Root nodule and ectomycorrhizae symbioses recruit mechanisms from the ancestral arbuscular mycorrhizae symbiosis

Summary

Plants establish mutualistic relationships with fungi/bacteria to adapt to nutrient-limited environments. This strategy has evolved over millions of years. The most ancestral symbiosis is the arbuscular mycorrhizae (AM) symbiosis, which facilitated the transition of plants from aquatic to terrestrial environments around 450 million years ago. This relationship involves intracellularly hosting of AM fungi in inner cortical cells, and there, they form symbiotic structure called arbuscule. These facilitate the exchange of nutrients between the plant and AM fungi. Another form of symbiosis occurs in tree species, ectomycorrhizae (ECM), that intercellularly host fungi partner within Hartig nets to facilitate nutrient exchange. The nitrogen-fixing root nodule symbiosis (RNS) is endosymbiosis between certain plant species and N2-fixing bacteria. This symbiosis allows plants to survive in nitrogen-poor environments.

In Chapter 2, it is studied whether genes conserved in AM symbiosis have been co-opted in the ECM symbiosis. By comparing genomes of several Fagales species, including ECM host species that have lost AM symbiosis, our phylogenomic analysis revealed that several AM-conserved genes are maintained in ECM-only hosts. These genes are involved in lipid metabolism, exocytosis, and signaling pathways in AM symbiosis. The transcriptome analysis of ECM roots, together with the detection of lipid droplets in ECM root, further supported the AM-derived mechanisms are recruited in ECM symbiosis. These findings support the hypothesis that ECM symbiosis, although distinct in its intercellular hosting, has an evolutionary link with the intracellular AM symbiosis.

In Chapter 3, the function of AP2/ERF transcription factors, ERN1 and ERA1 are described. They are crucial for intracellular accommodation in rhizobial and AM endosymbiosis, respectively. ERN1, which evolved from a duplication of an ancestor gene with ERA1, has a role in accommodation of rhizobia within nodule cells, in addition to the previously reported role in infection thread formation. By studying ern1 mutants in Parasponia, a more ancestor nodulation species, it is shown that the intracellular accommodation within nodule cells is very likely the ancestral function of ERN1 in nodulation. ERA1 is responsible for intracellular infection/accommodation in AM symbiosis. The shared role of ERN1 and ERA1 intracellular accommodation involves regulation of expression of VPY, a key player in symbiotic exocytosis in both endosymbioses.

The research presented in this thesis highlights: 1) The recruitment of AM-conserved genes, particularly, the lipid metabolism-related genes, in ECM symbiosis gives insight in the evolution of ECM symbiosis. 2) The phylogenetic and functional studies on ERA1 and ERN1 in rhizobial and AM endosymbioses give clues that these TFs could be the missing link between the common symbiotic signaling pathway (CSSP) and symbiotic exocytosis, because ERN1 is the target of CYCLOPS, a component of CSSP.