Ectomycorrhizal fungi transfer organic phosphorus to plants within a common mycorrhizal network based on plant host inorganic phosphorus availability

Background/Question/Methods

Ectomycorrhizal fungi (ECM) are a dominant group of soil microbes that form a mutualistic symbiosis with plant roots and enhance the ability of host plants to acquire scarce soil nutrients. In nature, ECM fungi are often found in a common mycorrhizal network (CMN); consisting of two or more plant individuals connected by a shared mycorrhizal fungus. However, the mechanisms that control preferential nutrient transfer from ECM fungi to each individual plant host are not well understood. We hypothesized that plant nutrient status might drive this process. We expected this because plants with higher P availabilities may supply less photosynthate to the CMN; which in turn, may lead to less cooperation from the ECM fungi. To test our hypothesis, petri plates were set up containing two lodgepole pine seedlings in a CMN with the ECM fungus Suillus tomentosus. The CMN was grown in ¼ MMN media containing no carbon (C), and 1/40 the amount of P as 1X MMN. Each petri plate contained a “hyphal well” that only hyphae were allowed to grow within. Each hyphal well contained 100 times more P than what was available to the seedlings. After ~90 days of growth, the foliage of each seedling was treated with either a P solution (equvialent to 2.5X MMN), or sterile water. After three days, a solution of either inorganic P (sodium ³²P phosphate), or organic P (serine ³²P phosphate), was injected into the hyphal well. 

Results/Conclusions

Our preliminary results suggest that when a plant’s P availability increases within a P-deprived CMN, the transfer of organically-derived P to that particular host plant (via the mycorrhizal pathway) will significantly decrease. Remarkably, however, our microcosms did not exhibit these same host preferences when the hyphae were supplied with inorganic P. Rather, in the inorganic P treatments there was no significant difference in P uptake between the P-enriched and water-control plants. Our results suggest that preferential allocation of P to host plants by their associated mycorrhizal fungi is likely driven by substrate type and host nutrition status. Plants may invest fewer resources to their mycorrhizal associates when inorganic P is more available. Because more recalcitrant organic P substrates may have higher associated cost during acquisition, ECM fungi may only transfer these nutrients to the more contributive plant host.