Mycorrhizal ecology: in the land of the one-eyed king

Unlike most of the other disciplines in microbial ecology, a substantial fraction of the theory on mycorrhizal ecology originates from times when assaying microbes was laborious and inefficient. Most of the resulting hypotheses target, as a result, the plant partner of the symbiosis, or at best treat the two mycorrhizal partners as a unified organism, a holobiont. I here address the legacy of this era of mycorrhizal ecology, as a means of systematizing our understanding of the discipline, but also identifying gaps in knowledge. First, I pair and review hypotheses that align with the holobiont concept with complementary hypotheses that explicitly consider the fitness of the mycorrhizal fungus. Second, I generate a hierarchy of hypotheses in mycorrhizal ecology to showcase the high potential for classifying theory into either hypotheses that treat the mycorrhiza as a holobiont or hypotheses considering mycorrhiza as an association of two individual partners. Third, I identify settings that might dictate when to better abstract mycorrhizas into holobionts and when to consider all their partners individually to foster research progress. I conclude the review with suggestions on how to further unify expectations in mycorrhizal ecology.