Background
The authors write that "virtually all land plant taxa investigated have well-established symbioses with a large variety of microorganisms (Nicolson, 1967; Brundrett, 2009)," some of which "are known to support plant growth and to increase plant tolerance to biotic and abiotic stresses (Bent, 2006)." Many of these microorganisms colonize the rhizosphere (Lugtenberg and Kamilova, 2009), while others "enter the root system of their hosts and enhance their beneficial effects with an endophytic lifestyle (Stone et al., 2000)." This is the case, as they put it, "for plant growth-promoting fungi such as arbuscular mycorrhizae, ectomycorrhizae and other endophytic fungi," as well as for plant growth-promoting bacteria and the more specialized plant growth-promoting rhizobacteria, many members of the first two of which categories, in the authors words, "are applied as biocontrol agents, biofertilizers and/or phytostimulators in agriculture (Vessey, 2003; Welbaum et al., 2004) or as degrading microorganisms in phytoremediation applications (Denton, 2007)."

What was done
In order to determine how the world's beneficial plant growth-promoting microorganisms might be impacted by continued increases in the air's CO2 content, as well as by possible concomitant changes in climate, Compant et al. reviewed the results of 135 different studies that investigated the effects of CO2 and changes in various climatic factors on "beneficial microorganisms and their interactions with host plants."

What was learned
The three researchers state that "the majority of studies showed that elevated CO2 had a positive influence on the abundance of arbuscular and ectomycorrhizal fungi," which, in their words, "is generally in agreement with meta-analyses performed by Treseder (2004) and by Alberton et al. (2005)." But they found that "the effects on plant growth-promoting bacteria and endophytic fungi were more variable." Nevertheless, they state that "in most cases, plant-associated microorganisms had a beneficial effect on plants under elevated CO2." In addition, they report that "numerous studies indicated that plant growth-promoting microorganisms (both bacteria and fungi) positively affected plants subjected to drought stress." Temperature effects, on the other hand, were more of a wash, as Compant et al. state that "the effects of increased temperature on beneficial plant-associated microorganisms were more variable, positive and neutral," and that "negative effects were equally common and varied considerably with the study system and the temperature range investigated."

What it means
The stress of drought is disadvantageous for nearly all terrestrial vegetation; but plant growth-promoting microorganisms should help earth's land plants overcome this potentially negative aspect of future climate change, as long as the air's CO2 content continues to rise. Temperature effects, on the other hand, would appear to be no more negative than they are positive in a warming world; and when they might be negative, continued atmospheric CO2 enrichment should prove to be a huge benefit to earth's plants by directly enhancing their growth rates and water use efficiencies. And under the best of climatic conditions, atmospheric CO2 enrichment should bring out the best of earth's plants, plus the best of the great majority of plant growth-promoting microorganisms that are there to biochemically "cheer them on."

References
Alberton, O., Kuyper, T.W. and Gorissen, A. 2005. Taking mycocentrism seriously: mycorrhizal fungal and plant responses to elevated CO2. New Phytologist 167: 859-868.

Bent, E. 2006. Induced systemic resistance mediated by plant growth-promoting rhizobacteria (PGPR) and fungi (PGPF). In Tuzun, S. and Bent, E. (Eds.) Multigenic and Induced Systemic Resistance in Plants. Springer, Berlin, Germany, pp. 225-258.

Brundrett, M.C. 2009. Mycorrhizal associations and other means of nutrition of vascular plants: Understanding the global diversity of host plants by resolving conflicting information and developing reliable means of diagnosis. Plant and Soil 320: 37-77.

Denton, B.D. 2007. Advances in phytoremediation of heavy metals using plant growth promoting bacteria and fungi. Microbiology and Molecular Genetics 3: 1-5.

Lugtenberg, B. and Kamilova, F. 2009. Plant-growth-promoting rhizobacteria. Annual Review of Microbiology 63: 541-556.

Nicolson, T.H. 1967. Vesicular-arbuscular mycorrhiza -- a universal plant symbiosis. Science Progress 55: 561-581.

Stone, J.K., Bacon, C.W. and White, J.F. 2000. An overview of endophytic microbes: endophytism defined. In: Bacon, C.W. and White, J.F. (Eds.) Microbial Endophytes. Marcel Dekker Inc., New York, New York, USA, pp. 3-29.

Treseder, K.K. 2004. A meta-analysis of mycorrhizal responses to nitrogen, phosphorus and atmospheric CO2 in field studies. New Phytologist 164: 347-355.

Vessey, J.K. 2003. Plant growth promoting rhizobacteria as biofertilizers. Plant and Soil 255: 571-586.

Welbaum, G., Sturz, A.V., Dong, Z. and Nowak, J. 2004. Fertilizing soil microorganisms to improve productivity of agroecosystems. Critical Reviews in Plant Sciences 23: 175-193.