Aneela Shaheen


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This review contains: A brief historical overview of bacterial inoculants  research in the wide areas of rhizosphere biotechnology highlighting new bio inoculants has received great attention during nearby past, and right expert knowledge  have been developed. Plant growth-promoting bacteria (PGPB) are bacteria that can enhance plant growth and protect plants from disease and abiotic stresses through a wide variety of mechanisms. Several important bacterial characteristics, such as biological nitrogen fixation, phosphate solubilization, ACC deaminase activity, and production of sidero-phores and phytohormones, can be assessed as plant growth promotion (PGP) traits.

Key words Rhizosphere ,PGPB, Bacterial strain ,inoculation, Nitrogen fixation ,phosphate solublisation, siderophore, Acc deaminase, VOCs , phytohormone


The rhizosphere is the soil-plant root interphase and in practice consists of the soil adhering to the root besides the loose soil surrounding it. Plant growth-promoting rhizobacteria (PGPR) are potential agents for the biological control of plant pathogens (Babalola .,et. al 2010).The rhizosphere can be defined as the land field or soil region where processes mediated by microorganisms are specifically influenced  the root system this region includes the soil connected to the plant roots and often extends a few millimeters of the root surface, being an important environment for the plant and microorganism interactions  that exert beneficial effects on plant development known as plant growth-promoting rhizobacteria (PGPR) ( Gray and Smith, 2005;Souza .,et al 2015) . One of the basic requirements for the good effect  of PGPR is their ability to colonize and come in contact to the  hosts rhizosphere, rhizoplane, or the root interior (Glick et al., 2007:parry., et al 2016).  A formulation containing one or more beneficial bacterial strains (or species) in an easy -to-use and economical carrier material, either organic, inorganic, or synthesized from defined molecules. The inoculant is the means of bacterial transport from the factory to the living plant. The three fundamental and essential characteristics for all inoculants are to: (1) support the growth of the intended microorganisms, (2) support the necessary number of viable microbial cells in good physiological condition for an acceptable period of time (Stephens,2000,Bahsan., et al 2014 ) .Deliver enough microorganisms at the time of inoculation to reach a threshold number of bacteria that is usually required to obtain a plant response, i.e., the inoculant must contain enough viable bacteria after the formulation process (Date ,2001,Bahsan et al 2014)

PGPR-medi-ated plant growth promotion occurs by the changing of the whole microbial community in the rhizosphere niche by producing the  various substances (Kloepper and Schroth 1981; Parray .,et al  2016 ).  Some inoculants move into the root interior to establish endophytic populations with the power to adjust to  the niche and benefits to the host plants (Compant et al.,2005; Kloepper et al.,1999.,Adesemoye.,et al 2013) . Some bacteria increase root surface area, thus giving a greater value of nutrient uptake, and in turn, induce plant productivity.). Plant growth promoting rhizobacteria can affect plant growth by different direct and indirect mechanisms (Glick 1995 .,Kumar et al.,2015). PGPR effect direct growth promotion of plants by fixing atmospheric nitrogen, Solubilizing insoluble phosphates, secreting hormones such as IAA, , and  ACC (1-Aminocycloprapane-1- carboxylic acid) deaminase production  which helps in regulation of ethylene. (Kumar et al; 2015). Induced systemic resistance (ISR), antibiosis, competition for nutrients, parasitism, production of metabolites (hydrogen cyanide, siderophores) suppressive to deleterious rhizobacteria, are some of the mechanisms that in a indirectl effect the plant growth (kumaret al ;2015).

Brief Historical Background

In modern cultivation process unselective use of fertilizers, particularly the nitrogenous and phosphorus, has led to considerable pollution of soil, air and water. Use of these extravagant chemicals exerts bad effects on soil microorganism, affects the fertility position of soil and also pollutes environment (Yousaf.,et al 2014; Gupta  et al; 2015). The application of these fertilizers on a long term basis often leads to reduction in pH and exchangeable bases thus making them unavailable to crops and the productivity of crop declines. To prevent this problem and obtain higher plant yields, farmers have become increasingly dependent on chemical sources of nitrogen and phosphorus, as well as being costly, the production of chemical fertilizers depletes nonrenewable resources, the oil and natural gas used to produce these fertilizers, and these put harmful impact on human and environment (joshi., et al  2006;Gupta et al ;2015) Inoculation of plants with useful bacteria can be outline back for centuries. From experience, farmers knew  that  when  they mixed  soil  taken  from a  previous  legume  crop  with soil  in which non legumes were to be grown, as result crop yield improve. By the end of the 19th hundred , the experience of mixing “naturally inoculated” soil with seeds became a recommended and careful way of legume inoculation in the USA (Smith, 1992;Bashan ,1998).Latter on in U.S pure culture rhizobia will be used and start the commercialization of artificial inoculation their patent culture placed in market under the name nit Ragin and 1st pure culture of a desire strain of rhizobia grown in a flat glass medium containing only a small amount of gelatin medium this material either to be  supplied to the seed or mixed with soil and spread over the field and worked over the 7.6 cm(Nobbeet al ;1986;smith et al.,1992).



Key diagram: Showing the different mechanism by which PGRP bacteria effect the growth of plant.


Plant growth promoting bacteria effect the growth by the two different method direct and indirect mechanism.


Direct PGPR enhance plant growth in the absence of pathogens. In accordance with soil bacterial species in the plant rhizosphere which grow in, on, or around plant tissues activate plant growth by the large no of mechanisms. In addition to providing the mechanical support and enhancing the water and nutrient uptake, microbial activity in the rhizosphere affects rooting patterns and the supply of available nutrients to plants (Jan et al ;2016). Endophytic bacteria have more advantage over rhizospheric bacteria because they get the opportunity to stay in direct contact with the plant tissues. Also, they offer more beneficial effects to the plants as compared to bacteria residing outside plants. However, rhizospheric colonials have been found to be the major source of endophytic colonization. Some people even consider endophytes as subsets of rhizospheric micro biome (Araujo., 2002 . Chaturvedi et al ;2016).

Nitrogen fixation

Nitrogen is generally considered one of the major limiting nutrients in plant growth. The biological process responsible for reduction of molecular nitrogen into ammonia is referred to as nitrogen fixation. A wide diversity of nitrogen-fixing bacterial species belonging to most phyla of the Bacteria domain has the capacity to colonize the rhizosphere and to interact with plants. Leguminous and actinorhizal plants can obtain their nitrogen by association with rhizobia or Frankia via differentiation on their respective host plants of a specialized organ, the root nodule (Bhat  et al .,2015). Most instances bacteria colonize only the surface of the roots and remain vulnerable to competition from other rhizosphere micro-organisms, even when the nitrogen-fixing bacteria are endophytic, benefits to the plant may result from better uptake of soil nutrients rather than from endophytic nitrogen fixation (Cocking et al .,2003). The first industrial production of rhizobium inoculant began by the end of the 19th century. However, to sustain production of cereal crops, legumes and other plants of agricultural importance, the supply of nitrogenous chemical fertilizers has been regularly increasing since the Second World War. According to an FAO report, production of N fertilizer for 2007 was 130 million tons of N, and this should further increase in the coming years. Biofertilizers can add 20-200kg N ha (by fixation), liberate growth-promoting substances and increase crop yield 10_50% . (Ghany et al .,2013). They are cheaper, pollution free, based on renewable energy sources and also improve soil.( Two groups of nitrogen fixing bacteria have been studied alot, which includes Rhizobia and Frankia. (Frankia strains are N2-fixing actinomycetes whose isolation and cultivation were first reported in 1978. They induce N2-fixing root nodules on diverse nonleguminous (actinorhizal) plants that are important in ecological successions and in land reclamation and remediation. The genus Frankia encompasses a diverse group of soil actinomycetes that have in common the formation of multilocular sporangia, filamentous growth, and nitrogenase-containing vesicles enveloped in multilaminated lipid envelopes. The relatively constant morphology of vesicles in culture is modified by plant interactions in symbiosis to give a diverse array of vesicles shapes (Benson et al .,1993).

Table 1: Different type of bacterial specie involved in nitrogen fixation in different Plant

Phosphate solublisation bacteria and their role to improve crop

Biological nitrogen fixation is very important in enhancing the the soil fertility. In addition to biological nitrogen fixation,  Maximum crop yields require sufficient phosphorus fertilization. Only phosphate in a soluble ionic form (Pi) is effective as a mineral nutrient ( Goldestien et al ;1986) .Phosphate solubilization is equally important , Phosphorus (P) is major essential macronutrients for biological growth and development.the use of phosphate solubilizing bacteria as inoculants simultaneously increases P uptake by the plant and crop yield. However, direct inoculation of free phosphate solubilizing bacteria (PSB) into soil is not easy to maintain cells survival around plants roots because it is susceptible to a variety of environmental soil variation such as temperature, humidity and salt stress (Wu et al., 2012 . M. Schoebitz et al ;2013 ). Variability of PSB inoculation on plant is mainly due to the quality in the inoculants formulations containing an effective bacterial strain and can determine the success or failure of a biological agent. Therefore, the goal will be to find an adequate formulation to convert a promising bacteria strain into a commercial inoculant product (Bashan, 1998 Schoebitz et al ; 2013  ). Strains from the genera Pseudomonas, Bacillus and Rhizobium are among the most powerful phosphate solubilizers. The basic mechanism for mineral phosphate solubilization is the production of organic acids, and acid phosphatases play a great role in the mineralization of organic phosphorous in soil (Rodríguezet al; 1999). The use of phosphate-solubilizing bacteria (PSB) as inoculants simultaneously increases P uptake by the plant and crop yield. Ectorhizospheric strains from pseudomonads and bacilli, and endosymbiotic bacteria from rhizobia have been described as effective phosphate solubilizers. The production of organic acids is considered as the principal mechanism for mineral phosphate solubilization in bacteria. This assumption has been supported by the cloning of two genes involved in gluconic acid production: PQQ synthase and gabY  genes ( Mariano et al ;2001).

Table no.2 Showing Growth promoting substances released by PGPR
Fig no 1. Showing the different function of phosphate solublising bacteria

Siderophore production

Siderophores can be defined as small peptidic molecules containing side chains and functional groups that can provide a high-affinity set of ligands to coordinate ferric ion (Crosa and Walsh, 2002).). Iron is a vital element require by all living organisms for many cellular processes such as electron transport chain and as a cofactor for many enzymes(Litwin et al., 1993 ,Ali et al ;2013) .PGPR secrete siderophores. Siderophores are low molecular weight iron binding protein compounds involved in the process of chelating ferric iron (Fe (iii)) from the environment. When Fe is limited, microbial siderophores provide plants with Fe, enhancing their growth In addition, GrrA/GrrS, but not GacS/GacA, are involved in siderophore synthesis regulation in Serratia plymuthica strain IC1270, suggesting that gene evolution occurred in the siderophore-producing bacteria.(M., X. Liu.,et al 2004:Compant et al; 2015) . The plant growth promotion traits of Pseudomonas aeruginosa, P. fluorescens and Bacillus subtilis isolated from the maize (Zea mays L.) rhizosphere. In vitro studies refelact that isolates have the potential to produce indole acetic acid (IAA), hydrogen cyanide, phosphate solubilisation, and Siderophores are small organic molecules produced by microorganisms under iron-limiting conditions which enhance the uptake of iron to the microorganisms ( Saha M et al; 2016,  Karnawal 2017). Siderophore-producing microbial inoculants have direct effect on plant growth ,shown in various crop in the past. Pseudomonas aeruginosa, Pseudomonas fluorescensor Ralstonia metallidurans, able to produce siderophores, were inoculated in an agricultural soil containing Cr (488 mg kg−1) and Pb (382 mg kg−1) and maize was cultivated(Braud et al;2009) .In another experiment, inoculation of Arabidopsis thaliana with Pseudomonas fluorescens resulted in the uptake of the Fe-pyoverdin complex synthesized by the bacteria leading to an increase in the iron level inside the plant tissue and enhanced the  plant growth(Vansuyt et al; 2007 ).  Microbes produce variety of siderophores and a major class includes catechols and hydroxamate. Numerous strains of actinobacteria have been reported as siderophore producers (Wang et al,. 2014 Sathya et al ;2017)

Fig No 2 Showing that how different type of bacteria enter in root of the plant and improve the growth

Bacteria with ACC deaminase help to promote the growth of plant

Plant growth-promoting bacteria that contain the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase pomote plant growth and development by decreasing plant ethylene levels, especially following a variety of environmental stresses (Glick,et al 2007). Certain plant growth promoting rhizobacteria (PGPR) contain a important  enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which regulates ethylene production by metabolizing ACC (an immediate precursor of ethylene biosynthesis in higher plants) into α-ketobutyrate and ammonia. Inoculation with PGPR containing ACC deaminase activity could be helpful in maintain the plant growth and development under stress conditions by reducing stress-induced ethylene production (saleem et al 2007).) . Bacteria that express ACC deaminase can lower the impact on plants of a range of different stresses . For example, both endogenous and exogenous ACC deaminase genes increase the symbiotic performance of many rhizobial strains (Ma et al., 2003a, 2004; Glick et al., 2014)  To study the effect of ACC deaminase on crown gall development, an ACC deaminase gene from the PGPB Pseudomonas putida UW4 was introduced into Agrobacterium tumefaciens C58, so that the effect of ACC deaminase activity on tumour formation in tomato and castor bean plants could be assessed. Plants were also coinoculated with A. tumefaciens C58 and P. putida UW4 or P. putida UW4-acdS- (an ACC deaminase minus mutant strain). In both types of experiments, it was observed that the presence of ACC deaminase generally inhibited tumour development on both tomato and castor bean plant.Different type of bacteria that having the     abiliy to produced  the       de aminase  ,  name  of bacteria that produced Acc deaminaseAchromobacter , Azospirillum , Agrobacterium , Achromobacter , Burkholderia , Enterobacter , Pseudomonas and Ralstonian..(Balha et al; 2006)

Fig no 3 Showing the combination of different mode of PGRP that improved the crop yield


 A wide range of microorganisms preseant in the rhizosphere having the ability  to produce substances that regulate plant growth and development. Plant growth promoting rhizobacteria produce phytohormones such as auxins, cytokinins, gibberellins and Ethylene can affect cell proliferation in the root structure by over production of lateral roots and root hairs by  increasing of the nutrient and water uptake (Gupta et al ;2015).

Indole acetic acid production ,produced by the combine approach of the chemical and genetic analysis. amyloliquefaciens FZB42 affects its ability to promote plant growth. Moreover, this ability is dependent on the presence of tryptophan, which is one of the main compounds present in several plant exudates (Kamilova et al. 2006,Elsorra et al; 2007). Inactivation of genes involved in tryptophan biosynthesis and in a putative tryptophan-dependent IAA biosynthesis pathway lead to reduction of both IAA concentration and plant-growth-promoting activity in the respective mutant strains (Elsorra et al ;2007). .Indole acetic acid production is a common trait of PGPR, and such bacteria is believed to be act againts the  salinity stress in plants.Morphological plant root changes have been observed repeat-edly upon Azospirillum inoculation and have been attributed to the production of plant-growth-promoting substances:auxins, cytokinins, and gibberellins, with auxin production being quantitatively the most important(Spaepen et al.,2008) Streptomyces isolate increased plant growth in wheat and produced indoleacetic acid and auxin in presence of salt (Sadeghi et al., 2012). The genetic mechanism of auxin biosynthesis and regulation by Pseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, and Azospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis (Costacurta et al.,1995).

 INDIRECT MECHANISM                           The major indirect mechanism of plant growth promotion in rhizobacteria is through acting as biocontrol agents Mechanisms of biological control by which rhizobacteria can promote plant growth indirectly, Indirect mechanisms include the biological control of pathogens  production of , Hydrogen cyanide (HCN) as well as siderophores which chelate iron and make it available to the plant roots .Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots.  During root colonization, these bacteria produce antifungal antibiotics (Hass et al; 2005).


The production of antibiotics(Germs killing drugs) is consider to be one of the  most powerful and studied biocontrol mechanisms  of plant growth promoting rhizobacteria against phytopathogens has become increasingly better understood over the past twenty year ( Gupta et al; 2015). There are  variety of antibiotics have been identified, including compounds such as , oomycin A, phenazine, pyoluteorin, amphisin, 2,4-diacetylphloroglucinol (DAPG),  tropolone, and cyclic lipopeptides produced by pseudomonads), pyrrolnitrin, tensin

 (Loper et al 2007,Gupta et al; 2015) .One problem associated with this mechanisam is that, depending too much on antibiotic-producing bacteria as biocontrol agents , the increased use of these strains, some phytopathogens may develop resistance to against the specific  antibitoics. To prevent this from happening, some researchers have utilized biocontrol strains that synthesize hydrogen cyanide as well as one or more antibiotics, is approach is effective because, while hydrogen cyanide may not have much biocontrol activity by itself, it appears to act synergistically with bacterially encoded antibiotic (Glick et al ;2012). For example, a seed application of a combination of three PGPR, Bacillus pumilus Meyer & Gotheil, Bacillus subtilis (Ehrenberg) Cohn and Curtobacterium flaccumfaciens (Hedges) Collins & Jones provided greater control of several pathogens on cucumber (Cucumis sativa L.) than when any were inoculated singly(Raupach and Kloepper, 1998,Whipps et al; 2001). The antibiotics phenazine-1-carboxylic acid (PCA) and 2,4-diacetylphloroglucinol (Phl) are major determinants of biological control of soil borne plant pathogens by various strains of fluorescent Pseudomonas spp.(raaijmakers ;et al 1997).

Induced systemic resistance (ISR)

 Induced systematic  resistance  came out as an important mechanism by which selected plant growth-promoting bacteria and fungi in the rhizosphere prime the whole plant body for enhanced defense against a broad range of pathogens and insect herbivores  nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance SAR (piterse et al; 2014). Rhizobacteria-mediated induced systemic resistance (ISR) has been demonstrated against fungi, bacteria, and viruses in Arabidopsis, bean, carnation, cucumber, radish, tobacco, and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated. Bacterial strains differ in their ability to induce resistance in different plant species ( L.C von loon et al.,1998). Plant growth promoting rhizobacteria (PGPR) belonging to fluorescent pseudomonads group were isolated from sugarcane rhizosphere. Selected strains were studied for the induced systemic resistance (ISR) against Colletotrichum falcatum Went causing red rot disease in the sugarcane . The talc based formulations of the PGPR strains were prepared and applied through different methods and in different growth phases of the crop in the field. All the tested PGPR strains have significantly reduced the disease development in the stalks (Viswanathan et al; 1999). Specific strains of the species B. amyloliquefaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B. sphaericus elicit significant reductions in the incidence or severity of various diseases on a diversity of hosts (Josep,2004). A wide variety of root-associated mutualists, including Pseudomonas, Bacillus, Trichoderma, and mycorrhiza species sensitize the plant immune system for enhanced defense without directly activating costly defense (zamioudi et al ;2014).

Production of the volatile organic compound

volatile organic compounds (VOCs) emission by certain plant-growth promoting rhizobacteria (PGPR) has been found to be involved in plant growth. However, little is known about the role of bacterial VOCs in plant developmental processes (Fransisca et al.,2010) .VOCs, the major source of secondary metabolites and important components in ecosystems , are intensively studied due to their availability as a biocontrol resource (Benthe.,et al 1997,Naznin et al ;2013). VOCs characterized by low molecular weight and high vapor pressure are produced by all organisms as part of their normal metabolism, and play important roles in communication within and between organisms (Santoro MVet al ;2014 Naznin et al ;2013). Many types of bacteria can regulate plant growth from a distance without any contact, suggesting the possibility that these bacteria emit invisible volatile compounds that promote or inhibit plant growth. Nearly 350 bacterial species have been reported to produce around 846 different potential VOCs, with 5431 synonyms  ,(Hafiz A et al 2017 : Lemfack et al., 2014). Several bacterials pecies,from diverse generaincludingBacillus,Pseudomonas,Serratia,Arthrobacter,andStenotrophomonas, produce VOCs thatinfluenceplantgrowth.Acetoinand2,3butanediolsynthesized by Bacillus are the best known of these compounds and are responsible for significant improvements in plant growth  (Ryu et al ;2003).

Table no 4.VOC production from the different bacteria
Table no 5.Production of plant growth regulators (PGRs) by rhizobacteria and crop response


Microbial inoculants have long been incorporated into field practices worldwide, with satisfactory results, especially for rhizobia. Plant growth promoting rhizobacteria (PGPR) are a heterogeneous group of bacteria that can be found  in the rhizosphere, at root surfaces and in association with roots, which can improve the extent or quality of plant growth directly and/or indirectly. In  the direct promotion by PGPR entails either providing the plant with plant growth promoting substances that are synthesized by the bacterium or facilitating  the uptake of certain plant nutrients from the environment. The indirect promotion of plant growth occurs when PGPR prevent deleterious effects of one or more phytopathogenic microorganisms. The exact mechanisms by which PGPR promote plant growth are not fully understood, but are thought to include the ability to produce a change in the pattern of growth of the plants.


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