Bacillus subtilis promotes plant phosphorus (P) acquisition through P solubilization and stimulation of root and root hair growth.

Bacteria can be applied as biofertilizers to improve crop growth in phosphorus (P)-limited conditions. However, their mode of action in a soil environment is still elusive. We used the strain ALC_02 as a case study to elucidate how Bacillus subtilis affects dwarf tomato cultivated in soil-filled rhizoboxes over time. ALC_02 improved plant P acquisition by increasing the size and P content of P-limited plants. We assessed three possible mechanisms, namely root growth stimulation, root hair elongation, and solubilization of soil P. ALC_02 produced auxin, and inoculation with ALC_02 promoted root growth. ALC_02 promoted root hair elongation as the earliest observed response and colonized root hairs specifically. Root and root hair growth stimulation was associated with a subsequent increase in plant P content, indicating that a better soil exploration by the root system improved plant P acquisition. Furthermore, ALC_02 affected the plant-available P content in sterilized soil differently over time and released P from native P pools in the soil. Collectively, ALC_02 exhibited all three mechanisms in a soil environment. To our knowledge, bacterial P biofertilizers have not been reported to colonize and elongate root hairs in the soil so far, and we propose that these traits contribute to the overall effect of ALC_02. The knowledge gained in this research can be applied in the future quest for bacterial P biofertilizers, where we recommend assessing all three parameters, not only root growth and P solubilization, but also root hair elongation. This will ultimately support the development of sustainable agricultural practices.

Differential influence of Bacillus subtilis strains on Arabidopsis root architecture through common and distinct plant hormonal pathways.

Plant growth-promoting microbes (PGPM) can enhance crop yield and health, but knowledge of their mode-of-action is limited. We studied the influence of two Bacillus subtilis strains, the natural isolate ALC_02 and the domesticated 168 Gö, on Arabidopsis and hypothesized that they modify the root architecture by modulating hormone transport or signaling. Both bacteria promoted increase of shoot and root surface area in vitro, but through different root anatomical traits. Mutant plants deficient in auxin transport or signaling responded less to the bacterial strains than the wild-type, and application of the auxin transport inhibitor NPA strongly reduced the influence of the strains. Both bacteria produced auxin and enhanced shoot auxin levels in DR5::GUS reporter plants. Accordingly, most of the beneficial effects of the strains were dependent on functional auxin transport and signaling, while only 168 Gö depended on functional ethylene signaling. As expected, only ALC_02 stimulated plant growth in soil, unlike 168 Gö that was previously reported to have reduced biofilms. Collectively, the results highlight that B. subtilis strains can have strikingly different plant growth-promoting properties, dependent on what experimental setup they are tested in, and the importance of choosing the right PGPM for a desired root phenotype.