Oxalotrophic bacteria in the ectomycorrhizosphere play an essential role in phosphorus mobilization

Background Ectomycorrhiza (EM)-associated bacteria play a pivotal role in hydrolyzing soil phosphorus (P). However, which taxa of bacteria and how they are enriched by EM fungi remains largely unexplored. Oxalotrophic bacteria (OxB) are highly abundant on mycorrhizal hyphae. Therefore, we hypothesized that EM hyphae release oxalate, which enriches OxB that mobilize soil less-available soil P. Results Using frc (formyl-coenzyme A transferase) as a biomarker gene of OxB, we found that more than 80% OxB genomes downloaded from NCBI harbor genes coding for P mobilization enzymes. The relative abundance of OxB in the mycorrhizosphere was increased compared with non-mycorrhizal samples under both laboratory and field conditions. The abundance of the frc gene was positively correlated with that of genes encoding P-mobilizing enzymes, like ppa, phoA, phoD, phoN and ugpQ. A number of OxB were isolated, and exogenous oxalate was used as a carbon source to support OxB and affect the bacterial extracellular phosphatase and phytase activity. Moreover, some OxB mobilized and utilized various less-available inorganic (e.g., apatite) and organic (e.g., adenine phosphate and glycerophosphoric acid) P forms, particularly DNA, hereby promoting EM fungal total P acquisition and growth. Conclusions Our findings indicate that EM-associated OxB play an essential role in soil P mobilization. This study reveals a new eco-physiological function of OxB, elucidates the mechanism of biotic interaction between bacteria-fungi-plant in P mobilization, and adjusts the old dogma that oxalate servers as a chemical compound to directly mobilize P, which provide novel insights into soil P cycling in forest ecosystems.