What was done
The authors grew Scots pine seedlings for four months in growth cabinets with atmospheric CO2 concentrations of 350 or 700 ppm.  In addition, two-thirds of the seedlings were inoculated with one of two species of mycorrhizal fungi, while the remaining third were left untreated, to determine the effects of elevated CO2 on mycorrhizal fungi and their interactive effects on plant growth.

What was learned
Atmospheric CO2 enrichment significantly increased seedling dry mass by an average of 45% regardless of fungal inoculation.  In addition, elevated CO2 increased the number of hyphal tips associated with seedling roots by about 62% for both fungal species.  Hyphal growth was accelerated by elevated CO2, and after 55 days of treatment, the mycorrhizal network produced by one of the fungal symbionts occupied 444% more area than its counterpart exposed to ambient CO2.

What it means
These results suggest that as the CO2 content of the air rises, fungal symbionts of Scots pine will likely receive greater allocations of carbon from their host.  This carbon can be used to increase their mycorrhizal networks, which, in turn, would enable the fungi to explore greater volumes of soil in search of minerals and nutrients to benefit the growth of its host.  In addition, by receiving greater allocations of carbon, fungal symbionts may actually prolong or keep photosynthetic down regulation from occurring as they provide an additional sink for leaf-produced carbohydrates.