Search

Abstract Background Bacterial auxin production is a mechanism that promotes plant growth. However, it can also be linked to deleterious rhizobacteria and plant-pathogenic bacteria. High levels of auxins have the potential to impede plant growth. Additionally, certain environmental factors can enhance auxin activity and impact the activity of plant growth-promoting rhizobacteria (PGPR). Results In this work, we boarded how the auxin-like effect in the soil–plant system modulates the plant growth-promoting activity of Lysinibacillus pinottii sp. nov. PB211. The effect of humified organic matter (HOM) soil was simulated with humic acids (HA) coming from vermicompost; the spectral configuration (13C-CP-TOSS-NMR and FTIR) and coleoptile elongation test indicated its bioactivity. Cucumber exhibits a PGP effect of PB211 and HA at a lower concentration compared to corn. This disparity agrees with the differential sensitivity to auxins of both plant models. Monocot plants, such as corn, generally exhibit greater resistance to exogenous auxin activity compared to eudicot plants, such as cucumber. The presence of HA in the growth substrate (sand) is found to modulate the PGPR activity of PB211, resulting in a loss of PGPR activity. Conclusions The findings highlight the importance of considering the cumulative auxin-like effects in the soil–plant system when utilizing bacterial inoculants for plant growth promotion in agriculture. Graphical Abstract

Abstract 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

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.