The Lotus Japonicus Acyl-acyl Carrier Protein Thioesterase FatM is Required for Mycorrhiza Formation and Lipid Accumulation of Rhizophagus Irregularis

Arbuscular mycorrhiza (AM) fungi establish symbiotic interactions with plants, providing the host plant with minerals, i.e. phosphate, in exchange for organic carbon. Arbuscular mycorrhiza fungi of the order Glomerales produce vesicles which store lipids as an energy and carbon source. Acyl-acyl carrier protein (ACP) thioesterases (Fat) are essential components of the plant plastid-localized fatty acid synthase and determine the chain length of de novo synthesized fatty acids. In addition to the ubiquitous FatA and FatB thioesterases, AM-competent plants contain an additional, AM-specific, FatM gene. Here, we characterize FatM from Lotus japonicus by phenotypically analyzing fatm mutant lines and by studying the biochemical function of the recombinant FatM protein. Reduced shoot phosphate content in fatm indicates compromised symbiotic phosphate uptake due to reduced arbuscule branching, and the fungus shows reduced lipid accumulation accompanied by the occurrence of smaller and less frequent vesicles. Lipid profiling reveals a decrease in mycorrhiza-specific phospholipid forms, AM fungal signature fatty acids (e.g. 16:1ω5, 18:1ω7 and 20:3) and storage lipids. Recombinant FatM shows preference for palmitoyl (16:0)-ACP, indicating that large amounts of 16:0 fatty acid are exported from the plastids of arbuscule-containing cells. Stable isotope labeling with [ C ]acetate showed reduced incorporation into mycorrhiza-specific fatty acids in the fatm mutant. Therefore, colonized cells reprogram plastidial de novo fatty acid synthesis towards the production of extra amounts of 16:0, which is in agreement with previous results that fatty acid-containing lipids are transported from the plant to the fungus.