PS 45-106 - Dominant foliar endophytes elicit growth in soybean under controlled conditions

Background/Question/Methods

Accurate predictions of how plants respond to our rapidly changing climate should consider symbiotic interactions with micro-organisms and how they change with the environment. Previous SoyFACE (free air CO₂ enrichment) studies show that elevated CO₂ levels shifted the assembly of foliar microbial communities in Glycine max (soybean). In particular, composition of three dominant endophytes changed such that an isolate of the nitrogen-fixing bacterium Methylobacterium decreased in abundance in elevated CO₂, while two fungal isolates (Colletotrichum sp. A2.15.12 and Colletotrichum sp. A2.10.2) increased in abundance. The purpose of this study was to determine the phenotypic effects of these endophytes in soybean under two conditions: a greenhouse study in sterile soil and a field study at the SoyFACE facility at ambient (416ppm) and elevated (550ppm) CO₂ levels. We grew fungal and bacterial isolates in the lab and inoculated them onto soybean both independently and in combination, then measured parameters of aboveground biomass, pod number, plant height, and chlorophyll content as indicators of plant phenotypic response to dominant endophytes.

Results/Conclusions

In the greenhouse, at ambient CO₂ with sterile soil, endophyte treatments increased soybean growth compared to the control. Furthermore, plants inoculated with Colletotrichum sp. A2.15.12 and Colletotrichum A2.15.12+Methylobacterium elicited the largest increase in plant biomass at 49% and 51%, respectively, compared to uninoculated control plants. However, the results were not consistent in the field where there were no significant differences in soybean biomass with endophyte treatment in either ambient or elevated CO₂. We can possibly conclude that these foliar endophytes, while commonly found in the field and having positive effects, might only affect plant phenotypes under controlled conditions when plants lack other microbial symbionts. Nitrogen fixation and other nutrient assimilation by mutualistic root endophytes may well outweigh the effect of foliar endophytes, and unless the soil is devoid of a microbial community, the foliar endophytes explored in this experiment may be mostly commensal. It is possible that they serve another function in plant defense or nutrient acquisition in non-agricultural plants. It will be important in future studies on plant-microbe responses to climate change to be mindful of the community context in any given environment.