Background
The authors write that "roots of a very large number of plant species are regularly colonized by a group of ascomycete fungi with usually dark-pigmented (melanized) septate hyphae (Mandyam and Jumpponen, 2005; Sieber and Grunig, 2006)" that are referred to as "dark septate root endophytic (DSE) fungi," with "most species belonging to the Leotiomycetes (Kernaghan et al., 2003; Hambleton and Sigler, 2005); Wang et al., 2006)."

What was done
Alberton et al. grew Scots pine (Pinus sylvestris) plants from seed for 125 days in Petri dishes -- both with and without inoculation with one of seven different species/strains of DSE fungi -- within controlled environment chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, destructively harvesting some of the seedlings at the 98-day point of the study and the rest of them at the experiment's conclusion.

What was learned
The three researchers report that "across all plants (DSE-inoculated and control plants) under elevated CO2, shoot and root biomass increased significantly by 21% and 19%, respectively, relative to ambient," with "higher values over the final four weeks (increases of 40% and 30% for shoots and roots, respectively)." In addition, they state that, "on average, shoot nitrogen concentration was 57% lower under elevated CO2," and that "elevated CO2 decreased root nitrogen concentration on average by 16%."

What it means
Alberton et al. acknowledge that their study "did not suggest a role for DSE fungi in increased nutrient uptake." In fact, they emphasize that "under elevated CO2, DSE fungi even reduced nitrogen content of the pine seedlings." And they further emphasize that "surprisingly, even under reduced nitrogen availability, elevated CO2 led to increases in both above-ground and below-ground plant biomass [italics added]."

So how did it happen? The Brazilian and Dutch researchers write that "a potential mechanism for the increase of plant biomass even when plant nutrient uptake decreases is the production of phytohormones by DSE fungi [italics added]," stating that "earlier authors noted that DSE fungi enhance plant growth by producing phytohormones or inducing host hormone production without any apparent facilitation of host nutrient uptake or stimulation of host nutrient metabolism (Addy et al., 2005; Schulz and Boyle, 2005)," further demonstrating that low levels of nitrogen availability need not be an insurmountable impediment to significant CO2-induced increases in plant growth and development.

References
Addy, H.D., Piercey, M.M. and Currah, R.S. 2005. Microfungal endophytes in roots. Canadian Journal of Botany 83: 1-13.

Kernaghan, G., Sigler, L. and Khasa, D. 2003. Mycorrhizal and root endophytic fungi of containerized Picea glauca seedlings assessed by rDNA sequence analysis. Microbial Ecology 45: 128-136.

Mandyam, K. and Jumpponen, A. 2005. Seeking the elusive function of the root-colonizing dark septate endophytic fungi. Studies in Mycology 53: 173-189.

Schulz, B. and Boyle, C. 2005. The endophytic continuum. Mycological Research 109: 661-686.

Sieber, T.N. and Grunig, C.R. 2006. Biodiversity of fungal root-endophyte communities and populations, in particular of the dark septate endophyte Phialocephala fortinii s.1. In: Schulz, B., Boyle, C. and Sieber, T.N. (Eds.) Microbial Root Endophytes [of series] Soil Biology 9: 107-132.

Wang, Z., Johnston, P.R., Takamatsu, S., Spatafora, J.W. and Hibbett, D.S. 2006. Toward a phylogenetic classification of the Leotiomycetes based on rDNA data. Mycologia 98: 1065-1075.