Rhizobia and arbuscular mycorrhizal fungi (AMF) as the microbiota potentially improving the growth of legumes in heavy metal polluted areas

The metalliferous tailings in southern Poland are spontaneously colonized by metal-tolerant Anthyllis vulneraria L. (Fabaceae), which can form simultaneously symbiotic association with nitrogen-fixing rhizobia and phosphorus-acquiring arbuscular mycorrhizal fungi (AMF). So far, the diversity of symbiotic microbiota of legumes colonizing the tailings have been poorly studied.

The aim of this study was to characterize the rhizobia, their plant growth promoting (PGP) traits, and AMF associated with A. vulneraria calamine ecotype. The results indicated that plants were nodulated by metal-resistant Bradyrhizobium liaoningense and Rhizobium metallidurans, which showed that PGP traits thus might induce A. vulneraria growth on metal contaminated sites directly by nitrogen fixation, IAA and ammonium production, phosphate solubilization, siderophore formation or lowering ethylene levels. Moreover, the nodulated roots where intensively colonized by AMF ( 86.3% of root mycorrhizal colonization and 20.53% of relative mycorrhizal intensity ) with the Arum-type of mycorrhiza. Molecular identification of AMF using PCR-DGGE analysis based on the 18S rDNA ribosomal gene by nested-PCR revealed Rhizophagus sp., R. fasciculatus, and R. iranicus in Anthyllis roots. Heavy metal excess had no negative effect on the number of AMF spores, amounts of glomalin-related soil proteins (GRSP) and AMF species composition. AMF might influence the plant growth directly providing nutrients and/or decreasing metal uptake and translocation by GRSP production. Metal ions were accumulated mainly in nodules and intraradically fungal structures rather than thick plant cell walls.

The results of this work indicate that the presence of unique symbionts, which can enhance A. vulneraria tolerance to heavy metal stress and plant adaptation to extreme conditions on calamine tailings.