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ISSN: 2276-7770
Association of
Bacteria with Plants: A
Potential Gateway to
Sustainable Agriculture
Prabhat N Jha
Garima Gupta
Prameela Jha
Rajesh Mehrotra
Greener Journal of Agricultural Sciences ISSN: 2276-7770 Vol. 3 (2), pp. 073-084, February 2013. 73
Research Article
Association of Rhizospheric/Endophytic Bacteria with
Plants: A Potential Gateway to Sustainable
*Prabhat N. Jha, Garima Gupta, Prameela Jha and Rajesh Mehrotra
Department of Biological Sciences, Birla Institute of Technology and Science, Pilani-333031 Rajasthan, India
*Corresponding Author’s Email: Tel.: +91 1596 245073 273; Fax: +91 1596
Application of associative bacteria for sustainable agriculture holds immense potential. These bacteria are known to
enhance growth and yield of plants by fixing atmospheric nitrogen, solubilization of phosphate, production of
phytohormones and siderophores, possession of antagonistic activity as well as reducing the level of stress ethylene in
host plants. Colonization of these bacteria can be tracked by tagging them with certain molecular markers such as β-
glucuronidase (gus) or green fluorescent protein (gfp) followed by electron microscopy or laser scanning confocal
microscopy. Associative bacteria and endophytes may express genes differentially to colonize and establish the plant
interior. They may also use ‘quorum sensing’ molecules for colonization process. Present review aims to highlight
various plant growth promoting properties, ecology and updates of molecular mechanisms involved in interaction
between associative bacteria and plants as well as immune responses triggered by these bacteria in plants.
Keywords: Associative bacteria, endophyte, diazotrophy, biocontrol, induced systemic tolerance, induced systemic
The over increasing population of the world has already touched the number of 6.8 billion. To feed this burgeoning
population, farmers heavily rely on the use of chemical fertilizers especially inorganic nitrogen. Application of
inorganic fertilizer has many repercussions, as it leads to ground and surface water contamination due to leaching
and denitrification, which is detrimental for human and animal health. Secondly, manufacturing of industrial nitrogen
fertilizer uses non-renewable resources like natural gas and coal and causes production of green house gases viz.,
CO2 and NO2 contributing to global warming (Bhattacharjee et al., 2008). Therefore, it’s high time to opt for
alternative fertilizers which can be used in sustainable agricultural practices without affecting the environment.
Application of plant growth promoting associative bacteria can be a potential option for enhancing growth and yield of
plant in sustainable manner.
On the basis of area of colonization, Plant Associated Bacteria (PAB) can be grouped into associative
bacteria that include rhizospheric (in vicinity of root) and rhizoplanic (on surface of root) bacteria and, endophytic
bacteria. Term ‘endophytic bacteria’ is referred to those bacteria, which colonizes in the interior of the plant parts, viz,
root, stem or seeds without causing any harmful effect on host plant (Hallmann et al., 1997). These bacteria may
promote plant growth in terms of increased germination rates, biomass, leaf area, chlorophyll content, nitrogen
content, protein content, hydraulic activity, roots and shoot length, yield and tolerance to abiotic stresses like draught,
flood, salinity etc. PAB can promote plant growth directly through Biological Nitrogen Fixation (BNF), phytohormone
production, phosphate solubilization, inhibition of ethylene biosynthesis in response to biotic or abiotic
stress (induced systemic tolerance) etc., or indirectly through inducing resistance to pathogen
(Bhattacharya and Jha, 2012). Present review aims to focus on plant growth promoting abilities of rhizospheric and
endophytic bacteria and their molecular aspects. PAB has been classified as the plant growth promoting bacteria on
the basis of basic mechanisms through which it stimulates plant growth as PGPB, which induces plant growth directly
and; bio-controller, which protects plants by inhibiting growth of pathogen and/or insect (Fig. 1) (Backman and
Greener Journal of Agricultural Sciences ISSN: 2276-7770 Vol. 3 (2), pp. 073-084, February 2013. 74
Sikora, 2008). In the present review, discussion regarding PGPB has excluded rhizobia associated with leguminous
Figure 1: Properties of associative/endophytic bacteria for plant growth improvement. Based on the properties,
associative/endophytic bacteria have been classified as Plant Growth Promoting Bacteria (PGPB) and biocontrol
bacteria. PGPB may benefit associated plants through providing nutrition (nitrogen, phosphorous and iron),
production of plant hormone and may enable plant tolerate abiotic stressors. Biocontrol bacteria (right panel in figure)
protect plants from invasion of pathogenic microorganisms through antagonism and/or induced systemic resistance.
Plant Growth Promoting Bacteria
Associative bacteria as well as endophytic bacteria use same mechanisms to influence plant growth. However, they
differ in efficiency through which they exert their beneficial effect. Based on various properties, plant growth
promoting bacteria can be classified as biofertilizers, rhizoremediators, phytostimulators and stress controllers.
Bacterial fertilizer is referred to the bacteria that supply nutrition to the associated plant. They may benefit plants by
providing utilizable nitrogen through fixation of atmospheric nitrogen or they make free phosphate available from
insoluble source of phosphate. Plant growth promotion due to solubilization of zinc compound driven by
Gluconoacetobacter has also been reported Beneficial properties of these bacteria are described below in brief
(Lugtenberg and Kamilova, 2009).
Biological Nitrogen Fixation: Many associative and endophytic bacteria are now known to fix atmospheric nitrogen
and supply it to the associated host plants. A variety of nitrogen fixing bacteria like Arthrobacter, Azoarcus,
Azospirillum, Azotobacter, Bacillus, Beijerinckia, Derxia, Enterobacter, Gluconoacetobacter, Herbaspirillum,
Klebsiella, Pseudomonas, Serratia and Zoogloea have been isolated from the rhizosphere of various crops, which
contribute fixed nitrogen to the associated plants. For instance, contribution of 20 Kg N ha-1 by Azotobacter paspali
was demonstrated using 15N dilution technique (Baldani and Baldani, 2005; Reinhold-Hurek and Hurek, 2011). In
recent years, application of endophytic bacterial inoculants supplying N requirement efficiently to the various host
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plants including cereal crops have drawn attention for increasing plant yield in sustainable manner. Additionally,
some of the rhizobial isolates have also been found to colonize non-legume plant as an endophyte and benefit the
associating host (Rothballer et al., 2008). In terms of benefiting through nitrogen fixation, endophytic bacteria are
considered to be better than that of rhizospheric one as they provide fixed nitrogen directly to their host plant and fix
nitrogen more efficiently due to lower oxygen pressure in the interior of plants than that of soil.
When diazotrophic bacteria establishes endophytic association with plants, total content of plant nitrogen
rises which may be due to the biological nitrogen fixation or increased ability of nitrogen uptake from soil. In a well-
organized study in Brazil suggested that 60-80% of the accumulated nitrogen in different varieties of sugarcane
namely, CB45-3, SP70-1143 and Krakatau, was contributed by BNF (Boddey, 1995). Combination of nitrogen-fixing
bacteria (viz.,Rhizobium. trifolii and Burkholderia MG43) and reduced amount of chemical fertilizer can achieve
overall yield equivalent to the yield that was obtained from recommended full dose of chemical fertilizer
(Bhattacharjee et al., 2008). Gluconoacetobacter diazotrophicus is the main contributor of endophytic BNF in
sugarcane, which according to nitrogen balance studies fix as high as 150 Kg N ha-1yr-1 (Muthukumarasamy et al.,
2005). However, contribution of BNF to host may vary with the genotype of host. Proteomic analyses of sugarcane
variety SP70-1143 grown with G. diazotrophicus revealed up-regulated expression of ammonia lyase which indicates
increased metabolism resulted from increased uptake of nitrogen contributed by bacteria (Lery et al., 2011). Up-
regulation of genes for nitrogen metabolism during plant-bacteria interaction was also evident in differential gene
expression studies carried out earlier (Nogueira et al., 2001). Another nitrogen-fixing endophyte of considerable
interest is Azoarcus. This diazotroph inhabits the roots of kallar grass (Leptochloa fusca), which yields 20-40 t of hay
ha-1 yr-1 without the addition of any N fertilizer in saline sodic, alkaline soils having low fertility (Ladha and Reddy,
2000). Percent contribution of plant nitrogen as a result of BNF by few associating endophytic bacteria has been
given in table 1.
TABLE 1: Contribution of biological nitrogen fixation by associative/endophytic bacteria
*Nitrogen derived from air
At the molecular level, role of endophytic bacteria supplying fixed nitrogen to host was ascertained using non-
nitrogen fixing Klebsiella pneumoniae where the rice plants inoculated with non- nitrogen fixing K. pneumoniae in
nitrogen-deficient media showed signs of nitrogen deficiency on the contrary to the wild type counterpart (Iniguez et
al., 2004). Nitrogen-fixation ability of endophytic bacteria ex-planta or in-planta is measured or detected on the basis
of nif genes, encoding nitrogenase enzyme or by immunological detection of nitrogenase using antibody raised
against nitrogenase enzyme (Nogueira et al., 2001). Presence of structural genes namely nifH or nifD in associative
as well as endophytic bacteria have been detected by polymerase chain reaction using pair of universal primers (Jha
and Kumar, 2009; Reinhold-Hurek and Hurek, 2011). Expression of nif genes has also been demonstrated by
reverse transcription PCR (RT-PCR) from plants inoculated with Azoarcus BH72 and plants growing in field and in
other associative diazotrophic bacteria (Terakado-Tonooka et al., 2008; You et al., 2005).
Phosphate Solubilization: Phosphate is known to be the second most limiting compound for plant growth. Although
most of the soil is rich in phosphate but they are in insoluble form and cannot be utilized by plants or other soil
organisms. A vast number of PGPB with phosphate solubilizing property have been reported which include members
belonging to Burkholderia, Enterobacter, Pantoea, Pseudomonas, Citrobacter and Azotobacter (Park et al., 2010).
Some plant growth promoting bacteria solubilize phosphate from organic or inorganic bound phosphates and
Endophytic bacteria
Rhizobium leguminosarum
Rice 19 to 28 Yanni et al., 1997; Biswas
et al., 2000
Rice 31 Baldani and Baldani, 2005
Rice 19-47 Ladha and Reddy, 2000
Rice 19-47 Ladha and Reddy, 2000
G. diazotrophicus, H. seropedicae, H.
rubrisubalbicans, A. amazonense and
Burkholderia sp
Sugarcane 29 Oliveira et al., 2002
K. pneumoniae
Rice 42 Iniguez et al., 2004
Burkholderia vietnamiensis
Rice 40-42 Govindrajan et al., 2008
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facilitate plant growth. Possible mechanisms for solubilization from organic bound phosphate involve either enzymes
namely C-P lyase, non- specific phosphatases and phytases. However, most of the bacterial genera solubilize
phosphate through the production of organic acids such as gluconate, ketogluconate, acetate, lactate, oxalate,
tartarate, succinate, citrate and glycolate (Khan et al., 2009). Type of organic acid produced for P solubilization may
depend upon the carbon source utilized as substrate. Highest P solubilization has been observed when glucose,
sucrose or galactose has been used as sole carbon source in the medium (Khan et al., 2009; Park et al., 2010).
Genetics and biochemical basis of acid secretion specifically gluconic acid in bacteria such as Erwinia herbicola,
Pseudomonas cepacia and Enterobacter asburiae have been reviewed by Rodriguez et al. (Rodrıguez et al., 2006).
Production of gluconic acid results from the conversion of glucose to gluconic acid by an enzyme glucose
dehydrogenase (Gcd). Gcd is a cell-envelope bound enzyme which depends on cofactor pyrroloquinoline quinine
Production of Phytostimulating Compounds
PGPB exert its effects through the production of substances which stimulate plant growth. These substances include
phytohormones namely auxins, cytokinins, gibberellins, certain volatiles and the cofactor pyrroquinoline quinine
(PQQ). Several associative bacteria have been shown to produce auxins chiefly IAA, which enhances lateral root
growth formation and thus increase nutrient uptake by plants and root exudation, which in turn stimulates bacterial
colonization and thus amplify the inoculation effect. Plant growth promotion as a result of IAA has been documented
in several plants in recent years (Spaepen et al., 2007). However, beneficial effects of bacterial IAA depend upon the
optimum concentration, which may vary for different plants. The role of phytohormone produced by associative
bacteria in the promotion of plant growth during stress conditions such as salinity or draught has also been
demonstrated recently (Egamberdieva, 2009). Since, indigenously produced phytohormone in plants declines in salt
stress condition, salt tolerant associative bacteria may enhance plant growth by supplying phytohormones
synthesized by them. Similarly, IAA producing bacteria may enhance growth of plant in drought condition by
stimulating formation of well- developed root system enough for providing sufficient water from soil. Moreover, the
role of IAA in response to stress is evident from its increased production of IAA in Azospirillum sp. during carbon
limitation and acidic pH (Spaepen et al., 2007).
In addition to IAA, some of the associative bacteria have ability to produce other phytohormones such as
cytokinin and gibberellin. Cytokinin produced by Bacillus megatarium UMCV1, a rhizospheric bacterium, was found
to promote biomass production in Arabidopsis thaliana through the inhibition of primary root growth followed by
increased lateral root formation and root hair length of host plant (López-Bucio et al., 2007). Interestingly, few
isolates are capable of producing more than one phytohormone. Moreover, few bacteria namely B. subtilis, B.
amyloliqufaciens and E. cloacae promote plant growth through the production of volatile organic compounds (VOCs)
such as acetoin and 2,3-butanediol. VOCs of PGPR were found to enhance plant growth by regulating auxin
homeostasis in plants which was evident from induction of genes encoding enzymes of metabolism of IAA (Zhang et
al., 2008).
Induced Systemic Tolerance
A few PGPB enable the associating plants to tolerate abiotic stresses such as drought, salt, nutrient deficiency or
excess, extremes of temperature and, presence of toxic metals. Thus, physical and chemical changes in plants
resulted from PGPB-induced tolerance to abiotic stresses has been termed recently as ‘Induced Systemic Tolerance’
(IST). IST is elicited through the production of bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase,
antioxidants, cytokinin or VOCs (Fig. 2).
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Figure 2: Various components of induced systemic tolerance. PGPB may help associated plant reduce effect of
stressors through reducing level of stress ethylene due to presence of ACC deaminase activity, release of
antioxidants, volatile organic compounds and plant hormone cytokinin.
PGPB equipped with ability to synthesize ACC deaminase (ACCD) reduce level of stress ethylene produced in plants
in response to various biotic and abiotic stressors. ACCD degrades ACC, an immediate precursor of ethylene, to α-
ketobutyrate and ammonia (Yang et al., 2009). In addition to ACC deaminase mediated IST, other mechanisms also
exist to confer IST in response to stresses. In salt stress, level of Na+ elevates, which decreases plant growth and
productivity. The ion transporter high-affinity K+ transporter 1 (HKT1) regulates Na+ import in roots. VOC of Bacillus
subtilis GB03 confer salt tolerance by down- and up-regulating HKT1 in roots and shoots respectively, and result in
low Na+ accumulation throughout the plant in comparison to control. Other PGPB mediated IST include production of
cytokinin which affects abscicic acid (ABA) signaling of plants during stress and augmented production of antioxidant
catalase (Yang et al., 2009).
Bacteria with the ability to degrade organic pollutant can be used for remediation of soil. Although pollutant degrading
bacteria characterized in laboratory environment may not thrive well in pollutant rich natural environment due to
requirement of energy for primary metabolism. Aforementioned problem can be overcome with the use of associative
and endophytic bacteria possessing ability to degrade soil pollutant. Since, PGPR colonizes in rhizosphere or
rhizoplane; they obtain their source of energy from root exudates for primary metabolism and degrade efficiently
organic xenobiotics present in the vicinity. For instance, P. putida PCL1444 effectively utilizes root exudates,
degrades naphthalene around the root, protects seeds from being killed by naphthalene, and allows the plant to grow
normally. Similarly, in-situ inoculation of P. putida W619-TCE reduced evapotranspiration of trichloroethylene by 90%
under field condition (de Bashan et al., 2012). In a recent report, endophytic bacteria isolated from seeds of Nicotiana
tabacum has been found to be potential candidate for reducing cadmium phytotoxicity (Mastretta et al., 2009).
Application of endophytic bacteria for degrading the pollutants like petroleum, toluene and other organic solvent as
well as protecting the plants from metals is of significant importance. In addition, endophytic bacteria engineered with
genes encoding enzymes for degradation of pollutants can be better exploited for remediation of soil (de Bashan et
al., 2012).
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World agriculture faces a great loss every year incurred from infection by pathogenic organisms. Application of
microorganism for the control of diseases seems to be one of the most promising ways. Biocontrol systems are eco-
friendly, cost-efficient and involved in improving the soil consistency and maintenance of natural soil flora. To act
efficiently, the biocontrol agent should remain active under large range of conditions viz., varying pH, temperature
and concentrations of different ions. Biocontrol agents limit growth of pathogen as well as few nematodes and
insects. Biocontrol bacteria can limit pathogens directly by producing antagonistic substances, competition for iron,
detoxification or degradation of virulence factors; or indirectly by inducing Systemic Resistance (ISR) in plants
against certain diseases, signal interference, competition for nutrients and niches and interference with activity,
survival, germination and sporulation of the pathogen (Lugtenberg and Kamilova, 2009).
Antagonism: Associative/endophytic bacterial biocontrol agents may inhibit growth of fungal pathogens by one or
more of the several mechanisms, which include production of antibiotics, siderophore and lytic enzymes.
A vast array of antagonistic chemical compounds has been identified in bacterial biocontrol agents. Gram negative
biocontrol agents such as Pseudomonas produce HCN, pyoleutorin (PLT), pyrrolnitrin (PRN), 2,4-
diacetylphloroglucinol (2-DAPG) and phenazines (PHZ) chiefly phenazine-1-carboxylic acid and phenazine-1-
carboxamide Lugtenberg and Kamilova, 2009). The Role of each antibiotic produced by bacterial biocontrol agent in
conferring control of fungal pathogen may vary in different species. Control of Sclerotinia sclerotiorum by P.
chlororaphis PA23 is primarily executed by PRN while PHZ (phenazine-1-carboxylic acid, 2-hydroxyphenazine) helps
in the development of biofilm formation (Selin et al., 2010). On the contrary, PHZ (phenazine-1-carboxamide)
produced by P. chlororaphis strain 1391 was identified to be responsible for controlling tomato fruit and root rot
caused by Fusarium oxysporum f. sp. radicis-lycopersici. Few other biochemicals having pathogen inhibiting activity
include gluconic acid, 2-hexyl-5-propyl resorcinol, munumbicin, and few VOCs (2,3-butanediol) produced by
biocontrol agent (Backman and Sikora, 2008). The level of antibiotic synthesis depends upon nutritional factors viz.,
type of carbon source utilized, trace elements and availability of other nutrients as well as non-nutritional factors like
environmental influences. Regulation of antibiotic production in biocontrol bacterial agents involves GacA/GacR or
GrrA/GrrS, RpoD, and RpoS, N-acyl homoserine lactone (AHL) derivatives, and positive auto regulation (Compant et
al., 2005).
Under iron-limiting condition, some of the biocontrollers secrete siderophore, which chelates available iron of
the soil and sometime from cohabiting microorganism, and deprive pathogenic fungi from this element (Compant et
al., 2005). In addition to the role of siderophore in biocontrol, bacterial siderophore has been implicated in iron
nutrition of crop plants and heavy metal phytoextraction. Production of siderophore by diazotrophic bacteria seems
physiologically more important since the role of catecholate type of siderophore has been implicated in transport of
Mo under iron starved condition in Azospirillum lipoferum. Because nitrogen-fixing bacteria require both iron and Mo
for the activity of nitrogenase, the role of siderophore seems pivotal for any diazotrophic bacteria especially under
iron deficiency (Rajkumar et al., 2010).
Bacteria may limit growth of other microorganisms also through the production of hydrolytic enzymes such as
chitinase, β-1, 3-glucanase, protease and, laminarinase etc. For instance, Serratia marcescens and Paenibacillus sp.
secrete chitinase to exert antifungal activity against Sclerotium rolfsii and Fusarium oxysporum f. sp. cucumerinum
respectively. Bacillus cepacia destroys Rhizoctonia solani, R. rolfsii, and Pythium ultimum through the production of
β-1, 3-glucanase. Secretion of protease and chitinase was found to be the possible mechanism for antagonistic
activity of endophytic bacteria Enterobacter and Pantoea against fungal pathogen Fusarium oxysporum f.sp.
vasinfectum (Backman and Sikora, 2008; Compant et al., 2005).
Induced Systemic Resistance: Certain bacterial interactions with root enables the associated plant to develop
resistance against potent pathogens. This phenomenon is termed as Induced Systemic Resistance (ISR) and has
been noted to be exhibited by both associative and endophytic bacteria (Table 2) (van Loon, 2007). It was first
noticed in carnation and cucumber where inoculation with selected PGPB (rhizobacteria) reduced susceptibility to wilt
and foliar disease respectively. In contrast to many biocontrol mechanisms, extensive colonization of the root system
is not required for ISR to be exerted (Lugtenberg and Kamilova, 2009).
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TABLE 2: Biocontrol activity of associative/endophytic bacteria
The bacterial products that elicit induction of ISR are diverse and can induce in plants which possibly possess
receptor for respective ligands. These elicitors may be lipopolysaccharides, flagella, siderophores, antibiotics, VOCs
or quorum-sensing signals. Majority of ISR activated by PGPB is mediated by jasmonate or ethylene (van Loon,
2007). Mechanisms of ISR in Pseudomonas has been reviewed recently (Jankiewicz and Kołtonowicz, 2012). In a
recent study, plant growth promoting Bacillus cereus AR156 was found to trigger ISR in A. thaliana through SA- and
JA/ET-signaling pathways in an (Non-expressor of PR1) NPR1-dependent manner (Niu et al., 2011). Development of
ISR may induce various genes to strengthen the host plant mechanically or metabolically. It involves fortification of
plant cell wall strength, alteration of host physiology or metabolic responses and, enhanced synthesis of plant
defense chemicals such as phenolic compounds, pathogenicity related protein (PR-1, PR-2, PR-5), chitinases,
peroxidases, phenyl alanine ammonia lyase, phytoalexins, oxidase and/or chalcone synthase. These metabolic
products protect the host plant from future infections from pathogens. Local immune response induced by PGPR has
also been demonstrated in few studies. However, pattern of local immune response depends on genotype of plants
and respective bacterial species associated with them (Compant et al., 2005).
Biocontrol against Nematode: Few rhizobacteria acting as a biological control agent against plant-parasitic
nematodes have also been reported (Tian et al., 2007). Antagonistic activity by aerobic endospore-forming bacteria
(AEFB) (mainly Bacillus spp.) and Pseudomonas spp against nematodes is well known. It is mainly exerted by the
means of metabolic by-products, enzymes and toxins including 2, 4-DAPG (P. fluorescens), hydrogen sulphide,
chitinase, and hydrogen cyanide.
Colonization of bacteria in rhizosphere or on plant surface is a complex process, which involve interplay between
several bacterial traits and genes. The colonization is multi-step process and includes (a) migration towards root
surface, (b) attachment, (c) distribution along root and (d) growth and survival of the population. For endophytic
bacteria one additional step is required that is entry into root and formation of microcolonies inter-or intracellularly.
Each trait may vary for different associative/endophytic bacteria. Colonization of bacteria is traced by tagging the
putative colonizing bacteria with a molecular marker such as auto fluorescent marker (e.g., green fluorescent protein
(gfp)) or β-glucosidase (gus) followed by microscopy (electron or confocal laser scanning microscopy) (Reinhold-
Hurek and Hurek, 2011). Fluorescent in-situ hybridization with real time PCR analysis can also be used for tracking
bacterial colonization and its quantification (Lacava et al., 2006). Understanding of molecular mechanism involved in
associative or endophytic colonization process is not well understood. Recent reports based on the genomic data
and other similar reports have suggested resemblance of colonization methods between pathogenic bacteria and
PGPB (Hardoim et al., 2008).
Endophytic Isolates
ogenic Fungi/Bacteria
P. fluorescens
Colletotrichum falcatum
Burkholderia phytofirmans
Botrytis cinerea
Burkholderia phytofirmans
Verticllium dahlia
P. Denitrificans
Ceratocystis fagacearum
P. puti
Ceratocystis fagacearum
P. fluorescens
F. oxysporum
f. sp.
P. fluorescens
Pythium ultimum
F. oxysporum
f. sp.
Bacillus pumilus
F. oxysporum
f. sp.
Bacillus pumilu
F. oxysporum
f. sp.
Sp. Strain ORS278
transcriptome analysis based study
Paenibacillus alvei
A. thaliana
Verticillium dahlia
A. thaliana
Quantitative PCR analysis based study
Bacillus cereus
A. thaliana
Pseudomonas syringae
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Root Colonization
Root colonization is the first and the critical step in establishment of plant-microbe association. Microorganisms move
towards rhizosphere in response to root exudates, which are rich in amino acids, organic acids, sugars, vitamins,
purines/ pyrimidines and other metabolic products. In addition to providing nutritional substances, plants start cross-
talk to microorganisms by secreting some signals which cause colonization by some bacteria while inhibits the other
(Bais et al., 2006; Compant et al., 2011). The patterns of chemoattractant especially organic acids may vary in
different isolates/strains. Malate, succinate and fructose are considered to be the strongest chemoattractants.
Exudate composition is in turn influenced by physiological status of plant, the presence of microbes and products
from rhizobacteria such as phenazines, 2,4-DAPG, zearalenone and exopolysaccharide. Sloughed up root cap cells
also have large impact on plant-microbe interaction. In addition to chemotaxis, electrotaxis (electrogenic ion transport
at the root surface) is also considered as a possible mechanism for initiating rhizobacterial colonization. Root hair
regions and emergence points are preferred site for colonization (Lugtenberg and Kamilova, 2009).
Colonization of root by microorganism may further induce release of exudates, and create ‘biased’
rhizosphere by exudating specific metabolic products. In some rhizospheric bacteria, root exudates induce flagellar
motility that leads their colonization on plant surfaces. During root colonization process, movement of associative
bacteria is followed by their adhesion on plant root which may be mediated by glycosylated polar flagellum, Role of
bacterial major outer membrane protein (MOMP) in early host recognition has been recognized in earlier report,
where MOMPs from Azospirillum brasilense showed stronger adhesion to extracts of cereals than extracts of
legumes and tomatoes. It suggests involvement of MOMPs in adhesion, root adsorption and cell aggregation of the
bacterium (Lugtenber and Kamilova, 2009). On the other hand, involvement of type IV pili and twitching motility has
been identified in tomato root colonization by Pseudomonas using pilA and pilT mutant, pilA is the gene encoding
prepilin, structural component of type IV pili and pilT encodes for protein required for pilus contraction that is
responsible for twitching motility (Lugtenberg and Kamilova, 2009; Reinhold-Hurek and Hurek, 2011). Preston et al.
(2001) identified SSIII secretion system III (SSIII) (hrp) in P. fluorescens SBW25 that is by in-vitro expression
technology (IVET), a promoter trapping technique. Moreover, role of two component regulatory system ColR/ColS in
competitive root colonization in P. fluorescence has been demonstrated. ColR/ColS system regulates
methyltransferase/WapQ operon, and thus maintains the integrity of outer membrane for efficient colonization (de
Weert et al., 2009).
Endophytic Colonization
Primary mechanism for colonization of endophytic bacteria is similar to that of associative one. Twitching motility and
type IV pile were found to be essential for successful colonization of Azoarcus, obligate endophytic bacteria (Böhm et
al., 2007; Reinhold-Hurek and Hurek, 2011). In addition, Bilal et al. (1993) suggested that cell-surface protein and
Ca2+ dependent twitching motility may be implicated in specific interaction with plants. Chemical composition of
lipopolysaccharides (LPS) present on the surface of bacteria might be determinative for successful colonization in
host plants (Serrato et al., 2010). Requirement for plant signal such as flavonoid present in root exudates of host
plant was also observed for stimulation of endophytic colonization of wheat and Brassica napus plants by
Azospirillum brsilense and A. caulinodans respectively (Lugtenberg and Kamilova, 2009).
Majority of natural isolates associated with plants form biofilm in the rhizosphere, on the surface of plant as
well as in the endorhizosphere. LapA (large adhesion protein A), a cell surface protein, or its homologue is supposed
to be putative adhesion needed for the adhesion of Pseudomonads on plant roots (Lugtenberg and Kamilova, 2009).
Entry of endophytic bacteria in plant roots is known to occur (a) through wounds particularly where lateral or
adventitious roots occur; (b) through root hairs and (c) between undamaged epidermal cells (Harodoim et al., 2008).
Chi et al. (2005) demonstrated that the colonization of gfp-tagged rhizobia in crop plants begin with surface
colonization of the rhizoplane at lateral root emergence, followed by endophytic colonization within roots, and then
ascending endophytic migration into the stem base, leaf sheath, and leaves where they develop high populations.
Azospirillum may also colonize endophytically through wounds and cracks of the plant root (Preito et al., 2011;
Reinhold-Hurek and Hurek, 2011).
Endophytic bacteria may colonize root tissues and spread actively in aerial parts of plants through
expressing moderate amount of degradative enzymes such as pectinases and cellulases. Utilization of aforesaid
enzymatic activities for colonization by Azospirillum irakense, Azoarcus sp. and others has been demonstrated as
one of the efficient methods to get entry into the host plant. Endoglucanase is one of the major determinants for the
colonization of endorhizosphere, which was evident from the observation that Azoarcus strain lacking endoglucanse
was not effective in colonizing the rice plants. The endoglucanase loosen larger cellulose fibers, which may help
entering to the plant. A homologue of endoglucanase gene has also been identified in P. stutzeri A1501, which
Greener Journal of Agricultural Sciences ISSN: 2276-7770 Vol. 3 (2), pp. 073-084, February 2013. 81
occasionally colonizes cortex of crop plants. In addition to endoglucanse, exoglucanases may also help in
colonization process. An exoglucanase having cellobiohydrolase and β- glucosidase activity was identified to be key
player in colonization process of Azoarcus sp. BH72 (Reinhold-Hurek and Hurek, 2011). In Elaegnus and Mimosa,
the endophyte penetrates the radial walls presumably by digesting the middle lamella and then proceeds between
cells and through intercellular spaces. In contrast to above examples, genes encoding plant cell wall degrading
enzymes has not been found in endophytic bacteria Herbaspirillum seropedicae strain SmR1 (Pedrosa et al., 2011).
Azoarcus sp., an obligate endophyte of Kallar grass, has been critically studied by using transposon mutant
expressing β-glucuronidase (GUS) constitutively as a reporter gene (in Azoarcus sp. BH72). Azoarcus sp. BH72
colonize apical region of roots behind the meristem intensively and penetrate the rhizoplane preferentially in the zone
of elongation and differentiation. They colonize in the cortex region both inter- and intracellularly. In older parts of the
roots, they also occur in air spaces. Azoarcus sp. is capable of invading even the stele of rice and xylem vessels
suggesting systemic spreading into shoots through the transport in vessels (Hurek and Reinhold-Hurek, 2003). On
the contrary, shoot colonization of Gramineae appears to be more pronounced by G. diazotrophicus and H.
seropedicae (Jha et al., 2004). Furthermore, Compant and associates reported colonization of endophytic bacteria
Burkholderia phytofirmans in epidermis and xylem of even reproductive organ of grapevine. In another study Preito
and associates suggested that endophytic bacteria are confined within an organelle most likely vacuole which arises
by narrowing of an internal membranous structure in roots (Preito et al., 2011).
Endophytic colonization is not as specific as of Rhizobia but successful endophytic colonization does involve
a compatible host plant (Ryan et al., 2008). However, endophytic colonization indeed depends upon the physiological
changes in plants and is restricted or slowed down by defense mechanism (Rosenblueth and Martínez-Romero,
2006). Colonization of G. diazotrophicus was found to be diminished in plants grown under high nitrogen fertilizer
regime. This reduction in colonization was explained as a result of altered plant physiology in the presence of
nitrogen fertilizer, which reduces sucrose concentration to be utilized by endophytic bacteria. Influence of organic
amendment on endophytic population has also been demonstrated (Hallman et al., 1997). Plant defense response
plays critical role in regulating colonization of endophytic bacteria. In dicotyledonous plants, salicylic acid (SA) and
ethylene restricts endophytic colonization. Ethylene, a signal molecule of ISR in plants decreases endophytic
colonization as observed in Arabidopsis thaliana inoculated with K. pneumoniae 342 (Iniguez et al., 2005). However,
proteomic approach used to study colonization by bacteria indicated that jasmonic acid, not ethylene and SA,
contribute in restricting endophytic colonization in grasses (Miché et al., 2006). Expression of jasmoic acid (JA)
induced PR proteins (defense proteins) depends upon the compatibility of plant variety and endophytic bacteria.
Antimicrobial peptides synthesized by some plants like rice and maize may reduce endophytic colonization (Hurek
and Reinhold-Hurek, 2003). Understanding of molecular mechanism and conditions limiting the colonization process
need to be elucidated for exploiting the beneficial endophytic or associative interaction with plants.
Future Prospects and Challenges
A thorough exploration of associative/endophytic bacteria and their obvious abilities to enhance plant growth and
productivity indeed indicate the existence of natural associations of these bacteria and their beneficial impact which
can be exploited to feed burgeoning population of the world. Despite the fact that a large number of associative and
endophytic bacteria have shown plant growth promoting properties at laboratory and green house level, these
bacteria fail to exhibit consistent performance under natural conditions. The factors that affect colonization and thus
PGPB derived benefit to plant may be soil type, nutritional status of soil, host plant genotype and age as well as
climatic conditions (Bhattacharya and Jha, 2012). High amount of available utilizable nitrogen reduces colonization of
PGPB in natural condition and it may also reduce the process of nitrogen fixation due to regulatory mechanism acting
in the diazotrophic isolates. Therefore, a challenge is posed for systematic optimization for the application of suitable
PGPB isolates and the amount of fertilizer to be added to obtain maximum output. Use of compost may be useful at
some extent which provides utilizable nitrogen to support growth of microorganism and make the plant evade from
negative effects of PGPB colonization on it.
One of the major challenges includes selection of plant genotype and age, and compatible associative
bacteria. Understanding of this compatibility would help to enhance productivity by using specific strain for
inoculation. Since, the colonization of associative bacteria also depends upon seasonal changes and soil hydric
stress, multiples field trials are required to optimize parameters for obtaining the maximum output. Another factor
which is to be studied in details is the plant defense response which may limit or reduce the colonization of
associative bacteria. In addition, colonization mechanism is still not well understood. Intelligent analysis of genomic
and functional genomics studies can help manipulate the conditions to enhance colonization process and increased
plant growth properties.
Lastly and most importantly, extensive and intensive research on the understanding of associative and
endophytic ecology will be major determinant to maximize benefit from these bacteria.
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... Use of endophytic bacteria presents a special interest for development of agricultural applications that ensure improved crop performance under cold, draught or contaminated soil stress conditions or enhanced disease resistance (Inga Miliute et al., 2015) [7] . Recenty Jha et al., 2013 [8] reported that endophytic microorganisms resides inside the plant to for improve plant performance in integration with plant disease management systems. Many bioactive metabolites are originated from microbial organisms, bacteria are the heart important groups of eukaryotic organisms that have wide capacity to produce numerous metabolites with antimicrobial activities and possess potential application as drugs. ...
... Use of endophytic bacteria presents a special interest for development of agricultural applications that ensure improved crop performance under cold, draught or contaminated soil stress conditions or enhanced disease resistance (Inga Miliute et al., 2015) [7] . Recenty Jha et al., 2013 [8] reported that endophytic microorganisms resides inside the plant to for improve plant performance in integration with plant disease management systems. Many bioactive metabolites are originated from microbial organisms, bacteria are the heart important groups of eukaryotic organisms that have wide capacity to produce numerous metabolites with antimicrobial activities and possess potential application as drugs. ...
... diazotrophicus desde su aislamiento (Cavalcante & Döbereiner, 1998). Existen referencias donde se asocia la posibilidad de crecimiento de los microorganismos en medios semisólidos carentes de nitrógeno combinado con el proceso de fijación biológica de nitrógeno (Pérez et al., 2014;de la Fé et al, 2015), La estimulación de indicadores del crecimiento por la inoculación de bacterias promotoras puede estar asociada, además de la contribución de nutrientes realizada por la fijación biológica de nitrógeno y la solubilización de fósforo, a otros factores como la liberación de fitohormonas (Pazos et al., 2016) y la colonización del ambiente rizosférico y endófito (Jha et al., 2013). Para el caso de G. diazotrophicus se describe la capacidad de liberar hormonas, fundamentalmente ácido indolacético, aspecto que también puede ser afectado por el medio de cultivo utilizado para el crecimiento de la bacteria (Patil et al., 2011), produciendo la estimulación particular de algún órgano de la planta. ...
... There are references where the possibility of growth of microorganisms in semi-solid media without combined nitrogen with the process of biological nitrogen fixation is associated (Pérez et al., 2014, de la Fé et al., 2015. This criteria was used in the study for determine the maintenance of this characteristic (Pazos et al., 2016 ) and colonization of the rhizospheric and endophytic environment (Jha et al., 2013). For G. diazotrophicus, the ability to release hormones, mainly indoleacetic acid, is described, which can also be affected by the culture medium used for the growth of the bacteria (Patil et al., 2011). ...
M. M. (2019). The culture medium effect in plant growth promotion activity of Gluconacetobacter diazotrophicus in carrot and sugar beet. Revista Bio Ciencias 6, e470. doi: Para el crecimiento de Gluconacetobacter diazotrophicus se han utilizado diferentes medios de cultivos, aunque existen pocos trabajos que ilustran cómo estos afectan las características del microorganismo y su efecto en la planta. El objetivo de esta investigación fue evaluar en condiciones in vitro e in vivo el efecto de cinco medios de cultivos sobre el potencial como promotor del crecimiento vegetal de esta especie bacteriana en zanahoria y remolacha. La concentración del microorganismo después de 48 horas de fermentación en zaranda rotatoria se mantuvo en el orden de 10 12 CFU. mL-1 para todos los medios de cultivo evaluados. Al finalizar el proceso fermentativo la bacteria presentó un crecimiento estable en un medio carente de nitrógeno combinado; sin embargo, las diferencias en los halos de solubilización en el medio NBRIP indican una afectación en la capacidad de solubilización de fósforo. El efecto de la aplicación de los productos finales de la fermentación sobre zanahoria y remolacha depende del medio utilizado para el crecimiento del microorganismo. Variantes como el SG y SYP producen una mayor influencia sobre el follaje, mientras que el medio MA (elaborado a base de arroz) sobresale por su efecto Article Info/Información del artículo The culture medium effect in plant growth promotion activity of Gluconacetobacter diazotrophicus in carrot and sugar beet El medio de cultivo influye en la actividad promotora del crecimiento vegetal de Gluconacetobacter diazotrophicus en zanahoria y remolacha For Gluconacetobacter diazotrophicus cultivation have been used different media, but a few researches shows the changes in the plant growth promoting traits of this microorganism. The aim of this work was to evaluate in vitro and in vivo conditions the effect of five culture media in plant growth promotion activity of this bacterium in carrot and sugar beet. The bacterium concentration after 48 hours of fermentation in a shaker was 10 12 CFU.mL-1 in all media. After the fermentation process, the bacterium has stable growth in culture medium without combine nitrogen; but it has difference in the solubilization halo in NBRIP medium, indicating the affectation of phosphorous solubilization ability. The effect of final fermentation products application over carrot and sugar beet also depend of the medium used for microorganism growth. Media like SG and SYP where better for the action over lives and MA medium (rice) was 2 Culture of Gluconacetobacter diazotrophicus./Cultivo de Gluconacetobacter diazotrophicus. Revista Bio Ciencias 6, e470.
... For phosphorus solubilization, K solibilization, zinc solubilization and plant growth promotion endophytic bacterias like Arthrobacter, Bacillus, Beijerinckia, Microbacterium and Psedomonas (Oteino et al., 2015;Srinivasan et al., 2018;Meena et al., 2016;Gontia-Mishra et al., 2016;Katiyar and Goel, 2004;Mehnaz et al., 2001;Anton et al., 1998) found to be most useful. In plant disease management (PDM) the bacteria streptomyces exfoliates showed antagonistic activity against lettuce sclerotinia and Burkholderia phytofirmans found effective against Botrytis cinerea (Jha et al., 2013). ...
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Nutrition concerns with the chemicals required by an organism for its growth, reproduction, tissue replacement and the energy necessary for maintaining these functions. Achieving optimal nutrition involves a complex interplay between feeding behaviour and post-ingestive processing of foods. An estimated 10 percent of all insects utilize diets that are nutritionally so poor or unbalanced that they depend on resident, beneficial micro-organisms for sustained growth and reproduction. Although the taxonomic identity of symbiotic microorganisms varies among insect orders, they are thought to provide essential amino acids, vitamins, carbohydrates, sterols, and aids in the nitrogen recycling process. They also provide the ‘missing enzymes’ required for the completion of complex food material digestion, in addition to direct nutritional value. Both ecto and endosymbionts have their own unique ways to develop reasonable symbiotic relationships with insects. Furthermore, the remarkable and specific capabilities of insects to transmit microbes from one generation to the next and the knowledge of cultivation of microbes in a yieldable way to ensure the nutritional benefits prove the long existed phylogenetic and symbiotic relationships between them. There is still a lot of information about nutritional relationship between insect and microorganism symbiosis is missing and unclear. However, recent researches on the alternation of nutrition by microbiota of insects mainly focus on the gene specificity, cytological studies and humoral response to identify the interaction and effectiveness of these tiny creatures on the nutrition of the host. The demystification of molecular networks governing insect–microbial symbiosis will help us to reveal the perplexing behaviours in digestion of insects that can be utilized in managing insect pests. But these findings await confirmation and need to be investigated minutely before establishment.
... Plant growth-promoting bacteria (PGPB) can act on plant growth either through direct mechanisms (biological nitrogen fixation, phytohormone productions, phosphorus solubilization, and iron sequestration by siderophore producers) or indirect mechanisms (induction of systemic resistance and competition and production of antibiotics, among others) (Olanrewaju et al., 2017;Afzal et al., 2019). Several studies for the isolation and inoculation of bacteria have shown a large number of endophytic bacteria colonizing specific niches inside plants with different responses for plant growth (Jha et al., 2013;Brusamarello-Santos et al., 2017). ...
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The objective of this work was to isolate endophytic bacteria from peach palm (Bactris gasipaes var. gasipaes) plants and to evaluate the effects of their inoculation on the plant seedlings. Bacteria were isolated from the leaves and roots of the seedlings and from the meristems of peach palm plants in vitro. The isolates were characterized phenotypically and, then, 15 of them, representing different phenotypic groups, were selected and identified by partial sequencing of the 16S rRNA gene. Afterward, these isolates and two commercial strains of Azospirillum brasilense (Ab-V5 and Ab-V6) were inoculated in the peach palm seedlings. After 76 days, the seedlings were evaluated for plant development. The following six genera were identified based on the sequencing: Pseudomonas, Enterobacter, Rhizobium, Stenotrophomonas, Klebsiella, and Erwinia. Out of the 15 inoculated isolates, 9 had a positive effect on the root dry mass of palm peach, with CNPF 77 (Enterobacter sp.), CNPF 100 (Rhizobium sp.), and CNP 179 and CNPF 277 (Stenotrophomonas sp.) standing out. Peach palm seedlings harbor endophytic bacteria which are able to increase root dry matter.
Agroforestry is a practice of combining food crops with tree crops to create a more dynamic, versatile, and long-term exploitation of resources available on land to fulfill the requirements of growing populations. To boost production, chemical fertilizers are being widely used, but this is depleting our land resources of nutrients and has negative consequences for soil, water, the ecosystem, and crop quality and yield. As a result, there is a pressing need to transit from inorganic to organic agriculture techniques and microbial biofertilizer treatments, as they are essential to assuring crop yield and environmental protection. These microbial biofertilizers can improve plant health by affecting making nutrient available to them, releasing plant growth regulators, and offering protection against various diseases, all while increasing crop output. Plant-beneficial bacteria are said to be enhanced by agroforestry systems as well. The current analysis focuses on proper land utilization in the form of agroforestry, for fulfilling 3F (food, fodder, and fuel) through microbial biofertilizer interventions while also addressing environmental and health concerns.
Microorganisms colonizing and living in internal tissues of their host plants perform a significant role in enhancing growth of the host plants by providing access to several essential nutrients to plants, plant growth-promoting phytohormone production, defence for biotic and abiotic stresses and many more, through a wide variety of mechanisms. One of these mechanisms is the synthesis and supply of various metabolites (primary and secondary metabolites) by these microorganisms. These metabolites are produced by microorganisms in different environmental conditions at a particular time period or a particular growth stage to improve their physical fitness by enhancing nutrient availability, giving protection in different environmental conditions, by warding off or killing predators and/or parasites, by displacing competitors, by interfering with chemical signals between microbial cells and/or by favouring the persistence of their host plants either by modulating their defence mechanisms or by acting as repellent or killing agents to different herbivores or pests. In arid ecosystems, the role of these plant growth-promoting microorganisms-derived metabolites increases by several folds because of several biotic and abiotic stress conditions. Endophytic PGPMs are also exploited for some high-value biochemicals such as antibiotics, bioactive peptides, etc. having commercial interest.
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Bean root rot (Fusarium solani) is an important disease in major areas of bean production in the world, including Iran, which causes significant damage to bean crops every year. To date, various methods have been used to control this fungus, including physical and chemical controls; however, due to the multiplicity of host and soil fungus, the use of biological control seems to be a suitable alternative and an important tool for managing this disease. A number of biological control methods have been implemented using endophytic bacteria, which have been effective in controlling and reducing this disease. In this study, the inhibitory effect of Pseudomonas fluorescens and Bacillus subtilis against bean root rot fungus, which were isolated from the root and crown of the bean plant using endophytic bacteria, was investigated in vitro. This experiment was performed with the help of cross-culture test, study of antifungal volatile compounds, extracellular material production test and antibiotic production. The studied bacteria were isolated and purified from bean fields in Lorestan province and were identified based on standard bacteriological tests and using PCR method and specific primers of 16S rRNA gene. In cross-culture test, Pseudomonas fluorescens and Bacillus subtilis isolates showed inhibition above 52% and 55%, respectively. In the production of antifungal volatile compounds, Pseudomonas fluorescens and Bacillus subtilis showed 27% and 29% inhibition, respectively. In the antibiotic production test, both isolates controlled up to 90% of the growth of the pathogenic fungus by producing high amounts of antibiotics. Pseudomonas fluorescens and Bacillus subtilis isolates were also effective in controlling the disease fungus by 29% and 36%, respectively. According to the results of laboratory tests, these isolates can be introduced as a biocontrol agent against Fusarium solani.
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The use of Information and Communication Technology (ICT, hereafter) has permeated almost every facet of human endeavours. In recent times, the world has seen the massive deployment of diverse technologies to facilitate work processes, and this is the thriving strength of the fourth industrial revolution (4IR). Besides, the recent outbreak of the Coronavirus (COVID-19) pandemic exacerbated the situation, as this has led to the discovery of some new technologies in the delivery of goods and services across all sectors, with education as a major beneficiary. However, some educational institutions, Technical and Vocational Education and Training Colleges (TVET, hereafter) inclusive, especially in developing countries remained closed during the lockdown period due to lack of capacity and infrastructure. Hence, this paper taps into the affordances of the multi-stakeholder collaborations framework of the Quadruple Helix Innovation Model to review published articles, policy documents and conference papers among others on possible ways to foster digital inclusion in TVET. Highlights from the review point to the following benefits of multi-stakeholder partnerships namely, collaborative designing and creation of digital policy, access to digital resources and infrastructure, and increased affordability and sustainability of digital inclusion of training programmes. Thus, it was concluded that these highlighted benefits resulting from such collaboration can be translated into the planning and implementation of digital inclusion in TVET. Keywords: Digital inclusion; Quadruple Helix Innovation Model; Teacher Training; TVET
Endophytes are ubiquitous symbiotic microorganisms living with different plant tissues without any harmful effects. Endophytes usually establish around the roots or cuticle surface of the leaf, while some minute fractions are also associated with seeds. Fungi and bacteria act as endophytes that are essentially involved in plant growth and development processes such as nitrogen fixation, phytohormone signaling, phytoremediation, secondary metabolite secretion, and regulation of plant metabolism. These interactive mechanisms among endophytes and host plants are essential to combat biotic and abiotic stress responses in plants. As the scenario of current agricultural practices is to apply environment-friendly approaches to combat biotic and abiotic stress, therefore, the current research programs channel towards beneficial endophytic-plant associations for biocontrol of plant diseases. The critical regulators for the association between microorganisms and host plants are biomolecules, including phytohormones. Several biomolecules such as antifungal compounds, compatible solutes, osmoprotectants, enzymes, siderophores, and secondary metabolites are contributed by endophytes during microbial-host association. Similarly, phytohormones such as auxin, cytokinin, gibberellins, ABA (abscisic acid), and SA (salicylic acid) are released by endophytes to interact with the host system where phytohormone and biomolecule-mediated signaling governs several kinds of mechanisms for plant disease control. The mechanisms include signaling for nutrient uptake, nutrient utilization, and abiotic and biotic stress tolerance. The current chapter discusses the role of different biomolecules and phytohormones released from endophytes in encountering stress response and disease management. Various signaling mechanisms supporting these symbiotic associations in plants are also discussed.
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Associative microorganisms live in the zone of direct influence of plants namely in the soil, which is in close contact with the roots. They form complex communities on the roots of plants in terms of taxonomic composition and structural and functional o8rganization, which have polyfunctional effect on plants. Rhizosphere biotechnologies with the use of associative bacteria to a specific plant species allow increasing their productivity and quality, which contributes to the stability of agroecosystems. Therefore, the main goal of the current study was to isolate associative to Triticum aestivum L. bacteria from the southern chernozem (Haplic Chernozems), which was sampled in the steppe zone of the Crimea. The methodical approach was used to select associative microorganisms for a specific plant species. The number of microorganisms of the main ecological-trophic groups for three varieties T. aestivum were determined. The maximum number of rhizosphere microorganisms was identified for the Ermak variety, as well as the number of morphotypes isolated from the apical part of the root. As a result of the research, six strains of associative bacteria were obtained. They increased the seed quality of the studied varieties by 5% and the biomass of the seedlings by 80%. Identification by the 16S rRNA gene showed their belonging to different bacterial genera. Thus, the quantitative composition of the chernozem southern of the rhizosphere of three varieties of T. aestivum was determined. Five strains of associative bacteria promising for further biotechnology of the agrocenosis rhizosphere were isolated.
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Plants acquire phosphorus from soil solution as phosphate anion. It is the least mobile element in plant and soil contrary to other macronutrients. It precipitates in soil as orthophosphate or is adsorbed by Fe and Al oxides through legend exchange. Phosphorus solubilizing bacteria play role in phosphorus nutrition by enhancing its availability to plants through release from inorganic and organic soil P pools by solubilization and mineralization. Principal mechanism in soil for mineral phosphate solubilization is lowering of soil pH by microbial production of organic acids and mineralization of organic P by acid phosphatases. Use of phosphorus solubilizing bacteria as inoculants increases P uptake. These bacteria also increase prospects of using phosphatic rocks in crop production. Greater efficiency of P solubilizing bacteria has been shown through co-inoculation with other beneficial bacteria and mycorrhiza. This article incorporates the recent developments on microbial P solubilization into classical knowledge on the subject.
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Plant growth-promoting bacteria (PGPB) are soil and rhizosphere bacteria that can benefit plant growth by different mechanisms. The ability of some microorganisms to convert insoluble phosphorus (P) to an accessible form, like orthophosphate, is an important trait in a PGPB for increasing plant yields. In this mini-review, the isolation and characterization of genes involved in mineralization of organic P sources (by the action of enzymes acid phosphatases and phytases), as well as mineral phosphate solubilization, is reviewed. Preliminary results achieved in the engineering of bacterial strains for improving capacity for phosphate solubilization are presented, and application of this knowledge to improving agricultural inoculants is discussed.
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Endophytic bacteria are ubiquitous in most plant species, residing latently or actively colonizing plant tissues locally as well as systemically. Several definitions have been proposed for endophytic bacteria; in this review endophytes will be defined as those bacteria that can be isolated from surface-disinfested plant tissue or extracted from within the plant, and that do not visibly harm the plant. While this definition does not include nonextractable endophytic bacteria, it is a practical definition based on experimental limitations and is inclusive of bacterial symbionts, as well as internal plant-colonizing nonpathogenic bacteria with no known beneficial or detrimental effects on colonized plants. Historically, endophytic bacteria have been thought to be weakly virulent plant pathogens but have recently been discovered to have several beneficial effects on host plants, such as plant growth promotion and increased resistance against plant pathogens and parasites. In general, endophytic bacteria originate from the epiphytic bacterial communities of the rhizosphere and phylloplane, as well as from endophyte-infested seeds or planting materials. Besides gaining entrance to plants through natural openings or wounds, endophytic bacteria appear to actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase. Since these enzymes are also produced by pathogens, more knowledge on their regulation and expression is needed to distinguish endophytic bacteria from plant pathogens. In general, endophytic bacteria occur at lower population densities than pathogens, and at least some of them do not induce a hypersensitive response in the plant, indicating that they are not recognized by the plant as pathogens. Evolutionarily, endophytes appear to be intermediate between saprophytic bacteria and plant pathogens, but it can only be speculated as to whether they are saprophytes evolving toward pathogens, or are more highly evolved than plant pathogens and conserve protective shelter and nutrient supplies by not killing their host. Overall, the endophytic microfloral community is of dynamic structure and is influenced by biotic and abiotic factors, with the plant itself constituting one of the major influencing factors. Since endophytic bacteria rely on the nutritional supply offered by the plant, any parameter affecting the nutritional status of the plant could consequently affect the endophytic community. This review summarizes part of the work being done on endophytic bacteria, including their methodology, colonization, and establishment in the host plant, as well as their role in plant–microbe interactions. In addition, speculative conclusions are raised on some points to stimulate thought and research on endophytic bacteria.Key words: endophytic bacteria, methods, localization, diversity, biological control.
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Rice (Oryza sativa L.) is one of the world's most important crops. The present investigation was desired to assess the range of growth-promoting activities of various diazotrophic bacteria on rice seedling vigor, its cadaver effect on straw and grain yield, and the persistence of an inoculant strain on rice roots under greenhouse conditions. Growth responses to inoculation exhibited bacterial strain-rice variety specificity that were either stimulatory or inhibitory. Growth responses included changes in rates of seedling emergence, radical elongation, height and dry matter, plumule length, cumulative leaf and root areas, and in and straw yields. Most notable were the inoculation responses to Rhizobium leguminosarum bv. trifolii E11 and Rhizobium sp. IRBG74, which stimulated early rice growth resulting in a carryover effect of significantly (P = 0.05) increased grain and straw yields at maturity, even though their culturable populations on roots diminished to below detectable values at 60 d after planting. The test strains were positive for indole-3-acetic acid production in vitro, but only some reduced acetylene to ethylene in association with rice under laboratory growth condition. These studies indicate that certain strains of nonphotosynthetic diazotrophs, including rhizobia, can promote growth and vigor of rice seedlings, and this benefit of early seedling development can carryover to significantly increased grain yield at maturity.
Non-pathogenic soilborne microorganisms can promote plant growth, as well as suppress diseases. Plant growth promotion is taken to result from improved nutrient acquisition or hormonal stimulation. Disease suppression can occur through microbial antagonism or induction of resistance in the plant. Several rhizobacterial strains have been shown to act as plant growth-promoting bacteria through both stimulation of growth and induced systemic resistance (ISR), but it is not clear in how far both mechanisms are connected. Induced resistance is manifested as a reduction of the number of diseased plants or in disease severity upon subsequent infection by a pathogen. Such reduced disease susceptibility can be local or systemic, result from developmental or environmental factors and depend on multiple mechanisms. The spectrum of diseases to which PGPRelicited ISR confers enhanced resistance overlaps partly with that of pathogen-induced systemic acquired resistance (SAR). Both ISR and SAR represent a state of enhanced basal resistance of the plant that depends on the signalling compounds jasmonic acid and salicylic acid, respectively, and pathogens are differentially sensitive to the resistances activated by each of these signalling pathways. Root-colonizing Pseudomonas bacteria have been shown to alter plant gene expression in roots and leaves to different extents, indicative of recognition of one or more bacterial determinants by specific plant receptors. Conversely, plants can alter root exudation and secrete compounds that interfere with quorum sensing (QS) regulation in the bacteria. Such two-way signalling resembles the interaction of root-nodulating Rhizobia with legumes and between mycorrhizal fungi and roots of the majority of plant species. Although ISR-eliciting rhizobacteria can induce typical early defence-related responses in cell suspensions, in plants they do not necessarily activate defence-related gene expression. Instead, they appear to act through priming of effecti ve resistance mechanisms, as reflected by earlier and stronger defence reactions once infection occurs.
Plant growth-promoting bacteria (PGPB) are commonly used to improve crop yields. In addition to their proven usefulness in agriculture, they possess potential in solving environmental problems. Some examples are highlighted. PGPB may prevent soil erosion in arid zones by improving growth of desert plants in reforestation programs; in turn, this reduces dust pollution. PGPB supports restoration of mangrove ecosystems that lead to improve fisheries. PGPB participate in phytoremediation techniques to decontaminate soils and waters. These include: phytodegradation, phytotransformation, bioaugmentation, rhizodegradation, phytoextraction, phycoremediation, and phytostabilization, all leading to healthier environments. This review describes the state-of-the-art in these fields, examples from peer-reviewed literature, pitfalls and potentials, and proposes open questions for future research.
Root colonization studies, employing immunofluorescence and using locally isolated strains, showed thatEnterbacter sp. QH7 andEnterobacter agglomerans AX12 attached more readily to the roots of most plants compared withAzospirillum brasilense JM82. Heat treatment of either root or inoculum significantly decreased the adsorption of bacteria to the root surface. Kallar grass and rice root exudates sustained the growth ofA. brasilense JM82,Enterobacter sp. QH7 andE. agglomerans AX12 in Hoagland and Fahraeus medium. All the strains colonized kallar grass and rice roots in an axenic culture system. However, in studies involving mixed cultures,A. brasilense JM82 was inhibited byEnterobacter sp. QH7 in kallar grass rhizosphere and the simultaneous presence ofEnterobacter sp. QH7 andE. agglomerans AX12 suppressed the growth ofA. brasilense JM82 in rice rhizosphere. The bacterial colonization pattern changed from dispersed to aggregated within 3 days of inoculation. The colonization sites corresponded mainly to the areas where root mucigel was present. The area around the point of emergence of lateral roots usually showed maximum colonization.
Scope and background of this compilation: In 2003, the Biological Control Committee of the American Phytopathological Society (APS) suggested that the time was right to develop a symposium on endophytes for the annual meeting of the society to be held in 2005. We were charged with developing a series of topics and speakers that would address the status of endophytes for biological control of plant diseases. That symposium was held in the 2005 meeting of APS, July 30–August 3 in Austin, Texas, where it generated very strong attendance. Preliminary abstracts of presentations were published for that meeting [Various, 2005. Endophytes, an emerging tool for biological control (six abstracts). Phytopathology 95(6), S138 (Suppl. 1)]. Authors from these presentations are largely represented in this compilation. In addition, we have added additional papers on fungal endophytes for plant disease and insect management.