AUTHOR=Battenberg Kai , Potter Daniel , Tabuloc Christine A. , Chiu Joanna C. , Berry Alison M. TITLE=Comparative Transcriptomic Analysis of Two Actinorhizal Plants and the Legume Medicagotruncatula Supports the Homology of Root Nodule Symbioses and Is Congruent With a Two-Step Process of Evolution in the Nitrogen-Fixing Clade of Angiosperms JOURNAL=Frontiers in Plant Science VOLUME=9 YEAR=2018 URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01256 DOI=10.3389/fpls.2018.01256 ISSN=1664-462X ABSTRACT=
Root nodule symbiosis (RNS) is a symbiotic interaction established between angiosperm hosts and nitrogen-fixing soil bacteria in specialized organs called root nodules. The host plants provide photosynthate and the microsymbionts supply fixed nitrogen. The origin of RNS represents a major evolutionary event in the angiosperms, and understanding the genetic underpinnings of this event is of major economic and agricultural importance. Plants that engage in RNS are restricted to a single angiosperm clade known as the nitrogen-fixing clade (NFC), yet occur in multiple lineages scattered within the NFC. It has been postulated that RNS evolved in two steps: a gain-of-predisposition event occurring at the base of the NFC, followed by a gain-of-function event in each host plant lineage. Here, we first explore the premise that RNS has evolved from a single common background, and then we explore whether a two-step process better explains the evolutionary origin of RNS than either a single-step process, or multiple origins. We assembled the transcriptomes of root and nodule of two actinorhizal plants,