Survival strategies of Bacillus spp. in saline soils: Key factors to promote plant growth and health.

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.

Genome mining, phylogenetic, and functional analysis of arsenic (As) resistance operons in Bacillus strains, isolated from As-rich hot spring microbial mats.

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 As(V) (up to 64 mM) and arsenite As(III) (up to 32 mM). Phylogenetic analysis based on 16 S 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 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 As(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.