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ISSN 00036838, Applied Biochemistry and Microbiology, 2015, Vol. 51, No. 3, pp. 271–277. © Pleiades Publishing, Inc., 2015. Original Russian Text © V.K. Chebotar, N.V. Malfanova, A.V. Shcherbakov, G.A. Ahtemova, A.Y. Borisov, B. Lugtenberg, I.A. Tikhonovich, 2015, published in Prikladnaya Biokhimiya i Mikrobiologiya, 2015, Vol. 51, No. 3, pp. 283–289. Endophytic Bacteria in Microbial Preparations that Improve Plant Development (Review) V. K. Chebotara, b, N. V. Malfanovaa, c, A. V. Shcherbakova, b, G. A. Ahtemovaa, A. Y. Borisova, B. Lugtenbergc, and I. A. Tikhonovicha a AllRussia b Research Institute for Agricultural Microbiology, Pushkin, 196608 Russia International Research Center “Biotechnology of Third Millennium,” Saint Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University), St. Petersburg, 191002 Russia c Institute of Biology, Leiden University, Leiden 2333, BE 8 Netherlands email: [email protected] Received August 20, 2014 Abstract—In this review data on the possibility of using endophytic bacteria for improving crop yields and quality are discussed. Keywords: endophytic bacteria, plant associations with microorganisms, inoculation of endophytic microor ganisms DOI: 10.1134/S0003683815030059 INTRODUCTION The association of plants with microorganisms that do not suppress or even stimulate their development attracts the attention of scientists, not only as the object of study with respect to the fundamentals of the coexistence and interaction of different organisms but also because of their possible use in the practice of an environmentally oriented production of agricultural products. Every microorganism associated with a plant organism is an evolutionarily verified component of a complex plant–microbial system beyond the scope of a single plant [1], and it has a significant impact on the biological structure and functioning of the entire sys tem [2]. Microorganisms existing within the plant, including aboveground and underground parts and seeds, that positively affect plant development are defined as endophytic. They use the internal environ ment of the plant (endosphere) as a unique ecological niche, protecting them from changes in the environ ment, and they formed as a result of hundreds of mil lions of years of evolution [3, 4]. These organisms can be transmitted through generations, from ancestor to descendent, as an integral part of the plant organism endosphere. Only microorganisms capable of colonizing the internal tissues of plants without causing disease and adversely affecting plant development are endophytic [5, 6]. In the world there are about 300000 species of plants, and each species may be a host for one or more species of endophytic microorganisms. However, until recently only some microorganisms across several plant species have been sufficiently studied [7, 8]. Cur rently, the attention of scientists is devoted to endo phytic fungi [9] and bacteria [10, 11]. In the last decade, a majority of agriculturally ori ented research has been aimed at studying rhizosphere microorganisms [12–14]. Among these organisms, bacteria are the most technologically advanced, both in terms of production and the use of microbial prep arations in agriculture. [15] This opens up prospects for the use of prokaryotic microorganisms beneficial for plant development under different conditions [10]. Endophytic bacteria colonize the same ecological niche in the plant as pathogens, and therefore they are used as a promising biological method to control plant pathogens, i.e. they are socalled “biocontrol” agents [14, 16, 17]. It was established that bacterial endo phytes are capable of inhibiting the development of phytophagous insects [18] and nematodes [19, 20] via the synthesis of biologically active compounds with “antipathogenic” action. The study of the “biochem ical weapons” of such bacteria will allow the isolation and purification of those chemical compounds that can be used for the production of new preparations to combat plant, animal, and even human diseases [7, 8]. Some strains of endophytic bacteria are used for the phytoremediation of soils, i.e. the purification of tech nologically contaminated areas by the production of special plant–bacterial systems [10, 21–25]. This review discusses the literature data on endo phytic bacteria and their use in the development of new microbial preparations that can be used in agri culture. 271 272 CHEBOTAR et al. Biodiversity of Endophytic Bacteria The endophytes’ niche can be occupied only by those bacteria that are able to colonize different parts of the intercellular spaces of plants, even seeds. The classic biodiversity studies of endophytic bacteria are based on characteristics of isolates obtained from var ious internal tissues of different plants after steriliza tion of their surface [26, 27]. Bacterial endophytes include both Grampositive and Gramnegative spe cies isolated from a wide range of host plants [28, 29]. Endophytic bacteria were isolated from both mono cots and dicots; woody plants, such as oak (Quercus L.) and pear (Pyrus L.); and herbaceous plants, such as sugar beet (Beta vulgaris L.) and maize (Zea mays L.) [30]. Thus, analysis of the tissues of the aboveground parts of spring crocus (Crocus vernus (L.) Hill) demon strated the presence of a previously known bacterial endophyte community, as well as those that had not yet been identified [31]. A study of microbial endophyte communities that inhabit the stems, roots, and tubers of agricultural crops with an analysis of 16S RNA sequences and fatty acids compositions, as well as the utilization of various carbon sources, revealed that they are represented by the genera Cellulomonas, Clavibacter, Curtobacterium, Pseudomonas, and Microbacterium [32]. The high density of cultured endophytic bacteria was detected in seedlings of poplar (Populus L.), spruce (Picea A. Dietr.), and larch (Larix Mill.) grown from tissue cultures [33]. Based on the analysis of 16S RNA sequences, most of these isolates were assigned to the genus Paenibacillus, to a species close to P. humicus, which were accumulated in tissues in vitro without any apparent adverse effect on plants. Endophytic bacteria of the genera Methylobacterium, Stenotrophomonas, and Bacillus were also found. Sequence analysis of the 16S RNA gene from the endophytic bacteria of rice (Oryza sativa L.) showed that they belong to different Proteobacteria subclasses (including the genera Cytophaga and Flexibacter), as well as the groups DeinococcusThermus, Acidobacte ria, and Archea. The most numerous were the Betapro teobacteria group (27% of all isolates), in which the genus Stenotrophomonas dominated. About 15% of all isolates belonged to uncultivated bacterial species [34]. A study of endophytic bacteria in the stem of maize grown in Indian tropics [35] showed that the bacteria are present for the whole growing period and the bac terial count is 1.36–6.12 × 105 CFU/g raw biomass (CFU is the colonyforming unit, e. g. the number of cells capable of growth on artificial media). The fact that Bacillus pumilus, B. subtilis, Pseudomonas aerugi nosa, and P. fluorescens dominated in bacterial isolates was established based on a chromatographic analysis of fatty acids [35]. As the result of the study of maize (Zea mays L.), sorghum (Sorghum vulgare Moench), soybean (Gly cine max (L.) Merr.), wheat (Triticum vulgare L.), and a number of wild plants (grasses and leguminous grasses), 853 bacterial endophytes strains were iso lated; about half of these strains were Grampositive, and the rest were Gramnegative bacteria. An analysis of the fatty acid composition made it possible to assign the endophyte isolates to 15 genera: Agrobacterium, Bacillus, Bradyrhizobium, Cellulomonas, Clavibacter, Corynebacterium, Enterobacter, Erwinia, Escherichia, Klebsiella, Microbacterium, Micrococcus, Pseudomo nas, Rothia and Xanthomonas. Bacillus, Corynebacte rium, and Microbacterium were dominant [36]. Endo phytic bacteria of the genera Cellulomonas, Clavi bacter, Curtobacterium, and Microbacterium, which were isolated from both cultivated and wild plants, colonized mainly maize and sorghum [36]. Thus, the data presented above make it possible to evaluate the extent of the diversity of bacterial endo phytic microbial flora existing in complex mutually beneficial plant–microbe systems and the number of isolates capable of growth on artificial media. Artificial Inoculation of Endophytic Bacteria and Colonization of Plant Endosphere The study of artificial inoculation of plants with bacterial endophytes previously isolated as a culture is the basis for the selection of the most promising strains for technological applications. Special meth ods were developed to analyze the ability of endo phytic bacteria to colonize the endosphere after the inoculation of leaf surfaces and the fruit cocoa tree (Theobroma cacao L.) [37]. It turned out that, after the inoculation of maize with the dominant bacterial endophyte strains (B. pumilus, B. subtilis, P. aeruginosa and P. fluorescens), the total bacterial count in seedlings grown in the greenhouse was lower (1.6–3.1 × 105 CFU/g raw stem biomass) than in seedlings grown under field conditions (1.8– 3.6 × 105 CFU/g). The highest density of endophytic bacteria (3.1 × 105 CF /g) was observed 28 days after inoculation with B. subtilis [35]. This finding indicates the regulation of interactions with certain endophytic bacteria by plants and the existence of specific pairs of bacteria host plants. Methods of in vivo analysis of biological material are required for research on the structural changes of plant endospheres and the spatial distribution of microorganisms in the course of development and operation of plant–microbe systems. One such marker system includes labeling with green fluorescent protein (GFP), which allows the detection and counting of microbial organisms in situ on the surface and inside plants in various laboratory, field, and environmental studies [38–41]. GFPtagged bacterial cells can be eas ily identified by epifluorescent microscopy with a con focal laser scanning microscope [23, 42]. This system proved to be very informative for monitoring the colo nization of internal plant tissues with pseudomonads [39, 40]. APPLIED BIOCHEMISTRY AND MICROBIOLOGY Vol. 51 No. 3 2015 ENDOPHYTIC BACTERIA IN MICROBIAL PREPARATIONS Endophytic bacteria cells colonizing internal plant tissues can also be visualized with the βglucuronidase (GUS) reporter system. The GUStagged strain Herbaspirillum seropedicae Z61 was used for artificial inoculation of rice seedlings. The most intensive GUSstaining was observed in coleoptiles, lateral roots, and lateral root connections to the main root. This H. seropedicae strain colonized intercellular spaces, aerenchyma, and cortical cells, and some microbial cells penetrated to the stele and further into the vascular tissue [43]. The most successful colonization of the plant by endophytic bacteria requires a corresponding (“the most complementary”) host plant. Thus, it was shown that the obligate nitrogenfixing endophyte Azoarcus sp. BH72 strain induces the defense mechanisms of the host plant and complicates the colonization of rice by other endophytic bacteria [44]. In other words, the system endophyteplant acquires new adaptive fea tures not typical for organisms outside the system. Influence of Bacterial Endophytes on Plant Growth and Development An investigation of the influence of endophytic bacteria on plant development [10] showed that endo phytes are different from biocontrol strains of rhizo sphere bacteria, since they not only inhibit the growth of pathogenic microorganisms but also stimulate the growth of plants. Endophytic bacteria are able to improve the nitrogen and phosphorus nutrition of plants [45–48], produce auxins [49], and synthesize vitamins [50] and siderophores [51]. Furthermore, it was shown that endophytic bacteria can regulate osmotic pressure and operation of the stomata or modify plant root development [33, 47], improving the general condition of the plants. Therefore, it is advisable to develop economically viable methods of exploiting this ability of endophytes in various fields of human activity associated with the cultivation of plants, such as crop production, forestry, landscaping, and others. The Ability of Endophytic Bacteria to Control the Development of Plant Diseases (“Biocontrol Activity”) The “biocontrol activity” of microorganisms is defined as their ability to reduce populations of target species of antagonistorganisms via a variety of eco logical mechanisms, including pathogenesis, compe tition within the ecological niche, and the production of various compounds that slow their growth and development [52]. Endophytic bacteria are capable of reducing or preventing adverse effects of phytopathogens on plants [33, 53]; therefore, the artificial inoculation of plants with endophytic bacteria can significantly reduce the effect of pathogenic fungi, bacteria, viruses, insects, APPLIED BIOCHEMISTRY AND MICROBIOLOGY 273 and nematodes [29, 54–56]. It was suggested that cer tain types of endophytic bacteria activate plant defense mechanisms, which ensure their overall resistance to microorganisms and parasites [10, 57]. It was shown that 9 out of 137 isolates of bacterial endophytes isolated from the tissues of stems, roots, and nodules of soybean in vitro inhibited the growth of pathogenic fungi Rhizoctonia bataticola, Macrophomina phaseolina, Fusarium udum and Sclerotium rolfsii [58]. Other studies have shown that 36 out of 60 bacterial endophytes isolated from nodules of the wood legume Sophora alopecuroides L. inhibited the growth of the fungal pathogen Verticillium dahlia on artificial media [59]. It was also demonstrated that both living cultures and the thermostable components of the culture fluids of 4 endophyte isolates of B. subtilis and one isolate of Burkholderia sp. inhibited the development of F. circi natum, inducing cancer of the pine (Pinus taeda L.) [60]. During the development of the biological control methods of the nematode Meloidogyne incognita, it was found that 4 out of 19 bacterial strains of endophytes (genera Bacillus, Pseudomonas and Methylobacterium) isolated from various agricultural crops had a negative effect on its development. Even filtrates of their culture supernatant reduced the quantity of “adult females” and egg weight [61]. However, the search for endophytic bacteria of coffee plants (Coffea arabica L. and Coffea robusta L.) that suppress the development of the leaf pathogen fungus Hemileia vastatrix Berk. Br. revealed that iso lates can suppress or enhance the development of the disease [62]. Thus, when using hightechnological endophytic bacteria from different plant species for biological control, we should remember that pathogens are evo lutionarily verified components of complex plant microbe systems, and not all endophytes are their antagonists. In recent years, a number of papers were published that describe the isolation of bacterial endophytes and the characterization of their biocontrol activity [10, 11]. Some of them contain analyses of the biocontrol activ ity of various combinations of endophytic fungi and bacteria [63]. However, the possibility of using micro bial preparations based on such organisms in agricul ture, forestry, and landscaping and the technology of their production currently remain understudied. Endophytic Bacteria—Producers of “Secondary Metabolites” Many endophytes represent soil bacteria of the genera Pseudomonas, Burkholderia, and Bacillus [27], which are well known as producers of “secondary metabolites” such as antibiotics, anticancer drugs, volatile organic compounds, and fungicidal, insecti cidal, and immunosuppressive agents. An investigation of the “secondary metabolites” of the endophytic bacteria of medicinal plants [64], based Vol. 51 No. 3 2015 274 CHEBOTAR et al. on the hypothesis that endophytes determine their healing properties, may open up new possibilities for “biochemical” pharmacology. The chemical nature of the biologically active sub stances produced by bacterial endophytes is very diverse, and a discussion of the whole spectrum within this review, which is dedicated to the use of endophytic bacteria in drugs improving plant growth, is not possi ble. More details of this aspect are discussed in other reviews [10, 11, 65]. It should be noted that the term “secondary metab olites,” e.g., “organic compounds not involved directly in the normal growth, development, and reproduction of organisms,” [66] should be used, since there is noth ing that is “nonfunctional” or secondary in nature. The possibility of biosynthesis of the substance in the body indicates that this substance is necessary for the body under conditions that are “normal” and evolutionarily determined. Otherwise, the biochemical synthesis of such substances would not be supported by this organ ism in the course of the evolution of the biosphere [65]. Microorganismproducers of “secondary metabo lites” in cell cultures are used in biotechnological pro duction [67]. However, until now endophytic bacteria forming biologically active substances were not suffi ciently used as producers of chemical compounds [10]. This review is devoted to a discussion of microbial agents based on endophytic bacteria, i.e. biological preparations containing live forms of microorganisms that are effective preparation agents [33, 45–51,52]. Taking into account the multifaceted relationship of a bacterial endophyte with plants, other endophytes, and the environment, including the plant endosphere, one can imagine how difficult it is for the metabolism to be adjusted. Even the use of tap water instead of dis tilled water for the preparation of the nutrient medium for cultivation of microorganisms causes the appear ance of new “secondary metabolites” [68]. Prospects for the Use of Endophytic Bacteria in Microbial Preparations that Improve Plant Growth At the beginning of the development of a new prep aration based on microbial endophytic bacteria, which could be used to stimulate plant development and pro tect plants from diseases, it is necessary to define both the targeted agricultural plants and pathogens con trolled by this preparation. The biodiversity of bacterial endophytes and host plants makes it possible to obtain endophytic bacteria with desired properties, which may be determined based on a biocontrol activity test in vitro [10, 11, 14, 27, 47, 48]. Such a task—“to find the organism with the desired properties in nature, multi ply, and return it back”—is an alternative to the inten sification of plant production that is based on a wide variety of agricultural chemicals and the genetic modi fication of organisms. The development of such a biological preparation includes the selection of the desired microorganism and the confirmation of its positive impact on the tar geted and/or model plants after artificial inoculation. Afterwards, certification/registration of the prepara tion is necessary: (1) to determine the chemical struc ture of the produced biologically active substances determining the desired properties of microorganism, (2) to perform taxonomic identification through DNA technology [69], and (3) to discover the methods of colonization of the endosphere. These three tasks are performed in parallel with the aim of creating eco nomically feasible technologies for the production and application of microbial preparations. Thus, the fol lowing characteristics are extremely important for production: simple composition of the nutrient media, the possibility of obtaining liquid bacterial cul tures with high density (109 CFU/mL), and informa tion about the cultivation parameters for the produc tion of maximally active bacterial cells with the desired properties. For the use of microbial preparations, the following characteristics should also be considered: shelf life (at least six months) without loss of properties (bacteria with a resting stage is preferable), prepara tion form (liquid/dry) and ease of integration into existing technologies for plants cultivation, the cost of the preparation market form (delivery costs), and the presence of aftereffects [70]. Isolated strains of endophytic bacteria can be used directly to inoculate seeds and seedlings. This will allow a reduction of the negative impact of stress biotic and abiotic factors on the plant as a result of the active colonization of internal tissues of plants and the subse quent positive biochemical and physiological impact on them. The endosphere provides endophytes with a significant advantage over organisms in the plant rhizo sphere and phyllosphere, including stable pH levels and moisture, as well as the flow of nutrients and the lack of competition from a large number of microor ganisms [71]. The endophytes colonizing a plant’s endosphere niche are evolutionarily verified microsym bionts selected by the plant. Assimilates and other organic compounds used by plants for the production of endophytic bacteria biomass are adequately com pensated by stimulating the development and physio logical state of the host plant [72]. Artificial inoculation of a plant with endophytic bacteria should not require large amounts of inoculum because of the specificity of plant–microbe systems and the competitiveness of endophytic bacteria. This property is crucial to a certain extent for economically justified biotechnological production, and it makes it possible to replace traditional chemical pesticides. The presence of the vertical (from ancestors to decedents) transmission of endophytic bacteria through the seed endosphere and the high potential of the biodiversity of seed endophytes [73] allow the clas sification of biotechnology based on endophytic bac teria, which is ideal. Indeed, after the isolation of bac APPLIED BIOCHEMISTRY AND MICROBIOLOGY Vol. 51 No. 3 2015 ENDOPHYTIC BACTERIA IN MICROBIAL PREPARATIONS terial endophyte of seeds with desirable properties from one variety/genetic line/plant species and the inoculation of target varieties/species of agricultural plants, these bacteria will be transferred from genera tion to generation through the endosphere and will ensure the presence of endophytes in the seeds. The application of combinations of endophytes with commercial pesticides used for the treatment of seeds or seedlings can result in a synergistic effect against one or several pathogens. While chemical pes ticides have a shortterm inhibitory effect on phyto pathogenic microorganisms, biological agents can have an adverse effects on phytopathogens throughout the growing season [71]. The fact that the application of endophytic bacteria preparation in agricultural agrocenoses can insignifi cantly adjust plant–microbe systems formed in the course of evolution should be considered. The total elimination of plant diseases and increasing crop yields by orders of magnitude should not be expected. Since the plant plays an active role in interactions with endophytes, not only the selection of bacteria from plant endospheres but also the selection/breeding of highly complementary (susceptible) species/varieties of agricultural plants should be performed for maximal effect of microbial preparations based on endophytes. Even 20 years ago, the data on the endophytic microflora of plants were fragmentary, while today there are university courses based on the isolation and characterization of primary endophytes of plants [74]. This review focuses on bacterial endophytes as the most technologically advanced microorganisms of the endosphere, although the variety of plant endophytes impress with their enormous potential [9, 11]. Unfor tunately, among the bacteria associated with plants, there are groups of bacteria associated with human diseases, the socalled “risk groups” [14, 75]. Endophytic microorganisms with the desired prop erties that are isolated from plants and capable of growth on artificial media can be currently used to develop new microbial preparations. The presence of uncultured endophytic bacteria [34, 76] provides a task for researchers to identify opportunities for the use of these microorganisms in crop production in the future. The goal of future research will be to study the control of plant microbial communities of endospheres by optimizing the conditions for the functioning of plant– microbe systems in phytocenoses. Such studies will undoubtedly be of great importance for the further development of microbiology, biology, plant–microbe systems, and ecology, and will provide a strong eco nomic effect from the application of endophytic bacte ria–based microbial preparations for the improvement of the plant development for different purposes. APPLIED BIOCHEMISTRY AND MICROBIOLOGY 275 ACKNOWLEGMENTS This work was supported by the Russian Science Foundation (project no. 141600146) and state sup port for the leading universities of the Russian Feder ation (grant 074U01). REFERENCES 1. Clay, K. and Holah, J., Science, 1999, vol. 285, no. 5434, pp. 1742–1745. 2. Omacini, M., Chaneton, E.J., Ghersa, C.M., and Muller, C.B., Nature, 2001, vol. 409, pp. 78–81. 3. 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