DIVERSITY OF PGPB

The microbial endophytes have commonly been found in every plants species. Partida-Martinez and Heli (2011) concluded that the existence of microorganisms in and on plants must be considered as the rule, rather than the exception. Endophytic organisms have been isolated from different parts of the plant and from both monocotyledonous and dicotyledonous plants, ranging from woody tree species, such as oak and pear, to herbaceous crop plants such as sugar beet and maize (Ryan et al., 2008). Although under various stresses such as heavy metals and salinity, some endophytic bacteria have been observed (Wani et al., 2007a, b; Sheng et al., 2008; Sun et al., 2010; Zhang et al., 2011; Babu et al., 2015; Li et al., 2016a; Zhao et al., 2016). Many factors, plant genotype, growth stage, and physiological status, type of plant tissue, environmental (soil) conditions, and agricultural practices also define endophytic colonization and endo- sphere community structures (Hardoim et al., 2008). The relatively few studies that have analyzed the influences of diverse plant gr owth stage on endophytic diversity have been reported. According to Shi et al. (2014), PCR-based Illumina pyrosequencing w'as used to examine the endophytic bacterial diversity and space-time dynamics in sugar beet (Beta vulgaris L.) in China. They reported that the plant genotype and plant growth stage of sugar beet influenced to the dynamics of endophytic bacteria communities. The great numbers of endophytic bacteria were detected during rosette formation and tuber growth and were reduced during seedling gr owth and sucrose accumulation, as observed by OTUs (Operational Taxonomic Units). Moreover, Kang et al. (2016) studied the communities’ structures of the endophytic bacteria associated with surface-sterilized pepper plants grown in the different field soils by using culture-independent (PCR- DGGE) and culture-dependent (plating) methods. Field soils used in this study were collected from two different geographic locations; Deckso and Gw’angyang in Korea. The results showed that in the same plant species, when propagated in diverse soil environments, the endophytic communities were different. Besides the determinants as described above, intrinsic bacterial traits significant for colonization have major roles as factors of endophytic diversity. Therefore, comparisons between various plant endophyte coimnunities are difficult. According to Reinhold-Hurek and Hurek (2011), the majority of the culturable isolated endophytic bacteria species are members ofProteobacteria, while Firmicutes, Actinobacteria, and also Bacteroides are little. The presence of bacterial endophyte coimnunities depends on their growth competence on a synthetic medium. Endophytic species are closely related to epiphytic species, and belong mainly to the alpha-, beta-, and gamma -Proteobacteria subgroups (Kuklinsky-Sobral et al., 2004). Therefore, the diversity of endophytic bacteria in alpha-, beta-, and gamma -Proteobacteria subgroups are reported from different host plant species (Table 3.2).

PLANT GROWTH-PROMOTING ACTIVITY AND PRODUCTS

PGPB stimulates plant growth in two mechanisms as direct and indirect mechanisms (Bashan and de-Bashan, 2005). The direct mechanism of PGPB is the major step involved to support plant growth in a forward and direct maimer that contained nitrogen fixation, phytohormones production, phosphate solubilization, and iron availability. The indirect mechanisms referred to the PGPB that acted as biocontrol, indirectly promote plant growth by producing antagonistic substances or inducing resistance to pathogens in order to prevent the deleterious effects of phytopathogenic microorganisms (bacteria, fungi, and viruses). Endophytic bacteria isolated from host plant species in Asian countries and their plant growth-promoting traits are shown in Table 3.3.

3.4.1 NITROGEN FIX A TION

In developing countries like South-East Asia and the developed world, agricultural practices that use high amounts of fertilizer have led to considerable increases in the productivity of food production, but high energy and environmental costs associated with fertilizer use necessitate the explore for alternative approaches of biological soil management. This fixation occurs through symbiotic and non-symbiotic N,-fixing bacteria possessing the nitrogenase enzyme that converts atmospheric elemental

Country

Species

Isolation Sources

References

Thailand

1Bradyrhizobium sp. SUTN9-2

Leguminous weed (Aeschynomene ameiicana L.)

Piromyou et al., 2015b

1Bradyrhizobium sp.

Rice (Oryza sativa)

Piromyou et al., 2015a

lNovosphingobium sediminicola,1Ochrobactnim intermedium

Industrial variety wild and chewing sugarcane

Muangthong et al., 2015

lNovosphingobntm nitrogenifigen,2Pseudomonas,2Klebsiella, 2Pan toe a

Rice

Rangjaroen et al., 2014

1Rhizobium lemnae

Duckweed (Lentma aequinoctialis)

Kittiwongwattana et al., 2014

China

1Rhizobium smilacinae

Traditional Chinese medicinal plant (Smilacina japonica)

Zhang et al., 2014a

lOchrobactrum anthropi Mnl strain

Roots of Jerusalem artichoke

Menget al., 2011,2014

1Sphingomodaceae, iBurkholdehaceae

Caragana microphylla grown in the desert

Dai et al., 2014

2Pseudoxanthomonas gei sp. nov.

Stem of Geiini aleppicum Jacq.

Zhang et al., 2014b

}Burkholdetia sp.

Dendrobium officinale

Yu et al., 2013

  • 1Brevundimonas diminuta,1 Methylobacteiium sp., 1Sinorhizobium terangae, lNovosphingobium tardaugens, lCaulobacter sp., 1Kaistina koreensis, 2Stenobophomonas maltophilia, 2Enterobacter sp., 2Pantoea sp., 2Stenotiophomonas sp., 2Acinetobacter baumannii,
  • 2Alkanindiges illinoisensis,2Methylophaga marina, 2Plesiomonas shigelloides, 2Pseudomonas stutzeh, }Achromobacter xylosoxidans, iBurkholdena fungirum, 2BurkhoIdena sp.,3Acidovoraxfacilis,
  • 1 Com am on as testosterone,3 Curibacter gracilis,

Roots of rice (Oryza sativa L.)

Sun et al., 2008

Country

Species

Isolation Sources

References

China

(continued)

325elftia acidovorans, 'Delfiia tsuruhatensis, 'Herbaspirillum fiisin- gense, 'Hydrogenophaga taeniospiralis,3Variovorax sp., iDuganella violaceinigra,3Methyloversatilis universalis, 'Gallionella fenvginea, iSteroUbactrium denitrificans

'Agrobacterium

Legume species of Astragalus

Chen et al., 2015

2Pantoea sp. Sd-1

Rice seeds

Xiong et al., 2014

3Burkholderia sp. SaZR4. SaMRIO.3Variovorax sp. SaNRl, 1Sphingomonas sp. SaMR12

Sedum alfredii plants

Zhang et al., 2013

2Enterobacter, 2Serratia, 2Stenotrophomonas,2Pseudomonas

Peanut plants

Wang et al., 2013

2Raou1tella sp.

Sugarcane roots

Luo et al., 2016

2Pseudomonas spp.

Wheat

Yang et al., 2011

1Pelomonas,1Rhizobium,2Pseudomonas, 2Aeromonas,3Rhodoferax, 3Uliginosibacterium

Narrowleaf cattail (Typha angustifolia L.) roots

Li et al., 2011

Others: Sulfurospirillum, Hyobacter, and Bacteroides

1Sphingomonas,2Pseudomonas

Wild alpine-subnival plant species

Sheug et al., 2011

1Sphingomonas,2Pantoea,2Enterobacter

Roots of elephant gl ass

Li et al., 2016b

Others:

Bacillus

3 Variovorax paradoxus

Salicomia europaea grown under extreme salinity

Zhao et al., 2016

2Enterobacter sp.

Roots of Sorghum sudanense gr own in a Cu mine wasteland soils

Li et al., 2016c

Country

Species

Isolation Sources

References

China

(continued)

2Klebsiella vahicola strain DX120E

Sugar cane crops

Lin et al., 2015

1Rhizobium oiyzicola

Surface-sterilized rice roots

Zhang et al., 2015

1Ochrobactrum endophyticum

Roots of Glycyrrhiza ur alensis F.

Li et al., 2016a

1Ncn’osphingobium oiyzae

Rice roots

Zhang et al., 2016

1Rhizobium populi

Storage liquid in the stems of Populus euphratica trees

Rozalron et al., 2014

lParacoccus sphaerophysae

Root nodules of Sphaerophysa salsula

Deng et al., 2011

India

2Pseudomonas and 2Stenotrophomonas

Origanum vulgare

Bafana, 2013

2Enterobacter and 2Stenotrophomonas

Roots of Indian ecotype Pteris vittata

Tiwari et al., 2016

1Rhizobium

2Pseudomonas putida, 2Enterobacter cloacae

Saffron (Crocus sativus)

Shanna et al., 2015

Others:

Bacillus licheniformis, B. stibtilis, B. ceretis, B. humi, B. pumilus, B. safensis, Brevibacillus sp., Paenibacillus elgii, Staphylococcus hominis

2Pantoea sp. and1Pseudomonas sp.

Fruit tissue

Jasim et al., 2015

2Polaromonas sp. and lRalstonia sp.

lParacoccus sanguinis,

2Klebsiella pneumoniae, 2Cedecea davisae,2Klebsiella oxytoca and 2Erwinia tasmaniensis

Banana shoot-tips

Thomas and Sekhar. 2017

Country

Species

Isolation Sources

References

India

(continued)

2Pseudomonas pulida BP25

Root endosphere of Panniyur-5, black pepper

Sheoran et al., 2015

2Klebsiella pneumonia

Piper nigrum (Black pepper)

Jasirn et al., 2014

2Senatia $p.,2Pseudomonas sp., 2Pantoea $p. Others:

Lysinibacillus sp.. Bacillus sp.

Etlmomedicinal plants

Nougklilaw and Joslii. 2014

2Pseudomonas aeniginosa BP35

Black pepper

Kumar et al., 2013

2Pseudomonas spp. 2Erwinia spp.

lR/nzobnim, Agio bacterium

Root-nodule of chickpea (Greer arietinum L.) and mothbean (Vigna aconiiifolia L.)

Shanna et al., 2012

}Burkholdeiia n opica, B. unamae and B. cepacia

Lycpodium cemuum L.

Ghosh et al., 2016

Korea

2Pseudomonas

Ginseng plants of varying age

Yeudau et al., 2010

2Pseudomonas sp. HNR13

Roots of Chinese cabbage

Haque et al., 2016

}Burkholdena sp. strain KJ006

Rice

Kwak et al., 2012

2Seiralia sp. RSC-14

Roots of Solatium nigrum

Khanetal., 2015

2Pseudomonas koreensis AGB-1

Roots of Miscanthus sinensis gl owing in mine-tailing soil

Babu et al., 2015

2Klebsiella four species Others:

Microbactehum, two species of Paenibacillus, three Bacillus species

Leaves, stems, and roots of 10 rice cultivars

Jietal., 2014

'A gi obactei ium tumefaci ens

Pepper plants (Capsicum annuum L. cv. Nokkwang)

Kang et al., 2016

Country

Species

Isolation Sources

References

Korea

(continued)

1Sphingopyxis chi ten sis

2Enhydrobacter aerosaccus

1Mailelella endophytica sp. nov.

Root sample of a halophyte, Rosa rugosa

Bibietal.,2013

1Hoejlea suaedae sp. nov.

Root of a halophyte (Stiaeda maritima)

Chung et al., 2013

1Rhizobium halophytocola sp. nov.

Root of Rosa nigosa

Bibi et al., 2012

Japan

'Azospin Hum sp. B510

Stems of rice plants (Oryza sativa cv. Nipponbare)

Yasuda et al., 2009; Kaneko et al., 2010

'-Pseudomonas fluorescens Others:

Bacillus sp.. Streptomyces luteogriseus

Carex kobomugi roots

Matsuoka et al., 2013

Saudi

Arabia

2Enterobacter cloacae

Non-nodulating roots of Medicago sativa

Khalifa et al., 2016

Oman

2Enterobacter cloacae Others:

Paenibaci Hus xylanexedens

Date palm (Phoenix dactylifera L.) seedling roots

Yaish et al., 2015

1Sphingomonas sp. LK11

Leaves of Tephrosia apollinea

Khan et al., 2014

Pakistan

2Acinetobactei

Others:

Bacillus

Three gl asses. Lolium perenne, Leptochloa fusca, Brachiaiia mutica, and two bees, Lecucaena leucocephala and Acacia ampliceps

Fatima et al., 2015

Country

Species

Isolation Sources

References

Singapore

lMethylobacterium sp. strain L2-4

Jatropha curcas L.

Madliaiyau et al., 2015, 2018

Iran

-Pseudomonas fluorescens strain REN1

Roots of rice plants

Etesami et al.. 2014

Taiwan

lAchromobacter xylosoxidans strain F3B

Plants that are predominantly located in a constructed wetland, including reed

(Phragmites australis) and water spinach (Ipomoea aquatica)

Ho etal., 2012,2013

Malaysia

1Nwosphingobium sp. lAsticcacaulis sp.

Root tissues of Nipa Palm (Лура fi-uticans)

Tang et al., 2010

1Rhodobacter sp.

1Rhizobium sp.

lBurkho!deria vietnamiensis, B. tiopica, B. kururiensis, and B. ambifaiia

3Herbaspirillum sp.

2Pseudomonas sp. and Enterobacter sp.

Aloe vera

Akinsanya et al., 2015

Others: Bacillus sp.

Vietnam

2Pseudomonas aemginosa 23 (1-1)

Watermelon roots

Nga et al., 2010

'Alphaproteobacteiia, '-Gammaproteobacteha, }Betaproteobacteria.

PGPB

Plant Growth Promoting Trait

Beneficial Interactions

References

Ochrobactrum anthropi Mul

Nitrogen fixation

Root morphological optimization and entranced nutrient uptake

Meng et al., 2014

Burkholderia (Dominant genus)

Nitrogen fixation

Play potentially important roles in D. officinale

Yu et al., 2013

Raoultella sp.

Nitrogen fixation

Increased sugarcane plant biomass, total nitrogen, nitrogen concentration, and chlorophyll, and relieved nitrogen-deficiency symptoms of plants under a nitrogen- limiting condition

Luo et al., 2016

Variovorax paradoxus

Indole-3-acetic acid (IAA) production and phosphate-solubilizing activities

Excellent seed germination at high NaCl concentration and increased in plant shoot length, indicating that PGPB could protect S. europaea seedling from injury caused by salt stress

Zhao et al., 2016

Enterobacter sp., Sphingomonas sp., Pantoea sp., Bacillus sp.

ACC deaminase, IAA production, siderophores. and arginine decarboxylase

Four endophytic bacteria fr om elephant grass (Pennisetum puipureum Schumach) promoted plant growth and biomass yield, alleviated the harmful effects of salt stress on Hybrid Pennisetum

Li et al., 2016b

Enterobacter cloacae

IAA production

Enhance canola root elongation when grown under normal and saline conditions as demonstrated by a gnoto- biotic root elongation assay

Yaish et al., 2015

Pseudomonas fluorescens REN1

ACC deaminase activity and IAA production

ACC deaminase containing P fluorescens REN1 increased in vitro root elongation and eudophytically colonized the root of rice seedlings

Etesami et al., 2014

Klebsiella, Microbacterium, Bacillus, Paenibacillus

IAA production,

Siderophore producing activity, and Phosphate-solubilizing activity

Rice seeds treated with these PGPB showed improved plant growth, increased height and diy weight and antagonistic effects against fungal pathogens

Ji et al., 2014

PGPB

Plant Growth Promoting Trait

Beneficial Interactions

References

Sphingomonas SaMR12

IAA production and Zinc phytoremediation

SaMRl 2 inoculation significantly enhanced the efficiency of zinc phytoextraction by increasing S. alfredii biomass, promoting zinc absorption, improving root morphology, and enhancing root exudates

Chen et al., 2014

Sphingomonas sp. LK11

Gibberellins and IAA production

Increased growth attributes of tomato plants (shoot length, chlorophyll contents, shoot, and root diy weights) compared to the control

Khan et al., 2014

Senatia sp. RSC-14

Phosphate solubilization and IAA production

Relived the toxic effects of Cd-induced stress by significantly increasing root/shoot growth, biomass production, and chlorophyll content and decreasing malondialdehyde (MDA) and electrolytes content of Solanum nigrum host plant

Khanetal., 2015

Enterobacter cloacae

Phosphate solubility and IAA production

Inoculation оfPisum sativum with MSR1 significantly improved the growth parameters (the length and dry weight)

Khalifa et al., 2016

Burkholdeha tropica, B. unamae and B. cepacia

Phosphorus (Pi) solubilization

Supply soluble phosphate to the host plant to maintain its good health

Ghosh et al., 2016

Pseudomonas fluorescens, Bacillus sp., Streptomyces luteoghseus

Siderophore production and inorganic phosphate solubilization

Contribute to the Fe and P uptakes by C. kobomugi by increasing availability in the soil

Matsuoka et al., 2013

Pseudomonas aeruginosa strain BPS 5

Antagonistic effect

Protection to black pepper against infections by Phytophthora capsici and Radopholus similis

Kumar et al., 2013

Pseudomonas aeruginosa

Antagonistic effect

Antifungal compound, phenazine 1-carboxylic acid against Pythium mtyriotylum

Jasim et al., 2014

PGPB

Plant Growth Promoting Trait

Beneficial Interactions

References

Azospirillum sp. B510

Antagonistic effect

Enhanced resistance against diseases caused by the virulent rice blast fungus Magnaporihe oryzae and by the virulent Xanthomonas oiyzae

Yasuda et al., 2009; Isawa et al., 2010; Kaneko et al., 2010

Pseudomonas pulida BP25

Antagonistic effect

Volatile substances exhibited anti-pathogens, Phytophthora capsici, Pythium myriotylum, Giberella monilifonnis, Rhizoctonia solani, Athelia rolfsii, Colletottichum gloeosporioides and plant-parasitic nematode. Radopholus similis

Sheoran et al., 2015

dinitrogen (N2) into ammonia (Shiferaw et al., 2004). The nitrogen-fixing endophytic bacteria widely presented as symbionts are Rhizobium, Brady- rhizobiurn, Sinorhizobium, andMesorhizobium with the leguminous plants while Frankia with non-leguminous trees and shrubs (Zahran, 2001). However, there is some report about plant colonization by Bradyrhizobia found not only in leguminous plants but also in non-leguminous species such as rice (Piromyou et al., 2015a). Non-symbiotic nitrogen fixation is earned out by free-living diazotrophs and can enhance non-legume plant growth. The important free-living and associative nitrogen-fixing genera have been reported, are Azospirittum, Azotobacter, Gluconaceto- bacter. Azoarcus, Achromobacter, Burkholderia, Enterobacter, Herbaspi- rillum, Klebsiella, Mycobacterium, Pseudomonas, Rhodobacter, Serratia, Bacillus, Clostridia, and Citrobacter (Hayat et al., 2012). Nitrogenase (nif) genes, for nitrogen fixation are found in both symbiotic and free- living systems (Reed et al., 2011). Muangthong et al. (2015) reported the strains of Novosphingobium sediminicola and Ochrobactrum intermedium isolated from sugarcane in Thailand showing the nitrogenase activity, hi addition, Tang et al. (2010) reported that root-associating bacteria of the nipa palm (Nypa fruticans), Burkholderia vietnamiensis exhibited the activity of nitrogen fixation. Raoultella sp. strain L03 was isolated from surface-sterilized sugarcane root in China. This strain is proficient to fix nitrogen in association with the plant host. Inoculation of this strain increased sugarcane plant biomass, total nitrogen, nitrogen concentration, and chlorophyll, and relieved nitrogen-deficiency signs of plants under a nitrogen-limiting condition (Luo et al., 2016). The Ochrobactrum anthropi Mill strain stimulated the growth of Jerusalem artichoke host plant by plant growth-promoting effect as symbiotic nitrogen fixation, root morphological optimization, and improved nutrient availability and plant uptake (Meng et al., 2014).

3.4.2 PHYTOHORMONES

Phytohomiones play key roles as signals and regulators of growth and expansion in plants. The capacity to produce them is frequently considered as a trait of the plant kingdom. While this strategy is probably successfiil, soil, and plant-associated prokaryotes may also produce or transform phyto- hormones under in vitro (Costacurta and Vanderleyden, 1995). Various PGPB are known to produce phytohomione, namely, auxins, cytokinin, and gibberellins (Pliego et al., 2011; Hayat et al., 2012). Tiwari et al. (2016) reported that the arsenic resistance endophytes obtained from the roots of Pteris vittata belonged to group Proteobacteria as Rhizobium sp. strain El, Enterobacter sp. strain E3 and E5 including Stenotrophomonas sp. strain E7 showed the capability of indole-3-acetic acid (IAA) production. The Variovorax paradoxus belonged to p-Proteobacteria and strain of Bacillus endophyticus, Bacillus tequilensis, Planococcus rifietoensis, and Arthro- bacter agilis could enhance halophyte Salicornia europaea plant growth under saline stress environments by IAA production together with the other plant growth-promoting traits. All strains show’ed excellent seed germination at high NaCl concentration and also show'ed a significant increase in plant shoot length, indicating that PGPB could protect S. europaea seedling from injury caused by salt stress (Zhao et al., 2016). According to Li et al. (2016b), four bacterial genera: Sphingomouas (strain ppO 1), Pautoea (strain pp02), Bacillus (strain pp04) and Enterobacter (strain pp06) w'ere able to produce IAA at the range of 10.50-759.19 mg/L, where Enterobacter sp. established the highest value. The four endophytic bacteria isolated from elephant grass (Pennisetum purpureum Schumach) significantly enhanced plant growth and biomass yield, alleviated the harmful effects of salt stress on hybrid Pennisetum. An endophytic bacteria Serratia sp. RSC-14 could tolerate to cadmium and secreted phytohonnones such as IAA (54 pg/ mL). RSC-14 inoculation relived the toxic effects of Cd-induced stress by significantly increasing root/shoot growth, biomass production, and chlorophyll content and decreasing malondialdehyde (MDA) and electrolytes content of Solanum nigrum host plant (Khan et al., 2015). Enterobacter cloacae MSR1 isolated from the non-nodulation roots of Medicago sativa and Enterobacter cloacae PD-P6 isolated from date palm (Phoenix dacty- lifera L.) possessed the multiple plant-growth promoting characteristics, one of them is IAA production (Yaish et al., 2015; Khalifa et al., 2016). Inoculation of Pisum sativum with strain MSR1 significantly enhanced the growth parameters (the length and diy w’eight) of this economically significant grain legume compared to the non-treated plants. PD-P6 (Enterobacter cloacae) wras able to enhance canola root elongation when grown under normal and saline conditions as demonstrated by a gnotobiotic root elongation assay. Sphingomouas sp. LK11 isolated from the leaves of Teph- rosia apollinea showed significantly increased growth attributes (shoot length, chlorophyll contents, shoot, and root dry weights) of tomato plants by producing two kinds of phytohonnones as gibberellins and IAA (Khan et al., 2014). Chen et al. (2014) reported about the endophytic bacterium,

Sphingomonas SaMR12 isolated from Sedum alfredii seems to increase plant biomass by IAA production and zinc pliytoextraction. In addition, Jasim et al. (2014) observed that the phytochemicals from Piper nigrum had a stimulating effect on IAA production by Klebsiella pneumoniae isolated from the same plant. Sharma et al. (2012) found the most isolates belonged to genus Pseudomonas associated with chickpea (Cicer arietinum L.) and moth bean (Vigna aconitifolia L.) that could produce IAA. Ten strains of genus Klebsiella (Gammaproteobacteria), Bacillus, and Paenibacillus (Bacilli) and Microbacterium (Actinobacteria) isolated from Korean rice cultivars showed the highest IAA production and rice seeds treated with these PGPB could enhance plant growth, increase height and dry weight (Ji et al., 2014).

3.4.3 MINERAL NUTRIENT SOLUBILIZATION

Most of this phosphorus is found in insoluble forms, therefore, not available for plant growth. Mineral phosphate solubilization by endophytic bacteria could aid in the availability of phosphate to host crops during initial colonization and subsequently promote plant growth (Kuklinsky- Sobral et al., 2004). The principal mechanism for mineral phosphate solubilization has been proposed. Organic acids and acid phosphatases produced by microorganisms play a major role in the mineralization of organic phosphorus in soil (Rodriguez and Fraga, 1999). The most proficient PGPB has been described as the potential phosphate solubilization to stimulate growth of host plant together with IAA production belonging to genera as follows: group у-Proteobacteria - Pseudomonas, Enterobacter, Klebsiella, Serratia, Envinia', group a-Proteobacteria - Phyllobacte- rium, Rhizobium, Agrobacterium', group p-Proteobacteria - Variovorax (Sharma et al., 2012; Ji et al., 2014; Khan et al., 2015; Khalifa et al., 2016; Zhao et al., 2016). According to Ghosh et al. (2016), Burkliolderia tropica, Burkholderia unamae, and Burkliolderia cepacia associated with Lycopodium cernuum L. from phosphate starved red lateritic soil of West Bengal showed the phosphate solubilizing capability. The isolated strains could release of bound phosphates from ferric phosphate (FeP04), aluminum phosphate (A1P04), and four different complex rock phosphates. The results indicated that their phosphate solubilizing efficacy is veiy good. Moreover, all strains were effective to supply soluble phosphate to the host plant to maintain its good health.

3.4.4 SIDEROPHORE PRODUCTION

Iron is a vital bioelement for virtually all forms of life. All microorganisms known so far, with the exception of certain lactobacilli, require non. In aerobic conditions, microorganisms need iron for a variety of functions, including reduction of oxygen for the ATP synthesis, for heme formation, and for other essential purposes. The aerobic atmosphere of the planet has caused the surface iron exists predominantly in its ferric state and reacts to form highly insoluble hydroxides and oxyliydioxides, making it largely unavailable to microorganisms. To acquire adequate non, bacteria have developed active strategies to solubilize this metal for its proficient Fe uptake. Siderophores produced by bacteria are one of the most commonly found strategies. Bacterial siderophores are low-molecular-mass molecules with high specificity and affinity for chelating or binding Fe3+ (Neilands, 1983; Miethke and Marahiel, 2007; Rajkumar et al., 2010). Moreover, the biocontrol activity of PGPB can be achieved by siderophore production. Siderophores play an important role in the suppression of plant-pathogen by chelation of Fe thereby creating competition for iron. Under iron stress conditions confers upon these antagonistic organisms as an added advantage, resulting in the exclusion of pathogens due to non starvation (O’Sullivan and O’Gara, 1992). Eight isolates of endophytic root-nodule bacteria from chickpea (Cicer arietinum L.) and moth bean (Vigna aconiti- folia L.) belonging to Pseudomonas and Rhizobium exhibited positive results for siderophore production (Shanna et al., 2012). Ji et al. (2014) also revealed that six isolates associated with Korean rice cultivars, belonging to genera Klebsiella (Gammaproteobacteria), Bacillus, and Paenibacillus (Bacilli) and Microbacterium (Actinobacteria) showed high siderophore producing activity. Endophytic bacteria isolated from elephant grass (Penni- setum purpureum Schumach) were identified as Sphingomonas, Bacillus, and Enterobacter. These isolated strains showed siderophore production ability, where Bacillus recorded the optimum siderophore 64.06% units (Li et al., 2016b). Matsuoka et al. (2013) reported that Pseudomonas fluore- scens, Bacillus sp., and Streptomyces luteogriseus isolated from roots of Carex kobomugi exhibited siderophore production and inorganic phosphate solubilization under Fe or P limited condition. C. kobomugi showed higher Fe and P content. Colonization of root tissue by these bacteria contributes to the Fe and P uptakes by C. kobomugi by increasing availability in the soil. Tiwari et al. (2016) have detected two isolates positive for siderophore production belonging to Rhizobium and Bacillus that might have a presumptive role in Arsenic (As) uptake, accumulation, and detoxification in association with P vittata ecotype.

3.4.5 ANTAGONISTIC ACTIVITY AND PHYTOPA THOGEN BIOCONTROL

Plant pathogens are a major and chronic problem for crop production and global ecosystem stability. The PGPB prevents the plants from phytopathogens (fungal, bacterial, and viral diseases), including insect and nematode pests, by several mechanisms. PGPB colonization and defensive retention of host plants are enabled by production of siderophores (Miethke and Marahiel, 2007), cell wall lytic enzymes (Backman and Sikora, 2008), antibiotic metabolites (Schouten et al., 2004) and induction of systematic resistance in plants (Kloepper et al., 2004; Van Loon et al., 2004). According to Yasuda et al. (2009), Azospirillum sp. B510 isolated from the stem of rice plants had the ability of disease resistance in host plants. Inoculation Azospirillum sp. B510 with rice plants showed enhanced resistance against diseases caused by the virulent rice blast fungus Magnaporthe oiyzae and by the virulent bacterial pathogen Xanthomonas oryzae. Moreover, Pseudomonas sp. HNR13 including with Bacillus sp. and Microbacterium sp. isolated from the Chinese cabbage roots also showed strong antagonistic activity against the soil-bome fungal pathogens; Pythium ultimum, Phytophthora capsici, Fusarium oxysporum, and Rhizoctonia solani while most of the isolates are members of the genus Bacillus that could produce cell wall degrading enzymes, exhibited high antagonism against the tested food-bome pathogenic bacteria (Haque et al., 2016). The Pseudomonas putida BP25 inhibited a broad range of pathogens such as Phytophthora capsici, Pythium myriotylum, Giberella moniliformis, Rhizoctonia solani, Athelia rolfsii, Colletotrichum gloeosporioides and plant parasitic nematode, Radopholus siniilis by its volatile substances (Sheoran et al., 2015).

3.4.6 1-AMINOCYCLOPROPANE-l-CARBOXYLASE (ACC) DEAMINASE AND OTHER PRODUCTS

The enzyme ACC deaminase was first recovered by Honma and Shimo- mura (1978). This enzyme catalyzes the cleavage of ACC, the immediate precursor, to ammonia and a-ketobutyrate (KB), therefore reducing the amount of ethylene synthesized by the plant (Gamalero and Glick, 2015). Ethylene, a stress phytohormone, was reduced to allow the plant to be more resistant to a wide variety of environment stresses. It can function as an efficient plant growth regulator at a veiy low concentration as low as 0.05 pL/L (Abeles et al., 1992). PGPB belonging to Proteo- bacteria subgroup, Azospirillum, Rhizobium, Agrobacterium, Achromo- bacter, Burkholderia, Ralstonia, Pseudomonas, and Enterobacter have been reported to possess ACC deaminase activity/genes (Blaha et al., 2006). PGPB act as a sink for ACC with the consequence that lowering plant ACC levels, decreasing the amount of ACC within the plant that can be converted into ethylene (Santoyo et al., 2016). ACC deaminase containing PGPB can significantly decrease the extent of plant growth inhibition that accrues from the stresses such as flooding, high salinity, drought, the presence of fungal and bacterial pathogens, nematode damage, the presence of high levels of metals and organic contaminants including extremes of temperature (Gamalero and Glick, 2015). Pseudomonas stutzeri A1501 carries a single gene encoding ACC deaminase, designated acdS gene. The acdS mutant lacking of ACC deaminase activity was less resistant to NaCl and NiCl, compared with the wild type. Moreover, the nitrogenase activity of the lack of ACC deaminase- inactivated mutant was greatly impaired under salt stress condition (Han et al., 2015). The four strains tested, Sphingomonas (strain ppOl), Pantoea (strain pp02), Bacillus (strain pp04) and Enterobacter (strain pp06) demonstrated relatively high levels of ACC deaminase activity, where Pantoea sp. presented the highest ACC deaminase activity at 1106.66±78.59 nmol KB/h/mg (Li et al., 2016b). Jasim et al. (2015) isolated and identified the endophytic bacteria from the fruit tissue of Elettaria cardamomum by using both culture-based and PCR based methods. PCR-based screening identified the isolates EcB 2 (Pantoea sp.), EcB 7 (Polaromonas sp.), EcB 9 (Pseudomonas sp.), EcB 10 (Pseudomonas sp.) and EcB 11 (Ralstonia sp.) as positive for ACC deaminase. Pseudomonas fluorescens REN1 containing ACC deaminase isolated from roots of rice, increased in vitro root elongation, and endophytically colonized the root of rice seedlings significantly, as compared to control under constant flooded conditions (Etesami et al., 2014).

Generally, plant growing in a unique environmental setting having special etlmobotanical uses having extreme age or interesting endemic location possesses novel endophytic microorganisms which can supply new leads. Recently, new endophytic bioactive metabolites, having a wide variety of biological activities as an antibiotic, antiviral, anticancer, antiinflammatory, and antioxidant have been characterized (Strobel and Daisy,

2003). Pnndir et al. (2014) isolated endophytic bacteria from tomato, Aloe vera, chilli, radish, cauliflower, cabbage, arjun, pomegranate, grass, carrot, coriander, guava, stevia, mint, garlic, peas, giloy, turmeric, neem and rose that were collected from different areas of Ambala, Haryana. Bioprospecting potentials as antimicrobial activity, antibiotic susceptibility pattern, enzyme activity, and dye degradation ability of thirty isolates were observed. Twenty isolates exhibited both antifungal {Aspergillus fumigatus, Aspergillus sp. and Candida albicans) and antibacterial activity {Staphylococcus epidermidis, Bacillus amyloliquefaciens, Escherichia coli, and Salmonella enterica ser. typhi). Moreover, 33.33% of isolates showed urease activity, 66.66% amylase activity, 50% esterase activity, while 53.33% malachite green degradation. According to El-Deeb et al. (2013), among 28 endophytic bacterial isolates form organs of Plec- tranthus tenuiflorus belonging to genus Pseudomonas, Acinetobacter, Bacillus, Micrococcus, Paenibacillus showed the stronger activities in extracellular enzymes, for example, amylase, esterase, lipase, protease, pectinase, xylanase, and cellulase. Shanna et al. (2015) reported about the diversity of culturable bacterial endophyte associated with Saffron that is a medicinally important plant in India. Fifty-four bacterial strains were identified as Bacillus, Paenibacillus, Pseudomonas, Brevibacillus, Entero- bacter, and Staphylococcus, 81% isolates showed lipase activity, 57% cellulase, 48% protease, 38% amylase, 33% chitinase and 29% showed pectinase activity. Moreover, Akinsanya et al. (2015) revealed for the first time the endophytic bacteria communities in Aloe vera that were mostly Pseudomonas sp., Enterobacter sp., and Bacillus sp. and these isolates produced bioactive compounds with good antimicrobial and antioxidant activities.

 
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