Rhizobacteria: Legendary Soil Guards in Abiotic Stress Management

Abstract

All plants are continuously subjected to various types of biotic and abiotic stress factors from the time they have been planted in the field up to the time of harvesting, transport, storage, and consumption of the plant or plant-based products. These stresses result in the negative and deleterious effects on crop health and also cause enormous losses across the globe. To reduce the intensity of the losses produced by these stress factors, researchers all across the world are involved in inventing new management practices which may include traditional genetics methodology and various techniques of plant breeding. The use of microorganisms to mitigate both abiotic and biotic stress can provide an economical, eco-friendly solution to the problem of losses due to abiotic and biotic stresses. One such category of microorganisms is root-colonizing nonpathogenic bacteria like plant growth-promoting rhizobacteria (PGPR) which can increase the plant’s resistance to biotic and abiotic stress factors. PGPR is the bacteria residing in the rhizosphere region and is involved in promoting plant growth and suppressing stress components. PGPR colonize the rhizosphere for nutrition which they acquire from plant root exudates. The mechanism by which plant growth-promoting rhizobacteria can accomplish the abovementioned task includes increment in plant growth by enrichment of soil nutrients through nitrogen fixation, solubilization of phosphates, production of metal ion chelators, and elevated production of plant growth-promoting hormones. The mechanism also focuses on elevated protection of the plants through influencing the levels of production of cellulases and β-1,3-glucanases which result in the activation of the defense mechanism of plants against pests and pathogens. PGPR also contains useful variation for making plant tolerant to abiotic stress factors like temperature extremes, pH variations, salinity and drought, and heavy metal and pesticide pollution. Enrichment of plant rhizosphere with such potential stress-tolerating PGPR is expected to provide enhanced plant growth and high yield of plant products in stress-affected areas. This chapter summarizes the research related to PGPR and its benefits and also throws light on the involvement of PGPR in abiotic stress management.

Full text

Series Editor: Naveen Kumar Arora
Microorganisms for Sustainability 12
R. Z.Sayyed
NaveenKumarArora
M. S.Reddy Editors
Plant Growth
Promoting
Rhizobacteria for
Sustainable Stress
Management
Volume 1: Rhizobacteria in Abiotic
Stress Management

Editors
R. Z. Sayyed
Department of Microbiology
PSGVP Mandal’s ASC College
Shahada, Maharashtra, India
M. S. Reddy
Department of Entomology & Plant
Pathology
Auburn University
Auburn, Alabama, USA
Naveen Kumar Arora
Department of Environmental
Microbiology, School of Environmental
Sciences
Babasaheb Bhimrao Ambedkar University
Lucknow, Uttar Pradesh, India
ISSN 2512-1901 ISSN 2512-1898 (electronic)
Microorganisms for Sustainability
ISBN 978-981-13-6535-5 ISBN 978-981-13-6536-2 (eBook)
https://doi.org/10.1007/978-981-13-6536-2
© Springer Nature Singapore Pte Ltd. 2019
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327
© Springer Nature Singapore Pte Ltd. 2019
R. Z. Sayyed et al. (eds.), Plant Growth Promoting Rhizobacteria for Sustainable
Stress Management, Microorganisms for Sustainability 12,
https://doi.org/10.1007/978-981-13-6536-2_15
A. Khan
Department of Biotechnology, HPT Arts and RYK Science College,
Nashik, Maharashtra, India
R. Z. Sayyed (*)
Department of Microbiology, PSGVP Mandal’s ASC College, Shahada, Maharashtra, India
S. Sei
Department of Agriculture, Payame Noor University, Tehran, Iran
15
Rhizobacteria: Legendary Soil Guards
inAbiotic Stress Management
AfreenKhan, R.Z.Sayyed, andSoniaSeifi
Abstract
All plants are continuously subjected to various types of biotic and abiotic stress
factors from the time they have been planted in the eld up to the time of harvest-
ing, transport, storage, and consumption of the plant or plant-based products.
These stresses result in the negative and deleterious effects on crop health and
also cause enormous losses across the globe. To reduce the intensity of the losses
produced by these stress factors, researchers all across the world are involved in
inventing new management practices which may include traditional genetics
methodology and various techniques of plant breeding. The use of microorgan-
isms to mitigate both abiotic and biotic stress can provide an economical, eco-
friendly solution to the problem of losses due to abiotic and biotic stresses. One
such category of microorganisms is root-colonizing nonpathogenic bacteria like
plant growth-promoting rhizobacteria (PGPR) which can increase the plant’s
resistance to biotic and abiotic stress factors. PGPR is the bacteria residing in the
rhizosphere region and is involved in promoting plant growth and suppressing
stress components. PGPR colonize the rhizosphere for nutrition which they
acquire from plant root exudates. The mechanism by which plant growth-
promoting rhizobacteria can accomplish the abovementioned task includes
increment in plant growth by enrichment of soil nutrients through nitrogen xa-
tion, solubilization of phosphates, production of metal ion chelators, and elevated
sayyedrz@gmail.com

328
production of plant growth-promoting hormones. The mechanism also focuses
on elevated protection of the plants through inuencing the levels of production
of cellulases and β-1,3-glucanases which result in the activation of the defense
mechanism of plants against pests and pathogens. PGPR also contains useful
variation for making plant tolerant to abiotic stress factors like temperature
extremes, pH variations, salinity and drought, and heavy metal and pesticide pol-
lution. Enrichment of plant rhizosphere with such potential stress- tolerating
PGPR is expected to provide enhanced plant growth and high yield of plant prod-
ucts in stress-affected areas. This chapter summarizes the research related to
PGPR and its benets and also throws light on the involvement of PGPR in abi-
otic stress management.
Keywords
Rhizobacteria · Stress tolerance · Salt stress · Drought stress · Pesticide stress ·
Heavy metal stress
15.1 Introduction
The major limiting factor for agricultural productivity is exposure of crops to vari-
ous abiotic stresses. To survive the harmful external pressure induced by various
environmental conditions, plants must modify their biological mechanisms; failure
in the same results in reduced plant development and productivity. The indigenous
microora of any diverse environmental niche shows extensive metabolic capabili-
ties to alleviate abiotic stresses observed in the environment to which they
belong(Kumar etal. 2018). Various types of microbial interactions are observed
with plants, and they are an essential segment of the ecosystem; hence, the natural
microora is believed to regulate the local and systemic reactions of plant defense
mechanism which can denitely increase the chances of survival of the plant in
stress-affected area (Meena etal. 2017). Productivity in principal crops is witness-
ing great reduction all over the world due to increased incidences of abiotic and
biotic stresses (Grover etal. 2011). Plant resistance to these biotic and abiotic stress
factors can be improved by inoculation with root-colonizing pathogenic bacteria
which can be applied as biofertilizers and can enhance the effectiveness of phytore-
mediation. Inoculation of plants with nonpathogenic bacteria can also provide “bio-
protection” against biotic stresses, and some root-colonizing bacteria can increase
tolerance against abiotic stresses such as drought, salinity, and metal toxicity. Any
disparity in nitrogen (N) cycling and nutritional status of the soil, the occurrence of
phytopathogens, alteration in climatic conditions, and occurrence of abiotic stresses
are the interwoven factors for a reduction in productivity of an agricultural eld.
However, the rapid increase in land degradation by numerous man-made activities
leads to an estimated loss of 24 billion tons of fertile soil worldwide (FAO 2011).
The early 1990s experienced heightened interest in bacterial endophytes which
further increased multiple times with results that provide conrmation to the fact
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329
that inoculation of plants with nonpathogenic rhizospheric bacteria induces positive
changes in plant growth and productivity. Hence currently a mixed population of
bacterial inoculants is commercially available for use as protection against biotic
and abiotic stresses (Dimkpa etal. 2009a, b).
Plant growth-promoting rhizobacteria (PGPR) are associated with plant roots
and hence have a major role in alleviating the effects of abiotic stresses such as
drought, low temperature, salinity, metal toxicity, high temperatures, etc. on plants
through various mechanisms like induced production of osmoprotectants and HSPs,
i.e., heat shock proteins. During the crop production, microorganisms can be used
as indicators of soil biodiversity and quality and can also contribute to reducing the
effects of negative stress caused in plants by abiotic factors (Milosevic etal. 2012).
A range of examples of stress tolerance mediated by PGPR can be found in the
previous study; the modes of action remain less elaborative, as most of the studies
and results are based on the lab-scale studies and do not replicate the same effects
in the agricultural elds. Some of the bacterial strains which reduce the effects of
abiotic stress are also shown to provide protection against stress induced by biotic
factors. Thus for sustainable agricultural systems, bacterial inoculants which pro-
vide cross-protection against both biotic and abiotic stress factors will be extremely
benecial. Inoculation of agricultural elds with stress-tolerant PGPR would
become more effective with detailed information about the concept of cross-
protection. Hence this chapter highlights the benets of colonization of plant rhizo-
sphere with PGPR in increased agricultural productivity (Dimkpa etal. 2009a, b).
15.2 Beneficial Effects ofRhizobacteria
A major part of total organic carbon (approximately 85%) in the rhizosphere comes
from sloughing of the root cells and tissues. Hence indigenous microora of the
rhizosphere alters their metabolic activities for obtaining the nutrients through the
exudates. In this view, it is essential to study the bacterial motility during interaction
with the plant. Microorganisms are the most diverse and elemental living system on
earth. As an essential living component of the rhizosphere, they are an important
component of the agricultural production systems. As natural inhabitants of seeds,
microorganisms aid in the proliferation of the seeds and establishment of diverse
symbiotic associations(Chakraborty etal. 2015). Natural inhabitants of the plant
help in supporting the plant during nutrient acquisition, providing better resistance
against various plant diseases and tolerating abiotic stresses. Intrinsic metabolic
activities of the rhizospheric bacteria and potent genetic capabilities make them
good candidates for ghting adverse environmental conditions (Singh 2016;Singh
etal. 2016). Vivid evidence to essential attributes of the plant-microbial interactions
can be provided by regulation of cellular, biochemical, and molecular mechanisms
which are closely related to stress tolerance (Bakker etal. 2013). Microorganisms
colonize the plant rhizosphere in high density. Hence rhizospheric soil which is
inuenced by root composition is highly enriched with amino acids, fatty acids,
nucleotides, organic acids, phenols, and phytohormones. The highly enriched
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330
nutrient composition of the soil results in colonization of the rhizospheric soil with
microora such as bacteria, fungus, algae, and protozoa. The extent of colonization
in rhizospheric soil is 10–100 times more than bulk soil. Among all the natural
inhabitants of the rhizosphere, bacterial inuence toward better plant productivity
and elevated defense is most signicant. Plant rhizobacteria can be categorized
based on their proximity to the roots as (1) bacteria living rhizosphere, (2) bacteria
colonizing the rhizoplane, (3) bacteria found in root tissues (endophytes) which also
colonize spaces between cortical cells, and (4) bacteria living inside specialized root
structures (nodules) which includes the legume-associated rhizobia and the woody
plant-associated Frankia sp. Bacteria that belong to any of the abovementioned cat-
egories and are involved in plant growth promotion directly through nitrogen xa-
tion, phosphate solubilization, iron chelation, etc. or are involved in indirect growth
promotion through suppression of plant diseases and induced resistance toward abi-
otic stresses are referred to as plant growth-promoting rhizobacteria (PGPR)
(Gopalakrishnan etal. 2015). Specicity of interactions between plant and rhizo-
spheric bacteria is determined by soil composition and extent of root exudates avail-
able. Rhizospheric bacteria which have exhibited benecial effects on plants include
species of the genera Bacillus, Enterobacter, Arthrobacter, Azotobacter,
Azospirillum, Pseudomonas, and Serratia, as well as Streptomyces species (Dimkpa
etal. 2008, 2009a). The details of denite mechanisms of plant growth promotion
remain largely elusive, as it is related to bacterial strains and most importantly is
based on the different compounds released by the various rhizospheric microorgan-
isms. The studies suggest that production of the primary plant growth-promoting
hormones such as auxins, cytokinin, gibberellins, abscisic acid (ABA), and ethylene
has a large share in the direct promotion of plant growth. These hormones can
directly, or, in combination with other bacterial secondary metabolites, stimulate
plant growth usually, in a concentration-dependent manner (Patten and Glick 2002).
Rhizobacteria can be elucidated as bacteria inhabiting the rhizosphere including
bacteria colonizing the root proximities, and the rhizoplane (exo-root) also incorpo-
rates the bacteria that penetrate into the root cortex (endo-root). Most of the rhizo-
spheric bacteria that have plant growth-promoting properties are endophytic in
nature (Schmidt and Baldwin 2008). Bacillus, Pseudomonas, Enterobacter,
Klebsiella, Serratia, and Streptomyces are among the most predominant rhizo-
spheric bacteria. Endophytes are found within the roots but are also observed in
other parts of the plants such as stems, seeds, tubers, and unopened owers.
Endophytic PGPR can further be differentiated as extracellular endophytic PGPR
(ePGPR) and intracellular endophytic PGPR (iPGPR). iPGPR can enter inside the
plant cell and are able to produce specialized structures called nodules. ePGPR are
prominently found in the rhizosphere or rhizoplane or within the apoplast but are
never observed inside the plant cells. According to their vicinity to the roots, ePGPR
can be further divided into (a) those colonizing root zone but are not in actual con-
tact of the roots, (b) those colonizing rhizoplane, and (c) those living in the spaces
between cortical cells of the roots (Dimkpa etal. 2009a, b).
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15.2.1 Induced Systemic Resistance VersusInduced Systemic
Tolerance
Various plant growth-promoting activities have been associated with PGPR which
affects the plant growth and promotion directly and indirectly. Plant growth promo-
tions through direct mechanisms involve enhanced release of phytohormones and
mobilization of nutrients by the PGPR strains in the rhizospheric regions which can
further be absorbed by host plant, thereby positively affecting their growth. Plant
growth promotions through indirect mechanisms occur when rhizobacteria prevent
the effect of phytopathogens (Kloepper etal. 2004).
Few strains of PGPR can also result in suppression of plant diseases caused by a
variety of pathogens through production of physical and chemical changes associ-
ated with plant defense, and this process is called induced systemic resistance
(ISR) (Lucas et al. 2014). Recent reports suggest that PGPR also play a role in
increasing plant response to abiotic stresses such as drought, salinity, high and low
temperature, etc. This phenomenon was termed as “Induced Systemic Tolerance”
(IST) (Yang etal. 2016).
15.2.2 Mechanisms ofStress Tolerance Mediated by PGPR
The key to the adaptation and survival of crop-plant and associated rhizobacteria is
the establishment of fruitful interactions between both the partners. Hence induced
systemic tolerance (IST) is the term applied to explain the microbe-mediated induc-
tion of abiotic stress responses (Fig.15.1). The role played by microorganisms to
reduce the deleterious effects of abiotic stresses in plants has been the area of con-
cern from the last few decades (Sharma etal. 2016; Sirari etal. 2016;Meena et.al.
2017).
Table 15.1 summarizes a few of the examples published on benecial effects of
bacteria on plants under various abiotic stress, bacteria involved in the interaction,
and the plant species to which they are applied. Common adaptation mechanisms
of plants exposed to environmental stress such as water and nutrient deciency or
toxicity due to heavy metal exposure generally include changes in root morphol-
ogy. The process of change or any alteration in root morphology has major involve-
ment of phytohormones such as auxin. Auxin particularly indoleacetic acid (IAA)
are produced in the plant shoot region and are then transported downward to root
tips, where they result in enhancement of cell elongation which results in better
root growth. Auxins also result in the promotion of the lateral root initiation. The
majority of rhizobacteria that exhibit a benecial effect on plant growth have been
shown to produce elevated levels of IAA. Hence inoculation of stress-affected
plant species with such bacteria will result in better growth of roots and enhanced
lateral root and root hair formation (Kajic etal. 2016;Damodara etal. 2018;Dimkpa
etal. 2009a, b).
Promotion of root growth results in a larger root surface and can, therefore, have
positive effects on water acquisition and nutrient uptake. In addition to all the
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332
abovementioned characteristics, rhizospheric bacteria contribute to the regulation
of ACC deaminase activity which further helps in the survival and growth of crop
plants under abiotic stress(Glick etal. 2007;Bargaz etal. 2015). Various mecha-
nisms which aid in elevated tolerance against abiotic stresses will be further
explained in detail.
15.3 Rhizobacteria-Mediated Salt Tolerance
Salinity is one of the most serious factors which limit the productivity of agricul-
tural crops, with adverse effects on germination, plant vigor, and crop yield world-
wide, more than 45 million hectares of irrigated land has been damaged by salt, and
1.5 million hectares are taken out of production each year as a result of high salinity
levels in soil. High salinity affects plant in various ways which include water stress,
ion toxicity, nutritional disorders, oxidative stress, alteration of metabolic processes,
membrane disorganization, and reduction of cell division and expansion of
genotoxicity.
All the vital processes such as photosynthesis, protein synthesis, and metabolic
processes are majorly affected during the establishment of salt stress. During initial
exposure to salinity, the rst symptom which occurs is water stress experienced by
crop plants which result in reduced leaf expansion. Stress due to increased salinity
Fig. 15.1 Induced systemic tolerance (IST) elicited by PGPR against drought, salt, and fertility
stresses underground (root) and aboveground (shoot)
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also results in an imbalance in osmotic pressure and further hampers cell expansion
and cell division and also inhibits stomatal functioning.
With prolonged exposure to salt stress, plants experience ionic stress which fur-
ther leads to early senescence of adult leaves and results in a reduction of leaf area
available for photosynthesis for supporting continuous growth. Excess of Na+ ions
and Cl− can affect plant enzymes and leads to cell swelling, reduced energy produc-
tion, and various other physiological changes. Various studies suggest that inocula-
tion with rhizobacteria can mitigate the deleterious effects of salt stress in different
plant species (Barassi et al. 2006). Many reports suggest that Azospirillum-
inoculated seeds of lettuce (Lactuca sativa) showed elevated rates of germination
leading to better vegetative growth than non-inoculated control plants when sub-
jected to salinity stress(Asari 2015).
It is reported that sodium uptake remains unchanged when plants are inoculated
with rhizospheric bacteria. Furthermore, the inhibition of photosynthesis was less
Table 15.1 Benecial effects of inoculation with selective PGPR on plant growth under abiotic
stress conditions
Stress type Bacterial inoculate Plant species Reference
Salt Azospirillum brasilense Pea (Phaseolus vulgaris) Dardanelli etal.
(2008)
Salt Pseudomonas syringae Maize (Zea mays) Nadeem etal. (2007)
Salt P. uorescens Groundnut(Arachis
hypogaea)
Sarvana Kumar and
Samiyappan (2007)
Salt Azospirillum Maize (Z. mays) Hamdia etal. (2004)
Salt A. brasilense Chickpeas (Cicer
arietinum), faba beans
(Vicia faba L.)
Hamaoui etal. (2001)
Drought Osmotolerant bacteria
(not completely
characterized)
Rice (Oryza sativa) Yuwono etal. (2005)
Drought Achromobacter
piechaudii
Tomato (L. esculentum),
pepper (Capsicum
annuum)
Mayak etal. (2004b)
Drought Azospirillum Wheat (T. aestivum) Cecilia etal. (2004)
Drought A. brasilense Maize (Z. mays) Casanovas etal.
(2002)
Temperature Burkholderia
phytormans
Grapevine (Vitis vinifera) Barka etal. (2006)
Temperature B. phytormans Potato (Solanum
tuberosum)
Bensalim etal. (1998)
Temperature Aeromonas hydrophila,
Serratia liquefaciens
Soy bean (Glycine max) Zhang etal. (1997)
Nutrient
deciency
Bacillus polymyxa,
Mycobacterium phlei
Maize (Z. mays)Egamberdiyeva (2007)
Iron toxicity Bacillus subtilis,
Bacillus megaterium
Rice (O. sativa) Asch and Padham
(2005) and Terre etal.
(2007)
Bacillus sp.
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334
dreadful in the plants inoculated with PGPR as compared to uninoculated variants
under salinity stress(Hahm etal. 2017). For instance, tomato plants inoculated with
Achromobacter species exhibit comparatively less serious effects of salinity stress.
Though the exact mechanism remains elusive, it has been reported that other than
regulation of bacterial deaminase, increased absorption of phosphates and potas-
sium plays a key role in the management of salinity stress (Mayak etal. 2004a).
Most of the rhizospheric bacteria are the inhabitants of the root surface and are
also observed in spaces between rhizodermal layers and root hairs, while few are
found in rhizosphere without being in actual contact of the root system. Exudates of
the roots and sloughed-off cells are enriched with avonoids, phenolic compounds,
and organic acids which play an essential role in inducing benecial effects on
stress-affected plants. PGPR contributes in growth promotion of stress-affected
plants through elevated assimilation of nutrients, by regulating nitrogen xation,
and solubilization of phosphates and also controls plant pathogen through competi-
tion and antagonism (Ilangumaran and Smith2018).
Regulation of abiotic stress can be achieved by inoculation with PGPR through
direct and indirect mechanisms which further leads to the induction of systemic
tolerance. Different species of PGPR have been explored for their abilities to
improve plant water relations, ion homeostasis, and elevated photosynthesis ef-
ciencies. Alleviation of stress is achieved by complex interactions between signal-
ing events which occur during plant-microbial interactions (Smith et al. 2017).
Colonization of the root surface and extracellular polysaccharide matrix by PGPR
results in the formation of a protective barrier against salinity stress. Few extracel-
lular molecules which act as signaling agents result in manipulation of phytohor-
mone status of the crop plants. This leads to amplied root-to-shoot communication
which results in the improvement of water and nutritional balance and stomatal
conductance. When stimulation of osmolyte accumulation occurs, it may result in
retarded leaf senescence which contributes to photosynthesis. Regulation of water
potential and stomatal conductance is affected by hydraulic conductivity and rate of
transpiration. For instance, few reports suggest that maize plants which were inocu-
lated with Bacillus megaterium result in enhanced hydraulic conductivity compared
to uninoculated plants when subjected to salt stress. Elevated hydraulic activity is
shown to be connected with high expression of plasma-membrane protein—aqua-
porin. Rhizospheric bacteria results in the induction of enhanced osmolyte accumu-
lation and signaling of phytohormones which contributes to the survival of the
plants through initial salinity stress (Marulanda et al. 2010). PGPR restrict salt
uptake of the plant by capturing cations in the exopolysaccharide matrix, resulting
in alteration of root structure and further regulates expression of ion afnity trans-
porters. The mineral nutrient acquisition of both micro- and macronutrients is
enhanced due to inoculation with PGPR which mitigates the effects of the high
inux of Na+ and Cl−. The maintenance of ion homeostasis is regulated by PGPR by
reducing accumulation of Na+ and Cl− in leaves and other parts of the plants. PGPR
also improves the activity of high-afnity K+ transporters to alleviate salinity stress.
The literature suggests that inoculation of stress-affected plants with Azotobacter
strains results in elevated K+ uptake and Na+ exclusion leading to increased contents
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335
of chlorophyll, proline, and polyphenols which makes it evident that inoculation
with PGPR enhances plant’s response during stress (Rojas Tapias etal. 2012).
15.4 Rhizobacteria-Mediated Temperature Tolerance
Elevated temperature, a consequence of global climate change, also has an adverse
effect on crop productivity. Heat stress results in a negative inuence on photosyn-
thetic rate, plant water relations, and owering and fruiting in both tropical and
temperate crops(Drigo etal. 2008). Increased water requirements and decreased
yield in plants were reported due to a shift in maximum and minimum temperature.
Extreme changes in temperature results in a stress condition for plants. For instance,
root elongation normally takes place above species-dependent minimum tempera-
ture range and exhibits linear increase with increasing temperatures only up to spe-
cic temperatures above which the root elongation rapidly decreases resulting in
stunted development of root system. The favorable effects of different PGPR strains
on growth and physiological development of soybean plants under sub-optimal root
zone temperatures were checked, and it was observed that stimulation of rhizobac-
teria is interactively dependent on the temperature of the rhizosphere. It has often
been asserted that growth-promoting consequences are associated with nitrogen
xation, but the positive effects were observed and resulted in physiological changes
in the plants even before the commencement of the nitrogen xation pro-
cess(Govindasamy etal. 2008).
This proves that mechanisms which function for the alleviation of temperature
stress in rhizobacteria are independent of nitrogen status. The stimulation of genes
in response to elevated temperature stress is regulated by heat stress transcription
factors (Hsfs). Plant Hsfs have a highly composite gene family which consists of
approximately more than 20 members, and the appearance of heat shock-induced
Hsfs genes are reported to modulate transcription during the prolonged response to
heat shock (Baniwal etal. 2004). Breeding of cultivars which are heat-tolerant or
development of transgenic varieties for heat-tolerance is a time-consuming and less
protable approach (Vanaja etal. 2007). Hence an approach regarding inoculation
of plants under temperature stress with rhizobacteria can be useful. Thermotolerant
varieties of Pseudomonas putida according to Srivastava etal. (2012) are a result of
overexpression of stress sigma factor σs and improved the formation of biolm at
high temperature. It was also demonstrated that heat shock proteins (HsPs) that
stabilize the membrane are induced under stress condition and confer thermotoler-
ance to rhizobacteria and thus the plant at elevated temperatures. A thermotolerant
strain of Pseudomonas spp. (AKM-P6) exhibiting PGPR activities was identied by
Ali etal. (2009) from the rhizosphere of pigeon pea grown under arid and semi-arid
zones in India. The abovementioned strains of Pseudomonas sp. help sorghum seed-
lings to cope up with heat stress through induced biosynthesis of high-molecular-
weight proteins in higher levels which results in reduced injuries to cellular
membranes and enhanced contents of metabolites such as proline, chlorophyll,
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336
sugars, amino acids, and proteins. This thermotolerance indicated by Pseudomonas
sp. AKM-P6 is predicted to be due to the production of exopolysaccharides.
Bensalim et al. (1998) also investigated the heat stress-alleviating effects of
Burkholderia phytormans PsJN on 18 clones of potato plants grown under differ-
ent temperature zones (20°C day, 15°C night, 33 °C day, 25°C night). Results
were estimated from accurate measurements of stem length, shoot, and root bio-
mass. The abovementioned parameters of plants inoculated with high temperature-
tolerant varieties suggest that colonization of the potato plants with thermotolerant
strains of rhizobacteria plays a vital role in their adaptation to heat. It was found that
tuberization was improved by as much as 63% in rhizobacteria-treated clones of
potato. One more report suggests that inoculation of grapevine (Vitis vinifera) with
the strains of Burkholderia phytormans PsJN results in lowering the rate of bio-
mass reduction and leakage of osmolyte which are prominent indicators of cell
membrane injury due to heat shock.
Abiotic stresses result in a range of complex stimuli that possess many different
yet altered attributes, and every single stimulus provides plant cell with a different
array of information. For example, stress due to low temperatures results in mechan-
ical constraints, changes in macromolecular activity, and diminished osmotic poten-
tial in the cell. Cold stress affects the growth and development of crop plants in an
unfavorable way and thereby results in reduced expression of the full genetic poten-
tial of plants by limiting metabolic receptions and proper water uptake. Membranes
rigidication is one of the many ways through which plants identify chilling stress
caused due to reduced uidity of the cellular membrane (Chinnusamy etal. 2005).
Membrane rigidication results in the induction of cold-responsive (COR) genes.
Expression of COR genes initiates activation of expression of CBF3, CBF 2, and
CBF 1 (C-repeat binding factors) during cold acclimation which regulates singling
cascade required for alleviation of cold shock. The ability of plants to cope up with
the chilling stress can be enhanced, upon exposure to low but nonfreezing tempera-
tures intermittently. Among other physiological changes induced due to cold stress
is elevated contents of sugar, proline, and anthocyanin which can be observed dur-
ing cold acclimation or hardening procedures. This can be conrmed by studies
which report that grapevine plants inoculated with rhizobacteria (Burkholderia phy-
tormans) accumulated marginally higher amounts of carbohydrates as compared
to control plants which were uninoculated variants. In addition, plants also dis-
played increased levels of proline and phenols, photosynthetic rates, and deposition
of starch (Barka etal. 2006). Such physiological changes are also representative
indicators for ISR, and hence it is proposed that rhizobacteria-mediated tolerance to
cold temperatures stress is emphatically correlated with the induction with ISR.
15.5 Rhizobacteria-Mediated Drought Tolerance
Dehydration and reduced availability of cellular water represent a common stress
challenge which plants encounter under drought, salt, and cold conditions. As water
is one of the most essential factors which affects the growth and survival of micro-
organisms. And hence water decit is an essential abiotic factor that inuences the
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agricultural productivity with high intensity and affects plant development-related
aspects such as a decreased rate of photosynthesis and reduction in available leaf
area due to premature leaf senescence. Water deciency leads to drought stress
which limits crop growth and productivity, especially in arid and semiarid
regions(Hassen etal. 2016). Rhizospheric bacteria utilize different mechanisms to
alleviate the effects of drought stress on the plant (Table15.2).
Groover etal. (2001) have investigated some of the mechanisms which include
(1) alleviation of soil drought impact through the production of exopolysaccharides,
(2) induction of resistance genes, (3) increased circulation of water in plants, and (4)
synthesis of ACC deaminase, indoleacetic acid, and proline. PGPR are involved in
mitigating the impact of drought on plants through a process so-called induced sys-
temic tolerance (IST) which includes (a) cytokinin production, (b) production of
antioxidants, and (c) degradation of ethylene precursor ACC by bacterial ACC
deaminase (Milosevic etal. 2012).
Drought stress also results in activation of a large army of genes which are often
referred to as “stress genes.” Most of the genes which are activated in response to
drought stress are also responsive to other abiotic stresses such as salinity stress or
chilling stress. For instance, RD 29A rhizobacteria have been shown to result in
modication of the root sensitivity, growth of leaves, and also increased tolerance to
soil trying evidently by inuencing ethylene signaling pathway(Rubin etal. 2017).
The ACC deaminase activity of Achromobacter piechaudii has been reported to
provide tolerance against water decit in tomato and pepper plants, resulting in a
marginal improvement in fresh and dry weights of the stress-affected plants.
Ethylene production was signicantly reduced in the plants which were inoculated
with tolerant PGPR strains. It also results in improved recovery from water- decient
soils although inoculation did not inuence relative water contents at signicant
levels (Mayak etal. 2004a, b).
Table 15.2 Effects of rhizobacteria on mitigation of drought stress in crops
Microorganism Crop Mechanism
Pantoea agglomerans Wheat EPS production which affects the structure
of rhizospheric soil
Rhizobium sp. Sunower Production of EPS which affects the
structure of rhizospheric soil
Pseudomonas putida P45 Sunower Production of EPS which affects the
structure of rhizospheric soil
Azospirillum sp. Wheat Increased water circulation
Achromobacter piechaudii Tomato
pepper
Synthesis of ACC deaminase
ARV8
Variovorax paradoxus Pea Regulation of ACC deaminase
Pseudomonas sp. Pea Reduced ethylene production
AM fungi Sorghum Enhanced water circulation
Brome mosaic virus (BMV) Rice Unknown
Pseudomonas mendocina and
Glomus intraradices
Lettuce Increased antioxidative status
Bacillus megaterium and Glomus
sp.
Clover Production of indoleacetic acid and proline
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On exposure to drought tolerance, maize seedlings inoculated with Azospirillum
brasilense displayed enhanced relative and absolute water contents in comparison
to non-inoculated plants. Inoculation with Azospirillum also results in prevention of
signicant drop in water potential which is closely interlinked with root growth,
total aerial biomass, and foliar area and is also associated with proline accumulation
in leaves and roots. The effects of drought tolerance were more evident at a 75%
reduction in the water supply as compared to a 50% reduction. Thus, these results
suggest that PGPR play a key role in providing resistance and increasing adaptation
of plants to drought stress and have a vital role in solving future food deciency
problems. It is also reported that interaction between plants and rhizobacteria under
drought stress affects plants as well as leads to positively change the soil properties.
The mechanisms elicited by rhizobacteria such as triggering osmotic response and
induction of novel genes play a vital role in the survival of plants under drought
stress. The development of drought-tolerant crop varieties through genetic engi-
neering and plant breeding is essential, but it is a time-consuming process. PGPR
inoculation to alleviate drought stress in plants opens a new chapter in the applica-
tion of microorganisms in dryland agriculture (Varukonda etal. 2016).
15.6 Rhizobacteria-Mediated Pesticide Tolerance
Pesticide accumulation in soils beyond the recommended safety levels occurs either
by repeated application or due to their gradual degradation rate. The effect of pesti-
cide on plant growth occurs by an alteration in plant root’s architecture. This results
in the appearance of a number of root sites for infection by rhizobacteria and the
transformation of ammonia into nitrates. This process of the transformation of
microbial compounds to plants is made easier by the rhizobacterial infection. With
the abovementioned changes in plant growth and development, the activity of free-
living or symbiotic nitrogen-xing bacteria has also been positively affected through
rhizobacterial infection (Gopalakrishnan etal. 2015). Various strains of rhizobacte-
ria have the displayed ability of pesticide degradation due to the activation of deg-
radative genes carried by plasmids or anked by transposons/chromosomes (Kumar
etal. 1996). From the studies it was suggested that very few strains of rhizobacteria
have the ability to tolerate pesticide stress under actual eld conditions, and hence
research on isolation, identication, and characterization of such pesticide-tolerant
species of rhizobacteria needs to be pursued in detail as such rhizobacteria are
essentially required in present-day conditions of ever-growing pesticide contamina-
tion in elds and considering the magnitude of pesticide residue generated.
15.7 Rhizobacteria andHeavy Metal Resistance
Various industrial operations discharge multiple types of heavy metals and upon con-
sequent accumulation in ecological systems create a massive threat to the varied
agroecosystems. When heavy metals like arsenic, mercury, cadmium, and lead which
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are highly toxic to plants accumulate into the soil to abnormal levels, it causes a dra-
matic change in microbial composition and their activities (Cheung and Gu 2007)
which leads to a consequent loss in soil fertility. Once the cytosolic metal concentra-
tion in plants increases above the tolerable limit, phytotoxicity of heavy metal inhib-
its transpiration and photosynthesis, disturbs carbohydrate metabolism, and drives
the secondary stresses like nutrition stress and oxidative stress which collectively
affects the plant’s growth and development (Kraemer and Clemens 2005).
For differentiation between a standard and stress response against heavy metal
contamination, it is essential to characterize the minimum and maximum concentra-
tion of every metal for different varieties of soil (Carmen and Roberto 2011).
Responses of rhizobacteria toward some of these heavy metals have been well-
documented. Many rhizospheric bacteria release metal-chelating substances in rhi-
zosphere such as iron-chelating siderophores. Siderophore production by bacteria
has been shown to signicantly affect plant uptake of metals. Hence rhizobacteria
can positively affect the bioavailability of heavy metals that can prove to be
extremely toxic to plants even in low concentrations (Dimkpa etal. 2009a, b). Due
to variation in soil conditions, metal valences are also affected, which can be cor-
related to microorganism to be specic rhizobacteria which also alter the metal
bioavailability by acidifying the microenvironment and by signicantly affecting
redox potential. Autotrophic and heterotrophic leaching of heavy metals which
results in enhanced volatilization through methylation process and release of metal
chelators such as siderophores can help in the mobilization of heavy metals. This
way, sorption of heavy metals to cell components is essentially the result of intracel-
lular sequestration or precipitation as insoluble organic compounds which reduce
heavy metal toxicity to plants (Gadd 2004).
Barley plants which were grown on contaminated soil with high contamination
of cadmium obtained 120% higher grain yield and a twofold decrease in Cd con-
tents in grain when the plants were inoculated with commercially available PGPR
Klebsiella mobilize CIAM 880. Stimulation of these effects was studied with a
mathematical model which indicates migration of rhizobacteria from rhizoplane to
rhizosphere where they form a complex with the heavy metal, making it nonavail-
able for the plant uptake (Pishchik etal. 2002). High intracellular carbohydrates and
large cell inclusions increase the resistance of Rhizobium leguminosarum to cad-
mium, copper, nickel, and zinc, whereas production of those has also been shown to
counter heavy metal-induced oxidation. In Rhizobium-legume symbiosis, it is usu-
ally the plant that is the limiting factor regarding tolerance to metal toxicity for
metals such as aluminum, copper, iron, and cadmium. Nodules help plants survive
because bacteroids counter metal stress (Balestrasse etal. 2001).
15.8 Conclusion andFuture Perspective
In the present-day scenario when we are experiencing the threat of global warming,
the agricultural production methodology should be designed by considering the
ever-changing environmental conditions and the availability of different types of
15 Rhizobacteria: Legendary Soil Guards inAbiotic Stress Management
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stresses. Plant growth-promoting rhizobacteria can be utilized to mitigate the biotic
stresses and can confer elevated tolerance to abiotic stresses in the host
plant(Tabassum etal. 2017). Thus, identication and detailed analysis of rhizobac-
terial strains that have the capabilities of providing cross-protection against multiple
stress factors will be highly important (Dimkpa etal. 2009a, b). Induced systemic
response (ISR) in the crop plants may be critically important for the ability of rhi-
zobacteria to alleviate the effects of both biotic and abiotic stress. Thus, the infor-
mation obtained from a detailed analysis of ISR against plant pathogens will be
applicable in understanding signaling cascades induced by PGPR which results in
elevated tolerance to abiotic stresses. The rhizosphere is a unique environmental
niche which provides habitats and nutrients to rhizospheric bacteria which in return
provides numerous benets of better plant growth, defense against infections against
phytopathogens, and survival of plants under different types of stress.
However, the amount of success in obtaining the benets of PGPR tends to
decrease as it moves from laboratory experiments to the greenhouse and nally to
elds, which suggests that there is need of research on the various aspects of PGPR
under eld conditions. Therefore, generation of research data and knowledge on
screening protocols and strain improvement of ideal rhizobacterial strain for rhizo-
spheric competence and sustainability is the current need to enhance eld level
successes (Gopalakrishna etal. 2015). The application of PGPR to help plants cope
up with the stress in the agricultural eld seems laborious, yet a lot is left to be uti-
lized (Ilangumaran and Smith 2018). As various types of abiotic stresses are serious
threats to total crop yield worldwide, agricultural experts are working to nd quicker
and reliable solutions as annual crop production is seriously affected by higher
degree from abiotic stresses. Hence at the moment, expanding the geographical
area, nding new strategies for breeding for abiotic stress tolerance, and detailed
analysis of rhizobacteria-mediated alleviation of abiotic stresses are essential areas
of focus. Among all of this, PGPR-mediated abiotic stress management has gained
enormous popularity and has attracted a lot of interest as it has the ability to serve
the purpose in an economical manner. This way, indigenous microbes should be
provided with prime importance for the successful achievement of the task as they
have better acclimation ability over an imported strain (Sarma etal. 2012).
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