A Model for Nitrogen Fixation in Cereal Crops

A novel mechanism for nitrogen fixation in an indigenous maize landrace was recently reported that focused on a nitrogen-fixing microbiota supported by an abundant secreted mucilage. The structure of a complex polysaccharide comprising the mucilage suggested that terminal fucose, arabinose, and xylose may provide the energy source to fuel nitrogenase activity. Nitrogen-fixing microbial associations with cereals have been of intense interest for more than a century (Roesch et al., Plant Soil 2008;302:91–104; Triplett, Plant Soil 1996;186:29–38; Mus et al., Appl. Environ. Microbiol. 2016;82:3698–3710; Beatty and Good, Science 2011;333:416–417). A recent report demonstrated that an indigenous Sierra Mixe maize landrace, characterized by an extensive development of aerial roots that secrete large amounts of mucilage, can acquire 28–82% of its nitrogen from atmospheric dinitrogen (Van Deynze et al., PLoS Biol. 2018;16:e2006352). Although the Sierra Mixe maize landrace is unique in the large quantity of mucilage produced, other cereal crops secrete mucilage from underground and aerial roots and we hypothesize that this may represent a general mechanism for cereals to support associations with microbial diazotrophs. We propose a model for the association of nitrogen-fixing microbes with maize mucilage and identify the four main functionalities for such a productive diazotrophic association. Nitrogen-fixing microbial associations with cereals have been of intense interest for more than a century (Roesch et al., Plant Soil 2008;302:91–104; Triplett, Plant Soil 1996;186:29–38; Mus et al., Appl. Environ. Microbiol. 2016;82:3698–3710; Beatty and Good, Science 2011;333:416–417). A recent report demonstrated that an indigenous Sierra Mixe maize landrace, characterized by an extensive development of aerial roots that secrete large amounts of mucilage, can acquire 28–82% of its nitrogen from atmospheric dinitrogen (Van Deynze et al., PLoS Biol. 2018;16:e2006352). Although the Sierra Mixe maize landrace is unique in the large quantity of mucilage produced, other cereal crops secrete mucilage from underground and aerial roots and we hypothesize that this may represent a general mechanism for cereals to support associations with microbial diazotrophs. We propose a model for the association of nitrogen-fixing microbes with maize mucilage and identify the four main functionalities for such a productive diazotrophic association.