Disentangling the complexities of how legumes and their symbionts regulate plant nitrogen access and storage

Nitrogen (N) availability strongly influences the structure and function of ecosystems (e.g. Vitousek & Howarth, 1991), but only a relatively small number of microbial groups have the ability to convert the N₂ in our atmosphere into biologically available forms.This process, N₂ fixation, is the dominant source of new N to the biosphere outside of anthropogenic inputs (Vitousek et al., 2013).Some N₂-fixing microorganisms live independently on plant leaves, on decomposing organic material, and in soil (Reed et al.,2011), while others have co-evolved with a few higher plant taxa to form symbioses that fix N₂ in root nodules (e.g. Sprent & Raven,1985). The relationship between these legumes and their root nodule symbionts (rhizobia) is one of the most well studied plant –microbe symbioses. Yet, many important questions about the controls, interactions, and implications of legume N₂ fixation remain unanswered. In this issue of New Phytologist (pp. 690–699),Wolf, Funk, & Menge elegantly address a fundamental set of questions about N₂ fixation in their examination of how herbaceous legumes, their symbionts, and external N availability interact to govern legume access and storage of N.