Molecular identification of endophytic Methylobacterium spp. (PPFMs) and their potential use as microbial plant growth-promoting agents

In this study, the plant growth-promoting properties (PGPB) of 40 Methylobacterium spp. (PPFM) strains isolated from different annual plants were evaluated in terms of phosphate solubilization, IAA production, siderophore production, ACC deaminase activities, and nitrogen fixation. Using TOPSIS analysis, the four most promising strains (M12, M15, M26, M34) were selected. The M26 strain exhibited the highest phosphate solubilization activity (253.23 µg/mL), while the M12 strain showed the highest IAA production (110.85 µg/mL). All strains increased their IAA production as methanol levels in the medium decreased, indicating that methanol functions not only as a carbon source but also as a signaling molecule. The M34 strain demonstrated the highest ACC deaminase activity (82.50 µmol α-ketobutyrate/hour/mg protein). Additionally, all strains exhibited similar levels of nitrogen fixation activity (14 µg/mL). In our study, it was revealed that the 16S rRNA gene region was insufficient for species identification, and the mxaF gene, which was considered to provide higher resolution for species identification, also failed to deliver the expected phylogenetic distinction. For the accurate identification of all strains, advanced methods such as whole-genome sequencing, chemotaxonomic analyses, and multilocus sequence analyses (MLSA) using marker genes (e.g., gyrB, rpoD) need to be applied. In pot experiments, the M15 strain showed the most positive results in plant growth parameters (fresh weight, dry weight, and plant height). Furthermore, it was demonstrated that while the M15 and M26 strains did not alter the richness of the rhizospheric soil microbiota, they changed the relative abundance in the taxonomic composition. These findings support the potential use of Methylobacterium spp. as microbial plant growth-promoting agents in sustainable agricultural practices.