Implications and Future Research Directions
Increasingly researchers, using a wide range of experimental systems, are exploring microbe-mediated adaptation (Kohl et al. 2014; Weigel & Erwin 2017; Sison-Mangus et al. 2018; Moeller et al. 2019). If microbes commonly influence adaptation through microbe-mediated local adaptation or microbe-mediated adaptive plasticity, then our understanding of the traits and mechanisms underlying adaptation may be incomplete, therefore making our responses to environmental and agricultural challenges potentially inappropriate.
Just as Clausen, Keck, and Heisey (1948) realized 70 plus years ago, plants are excellent study systems to perform reciprocal transplants and partition the variation of plant phenotypes into genetic and environmental components (Núñez-Farfán & Schlichting 2001). However, early pioneers were unaware that microbes can be heavily influenced by their host’s genotype (Bulgarelli et al. 2012; Lundberg et al. 2012; Gehring et al. 2017) and have large effects on plant phenotypes (Friesen et al. 2011; Friesen 2013; Wagner et al. 2014; Giauque et al. 2019). Consequently, in early studies many microbial effects may have been erroneously attributed to plant genotype and at other times erroneously attributed to environment effects (e.g. Chanway et al. 1989). In many cases, such bookkeeping may not fundamentally change outcomes. For example, in the Gehring et al. (2017) case study the mycorrhizal community is so tightly under plant control, it functions as an extended phenotype of the plant host. However, even in this case, one must wonder what will happen to those plant genotypes enriching for drought tolerance promoting mycorrhizae if the ectomycorrhizal community is eroded by other forces? Below we discuss how explicitly considering microbial effects may substantially alter our views of the spatial scales, pace, and consequences of plant adaptation.