MECHANISMS UNDERLYING BACTERIAL ENDOPHYTES- PLANT INTERACTIONS

2.5.1 PHYTOSTIMULA TION

Phytostinrulation is the promotion of plant growth via the production of phytohomrones (Bloemberg and Lugtenberg, 2001). They have a complex, diverse, and significant role that combines both assumed developmental pathways and environmental dynamic responses in plant growth (Durbak et al., 2012). Certain bacteria produce phytohomrones, i.e., auxins (i.e.,

IAA), gibberellins (GAs), and cytokinins (Bottini et al., 2004; Tsavkelova et al., 2006; Kudoyarova et al., 2014). It could be assumed that phyto- lionnones (IAA and ethylene biosynthesis) have been exploited as signal molecules between bacteria and plants (Yuan et al., 2008). Moreover, phytohonnone production can be also regulated by bacterial action on the plant. The production of jasmonic acid (JA) (Forchetti et al., 2007) and salicylic acid (SA) (De Meyer et al., 1999) were reported in endophytic bacteria isolated from sunflower. The auxin synthesis that plays a key role in plant root development has been identified in many bacterial strains belong to the genera of Azospirillum, Pseudomonas, and Bacillus (Dodd et al., 2010). Cytokinins can be produced by Arthrobacter, Azospirillum, Azotobacter, Bacillus, Rhizobium, and Pseudomonas strains. When the plants inoculated with B. subtilis that produce cytokinin showed enhanced chlorophyll content and cytokine accumulation. Finally, this inoculation increased the weight of shoots and roots (Arkhipova et al., 2007). PGPBs such as Azospirillum, Brevibacterium, Bacillus, Lysinibacillus, and Pseudomonas have been shown to synthesize abscisic acid (ABA) and affect its level in plants, especially under stressful conditions like salinity (Belimov et al., 2014). For instance, under drought conditions, when maize inoculated with ABA producer strain Azospirillum lipoferum USA 59b, this interaction support to the plant growth by inducing high ABA production in the plants (Cohen et al., 2009).

The optimal functioning of PGPBs includes the synergistic interaction between ACC deaminase and both plant and bacterial auxin, IAA. These bacteria favor to plant growth and help to plants against biotic and abiotic stress conditions. ACC deaminase, responsible for the cleavage of the plant ethylene precursor (Homna and Shimomura, 1978), is closely related to the stimulation of plant growth by microorganisms (Glick et al.,

1998). Therefore, the PGPB have a role as a sink for ACC, thus lowering plant ACC levels, decreasing the amount of ACC within the plant that can be converted into ethylene (Belimov et al., 2009). Medicago truncatula (M truncatula) and A. thaliana mutants were used to analyze the effect of plant defense/response pathways in controlling the number of endophytic bacteria (Iniguez et al., 2005). They reported that the amount of Klebsiella sp. strain Kp342 and Salmonella typhimurium (S. typhimurium) strain decreased by ethylene, a signal molecule which induces plant SR. While the ethylene precursor, ACC, reduced colonization of endophytic bacteria, an ethylene-insensitive M. truncatula mutant was hyper-colonized by Klebsiella sp. strain Kp342. Various endophytes, including Arthrobacter

spp. and Bacillus spp. in pepper plants (Sziderics et al., 2007), as well as Pseudomonasputida and Rhodococcus spp. in peas (Belimov et al., 2001) release ACC deaminase to increase plant growth. Even though the mechanism of plant growth promotion is unknown, ACC deaminase production may reduce abiotic stress by balancing the level of ethylene in the plant since the elevated amount of ethylene inhibit cell division, DNA synthesis, and root/shoot growth (Burg, 1973). Moreover, the production of other plant hormones, including IAA, JA, and ABA by bacterial strains may also stimulate plant growth (Forchetti et al., 2007). When Miscanthus seedlings wrere inoculated with Herbaspirillum frisingense (.H. frisingense) GSF30T, elicited the root and the shoot growth. Transcriptomic analysis has provided detailed information on ethylene and jasmonate signaling regulation and showed that the induction of plant growth is promoted by the activation of phytohormones (Straub et al., 2013).

Rothballer et al. (2008) show'ed that H. frisingense GSF30T produce IAA and similar positive effect of IAA was observed in wheat inoculated with B. subtilis (Egorshina et al., 2012). In the case of Azospirillum spp., while root stimulation is increased by auxin, production of the other phytohormones had a slightly effect on nitrogen fixation and production (Steenhoudt and Vanderleyden, 2000). This strain can also be used in the field, for example, re-inoculation of seedlings with Azospirillum sp. strain B510 that was previously isolated from surface-sterilized stems of rice, enhanced root proliferation and mass production in field conditions (Isaw'a et al., 2010). In addition, three Pseudomonas strains stimulated wdieat growth and its spike length both in laboratory and field conditions (Iqbal and Hasnain, 2013). In both cases, these effects were come up with phytohormone production rather than nitrogen fixation. Thus, PGPBs alter the phytohormone level, affecting the hormonal balance of the plant and its resistance to stress.

2.5.2 PHYTOREMEDIATION

Some bacterial endophytes promote plant growth by increasing plant tolerance or resistance capacity to high concentrations of pollutants. The remediation of organic compounds and toxic metals is probably through effective plant-bacterial interactions, and the cleaning of contaminated soils with plants is called phytoremediation. The abilities of the PGPBs mentioned under these conditions are to absorb heavy metal ions from the extracellular space, to accumulate them in the cell wall, or to convert them into less toxic forms. The inclusion of bacteria interacting with plants in the phytoremediation process is increasingly accepted as an alternative to combat the inherent weaknesses of the plants (Abhilash et al., 2012). The microbial community in the rhizosphere has been shown to play a part in the degradation of trichloroethylene (TCE) contaminant in groundwater (Weyens et al., 2009a). In this sense, it has been proposed to use various endophytic bacteria for the degradation of hydrophobic compounds (Stenuit et al., 2010) or for the delivery of the plant N-feed or P, Fe, or hormones (Marchand et al., 2010).

There are some studies published on the degradation ability of endophytic bacteria on endophytes (Mitter et al., 2013b). The first report on bacteria isolated from plant inner tissue that had the ability to degrade hydrocarbons was published by Siciliano et al. (2001). Those bacteria were intensely located on the root interior rather than birlk soil. Density of endophytic bacteria was correlated with amount of hydrocarbons hr soil. Besides, some hydrocarbon degradable bacteria isolated from plant growing soil contaminated with crude oil were also identified. These bacteria have enormous potential to degrade alkane and aromatic hydrocarbons (Phillips et al., 2008; Yousaf et al., 2010). In addition, different bacterial strains that have the ability to degrade 2,4-dichlorophenoxyacetic acid (2,4-D) were also isolated from poplar internal tissues (Porteous Moore et al., 2006; Taghavi et al., 2011).

Endophytes like Burkholderia cepacia have been reported to improve both remediation and biomass production in the host (Weyens et al., 2009a, b). At the same time, some of the organic pollutants degrading endophytic bacteria have resistance to heavy metals, and they enhanced the capacity of phytorernediation of soil and water (Weyens et al., 2010b). For instance, five endophytic bacteria had ACC deaminase activity, solubilized phosphorus, and produced IAA and siderophore were isolated from Sedwn plumbizincicola, the Cd/Zn hyperaccumulator plant (Ma et al.,

2015). Moreover, it was found that the strains have high Cd, Pb, and Zn resistance. In addition, when plants were inoculated with Bacilluspumilus (В. pumilis) strain E2S2, increased Cd uptake and physiological properties (i.e., root and shoot length, fresh, and diy biomass) were observed in comparison to non-inoculated plants. Recently, Sinorhizobium meliloti (S. meliloti) strain CCNWSX0020, isolated from Medicago lupulina under copper stress was reported (Kong et al., 2015). According to the report, it elevated nitrogen content, Cu accumulation, and plant growth. Moreover, when plants inoculated with S. meliloti in high Cu levels, genes responsible for antioxidant response were also stimulated.

In pot experiments. Bacillus sp. SLS18 increased the biomass of sorghum grown in the presence of either manganese or cadmium. Also, the existent of a similar effect in dicotyledon species indicates broad host range applicability (Luo et al., 2012). For example, the inoculation of barley with bacterial strains Arthrobacter myosorens and Flavobacterium sp. reduced the mobility of Cd ions in soil and prevented contamination of the grain with heavy metal ions. On the other hand, the interaction of metallophilic bacteria, which function as accumulators or hyperaccumulators of heavy metals, with plants in cleaning up the contaminated soils will provide useful information for the phytoextraction of metals (Hu et al., 2007), such as the isolates from the rhizosphere of Alyssum murale plants that contained Ni-resistant bacteria Spliingomonas and Microbacterium which were capable of decreasing the content of mobile Ni in soil and inoculated plants. Moreover, the association of mustard (Brassica juncea) plants with Xanthomonas sp., Pseudomonas sp., Azomonas sp., and Bacillus sp. strains elevated the mobility of Cd ions (Belimov et al., 2011; Luo et al., 2012). Therefore, it is important to note that the capability of microorganisms for the accumulation of heavy metal ions may give optional or different ways to chemical methods of soil amendment.

2.5.3 BIOFER TIL IZA TION

The promotion of plant growth through the provision of basic nutrients or by increasing availability is called biofertilization (Bashan, 1998). Nitrogen fixation is the conversion of atmospheric nitrogen to ammonia, a well-studied form of biofertilization. Numerous PGPBs, including Azospirillum, Pantoea agglomerans, Acetobacter, Azoarcus, Herbaspirillum spp., Azoarcus spp. and Aeromonas have been extensively studied for their nitrogen fixation ability (Bloemberg and Lugtenberg, 2001; Verma et al., 2001; Hurek et al., 2002; Dobbelaerae et al., 2003).

Utilization of the potential of biological nitrogen fixation is assumed as a priority target for the development of novel cropping systems in 21s,-centuiy agriculture (Roesch et al., 2008). The difficulty underlying the more widespread use of biological nitrogen fixation in agricultural production is due to the complexity of nitrogenase, an ancient enzyme used by prokaryotes to convert atmospheric N, to NH3, responsible for atmospheric nitrogen fixation. Free-living endophytes with biological nitrogen fixation ability utilize the low oxygen environment created by the plant as it optimizes nitrogen activity (Elliott et al., 2009). Some endophytes also produce compounds such as triterpenes, while another plant-associated bacterium may require input from the host plant for the reaction to occur (Doty et al., 2009).

Several endophytic B. pumilus strains were isolated, which could reduce the acetylene from sunflower tissues and were able to make nitrogen fixation (Forchetti et al., 2007). Similar features have also been reported for Bacillus spp. strain isolated from the cactus Pachycereus pringlei (Puente et al., 2009). Also, various bacilli with the niJH encoding the small subunit of nitrogenase have been reported (Ding et al., 2005; Rashedul et al., 2009). The rate of nitrogen fixation from Bacillus species such as B. azotofixans, B. coagulans, B. holimixa, and B. macerans can be as high as 18.8% of the number of spore-forming bacteria in the soil (Melentev et al.,

2007). There is evidence that free-living and diatrophic bacteria from the genera Azotobacter, Azospirillum, Rhizobium, and Bradirhizobium as well as Rhizobacteria, Pseudomonas, and Bacillus can also be elicite nitrogen fixation (Dobbelaere et al., 2003).

Diazothropy is a symbiotic relationship between bacteria and their host plant. Conception of diazotrophy is important to understand the mechanisms of plant gr owth promotion. It is known that rhizobia fix the atmospheric nitrogen and serve to plants. Some PGPBs are diazotrophic and can fix nitrogen (Lodewyckx et al., 2002), but not all PGPBs are diazotrophic as they cannot fix atmospheric nitrogen or cannot fix the amount enough for their host (Hong et al., 1991). There are a large number of diazotrophic bacteria such as Acetobacter diazotrophicus, Herbaspirillum sp. and Azospirillum sp. in some agronomically important plants such as sugarcane (Saccharum sp.), rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays). At this point, endophytic diazotrophs have the advantage as compared to root-associated diazotrophs. For example, Azospirillum sp. and Azobacter sp. are located in the depths of the plant roots and can grow in the atmosphere with little oxygen, which is suitable for the study of the nitrogenase (Mclnroy and Kloepper, 1995; Triplett, 1996; Dobbelaere et al., 2003).

Furthermore, endophytic bacteria that fix nitrogen in the internal tissues of the plants without damaging them (Govindarajan et al., 2006, 2008) can provide up to 65% of plant nitrogen in poplar clones (Knoth et al.,

2014). Endophytes identified in tomato grown in the field, in sorghum

(Wong-Villarreal and Caballero-Mellado, 2010) and in mimosa (Elliott et al., 2009) were generally found as Burkholderia spp. while free-living and nitrogen-fixing Sphingomonas spp. were identified in cottonwood and willow growing in nutrient-poor conditions (Doty et al., 2009). The effectiveness of nitrogen fixation varies between different bacteria, such as seven free-living lhizospheric and endophytic Burkholderia spp. {B. unamae, B. tropica, B. silvatlantica, B. xenovorans, B. vietnamiensis, B. kururiensis, and B. sacchari) (Martinez-Aguilar et al., 2008).

Phosphorus ranks second important key element among the mineral nutrients the plant needs after nitrogen. Although both organic and inorganic forms of phosphorus are abundant in the soil, its availability is limited because mostly it is found in an insoluble form. Despite the P content in average soil is about 0.05% (w/w), only 0.1% of the total P is available to plant because of poor solubility and its fixation in soil (Illmer and Schinner, 1995). Certain PGPBs can increase the solubility and availability of phosphate. With the release of low molecular weight acids, chelating of the metal cation that adheres to phosphorus can be provided hence making it more accessible for plants (Kpomblekou and Tabatabai, 2003). For elimination of the phosphorus starvation of plants, endophytes are becoming important since they can dissolve phosphorus-containing substances with organic acids, phosphatases, or other metabolites (Dobbelaere et al., 2003). The phosphate dissolution capacity of the Pseudomonas fluorescens CHAO strain depends on its ability to produce gluconic acid (De Werra et al., 2009). Achromobacter xiloxidans and Bacillus pumilus in sunflower (Helianthus armuus) were identified as having the highest chelating capabilities (Forchetti et al., 2007). Yazdani and Bahmanyar (2009) showed that the use of PGPB in fertilizer treatments for com (Zea mays) decreased the need for phosphorus treatment by 50% without significant loss in grain yield.

2.5.4 BA CTERIA L SIGN A L INC MOL ECUL ESA ND BIOCON TROL

Bacteria can regulate gene expression through “quorum sensing” (QS), a term that refers to gene expression controlled by the response to signal molecules in a cell-density-dependent maimer (Fugua et al., 1994). Some bacteria use these molecules to create their own pathogenicity; others living in the same environment could break them down. So far, it has been indicated that the production of antibiotics, virulence factors, and plant cell wall-degrading exo-enzymes is activated in respond to QS (Von

Bodman et al., 2003). Noticeably, plants induce QS-regulated signals in bacteria and could also control its level (Bauer and Mathesius, 2004). For this reason, it would be interesting to investigate whether endophytes produce QS compounds that affect the growth of plants and then adaptation properties.

Some mechanisms employed by phytogenic bacteria (Vant Slot and Knogge, 2002) are also possible to be used by other endophytes. Many candidate genes with unknown functions were expressed differently during plant bacterial interaction (Rocha et al., 2007), especially inoculation of sugarcane with endophytic nitrogen-fixing bacteria altered the expression profile of shr5 (a plant receptor kinase) gene (Vinagre et al.,

  • 2006) . In the presence of beneficial endophytes, the genes of the ethylenesignaling pathway were also differently expressed (Cavalcante et al.,
  • 2007) . Genes such as carAB, which is required for degradation of a fatty acid signaling molecule, can be produced by recombinant biocontrol strains and they reduce virulence caused by Xmithomonas sp. and Xylello fastidiosa (Newman et al., 2008).

Plants produce secondary metabolites that mimic or inhibit QS molecules (Gao et al., 2003). For example, tobacco plants were engineered to synthesize acyl-homoserine lactones (AHLs) in the chloroplast (Scott et al.,

2006), and it was shown that AHL was transported and secreted on the phyl- losphere and in the rhizosphere. The virulence through Envinia carotovora was reduced by pretreatment of potato slices with Bacillus thuringiensis (Dong et al., 2004). Due to deterioration of QS molecules of E. carotovora with AHL lactonase, potato slices pretreated with B. thuringiensis lacked the ability to produce AHL-lactonase, but it did not reduce maceration. On the other hand, the plants that are manipulated to express AHL lactonase have exhibited a positive effect in the protection of plants agamst pathogens (Zhang, 2003). Recently, a gene encoding a new type of AHL- lactonase, which is nominated qsdA from Rhodococcus erythropolis, has been characterized. This gene was able to gain quorum quenching capacity to P. fluorescens that led to enhanced protection of potato tuber agamst the soft rot pathogen Pectobaterium carotovorum (Uroz et al., 2008). Rosmarinic acid, a highly antioxidant phenolic compound, has a potential antimicrobial activity against a range of soil-bome microorganisms, and it was induced in sweet basil hair root cultures after contact with Pythium ultimum (P. ultimurn) (Bais et al., 2002). In addition, recently endophytic actinobacteria has great potential, as it is the source of novel bioactive compounds, including antibiotics, antifungals, and antitumor compounds

(Qin et al., 2011). Endophytic bacteria inhibit bacterial communication and biofilm formation, and virulence in this way, without suppressing bacterial growth, both by producing specific bioactive products and by blocking pathogenic QS. Accordingly, it has been reported that cell-free lysates of endophytic bacteria suppress QS molecules and prevent biofilm formation in P. aeruginosa PAOl (Rajesh and Ravishankar, 2013). Therefore, this kind of “quorum quenching” gains importance as an alternative innovative approach in dealing with drug-resistant bacteria (Kusari et al., 2014).

Biocontrol is the promotion of plant growth through protection from plant pathogens. Endophytic bacteria are emerging as promising biological methods to control plant pathogens (Koumoutsi et al., 2004). Since the ultimate goal of the action mechanism of PGPBs is not pathogen destruction, they are different from the classical plant protection chemicals. It is used to stimulate SR in order to control the pathogens, which are present in large quantities in the environment. The majority of PGPBs are active to trigger the cascade of defense responses due to the production of various metabolites (Maksimov et al., 2011). Bacterial endophytes may interfere with the development of phytophagous insects and nematodes through the synthesis of biologically active compounds, termed antipathogenic action (Azevedo et al., 2000). Commonly used mechanisms of biocontrol mediated by PGPBs are antibiosis (Haas and Defago, 2005; Lugtenberg and Kamilova, 2009), induced systematic resistance (ISR) (Van Loon, 1998, 2007; Kloepper et al., 2004), competition for niches and nutrients (Kamilova et al., 2005; Validov, 2007) and predation and parasitism (Hannan et al., 2004). The ISR associated with PGPBs is effective against a broad spectrum of plant pathogens including oomycetes, fungi, bacteria, and viruses and even insects and herbivores (Van Oosten et al., 2007,

2008). This resistance was observed in a variety of plant species including Arabidopsis, beans, carnation, eucalyptus, radish, tobacco, tomato, rice, maize, and wheat (Kloepper et al., 2004; Van Der Ent et al., 2009; Bene- duzi et al., 2012; Yi et al., 2013). Development of PGPB-induced ISR in plants may involve the generation of reactive oxygen species (ROS) as a critical event in the formation of priming effect and after inoculation of a pathogen, plants treated with PGPB show early ROS formation (Comath et al., 2006).

Pathogen-induced systemic acquired resistance (SAR) co-exists with coordinated activation of genes associated with pathogenesis (PR), and many of them encode proteins with antimicrobial activity (Van Loon et al.,

  • 2007). This resistance is controlled by the redox-regulated protein NPR1, which is activated by SA, and functions as a transcriptional co-activator for a large part of the PR genes (Pieterse et al., 2012). SAR induced by endophytic bacteria is phenotypically similar to SAR associated pathogenesis, except activation of JA and ethylene synthesis (Pieterse et al.,
  • 2014). B. pumilus SE 34 induces SR against Fusarium wilt on tomatoes (Benhamou et al., 1998). After pre-inoculation of Arabidopsis seedlings with two strains of Streptomyces sp., seedlings were inoculated with Erwinia caratovora (E. caratovord). The plants found to be protected from disease symptoms as endophyte-ffee plants decayed within 5 days. ISR is conducted by one strain and SAR is achieved by the other strain due to the fact that gene stimulation in Arabidopsis was specific to strain (Conn et al., 2008).

Several microbial components, which are referred to as microbe- associated molecular patterns (MAMPs), are recognized by plants and act as elicitors, triggering a generalized MAMP-triggered immunity (MTI). Despite generally described in the context of pathogenicity, MAMPs are highly conserved in whole classes of microbes, including endophytes. MTI includes the production of reactive oxygen and nitrogen species (Newman et al., 2013). The antibiotic iturin A is produced by Bacillus sp. CY22 and it suppresses the root rot of balloon flower caused by Rhizoctonia solani (R. solani) (Cho et al., 2003). Also, Pseudomonas fluorescens carrying the chitinase-encoding gene chi A, can control the phytopathogenic fungus R. solani on bean seedlings (Downing and Thomson, 2000).

Endophytes offer tremendous promise to discover natural products with therapeutic value (Xiong et al., 2013). One of the interesting examples of bioactive plant-associated metabolites by endophytes is hypericin, which originally isolated from the plant Hypericum perforatum (Nahrstedt and Butterweck, 1997). This compound has many pharmacologically interesting properties, such as its distinctive antidepressant properties, antiinflammatory, antimicrobial, and antioxidant activities (Tammaro and Xepapadakis, 1986). Recently a Streptomyces sp. was isolated from an annual plant Lolitim perenne (Guemey and Mantle, 1993). This endophyte contains a diketopiperazine called methylalbonoursin and consists of leucine and phenylalanine. Moreover, a number of new antibiotics identified as munumbicins А, В, C, and D, were isolated from a streptomycete within a snake vine plant (Castillo et al., 2002). These are broad-spectrum antibiotics that can be active against several humans as well as plant pathogenic fungi and bacteria.

Endophytic bacteria isolated from rice seeds showed vast fungal activity against Gaeumannomyces graminis, P. myriotylum, Heteroba- sidium annosum, and R. solani (Mukhopadhyay et al., 1996). Entero- bacter cloaca, an endophyte isolated from com also has antibiotic ability against Fusarium mondiforme (Hinton and Bacon, 1995). This ability was confirmed by Chen et al. (1995) on cotton plants in which it reduced wilt disease symptoms caused by a Fusarium sp. Van Buren et al. (1993) reported that pathogen-endophyte antagonistic activity on Clavibacter michiganensis subsp. sepedonicum (Van Buren et al., 1993) and P.fluo- rescens 89B-27 and Serratia marcescens 90-166 activity against to Pseudomonas syringae pv. Jachrymans (Liu et al., 1995). Additionally, control of plant-parasitic nematodes (Hallmann et al., 1995) and insects (Dimock et al., 1988) via rhizospheric and endophytic bacteria were also observed (Kloepper et al., 1991).

Iron (Fe) is an essential element that plays a role as a catalyst in enzymatic processes, oxygen metabolism, electron transfer, and DNA/ RNA synthesis (Barry et al., 2009). Since it regulates surface motility and stabilizes the polysaccharide matrix, Fe is also essential for biofilm formation. Under iron-deficient growth conditions, the microbial surface hydrophobicity is reduced, which causes the surface protein composition to be altered to limit the biofilm formation. Due to the low Fe availability in the environment, microorganisms have developed specific uptake mechanisms such as the production of siderophores. Siderophores such as pyrethroids and SA produce iron, compete with phytopathogens for trace metals, and indirectly contribute to disease control (Duffy and Defago,

1999). One of the most studied siderophores with a strong antifungal effect is pseudobacin produced by P. putida BIO, and it suppressed the development of oxyspomm in iron-depleted soils (Kloepper et al., 1980). In addition, pseudobacin produced by P. putida WCS 358 inhibited the growth of Ralstonia solanacearum in eucalyptus, the growth of Em inia carotovora in tobacco and of B. cinerea in tomatoes (Bakker et al., 2007). Antibiotics can affect microorganisms through not only the inhibition of cell wall synthesis and respiratory enzymes but also the alteration of membrane functions and protein synthesis. As an example, P.fluorescens CHAO produces 2,4-diacetylphloroglucinol (DAPG) and this compound destmcts the membranes and suppress the germination of zoospores of the oomycete Pythium spp. (Melentev et al., 2007). Therefore, PGPBs produce antimicrobial metabolites such as DAPG and enhance the disease suppression in plants. For example, when seeds were inoculated with

DAPG-producing endophytic isolates, eggplant wilt caused by R. sola- nacearum was reduced by 70% (Ramesh et al., 2009). Similarly, endophytic bacteria belonging to genus Bacillus are known to produce various antifungal and antibacterial lipopeptides, including iturins, bacillomycins, fengycins, and surfactins (Gond et al., 2014). It is known that antibiotics from Bacillus spp. stimulate the swellings of hyphal apices in Sclerotima sclerotiorum, Alternaria alternate, Drechslera oryzae, Fusarium roseum, and pathogenic Puccinia graminis in hyphal apicals (Duffy et al., 2003).

Why Bacillus endophytes frequently occur in the natural populations of plants? One of the possible reasons can be the disease protection of Bacillus endophytes by using its lipopeptides, compliments in several wild populations (White et al., 2014b). It was known that endophyte infection might induce the expression of disease resistance genes in host plants. With regard to this, Bacillus endophytes in com resulted in enhanced expression of defense-related genes (Gond et al., 2015). Therefore, it is known that the efficiency of antibiotic production in many endophytic bacterial strains has been shown to increase the resistance of plants to pathogens. However, the exact details of the interaction between endophytes and host plants leading to the induced expression of resistance genes are presently unknown.

Volatile organic compounds (VOCs) evaporate under high vapor pressures, and under normal conditions, they can enter the atmosphere. Low molecular weight (<300 g/mol'1) compounds such as alcohols, aldehydes, ketones, and hydrocarbons take part in this class. VOCs help the plant growth and response ISR and 1ST in plants (Ryu, 2015). Volatile substances such as acetoin and 2,3-butanediol released from some PGPBs, can change plant-bacteria interactions and promote plant growth. In several numbers of mechanisms that PGPBs use to interact with plants, VOC emissions are an important participant. VOCs have mainly play role on antibiosis and biocontrol of plant pathogens. Recent studies using gas chromatography and mass spectrometry have revealed the capacity of bacteria to produce outstanding VOCs. Bacteria can produce different volatiles such as ammonia, alcohols, butyrolactones, phenazine-

1-carboxylic acid, HCN, and some of them have antifungal activity on various fungal species (Trivedi et al., 2008). Different kinds of VOCs can be produced based upon growth conditions and environment (Schulz and Dickschat, 2007). Moreover, B. subtilis GB03 emitted VOCs, which promoted various hormone signaling pathways in A. thaliaua, including auxin, cytokinins, brassinosteroids, gibberellins, and SA. Therefore, this study led to new opportunities on the role of volatiles during plant- microbe interaction accompanied by plant growth (Zhang et al., 2007).

As defensive mechanism of plants is triggered ISR occurs and leads to resistance against to pathogen infection. Regarding this, PGPB VOCs could behave as bioprotectants by ISR (Ryu et al., 2014). In addition, ISR is used for enhancing tolerance to abiotic stress by chemical and physical changes elicited by PGPBs. Zhang et al. (2008) showed the role of VOCs emitted from B. subtilis GB03 on ISR to salt stress in Arabidopsis. In another hand, endophytic bacteria have been detected in English oak (Q и ere us robur), common ash (Fraxinus excelsior), and can degrade VOCs such as TCE (Weyens et al., 2010a; Kang et al., 2012) that could have a role in phytoremediation. Recent studies on these mechanisms are shown in Table 2.1.

 
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