The Encephalartos natalensis-cyanobacterial coralloid root partnership for nitrogen acquisition

Abstract

Plant-microbe symbioses have important environmental and ecological implications because of the role they played in plant diversification as well as their ongoing impact on nutrient acquisition in nutrient-poor environments. Land plants, the foundation of most terrestrial ecosystems today, are the descendants of ancestral aquatic algae that transitioned to land approximately 443 to 470 million years ago. The consensus is that symbiotic associations facilitated plant colonisation. Over 450 million years of coevolution has led to a tremendous diversity of mutualistic symbioses, which could have aided the eventual domination of angiosperms. Plant-microbe evolution studies have mainly focused on the angiosperms, especially focusing on the two well-known symbioses commonly established by them; the arbuscular mycorrhizal (fungal) and the root-nodule (bacterial) symbioses. Although they make up ~80% of extant land plant species, angiosperms represent but a single plant lineage, therefore, invaluable knowledge can be gained from studying the unique biology of symbioses in other land plant lineages. For example, cyanobacterial symbiosis independently evolved across unrelated land plant lineages (bryophytes, ferns, gymnosperms and angiosperms), and is amongst the major evolutionary innovations linked to the acquisition of nitrogen via partnerships with microorganisms. However, since the majority of published literature on cyanobioses are dated, information for these partnerships have remained inadequate – especially at a molecular and cellular resolution. Using an integrated approach and utilising multiple techniques including microscopy, de novo transcriptome assembly and quantification, and comparative genomics, the objectives of this MSc project were to investigate gene expression in coralloid root and control root tissues from the cycad, Encephalartos natalensis, and analyse genes preferentially expressed within these tissues as well as genes involved in the more commonly studied symbiotic associations. For comparative purposes, the symbiotic and control tissues from three other cyanobiosis-forming plant lineages were also analysed. By integrating multiple techniques and scientific fields, this project is one of the first to show possible neofunctionalisation of common symbiotic pathway genes for cyanobacterial partnerships from pre-existing arbuscular mycorrhizal symbiosis in a similar way as that which occurred in nodule symbioses. While adding integral knowledge to the field of plant-microbe evolution, major outputs of this study also include transcriptomic resources for two cyanobacterial hosts, a comprehensive candidate gene list as well as gene expression profiles.