The role of Lotus japonicus thioesterases in lipid metabolism and carbon exchange with the arbuscular mycorrhizal fungus Rhizophagus irregularis

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Plants of the Fabaceae (e.g. Lotus japonicus, Medicago truncatula) undergo symbiotic interactions with arbuscular mycorrhizal fungi (e.g. Rhizophagus irregularis). The fungus provides minerals, in particular phosphate, in exchange for metabolites from the plant root. Previous experiments suggested that the fungus takes up hexoses from the roots. Transcriptome and full genome sequences are available for Rhizophagus. A major result of these sequencing efforts which was recognized only later by us was the fact that Rhizophagus is devoid of genes for de novo fatty acid synthesis and that it is therefore incapable of producing fatty acids from acetyl-CoA. For this reason, it is possible that the plant produces lipids which are transferred to the fungus as a carbon source. Several mutants of Lotus and Medicago were recently identified which are affected in genes possibly involved in the transfer of lipids from the root to the plant surface or to the fungus. We will focus on Lotus genes encoding acyl-ACP thioesterases (Fat) which are involved in regulating fatty acid de novo synthesis in plants. Phylogenetic analyses revealed that Lotus contains four Fat genes (FatA, FatB, FatC, FatD). Expression of FatC is induced during mycorrhization. In this project, we will express the cDNAs for all four thioesterases from Lotus in Escherichia coli and determine their substrate specificities. The corresponding Lotus mutants (fatA, fatB, fatC, fatD) will be isolated from insertional mutant populations and their fatty acid compositions in different organs will be measured. The mycorrhization efficiency of the mutants, in particular of fatC, will be studied. We will measure lipid precursors and perform lipid labeling experiments to follow the flux of fatty acids from the plant root to the fungus. In collaboration, we will measure lipids in additional Medicago mutant lines affected in lipid metabolism. These experiments will be instrumental to unravel the role of lipids for the establishment of arbuscular mycorrhiza interactions.