Role of Root Associated Bacteria Derived Flavins in Plant Growth Promotion

Abstract

Plant Growth Promoting (PGP) microbes have many mechanisms to promote plant growth, health and development. However, the role of bacterial derived flavins (FLs) in plant growth promotion is not clearly understood. This study investigated the role of bacterial derived FLs in plant growth promotion. Based on the bacterial culture fluorescence, 26 potential FLs secreting bacteria with 11 genetically different strains commonly from orders Rhizobiales and Burkholderiales were isolated from apple (Malus domestica [Suckow] Borkh) roots. Isolates also exhibited phosphate solubilization, indole-3-acetic acid production, 1-aminocyclopropane-1-carboxylic acid deaminase enzyme activity and at least 25% more dry mass production with the inoculation in alfalfa (Medicago sativa). Inoculation of FLs secreting S. meliloti 1021 (FL+) and its mutant 1021ΔribBA, with a limited ability to secrete FLs (FL−) to kale (Brassica oleracea var. acephala) and lettuce (Lactuca sativa) seedlings showed increased total root length by 19% and 14%, respectively, with the inoculation of 1021 compared to 1021ΔribBA. S. meliloti 1021 inoculation also improved kale and lettuce plant growth, photosynthetic rate and yield than 1021ΔribBA. In lettuce, total phenolics and flavonoids were significantly (p < 0.05) increased by 102% and 57% respectively, and in kale, the increments were 6% and 23% respectively, with 1021 than 1021ΔribBA inoculation. Results from microbiome analysis showed that cultivated apple orchard soil microbial community structure and composition were significantly (p < 0.05) different than that of the uncultivated soil. The orchard soil had several potential PGP microbes or biocontrol agents to fight against phytopathogens. Findings from this thesis indicate that isolation of FLs secreting bacteria from apple roots was feasible by bacterial culture fluorescence technique and FLs secreting Plant Growth Promoting Bacteria (PGPB) have the potential to promote plant growth, physiology, yield and phytochemical property of the plant. Future studies will use molecular mechanisms following the FLs to promote plant growth.