Isolation and Characterization of Plant Growth Promoting Bacteria Isolated from Andean Soil as Potential Inoculants of Soybean Seeds

Argentina is the leading exporter of soybean oil and flour, and the third largest producer of grain. Since, the crop is a matter of great importance to the national economy. Their production depends on the soil as their main resource to ensure a good productive capacity, so it is necessary to preserve the physical, chemical and biological properties of the soil. Although, the indiscriminate use of chemical fertilizers, disturb them. In recent years, there has been a trend towards cleaner production to reduce the use of chemical. One of the alternatives involves biological means through the use of plant growth promoting bacteria. These group of bacteria colonize the rhizosphere of plants and stimulate the plant growth by several mechanisms. The objective of this work was to characterize, identify and evaluate the growth promoting effect of 13 strains isolated from the Andean vegetation rhizosphere. The bacterial isolates were Enterobacteria, Stenotrophomonas, Pseudomonas, Nocardiodes, Bacillus, Exiguobacterium, Acinetobacter and Lactococcus genera. The results of the biochemical characterization determined that from the 13 bacterial strains, which produce siderophores, 11 possess the catalase enzyme, 10 fixate nitrogen, 12 produce the protease enzyme, 12 solubilize phosphorus, and 11 produce indoleacetic acid. The application of different inoculums to the seeds, allowed to obtain plants with longer stem length, more developed roots, larger and more intense coloration leaves than the control plants. The results encourage deeper studies to achieve the formulation of inoculums to use as a biofertilizer, which would replace chemical fertilizers or reduce their doses.


Introduction
Soil is an important natural resource that needs to be preserved and improved its quality and productive capacity (Pascual et al., 2000).It is considered a heterogeneous compound defined by its physical, chemical and biological properties, which in the environment maintains the interaction and dynamic equilibrium between its components.It is known that the performance of an edaphic ecosystem depends on a large extent of the microbial activity of the soil, since they play an important role in nutrient cycling (Paul & Clark, 1996).
In recent years, there has been a trend towards clean production, which aims to reduce the use of chemical inputs for the fertilization and control of phytopathogens.Although, chemical fertilizers represent between 20 -30% of the costs of production of a crop and when are properly used increase productivity and profitability, its indiscriminate use has a severe environmental impact.These Agrochemicals are involved in the alteration of the natural microbiota of the soil, reduce and significantly harm the beneficial interactions between the microorganisms and the plant.
The environmental problems are arising from the use of Agrochemicals and the continuous increase in their price; there is a need to look for new alternatives that reduce the fertilizer levels.However, such reductions could represent an abiotic stress on the plants.Hence, there is a need to develop new alternatives that do not damage the environment.One potential way to decrease the negative environmental impact involves biological methods because contribute to improve or maintain the soil quality and biodiversity (Abiala et al., 2015, Hungria et al., 2010, 2013;Adesemoye et al., 2009;Alves et al., 2004).
The term plant growth promoting rhizobacteria (PGPR) was first defined by Kloepper and Schroth (1978) to describe an heterogeneous group of soil bacteria that colonize the rhizosphere of plants, growing in, on or around plant tissues, which stimulate plant growth by several mechanisms (Ahmad et al., 2006;Pérez-Montaño et al., 2014).
An "extreme" habitat is defined as an area where physical conditions are far from those optimal for human life.From a less anthropocentric point of view, volcanoes and geysers, marine depths, salt flats, deserts, ice and eternal snows, and alkaline lakes are considered extreme ecosystems while the microorganisms that colonize these environments are called "extremophiles" (Rothschild & Mancinelli, 2001); they have adapted their genotypes and phenotypes to survive these unusual conditions, which are not "extreme" for them.For this reason, extreme microorganisms offer diverse potential applications in various fields of biotechnology.Soybeans are also known as "Manchuria bean" or "China pea" that is an annual dicotyledonous herbaceous plant of the legume family (Fabaceae), subfamily Papilionoideae (L.Rapela 2013).These names have their origins in the areas of central and northern China and are characterized by their high protein content and nutritional quality.The soybeans occupy an intermediate position between pulses and oleaginous grains, contains more protein than most legumes and less fat than most oilseeds (Toledo, 2009).They are of great agricultural and industrial interest.
In Argentina, around 32 million hectares are planted with 17 million soybeans.It is the crop that has grown the most in the last 25 years and has a distinctive characteristic with respect to the other grains: it is exported almost entirely.Also, our country is the third world producer of soybeans with around 48 million tons, after the United States with 80 million, Brazil with 60 million and ahead of China with 18 million (FAO 2016).
The soybean crop is highly demanding and also is an extractive of nutrients.Because, it is the crop that produces more nitrogen, phosphorus, sulfurs and potassium extracts with each ton of grain (García, 1999).Therefore, most farmers tend to use fertilizers, improve soil quality, and increase production in their crops.
Nowadays, it is urgent to maintain that high productivity, but it is necessary to alter as little as possible the environment.Hence, the aims in this work were characterizing isolated bacteria from Andean soils and select those with plant growth promotion traits and finally study their effects on soybean.

Sampling and Isolation
The samples were taken in July 2015, from Tocorpuri Peatland, a place located in Second Region, Antofagasta, Chile.596.149E; 7.511.532S.
For isolation, ten grams of soil material were rinsed with 90 mL phosphate buffer saline solution, followed by vortexing.Serial dilutions were prepared and spread plating on differential selective culture medium (AGEL, MLR, TSB and soil extract).The inoculated plates were incubated at 30°C for 7 days.After incubation, single colonies were subcultured onto fresh broth medium.

DNA Extraction From Pure Cultures and 16S Rdna Gene Sequence Analysis
Chromosomal DNA of the 13 isolates was prepared as described by Pospiech and Neumann (1995).The 16S rDNA was amplified using the primer set F27/R1492 (Lane 1991), see Table 1.Polymerase chain reaction (PCR) products were checked in 0.8% (w/v) agarose gels, and DNA sequencing was performed by Macrogen (Korea).

Characterization of Isolated as PGPB
Experiments were performed in triplicate.

Catalase Test
The ability to produce catalase enzyme was tested.Place a small amount of overnight culture onto a clean microscope slide and a drop of hydrogen peroxide (5%) was added.Positive reactions are evident by immediate effervescence (bubble formation).
2.3.2Production of Indole Acetic Acid (IAA) The method used for the quantitative and qualitative estimations in the determination of Indoleacetic Acid (IAA) was the Salkowski colorimetric technique.
The bacteria were grown in modified LB medium with 1 mg/ml of tryptophan (Atlas 1946) and were left under constant stirring at 150 rpm for 72 hours at 30 °C to produce Indoleacetic acid (Sachsen 2009).Cultures were centrifuged at 5000 rpm for 15 min followed by removing and mixing the supernatant with Salkowski's reagent in the ratio 4:1.The mixture was incubated at room temperature for 30 min, and absorbance values were measured at 530 nm, using of distilled water as a blank.This experiment was performed in triplicate.

Phosphate Solubilization
The phosphate solubilization was determined according to Vázquez et al., (2000), each pure culture was spread on Sundara Rao Sinha Medium (SRSM), specific medium that containing insoluble calcium phosphate.Plates were incubated at 30°C for 96hrs.Phosphorus solubilizing strains were evidenced by the change in color of the indicator to yellow.

Siderophore-Producing Strains
The production of siderophores was tested in agar medium with chromium azurol sulfonate (CAS), according to the methodology of Louden BC (2011).Strains were grown in a minimal medium without iron source for 24 hours at 30°C, and 200 rpm.
After 24 hours, 1ml of each culture was taken and centrifuged at 12.000 rpm for 15min.From the cell-free supernatant, 35 μl were taken and inoculated in each of the holes of the plate with the CAS-AGAR medium.
The plates were incubated 24 hours at 30 °C, the formation of orange haloes around the well indicating siderophore activity.

Protease Production
The protease production was determined according to Abo-Aba et al., 2006 with modifications.Plates were inoculated with 1μL of pure bacterial culture in a Petri dish containing 3% agar milk.Plates were incubated at 30 °C for 24 hours.The positive result was evidenced by the formation of transparent haloes around each colony.

Nitrogen Fixing Activity
The pure bacterial cultures were inoculated in plates with Nfb medium (Cadena & Martinez, 2011), with NH 4 Cl as a unique nitrogen source.Plates were incubated 28±2°C for 7 days; a color change from green to blue qualitatively indicates the positive effect of N 2 -fixing activity.

Inoculum Preparation
Thirteen strains isolated from Tocorpuri Vegas were used in this work.After incubation, the cells were centrifuged at 10,000 x g for 10 min and washed three times with distilled water to remove any culture medium residue that may interfere with the growth promoting effect on soybean plants.Soybean seeds (Glycine Max 2000) were sterilized in surface with ethanol 3 times during 15 min and rinsed with distilled water 5 times.Later, the seeds were treated with the bacterial suspensions at the concentration of 10 5 CFU ml -1 for 50 min to 200 rpm under sterilized conditions.The control of seeds was realized soaking them with sterilized and distilled water.

Pot Experiment
The sterilization of the soil was carried out in an autoclave at 121 °C for 60 minutes, the process was repeated three times to ensure sterility (Castro & Roa, 2006).
Germination pots of 6 cm deep and 4.5 cm wide were filled at the top with 4.5 cm of sterile soil.Three seeds (previously treated) were placed per well, as a total of 9 seeds per treatment in each experiment.Later the seeds were covered one cm with earth; and the plants were watered every day with 5 mL of sterile distilled water.
At the end of the experiment period (30 days), the plants were uprooted and different growth parameters such as dry weight of root, stem and leaf were measured.The biomass was dried to constant weight in an oven-dried for 4 days at 65 °C.All data were recorded on 10 plants per replicate and the experiment was repeated 3 times.Data were subjected to variance (ANOVA).Significance of P<0.05 was tested by Duncan's multiple range test using the InfoStat statistical software (Version 2016).

Results and Discussion
The plant promoting traits is in Table 2. Of the 13 isolated, 13 produce siderophores, 11 holds the catalase enzyme and 12 produce the protease enzyme, 10 fixate nitrogen, 12 solubilize phosphorus and 11 produce indoleacetic acid.
As shown in Table 2, 10 isolated are able to fix Nitrogen (N) and 12 are able to solubilize phosphate demonstrated by the color change of culture medium used (data not shown).Nitrogen is a major nutrient; it has many functions in the growth and development of crop plants.All organisms require N to synthesize biomolecules such as proteins and nucleic acids.It is also a constituent of compounds as chlorophyll and alkaloids.The nitrogen improves root systems, which has special significance in absorption of water and nutrients (Fageria & Baligar, 2005).The nitrogen deficiency is the most important nutritional disorder limiting crop yields worldwide.Hence, efficient use of N in crop production is crucial for increasing crop yield and quality, environmental safety, and economic considerations (Campbell et al., 1995;Grant et al., 2002).The main reserve of nitrogen in the biosphere is molecular nitrogen from the atmosphere.Nevertheless, plants cannot directly assimilate the molecular nitrogen, but it becomes available through the biological nitrogen fixation process "BNF" (Newton, 2000;Franche et al., 2009).Many species of microorganisms are used in the cultivation of plants, facilitating the host plant growth without the use of nitrogenous fertilizers, becoming economic interest.
In the case of legumes, there are several nitrogen-fixing microorganisms, such as: Rhizobium, Bradyrhizobium, and Actinomyceto (Paredes, 2013).The production of soybean (Glycine max L.) in Brazil is an excellent example of the efficiency of BNF through the use of different strains of Bradyrhizobium sp., such as B. japonicum and B. elkanii (Alves et al., 2004;Torres et al., 2012).Although we did not work with these genera, our results revealed that Exiguobacterium, Enterobacter, Stenotrophomona, Lactoccocus, and Nocardiales were able to fix nitrogen.
The BNF is an important alternative for the recovery of soil fertility, especially now since fertilizer application is an expensive procedure that can also increase contamination (Zahran, 1999).
After Nitrogen, Phosphorus (P) is one of the major essential macronutrients for biological growth and development.It participates as a structural component of nucleic acids, phospholipids and adenosine triphosphate (ATP), as a key element of metabolic and biochemical pathways, particularly important for the BNF and photosynthesis (Khan et al., 2009;Richardson & Simpson, 2011).Agricultural soils contain large reserves of phosphorus, which have been accumulated mostly as a result of regular applications of P fertilizers.However, a substantial portion of soluble inorganic phosphate in fertilizers is immobilized rapidly in the soil and becomes unavailable to plants (de Souza et al., 2015;Rodríguez & Fraga 1999).
Phosphate-solubilizing bacteria (PSB) mobilizes insoluble inorganic phosphates from their mineral matrix to the bulk soil where they can be absorbed by the plant roots (Shashidhar & Podile 2010).Microorganism can transform insoluble phosphates into soluble forms, through the process of acidification, chelation, exchange reactions and the production of gluconic acid (Rodriguez et al., 2004;Chung et al., 2005).Pseudomonas, Rhizobium, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Aerobacter, Flavobacterium, Acinetobacter, Erwinia and Pantoea have been recognized as the genera of PSB (Rodríguez & Fraga 1999;Torres et al., 2008;Peix et al., 2009); being Pseudomonas and Bacillus the genera with the greatest potentiality of use in agriculture (Franco, 2015).Among our isolates, only one strain (Exiguobacterium sp.S58) was not able of solubility.
On the other hand, Iron (Fe) is practically one of the essential micronutrients of all living organisms.It is involved in cellular metabolism, as a cofactor of numerous enzymes (Wandersman & Delepelaire, 2004), and intervenes in various functions of biological processes, such as oxygen transport, DNA synthesis, nitrogen fixation, respiration and photosynthesis (Greenshields et al., 2007); it is also responsible for the green color of plants, fundamentally in the production of chlorophyll (Santacruz et al., 2012).Although it is the fourth most abundant metal in soils, (Crichton & Charloteaux-Wauters, 1987) in the presence of oxygen and neutral pH (physiological conditions), is inaccessible due to the rapid oxidation of Fe 2+ to Fe 3+ and the subsequent formation of insoluble hydroxides (Harrington & Crumbliss, 2009).In conditions of Iron deficiency, the secretion of siderophores by bacteria might stimulate plant growth, thereby improving nutrition through the sequestration of Fe from the environment (de Souza et al., 2015).This positively influences the growth of plants because it allows them to survive in soils with low iron availability, thus improving their conditions.Eleven isolates were positive for siderophore production, showing a yellow zone on CAS-agar medium plate, a Pseudomonas spp. was used as a positive control (data not shown).Although Pseudomonas and Bacillus have been the most studied genera for their ability to produce siderophores, the present study showed that the genera Exiguobacterium, Lactococcus, Enterobacter, Nocardiodes, Acinetobacter and Stenotrophomonas were also able of synthesizing.
Indolic compounds, such as the indole-3-acetic acid (auxin phytohormone) (IAA), present great physiological relevance for bacteria-plant interactions, varying from pathogenesis to phytostimulation (Spaepen et al., 2007).Among plant growth regulators, indole-3-acetic acid (IAA) is the most common natural auxin found in plants, and the 80% of bacteria is able to produce it (Patten & Glick 1996;Khalid et al., 2004).; because of its positive effect on root growth and the morphology, is believed that increases the access to more nutrients in the soil (Vessey, 2003); it is also widely used by farmers to accelerate the growth of plants, to promote the initiation of adventitious roots, as well as the flowering, fruit setting and number of leaves (Franco, 2008).Among the most prominent genera are Azospirillum, Azotobacter, Pseudomonas, Rhyzobium and Bacillus among others (Patten & Glick, 1996).As can be seen in Table 2 Bacillus sp.TSB9 was able to produce the highest concentration of IAA (60.86 μg / ml).There are studies carried out on plants of Vigna radiata that demonstrate its high capacity to synthesize this hormone, favoring the elongation of shoots and number of roots (Vega-Celedón et al., 2016).
On the other hand, Exiguobacterium N31 and N24 were also able to produce high concentrations of AIA 54.66 μg / ml and 42.93 μg / ml, respectively.Kasana and Pandey (2017) report Exiguobacterium strains with growth-promoting properties.
Several studies demonstrated that environmental stresses such as drought, salinity, chilling, metal toxicity and UV-B radiation as well as pathogens attack, lead to enhanced generation of reactive oxygen species (ROS) in plants.Extracting or detoxification the excess ROS is achieved by an efficient antioxidative system as the catalase enzyme (Sharma et al., 2012).Our results indicate that all evaluated strains have positive catalase activity.
Finally, protease production was detected in 12 of 13 evaluated strains (data not shown).The microorganisms capable of secreting proteases penetrate plant cells, and in this way influence the colonization of the roots.Furthermore, they also regulate the ecological balance to exercise control over the pathogen and promote the growth of plants due to a bio-control of diseases, nematodes, detoxifying and degrading virulence factors produced by phytopathogens (Velivelli et al., 2015).
The soil sample used for the experiments was silt loam (sand, 10.

Figure
Figure 2. R

Table 1 .
PCR Primers and amplification programs