Date Published: June 3, 2016
Publisher: Springer Berlin Heidelberg
Author(s): Fakhra Liaqat, Rengin Eltem.
Endophytes are microorganisms which live symbiotically with almost all varieties of plant and in turn helping the plant in a number of ways. Instead of satisfactory surface sterilization approaches, repeatedly occurring bacterial growth on in vitro rootstock cultures of peach and pear was identified and isolated as endophytic bacteria in our present study. Five different isolates from peach rootstocks were molecularly identified by 16S rRNA gene sequencing as Brevundimonas diminuta, Leifsonia shinshuensis, Sphingomonas parapaucimobilis Brevundimonas vesicularis, Agrobacterium tumefaciens while two endophytic isolates of pear were identified as Pseudoxanthomonas mexicana, and Stenotrophomonas rhizophilia. Identified endophytes were also screened for their potential of plant growth promotion according to indoleacetic acid (IAA) production, nitrogen fixation, solubilization of phosphate and production of siderophore. All seven endophytic isolates have shown positive results for IAA, nitrogen fixation and phosphate solubilization tests. However, two out of seven isolates showed positive results for siderophore production. On the basis of these growth promoting competences, isolated endophytes can be presumed to have significant influence on the growth of host plants. Future studies required to determine the antimicrobial susceptibility profile and potential application of these isolates in biological control, microbial biofertilizers and degradative enzyme production.
Endophytes are bacteria or fungus which colonize healthy plant tissue, residing within a plant cell or between plant tissues with no apparent symptoms of disease (Nair and Padmavathy 2014). These endophytic microorganisms can spend their life cycle or a part of it while invading living tissue of the host plant deprived of producing any harm, with sometimes causing unapparent and asymptomatic infections (Kumala and Siswanto 2007). Endophytic bacteria frequently reside most of the plant and not a single plant species studied till date have been found free of endophytic bacteria (Nair and Padmavathy 2014). It is worth mentioning that, a huge number of existing plant species are host to more than one types of endophytes. Endophytic biology of all those plants is not fully studied. Therefore, it is a considerable opportunity to discover novel and valuable microorganisms among these plants (Muzzamal et al. 2011). While living symbiotically with the plants, these bacteria are known to stimulate plant growth by different ways including the production of phytohormones, solubilization of inorganic minerals like phosphate, fixation of atmospheric nitrogen and sequestration of iron. Additionally, they sometimes offer protection against pathogenic microorganisms and augmentation of ecological constraints such as drought, salinity and heavy metals (Khalifa et al. 2015). On the other hand, the presence of bacteria growth in in vitro plant tissue culture is generally declared as contaminants, which must be prohibited and eradicated (George et al. 2008; Abreu-Tarazi et al. 2010). Detecting the presence of endophytes in in vitro plant tissue culture and micropropagated plants is not extensively studied spot. Rarely, a few studies reported the manifestation of endophyte in in vitro plant tissue cultures (Almeida et al. 2009; Dias et al. 2009; Abreu-Tarazi et al. 2010; Moraes et al. 2012). The presence of valuable endophytic bacteria in plant tissue cultures and in their micropropagations may be more frequent than that reported (Abreu-Tarazi et al. 2010). In vitro plant tissue culture may offer a useful system and source to recover beneficial microorganisms resides within specific organs (Moraes et al. 2012). For a successful recovery of endophytes from plant tissues, it is foremost important to discriminate and eliminate any surface contaminants of plants, because endophytes purely reside inside the plant. In in vitro explant cultivation, plant materials are generally extensively surface sterilized, but inner flora of plant tissues cannot be avoided by surface sterilization approaches. The present study was, therefore, undertaken to identify and characterize endophytic bacteria appeared in in vitro explant cultures of GF677 (Prunus amygdalus × P. persica) peach rootstocks and OHF333 (Pyrus communis L.) pear rootstocks by culture-dependent technique; further, these isolates were also examined to explore the different characteristics predominantly plant growth promoting properties of endophytic isolates.
Rootstocks of peach and pear plants were observed to comprise endophytic bacteria appeared during in vitro cultivation on MS medium. Surface sterilization method for isolation of endophytic bacteria was found adequate as control plate has not shown any growth. Therefore, bacterial colonies appeared on sample plates can be well thought-out as endophytic bacteria of peach and pear rootstocks. To the best of our knowledge, there is no report found on the isolation of endophytes from peach and pear in vitro rootstock cultures. Though, there are few studies reported for the isolation of endophytic microorganisms from micropropagated plants (Almeida et al. 2009; Dias et al. 2009).
In the end of our study, we do not claim the isolation and detection of every endophytic strain in peach and pear rootstocks. However, we concluded that some of the bacterial strains isolated from peach and pear have the ability to produce the growth regulator IAA, to fix nitrogen and to solubilize phosphate. Two of the strains also have the ability to produce siderophore. Five out of seven isolates belong to same phylum, thus we can relate such similarities to the endophytic ecology of bacterial species, and also to plant metabolism and nutrient accessibility. Further studies are required to reveal the potential of these endophytes as biofertilizers. Moreover, greenhouse and field investigations are recommended for confirmation of this potentiality. Antimicrobial susceptibility profiles of these endophytes could suggest the best options for their control in in vitro plant tissue culture.