya et al. (2010b)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 enhancedproline accumulation in plants by up regulating its biosynthetic pathway hence preserving cell water potential, stabilizing the cell membrane and protein during drought stress105. It has been reported that enhanced concentration of osmolytes like proline, betaine, glutamate, glycine and trehalose stimulated by Azospirillum help plants to overcome osmotic stress106.Similarly, A. lipoferum metabolic activities lead to accumulation of free amino acids and soluble sugars thus improving maize growth in drought107.Pseudomonas putida GAP-P45 enhance plant biomass, relative water content and leaf water potential by stimulating accumulation of proline in maize plants in drought conditions97. Azospirillium spp. z19 made maize seedling to tolerate drought stress to a higher level as compared to uninoculated plants due to higher proline levels108. 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 stress111. Phaseolus vulgaris plants inoculated with Rhizobium showed improved metabolism of carbon and nitrogen and upregulation of trehalose-6-phosphate synthase gene112,113. 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 stress114,98.

Table (8):
Effect of osmolytes on plants growth under drought stress.

S.No. PGPR Plant Impact on plant Modifications Reference Year
1 Pseudomonas putida GAP-P45 Arabidopsis thaliana Enhanced polyamine biosynthetic genes Sen et al114, Ghosh et al. 98 2018
2 Azospirillium spp AZ39 and AZ19 Maize Increased Proline Garcia et all08 2017
3 Bacillus polymyxa Lycopersicon esculentum Increased production of proline Shintu and Jayaram104 2015
4 Consortia of P. jessenii, P. synxantha and A. nitroguajacolicus Oryza sativa Improved plant growth because of proline accumulation Gusain et al103 2015
5 Klebsiella variicola, P. fluorescens and Raoultella planticola Maize Improved RWC in leaf due to gibberellin and choline accumulation Gou et al109 2015
6 B. thuringiensis Lavandula dentate Enhanced physiological, nutritional and metabolic activities Armada et al56 2014
7 Azospirillum lipoferum Maize Free amino acids and soluble sugars accumulation lead to improved growth of plant Bano et al107 2013
8 P. fluorescens Maize Improved growth of plant due to increased proline and phytohormones content Ansary et al111 2012
9 P. putida Maize Improved RWC, leaf water potential Sandhya et al.105 2010
10 Bacillus subtilis Arabidopsis Increased glycine, betaine and choline content Zhang et al110 2010
11 Azospirillum brasilense Maize Increased synthesis of trehalose Rodriguez et al106 2009
12 Rhizobium etli Phaseolus vulgaris Increased synthesis of trehalose Suarez et al.112 2008

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, H2O2 causes ethylene production. In response to the stress condition, antioxidant defense system is used by plants, in which plants produce various enzymatic and non-enzymatic antioxidants115. 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 effects116. Thus, it is important to maintain balance between ROS production and annihilation of free radicals produced117. 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 H2O2, malondialdehyde (MDA) in Sahbhagi (drought tolerance) and IR-64 (drought sensitive) rice crop103. 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 stress97. 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 conditions55. Streptomyces pactum Act12 treatment in wheat increased osmoregulation and antioxidant efficiency of plants. Bacillus pumilus DH-11 and B. firmus 40, induced ROS-scavenging 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 plants119,120. 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 plants122, maize96,98,123. 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 wheat124,119. These experimental evidences proves that PGPR have significant role in increasing plant tolerance towards drought
(Table 9).

Fig. 1. Strategies used by rhizobacteria to enhance plant growth under normal and stress conditions (a) Poor plant growth without PGPR and (b) Enhanced plant growth with PGPR

Table (9):
Role of antioxidant activity in drought tolerance in plants.

S.No. PGPR Plant Impact on plant Modifications Reference Year
1 Streptomyces pactum Wheat ABA accumulation Upregulation of drought resistant related genes Li et al.119 2019
2 Pseudomonas spp. Finger millet Prevent oxidative damage Chandra et al.81 2012
3 Pseudomonas putida MTCC5279 (RA) Chickpea Reduced/controlled the expression of stress response gene, increased ROS scavenging (CAT, APX, GST) Tiwari et al.47 2016
4 P. jessenii, P. synxantha, A. nitroguajacolicus Rice Enhanced growth of plants, induced SOD, CAT, POD, APX, reduced H2O2, MDA level Gusain et al.103 2015
5 B. thuringiensis Lavandula dentate and Salvia officinalis Enhanced growth of plant, reduced GR, APX activity Armada et al.Tiwa55 2014
6 EPS producing bacteria Maize Reduced APX, CAT and GPX activity Naseem and Bano94 2014
7 Pseudomonas sp. GGRJ21 Mung beans Enhanced CAT, POX and SOD activity Sarma and Saikia123 2014
8. Bacillus amyloliquefaciens 5113 Azospirillum brasilense N040 Wheat Increased fresh and dry weights, Antioxidant enzymes, enhanced of stress response genes APX1, SAMS1, and HSP17.8 Kasim et al124 2013
9 Serratia sp.,Bacillus cereus, B. subtilis Cucumber Chlorophyll content increased, increased CAT Wang et al122 2012
10 Bacillus sp. Maize Lower APX, GPX activity Vardharajula et al.96 2011
11 Pseudomonas sp., Bacillus lentus, A. brasilense, Pseudomonas sp., Ocimum basilicum L. Enhanced activity of CAT enzyme, Enhanced GPA and APX activity Heidari and Golpayegani118 2011
12. Pseudomonas fluorescens strain Pf1 Bacillus subtilis EPB5, EPB22, and EPB 31 Green Gram Stress-related enzymes  Proline content Saravana Kumar et al121 2011

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 etc125. 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 transduction126. 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 studies 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 tolerance125,128,129. Molecular networks of signal transduction genes are also involved in drought stress responses130,131.

There are different molecular techniques which give a 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 display132,133. This helps in exploring physiological functions of such genes and tolerance induced by PGPR134. Upregulation of EARLY RESPONSE TO DEHYDRATION 15 (ERD15) in Arabidopsis thaliana was seen when inoculated with Paenibacillus polymyxa B2 as investigated at transcriptional level135. Pepper plants when inoculated with Bacillus showed more than a 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 stress124. 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 observed137.

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 engineering138. Monsanto introduced GM crop MON 87460, a maize (Zea mays L), in 2009 which was drought stress tolerant. This crop increased production 5.5-folds from 50,000 ha in 2013 to 275,000 ha in 2014. Cold shock protein B (CSPB) inserted from Bacillus subtilis in MON 87460 expresses to imparted drought tolerance139,140. In bacteria, cold shock proteins  help in preserving normal cellular functions by stabilizing cellular RNA and enhancing gene expression under abiotic stress141. Similarly, the translation of CSPB have been reported to enhance tolerance to abiotic stress in Arabidopsis and rice142. Another important gene OsNLI-IF overexpressed by cold, heat, salt and drought stresses improved drought tolerance in transgenic tobacco plants143. 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 environment144.

Table (10):
Stress responsive genes induction by rhizobacteria with molecular techniques involved in its analysis.

S.No. PGPR Plant Technique involved Impact on plant Reference Year
1 Gluconobacter Diazotrophicus Sugarcane Illumina sequencing Activation of ABA dependent signaling genes Vargas et al.136 2014
2 P. chlororaphis Arabidopsis thaliana Microarray analysis Up regulation of transcripts of jasmonic acid-marker genes, pdf-1.2 and VSP1, salicylic acid regulated gene (PR-),Ethylene response gene (HEL) Cho et al.137 2013
3 Bacillus amyloliquefacienss, A. brasilense Wheat Real time PCR Stress related genes (APX1, HSP 17.8, SAMS1) up regulated Kasim et al.124 2013
4 B. licheniformis Pepper 2D-PAGE, DD-PCR Enhanced stress response  genes (Cadhn, sHSP, CaPR-10 and VA) Lim and Kim84 2013

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 is established that, members of the rhizospheric bacteria can alleviate abiotic stresses 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.

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.


Authors would like to express thanks to Amity University, Noida, Uttar Pradesh and ICAR-NBAIM, Maunath Bhanjan, Uttar Pradesh for support extended in writing this review.

The authors declare that there is no conflict of interest.

All authors have made substantial contribution to develop this manuscript.


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

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

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