Assessing the adaptability of the actinorhizal symbiosis in the face of environmental change

Human activity, and in particular industrial activity, has altered natural environments. Here we present an experimental approach adapted to study the actinorhizal symbiosis in alder trees and shrubs submitted to abiotic stress. We measured the impact of exogenous nitrogen on the establishment of the alder symbiosis with Frankia sp., and its primary function; nitrogen fixation. Results showed our version of the growth pouch method was functional, and corroborated the gradual inhibition of symbiosis in the presence of increasing exogenous nitrogen concentrations. In mountain alder (Alnus viridis ssp. crispa) there was a gradual and suppressive effect of nitrogen on the relative number or root nodules, while in black alder (Alnus glutinosa) results suggested a threshold effect at 45 ppm N. Shoot to root biomass ratios were increased in the presence of the microsymbiont, and this effect was generally maintained even in the presence of heavy metals (As, Se or V). Alders and the actinorhizal symbiosis were not heavily affected by the presence of heavy metals, confirming potential applications in soil rehabilitation, however the distribution of metals in plant tissues sometimes changed when high levels of metals were present. A. glutinosa plants exposed to high levels of As significantly increased the allocation of As to roots (≈90%), while those exposed to high levels of Se rose their aerial tissue Se allocation to roughly 86%. A. glutinosa plants exposed to high V levels did not change behavior: V was in all cases preferentially accumulated in underground tissues (≥90%). Our results detail the use of a high-throughput approach to study the plasticity of the actinorhizal symbiosis in the presence of fluctuating nitrogen and metal conditions. These methods are transposable to numerous actinorhizal studies in both fundamental and applied research.

► A method was adapted to study the actinorhizal symbiosis in the presence of abiotic stress. ► Inhibition of symbiosis in the presence of increasing exogenous nitrogen concentrations was gradual or sudden depending on alder species. ► Heavy metals did not impact the increase in shoot to root biomass ratio associated with the establishment of symbiosis. ► The presence of symbiosis influenced plant As and Se translocation patterns in A. glutinosa. ► Regardless of inoculation status, V was predominantly accumulated in A. glutinosa root tissues.