REVIEW

Beneficial bacteria of agricultural importance

Olubukola Oluranti Babalola

Received: 10 May 2010 / Accepted: 28 June 2010 / Published online: 16 July 2010

� Springer Science+Business Media B.V. 2010

Abstract 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. A biocontrol strain should be able to

protect the host plant from pathogens and fulfill the

requirement for strong colonization. Numerous com-

pounds that are toxic to pathogens, such as HCN,

phenazines, pyrrolnitrin, and pyoluteorin as well as,

other enzymes, antibiotics, metabolites and phyto-

hormones are the means by which PGPR act, just as

quorum sensing and chemotaxis which are vital for

rhizosphere competence and colonization. The pres-

ence of root exudates has a pronounced effect on the

rhizosphere where they serve as an energy source,

promoting growth and influencing the root system for

the rhizobacteria. In certain instances they have

products that inhibit the growth of soil-borne patho-

gens to the advantage of the plant root. A major

source of concern is reproducibility in the field due to

the complex interaction between the plant (plant

species), microbe and the environment (soil fertility

and moisture, day length, light intensity, length of

growing season, and temperature). This review listed

most of the documented PGPR genera and discussed

their exploitation.

Keywords Biocontrol � Exudate �PGPR � Phytohormones

Introduction

Plant growth-promoting rhizobacteria (PGPR) are

free-living bacteria of beneficial agricultural impor-

tance. The PGPR present encourage beneficial effects

on plant health and growth, suppress disease-causing

microbes and accelerate nutrient availability and

assimilation. Thus, in the quest to improve soil

fertility and crop yield and to reduce the negative

impacts of chemical fertilizers on the environment,

there is a need to exploit PGPR for continued

beneficial agricultural purposes. PGPR exist in the

rhizosphere and this is defined as the region around

the root. Seed inoculation with PGPR could be by

drench application, seed bacterization, or via dual

treatment. PGPR compensate for the reduction in

plant growth caused by weed infestation (Babalola

et al. 2007b), drought stress (Zahir et al. 2008), heavy

metals (Kumar et al. 2009), salt stress (Egamberdieva

2008; Kaymak et al. 2009) and some other unfavor-

able environmental conditions. One of the mecha-

nisms by which bacteria are adsorbed onto soil

O. O. Babalola (&)

Department of Biological Sciences, Faculty

of Agriculture, Science and Technology, North-West

University, Private Bag X2046, Mmabatho 2735,

South Africa

e-mail: olubukola.babalola@nwu.ac.za

123

Biotechnol Lett (2010) 32:1559–1570

DOI 10.1007/s10529-010-0347-0

particles is by simple ion exchange and a soil is said

to be naturally fertile when the soil organisms are

releasing inorganic nutrients from the organic

reserves at a rate sufficient to sustain rapid plant

growth. These bacteria belong to the genera

Acetobacter, Acinetobacter, Alcaligenes, Arthrobac-

ter, Azoarcus, Azospirillum, Azotobacter, Bacillus,

Beijerinckia, Burkholderia, Derxia, Enterobacter,

Gluconacetobacter, Herbaspirillum, Klebsiella, Ochro-

bactrum, Pantoae, Pseudomonas, Rhodococcus, Serra-

tia, Stenotrophomonas and Zoogloea and have been

subject of extensive research throughout the years.

Some PGPR may have more than one mechanism

for accomplishing plant growth (Ahmad et al. 2008).

Included in such mechanisms are antagonism to

pathogenic fungi, siderophore production, nitrogen

fixation, phosphate solubilization, the production of

organic acids and indole acetic acid (IAA), NH3,

HCN, the release of enzymes (soil dehydrogenase,

phosphatase, nitrogenase, etc.), and the induction of

systemic disease resistance (ISR). Similarly, the plant

beneficial effects of bacteria include attributes such

as the production of phytohormones, nitrate reduc-

tion, and nitrogen fixation. For that reason, potential

PGPR are sometimes screened in vitro for such

multiple plant growth promoting traits. PGPR as

these make use of the plant growth promoting traits to

improve the health of forest tree species (Barriuso

et al. 2008).

The use of beneficial soil microorganisms as

agricultural inputs for improved crop production

requires the selection of rhizosphere-competent

microorganisms with plant growth-promoting attri-

butes (Hynes et al. 2008). The beneficial bacteria

termed PGPR are disease-suppressive microorgan-

isms that improve plant health. Findings and docu-

mentation abound, and all point towards the need to

commercially exploit PGPR as biofertilizers for their

agricultural benefits. From a general perspective,

however, the problems of variability in colonization

efficiency, field performance and rhizosphere com-

petence are controversial issues. This review provides

(1) an overview of these PGPR mechanisms and their

applications for beneficial agricultural purposes, (2) it

highlights the current knowledge on PGPR and (3)

examines the interactions of bacteria used as biocon-

trol agents of bacterial and fungal plant pathogens;

consideration is given to biofertilizer, quorum sens-

ing, PGPR bioactive factors, plant growth promoters,

metabolites, release of enzymes, and production of

antibiotics among others.

PGPR/biofertilizer

PGPR are bacteria that produce hormones, vitamins

and growth factors that improve plant growth and

increase plant yield. The endorhizosphere and exo-

rhizosphere are the regions around the root. The

rhizosphere is richer in microflora and available

nutrients such as photosynthates from the roots than

the bulk soil. Manipulating microorganisms in the

rhizosphere is by diverse, competitive, complex,

highly regulated cell-to-cell communication and by

using signaling compounds to monitor their sur-

roundings and alter their activities. Plants are also

involved in these signaling mechanisms. Microbial

activity in the rhizosphere may affect the acquisition

of mineral nutrients by roots either directly via effects

on mobilization and/or immobilization or indirectly

via effects on root morphology and/or physiology. A

large proportion of manganese oxidizing microor-

ganisms in the rhizosphere may, therefore, either

decrease the risk of manganese toxicity in plants

growing in poorly aerated or waterlogged soils, or

increase the risk of manganese deficient in well-

aerated calcareous soils.

The term Plant Growth-Promoting Rhizobacteria

(PGPR) was coined over three decades ago; they are

non-pathogenic, strongly root colonizing bacteria on

the surface of plant’s roots which increase plant’s

yield by one or more mechanisms. The PGPR

enhance the availability and uptake of plant nutrients,

the production of substances promoting plant growth

and the suppression of deleterious bacteria (Table 1).

Unlike the adverse effect of the continuous use of

chemical fertilizers, PGPR when applied to the soil

improve the soil structure, leaving no toxic effects.

Microbial inoculants or biofertilizers are an important

component of organic farming (Table 1); accounting

for about 65% of the nitrogen supply to crops

worldwide. PGPR are known to fix atmospheric

molecular nitrogen through symbiotic and asymbiotic

or associative nitrogen fixing processes (Anjum et al.

2007). With more and more emphasis being placed

on organic farming, PGPR are finding increasing

applications today as biofertilizers (Table 1).

1560 Biotechnol Lett (2010) 32:1559–1570

123

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Biotechnol Lett (2010) 32:1559–1570 1561

123

Quorum sensing (QS)

In QS behavior small diffusible extracellular signal-

ing molecules meditate cell–cell communication. The

signaling molecules for Gram-negative bacteria are

named autoinducers, usually acylated homoserine

lactones (AHLs). The Gram-positive bacteria use

peptide-signaling molecules for QS. At a particular

population density, bacteria exert a concerted action;

the signals activate transcription factors that induce

specific genes in the bacteria. Examples of rhizobac-

teria that utilize AHL signals to regulate the expres-

sion of traits important in plant–microbe interaction

are Pantoea stewartii, Pseudomonas aureofaciens,

and Pseudomonas syringae. Studies indicate that host

plant can interfere with the QS of bacteria. The QS

molecules in Azospirillum lipoferum are strain-spe-

cific and are associated with rhizosphere competence

and adaptation to plant roots (Boyer et al. 2008).

Plant–microbe interactions such as biofertilization,

rhizoremediation, biocontrol and phytostimulation

may be QS-dependent. Some studies have high-

lighted the fact that bacteria form microcolonies on

plant roots and these are assumed to be suitable

setting for QS as a result of the high bacterial cell

density. In addition, the mucigel, secreted from the

roots, can cover the bacteria and this could retard

diffusion of N-acyl homoserine lactones required for

QS. The degradation of AHLs is a mechanism for

biocontrol. Two N-acyl homoserine lactone systems,

RhlI/R and LasI/R, in Pseudomonas aeruginosa

PUPa3 from rice rhizosphere are involved in the

regulation of plant growth-promoting traits (Steindler

et al. 2009).

Chemotaxis and mobility

Findings by some researchers using spinach roots

indicated that gravitational water flow in irrigation

plays an important role in the spread of the coloni-

zation of roots by PGPR Pseudomonas strains

(Urashima et al. 2004). When the colonization pattern

and the effect of bacterization on the early root

development of Pseudomonas fluorescens 5.014 on

tomato seeds were monitored, the inoculated strains

were distributed over the entire root and the distal

segment was more densely colonized compared with

the proximal and middle segments. The density of the

indigenous seed flora was uneven but was stabilized

by the application of PGPR. Azospirillum brasilense

confirms both chemotaxis and chemokinesis reaction

to the sequential gradients of diverse chemo-effec-

tors, thereby increasing the chance of root–bacterial

interactions. Genera Azospirillum needs bacterial

motility for establishment on plant root as demon-

strated using non-flagellated mutants and a

non-chemotactic mutant. The understanding that

chemotaxis is essential for colonization has aided

the selection of bacterial strains that are good

colonizers of cereal root. Examples of chemotactic

agents in the rhizosphere are amino acids and sugar.

The root exudates of monocots improved growth

yield because the root tips of monocots, e.g. perennial

ryegrass, support ten times more bacteria than those

of dicots, e.g. potato, radish and tomato (Lugtenberg

et al. 2001).

Bacteria can move only in water films and,

although many are smaller than 1 lm diameter, they

appear to be readily motile only in films appreciably

thicker than this, although they may well be capable

of growth at lower moisture contents. For all motile

bacteria, the rate of movement is dependent on the

moisture content of the soil and is thus dependent on

soil structure. Moisture content affects the activity of

the soil population in a number of ways: through the

thickness of the water films in the soil and conse-

quently its aeration; through the reduction in the free

energy of the water as the soil becomes drier and the

films become thinner, so affecting water availability

and the cell osmotic relationships; and finally through

the effects that can be demonstrated when soil

moisture fluctuates from the dry state to that of

becoming adequate. Movement becomes restricted as

the soil becomes drier, partly because the water film

pathway between two points in the soil becomes more

tortuous, and partly due to organisms becoming

trapped in moisture filled pores occluded by narrow

necks. Studies have indicated the non-uniform distri-

bution and the lognormal distribution of root-colo-

nizing bacteria in the rhizosphere. A number of areas

on the roots are virtually free from bacteria while

other areas can be densely inhabited. Root hairs

provide a hospitable biological surface for chemo-

taxis, migration and colonization due to microorgan-

isms (Pothier et al. 2007).

1562 Biotechnol Lett (2010) 32:1559–1570

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PGPR bioactive factors

PGPR bioactive factors are substances that impact on

growth. Examples include: root exudates, vitamins,

and amino acids.

Root exudates

Root exudates are chemical compounds photosynth-

ates, organic acid, sugar (Castro-Sowinski et al. 2007;

Kamilova et al. 2006), polyamine putrescine (Kuiper

et al. 2001) excreted from root tissues; the process is

called rhizodeposition. Indirect interactions between

plants and microbes occur in the rhizosphere due to

root exudates. Energy-rich organic acid materials

such as citric, lactic, succinic, amides, malic, oxalic,

pyruvic, aliphatic and aromatic acids, and fructose

(simple sugar), carbohydrates (glucose, xylose) loss

by exudation, secretion or autolysis of older root

cells; leaked moisture and root-growth-lubricant

mucigel secreted from the roots all provide nutrients

for microorganism growth (Alexandre et al. 2000). In

addition to such solubles and diffusibles, there is the

entire range of insolubles occurring in the root (e.g.,

cellulose, lignin, protein) and lost from it by cell

exfoliation and root pruning. Organic acids are 83%

of the total exudates and sugars roots increased

during plant growth (Kamilova et al. 2006). Some of

the matter lost by the roots has a low molecular

weight and is metabolized by the microbes. Rhizo-

sphere bacteria are ecophysiologically adapted to

these chemical metabolites. The abundant supply of

organic carbon contained in or emanating from plant

root makes the rhizosphere an oasis for growth and

maintenance of bacteria in the bulk soil. Some root-

associated populations may be more sensitive to a

change in exudates (as may occur during plant

growth) than others (Herschkovitz et al. 2005).

Apparently, chemical signals between plant roots

and microbes influence community structure and

functions. PGPR are exogenous bacteria introduced

into agricultural ecosystems that act positively upon

plant development (Castro-Sowinski et al. 2007)

showed that P. putida strain PCL1444 cultivated in

the presence of grass seedlings cv. Barmultra in soil

led to a tenfold increase of cells. Based on the

underlying principle that root exudate is the best

nutrient source available in soil.

Vitamins

Vitamin producing bacteria are widespread in the

rhizosphere because many bacteria excrete this com-

pound. It should however be mentioned that the

vitamins are at time not in sufficient quantity. A large

proportion of indigenous bacteria requires or is stim-

ulated by water-soluble B vitamins and amino acids.

They will not grow in simple laboratory media unless

supplemented with the appropriate substances. In the

absence of vitamins (thiamine, biotin, pantothenic

acid, folic acid, riboflavin, vitamin B12, nicotinic acid,

pyridoxine, and myoinositol), inoculation with a PGPR

P. putida strain G11-32 promoted the biomass of

soybean callus in a tissue culture bioassay. Similarly, a

thiamine auxotroph of P. fluorescens WCS365

appeared to be a poor competitor in the tomato

rhizosphere. The ability of PGPR to synthesize biotin

and/or thiamine appears to be an important coloniza-

tion trait (Lugtenberg et al. 2001). Thiamine is the most

frequently required vitamin, but biotin and vitamin B12

also are essential for a large number of bacteria.

Amino acids

The ability to synthesize amino acids is an important

colonization trait. Notable among the amino acids are

glutamine and asparagine with glutamic acid, alanine,

serine, valine, isoleucine, aspartic and leucine which

are also present. The loss of amino acids by exudation

has been reported (Alexandre et al. 2000); the amount

of loss may vary with plant’s species, age and

developmental stage. The role of amino acid is to

support growth of auxotrophs.

Growth promoters

Growth promoters are organic molecules required in

trace quantities for growth. The need for growth

factors has considerable ecological importance

because a species needing them will only grow in

habitats where the organic molecules are present.

Production of phytohormones

Phytohormones produced by bacteria contributed to

the host root respiration rate, metabolism and root

Biotechnol Lett (2010) 32:1559–1570 1563

123

abundance and hence improved the mineral and water

uptake in inoculated plants. Gibberellins (GAs) may

influence seed germination, stem elongation, flower-

ing, and fruit setting of higher plants. Four GAs (GA1,

GA3, GA4, and GA20) from seven species of bacteria,

Acetobacter diazotrophicus, A. lipoferum, A. brasi-

lense, Bacillus licheniformis, Bacillus pumilus, Herb-

aspirillum seropedicea and Rhizobium phaseoli, have

been identified (MacMillan 2002). Azotobacter chroo-

coccum, an inoculant that fixes atmospheric nitrogen,

produces on average 0.05 lg gibberellic acid (GA3)

equivalents per milliliter of culture in 14 days; and

when tomato seedling roots are treated with a culture,

growth is drastically changed and found similar to

treatment with authentic GA3. Bacterial inoculants,

when applied to plants, probably contain sufficient

growth regulators to influence future plant develop-

ment, but as they multiply around seedling roots,

possibly more hormone is produced and absorbed by

the plants at the critical differentiation stage.

PGPR synthesize auxins, and these influence root

growth, cell elongation, tissue differentiation, plant

growth promotion and responses to light and gravity.

Auxins are the most important plant hormone

produced by Azospirilum, Bacillus megaterium and

Pseudomonas spp. HPLC analysis confirmed the

presence of IAA and indole acetamide (IAM) as the

major auxins in the culture filtrates of some rhizo-

bacteria (Lopez-Bucio et al. 2007).

Regulating ethylene production in roots

Ethylene is a plant growth regulator. For many plants,

a burst of ethylene, a gaseous natural byproduct of

plant metabolism, is required to break seed dor-

mancy, increase the number of roots, shoot and root

growth differentiation, adventitious root formation,

leaf and fruit abscission, the induction of flowering

and increased femaleness in dioecious plants, flower

and leaf senescence, and fruit ripening (Babalola

et al. 2007b) but following seed germination, a

sustained high level of ethylene would inhibit root

elongation, and lead to abnormal root growth, which

would seriously impede plant growth and develop-

ment. Although ethylene is involved in the regulation

of numerous physiological processes in plants, it is

also produced by PGPR (Babalola 2010). Apart from

being a plant growth regulator, ethylene acts as a

stress hormone when under conditions such as those

generated by salinity, drought, waterlogging, heavy

metals and pathogenicity. Under stress conditions, the

endogenous production of ethylene is accelerated

substantially which adversely affects root growth and

consequently the growth of the plant as a whole

(Saleem et al. 2007). The ability of 1-aminocyclo-

propane-1-carboxylate (ACC)-utilizing PGPR to

ameliorate plant growth when inhibited by ethylene

through a decrease in ACC content and the enzymatic

removal of ethylene has been demonstrated. Another

way bacteria stimulate plant growth is with ACC

deaminase that hydrolyzes the ethylene precursor

ACC and thus presents a promising opportunity to

increase crop yields. The bacteria occurring on the

root surface degrade ACC to ammonium and

a-ketobutyrate for use as carbon and nitrogen sources.

The results of (Shaharoona et al. 2007) demonstrated

that ACC-deaminase activity is an efficient parameter

for the selection of promising PGPR under axenic

conditions. Ethylene plays a key role in inducing

multifarious physiological changes in plants at the

molecular level (Saleem et al. 2007).

Metabolites

Hydrogen cyanide (HCN), 2,4-diacetylphloroglucinol

(DAPG) and siderophores are antifungal compounds.

Their production assists in the control of soil-borne

diseases.

Diacetylphloroglucinol (DAPG) production

DAPG is a polyketide antibiotic that suppresses a wide

variety of soil-borne fungal pathogens, including

Gaeumannomyces graminis var. tritici and Fusarium

oxysporum f. sp. radicis-lycopersici (Duffy et al. 2004).

An example of DAPG producer is P. fluorescens Q2-87

(Duffy et al. 2004). Strains of P. fluorescens that

generate DAPG are efficient PGPR controlling root

and seedling diseases.

Production of siderophores

Low molecular weight siderophores synthesized by

PGPR can solubilize and sequester iron from the soil

1564 Biotechnol Lett (2010) 32:1559–1570

123

and then provide it to the plant cells. Ochrobactrum

anthropi TRS-2 (Chakraborty et al. 2009) isolated

from the rhizosphere of tea produces siderophore and

IAA in vitro. Pseudomonas spp. produces fluorescent

siderophores. These bacteria have biological control

and growth promoting capability.

Production of volatile compounds

Low molecular weight metabolites, such as HCN, a

biocide, with antifungal activity is released in a blend

of volatile organic compounds such as 2,3-butanediol

and acetoin that promote the growth of Arabidopsis

thaliana. In a comparison of the importance of HCN

and DAPG to P. fluorescens CHA0, HCN not

repressed by fusaric acid, played a more important

role in disease suppression than DAPG (Duffy et al.

2004). The production of HCN was a more common

trait of Pseudomonas (88.89%) (Ahmad et al. 2008).

Among other substances reported are pyoluteorin

(Baehler et al. 2006) and auxofuran (Riedlinger et al.

2006).

Release of enzymes

Fixation of N2 that is transferred to the plant

The atmosphere contains 78% nitrogen. Leguminous

plants have a symbiotic (mutually advantageous)

relationship with the bacteria that provide fixed

nitrogen. In the rhizosphere, nitrogen-fixing (diazo-

trophic) bacteria are also present. Examples of

nitrogen-fixing bacteria are Azotobacter vinelandii,

Beijerinckia derxii and Zoogloea strain Ky1

(Meunchang et al. 2006). Culturable diazotrophs

from two traditional Indian rice cultivars, Sataria and

Kartiki, are H. seropedicae, Azospirillum amazon-

ense, Burkholderia cepacia/vietnamiensis, and Pseu-

domonas spp. (Jha et al. 2009). Other bacterial genera

are Acetobacter, Arthrobacter, Alcaligenes, Bacillus,

Enterobacter, Klebsiella and Pseudomonas. These

are broadly divided into three categories, viz.,

symbiotic microorganism e.g. legume—Rhizobium

symbiosis, asymbiotic or free living e.g. Azotobacter

and associative symbiosis, e.g. Azospirillum.

Production of exo-enzymes that suppress

deleterious microbes

PGPR may synthesize some enzymes that modulate

plant growth and development. The enzymes are

usually for defence and they could be cell wall-

degrading hydrolytic enzymes. The production of

enzymes includes chitinase, cellulase, b-1,3-glucan-

ase, protease, lipase can lyse some fungal cells

(Muleta et al. 2007) and suppress deleterious rhizo-

bacteria. Examples of the best characterized defence

enzymes are peroxidase, polyphenol oxidase and

phenylalanine ammonia-lyase (Latha et al. 2009).

Organic and inorganic phosphate solubilization

Phosphorus is an essential nutrient for plants. There

are a number of microorganisms which can solubilize

the economical sources of phosphorus, for instance

rock phosphate. Among such phosphate-solubilizing

bacteria are Pseudomonas striata, Enterobacter,

Erwinia, B. megaterium and O. anthropi TRS-2

(Chakraborty et al. 2009). They solubilize the bound

phosphorus, mineralize organic phosphorus and

release soluble inorganic phosphate into soil through

decomposition of phosphate-rich organic compounds.

Production of antibiotics

Some Bacillus spp. produce antibiotics and promote

plant growth (Choudhary and Johri 2009). The

production of phenazine antibiotics contributes to

ecological fitness by competing with the resident

microflora. The modes of action of these antibiotics

are not well known but they do suppress root

diseases. O. anthropi TRS-2 (Chakraborty et al.

2009) and Bacillus sp. MEP2 18 and ARP (2) 3

exhibit antagonistic activity against some phytopath-

ogenic fungi (Principe et al. 2007). Also elaborated

are the antifungal properties of pyoluteorin, pyrrol-

nitrin and iturin.

Nematophagous bacteria

Nematophagous bacteria exhibit diverse modes of

action including promotion of plant health. They act

synergistically through the direct suppression of

nematodes, promoting plant growth, and facilitating

Biotechnol Lett (2010) 32:1559–1570 1565

123

the rhizosphere colonization and activity of microbial

antagonists in the rhizosphere.

Actinomycetes

Actinomycetes (Babalola et al. 2009), when associ-

ated to root systems, are considered rhizobacteria and

some behave as true PGPR, showing good potential

as inducer of ISR where the interaction has been

mentioned to interfere with the autoregulation of

nodulation in alfalfa (Solans et al. 2009). Streptomy-

ces successfully suppresses the growth of plant-

pathogenic fungi (Riedlinger et al. 2006). This is not

to say that all Streptomyces spp. are PGPR. The plant

growth-promoter as well as plant growth-retardant

constitutes the genus Actinomyces.

PGPR application methods

Treatments with PGPR include drench application

(Babalola et al. 2007b) and seed bacterization

(Babalola et al. 2007a; Kumar et al. 2009). Also

documented are seedling treatment (Babalola et al.

2007a), bioformulation, biopreparation and dual

treatment (Lavania et al. 2006). Coating seeds with

commercial inoculants reduces inconsistency in the

field, that notwithstanding, field-tested commercial

formulations, mostly based on dry powder (charcoal,

lignite, farmyard manure, etc.) have inherent prob-

lems of appropriate shelf-life and cell viability.

Biological control

The potential of PGPR for biological control can

result from one or more mechanisms, including

antagonism, competition, and the production of

antibiotics or siderophores or by inducing disease

resistance and/or direct growth by improving nutrient

uptake for plants by the alteration of plant hormone

levels (Anith et al. 2004). Combining multiple PGPR

types can suppress disease development in many crop

plants and protect them against a broad range of soil-

borne plant pathogens. The strategy to study root

growth is essential as it helps in increasing root

branching, root mass, root length, and/or the amount

of root hairs and therefore the creation of a greater

root surface area for nutrient absorption. To effec-

tively reduce disease symptoms, the bacteria must be

able to colonize the roots aggressively and have the

potential to dominate the ecological niche. PGPR

B. pumilus SE 34, P. putida 89B61, BioYield and

Equity Biological control has emerged as one of the

important methods in the management of soil-borne

plant pathogens (Anith et al. 2004). The dependence

on chemicals for plant disease control could be

reduced to non-economic and environmentally

friendly level.

Laboratory, screenhouse and field studies

Striga hermonthica and Orobanche are two of the

most devastating parasitic weeds. They require spe-

cific host factors to break seed dormancy, thereby

ensuring the immediate availability of a host root.

The most common germination stimulants are

sesquiterpene lactones, collectively referred to as

strigolactones, including strigol. It was earlier hypoth-

esized that Striga spp. has the same host range as

rhizospheric bacteria of the genus Azospirillum. In a

recent work, Babalola and colleagues (2007b)

describe the potential significance of PGPR for

agro-industry in promoting the growth of leguminous

crops with respect to their biomass and grain yield and

use of the PGPR to promote plant growth in Striga

infested soils and increase plant yield component.

Under optimal laboratory conditions, bacterial

cultures in a concentration equal to that in the

rhizosphere produce phytohormones to a level com-

parable to concentrations around field-grown maize

roots. In various studies, Pseudomonas, Klebsiella

oxytoca and Enterobacter sakazakki increased the

percentage germination of S. hermonthica over

the non-inoculated control (Babalola et al. 2007a).

The study demonstrated that there is some potential

in certain rhizosphere bacteria to induce the suicidal

germination of S. hermonthica.

Bacterial isolates were grown on Tryptone Soy

Agar (TSA) and King’s medium B (KB). All cultures

were grown for 48 h at 28�C on appropriate agar

plates as stated above. Individual plates were har-

vested by adding 10 ml of 0.01 M phosphate buffer

(PBS, pH 7.0) solution and scrapping with a sterile

cotton swab. The harvested bacteria were pooled

together according to the bacterial type, washed

twice, and adjusted to the desired concentration by

optical density. The bacteria were serially diluted in

buffer and plate counts were made on the inoculum

prior to inoculation. The average colony forming

1566 Biotechnol Lett (2010) 32:1559–1570

123

units (c.f.u.) from triplicate plates was determined by

counting after 48 h incubation, this was used to

express the quantity of inoculum added. A 250 ml,

24 h culture of each bacterial cell, free from culture

filtrate was drenched on the non-sterilized potted soil

in the screenhouse. Pots that received only water

served as the control. The results suggest that PGPR

were able to promote maize growth (Fig. 1) even in

S. hermonthica infested soil (Fig. 2). In the case of

Fig. 2, the soil was passed through a 10-mesh sieve

and thoroughly mixed with S. hermonthica seeds

within the first 0–10 cm depth at 120 seeds per kg

soil 14 days before planting. Growth promotion may

be as a result of the production of plant growth

producing substances, the suppression of deleterious

bacteria, improved soil structure due to PGPR, or the

alleviation of Striga induced stress on the maize host.

The yield effect of PGPR was plant cultivar-

dependent (Babalola et al. 2007b) (Table 2). The

application of bacterial inoculants as biofertilizers

has improved the growth and yield of cereal crops

(Babalola et al. 2007b). All the bacteria affected crop

growth similarly, frequently increasing seed germi-

nation rates, elongating stems, enlarging leaves and

causing early flowering and fruiting effects indicative

of a common cause, related not to mineralization but

to growth regulators. The bacterial inoculants were

more effective when used in conjunction with

mineral and organic fertilizers (Kumar et al. 2009)

and worked best in soils supporting increased plant

and microbial growth. Tests by various workers for

the production of growth regulators by the bacteria

used as inoculants have shown that all of them

produce traces of IAA (Chakraborty et al. 2009) and

gibberellin-like substances, sufficient to cause

changes in plant morphology, for only trace amounts;

if applied at the right stage of plant development, will

change growth completely.

To study whether PGPR also has a beneficial

effect on plants under Striga infestation, the growth

patterns of maize were investigated. The PGPR

E. sakazakii and Pseudomonas spp. promoted a

significant increase in the associated trap crop,

cowpea (Vigna unguiculata), pod weight, fresh

biomass, pod number and pod wall thickness (Babal-

ola et al. 2007b). The PGPR reduce the debilitating

effects of the root parasitic weed, S. hermonthica, on

host plant maize (Babalola et al. 2007a). The use of

bacteria for the biological control of S. hermonthica

is relatively new (Babalola et al. 2002). In the field,

results from the use of PGPR have been less

consistent and their ineffectiveness has often been

attributed to their inability to colonize plant roots.

This inconsistency in PGPR field results might be due

to many factors such as complex interactions among

hosts, rhizobacteria, pathogens, and climate. The

soil’s environmental conditions, e.g., texture, water

content, soil temperature and pH value could also

contribute to the inconsistency reported.

Fig. 1 A typical large pot screenhouse experiment with or

without added PGPR to maize planted in Striga-free soil.

Plantings were done in screenhouse potted soil. The soil was

not sterilized. A mixture of bacterial cells from 24-h-old

culture resuspended and washed clean of culture media were

drenched into the soil. Pots which did not receive bacterial

cells but only water, served as control

Fig. 2 A typical large pot screenhouse experiment of presence

or absence of added PGPR in maize planted in S. hermonthica-

infested soil

Biotechnol Lett (2010) 32:1559–1570 1567

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Conclusions and future perspectives

This review has shown that PGPR are free-living

bacteria of beneficial importance to agriculture and

abound in the rhizosphere, the region around the root.

Some PGPR have more than one mechanism of

accomplishing increased plant growth, such as the

production of enzymes, bioactive factors, antibiotics,

metabolites as well as growth promoters. The mech-

anisms of action presented are not an all-encompass-

ing list. Although variability in field performance is

common, PGPR are environmentally friendly, unlike

the overuse of chemical fertilizers. Certain bacteria

containing ACC deaminase can show both patho-

genic and growth-promoting properties in their

interaction with plants. Inconsistency in experimental

results lies as a result of fluctuations in environmental

conditions but role of biological factors cannot be

ruled out. Chemical fertilizers increase yield in

agriculture but are expensive and harm the environ-

ment. They deplete non-renewable energy via side

effects, such as leaching out, and polluting water

basins, destroying micro-organisms and friendly

insects, making the crop more susceptible to the

attack of diseases, reducing soil fertility, thereby

causing irreparable damage to the overall system.

The use of PGPR could be a better alternative to

chemical fertilizers. They are economical, not harm-

ful to the environment and could easily be found.

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