The role of resource mutualisms in plant adaptation to abiotic environments
Heterogeneous environments can drive differential selection on populations, leading to local adaptation. While many studies have documented local adaptation, the key selective agents driving local adaptation are often unclear. Moreover, given that organisms live in complex communities, interacting species, such as symbiotic microbes, may influence plant adaptation. Because of their short generation times, genetic diversity and dispersal ability, microbes may evolve rapidly to potentially ameliorate the negative fitness consequences of environmental stress for their plant hosts. As a result, microbial symbionts may facilitate plant adaptation to local environments. Here I investigated how resource mutualists influence plant adaptation to soil moisture in the field using the annual native legume Amphicarpaea bracteata and nitrogen-fixing symbiotic bacteria, Bradyrhizobium sp. I conducted a replicated reciprocal transplant experiment in three sites with wet soil moisture conditions and three sites with dry moisture conditions. I transplanted seedlings from each site back into their home site and also into replicated dry and wet sites. Plant survival and fitness were recorded throughout the growing season. Presence and absence of rhizobium containing root nodules were also recorded.
I found evidence for local adaptation to soil moisture conditions. In wet sites, plants originating from the wet sites significantly out performed the ones originating from the dry sites (Tukey HSD test: P < 0.001). In contrast, in dry sites, plants originating from dry and wet sites performed equally. I also found that that the proportion of nodulated plants depended on both source and destination sites (F1,1543 = 47.216, P < 0.001). In wet destination sites, plants originating from the wet sites were more likely to produce nodules than plants originating from the dry sites. In the dry destination sites however, plants originating from wet and dry sites were equally likely to nodulate. Furthermore, nodulated plants had significantly greater plant fitness than non-nodulated plants when they were grown in wet destination sites, but nodulated plants had lower fitness than non-nodulated plants when they were grown in dry destinations (F1,1176 = 4.22, P = 0.04). In sum, this study shows that soil moisture is an important selection agent driving local adaptation and that symbiotic rhizobia likely contribute to local adaption to soil moisture. Given the intimate relationship between plants and symbiotic microbes, other symbiotic microbes may also play an important role in plant adaptation and deserve further investigation.