Soil salinity is a major limiting factor for agricultural crops, which increases their susceptibility to pathogenic attacks. This is particularly relevant for tomato (Solanum lycopersicum), a salt-sensitive crop. Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici, is a significant threat to tomato production in both greenhouse and field environments. This study evaluated the salinity tolerance, biocontrol, and plant growth-promoting properties of Bacillus velezensis AF12 and Bacillus halotolerans AF23, isolated from soil affected by underground fires and selected for their resistance to saline conditions (up to 1000 mM NaCl). In vitro assays confirmed that both strains produced siderophores, indole-3-acetic acid (IAA), and proteases and exhibited phosphate solubilization under saline stress (100–200 mM NaCl). AF23 exhibited synergistic interactions with AF12, and inoculation with either strain individually or in combination significantly improved the growth of the Bonny Best tomato cultivar under 200 mM saline stress, leading to increased shoot and root weight, enhanced chlorophyll content, and higher total biomass. The biocontrol potential of AF12 and AF23 was evaluated in tomato plants infected with F. oxysporum. Both strains, individually or combined, increased shoot and root weight, chlorophyll content, and total biomass under non-saline conditions, promoting growth and reducing infection rates under saline stress (100 mM NaCl). Genomic analysis revealed that both strains harbored genes related to salt stress tolerance, biocontrol, and plant growth promotion. In conclusion, Bacillus strains AF23 and AF12 demonstrated strong potential as bioinoculants for enhancing tomato growth and providing protection against F. oxysporum in saline-affected soils.

Soil salinity is one of the most important abiotic factors that affects agricultural production worldwide. Because of saline stress, plants face physiological changes that have negative impacts on the various stages of their development, so the employment of plant growth-promoting bacteria (PGPB) is one effective means to reduce such toxic effects. Bacteria of the Bacillus genus are excellent PGPB and have been extensively studied, but what traits makes them so extraordinary to adapt and survive under harsh situations? In this work we review the Bacillus' innate abilities to survive in saline stressful soils, such as the production osmoprotectant compounds, antioxidant enzymes, exopolysaccharides, and the modification of their membrane lipids. Other survival abilities are also discussed, such as sporulation or a reduced growth state under the scope of a functional interaction in the rhizosphere. Thus, the most recent evidence shows that these saline adaptive activities are important in plant-associated bacteria to potentially protect, direct and indirect plant growth-stimulating activities. Additionally, recent advances on the mechanisms used by Bacillus spp. to improve the growth of plants under saline stress are addressed, including genomic and transcriptomic explorations. Finally, characterization and selection of Bacillus strains with efficient survival strategies are key factors in ameliorating saline problems in agricultural production.

Crop plants are subjected to diverse abiotic and biotic stresses, which could affect their health and production. Therefore, searching for novel stress-resistant microbial agents with plantbeneficial activities is considered an urgent in agriculture. Here, we report the new stress tolerant bacterium Streptomyces achromogenes subsp. achromogenes UMAF16, isolated from a soil subjected to underground fires. Taxonomic and Genomic analysis derived from ANI (average nucleotide identity) species delimitation cut-off values, GGDC (genome-to-genome distance calculator), and phylogenetic analysis, corroborated the affiliation of UMAF16 to Streptomyces species. Genome mining of UMAF16 shows an excellent arsenal of stress tolerant genes (e.g. temperature, osmotic, and oxidative stress), as well as antifungal and plant growth-promoting mechanisms. Additionally, antiSMASH revealed multiple biosynthetic genes with antimicrobial (e.g. Achromosin, Livipeptin, Neocarazostatin A) and PGP activities (e.g. siderophores like Desferrioxamin B/Desferrioxamine E). To further confirmed previous predicted activities, it was biochemically evaluated the antifungal and plant growth-promoting mechanisms, including production of indole-3-acetic acid (IAA), proteases, siderophores, biofilm formation, and phosphate solubilization. Our results also show that strain UMAF16 grows well at temperature 50 degrees C, tolerates dehydration stress simulated by polyethylene glycol, and exert antifungal action against pathogens like Alternaria alternata, Mucor fragilis and Fusarium brachygibbosum. Strain UMAF16 also showed growth-stimulating activities when interacting with tomato plants, evaluated under three different plant-growth systems. Root and shoot dry weight, as well as chlorophyll content were in

The geothermal zone of Araró, México, is located within the trans-Mexican volcanic belt, an area with numerous arsenic (As)-rich hot springs. In this study, the draft genome sequence of two endemic Bacillus strains (ZAP17 and ZAP62) from Araró microbial mat hot springs was determined, which were able to grow on arsenate (up to 64 mM) and arsenite (up to 32 mM). Phylogenetic analysis based on 16S rRNA and gyrB sequences, as well as genome sequence analysis based on average nucleotide identity (>96%) and digital DNA–DNA hybridization (>70%), indicated that these strains belong to the Bacillus paralicheniformis ZAP17 and Bacillus altitudinis ZAP62. Furthermore, through genome mining, it was identified two arsenic resistance operons, arsRBC, and arsRBCDA in both strains as potential determinants of arsenic (As) resistance. Predicted ArsA (arsenial pump-driving ATPase), ArsB (Arsenical efflux pump protein), ArsC (Arsenate reductase), ArsD (Arsenical efflux pump protein) and ArsR (Metalloregulator/ ars operon repressor) proteins, clearly grouped with their respective clades corresponding to other characterized bacterial species, mainly Firmicutes. To further evaluate the functionality of the ars operons in ZAP17 and ZAP62 strains, our results showed that arsRBC and arsRBCDA genes were expressed in the presence of arsenite (III). Finally, the presence of ars operons in the genome of Bacillus species residing in As-rich environments, such as the Araró hot springs, might be a potential mechanism to survive under such harsh conditions, as well as to design sustainable bioremediation strategies.

Plant growth-promoting bacteria (PGPB) are excellent biocontrol agents and stimulators of plant growth, nutrition, and production. Therefore, these plant-associated bacteria are considered an excellent alternative to reduce or eliminate the use of toxic agrochemicals. In this work, we review the current state of the beneficial mechanisms (direct and indirect), including the production of antibiotic compounds and enzymes, facilitation of resource acquisition, or production of stimulating phytohormones/metabolites. Some aspects of the formulation technology and bioinoculant efficiency of diverse PGPBs (e.g., rhizobacteria, phyllobacteria and endophytic bacteria) in the field are also discussed. However, the commercialization and application of these biological agents in agriculture occur mainly in developed countries, limiting their success in developing regions. The possible causes of the delay in the application of bioinoculants for sustainable agriculture and the plausible solutions are also discussed in this study. Finally, the use of PGPBs is currently a priority for sustainable production in agriculture.