Evaluation of the oxidative stress alleviation in Lupinus albus var. orden Dorado by the inoculation of four plant growth-promoting bacteria and their mixtures in mercury-polluted soils

Synthesis and Degradation of the Phytohormone Indole-3-Acetic Acid by the Versatile Bacterium Paraburkholderia xenovorans LB400 and Its Growth Promotion of Nicotiana tabacum Plant

Plant growth-promoting bacteria (PGPB) play a role in stimulating plant growth through mechanisms such as the synthesis of the phytohormone indole-3-acetic acid (IAA). The aims of this study were the characterization of IAA synthesis and degradation by the model aromatic-degrading bacterium Paraburkholderia xenovorans LB400, and its growth promotion of the Nicotiana tabacum plant. Strain LB400 was able to synthesize IAA (measured by HPLC) during growth in the presence of tryptophan and at least one additional carbon source; synthesis of anthranilic acid was also observed. RT-PCR analysis indicates that under these conditions, strain LB400 expressed the ipdC gene, which encodes indole-3-pyruvate decarboxylase, suggesting that IAA biosynthesis proceeds through the indole-3-pyruvate pathway. In addition, strain LB400 degraded IAA and grew on IAA as a sole carbon and energy source. Strain LB400 expressed the iacC and catA genes, which encode the α subunit of the aromatic-ring-hydroxylating dioxygenase in the IAA catabolic pathway and the catechol 1,2-dioxygenase, respectively, which may suggest a peripheral IAA pathway leading to the central catechol pathway. Notably, P. xenovorans LB400 promoted the growth of tobacco seedlings, increasing the number and the length of the roots. In conclusion, this study indicates that the versatile bacterium P. xenovorans LB400 is a PGPB.

Induction of phytoextraction, phytoprotection and growth promotion activities in Lupinus albus under mercury abiotic stress conditions by Peribacillus frigoritolerans subsp., mercuritolerans subsp. nov

Strain SAICEUPBMT was isolated from soils of Almadén (Ciudad Real, Spain), subjected to a high mercury concentration. SAICEUPBMT significantly increased aerial plant weight, aerial plant length and the development of secondary roots under mercury stress; increased twice the absorption of mercury by the plant, while favoring its development in terms of biomass. Similarly, plants inoculated with SAICEUPBMT and grown in soils contaminated with mercury, express a lower activity of antioxidant enzymes; catalase enzymes (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) for defense against ROS (reactive oxygen species). Whole genome analysis showed that ANI (95. 96 %), dDDH (72.9 %), AAI (93.3 %) and TETRA (0.99) values were on the thresholds established for differentiation a subspecies. The fatty acids analysis related the strain with the Peribacillus frigoritolerans species. And the synapomorphic analysis reveals a common ancestor with analysis related the strain with the Peribacillus frigoritolerans species. Results from genomic analysis together with differences in phenotypic features and chemotaxonomic analysis support the proposal of strain SAICEUPBMT as the type strain of a novel subspecies for which the name Peribacillus frigoritolerans subps. mercuritolerans sp. nov is proposed. The absence of virulence genes and transmissible resistance mechanisms reveals its safety for agronomic uses, under mercury stress conditions. The ability of Peribacillus frigoritolerans subsp. mercuritolerans subsp. nov to improve plant development was tested in a Lupinus albus model, demonstrating a great potential for plant phytoprotection against mercury stress.

A new Eosin Y-based 'turnon' fluorescent sensor for ratiometric sensing of toxic mercury ion (Hg2+) offering unaided eye detection and its antibacterial activity

A unique fluorescent molecule (ND-S) was obtained from Eosin Y in two simple yet high yielding steps (1). ND-S has special metal ion sensing ability, such that it can selectively detect toxic Hg2+ present in very low concentration in aqueous solutions in the presence of other competing metal ions. The host-guest complexation is ratiometric and is associated with significant increase in fluorescence during the process. Isothermal titration calorimetry (ITC) experiments provided thermodynamic parameters related to interaction between ND-S and Hg2+. Using inductively coupled plasma mass spectrometry (ICP-MS), the Hg2+(aq) removal efficiency of ND-S was estimated to be 99.88%. Appreciable limit of detection (LOD = 7.4 nM) was observed. Other competing ions did not interfere with the sensing of Hg2+ by ND-S. The effects of external stimuli (temperature and pH) were studied. Besides, the complex (ND-M), formed by 1:1 coordination of ND-S and Hg2+ was found to be effective against the survival of Gram-positive bacteria (S. aureus and B. subtilis) with a high selectivity index. Moreover, bacterial cell death mechanism was studied systematically. Overall, we have shown the transformation of a toxic species (Hg2+), extracted from polluted water by a biocompatible sensor (ND-S), into an effective and potent antibacterial agent (ND-M).

Reduced Antibiotic Resistance in the Rhizosphere of Lupinus albus in Mercury-Contaminated Soil Mediated by the Addition of PGPB

The emergence of antibiotic resistance (AR) poses a threat to the "One Health" approach. Likewise, mercury (Hg) pollution is a serious environmental and public health problem. Its ability to biomagnify through trophic levels induces numerous pathologies in humans. As well, it is known that Hg-resistance genes and AR genes are co-selected. The use of plant-growth-promoting bacteria (PGPB) can improve plant adaptation, decontamination of toxic compounds and control of AR dispersal. The cenoantibiogram, a technique that allows estimating the minimum inhibitory concentration (MIC) of a microbial community, has been postulated as a tool to effectively evaluate the evolution of a soil. The present study uses the metagenomics of 16S rRNA gene amplicons to understand the distribution of the microbial soil community prior to bacterial inoculation, and the cenoantibiogram technique to evaluate the ability of four PGPB and their consortia to minimize antibiotic resistance in the rhizosphere of Lupinus albus var. Orden Dorado grown in Hg-contaminated soils. Results showed that the addition of A1 strain (Brevibacterium frigoritolerans) and its consortia with A2, B1 and B2 strains reduced the edaphic community´s MIC against cephalosporins, ertapenem and tigecycline. The metagenomic study revealed that the high MIC of non-inoculated soils could be explained by the bacteria which belong to the detected taxa,. showing a high prevalence of Proteobacteria, Cyanobacteria and Actinobacteria.