Revealing the Role of Actinorhizal Symbioses in Ecosystem Nitrogen Dynamics

Symbiotic associations between plants and microorganisms are crucial to global biogeochemical cycling and ecosystem stability. Mycorrhizal fungi and nitrogen (N₂)-fixing bacteria are recognized as the two main groups of microorganisms involved in such symbiotic interactions. They not only constitute the most wide-spread symbiotic microorganisms, but also ensure plants to acquire additional N resources directly from the atmosphere. Although plant-microbial interactions, for example, the performance of AM-plant and rhizobia-legume plant symbioses, have been well studied and reviewed in detail previously, still less information is known about these processes in actinorhizal symbioses. The present review is aimed to summarize current knowledge of the interaction of partners in actinorhizal root symbioses, in particular the signalling processes during establishment of BNF, and the specificity of and dependency on different symbiotic partners in this interactions, based on evolution and distribution in the plant and microbial kingdom. The features of nutrient transfer in these root symbiotic relationships and the significance of actinorhizal symbioses for the performance of plants under environmental stress are discussed and compared with AM and rhizobia-legume symbioses. In addition, research gaps in actinorhizal root symbioses research are identified and future research avenues are suggested.