plant growth promoting rhizobacteria

275RESONANCE � March 2013

GENERAL � ARTICLE

Plant Growth Promoting RhizobacteriaPotential Microbes for Sustainable Agriculture

Jay Shankar Singh

KeywordsAgriculture, biofertilizers, PGPR,sustainable agriculture.

Plant Growth Promoting Rhizobacteria (PGPR) are a groupof bacteria that enhances plant growth and yield via variousplant growth promoting substances as well as biofertilizers.Given the negative environmental impact of artificial fertiliz-ers and their increasing costs, the use of beneficial soil micro-organisms such as PGPR for sustainable and safe agriculturehas increased globally during the last couple of decades.PGPR as biofertilizers are well recognized as efficient soilmicrobes for sustainable agriculture and hold great promisein the improvement of agriculture yields.

Agriculture contributes to a major share of national income andexport earnings in many developing countries, while ensuringfood security and employment. Sustainable agriculture is vitallyimportant in today’s world because it offers the potential to meetour future agricultural needs, something that conventional agri-culture will not be able to do. Recently there has been a greatinterest in eco-friendly and sustainable agriculture. PGPR areknown to improve plant growth in many ways when compared tosynthetic fertilizers, insecticides and pesticides. They enhancecrop growth and can help in sustainability of safe environmentand crop productivity. The rhizospheric soil1 contains diversetypes of PGPR communities, which exhibit beneficial effects oncrop productivity. Several research investigations are conductedon the understanding of the diversity, dynamics and importanceof soil PGPR communities and their beneficial and cooperativeroles in agricultural productivity. Some common examples ofPGPR genera exhibiting plant growth promoting activity are:Pseudomonas,Azospirillum, Azotobacter, Bacillus, Burkholdaria,Enterobacter, Rhizobium, Erwinia, Mycobacterium,Mesorhizo-bium, Flavobacterium, etc. This article presents perspectives onthe role of PGPR in agriculture sustainability.

Jay Shankar Singh is anAssistant Professor in theDepartment of Environ-mental Microbiology, BBAmbedkar (Central)University, Lucknow,

Uttar Pradesh, India. He isworking on the environ-mental problems related toecology of methane (CH4)-consuming bacteria(methanotrophs),

environmental remediationand sustainable agricul-ture development. He alsohas a strong devotion toteaching and is alwaysavailable to share withothers the ever-growingenthusiasm for hisresearch field.

1 The soil surrounding plant rootarea where intense biologicaland chemical activities are influ-enced by root exudates, and mi-croorganisms feeding on thecompounds.

276 RESONANCE ��March 2013

GENERAL � ARTICLE

PGPR as Biofertilizers

Free-living PGPR have shown promise as biofertilizers. Manystudies and reviews have reported plant growth promotion, in-creased yield, solubilization of P (phosphorus) or K (potassium),uptake of N (nitrogen) and some other elements through inocula-tion with PGPR. In addition, studies have shown that inoculationwith PGPR enhances root growth, leading to a root system withlarge surface area and increased number of root hairs.

A huge amount of artificial fertilizes are used to replenish soil Nand P, resulting in high costs and increased environmental pollu-tion. Most of P in insoluble compounds is unavailable to plants.N2-fixing and P-solubilizing bacteria may be important for plantnutrition by increasing N and P uptake by the crop plants, andplaying a crucial role in biofertilization. N2-fixation and P-solubi-lization, production of antibiotics, and other plant growth pro-moting substances are the principal contribution of the PGPR inthe agro-ecosystems. More recent research findings indicate thatthe treatment of agricultural soils with PGPR inoculation signifi-cantly increases agronomic yields as compared to uninoculatedsoils.

Mechanisms of Plant Growth Promotion by PGPR

Several mechanisms have been suggested by which PGPR canpromote plant growth; some important ones are as follows (Table 1):

Box 1.

Plant growth promoting rhizobacteria (PGPR): PGPR (rhizo-biofertilizers) are a group of bacteria thatactively colonize plant roots and enhance plant growth and yield.

Biofertilizer: Composition of living micro-organisms which, when applied to surface of plant, seed, or soil,colonize the plant rhizosphere or inside the plant body (as endophyte) and promotes plant growth throughenhancing the supply or availability of fundamental nutrients to crop plants.

Sustainable agriculture: Sustainable agriculture involves the successful management of agricultural re-sources to satisfy human needs while maintaining or enhancing environmental quality without exploiting thenatural resources of future generations.

Many studies andreviews have reported

plant growthpromotion, increasedyield, solubilizationofP (phosphorus) or K(potassium), uptakeof N (nitrogen), etc.through inoculation

with PGPR.


GENERAL � ARTICLE

Phytohormone Production

The enhancement in various agronomic yields due to PGPR hasbeen reported because of the production of growth stimulatingphytohormones (Table 2) such as indole-3-acetic acid (IAA),gibberellic acid (GA3), zeatin, ethylene and abscisic acid (ABA).

PGPR Crop parameters

Rhizobium leguminosarum Direct growth promotion of canola and lettuce

Pseudomonas putida Early developments of canola seedlings, growthstimulation of tomato plant

Azospirillum brasilense andA. irakense Growth ofwheat and maize plants

P. flurescens Growth of pearl millet, increase in growth, leafnutrient contents and yield of banana (Musa)

Azotobacter and Azospirillum spp. Growth and productivity of canola

P. alcaligenes, Bacillus polymyxa, Enhances uptake of N, P and K bymaize cropandMycobacteriumphlei

Pseudomonas, Azotobacter and Stumulates growth and yield of chick pea (CicerAzospirillum spp. arietinum)

R. leguminismarum and Improves the yield and phosphorus uptake in wheatPseudomonas spp.

P. putida, P. flurescens, A. brasilense Improves seed germination, seedling growth andandA. lipoferum yield of maize

P. putida, P. fluorescens, P. fluorescens, Improves seed germination, growth parameters ofP. putida, A. lipoferum, A. brasilense maize seedling in greenhouse and also grain yield

of field grown maize

Table 1. PGPR and their ef-fect on growth parameters/yields of crop/fruit plants.

Phytohormones PGPR

Indole-3-acetic acid (IAA) Acetobacter diazotrophicusandHerbaspirillum seropedicae

Zeatin and ethylene Azospirillum sp.

Gibberellic acid (GA3) Azospirillum lipoferum

Abscisic acid (ABA) Azospirillum brasilense

Table 2. Examples of differ-ent phytohormone-producingPGPR.


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Recent studies confirm that the treatment of seeds or cuttings withnon-pathogenic bacteria, such as Agrobacterium, Bacillus, Strep-tomyces, Pseudomonas, Alcaligenes, etc. induce root formationin some plants because of natural plant growth promoting sub-stances produced by the bacteria. Although the mechanisms arenot completely understood, root induction by PGPR is the ac-cepted result of phytohormones such as auxins, growth inhibitingethylene and mineralization. Environment-friendly applicationsin agriculture have gained more importance, in particular inhorticulture and nursery production. The use of PGPR for nurserymaterial multiplication may be important for obtaining organicnursery material because the use of all formulations of syntheticplant growth regulators, such as indole-3-butyric acid (IBA), areprohibited in organic agriculture throughout the world.

Phosphate Solubilization

Rhizobium and phosphorus (P) solubilizing bacteria are impor-tant to plant nutrition. These microbes also play a significant roleas PGPR in the biofertilization of crops. These bacteria secretedifferent types of organic acids (e.g., carboxylic acid) thus lower-ing the pH in the rhizosphere and consequently release the boundforms of phosphate like Ca3 (PO4)2 in the calcareous soils. Utili-zation of these microorganisms as environment-friendlybiofertilizer helps to reduce the use of expensive phosphaticfertilizers. Phosphorus biofertilizers could help increase the avail-ability of accumulated phosphate (by solubilization), increase theefficiency of biological nitrogen fixation and render availabilityof Fe, Zn, etc., through production of plant growth promotingsubstances.

Siderophore Production

PGPR are reported to secrete some extracellular metabolitescalled siderophores. For the first time Kloepper et al. (1980)reported the significance of siderophores produced by certaingenera of PGPR in plant growth promotion. Siderophores arecommonly referred to as microbial Fe-chelating low molecular

The enhancement inagronomic yields by

PGPR is due toproduction of growth

stimulatingphytohormones and

the consequentrelease of the boundform of phosphate,andextracellular

metabolites.


GENERAL � ARTICLE

weight compounds. Thepresence of siderophore-producing PGPRin rhizosphere increases the rate of Fe3+ supply to plants andtherefore enhance the plant growth and productivity of crop.Further, this compound after chelating Fe3+ makes the soil Fe3+

deficient for other soil microbes and consequently inhibits theactivity of competitive microbes.

PGPR as Biocontrol Agents

PGPR produce substances that also protect them against variousdiseases. PGPR may protect plants against pathogens by directantagonistic interactions between the biocontrol agent and thepathogen, as well as by induction of host resistance. In recentyears, the role of siderophore-producing PGPR in biocontrol ofsoil-borne plant pathogens has created great interest. Microbiolo-gists have developed techniques for introduction of siderophore-producing PGPR in soil system through seed, soil or root system.PGPR that indirectly enhance plant growth via suppression ofphytopathogens do so by a variety of mechanisms. These include:

• The ability to produce siderophores (as discussed above) thatchelate iron, making it unavailable to pathogens.

• The capacity to synthesize anti-fungal metabolites such asantibiotics, fungal cell wall-lysing enzymes, or hydrogen cya-nide, which suppress the growth of fungal pathogens.

• The ability to successfully compete with pathogens for nutri-ents or specific niches on the root; and the ability to inducesystemic resistance.

Among the various PGPRs identified, Pseudomonas fluorescensis one of the most extensively studied rhizobacteria, because of itsantagonistic actionagainst several plant pathogens.Banana bunchytop virus (BBTV) is one of the deadly virus which severelyaffects the yield of banana (Musa spp.) crop in Western Ghats,Tamil Nadu, India. It has been demonstrated that application ofP. fluorescens strain significantly reduced the BBTV incidencein hill banana under greenhouse and field conditions. Different

PGPR producesubstances thatalso protect themagainst variousdiseases


GENERAL � ARTICLE

Table3.PGPR as biocontrolagents against various plantdiseases.

PGPR Disease resistance

Bacillus pumilus, Kluyvera cryocrescens, Cucumber Mosaic Cucumovirus (CMV) of tomatoB. amyloliquefaciens and B. subtilus (Lycopersicon esculentum)

B. amyloliquefaciens, B. subtilis and Tomato mottle virusB.pumilus

B. pumilus Bacterial wilt disease in cucumber (Cucumissativus), Blue mold disease of tobacco (Nicotiana)

Pseudomonas fluorescens Sheath blight disease and leaf folder insect in rice(Oryza sativa), Reduce the Banana Bunchy TopVirus (BBTV) incidence, Saline resistance ingroundnut (Arachis hypogea)

B . subtilis and B. pumilus Downymildew in pearl millet (Pennisetumglaucum)

B. subtilis CMV in cucumber

B. cereus Foliar diseases of tomato

Bacillus spp. Blight of bell pepper (Capsicum annuum), Blightof squash

Burkholderia Maize (Zea mays) rot

B. subtilis Soil borne pathogen of cucumber and pepper (Piper)

Bacillus sp. andAzospirillum Rice blast

Fluorescent Pseudomonas spp. Rice sheath rot (Sarocladium oryzae)

PGPR species as biocontrol agents against various plant diseasesare given in Table 3.

PGPR as Biological Fungicides

PGPR and bacterial endophytes play a vital role in the manage-ment of various fungal diseases (Box 2). But one of the majorhurdles experienced with biocontrol agents is lack of appropriatedelivery system.

PGPR and bacterialendophytes play a

vital role in themanagement ofvarious fungal

diseases.


GENERAL � ARTICLE

Suggested Reading

[1] B R Glick, C L Patten, GHolguin and DM Penrose, Biochemical and geneticmechanisms used by plant growth promoting bacteria, Imperial College Press,London. 1999.

[2] A R Podile and K Kishore, Plant growth-promoting rhizobacteria. In: Plantassociated bacteria. Springer Netherlands, pp. 195–230, 2007.

Conclusion

Worldwide, considerable progress has been achieved in the areaof PGPR biofertilizer technology. It has been also demonstratedand proved that PGPR can be very effective and are potentialmicrobes for enriching the soil fertility and enhancing the agricul-ture yield. PGPR are excellent model systems which can providethe biotechnologist with novel genetic constituents and bioactivechemicals having diverse uses in agriculture and environmentalsustainability. Current and future progress in our understanding ofPGPR diversity, colonization ability, mechanisms of action, for-mulation, and application could facilitate their development asreliable components in the management of sustainable agricul-tural systems.

Address for CorrespondenceJay Shankar Singh

Department of EnvironmentMicrobiology

School for EnvironmentalSciences, BB Ambedkar(Central) University,

Raibarely Road, Lucknow226025

Uttar Pradesh, India.Email:

[email protected]

Box 2

Bacillus subtilis, Pseudomonas chlororaphis, endophytic P. fluorescens inhibit the growth of stem blightpathogen Corynespora casiicola. The seed treatment and soil application of P. fluorescens reduce root rotof black gram caused by Macrophomina phaseolina. Seed and foliar application of P. fluorescens reducesheath blight of rice. B. subtilis in peat supplemented with chitin or chitin-containing materials show bettercontrol of Aspergillus niger and Fusarium udum in groundnut and pigeon pea, respectively. Strains ofBurkholderia cepacia have been shown to have biocontrol of Fusarium spp.

It has been alsodemonstrated andproved that PGPRcan be veryeffective and arepotential microbesfor enriching the soiland enhancing theagricultureyield.

plant growth promoting rhizobacteria

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