Biofertilizers are an alternative to face the sustainability problem that chemical fertilizers represent in agriculture. Among them, plant growth-promoting rhizobacteria (PGPR) is a microbial group with high potential, but lack of reproducible results from their application is a bottleneck for its use in agricultural production. Here we highlight a factor that could partially explain this inconsistency: the total auxin level in the soil–plant system. Auxin production is recognized as a main mechanism for plant growth promotion by PGPR; however, the final effect of auxins depends on a fine balance of its content, and this will be a result of all the sources of auxin compounds in the system. In addition to the auxins produced by inoculated bacteria, the plant itself produces its own hormones as part of complex physiological processes, varying in amount and sensitivity. Also, soil organic matter displays like auxin activity, causing plant responses just like those produced by added auxins. Therefore, the inoculation of an auxin-producing PGPR on plants might cause a wide variety of responses, ranging from effective growth promotion to growth restriction, depending on the total auxin content in root tissue. We think this must be considered for the practical use of bacterial biofertilizers, in order to have better and more consistent results of inoculation. Graphical Abstract

Background: A new prototype of bio-conditioner useful in rehabilitation of degraded soils was performed. In order to obtain this aim two stages were established: production of biomass of Microbacterium sp. CSB3 and formulation of this inoculum in a sediment supplemented with low-rank coal (LRC). Materials and methods: The effect of agitation and pH on microbial growth was determined. As response variables, the final production of biomass (Xf) and yield (Yx/s) were determined. Growth dynamics of CSB3 in a 2-L reactor was also evaluated through Xf, Yx/s and the determination of kinetic parameters (specific growth rate [μ] and duplication time [Dt]). The formulation of CSB3 was evaluated; mixtures of several LRC proportions with a sediment from a municipal aqueduct were made. During 90 days, the viability of CSB3 was monitored by counting CFU. Results: The optimal pH and agitation for Xf and Yx/x were 7.5 and 232 rpm, respectively; the values of Xf, Yx/s, μ and Dt in 2-L reactor were: 1.5 gL −1 , 0.28 g/g, 0.0208 h −1 , 33.3 h, respectively. Regarding the formulation, the most suitable combination to conserve the viability of CSB3 was LRC 25%-sediment 75%; the heavy metals content of LRC allow to infer that the prototype of bio-conditioner does not represent a pollution risk for environment soil. Conclusions: It was possible to optimize the growth of CSB3 under laboratory conditions. The viability of CSB3 could be maintained by a formulation in a sediment supplemented with lignite; this formulation constitutes a new prototype of soil bio-conditioner.

Lysinibacillus is a bacterial genus that has generated recent interest for its biotechnological potential in agriculture. Strains belonging to this group are recognized for their mosquitocidal and bioremediation activity. However, in recent years some reports indicate its importance as plant growth promoting rhizobacteria (PGPR). This research sought to provide evidence of the PGP activity of Lysinibacillus spp. and the role of the indole-3-acetic acid (IAA) production associated with this activity. Twelve Lysinibacillus spp. strains were evaluated under greenhouse conditions, six of which increased the biomass and root architecture of corn plants. In most cases, growth stimulation was evident at 108 CFU/mL inoculum concentration. All strains produced IAA with high variation between them (20–70 µg/mL). The bioinformatic identification of predicted genes associated with IAA production allowed the detection of the indole pyruvic acid pathway to synthesize IAA in all strains; additionally, genes for a tryptamine pathway were detected in two strains. Extracellular filtrates from all strain’s cultures increased the corn coleoptile length in an IAA-similar concentration pattern, which demonstrates the filtrates had an auxin-like effect on plant tissue. Five of the six strains that previously showed PGPR activity in corn also promoted the growth of Arabidopsis thaliana (col 0). These strains induced changes in root architecture of Arabidopsis mutant plants (aux1-7/axr4-2), the partial reversion of mutant phenotype indicated the role of IAA on plant growth. This work provided solid evidence of the association of Lysinibacillus spp. IAA production with their PGP activity, which constitutes a new approach for this genus. These elements contribute to the biotechnological exploration of this bacterial genus for agricultural biotechnology.

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DDT is an organochlorine insecticide that is persistent and exhibits residuality in the environment. This study assessed the effect of the application of lignite [low rank coal (LRC)] and coal solubilizing bacteria (CSB), on the bioavailability of DDT in soil with low organic matter content. In doing this, three trials were designed; in the first trial, soil samples were treated with CSB and LRC for 30 days and, afterwards, they were immersed in a DDT solution at water solubility limit and, lastly, the remaining DDT in the aqueous solution was determined. In the second trial, soil samples previously contaminated with DDT were treated with LRC and CSB. After 30 days of this interaction, the soil samples were immersed in water and the remaining DDT in solution was subsequently determined. The third trial was similar to the latter, but the interaction lasted for six months. In the first experiment, treatments with LRC and LRC + CSB, showed 8.16 and 3.4 % of remaining DDT respectively, thus indicating the retention of the compound in the soil. In the second trial, the treatment with CSB greatly reduced the bioavailable DDT (0.007 ppm), compared to the control (0.014 ppm); this is possible since these bacteria use DDT as a carbon source. In the third trial, the highest reduction in the bioavailability of DDT took place in LRC and LRC + CSB treatments; this trial also detected DDD produced from DDT transformation, which showed the same behavior; the interaction timeframe favors adsorption and copolymerization of pollutants to humified organic matter (HOM) in soil. Use of LRC as a source of HOM represents a promising strategy for the treatment of soils with low organic matter content affected by persistent organic pollutants such as DDT.

Coal solubilizing bacteria (CSB) are microorganisms to able to bio transformed low rank coal, releasing humified organic matter in the process. On the other hand, these bacterial genera have reported previously as plant growth promoting bacteria. The aim of this work was to assess the Plant Growth Promoting Rhizobacteria (PGPR) capacity of five CSB strains: Bacillus pumilus (CSB05), B. mycoides (CSB25), Microbacterium sp. (CSB3), Acinetobacter sp. (CSB13) and B. amyloliquefaciens (CSB02). For this, the PGPR traits of CSB were evaluated under laboratory conditions: the biological nitrogen fixation capacity, the reduction of acetylene, the synthesis of indole acetic acid (IAA) and the solubilization of phosphates. In a second experiment under plant nursery conditions, PGPR activity of strain CSB05 was evaluated in common bean plants. Under laboratory conditions, it was evidenced that all the evaluated strains produced IAA, solubilized phosphate in a liquid medium, presented atmospheric nitrogen fixation capacity, and only the CSB3 and CSB13 strains reduced acetylene. In the plant nursery experiment, PGPR activity of strain CSB05 was detected in common bean plants, reflected in increases in the height of these plants. These results show that CSB are promising in the PGPR activity, which is interesting to the design of biological products with agricultural and environmental applications, for the management of crops in disturbed soils of the Colombian dry Caribbean.