SYMRK, an enigmatic receptor guarding and guiding microbial endosymbioses with plant roots

The research field of plant–microbe interactions experiences a surge of fundamental insights thanks to the adoption of a restricted number of model systems. However, to see the broader picture and understand the complexity of evolutionary relationships, it is equally important to study relevant non-model interactions. This task is (still) difficult because of the lack of genomics and genetics tools, but rewarding when the outcome proves some long-standing assumptions and suggests new ways to address old questions. A nice example is the work of Gherbi et al. (1) presented in this issue of PNAS. Their article deals with a non-model plant, Casuarina glauca, whose roots interact with both symbiotic fungi and nitrogen-fixing bacteria, and evidences the central and ancestral role of the plant receptor SYMRK in the establishment of the three major types of root endosymbioses.

Microbial associations with plant roots form underground networks of diverse and intimate interactions. Three major groups of endophytic root symbioses are of crucial importance for the geobiochemical cycle and the ecological equilibrium of our planet. Fungi of the order of the Glomeromycota form arbuscular myccorhiza (AM) in a ubiquitous ancestral symbiosis with roots of the majority of land plants leading to nutrient exchange at an extended fungus–plant membrane interface (arbuscules) inside cortical root cells (2). More recently in evolution, two types of root symbioses with nitrogen-fixing bacteria arose in the Eurosid I clade of dicots (3). Molecular dinitrogen can only be enzymatically reduced by prokaryotes. In symbiotic interactions, the bacteria directly deliver the fixed nitrogen to the host and provide nitrogen input in the ecosystem using the sun energy captured by the host via photosynthesis. Legume roots interact with Gram-negative rhizobia, whereas non-legume actinorhizal plants interrelate with Frankia bacteria that are Gram-positive filamentous actinomycetes (4, 5). Although based on different schemes, in both …

*E-mail: marcelle.holsters{at}psb.ugent.be