Plant growth-promoting microbes (PGPMs) can enhance crop yield as a supplementary tool in sustainable agriculture. However, selected PGPM candidates do not always perform as expected, which highlights the need for proper characterization of their mode-of-action. Strains belonging to the genus of Gram-positive Bacillus are commonly applied as PGPMs due to their positive effects in various crops and conditions. The aim of this PhD project was to use the strain “ALC_02” as a case study to elucidate how a Bacillus subtilis strain promotes the growth of plants. Specifically, we studied the effect of inoculating ALC_02 to phosphorus-deficient dwarf tomato (Solanum lycopersicum) cultivar Micro-Tom and further investigated the mode-of-action of root growth stimulation in the model plant Arabidopsis thaliana.

ALC_02 promoted the biomass and phosphorus (P) content of tomato cultivated in greenhouse with P-limited soil, which was associated with induced root growth. Inoculation with ALC_02 released P from native P pools in in vitro soil samples, and ALC_02 displayed both phytase activity, siderophore production, tricalcium phosphate solubilization, and produced auxin in liquid growth medium. To our knowledge it has not been studied before how bacteria affect root hairs of soil-grown plants, and how this relates to plant P acquisition, which we studied using rhizoboxes. The earliest observed plant response upon ALC_02 inoculation was elongation of root hairs, which ALC_02 colonized specifically, and this was associated with a subsequent increase in plant P content. We propose that root hair elongation is an important early response that facilitates P uptake in ALC_02-inoculated plants.

The mechanisms involved in root growth stimulation were investigated further in the model plant Arabidopsis. Here we included the B. subtilis strain 168 Gö, which has previously been described to contain mutations that lowers its ability to colonize roots. Both strains increased the root surface area in vitro, but through different root anatomical traits, and only ALC_02 promoted the root hair surface area. Noticeably, both strains seemed to increase auxin biosynthesis in the plant but interacted differently with plant ethylene signaling. 168 Gö had no effect on root growth in soil-grown Arabidopsis, indicating that root colonization is a prerequisite for B. subtilis to be effective in soil but not the in vitro setup.

Collectively, the present work contributed to the knowledge of the mode-of-action of ALC_02 on induced plant growth in phosphorus-limited conditions, which appears to involve both P release in the soil and stimulation of root and root hair growth and was associated with colonization of root hairs. These findings can aid in the future search and characterization of PGPMs, and thus contribute to the development of robust PGPM-based products as part of sustainable agricultural practices.