As the world’s population increasing
continuously and the environment gets damaged due to adverse condition like
global warming, it is impossible to produce sufficient food to feed the
population. For instance, the world’s population which is currently 7.8 billion
will increase up to around 10 billion in 2050, the increasing population also
increase demand of food, mostly it required more than double amount of food
[1-4]. To fulfil their need of food demand, it is absolutely indispensable to
increase the agriculture production either by using more land for crop
production or by means of different types of chemical fertilizers, pesticides,
insecticides etc. But the availability of land area is limited due to
urbanization and industrialization while available area already under
cultivation is simultaneously losing fertility due to different anthropogenic
activity7 like use of chemical fertilizers, pesticides. In recent time,
majority of the world is facing climate change due to global warming which
leads to different stress conditions like severe drought, very high
temperature, salinity, soil acidity etc., and ultimately decreases agricultural
production. Low water availability and extreme temperature adversely affect to
the plant growth and reduce the food production. In past decades, the intensity
of drought stress increased and became most destructive abiotic stress that
affect world’s food security. It is expected that by 2050, serious plant growth
problems generated for more than 50% of the arable lands by drought stress
[5,6]. Severe drought reduced gross primary productivity of Europe by 30% in
2003 [7] and maize production of United States by 25% in 2010 [8]. According to published data from 1980 to
2015, drought stress reduced the 21% wheat yield and 40% maize yield worldwide
[9].
Plant growth and development under drought
stress
As plants are
sessile, their normal physiological functions and metabolism easily get
affected by abiotic stresses such as extreme temperatures, excess water
flooding or deficient, salinity, as well as heavy metal. High temperature
generates excessive heat which produces reactive oxygen species (ROS) and
further leads todegradation of membranes, denaturation of
proteins as well as nucleic acids while at low temperature plants get
dehydrated due to the freezing condition. Inadequate nutrients in soil, change
in pH or microbial community of soil also affect the growth as well as
development of plant. The availability of water is the prominent factor
responsible for the promotion of growth and survival of plant as it adversely
affects the normal metabolic processes of plant [10]. Among all abiotic stress,
drought stress has major effect on the normal growth and metabolism [11,12],
physiological, biochemical and morphological characteristics of plant [13,14].
Insufficient water availability generates osmotic stress and cell dehydration
which hinder the cell division and cell elongation [15]. Drought stress also
affects various physiological characteristics like relative water content
(RWC), stomatal conductance, transpiration rate, relative electrical conductivity,
maximum quantum efficiency of photosystem II (Fv/Fm ratio), leaf water
potential, net CO2 assimilation rate, malondialdehyde (MDA) content, turgor
pressure etc. Furthermore, drought stress declines the photosynthetic capacity
of plant cells due to stomata closure which reduces the availability of CO2 and
also due to reduced turgor pressure and oxidative stress via ROS which reduced
the chlorophyll content in leaves [16-19]. Several species like sweet corn,
sunflower mung bean Paulownia imperialis bean and Carthamus tinctorius has been
studied with decreased chlorophyll content under drought stress. Growth of
several species such as barley maize rice and wheat has been reduced under
drought stress. Water conservation occurs in plant tissue due to less
availability of water under drought and salt stress, which results in low gas
exchange and stomatal conductance in leaves that cause damage and death of
tissues. As plant uptake the nutrient from soil through root by osmosis, the
diffusion rate as well as availability and transportation of many nutrients
like S, Mg, Ca, P and nitrate is reduced in drought stress [20-32]. Drought
also induces the generation of reactive oxygen species (ROS) such as hydrogen
peroxide, hydroxyl radicals, superoxide radicals, singlet oxygen that
diminishes the antioxidative defence and produce oxidative stress This
oxidative stress with high concentration of ROS causes various changes in plant
like degradation of proteins, lipids and nucleic acids, cell membrane
degradation, lipid peroxidation [33-35]. Overproduction of ROS in chloroplast,
mitochondria lead to reduction in CO2 uptake and affect the rate of
photosynthesis Seed germination and plant growth require enough quantity of
water but in drought condition plant size, leaf area and size remain small and
seedling development either stop completely or occurs delay. Drought stress
also affect to the various plant’s biochemical activities like nitrate
reductase activity due to limited uptake of nitrate from the soil by plant roots
it enhances the synthesis of ethylene, a plant growth hormone which adversely
affect to root and shoot growth of the plant [36-40]. Finally, the quantity and
quality of plant growth is negatively affected by the drought stress, so to
overcome this problem and increase the food production to fulfil the
requirement of increasing population, the alleviation of drought stress is
important.
Microbial biome thriving under drought
stress
Enormous number of microorganisms exists in the
rhizosphere and on the root surface which have a substantial influence on the
growth pattern of plant by producing different plant hormones and metabolites
The microbial community in rhizosphere may get change under the influence of
drought stress, like, population of gram-positive bacteria such as
actinobacteria, Bacillus, Pseudomonas get enhanced in comparison with
gram-negative bacteria such as Bacteroidetes, Proteobacteria The change in
community of the root associated bacteria in rhizosphere and rhizoplane may
lead to stress adapted bacterial colonization that enhance tolerance against
drought stress Altered distribution of bacteria in rhizosphere and root
associated soil was observed and compared to uncultivated soil of drought
sensitive pepper plant (capsicum annuum L.) indicates effect of metabolic
activity of plant on the microbial community [41-45]. In another study, pepper
plants inoculated with drought tolerant bacteria which were isolated from
desert revealed a more resistance against drought situation as compared to uninoculated
plant. The root growth increased by 40% in inoculated plant which enhanced
water uptake capability of plant Salicornia, a halotolerant plant and their
rhizobacterial microbiome showed tolerance against abiotic stress by performing
various plant growth promoting (PGP) traits and high colonization with root
indicating that halotolerant bacteria isolated from saline environment are
potent to enhance plant growth under both salinity and drought stress The
growth of Brassica rapa is reduced with change in chlorophyll content, number
of flowers and leaf with their simplified indigenous microbial community as
compared to complex indigenous rhizobacterial community. This suggest that
microorganisms present in soil help to mitigate the effect of abiotic stress by
modulating plant growth promoting traits .Comparatives studies of microbial
diversity in two soil samples one from agriculture soil and another from Egypt
desert soil showed difference in microbial population for instance,
extremophiles were present in desert soil while absent in agriculture soil,
whereas agriculture soil was found to be rich with Bacillus sp. and
Paenibacillus sp. which promotes plant growth [46-50]. This suggests that
microbiome of soil vary with variation in soil pH, salinity, water content,
temperature, metal ion concentration and it is unique to each ecosystem. The
synergistic interaction between plants and microbiome of rhizosphere helps to
mitigate abiotic stress by a different mechanism [51-53]. Among them, plant
growth promoting rhizobacteria (PGPR) can facilitate the plant growth via
different mechanism such as directly modulate the plant hormone level and
enhance uptake of micronutrient or indirectly prevent the deleterious effect of plant pathogen by
producing antagonistic response or by inducing resistance to pathogen . PGPR
cleave 1-aminocyclopropane-1-carboxylate (ACC), a precursor of plant ethylene
by producing ACC deaminase and modulate the level of ethylene in a plant in
response to stress condition [54-56]. Inoculation of ACC deaminase producing
Pseudomonas lini and Serratia plymuthica significantly increased leaf relative
water content, plant height, ROS scavenging enzymes, root and shoot dry weight,
soil aggregate stability, regulating IAA and ABA level, reducing malondialdehyde
in jujube plant and alleviate the adverse effect of drought stress. The drought
sensitive wheat (Triticum aestivum L.) when treated with Streptomyces pactum
Act12 under drought stress, it shows increased growth of wheat by increasing
21.3% fresh shoot weight, 10.3% shoot length, 13.6% root length. It also
increases total soluble sugar content, proline and also upregulate the
expression of drought resistance related genes like SnRK2, EXPA2, P5CS in
addition decrease the malondialdehyde content around 20.5% in wheat
[58].Microorganisms also exist as an endophyte which colonize stems, leaves,
roots, tubers and other organelles of the plant and provide protection to plant
against environmental stress by employing various mechanisms [57-60]. Endophytes
helps plant to survive in drought stress by enhancing relative water content,
antioxidants and root growth in addition to releasing various growth regulating
substances like IAA, ABA, ACC deaminase . The extent of colonization by
endophytes to host plant organs and tissues indicate the ability of PGPR to
adapt specific ecosystem and make favorable environment for their existence.
According to Chen et al, the endophytic bacterium Pantoea alhagi, which is
isolated from wheat enhance the drought tolerance as well as root length and
plant growth by adopting mechanisms like production of siderophores, IAA,
ammonia. Khan et al. reported that, when plants inoculated with drought
resistant endophytic bacteria Pseudomonas azotoformans ASS1 isolated from
Alyssum serpyllifolium leaves, enhance chlorophyll content, peroxidase,
proline, catalase, superoxide dismutase and decrease the malondialdehyde
content in plant under drought stress.
PGPR mitigating drought stress
Various microorganisms can be used to
increase plant growth under different stress conditions inexpensively. These
beneficial microorganisms can be used as bioinoculants to enhance plant growth
which mainly colonize the rhizospheric area and they can be classified into
three major categories: i) Plant growth promoting rhizobacteria (PGPR) ii)
Arbuscular mycorrhizal fungi (AMF) and iii) Nitrogen fixing rhizobia [61-66].
Plant growth-promoting rhizobacteria (PGPR) are the soil bacteria occupying on
the surface of plant roots (rhizoplane), within root tissues (endophytes) or in
soil near root area (rhizosphere) that can boost plant growth under various
abiotic stresses. In rhizosphere, plant and their associated bacteria establish
mutually beneficial relationship for instance PGPRs use plant root exudate for their
requirement of carbon, enzymes, nutrient, hormones to derive nourishment and in
return they enhance the availability and uptake of nutrients for plants and
promote healthy plant growth [67-72]. Plant roots make the rhizosphere a niche
for microbial activity and sustain the soil fertility by interacting with both
soil and microorganisms Different genera of PGPR decrease the damage caused by
environmental stresses and increase the tolerance in plants towards stress
condition by adapting different direct mechanisms such as, production of
phytohormones, phosphate solubilization, nitrogen fixation, iron sequestration,
exopolysaccharide production and indirect mechanisms such as production of
hydrogen cyanide (HCN), siderophore production, synthesis of ACC deaminase, production of antibiotics, Induced systemic
resistance (ISR), production of lytic enzymes, production of osmo-protectants
and antioxidants production (Figure 1).deleterious effect of plant pathogen by
producing antagonistic response or by inducing resistance to pathogen . PGPR
cleave 1-aminocyclopropane-1-carboxylate (ACC), a precursor of plant ethylene
by producing ACC deaminase and modulate the level of ethylene in a plant in
response to stress condition [54-56]. Inoculation of ACC deaminase producing
Pseudomonas lini and Serratia plymuthica significantly increased leaf relative
water content, plant height, ROS scavenging enzymes, root and shoot dry weight,
soil aggregate stability, regulating IAA and ABA level, reducing malondialdehyde
in jujube plant and alleviate the adverse effect of drought stress. The drought
sensitive wheat (Triticum aestivum L.) when treated with Streptomyces pactum
Act12 under drought stress, it shows increased growth of wheat by increasing
21.3% fresh shoot weight, 10.3% shoot length, 13.6% root length. It also
increases total soluble sugar content, proline and also upregulate the
expression of drought resistance related genes like SnRK2, EXPA2, P5CS in
addition decrease the malondialdehyde content around 20.5% in wheat
[58].Microorganisms also exist as an endophyte which colonize stems, leaves,
roots, tubers and other organelles of the plant and provide protection to plant
against environmental stress by employing various mechanisms [57-60]. Endophytes
helps plant to survive in drought stress by enhancing relative water content,
antioxidants and root growth in addition to releasing various growth regulating
substances like IAA, ABA, ACC deaminase . The extent of colonization by
endophytes to host plant organs and tissues indicate the ability of PGPR to
adapt specific ecosystem and make favorable environment for their existence.
According to Chen et al, the endophytic bacterium Pantoea alhagi, which is
isolated from wheat enhance the drought tolerance as well as root length and
plant growth by adopting mechanisms like production of siderophores, IAA,
ammonia. Khan et al. reported that, when plants inoculated with drought
resistant endophytic bacteria Pseudomonas azotoformans ASS1 isolated from
Alyssum serpyllifolium leaves, enhance chlorophyll content, peroxidase,
proline, catalase, superoxide dismutase and decrease the malondialdehyde
content in plant under drought stress.
PGPR mitigating drought stress
Various microorganisms can be used to
increase plant growth under different stress conditions inexpensively. These
beneficial microorganisms can be used as bioinoculants to enhance plant growth
which mainly colonize the rhizospheric area and they can be classified into
three major categories: i) Plant growth promoting rhizobacteria (PGPR) ii)
Arbuscular mycorrhizal fungi (AMF) and iii) Nitrogen fixing rhizobia [61-66].
Plant growth-promoting rhizobacteria (PGPR) are the soil bacteria occupying on
the surface of plant roots (rhizoplane), within root tissues (endophytes) or in
soil near root area (rhizosphere) that can boost plant growth under various
abiotic stresses. In rhizosphere, plant and their associated bacteria establish
mutually beneficial relationship for instance PGPRs use plant root exudate for their
requirement of carbon, enzymes, nutrient, hormones to derive nourishment and in
return they enhance the availability and uptake of nutrients for plants and
promote healthy plant growth [67-72]. Plant roots make the rhizosphere a niche
for microbial activity and sustain the soil fertility by interacting with both
soil and microorganisms Different genera of PGPR decrease the damage caused by
environmental stresses and increase the tolerance in plants towards stress
condition by adapting different direct mechanisms such as, production of
phytohormones, phosphate solubilization, nitrogen fixation, iron sequestration,
exopolysaccharide production and indirect mechanisms such as production of
hydrogen cyanide (HCN), siderophore production, synthesis of ACC deaminase, production of antibiotics, Induced systemic
resistance (ISR), production of lytic enzymes, production of osmo-protectants
and antioxidants production (Figure 1).fixation, solubilizing phosphate, siderophore
production which help in plant growth.
Production of Growth regulators
Plant produced
various phytohormones such as cytokinin, auxins, ethylene, gibberellins and
abscisic acid (ABA) which play a significant role in plant growth and
development and help to escape or tolerate the stressful abiotic conditions by
modifying nutrient allocation, source/sink transition [84-86]. PGPRs also
synthesize various phytohormones including cytokines, gibberellins, and IAA,
which further simulate plant growth under unfavourable environmental conditions
[78-88]. They have an ability to manipulate plant hormone balance and even
modify the plant hormone crosstalk by altering root to shoot signaling and
hormone concentration to provide protection against abiotic stress .The most
prominent and physiologically active phytohormone, which regulate the various
developmental processes in plant either directly or indirectly are auxins. In
auxins, the most important plant growth promotor is Indole-3-acetic acid (IAA),
which is produced by both plants as well as PGPRs and essential for
rhizobacteria – plant interactions [89-90]. IAA is the most abundant
phytohormones secreted by the bacteria which is produced by around 80% of
rhizospheric bacteria. IAA play important role in different plant activities
like, cell division, cell and tissue differentiation, lateral and adventitious
root initiation and development, vegetative growth, seed and tuber germination,
fruit development, leaf formation, abscission, embryo development etc. In
microorganisms, IAA mainly synthesized via tryptophandependent pathway. IAA production is a direct
mechanism of PGPR by which they enhance the growth of root system by increasing
number as well as surface area of root tips, adventitious root differentiation
from stem and thereby promoting uptake of water and minerals more efficiently under
drought stress [91-93].Wheat plant inoculated with Bacillus subtilis (LDR2)
under drought stress, it enhanced IAA content (80%), total biomass, Fv/Fm
value, net CO2 assimilation, stomatal conductance, transpiration rate and
decreased the ABA as well as ACC content. It also downregulates the AUX/IAA1
gene expression and upregulate the TaCTR1 and TaDREB2 gene expression in wheat
plant under drought stress as compared to uninoculated plant. According to
Jochum et al. [94], Bacillus spp. 12D6 and Enterobacter spp. 16i increased the
root length, root diameter and surface area, root branching by excreting IAA in
wheat and maize plant under water deficit condition as compared to the control.
Inoculation of wheat plant with IAA rhizobacterial strain of genus Bacillus,
Enterobacter, Moraxella and Pseudomonas lead to significant improvement of root
growth, shoot length, number of tillers and spikelet, spike length and grain
weight under drought conditions as compared to control condition. Combinations
of PGPRs significantly improved yield parameters in wheat plant as compared to
single PGPR [95]. PGPR isolated from rhizosphere of tea plant Enterobacteria
lignolyticus strain TG1 produced a noteworthy amount of IAA (92.5 ± 0.2 µg
mL?1). When this PGPR under greenhouse conditions inoculated with three
different important tea clones TV1, TV19 and TV20, it showed increase root
biomass (4.3-fold), root length (2.2-fold), shoot biomass (3.1-fold) and shoot
length (1.6-fold) in contrast to control plants. In plants, at low
concentration, IAA promote the elongation of primary root, while at higher
concentration it stimulates lateral root formation with decreasing primary root
length and also increase the growth of root hairs. Abscisic acid (ABA) is a
plant stress hormone which plays vital role in plant growth under environmental
stresses by modulating physiological process. ABA regulates water content of
plants under water deficit conditions by regulating opening and closing of
stomata in response to various signals like ROS, nitric oxide etc. ABA also play role in growth of root
branching to improve water uptake and induces leaf growth by providing water
movement. For long term response, ABA also regulates the expression of stress
responsive genes. When Arabidopsis plant inoculated with ABA producing
Azospirillum brasilense, it increased the resistance towards drought and other
abiotic stress as compared to Arabidopsis mutant aba2-1, defective in ABA
biosynthesis and wild type plant [96-99].
Osmolyte Synthesis (Osmoregulation)
Under drought stress, plant adjust their physiology
and metabolism such as protection of membrane integrity, cellular osmotic
adjustment, stabilization of proteins/enzymes, and detoxification of ROS by
regulating the secretion of several compatible solutes. They are highly
soluble, low molecular weight compounds and nontoxic at high cellular
concentrations. These solutes include soluble sugars (e.g., trehalose,
sucrose), polyamines, glycine betaines, polyhydric alcohols, organic acids
(e.g., malate), inorganic ions (e.g., calcium), quaternary ammonium compounds,
water stress proteins (e.g., dehydrins), proline and other amino acids which
are useful for plant growth and to adapt stress conditions. PGPRs also secrete
osmolytes, which synergistically act with plant’s osmolytes further to boost
plant growth under stress condition [100-102]. These compatible solutes protect
the genetic material, proteins, enzymes, membranes and organelles in plant
against oxidative stress due to drought condition. It has been suggested that
proline protect different proteins like chaperones from degradation and/or
mis-folding under water stress condition by forming a protective layer around
them and by maintaining their integrity. Plants store some ions and metabolites
like proline in their vacuoles, which help plants to maintain high turgor
pressure, decrease the osmotic potential as well as maintain their
physiological and metabolic activity. Proline also provide stability to the
cell membrane by interacting with membrane phospholipids. It also regulates
cytosolic pH and NAD/NADPH ration under drought stress. Inoculation of
Arabidopsis thaliana with PGPR Pseudomonas putida GAP-P45 modulate the proline
metabolism by modulating the gene expression of both proline catabolic and biosynthetic
activity under water stress [103-106]. Soya bean plants shows increased drought
tolerance when inoculated with endophyte Sphingomonas sp. LK11 isolated from
the leaves of Tephrosia apollinea, which produces sugars and amino acids like
proline, glutamate and glycine . Shahzad et al.
Reported that when rice plants inoculated with endophytic bacteria
Bacillus amyloliquefaciens enhanced content of amino acids like proline,
cysteine, aspartic acid, glutamic acid etc. Apart from proline, PGPRs also synthesize
other osmolytes like soluble sugar such as trehalose (?-D-glucopyranosyl-1,
1-?-D-glucopyranoside), proteins such as dehydrins. The concentration of these
osmolytes synthesized by PGPR are increased under drought stress and prevent
the cell destruction and cell death. Sugar content in the leaves of plant is
reduced under extreme stress condition and it leads to destructive for the
macromolecules in cell and cell membrane. In this type of condition, trehalose
or sucrose, a non-reducing disaccharide produced by PGPR act as an
osmo-protectant and protect the plant from cell membrane and macromolecules
degradation by stabilizing degrading enzymes .In chickpea plant, the
photosynthetic efficacy and water balance maintained by inducing higher soluble
sugar accumulation by the application of combination of plant growth regulators
and consortium of PGPRs as, Bacillus subtilis, Bacillus thuringiensis and
Bacillus megaterium. It also enhanced leaf chlorophyll as well as proline
content and reduced the lipid peroxidation, antioxidant enzyme activities which creates drought
tolerance in chickpea plant [107-109]. Elevated sugar level in leaves because
activation and expression of several genes involved in photosynthesis and as a
result it increases the rate of photosynthesis in leaves under drought
condition. To cope up with the water stress condition, high sugar levels in
leaves act as signalling molecule and control several physiological processes
such as flowering, germination, photosynthesis, senescence. Trehalose
biosynthesis gene transformed Azospirillum brasilense inoculated with maize
plant, 85% maize plants survived under drought stress as compared to wild type
strain only 55% plants were survived. It significantly increases total biomass
(73%), as well as length and biomass of root and leaf under water deficit
condition by producing higher level of trehalose in maize plant inoculated with
transformed strain in contrast to inoculation with wild type strain. Choline
also plays an important role to develop water stress resistance in plant,
mainly by synthesis and accumulation of glycine betaine (GB) by acting as
osmo-protectant. GB can be synthesized by plant, animal and microorganisms by
utilizing choline or glycine as a precursor molecule with diverse mechanism. GB
protect the plants under different environmental stress condition by various
mechanisms such as, it plays vital role in maintaining the ROS level under
control by stabilizing the activities of ROS scavenging enzymes, it function as
chemical chaperone and protect the activities of malate dehydrogenase and
Rubisco enzyme under salt stress, GB also serve as molecular chaperones which
regulate the denaturation and disaggregation of proteins as well as assist
refolding of proteins and provide thermal stability to them [110-114]. Plant
growth promoting rhizobacteria Raoultella planticola inoculated in roots of two
maize variety Zheng Dan 958 (drought tolerant) and Jun Dan 20 (drought
sensitive), it was found that it accumulates higher choline and GB content in
plant cells and promoted seed germination by providing osmo-regulation under
drought stress. This higher GB content was regulated by activity of key enzymes
such as, phosphor-ethanolamine N-methyltransferase (PEAMT), choline
monooxygenase (CMO), and betaine aldehyde dehydrogenase (BADH) are involved in
the synthesis of GB in chloroplast. When maize plant inoculated with three PGPR
strains i.e., Klebsiella variicola F2 (KJ465989), Raoultella planticola YL2
(KJ465991) and Pseudomonas fluorescents YX2 (KJ465990) under varying degree of
drought stress, it enhanced the leaf relative water content and dry matter
weight with greater osmotic regulation as compared to control inoculated plant
through the improved content of choline and GB in plant cells.
Upregulation of ACC deaminase
Environmental factors
including temperature, light, nutrition, various biotic and abiotic stress as
well as concentration and presence of other phytohormones affect the synthesis
of ethylene in a plant [115-118]. Ethylene is a phytohormone which is concerned
with several physiological processes in plants like abscission, fruit ripening,
leaf and flower senescence, aging, initiation of root and root nodule
formation. Ethylene is synthesized from its precursor 1-aminocyclopropane-1
carboxylate by ACC-oxidase which is produced from S-adenosylmethionine
(S-AdoMet) by 1-aminocyclopropane-1- carboxylate synthase (ACS). Under
environmental stress conditions like salinity, drought, water logging, heavy
metal contamination, synthesis of ACC has been reported to increase further
increasing the concentration of ethylene which reduce the vegetative growth of
plant by restraining root and shoot development process and leaf expansion.
Plant use ACC deaminase produced by PGPR in rhizosphere to degrade the ACC,
resulting in reduction of ethylene production and aid the normal vegetative
growth and development of plant root and shoots. ACC deaminase produced by PGPR converts Plant
ACC (the precursor of ethylene) into ammonia and ?-ketobutyrate, there by halting
ethylene production and enhancing growth of plant even under environmental
stress condition [119 -126].ACC deaminase producing bacterial strain
Enterobacter mori, E. asburiae and E. ludwigii alleviate the effect of water
stress on wheat by reducing the release of ethylene in leaves of wheat
concomitantly reducing the activity of ACC synthase and ACC oxidase activity as
compared to non-stress condition [127]. When velvet bean (Mucuna pruriens)
inoculated with ACC deaminase producing rhizobacteria Enterobacter HS9 and
Bacillus G9, it significantly reduced the ethylene production around 50% by
reducing concentration of ACC in the roots and leaves and improved root and
shoot length as well as biomass of velvet bean under severe drought stress
condition while greater concentration of ACC was found in root and leaf of
uninoculated plant. According to Zarei er al,
sweet corn plant inoculated with combination of four ACC deaminase
producing bacteria Pseudomonas fluorescents strains i.e., P. fluorescens P1, P.
fluorescens P3, P. fluorescens P8, P. fluorescens P14 under water stress
condition. The result showed significant increase in total sugar content,
chlorophyll content, proline, photochemical efficiency of photosystems, root
growth, catalase and peroxidase activity along with it also increased the yield
traits like canned seed (27%) and ear yield (44%). When maize plant inoculated
with plant growth-promoting rhizobacteria Pseudomonas aeruginosa, Enterobacter
cloacae, Achromobacter xylosoxidans and Leclercia adecarboxylata, it showed
increase in root and shoot length, dry and fresh weight of root and shoot,
amount of nitrogen, potassium as well as phosphorous and improved the plant
growth with reducing accumulation of ethylene in plant which indicates ACC deaminase
producing ability of PGPRs [128-130]. Furthermore, Danish et al, inoculated
maize plant with Pseudomonas aeruginosa, Enterobacter cloacae, Achromobacter
xylosoxidans and Leclercia adecarboxylata with timber waste biochar, which
further increase the plant growth by increasing photosynthetic rate, stomatal conductance, content of chlorophyll and
carotenoids, grain yield as compared to control plant.
P1
are studied for their EPS producing capacity. Under non-stress condition all
four strain secrete same level of EPS while under osmotic stress condition EPS
production by Pseudomonas aeruginosa ZNP1 was increased about 2.5-fold and
Bacillus endophyticus J13 was increased about 1.5-fold. There is no change in
EPS level in case of Bacillus tequilensis J12 while it is adversely affected in
case of Pseudomonas aeruginosa PM389. When Arabidopsis thaliana inoculated with
all these four stains separately under osmotic stress and normal condition, the
fresh weight, dry weight and plant water content increased significantly under
osmotic stress as compared to no stress condition.
EPS producing bacteria, Pseudomonas putida
strain GAP-P45 when inoculated with sunflower plant, it increased RAS/RT, root
aggregation, high relative water content of leaves, plant biomass in addition
it also forms biofilm on the surface of roots of sunflower seeding and provide
the drought resistance to sunflower plants . When wheat plant treated with
EPS-producing bacteria Bacillus cereus strain P2 and Planomicrobium chinenese
strain P1 under drought stress, it showed increase in leaf protein content
(56%), leaf sugar content (69%), shoot and root dry weight as well as fresh
weight, relative water content and soil moisture content. On the other hand, it
reduced lipid peroxidation, phenolic content of leaves and the activity of
different antioxidant enzymes like catalase, peroxidase, ascorbate peroxidase
[131-140]. Maize plant inoculated with EPS producing rhizobacterial strains
Proteus penneri (Pp1), Pseudomonas aeruginosa (Pa2), and Alcaligenes faecalis
(AF3) showed increase soil moisture content, leaf.
Rhizobacterial exopolysaccharide (EPS)
synthesis
Lack of irrigation and inadequate rainfall
severely affects agriculture production. Water stress leads to inadequate soil
moisture, alters the physicochemical and biological properties of soil often
resulting in decreased soil microbial activity, affecting functional niche and
also reduce the growth and production of crop plants. Water unavailability
alters soil structure indirectly as microorganism’s consumption rate of
proteins and polysaccharides from soil changes in absence of water. In this
type of unfavourable environmental condition where water availability is
scanty, PGPRs secrete high molecular weight complex organic compound termed
exopolysaccharide (EPS), in the environment.
EPSs are synthesized by bacteria during late logarithmic or stationary
phase of growth and present on the outer surface of the bacterial cell which
provides stability to the cell membrane against environmental stress condition
and enables their survival. EPS play important role in surface attachment,
biofilm formation, bioremediation, plant-microbes interaction, protection,
microbial aggregation and root adhering soil per root tissue (RAS/RT) ratio.
EPS provides the protection to the microbes against desiccation by increasing
water retention and by regulating diffusion of organic carbon, it also prevents
the soil drying by holding the water in soil surrounding the plant root. Under
water stress condition, plants showed increase in proteins, proline, and sugar,
several compatible solutes, relative water content as well as activity of
different antioxidant enzymes when inoculated with EPS producing bacteria. When
sunflower plant treated with EPS-producing rhizobacterial strain YAS34 under
drought stress, it exhibited significant increase in RAS/RT ratio in
rhizosphere [136,137]. According to Ghosh et al, four bacterial strains,
namely, Pseudomonas aeruginosa PM389, Bacillus endophyticus J13, Bacillus
tequilensis J12 and Pseudomonas aeruginosa ZNP1 are studied for their EPS
producing capacity. Under non-stress condition all four strain secrete same
level of EPS while under osmotic stress condition EPS production by Pseudomonas
aeruginosa ZNP1 was increased about 2.5-fold and Bacillus endophyticus J13 was
increased about 1.5-fold. There is no change in EPS level in case of Bacillus
tequilensis J12 while it is adversely affected in case of Pseudomonas
aeruginosa PM389. When Arabidopsis thaliana inoculated with all these four stains
separately under osmotic stress and normal condition, the fresh weight, dry
weight and plant water content increased significantly under osmotic stress as
compared to no stress condition. EPS producing bacteria, Pseudomonas putida
strain GAP-P45 when inoculated with sunflower plant, it increased RAS/RT, root
aggregation, high relative water content of leaves, plant biomass in addition
it also forms biofilm on the surface of roots of sunflower seeding’s and
provide the drought resistance to sunflower plants . When wheat plant treated
with EPS-producing bacteria Bacillus cereus strain P2 and Planomicrobium
chinenese strain P1 under drought stress, it showed increase in leaf protein
content (56%), leaf sugar content (69%), shoot and root dry weight as well as
fresh weight, relative water content and soil moisture content. On the other
hand, it reduced lipid peroxidation, phenolic content of leaves and the
activity of different antioxidant enzymes like catalase, peroxidase, and
ascorbate peroxidase. Maize plant inoculated with EPS producing rhizobacterial
strains Proteus penneri (Pp1), Pseudomonas aeruginosa (Pa2), and Alcaligenes
faecalis (AF3) showed increase soil moisture content, leaf area, relative water
content of leaves, root and shoot length, dry and fresh weight of root as well
as shoot under water stressed condition [138-141].
Antioxidative defense System in Stress
Management
Under optimal growth condition, plant cells produce
minimal amount of reactive oxygen species (ROS). While in water deficient condition,
various oxidative pathways in plants leads to generation of ROS namely,
hydrogen peroxide (H2O2), super oxide free radicals (O2-), hydroxyl radicals
(OH-), alkoxy radicals (RO) and singlet oxygen (O-). This ROS cause oxidative
damage and hinder the normal functions of plant cells by reacting with the macromolecules like deoxyribonucleic acid
(DNA), proteins, and lipids and even causes their denaturation or degradation.
ROS enhances the lipid peroxidation of membrane phospholipids and damage the
cell and organelle membrane by producing malondialdehyde, a final product of
peroxidation under stressful condition. An antioxidative system plays a crucial
role to cope up with ROS and for maintenance of physiological system in plant
cells. Under drought condition, to reduce the oxidative damage and accumulation
of ROS in plant cells, plant develop antioxidative defense system consisting of
various enzymatic and non-enzymatic mechanism. Various PGPRs also synthesize
such antioxidative molecules to protect the plants against various oxidative
stress conditions. Antioxidative enzymatic mechanism developed under adverse
environmental condition includes superoxide dismutase (SOD), glutathione
reductase (GR), catalase (CAT), peroxidase (POD), glutathione peroxidase (GPX)
and ascorbate peroxidase (APX). Plants also developed some non-enzymatic
mechanism including ascorbic acid, cysteine and glutathione which generate
tolerance against oxidative damage and provide protection to plant cells
[143-145].Five drought tolerant plant growth promoting Pseudomonas spp. Strains
i.e., P. putida strain GAP-P45, P. syringae strain GRFHYTP52, P. stutzeri
strain GRFHAP-P14, P. monteilli strain WAPP53 and P. entomophila strain BV-P13
inoculated with maize plant, it showed increase in relative water content of
leaves, proline (up to 6.3 fold), starch, protein, root length (37.12 –
48.08%), shoot length (36.06 – 44.11%), total dry biomass (37.80 – 60.36%),
soluble sugar, RAS/RT ratio (26.0 – 56.72%), leaf water potential, soil
aggregate (27 ± 1 – 70 ± 3%) and significantly lower the activity of
antioxidant enzymes such as CAT, SOD, APX, GPX with decreasing electrolyte
leakage as compared to uninoculated plant under drought stress. According to Khan et al. [109], when chickpea
plant inoculated with consortium of Bacillus subtilis, Bacillus thuringiensis,
and Bacillus megaterium under drought stress, it decreased the activity of
antioxidant enzymes like, SOD, CAT, POD, and APX and promoted the plant growth
by increasing the protein, chlorophyll, sugar content. As it compared to
uninoculated plant activity of SOD, CAT, POD and APX get elevated in response
to drought stress. Potato plant showed increase leaf area, tuber weight, tuber
number, dry matter production, total soluble sugar, proline, total protein with
less decrease in chlorophyll content, leaf relative water content, membrane
stability, MDA and ROS production, tuber yield and higher enzymatic activity of
POD, SOD and CAT, when inoculated with drought tolerant plant growth?promoting rhizobacteria (PGPR) Bacillus
subtilis HAS31 under drought stress as compared to uninoculated plant. Drought
mitigating PGPR Pseudomonas putida GAP-P45 reduced the accumulation of ROS and
also significantly reduced the activities of all ROS scavenging enzymes in
Arabidopsis thaliana under water stressed condition as compared to uninoculated
plant [146,147].
Volatile organic compound secretion
In addition to various mechanism describe
above, PGPR in rhizosphere also produce some gaseous organic compounds known as
volatile organic compounds (VOCs) to enhance the growth of plant and to provide
protection against stress condition. These VOCs are low molecular weight (?300
Da) containing organic compounds which get evaporated at normal temperature and
pressure [148]. When PGPRs release VOCs in soil, it stimulates plant defense
system against bacterial and fungal pathogens by evoking an induced systemic
resistance/tolerance (ISR/IST) response in plants [149-151]. VOCs produced from rhizobacteria can maintain
plant health by interacting with plant roots. This microbial volatile organic
compound (mVOCs) includes alcohols, sulfides, pyrazines, alkanes, ketones,
benzenoids, terpenes [152-155]. Microbial VOCs can provide protection to plants
against phytopathogens as well as stress caused by drought and heavy metal by
signalling inhibiting microbial growth and activity like raising pH of medium,
biofilm formation, modifying drug resistance or by eliciting ISR and IST in
plants. These VOCs could be used as alternative technique for sustainable crop
improvement because of their eco-friendly nature. Bacteria are the predominant
microbes in soil which generate induced systemic resistance in plant by
producing large variety of VOCs. Bacterial VOCs includes alkanes, ketones,
terpenoids, sulfur compounds, esters and also ammonia, alcohols,
phenazine-1-carboxylic acid, HCN which comprises antifungal activity that contribute
to the biocontrol activity of PGPRs. Hydrogen cyanide (HCN) is blocking the
cytochrome oxidase and there by inhibit electron transport chain and energy
supply to the cells, and leading to death of cells especially all aerobic
microorganisms at pico-molar concentration.
Co-inoculation of PGPR
Single strain of PGPR
provide benefit to the plant against drought stress by generating tolerance or
resistance against them. In addition to this, the combination of more than one
PGPR or with mycorrhizae provide better protection to the host plant against
different stress condition and promote plant growth as compared to single PGPR
strain. Dasgupta et al. studied PGPR from the Rhizosphere of Sesbania bispinosa
and selected 12 isolates. From that three-strain showed PGPR traits that are
Escherichia coli DACG2, Pseudomonas fluorescens strain DACG3 and Burkholderia
sp. DACG1. The combination of all three strain gives maximum rates of plant
height, number of leaves, pod bearing branches and seed weight in pot study as
compared to single inoculant and double inoculants. Co-inoculation of tobacco
(Nicotiana tabacum L.) with plant growth promoting rhizobacteria, Bacillus
methylotrophicus and Arbuscular mycorrhizal fungi, Glomus versiforme
significantly increased growth and biomass of tobacco plant by improving
accumulation of flavonoids (71.74%), macromolecules like deoxyribonucleic acid
(DNA), proteins, and lipids and even causes their denaturation or degradation.
ROS enhances the lipid peroxidation of membrane phospholipids and damage the
cell and organelle membrane by producing malondialdehyde, a final product of
peroxidation under stressful condition. An antioxidative system plays a crucial
role to cope up with ROS and for maintenance of physiological system in plant
cells. Under drought condition, to reduce the oxidative damage and accumulation
of ROS in plant cells, plant develop antioxidative defense system consisting of
various enzymatic and non-enzymatic mechanism. Various PGPRs also synthesize
such antioxidative molecules to protect the plants against various oxidative
stress conditions. Antioxidative enzymatic mechanism developed under adverse
environmental condition includes superoxide dismutase (SOD), glutathione
reductase (GR), catalase (CAT), peroxidase (POD), glutathione peroxidase (GPX)
and ascorbate peroxidase (APX). Plants also developed some non-enzymatic
mechanism including ascorbic acid, cysteine and glutathione which generate
tolerance against oxidative damage and provide protection to plant cells
[143-145].Five drought tolerant plant growth promoting Pseudomonas spp. Strains
i.e., P. putida strain GAP-P45, P. syringae strain GRFHYTP52, P. stutzeri
strain GRFHAP-P14, P. monteilli strain WAPP53 and P. entomophila strain BV-P13
inoculated with maize plant, it showed increase in relative water content of
leaves, proline (up to 6.3 fold), starch, protein, root length (37.12 –
48.08%), shoot length (36.06 – 44.11%), total dry biomass (37.80 – 60.36%),
soluble sugar, RAS/RT ratio (26.0 – 56.72%), leaf water potential, soil
aggregate (27 ± 1 – 70 ± 3%) and significantly lower the activity of
antioxidant enzymes such as CAT, SOD, APX, GPX with decreasing electrolyte
leakage as compared to uninoculated plant under drought stress. According to Khan et al. [109], when chickpea
plant inoculated with consortium of Bacillus subtilis, Bacillus thuringiensis,
and Bacillus megaterium under drought stress, it decreased the activity of
antioxidant enzymes like, SOD, CAT, POD, and APX and promoted the plant growth
by increasing the protein, chlorophyll, sugar content. As it compared to
uninoculated plant activity of SOD, CAT, POD and APX get elevated in response
to drought stress. Potato plant showed increase leaf area, tuber weight, tuber
number, dry matter production, total soluble sugar, proline, total protein with
less decrease in chlorophyll content, leaf relative water content, membrane
stability, MDA and ROS production, tuber yield and higher enzymatic activity of
POD, SOD and CAT, when inoculated with drought tolerant plant growth?promoting rhizobacteria (PGPR) Bacillus
subtilis HAS31 under drought stress as compared to uninoculated plant. Drought
mitigating PGPR Pseudomonas putida GAP-P45 reduced the accumulation of ROS and
also significantly reduced the activities of all ROS scavenging enzymes in
Arabidopsis thaliana under water stressed condition as compared to uninoculated
plant [146,147].
Volatile organic compound secretion
In addition to various mechanism describe
above, PGPR in rhizosphere also produce some gaseous organic compounds known as
volatile organic compounds (VOCs) to enhance the growth of plant and to provide
protection against stress condition. These VOCs are low molecular weight (?300
Da) containing organic compounds which get evaporated at normal temperature and
pressure [148]. When PGPRs release VOCs in soil, it stimulates plant defense
system against bacterial and fungal pathogens by evoking an induced systemic
resistance/tolerance (ISR/IST) response in plants [149-151]. VOCs produced from rhizobacteria can maintain
plant health by interacting with plant roots. This microbial volatile organic
compound (mVOCs) includes alcohols, sulfides, pyrazines, alkanes, ketones,
benzenoids, terpenes [152-155]. Microbial VOCs can provide protection to plants
against phytopathogens as well as stress caused by drought and heavy metal by
signalling inhibiting microbial growth and activity like raising pH of medium,
biofilm formation, modifying drug resistance or by eliciting ISR and IST in
plants. These VOCs could be used as alternative technique for sustainable crop
improvement because of their eco-friendly nature. Bacteria are the predominant
microbes in soil which generate induced systemic resistance in plant by
producing large variety of VOCs. Bacterial VOCs includes alkanes, ketones,
terpenoids, sulfur compounds, esters and also ammonia, alcohols,
phenazine-1-carboxylic acid, HCN which comprises antifungal activity that contribute
to the biocontrol activity of PGPRs. Hydrogen cyanide (HCN) is blocking the
cytochrome oxidase and there by inhibit electron transport chain and energy
supply to the cells, and leading to death of cells especially all aerobic
microorganisms at pico-molar concentration.
Co-inoculation of PGPR
Single strain of PGPR
provide benefit to the plant against drought stress by generating tolerance or
resistance against them. In addition to this, the combination of more than one
PGPR or with mycorrhizae provide better protection to the host plant against
different stress condition and promote plant growth as compared to single PGPR
strain. Dasgupta et al. studied PGPR from the Rhizosphere of Sesbania bispinosa
and selected 12 isolates. From that three-strain showed PGPR traits that are
Escherichia coli DACG2, Pseudomonas fluorescens strain DACG3 and Burkholderia
sp. DACG1. The combination of all three strain gives maximum rates of plant
height, number of leaves, pod bearing branches and seed weight in pot study as
compared to single inoculant and double inoculants. Co-inoculation of tobacco
(Nicotiana tabacum L.) with plant growth promoting rhizobacteria, Bacillus
methylotrophicus and Arbuscular mycorrhizal fungi, Glomus versiforme
significantly increased growth and biomass of tobacco plant by improving
accumulation of flavonoids (71.74%), (TiO2) nano particle, it gets adhered to the
plant root and protect the plant Brassica napus from fungal infection by acting
as biocontrol agent. Therefore, research effort should be focused on the
development of nanoparticles, which can be used with PGPR in the formation of
bio fertilizers. Due to advancement in omics technology like genomics,
proteomics, transcriptomics, metabolomics, it can be possible to identify genes
which is responsible for drought tolerances as well as different proteins like
heat shock proteins, chaperones, which gives protections to plants and microbes
against stressful condition, and also can be transferred to beneficial PGPR
which enhances their ability to survive in various adverse condition. By using
molecular techniques like microarray, RNA sequencing, the function of different
upregulated or down regulated genes can be studied out under abiotic stress.
The overexpression of genes responsible for production of osmolytes like
trehalose, proline which reduced the effect of ROS can enhance the tolerance
towards stress condition of bacterial strains as well as plants.