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The role of lipids during nodulation and arbuscular mycorrhiza formation

Prof. Dr. Peter Dörmann

Plants of the Fabaceae (legume family) are capable to form nodules after infection of the roots with rhizobial bacteria. The rhizobia harbor the enzyme machinery for the conversion of N2 into ammonia (NH3). Therefore, symbiotic nitrogen fixation by the root nodule bacteria contributes to the supply of ammonia to plant metabolism. We employ the model system Lotus japonicus (infected by Mesorhizobium loti) to study the role of lipids during nodulation (Figures 3, 4) (Gaude et al., 2004). The bacteria form bacteroids inside the cytosol of the plant cell and are surrounded by the plant derived, so-called peribacteroid membrane.

Furthermore, many plants undergo symbiotic interactions with arbuscular mycorrhizal fungi such as Rhizophagus irregularis (syn. Glomus intraradices), a soil fungus that infects the roots and provides nutrients, in particular phosphate, to the plant. The fungus forms arbuscules inside the plant cell which are surrounded by the plant-derived, so-called periarbuscular membrane. We study the lipid changes during the infection process of the fungal and the plant cells.

Figure 4

Figure 4:
Lotus japonicus, a model plant of the Fabaceae (legume) family is used to study root nodule formation and arbuscular mycorrhization. A: Lotus infected with Mesorhizobium loti, forming root nodules. B: Uninfected Lotus plants. The two plants were grown under nitrogen limitation. Nodulated plants grow better because of the nitrogen supplied by the nodule bacteria.

Figure 5
Figure 5: Root infection with Rhizobia. A Root hair curling is initiatedafter the release of bacterial Nod factors. Rhizobia are trapped in the root hairs and enter the root via an infection thread. B When the bacteria reach the root cortex they invade plant cells by endocytosis . The bacteroids are surrounded by the peribacteroid membrane, which is derived from the plant plasma membrane.

Figure 6
Figure 6: Lotus japonicus, a model plant of the Fabaceae (legume) family to study root nodule formation and arbuscular mycorrhization.

Figure 7

Figure 7: Infection of roots with rhizobial bacteria (left) or with arbuscular mycorrhizal fungi (right). In the two symbiotic interactions, the microorganisms form structures inside the cytosol of the host plant cell, surrounded by a plant-derived membrane.

Figure 8

Figure 8: Arbuscular Mycorrhiza Colonization of Plant Roots. A: Lotus japonicus root colonized with Rhizophagus irregularis. Fungal structures were stained with ink and vinegar. B: Arbuscules C: Vesicles D: Rhizophagus irregularis spores and hyphae in axenic root culture with Agrobacterium rhizogenes transformed chicory roots. E: Process of colonization. Rhizophagus spores germinate and hyphae grow towards the root after perception of strigolactones. The fungus forms hyphopodia on the root surface and invades the plant via rhizodermal cells. The hyphae enter the apoplast when they reach the root cortex and form arbuscules inside inner cortical cells. Vesicles are formed inside the apoplast. a: arbuscule, h: hypha, s: spore, v: vesicle.

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