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. These microbial communities may benefit plants by promoting adaptive plasticity or by altering selection on plant traits. We investigated how microbes, and microbial responses to stress, influence adaptive plant responses using pedigreed populations of Chamaecrista fasciculata grown in mesocosms in the greenhouse. To manipulate microbe history, we inoculated each mesocosm with field rhizosphere soil from plants that received one of four stress treatments: salt, herbicide, simulated herbivory (a non-global change induced stress), or no stress. We also included a sterile inoculum treatment as a control. We then manipulated the contemporary environment by applying salt, herbicide, simulated herbivory, and no stress in the greenhouse in a full factorial design with microbe history. We quantified environment- and microbe-induced plasticity and selection on three plant traits - early growth, flowering time, and specific leaf area (SLA).
Results/Conclusions
Preliminary results show that both the abiotic environment (P < 0.05) and microbe history (P < 0.001) influenced plant flowering time, but the direction of these plastic shifts did not always align. For example, contemporary herbivory delayed flowering relative to control treatments (P < 0.001), but microbes originating from field plots with a history of herbivory accelerated flowering (P = 0.02). In this case, stress altered microbial communities and these altered microbial communities caused plant flowering time to change in a way that counteracted the direct plant response to stress. We are currently estimating selection gradients for flowering time in each treatment, which will allow us to assess the adaptiveness of observed plastic shifts and predict the strength and direction of selection imposed by both the contemporary environment and microbe history. We are also analyzing early growth and SLA as additional traits. This work will help us understand how microbial communities, and their response to stress, influence adaptive plant responses. If microbial effects are adaptive, then plants may be able to avoid extinction more readily than current models predict based on plant responses alone; on the other hand, if they are maladaptive, then the negative effects of global change may be greater than anticipated.