Evolution-based discovery of symbiotic molecular networks in plants – EVOLSYM

The acquisition of nitrogen and phosphorus by plants is a major constraint in agriculture, leading to an increasing dependence on chemical fertilizers in the developed world and poor crop yields in the developing world, where the cost of these fertilizers becomes prohibitive. It is imperative to find alternatives to inorganic fertilizers if we are to improve the sustainability of our agricultural systems. In natural ecosystems most land plants form symbiotic association with arbuscular mycorrhizal (AM) fungi that aid the plant in the capture of these nutrients. However, a wide variety of symbiotic responses are found in plants, with the interaction varying from highly beneficial to only limited or even having a negative impact on plant growth. Establishment of this symbiosis requires a deep re-wiring of the plant secondary metabolism leading to the production of dozens of new molecules that are hypothesized to finely tune the symbiosis. The role played by the large majority remain unknown. Identification of genes required for the biosynthesis of these metabolites, or for the regulation of this biosynthesis, is necessary to characterize their function and the mechanisms governing their production, thus providing novel strategies to improve the efficiency of the arbuscular mycorrhizal symbiosis. Recently, in addition to well-known symbiotic genes, I identified more than 170 novel putative symbiotic genes using a phylogenomic approach. These new genes, that had escaped the classical screens of forward genetics, are relevant candidates as they are highly conserved in host species and absent in species that have lost the AM symbiosis. Among them, eight, were found up-regulated during the AM symbiosis in both the model legume Medicago and in liverworts, the earliest diverging land plant lineage, including five associated with the plant metabolism. Here, I propose to examine the symbiotic role of these eight candidates by reverse genetics in Medicago and Marchantia and to determine their link with the metabolic and transcriptomic rewiring observed during the AM symbiosis.