Does the physiology of fungal symbionts drive specificity in Alnus-ectomycorrhizal assemblages

Background/Question/Methods

The importance of microbial symbioses for plant nutrition is well documented, however the mechanisms driving these partnerships are not clear. For example, the plant genus Alnus forms a tripartite symbiosis with both mycorrhizal fungi and nitrogen-fixing actinobacteria, and it has been shown that the relationship between Alnus and its ectomycorrhizal (ECM) fungal partners is uncommonly specific. We hypothesized that the specificity of the Alnus-ECM assemblage was shaped partly by the physiology of fungal symbionts. Specifically, we proposed that the fungi forming ectomycorrhizas with Alnus are proficient at obtaining nutrients other than nitrogen (e.g. phosphorus) for their host. To analyze this hypothesis, we compared the enzyme activity of ECM root tips sampled from the fine roots of Alnus rubra and Pseudotsuga menziesii in pure stands at the Cascade Head Experimental Forest in Western Oregon. Nine root tips were sampled from each of eight soil cores taken from each of two replicate plots for both tree types.  Excised root tips were tested for the activity of B-glucosidase (cellulose), N-acetylglucosaminidase (chitin), leucine aminopeptidase (protein), and acid phosphatase (phosphorus), and DNA was extracted for molecular identification.

Results/Conclusions

We were able to molecularly identify 65% and 80% of the ECM root tips sampled from A. rubra and P. menziesii plots, respectively. The ITS sequences of 7 different ECM taxa were recovered from Alnus plots, while 32 ECM taxa were detected on Pseudotsuga roots. Overall enzyme profiles differed between Alnus and Pseudotsuga ECM fungal communities: B-glucosidase, N-acetylglucosaminidase, and leucine aminopeptidase activity were all significantly higher at Pseudotsuga plots. Contrary to our original hypothesis, Alnus-associated ECM fungi did not have higher acid phosphatase activity than the ECM fungi associated with Pseudotsuga.  Exoenzyme activity also differed among individual ECM fungal taxa. Pseudotsuga-associated A. byssoides demonstrated a significantly greater rate of acid phosphatase activity than all other taxa at either plot, while Lactarius brunneohepaticus had significantly higher N-acetylglucosaminidase activity than all other dominant fungal taxa forming ECM on Alnus. Rhizopogon spp. contributed significantly more to N-acetylglucosaminidase production than other dominant ECM fungi in Pseudotsuga plots. While we were not able to confirm that Alnus-associated ECM fungi were more effective at obtaining nutrients other than nitrogen for their host, we did detect a range of enzyme activity among co-occurring fungi that suggests functional complementarity in both the Alnus and Pseudotsuga ECM assemblages.