Salt-tolerant Bacillus species as a promising strategy to mitigate the salinity stress in wheat (Triticum turgidum subsp. durum)

Salinity negatively impacts the bacterial diversity associated with crops in commercial fields; however, Bacillus species -due to their great metabolic/genomic background and spore formation-have shown high resilience to this abiotic stress enhancing the tolerance of plants to saline soils. This work aimed to assess the diversity of cultivable salt-tolerant Bacillus species associated with wheat commercial fields in the Yaqui Valley, Mexico, as well as to identify native salt-tolerant Bacillus strains as promising alternatives to mitigate the negative effects of saline soils on wheat. Thus, based on morphological and molecular (sequencing the 16S rRNA gene) traits related to this genus, fifty-one Bacillus strains were isolated from thirty-six 1 ha-wheat commercial fields in the Yaqui Valley, Mexico [soils showed a gradient of electrical conductivity (E.C.) from 0.6 to 6.4 dS m−1]. These strains were grouped into ten Bacillus species: 1) B. subtilis (25.5%), 2) B. megaterium (25.5%), 3) B. cereus (19.6%), 4) B. amyloliquefaciens (7.8%), 5) B. pumilus (5.9), 6) B. licheniformis (5.9%), 7) B. endophyticus (3.92%), 8) B. atrophaeus (1.96%), 9) B. safensis (1.96%), and 10) B. mojavensis (1.96%). The soil salinity in wheat fields (E.C. 0.6–6.4 dS m−1) and the in vitro salt-tolerance assay (E.C. 6.4 dS m−1) negatively impacted the abundance and diversity of Bacillus species; however, B. licheniformis TRQ65 (isolated from soils having 4.1–5.0 dS m−1) showed the highest tolerance to in vitro saline stress (6.4 dS m−1), increasing its growth 223% under saline vs. non-saline conditions. Finally, the inoculation of B. licheniformis TRQ65 was able to mitigate the negative effects of saline soil conditions on wheat (30 days post-inoculation), since the growth promotion of inoculated seedlings under a saline soil condition (6.4 dS m−1) was similar (no significant differences, p > 0.05) to inoculated wheat seedlings under a non-saline soil condition, which was higher (significant differences, p < 0.05) than un-inoculated wheat seedlings under a non-saline and saline soil. Bacillus licheniformis TRQ65 (recently taxonomically affiliated as B. paralicheniformis TRQ65) is a promising native salt-tolerant strain that needs to be studied in the future for improving wheat growth and yield, under saline or non-saline soils.