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The nitrogen-fixing symbiosis has evolved by recruiting genes involved in an older symbiosis

The nitrogen-fixing symbiosis has evolved by recruiting genes involved in an older symbiosis
Rhizobial soil bacteria and arbuscular mycorrhizal fungi, two very different types of microorganisms, produce similar signal molecules named lipo-chitooligosaccharides (LCOs). The authors of this article published in Current Biology show that plant-derived LCO receptor, involved in the establishment of the old arbuscular mycorrhizal arbuscular symbiosis, have been recruited during evolution to allow establishment of a root symbiosis with rhizobia within root organs capable of fixing atmospheric nitrogen, and allowing the plant legumes such as pea, soybean and bean to grow and overcome the external supply of nitrogenous nutrients.

The arbuscular mycorrhiza is a very old mutualistic symbiosis (appeared at the time of the colonization of the continents by the plants 450 million years ago) found between the fungi Glomeromycota and the majority of the terrestrial plants and in which fungi provide plants with the nutrients they gain in the soil in exchange of carbohydrates and lipids. To colonize plant roots, arbuscular mycorrhizal fungi first penetrate in the epidermal and cortical cells, and then propagate inter-or intracellularly in the roots. Inside cortical cells of the internal root, these fungi form highly branched structures called arbuscules, in which most of the nutrient exchanges take place.

In the more recent nitrogen-fixing symbiosis between legumes and rhizobia bacteria, the bacterium can fix nitrogen within root nodules. Bacterial LCOs are key signaling molecules of nodule organogenesis and bacterial colonization that allow legume nitrogen nutrition. Although microorganisms differ between these two endosymbioses, the current hypothesis is that nitrogen-fixing symbiosis has evolved by recruiting genes involved in older arbuscular mycorrhizal symbiosis.

LCO receptors involved in arbuscular mycorrhizae are functional for the perception of Rhizobia in legumes

Researchers at the Laboratory for Plant-Microorganism Interactions (LIPM - UMR CNRS / INRA) and the Plant Science Research Laboratory (LRSV - UMR University of Toulouse / CNRS / UPS) have recently shown that LCOs are produced by arbuscular mycorrhizal fungi. However, the corresponding receptors on the plant side were unknown to date. The authors of this paper show that LYK10 proteins in tomato and petunia are LCO receptors and participate in the establishment of arbuscular mycorrhiza. By introducing the promoters of the tomato and petunia LYK10 genes into legumes, they observe an activity similar to that observed with the endogenous orthologous gene (derived from a single sequence present in the common ancestor to both species). In addition, the introduction of the coding sequence for the LYK10 gene makes it possible to restore nodulation in mutants of the legume orthologous gene.

Combination of genetic and biochemical approaches has allowed the authors to designate LYK10 as part of an ancestral device for the perception of signals implicated in the establishment of arbuscular mycorrhiza. A mechanism that might have then been recruited during evolution for nitrogen-fixing symbiosis in legumes.

See also

Ariane Girardin et al. LCO Receptors Involved in Arbuscular Mycorrhiza Are Functional for Rhizobia Perception in Legumes. Current Biology. 29, 1–11, December 16. https://doi.org/10.1016/j.cub.2019.11.038