8/12/2019 Pgpr Poster
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Emergence of Plant Growth Promoting Rhizobacteria for an
Environmentally Sustainable AgricultureNeha Upadhyay, Radha
Rani, Shivesh Sharma*
Department of Biotechnology,
MNNIT Allahabad, Allahabad -211004 U.P.Abstract
Plant growth promoting rhizobacteria (PGPR) are valuable
bacteria commercialized as biofertilizer and biopesticides. Any
technology that deals with natural environment is always turning to
an innovative
way and PGPR isolates are categorized as an eco-friendly
microbial tool. PGPR inoculants are emerged as a novel technique
that reduces the excessive use of chemical fertilizers and
pesticide. The industrial
requirement for PGPR is progressively increased, as it propounds
attractive way to maintain a sustainable agriculture system. The
successful commercialization of PGPR strains depend on the
association
between the scientific organizations and industries. A number of
plant growth promoting rhizobacterial isolates has been reported
that enhance plant growth either directly or indirect ly by
inhibiting the growth
of plant pathogen. Production of phytohormones (IAA),
solublizing phosphate and nitrogen fixation are some of the
mechanism that directly involve in plant growth and promotion.
Indirect mechanism for
plant growth promotion involves production of antifungal
substances, HCN production, production of s iderophores and cell
wall degrading enzymes. The miscellaneous and favourable
interactions between
plant and PGPR inoculants lead towards a hopeful solutions for
environmentally susta inable agr iculture. Recent progress renders
the use of PGPR as biopesticides as well as biofertilizers. Its
long term
benefits for successful agriculture are presented here.
IntroductionFertilizers are essential components of modern
agriculture because they provide
essential plant nutrients. However, overuse of fertilizers and
pesticide can cause
unanticipated environmental impacts such as accumulation of
toxic chemical
substances in the soil, depletion of organic carbon content,
micro flora and fauna,
soil fertility having deleterious effects on crop productivity.
One potential way to
decrease negative environmental impacts resulting from continued
use of chemical
fertilizers is inoculation with plant growth-promoting
rhizobacteria (PGPR). These
bacteria influencing root growth and morphology or by aiding
other beneficial
symbiotic relationships exert beneficial effects on plant growth
and development
and many different genera have been commercialized for use in
agriculture. During
the past couple of decades, the use of plant growth promoting
rhizobacteria (PGPR)
for sustainable agriculture has increased tremendously in
various parts of the world.
Significant increases in growth and yield of agronomically
important crops inresponse to inoculation with PGPR have been
repeatedly reported (1, 2, 5).
Plant Growth Promoting Rhizobacteria (PGPR)
PGPR (Biofertilizers & Biofungicides) are Plant
Growth-Promoting Rhizobacteria defined as root-
colonizing bacteria that exert beneficial traits on
plant growth and development. Root colonization
comprises the ability of PGPR to establish on or in
the root or rhizosphere to multiply, survive and
colonize along the growing root in the presence of
the indigenous microflora. Genera of PGPR
generally include Acinobacter, Agrobacter,
Arthrobacter, Azospirillum, Bacillus,
Bradyrhizobium, Frankia, Pseudomonas,
Rhizobium, Serratia, Thiobacillus, and others. In
addition to plant growth promotion, PGPR are also
used for controlling several plant pathogens,
enhancement of nutrient up-take and in
rhizomediation.
.
Mechanism of actionPGPR has a significant impact on plant
growth
and development in both indirect and direct
ways. the direct promotion of plant growth by
PGPR generally entails providing the plant withcompound that is
synthesized by the bacterium
or facilitating the uptake of nutrients from the
environment On the other hand, indirect
promotion of plant growth occurs when bacteria
prevent some of the deleterious effects of a
phytopathogenic organism by one or more
mechanisms. (Glick, 1995; Glick et al.1999).
Phosphate solublizationPGPR stimulate plant growth directly
through increase in nutrition
acquisition, such as phosphate solubilization, or by rendering
the
inaccessible nutrients available to the plants. Phosphorous is
taken
up from soil solution as phosphate (Pi, H2PO4
_). Although soils
generally contain a large amount of total P but only a small
proportion is available for uptake by the plants. Several
phosphate
solubilizing microorganisms (PSMs) are now recorded to
convert
the insoluble form of phosphorus to soluble form through
acidification, secretion of organic acids or protons (7) and
chelation
and exchange reactions. Phosphate-solubilizing
microorganisms
(PSM) include a wide range of symbiotic and non symbiotic
organisms, such as Pseudomonas, Bacillus, and
Rhizobiumspecies;
actinomycetes; and various fungi-like Aspergillus and
Penicillium
species (7).
Siderophore ProductionIron is an essential nutrient of plants,
but it is relatively insoluble in soil
solutions. Siderophores are low molecular weight, extracellular
compounds
with a high affinity for ferric iron, that are secreted by
microorganisms to take
up iron from the environment and their mode of action in
suppression of
disease were thought to be solely based on competition for iron
with the
pathogen (Bakker et al., 1993; Duijff et al., 1997). Plant roots
prefer to absorb
iron as the more reduced ferrous (Fe2+) ion, but the ferric
(Fe3+) ion is more
common in well aerated soil although it is easily precipitated
in iron-oxide
forms (Salisbury and Ross, 1992). Plants commonly excrete a
soluble organic
compound (chelators and phytosiderophores) which binds Fe3+ and
helps to
maintain it in solution. Chelators deliverthe Fe3+ to the root
surface where it
is reduced to Fe2+ and immediately. Some rhizospheric bacteria
also produce
siderophores and there is evidence that a number of plant
species can absorb
bacterial Fe3+-siderophore complexes (Wang et al., 1993).
Majority of
research on microbial siderophores in the rhizosphere is
associated with their
biocontrol activities due to their competitive effects with
plant pathogens.
Production of PhytohormonesPGPR facilitates plant growth by the
production of plant growth regulators or phytohormones like IAA,
gibberllic
acid and cytokinins (5). Tryptophan has been identified as main
precursor molecule for biosynthesis of IAA in
bacteria. IAA controls a diverse array of functions in plant
growth and development and acts as a key component
in shaping plant root architecture such as root vascular tissue
differentiation, regulation of lateral root initiation,
polar root hair positioning, and root gravitropism (4).
Development of PGPR
inoculant
Challenges in Field Application of PGPRThe application of PGPR
for control of fungal pathogens in greenhouse systems shows
considerable promise (30), due in
part to the consistent environmental conditions and high
incidence o f fungal disease in greenhouses. Achieving
consistent
performance i n the field where there is heterogeneity of
abiotic and biotic factors and competition with indigenous
organisms is more difficult. Knowledge of these factors can aid
in determination of optimal concentration, timing and
placement of inoculant, and of soil and crop management
strategies to enhance survival and proliferation of the i
noculant.
The concept of engineering or managing the rhizosphere to
enhance PGPR function by manipulation of the host plant,
substrates for PGPR, or through agronomic practices, is gaining
increasing attention (8 ). Development of better
formulations to ensure survival and activity in the field and
compatibility with chemical and biological seed treatments
isanother area of focus; approaches include optimization of growth
conditions prior to formulation and development of
improved carriers and application technology (8,3).
ConclusionAs our understanding of the complex environment of
the rhizosphere, of the mechanisms of action of PGPR,
and of the practical aspects of inoculant formulation and
delivery increases, we can expect to see new PGPR
products becoming available. The success of these
products will depend on our ability to manage the
rhizosphere to enhance survival and competitiveness of
these beneficial microorganisms (4). Rhizosphere
management will require consideration of soil and crop
cultural practices as well as inoculant formulation and
delivery (3,8). The use of multi-strain inocula of PGPR
with known functions is of interest as these
formulations may increase consistency in the field (8).
They offer the potential to address multiple modes of
action, multiple pathogens, and temporal or spatial
variability . PGPR offer an environmentally sustainable
approach to increase crop production and health. The
application of molecular tools is enhancing our ability
to understand and manage the rhizosphere and will lead
to new products with improved effectiveness.
References1. Ahemad, M., Khan, M.S., 2009a. Effect of
insecticide-tolerant and plant growth promoting Mesorhizobium
on
the performance of chickpea grown in insecticide stressed
alluvial soils. J. Crop Sci. Biotechnol. 12, 213 222.
2. Ahemad, M., Khan, M.S., 2011k. Pseudomonas aeruginosa strain
PS1 enhances growth parameters of greengram[Vigna radiata (L.)
Wilczek] in insecticide-stressed soils. J. Pest Sci. 84,
123131.
3. Bashan, Y. 1 998. Inoculants of plant growth-prom oting
bacteria for use in agriculture. Biotechnol. Adv . 1 6:7 2
9-7 7 0.
4. Bhattacharyya, P.N., Jha, D.K., 2012. Plant growth-promoting
rhizobacteria (PGPR): emergence in agriculture
World J Microbiol Biotechnol) 28:13271350 DOI
10.1007/s11274-011-0979-9
5. Kloepper JW, Schroth MN (1981) Relationship of in vitro
antibiosis of plant growth promoting rhizobacteria to
plant growth and the displacement of root microflora.
Phytopathology 71:102010246. Glick, B.R., C.L. Patten, G.Holguin
and D.M. Penrose, 1999. Biochemical and genetic Mechanism used by
plant
growth Promoting Bacteria. Imperial College Press, London, UK.7.
Richardson AE, Barea JM, McNeill AM, Prigent-Combaret C (2009)
Acquisition of phosphorus and nitrogen in
the rhizosphere and plant growth promotion by microorganisms.
Plant Soil 321:305339.8. Nelson, L.M. Plant Growth Promoting
Rhizobacteria (PGPR): Prospects for New Inoculants 2004 Plant
Managem ent Network. Accepted for publication 14 January 2004.
Published 1 March 2004.
Selection and characterization of
PGPR strainsPGPR strains have diverse applications in
agriculture, horticulture and forestry. Specific
PGPR strains are initially selected from several
hundreds of root-colonizing bacteria isolated
from excised roots of field grown plants. During
the experiment, those PGPRs that consistently
caused statistically significant increases in root
or shoot development or both are selected for
further testing in agricultural field.
Recently, selections of efficacious PGPR
strains have been made by mass screening
technique (4). Primary screenings of new
isolates are done based on physiological,
nutritional and biochemical characteristics as in
BergeysManual of Determinative Bacteriology.
While DNA and RNA homology tests are also
considered as most reliable tools for the
characterization of potent PGPR strains (4).
Table: Examples of plant growth promoting
rhizobacteria tested for various crop types
Nitrogen
FixationNitrogen is one
Nitrogen is one of
the most common
nutrients required
for optimal plant
growth and
productivity. Approx
60 % of the earths
available nitrogen is
fixed by the process
of biological
nitrogen fixation and
represents an
economically
beneficial and
environmentally
sound alternative tochemical fertilizers
(Ladha et al., 1997.
The process of N2fixation is carried
out by nitrogenase
enzyme coded by nif
genes (Kim and
Rees, 1994).
Fig.3 Mechanism of P solublization
Fig.2 mechanism of Plant growth promotion by PGPR
Fig.1 Schematic representation of PGPR
Fig.4 Function of siderophores
