Global change is occurring at such a rapid pace that there is serious doubt as to whether plants can adapt fast enough to avoid extinction. One possible solution is that they rely on more rapidly changing microbial communities, which can respond to environmental change through rapid shifts in community composition or through the evolution of key taxa. Experiments have shown that microbial communities can rapidly respond to drought stress in ways that benefit plants, but the mechanism underlying these intriguing patterns is unknown. One such mechanism might be fitness alignment - i.e., traits that favor the survival and reproduction of microbes also benefit plant hosts. I identified two microbial traits that are adaptive for microbes under drought stress (and would therefore become more prevalent in the community under drought), and that I hypothesized could benefit plants under drought: biofilm production and low optimum water potential (i.e. “dry-adapted” microbes). I inoculated 14 bacterial taxa that span a range of each of these traits individually onto Chamaecrista fasciculata plants in the greenhouse under simulated drought and well-watered conditions. I measured plant traits (chlorophyll, early growth, size at reproduction, flowering time, specific leaf area) and biomass as a proxy for plant fitness.
Results/Conclusions
Bacterial traits affected plant growth and plastic responses to drought. Dry-adapted microbes significantly increased plant early growth under drought while wet-adapted microbes tended to increase early growth in well-watered soils (Water x Optimum: P = 0.03), and high biofilm-producing isolates tended to increase plant chlorophyll under drought (Water x Biofilm: P = 0.03). However, bacterial traits did not influence plant final biomass. Bacterial traits also promoted plastic shifts in flowering time and size at reproduction in response to soil moisture. In wet soils, wet-adapted microbes caused plants to flower earlier and at a significantly larger size, while in dry soils, dry-adapted microbes did not influence plant flowering time but caused plants to flower at a larger size (Flowering Time: Water x Optimum: P = 0.04; Size at Reproduction: Water x Optimum: P = 0.04). I am currently analyzing specific leaf area data. Overall, these results suggest that microbial traits may promote plant growth and plastic responses to soil moisture through fitness alignment. If these patterns are generally true, and microbe-mediated plastic shifts are adaptive, then fitness alignment between plants and their soil microbiome may help plants avoid extinction more readily than current models predict based on plant responses alone.