A first glimpse at genes important to the Azolla–Nostoc symbiosis

Abstract

© 2019, Springer Nature B.V. Azolla is a small genus of diminutive aquatic ferns with a surprisingly vast potential to benefit the environment and agriculture, as well as to provide insight into the evolution of plant-cyanobacterial symbioses. This capability is derived from the unique relationship Azolla spp. have with their obligate, nitrogen-fixing cyanobacterial symbiont, Nostoc azollae, that resides in their leaves. Although previous work has specified the importance of the exchange of ammonium and sucrose metabolites between these two partners, we have yet to determine the underlying molecular mechanisms that make this symbiosis so successful. The newly sequenced and annotated reference genome of Azolla filiculoides has allowed us to investigate gene expression profiles of A. filiculoides—both with and without its obligate cyanobiont, N. azollae—revealing genes potentially essential to the Azolla-Nostoc symbiosis. We observed the absence of differentially expressed glutamine synthetase (GS) and glutamate synthase (GOGAT) genes, leading to questions about how A. filiculoides regulates the machinery it uses for nitrogen assimilation. Ushering A. filiculoides into the era of transcriptomics sets the stage to truly begin to understand the uniqueness of the Azolla-Nostoc symbiosis.