In the last 20 to 25 years, many researches are published on the colonization of endophytic bacteria inside in vitro plant cultures (Reed et al., 1995; Reed and Tanprasert, 1995; Dunaeva and Osledkin, 2015). These are associated with free-living bacteria, plant-inhabiting, animal-related, and processed foods, sewage, or human pathogens (Cassells and Tahmatsidou, 1996; Fletcher et al., 2013). There were 14 isolates of bacteria belonging to 9 genera, including hi the animal- or human-related ones, found at the 1 cm long shoot apices of papaya after the surface sterilization (Tomas et al., 2007). Despite the surface sterilization of the initial explants, the plant cultures were not necessarily free of bacteria. While the presence of some bacteria may be evident at the beginning, sometimes the contaminants may not be detected until the proliferation stage, and often even after the suboptimal conditions or acclimation (Pirttila et al., 2008; Thomas, 2011). For instance, pathogenic Xanthomonas axonapadis was symptomless in Anthurium cultures for one year (Norman and Alvarez, 1994), and pathogenic Agrobacterium vitis was present in the latent stage of in vitro Vitis vinifera shoot cultures for 14 weeks (Poppenberger et al., 2002).

Many isolates from plant tissue culture have promoted plant growth rates or influenced morphogenesis in vitro. Regulation of optimum conditions, growth, and organogenesis ofplantlets are easier in in-vitro compared to ex vitro plant production. These are used in both research and industry. Some plant varieties, such as recalcitrant genotypes do not effectively multiply or regenerate rooting in vitro. However, if they are inoculated with endophytic beneficial bacteria and supported by exogenous regulators and metabolites, these plant varieties can overcome culturing challenges. For example, rodestrine, a phytohormone produced by Rhodobacter sphaer- oides, promote the rooting of mulberry microshoots (Sunayana et al.,

2005) and Bacillus spp. produce indole-3-acetic acid (IAA) that enhances rooting of strawberry (Dias et al., 2009). Endogenous bacteria related with micropropagated Primus avium genotypes were found to influence plant growth rate (Quambusch et al., 2014).

The beneficial effects of several bacterial strains on the explants in vitro have led to the biotization of plant cultures with usefiil bacteria (Nowak, 1998). One of the best-known examples for biotization is Burk- holderia phytopharmans PsJNTM strain, previously known as Pseudomonas spp. PsJN, and it does not grow on the plant medium without plant explants (Sessitsch et al., 2005). It stimulates the growth of microshoots and microroots as colonized on both surface and internal tissues (Nowak, 1998), allows for more effective use of water and enhances the resistance of the plant to pathogens (Theocharis et al., 2012) and cold stress (Fernandez et al., 2012). Another bacteria, Pseudomonas spp., has been reported to produce polysaccharides that can prevent the overhydration of oregano (Shetty et al., 1995), raspberry (Ueno et al., 1998) and anise (Bela et al., 1998) cultures. Methylobacterium sp. DIO and Methy- lophilus glucoseoxidans elicite the morphogenic callus production from wheat embryos (Kalyaeva et al., 2003). The stimulation of callus somatic embryogenesis, which is derived from geranium hypocotyls, is mediated by the Bacillus circulans strain (Murthy et al., 1999); Curtobacterium citreum promotes the development of axillary shoots in geranium cultures (Panicker et al., 2007). Bacillus spp. promotes root growth, Azotobacter cliroococcum enhances the number of shoots in wheat (Andressen et al., 2009), Sphingomonas spp. facilitates the adaptation to climate change of micropropagated strawberries in greenhouse conditions (Dias et al., 2009), Azospirillum brasilense 243 increases the compatibility of micropropagated fruit rootstocks (Vettori et al., 2010). Scherling et al. (2009) reported that the interaction between Paenibacillus P22 and poplar shoot explants, which can assimilate atmospheric nitrogen, causes significant changes in plant metabolism, consistent with experiments on bacterial interaction with plant growth during in vitro conditions (Russo et al., 2012).

Moreover, methylobacteria generally found in soil and plant surfaces; several beneficial interactions of these group bacteria with plants as endophytes were identified (Madhaiyan et al., 2011). For example,M. extorquens that produces a pink pigment (Clmstoserdova et al., 2003) was determined in meristematic cells of Pinus silvestris. When this strain was inoculated on the plant callus, it affected the growth and regeneration of that plant through diverse mining (Pirtila et al., 2008). It is known that many strains isolated from different places such as soil, lhizosphere water, plant parts (Lorentz et al., 2006), and in vitro plant cultures (Ulrich et al., 2008) secrete different metabolites and enzymes with plant growth regulators (Timmusk et al., 1999). For instance, P. glucanolyticus isolated from the black pepper roots is able to solubilize potassium (Sangeeth et al., 2012). In addition, Lorentz et al. (2006) reported that many Paenibacillus strains, which have antimicrobial activity, belonging to the various species.

Beneficial effects of bacteria can occur under stress conditions. For instance, in harmony with the greenhouse conditions, those bacteria can protect plants from losing water, microbial threats; also they can activate the growth of conductive tissues in plants to supply the absorption of water and nutrients (Chandra et al., 2010). One of the most potential applications for using beneficial bacteria in in-vitro plant propagation is, of course, the improvement of acclimatization (Digat et al., 1987). In potato and strawberry, inoculated microshoots before rooting with Pseudomonas aureofaciens strain have been shown to grow better (Zakharchenko et al., 2011). Moreover, Azospirillum brasilense is a strain on Primus cerasifera promoted rooting and climate adaptation (Russo et al., 2008).

Furthermore, some contaminants can also aid the maintenance of the plant tissues without the addition of any plant growth regulators. Habituated cultures can continue to grow at a specific point of their' growth without adding the exogenous plant growth regulators. Compared to the transcripts of habituated and non-habituated Arabidopsis thaJiana (A. thaliana) cells, it showed various expressions of 800 genes, including up-regulation of genes encoding cytokine receptors (Pisclrke et al., 2006). At this point, up to current knowledge, it can be stated that the auxin and cytokinin balance in plant tissues could be changed by endophytic bacteria with their metabolic activity. The balance is influenced by the activity of endophytes producing auxins, gibberellins, cytokinirrs, and other plant hormones (Arslrad and Frankenberger, 1991).

Also, recently in our study, the presence of plant growth-promoting putative endophytic bacterium associated with the microslroots of Fraser photinia has been identified (Seker et al., 2017). This plant-bacteria relationship has ensured healthy and vigor microslroots together with the promotion of rooting without supplying auxin to the plant medium in in-vitro conditions.

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