Heather R. Kates

Abstract Root nodule symbiosis (RNS) is a complex trait that enables plants to access atmospheric nitrogen converted into usable forms through a mutualistic relationship with soil bacteria. Pinpointing the evolutionary origins of RNS is critical for understanding its genetic basis, but building this evolutionary context is complicated by data limitations and the intermittent presence of RNS in a single clade of ca. 30,000 species of flowering plants, i.e., the nitrogen-fixing clade (NFC).

1 Introduction The story of angiosperm diversification can be seen from the point of view of environmental challenge: how plants left their ancestral mesic tropical origins and gained adaptations to confront the stresses of, and dominate in, increasingly harsh and dynamic environments (Edwards et al., 2010; Arakaki et al., 2011; Folk et al., 2019, 2020; Soltis et al., 2019).