Original articleCharacterization of plant growth-promoting traits of bacteria isolated from the rhizosphere of grapevine grown in alkaline and acidic soils
Highlights
► We isolated and identified rhizobacteria from acidic and alkaline soil wild grape. ► We evaluated their N2-fixing, P-solubilising, siderophores-producing and other traits. ► rhizosphere wild grape surveyed first time as P-solubilizers, N2-fixers and others. ► 95 isolates fix N, 12 fixe N and solubilize P, 12 fixe N and produce siderophores. ► Some of isolates may be represent new nitrogen-fixing and P-solubilizing species.
Introduction
Microorganisms colonizing the rhizosphere can affect plant growth both positively and negatively, the term plant growth promoting rhizobacteria (PGPR) often describes beneficial rhizobacteria that stimulate plant growth. PGPR in turn having a great impact on root biology, influence plant growth, nutrition and development are important for long-term sustainability. Plant-associated N2-fixing and P-solubilizing bacteria are regarded as a possible alternative for inorganic nitrogen fertilizers, and PGPR strains have previously been attracted the attention of agriculturists as soil inoculums to improve the plant growth and yield [1], [10], [13], [23], [35]. Apart from fixing N2, PGPR can influence plant growth directly by the synthesis of phytohormones, antibiotics, vitamins, enzymes and/or fungicidal compounds, inhibition of plant ethylene synthesis, improvement of nutrient uptake, enhanced stress resistance, production of siderophores, solubilization of inorganic phosphate, and mineralization of organic phosphate [19], [27].
The thin layer of soil surrounding crop roots and the volume of soil occupied by roots is known as the rhizosphere. The rhizosphere is well known to host a variety of PGPR and the majority of plant-associated bacteria derive from the soil environment [17]. One strategy that may contribute to the establishment of pre-selected beneficial organisms in the root zone of soils is through early establishment of selected communities of bacteria in the rhizosphere. In the last few years, the number of PGPR that have been identified has seen a great increase, mainly because the role of the rhizosphere as an ecosystem has gained importance in the functioning of the biosphere and also because mechanisms of action of PGPR have been deeply studied. Rhizosphere soil is influenced by plant roots which select for specifically adapted microbial communities [2], [4]. Several studies have demonstrated that the bacterial diversity in rhizospheres can be influenced by a number of different factors, i.e., the plant species, varietal differences within a species, plant age, plant genotype, agricultural management, or soil properties [7], [12], [18], [30], [39]. An important factor to be considered when screening new isolates is their activity in the range of environments in which they would be expected to be used; in particular different plants and soil types. Previous isolations of N2-fixing bacteria have revealed a broad diversity of diazotrophs to inhabit the crop rhizosphere [7], [11]. Nevertheless, many rhizospheric, N2-fixing and P-solubilizing bacterial species remain unknown and more studies are needed to reveal the high biodiversity of these bacteria.
Isolating of native strains adapted to the environment and their study may contribute to the formulation of inoculants to be used in region crops. Native isolates may be preferred in the selection of bacteria for inoculation, as they are adapted in the environment and can be more competitive than the foreign bacteria. The advantage of using natural soil isolates is the easier adaptation and succession when inoculated into the plant rhizosphere [13]. On the other hand, characterization and identification of these bacteria are necessary for wide ecological studies of the plant rhizosphere. Some endophytic bacteria colonize upper grapevine organs, especially berries, flowers, fruits and seeds on grapevine [16] and inoculation of Vitis vinifera explants with PGPR strain increased grapevine growth and physiological activity at low temperature [1]. Though much information is available on the activity of soil microorganisms and nitrogen fixation and phosphate solubilization for annual crops, information about the characteristics of composition and diversity of the bacterial community in perennial crops soil ecosystems is scarce. The diversity of PGPR species in grapevine soils remains unknown, especially of those strains that can fix nitrogen and solubilise phosphate besides having several plant growth promoting and biocontrol properties. Also, there is not sufficient knowledge on rhizosphere microbiology of the wild or cultured grape regions. For this reason it is important to study native strains isolated in the wild or cultured grape-growing regions where they may be used as berries and/or grapevine inoculants. The study was focused on rhizosphere bacteria as these represent an important group of soil microorganisms interacting with plant nutrition and health. The purpose of the present work was to isolate and identify PGPR of rhizosphere soils of native wild grapevines at three different region locations in Turkey and evaluation of their N2-fixing, P-solubilising, siderophores-producing status and other plant growth traits.
Section snippets
Soil samples and isolation of bacteria
We conducted a survey of PGPR, naturally colonizing a mild and continental climate and mostly acidic and alkaline rhizospheric soil of native grape; in the mild climate with high precipitation and acidic environmental area of the Storm (Fırtına) Valley in the Black Sea Region, a hard, grey and limestone alkaline soil of Akdamar Island in the Eastern Anatolia and Mediterranean climate of the Kemalpaşa in the Aegean Region have never been studied before. These regions have very widespread and
Results
The number of cultivable N2-fixing bacteria expressed as colony-forming units (CFU) ranged between 4.0 ± 0.4 × 105 and 4.2 ± 0.9 × 107 CFU g−1 dry soils in the sampled the rhizosphere of wild grape-growing zones of various agro climatic regions in Turkey. Morphological, biochemical and physiological tests showed that 95 rhizobacterial isolates obtained from the wild grape rhizosphere have several common characteristics (Table 2). For example, 44.2 and 27.4% of them were oxidase and sucrose
Discussion
The taxonomic identities of 27 genera and 44 species from approximately 95 rhizospheric root-associated bacteria isolated from rhizospheric soil samples of grape, grown at 3 sites were determined. Identification of the bacterial isolates was more successful in the grapevine rhizosphere samples expressing an overall identification of about 74.2% of the total isolates. Characterization of the isolates on the basis of their FAME profiles revealed the presence of both Gram-positive and
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