Phytohormone signaling in arbuscular mycorrhiza development

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Arbuscular mycorrhiza (AM) is a widespread symbiosis of plants with fungi of the Glomeromycota that is based on the exchange of nutrients between the symbionts: The plant benefits from increased mineral nutrient uptake while the biotrophic fungus receives photosynthetically fixed carbon. Colonization of plant roots by AM fungi involves mutual recognition through diffusible signal molecules, followed by hyphal docking to the root surface, penetration of the outer cell layers, and formation of branched arbuscules inside cortex cells. Intracellular colonization is steered by an evolutionary ancient signal transduction program called common SYM pathway. The plant also dynamically controls the extent of colonization through unknown mechanisms to optimize the symbiotic interaction according to environmental conditions and its resulting physiological state. Recent reports highlight a prominent role of phytohormones, regulators of plant development, in the control of AM colonization. However, this knowledge is still rudimentary and mechanistic approaches are needed to unravel the molecular basis for the obvious interconnections between AM and whole plant development. This proposal is designed to study mechanisms and outcomes of plant hormone signaling in AM development. It builds on my previous discovery that two components of the karrikin receptor complex, the alpha/beta-fold hydrolase D14L and the F-box protein MAX2, are essential for the colonization of roots by AM fungi. Karrikins are newly described butenolides isolated from smoke and originally found to trigger plant germination in post-fire environments. They also display hormonal activity in plants that germinate independently of fire and the wide phylogenetic distribution of D14L and karrikin-responsiveness suggests that plants produce an endogenous karrikin-like substance. My surprising finding that the karrikin-receptor module is involved in the everyday interaction of plants with symbiotic fungi shows that it is more broadly implicated in regulating plant life than previously assumed. Here I aim to 1. elucidate the dynamics, perception and signaling mechanisms of the D14L MAX2 module during AM development; 2. unravel how D14L signaling specificity is determined; 3. identify downstream targets of D14L MAX2 signaling required for AM development; 4. uncover interactions of the D14L MAX2 module with other phytohormone signaling pathways. The proposed research will combine reverse genetics, physiological and AM assays, transcriptomics, protein interaction studies, and spatiotemporal localization by fluorescence live imaging of D14L MAX2 signaling components and downstream targets during AM development. It will use the well-established root symbiosis model plant Lotus japonicus. I expect that this work not only will reveal mechanisms that control AM symbiosis via hormone signaling but will contribute to the general understanding of hormone function in plant development beyond the AM field.