Evolution and biogeography of actinorhizal plants and legumes: A comparison

Abstract

Abstract The symbiosis between plants and nitrogen‐fixing bacteria is widespread among legumes and actinorhizal plants within the nitrogen‐fixing root nodule (NFN) clade. However, there are major differences, as well as similarities, in the symbioses between actinorhizal plants and Frankia and those of legumes and their associated rhizobia. This review provides an overview of NFN symbioses. We outline the evolution and biogeography of actinorhizal plants and legumes and compare and contrast their microsymbionts and symbiotic processes. Within the NFN clade, a far greater number of nodulated legumes exists, compared with actinorhizal plants, and legumes have a much wider biogeographical distribution. There are genetic and physiological differences between free‐living diazotrophic Frankia and the phylogenetically diverse rhizobia, most strains of which are unable to fix N2 ex planta. Actinorhizal nodules are modified lateral roots with a central vascular system, whereas legume nodules are stem‐like organs with peripheral vascular systems. Most legumes contain their microsymbionts within symbiosomes, rather than the infection threads found in actinorhizal nodule cells. Legumes have greater control of their microsymbionts, and those within the Inverted Repeat Lacking Clade impose terminal differentiation on their bacteroids. Legumes also have effective processes for autoregulation of nodulation and downregulation of N2 fixation in response to high levels of soil N. These features of the legume‐rhizobia symbiosis have led to increased efficiencies in N2 fixation. Synthesis. We suggest that these characteristic features of the legume‐rhizobia symbiosis, specifically legumes' greater flexibility in the choice of microsymbiont partner and the evolution of increased efficiencies in N2 fixation, are factors that can explain why the majority of species within the Leguminosae have retained the ability to nodulate and how this has contributed to their evolutionary success.