Do stressful environments promote mutualism?: A study of teosinte and mycorrhizae

Background/Question/Methods

Outcomes of species interactions can vary, even changing from costly to beneficial. The stress-gradient hypothesis predicts these changes: as stress from environmental conditions increases, aspects of interactions that ameliorate stress become relatively more important than costly aspects. Benefits from arbuscular mycorrhizal fungi (AMF), which exchange nutrients for carbon from plants, are conditional on soil nutrients, water and temperature. Since cold soils exacerbate nutrient and drought stress in plants, the stress-gradient hypothesis predicts that the value of AMF to plants could increase at dry, cool sites. If positive interactions are more favored in stressful environments, we predict that selection will favor more cooperative and less exploitative partners in this mutualism.

To test these hypotheses, we measured root colonization by AMF and plant size in wild populations of teosinte, and investigated if environments were associated with changes in the relationship between plant size and root colonization. We then grew populations of teosinte with a common AMF mix in the greenhouse, and asked if plants from dry, cool sites showed greater mutualistic responses to mycorrhizae. To ask if phenotypes of mycorrhizae from cool, dry environments are more mutualistic, we grew B73 maize in the greenhouse with collections of mycorrhizae from different environments.

Results/Conclusions

In the field, AMF colonization and plant size are positively correlated at dry, cool sites but negatively correlated at wet sites, suggesting that AMF may be more beneficial to teosinte in dry, cool sites, as the stress-gradient hypothesis predicts.

Greenhouse conditions were extremely low in nutrients, and mycorrhizal mixes were costly to all teosinte populations. Populations from dry, cool environments were most negatively affected per unit of colonization. If dry, cool environments promote adaptation towards mutualistic treatment of mycorrhizae by teosinte, then we expect that these populations are most naïve in response to poor partners.

B73 maize grew slowest with inocula from cool, dry teosinte populations in the greenhouse. These mycorrhizae are worse partners for B73 than other mycorrhizae.

In conclusion, we hypothesize that mycorrhizae should offer the greatest benefits to teosinte in places that are environmentally stressful. Teosinte populations may respond to this benefit of the interaction by adapting to invest more in partnerships, which has negative outcomes when partners are costly. However, mycorrhizae from cool, dry sites may not be universally beneficial partners. Benefits that these mycorrhizae provide in the field may instead depend strongly on genotypes of plants, environments, or on genotype by environment effects.