Plant growth-promoting mechanisms of endophytes

Microbial Endophytes. http://dx.doi.org/10.1016/B978-0-12-819654-0.00003-X
Copyright © 2020 Elsevier Inc. All rights reserved.
Plant growth-promoting
mechanisms of endophytes
Aswathy Jayakumara
, Veena P. Kumara
, Meritta Josepha
, Indu C. Nairb
,
Remakanthan A.c
, Radhakrishnan E.K.a
a
School of Biosciences, Mahatma Gandhi University, Kottayam, Kerala, India;
b
Department of Biotechnology, SAS SNDP Yogam College, Konni, India; c
Department of
Botany, University College,Thiruvananthapuram, India
Chapter outline head
3.1 Introduction 57
3.2 Bacterial endophytes and their diversity 58
3.3 Current understanding on the mechanisms of plant growth promotion by
bacterial endophytes 59
3.4 Various mechanisms 59
3.5 Microbial production of IAA 59
3.6 ACC deaminase production 63
3.7 Phosphate solubilization 64
3.8 Nitrogen fixation 64
3.9 Siderophore production 65
3.10 Biocontrol 65
3.11 Competition 65
3.12 Antibiotics 66
3.13 Lytic enzymes 67
3.14 Induced systemic resistance 67
3.15 Ethylene 68
3.16 Quorum quenching 68
3.17 Plant probiotics 68
3.18 Conclusions 69
References 70
3.1 Introduction
The global changes in climate and increasing population have unfortunate effects in
food production and will become insufficient to feed the world. The green revolution
could alleviate poor crop production by using high yielding varieties and use of chemi-
cal fertilizers and agrochemicals. But excessive use of chemical fertilizers and agro-
chemicals has resulted in the deterioration of soil fertility. Hence, agronomic practices
are moving toward sustainable and environment friendly approach. Now a days sev-
eral approaches like organic farming, environment friendly fertilizers and pesticides,
3

58 Microbial Endophytes
plant growth-promoting rhizobacteria and endophytic bacteria, highly efficient trans-
genic plants are in the mainstream of agriculture. Plant microbe interactions have been
studied for many decades. Among them, the interaction between endophytic bacteria
and its host have of great physiological and ecological significance. Endophytic bacte-
ria are those organisms that reside in plants without causing any harmful effect to the
host plant. They are capable of colonizing in any part of a plant like root, stem, leaf,
flowers, and nodes. They gain entry into the plant by multiple ways except for the
endophytes present in the seeds (Santoyo et al., 2016). The nutrient rich rhizosphere
soil harbors wide number of microorganisms, from which the selected ones colonize
within the plant. The metabolites secreted or leaked out from the plant determine the
characteristics and species of organism get attracted and recruited. The primary entry
into plant tissues is through the cracks in roots due to the emergence of lateral roots
and different wounds, which allow the leakage of metabolites and attracts the bacteria
toward the plant. The other site of entry in aerial portion is via stomata, particularly of
leaves and young stems (Roos and Hattingh, 1983). The bacteria also enter through the
root hair cells (Huang, 1986) (Fig. 3.1). Rhizobia spp. also colonizes the internal plant
tissues and form nodules, where the nitrogen fixation process is carried out. The endo-
phytic bacteria in to the entering host moves to different plant parts for colonization,
and continue to reside with in the plant by providing necessary support for its survival.
3.2 Bacterial endophytes and their diversity
Bacterial endophytes are found in almost all species of plants that have been analyzed.
Endophyte free plant is likely to be absent, because such plant will be more susceptible
to both biotic and abiotic stress. Several endophytes are associated with different plant
Figure 3.1 Mode of entry of endophytic bacteria.

Plant growth-promoting mechanisms of endophytes 59
organs and are diverse. Within the roots the number of bacterial cells could be found
in the range of 104
–108
per gram of root tissue, which is less than that in the rizosphere
soil. This observation indicates that the roots are effective in selecting and limiting the
bacteria in the root endobiome (Bulgarelli et al., 2013). Root endophytes are domi-
nated by Proteobacteria followed by Actinobacteria, Firmiculates, and Bacteroidetes.
Studies suggest that the leaf or shoot microbiome are mainly recruited from the soil
and translocated to respective tissues via apoplast pathways. There are evidences sug-
gesting the overlapping of these shoot and root microbiome at both taxonomic and
functional levels (Bai et al., 2015). The bacterial components present in the interior
tissues of plant are harmless to their host. Mostly changes occur in composition and
diversity, which could be determined by the ecological factors of plant and soil.
3.3 Current understanding on the mechanisms of plant
growth promotion by bacterial endophytes
After successful colonization in host plant, endophytes promote the growth of the
plant by several mechanisms (Table 3.1). They are sheltered from the majority of biot-
ic and abiotic stress factors by the host plant, which confirms the mutual relationship.
The direct mechanisms of growth promotion involve the production of plant benefi-
cial compounds like phytohormones, ACC deaminase, sequestration of iron, and the
solubilization of phosphate (Glick, 2012). The deleterious phytopathogens and pests
are prevented from attacking the plants by certain indirect methods and involve the
production of antibiotic, chelation of iron, and the synthesis of extracellular enzymes
for the lysis of fungal cell wall (van Loon, 2007) (Fig. 3.2).
3.4 Various mechanisms
Bacterial endophytes have advantages over bacteria inhabiting the rhizosphere. By vir-
tue of being within the tissue, they are having direct contact with the plant and hence
easy communication between cells can occur. Therefore, they could exert a direct
beneficial effect on host. In this process, compound produced by the bacteria directly
influence the physiological activities of the host plant and may result in enhanced
biomass production. Bacteriogenic substances include hormones, siderophore, ACC
deaminase, etc. resulting in plant beneficial processes such as phosphate solubiliza-
tion, fixation of atmospheric nitrogen, and chelation of metal ions in absorbable form.
3.5 Microbial production of IAA
Indole acetic acid is a phytohormone, which is known to be functionalized in cell
division, elongation, and differentiation of plants. Also it aids in the germination of
seeds, tubers, and initiates adventitious and lateral root formation. IAA production by

Table 3.1 Plant growth-promoting properties of bacterial endophytes.
Endophytic bacteria Source of isolation
Plant growth-
promoting properties References
Gluconacetobacter
diazotrophicus PaI5
Saccharum offici-
narum, Camellia
sinensis, Oryza
sativa
Nitrogen fixation, auxin
synthesis
Bertalan et al.
(2009)
Klebsiella pneumoniae
342
Zea mays Nitrogen fixation Guttman et al.
(2008)
Bacillus cereus and B.
subtilis
Teucrium polium PGP properties Hassan (2017)
Bacillus spp. and
Pseudomonas spp
Malus domestica Shoot growth, cellular
redox balance, and
protein expression
under in- vitro
conditions
Tamošiu¯nė
et al. (2018)
Enterobacter spp. strain
PDN3
Populus deltoides Endophyte-assisted
phytoremediation of
Trichloroethylene
Doty et al.
(2017)
Pseudomonas stutzeri
A15
Oryza sativa PGP properties Pham et al.
(2017)
Serratia grimesi BXF1 Pine pinaster,
Solanum
lycopersicum and
Cucumis sativus
promotes early
nodulation and growth
of common bean
Tavares et al.
(2018)
Enterobacter spp. Eleusine coracana Suppressing Fusarium
graminearum in plant
tissues and reduction
of deoxynivalenol
mycotoxin
Mousa et al.
(2015)
Pseudomonas poae
RE*1-1-14
Beta vulgaris Production of novel
lipopeptide Poaeamide
suppressing
Phytophthora capsici
and P. infestans
zoospores
Zachow et al.
(2015)
Azoarcus spp. BH72 Oryza sativa Nitrogen fixation Krause et al.
(2006)
Azospirillum lipoferum
4B
Oryza sativa, Zea
mays, Triticum
Nitrogen fixation,
phytohormone
secretion
Richardson
et al. (2011)
Bacillus mojavensis B. monnieri Biocontrol mechanisms Jasim et al.
(2016c)
Azospirillum spp. B510 Oryza sativa Nitrogen fixation,
phytohormone
secretion
Kaneko et al.
(2010)

Plant growth-
Burkholderia
phytofirmans PsJN
Solanum tuberosum,
Zea mays,Solanum
lycopersicum,
Hordeum
vulgare,Allium
cepa,
IAA synthesis, ACC
deaminase
Weilharter et al.
(2011)
Burkholderia spp.
KJ006
Oryza sativa ACC deaminase,
nif gene cluster,
antifungal action
Kwak et al.
(2012)
Klebsiella sp. P. nigrum Phosphate solubilization,
ACC deaminase,
Siderophore
Jasim et al.
(2013b)
Enterobacter spp. 638 Populus Siderophore, IAA,
acetoin and
2,3-butanediol
synthesis, antifungal
action
Taghavi et al.
(2008)
Burkholderia
phytofirmans
Allium cepa Growth enhancement Compant et al.
(2005b)
Burkholderia
phytofirmans
Allium cepa Growth enhancement Kim et al.
(2012)
Burkholderia
phytofirmans
Allium cepa Growth enhancement,
increased chlorophyll
content
Zúñiga et al.
(2013)
Ralstonia sp. and
Bacillus sp
Musa accuminata
AAA cv. Grand
Nain
Growth enhancement
effect
Jimtha et al.
(2014)
Rhizobium spp. Solanum
lycopersicum
Growth enhancement Tian et al.
(2017)
Rhizobium spp. Zea mays Growth enhancement Patel and
Archana
(2017)
Bacillus sp. C. annuum Biocontrol Jasim et al.
(2016b)
Rhizobium spp. Zea mays, Sorghum
bicolor, Oryza
sativa
Growth enhancement Riggs et al.
(2001)
Pseudomonas sp. Zingiber officinale IAA, ACC deaminase
and siderophore
Jasim et al.
(2013c)
Ralstonia spp. Zea mays, Sorghum
bicolor, Oryza
sativa
Growth enhancement Patel and
Archana
(2017)
Pseudomonas spp. Solanum
lycopersicum
Growth enhancement Tian et al.
(2017)
Table 3.1 Plant growth-promoting properties of bacterial endophytes. (Cont.)
(Continued)

Plant growth-
Burkholderia
vietnamiensis,
Rhanella spp.,
Acinetobacter spp.,
Herbaspirillum spp.,
Pseudomonas putida,
Sphingomonas spp.
Populus deltoides Growth enhancement,
increased CO2
assimilation
Knoth et al.
(2013)
Burkholderia
vietnamiensis,
Rhanella spp.,
Enterobacter spp.,
Pseudomonas
graminis,
Acinetobacter spp.,
Sphingomonas
yanoikuyae
Populus deltoides Growth enhancement Knoth et al.
(2014)
Paenibacillus sp. Curcuma longa IAA production Aswathy et al.
(2012)
Herbaspirillum
seropedicae
Zea mays Growth enhancement Riggs et al.
(2001)
Pseudomonas
aeruginosa
Zingiber officinale Biocontrol Jasim et al.
(2013a)
Herbaspirillum
seropedicae
Zea mays Increased rooting,
change in gene
expression
do Amaral et al.
(2014)
Bacillus sp. Elettaria
cardamomum
Plant growth
enhancement
Jasim et al.
(2015)
Herbaspirillum
seropedicae
Zea mays Nitrogen fixation Roncato-
Maccari et al.
(2003)
Bacillus
amyloliquefaciens
Bacopa monnieri Biocontrol Jasim et al.
(2016a)
Pseudomonas
fluorescens
Miscanthus sinensis Growth enhancement
in phosphate limited
conditions
Oteino et al.
(2015))
Pseudomonas
fluorescens
Brassica napus Growth enhancement,
increased Pb uptake,
root elongation
Oteino et al.
(2015)
Burkholderia spp. Capsicum frutescens Plant probiotic function Sabu et al.
(2018)
Pseudomonas
fluorescens
Solanum nigrum Growth enhancement Ausubel et al.
(2008)

endophytic bacteria occurs through tryptophan dependent and tryptophan independent
mechanisms. Generally microbial production of IAA occurs via indole-3-acetonitrile
(IAN) pathway, indole-3-acetamide (IAM) pathway, and the indole-3-pyruvate (IPyA)
pathway (Li et al., 2018). Among these IAM pathway is mainly attributed to the phy-
topathology, and the IPA pathway is related to epiphytic and rhizosphere fitness. En-
dophytic IAA can contribute to the increase in shoot length, root length, root number,
and also can prevent the plant from pathogenic invasion (Jayakumar et al., 2018).
Several other studies also supported the activity of endophytic IAA in plant growth
promotion (Bhutani et al., 2018; Lata et al., 2006; Liu et al., 2017).
3.6 ACC deaminase production
Ethylene is an important plant hormone produced by higher plants in association with
fruit ripening, senescence, and stress response. The presence of increased level of
Figure 3.2 Plant growth-promoting mechanisms of endophytic bacteria.
Plant growth-
Methylobacterium
spp., and
Brevundimonas spp.
Camellia sinensis Plant probiotic function Yan et al.
(2018)
Bacillus sp. Curcuma longa IAA production, ACC
deaminase production,
Nitrogen fixation
Jayakumar et al.
(2018)
Bacillus cereus and
Enterobacter cloacae
Zea mays L. IAA production Abedinzadeh
et al. (2019)

ethylene causes stress on plants and thereby inhibits the growth of vegetative tissues.
ACC deaminase is an enzyme produced by many plant growth-promoting bacteria
(PGPB), which have the ability to uptake ACC and converts to α-ketobutyrate and
ammonia. This lowers the ACC levels and thereby decreasing the level of ethylene,
which inturn minimize the plant stress level. Several studies reported that the inocula-
tion of ACC deaminase producing bacteria can protect the plant against flooding, sa-
linity, drought, heavy metal toxicity, and the presence of phytopathogens (Glick, 2014;
Santoyo et al., 2016; Zhang et al., 2011).
3.7 Phosphate solubilization
Phosphorus is an essential macro nutrient, which aids in the growth and development
of plants and is present at 400–1200 mg concentration per kg of soil. The soluble
phosphate concentration present in soil is very low and is about 1 ppm. Plant absorb-
able forms of phosphate include monobasic and the dibasic ions. Most of the elemen-
tal phosphorous is found to be immobilized in various living organisms and or locked
up in sediments. Microbes play an important role in the release and cycling of immo-
bilized phosphorous. These microbes solibilize phosphate by acidification, secretion
of organic acids, and through the chelation-based mechanisms. Many bacterial gen-
era are reported as phosphate solubilizers such as Azotobacter, Bacillus, Beijerinckia,
Burkholderia, Enterobacter, Erwinia, Azospirillum, Serratia, Flavobacterium, Pseu-
domonas, Microbacterium, and Rhizobium (de Abreu et al., 2017; Huang et al., 2010;
Oteino et al., 2015; Zaidi et al., 2009). Several studies have reported that the inocula-
tion of phosphate solubilizing endophytic bacteria can contribute to the enhancement
of growth in plants (Emami et al., 2019; Oteino et al., 2015).
3.8 Nitrogen fixation
Nitrogen is one of the macronutrients for the growth and development of plants.
About 78% of the nitrogen is in its gaseous form, which is not readily available
to plants. And also the nitrogen deficiency in soil has necessitated the use of vari-
ous nitrogenous fertilizers. Although many endophytes are present in nature, only
certain bacteria have the capacity to fix nitrogen. This is because of the inabil-
ity of these bacteria to produce nitrogenase enzyme. Endophytic diazotropic bac-
teria have been reported to be present in agriculturally important plants such as
Brassica napus, Leptochloa fusca, Oryza sativa, Pennisetum glaucum, Musa acu-
minata, Saccharum officinarum, and Zea mays (Anand and Chanway, 2013; An-
drade et al., 2014; Araújo, 2013; Gupta et al., 2013). Several nitrogen fixers with
plant growth enhancement effect have been reported and include Azospirillum spp.,
Herbaspirillum spp., Burkholderia spp., Enterobacter cloacae, Klebsiella oxytoca,
Klebsiella pneumoniae, Pantoea sp., and Bacillus spp. (Govindarajan et al., 2006;
Islam et al., 2009; Loiret et al., 2004).

3.9 Siderophore production
Iron (Fe) is the most abundant element on earth, however obtaining sufficient amount
of iron is more problematic in the rhizosphere where plants and microorganisms com-
petes for iron. Under such iron limiting conditions, many bacteria produce low molec-
ular weight (∼400–1500 Da) iron-specific ligands, termed as siderophores. Because
of its iron chelating activity, siderophores are known as the vehicle for the transport
of Fe3+
into microbial cells. The uptake of Fe3+
by microoorganisms is successfully
carried through the Fe-siderophore receptors because of the high affinity of these mol-
ecules for Fe3+
. There are over 500 types of siderophore, out of which the chemical
structure of 270 is studied well. The known siderophores are belongs to three main
groups like the catecholates, hydroxamate, and carboxylates.
3.10 Biocontrol
Biocontrol or biological control can be defined as the reduction or complete inhibi-
tion of phytopathogens by the endophytic bacteria. The most studied and commonly
reported mechanism is antagonism. It includes most specific mechanisms like compe-
tition, antibiosis, hydrogen cyanide production, and siderophore production.Various
other mechanisms are also reported like “induced systemic resistance” (ISR) and “sys-
temic acquired resistance” (SAR). ISR is elicited by certain non pathogenic microor-
ganisms, whereas SAR is elicited via pathogens or chemical compounds.
3.11 Competition
The greater incidence of disease can be limited by the competition between patho-
genic and nonpathogenic strains of bacteria for colonization. The root surface and
rhizosphere soil contain carbon sinks with 40% of photosynthate allocation. Rapid
colonization of abundant nonpathogenic bacteria in this nutrient rich area prevents the
growth of pathogenic strains. Overall environment of soil is dependent on nutrient rich
niches that attract wide variety of microorganisms, forming relationships such as asso-
ciative, symbiotic, neutralistic, or parasitic. The various parameters that determine the
effectiveness of PGPB mediated processes include the strain competence and persis-
tence, root colonizing capacity, ability to synthesize and release various metabolites,
plant species, and genotype specificity of the bacterial strain. The crucial and complex
process of root colonization is the prerequisite for its effective application like bio-
fertilization, phytostimulation, bicontrol, and phytoremediation. The chemotactic and
motile microorganisms are efficient root colonizers, whereas the nonmotile ones are
less efficient. The organic acids, amino acids, and specific sugars present in the root
exudates attract these organisms to the root. They reach the site of entry by active
mobility in response to the chemotactic substance (de Weert et al., 2002). PGPB may
be uniquely equipped to sense chemoattractants, for example, rice exudates induce

stronger chemotactic responses of endophytic bacteria than from nonPGPB present in
the rice rhizosphere (Bacilio-Jiménez et al., 2003).
Pseudomonas and Bacillus spp. are the most common colonizers in agricultural
crops. The exudates may contain antimicrobial compounds of great ecological im-
portance, which inhibit the pathogens. The quantity and composition of these nutrient
niches vary with the species of plants, possess a challenge to the colonizing bacteria.
Here the colonization completely depends on the bacterial competence to the com-
pounds by taking it as an advantage or by getting adapted to the specific changes in the
environment (Bais et al., 2004). Bacterial lipopolysaccharides (LPS), the O-antigen
chain, can also contribute to root colonization (Dekkers et al., 1998). The importance
of LPS in colonization are strain dependent as the O-antigenic side chain of Pseudo-
monas fluorescens WCS374 does not contribute to potato root adhesion, whereas the
O-antigen chain of P. fluorescens PCL1205 is involved in tomato root colonization.
Furthermore, LPS, O antigen does not contribute to rhizoplane colonization of tomato
by the plant beneficial endophytic bacterium P. fluorescens WCS417r, but they were
involved in endophytic colonization of roots (Compant et al., 2005a).
3.12 Antibiotics
The natural products produced by the endophytes can help in protection of the host
against pathogen invasion. These chemicals are also of great significance in pharma-
ceutical, agrochemical, and biotechnological industries too (Harrison et al., 1991). Re-
searches on antibiotics and other microbial natural products are pivotal for global fight
against the growing problem of antibiotic resistance. It is necessary to find new antibi-
otics to tackle this problem, and currently endophytic bacteria are one of the potential
sources of novel antibiotics (Christina et al., 2013). Many natural products produced
by endophytes have proven to be antibacterial, antifungal, antidiabetic, antioxidants,
and immunosuppressives. Thus, endophytes are viewed as great novel sources of bio-
active natural products. They are one of the untapped potential sources with majority
of them producing different kinds of antibiotics, which has unusual amino acids in it.
A wide variety of antibiotics are being produced by plant growth-promoting bac-
teria (PGPB). Most of them not only inhibit phytopathgens like bacteria, fungus, vi-
rus, but also helps in growth enhancement of the plant. They include Bacillus spp.,
Pseudomonas spp., Azospirillum spp., Rhizobium spp., and Serratia spp. (Haas and
Keel, 2003). Antibiotics produced by PGPB include 2,4 diacetylphloroglucinol,
phenazine-1-carboxyclic acid, phenazine-1-carboxamide, pyoluteorin, pyrrolnitrin,
oomycin A, viscosinamide, butyrolactones, kanosamine, zwittermycin-A, aerugine,
rhamnolipids, cepaciamide A, ecomycins, pseudomonic acid, azomycin, antitumor
antibiotics FR901463, cepafungins, and antiviral antibiotic karalicin. These antibiotics
are known to possess antiviral, antimicrobial, insect and mammalian antifeedant, anti-
helminthic, phytotoxic, antioxidant, cytotoxic, antitumor, and plant growth-promoting
activities (Fernando et al., 2006). The mechanism behind the biocontrol by these com-
pounds include cell distruption and suppression of pathogens, hence it is commer-
cialized and greatly significant to pharmaceutical field (Compant et al., 2005a). The

free living and endophytic bacteria release allelochemicals, which act antagonistically
with the pathogens likewise all other microorganisms (Saraf et al., 2014). Phenazines
and pyrrolnitrin are antifungal products that are produced by Pseudomonads. Each
antibiotic exhibits a different antifungal mechanism, pyrrolnitrin had shown to have
antagonism against Botrytis cinerea, Rhizoctonia solani, and Sclerotinia sclerotio-
rum. The phenazines have proven to be effective against Gaeumannomyces grami-
nis var. tritici. The antibiotics from Bacillus spp. include lipopeptides, polymyxin,
circulin, and colistin are active against Gram-positive and Gram-negative bacteria
and pathogenic fungi (Maksimov et al., 2011). The UW85 strain of B. cereus sup-
pressed oomycete pathogens through the production of the antibiotics zwittermicin A
(aminopolyol) and kanosamine (aminoglycoside), which contributed to the biocontrol
of alfalfa damping off (Beneduzi et al., 2012; Silo-Suh et al., 1994). The PGPB an-
tibiotics were mostly used for the crop plants, where a single infection could badly
affect the yield. Gradually the excessive use of these antagonisis lead to development
of the resistant strains. To tackle the menace of resistance bicontrol agents that pro-
duce cyanides, alcohols, and ketones as secondary metabolites were used. Cyanide
is a secondary metabolite produced by Gram-negative P. fluorescens, P. aeruginosa,
and C. violaceum. The aerobic, root colonizing biocontrol bacterium CHA0 protects
several plants from root diseases caused by soil borne fungi through the production of
diverse metabolites (Fernando et al., 2006; Voisard et al., 1994). Antifungal volatiles
of P. chlororaphis (PA23) isolated from soybean roots include aldehydes, alcohols,
ketones, and sulfides, which were inhibitory to all the stages of S. sclerotiorum root
pathogen (Fernando et al., 2006).
3.13 Lytic enzymes
Apart from antibiotics, many endophytes produce certain cell wall degrading enzymes
to control phytopathogens. These degradative enzymes have the capacity to alter the
structural integrity of the fungal cell wall and thereby inhibit or kill the pathogens.
These include β-1,3-glucanase, chitinase, cellulase, and protease which inhibit the
growth of pathogens in alone and in combination with other biocontrol strategies.
Chitinase mediates the degradation of chitin, which is the major cell wall component
of fungus. Along with other enzymes, it has been found to be active against vari-
ous phytopathogens like Botrytis cinerea, Sclerotium rolfsii, Fusarium oxysporum,
Phytophthora spp., Rhizoctonia solani, and Pythium ultimum (Aktuganov et al., 2003;
Quecine et al., 2008; Zhang et al., 2015).
3.14 Induced systemic resistance
ISR caused by plant beneficial bacteria by inducing resistance mechanism in the plant.
The plant develops an enhanced defensive state against the pathogen, when stimulated
appropriately. It is not pathogen specific, and is effective in controlling disease caused
by several phytopathogens by ethylene and jasmonate signaling pathways.

Pseudomonas and Bacillus spp. are the most studied microbes that trigger ISR
and develop resistance against several plant pathogens, including fungal, bacterial and
viral pathogens, nematodes, and insects. Several elicitors are reported other than jas-
monate and salicylate and include the O-antigenic side chain of outer membrabe of
bacteria, chitin, cyclic lipopeptides, flagellar proteins, β-glucans, and pyoverdine.
3.15 Ethylene
The smallest simple structured gaseous phytohormones, which allows plant–plant
communications. It is a multifaceted hormone, which has various roles in the regula-
tion of leaf development, senescence, fruit ripening, stimulation of germination, etc.
It is mainly produced in response to multiple environmental stresses, both abiotic
and biotic and acts as a bridge between a changing environment and developmental
adaptation. Ethylene synthesis triggering abotic stresses include submergence, heat,
shade, exposure to heavy metals and high salt, low nutrient availability, and water
deficiency (Dubois et al., 2018). ACC deaminase is the rate determining enzyme that
regulates ethylene in plants. The plant growth promotion is directly linked to the lev-
els of ethylene in plants, which is highly produced during stress conditions. The ACC
deaminase positive bacterial endophytes are excellent growth promoters because they
ameliorate plant stress by blocking the production of ethylene. The ACC deaminase
activity and its role in plant growth induction was well demonstrated using endophytic
Burkholderia phytofirmans.
3.16 Quorum quenching
Quorum sensing mechanism is required for the survival of most of the microorgan-
isms. It is thought to regulate the physiological activities such as cell to cell commu-
nication, reproduction, adaptation, biofilm formation, and competence. Endophytic
bacteria have been reported to be involved in the quorum sensing quenching mecha-
nism as a strategy to control certain phytopathogens. For example, endophytic bacte-
ria from Cannabis sativa L. have been found to disrupt the cell to cell communication
of Chromobacterium violaceum (Kusari et al. 2014).
3.17 Plant probiotics
The ecofriendly approach for sustainable agricultural practices has of great signifi-
cance in day-to-day life. For this, formulations based on endophytic bacteria have of
great interest. Bioprimed plant always shows enhanced plant growth and is free from
many of the environmental stresses (Mahmood and Kataoka, 2018) (Fig. 3.3). The
big challenge associated with the development of such formulations are the isolation,
characterization, and studying the field potential of promising bacteria. Formulations
with increased shelf life, broad spectrum action, and with good performance under

field conditions can be commercialized. The commercialization and application of
such potential candidate depend mainly on the selection of compatible carriers. Cur-
rently organic and inorganic carriers are available with the potential to protect the
bacteria from stress conditions. Many inorganic and organic carriers such as talc, al-
ginate, peat, vermiculate, sawdust, zeolite, pyrophyllite, and montmorrilonite are used
(Malusá et al., 2012). The shelf life of formulations varies based on the bacteria and
the carrier type. Most suitable carrier material, which can extent the bacterial viability
can be selected to transform the bacterial formulations into plant probiotics for agri-
cultural field application.
3.18 Conclusions
Endophytic bacteria are microorganisms that colonize the interior part of plant with-
out causing any harmful effects. Being inside, they may promote the growth of plants
by several direct and indirect mechanisms. Several bacteria have been reported to en-
hance the growth of plants, and many of them are uder investigation. Currently several
endophytic bacteria-based formulations have been developed, and some of them are
under commercialization. Hence these endophytic bacteria can replace majority of
the chemical fertilizers and pesticides for better agronomic practices and sustainable
agricultural productivity.
Figure 3.3 Effect of endophytic bacterial priming on plants .

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