LAUSR

Root symbiotic associations with Nfixing bacteria and mycorrhizal fungi are important evolutionary adaptations of plants to compete for nutrients. Nitrogenfixing plant–bacterial associations are widely distributed across all terrestrial biomes and continents apart from Antarctica. Nodulated plants form important components of plant communities, especially in Nlimited early successional ecosystems, riparian habitats and tropical savanna and shrubland biomes (Cleveland et al., 1999). In early successional habitats, Nfixing plants and their root symbiotic microbes contribute to soil development and facilitate recruitment of other plant species and consumers (Walker, Clarkson, Silvester, & Clarkson, 2003). The global symbiotic biological N fixation amounts roughly to 45 Mt annually, which is the main contributor to natural terrestrial N sources (Vitousek, Menge, Reed, & Cleveland, 2013). Nitrogenfixing mutualistic relationships between plant roots and bacteria have evolved multiple times in both partners (Rai, Söderbäck, & Bergman, 2000; Santi, Bogusz, & Franche, 2013; Werner, Cornwell, Sprent, Kattge, & Kiers, 2014; Doyle, 2016). The differentiated forms of associations occur as root (or additionally stem) nodules, but in multiple instances plants host Nfixing bacteria in undifferentiated leaf, stem or root tissues (Vessey, Pawlowski, & Bergman, 2005; Santi et al., 2013). Rhizobiaceae (αproteobacteria) and Burkholderiaceae (βproteobacteria) are the most well known Nfixing bacterial groups that nodulate mostly legumes (Fabaceae; Sprent, Ardley, & James, 2017). A small genus Parasponia (Cannabaceae) has evolved independently symbiotic associations with Rhizobiaceae (Trinick, 1980). In addition, rhizobial root nodules have been reported in three zygophyllaceous genera, Tribulus, Fagonia and Zygophyllum (Mostafa & Mahmoud, 1951), but Received: 20 November 2017 | Accepted: 12 February 2018 DOI: 10.1111/jvs.12627