Spore inoculation of Bacillus pumilus TUAT1 strain, a biofertilizer microorganism, enhances seedling growth by promoting root system development in rice

This study was conducted to determine the effects of plant growth-promoting rhizobacteria (PGPRs) on growth, root system development, nutrient absorption, and expression of genes related to root development in rice seedlings. Bacillus pumilus TUAT1, a beneficial PGPR bacterium that is used as a component of commercially available biofertilizer, was inoculated onto rice seedlings once weekly for 3 weeks, and effects on growth parameters were determined. Our results showed that inoculation of B. pumilus TUAT1 spores at a density of 1 × 10⁷ colony-forming units (CFU) per mL yielded a significant increase in all traits compared to the control (water inoculum). In contrast, inoculation with vegetative cells did not yield significant effects (compared to control) in several traits, including the numbers of crown roots and lateral roots. CFUs increased with time and maintained to a higher number in the root zone than at the stem base of rice seedlings. Real-time quantitative reverse transcriptase-polymerase chain reaction of several genes known to be involved in root development revealed that B. pumilus TUAT1 inoculation led to increased accumulation of CRL5 transcripts and decreased accumulation of WOX11 transcripts in the basal stem, where crown root initiation occurs. Thus, our results suggest that spore inoculation of B. pumilus TUAT1 promotes rice crown root formation through suppression of cytokinin signal transduction by transcriptional control of CRL5 and WOX11. Consequently, the use of this bacterial strain may provide a means of improving growth, especially in root systems.