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

豆科植物（如百脉根、蒺藜苜蓿）与丛枝菌根真菌（如Rhizophagus irregularis）共生。真菌提供矿物质，特别是磷酸盐，以交换植物根部的代谢物。以前的实验表明，真菌从根部吸收己糖。可获得噬根虫的转录组和全基因组序列。这些测序工作的一个主要结果是我们后来才认识到的事实，即Rhizophagus缺乏从头脂肪酸合成的基因，因此它不能从乙酰辅酶A产生脂肪酸。由于这个原因，植物可能产生脂质，这些脂质被转移到真菌作为碳源。最近鉴定了莲花和苜蓿的几种突变体，这些突变体在可能涉及脂质从根转移到植物表面或真菌的基因中受到影响。我们将集中在莲花基因编码的酰基-ACP硫酯酶（脂肪），参与调节植物脂肪酸从头合成。系统发育分析表明，莲花含有四个脂肪基因（FatA，FatB，FatC，FatD）。FatC的表达在菌核化过程中被诱导。在这个项目中，我们将表达所有四个硫酯酶的cDNA从莲花在大肠杆菌中，并确定其底物特异性。将从插入突变体群体中分离相应的Lotus突变体（fatA、fatB、fatC、fatD），并测量它们在不同器官中的脂肪酸组成。将研究突变体，特别是fatC的菌根化效率。我们将测量脂质前体，并进行脂质标记实验，以跟踪脂肪酸从植物根部到真菌的流量。在合作中，我们将测量脂质代谢受影响的其他苜蓿突变株系中的脂质。这些实验将有助于解开的作用脂质的建立丛枝菌根相互作用。

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.