Fig 1.
Ultrastructure of symbiosomes in the RNS between Lotus japonicus and Mesorhizobium loti.
(A) Schematic drawing of a L. japonicus root with root nodules and a cross-section through a colonized nodule cell filled with symbiosomes. (B) TEM image of a densely colonized zone in a root nodule cell with several bacteroids packed in symbiosomes and surrounded by plant cytoplasm. In case of the bacterium M. loti, in symbiosis with L. japonicus, the bacteroid ultrastructure is similar to the free-living state [13,14]. The bacteroids maintain the rod shape with a length of around 1 µm. However, depending on whether the bacteroids were cut longitudinally or transversally, their visible shape can range from rod to round shaped. The slightly wavy appearance of the bacteroid membranes is caused by minimal shrinkage of the cells within chemical fixation. In some cases the bacteroid membranes are difficult to be recognized and to be separated from each other. Scale bar: 2.5 µm. (C) TEM image of a symbiosome and (D) corresponding schematic drawing. The symbiosome in C and D consists of four bacteroids, the PBS and the PBM. The symbiosome is surrounded by plant cell cytoplasm and other symbiosomes. The plant-derived PBM and the electron-translucent PBS represent the envelope of bacteroids. The bacteroid outer and inner membrane surrounds the cytoplasm of the M. loti cell and separate it from the surrounding PBS. Scale bar: 1 µm; (E) Close-up image of panel (C), with details of the plant-microbe interface and the membranes of both organisms. Scale bar: 250 nm.
Fig 2.
Ultrastructure of arbuscule branches formed by the AM fungus Rhizophagus irregularis in symbiosis with the legume host Lotus japonicus.
(A) Schematic drawing of a L. japonicus root with hyphopodia an of AM fungus (AMF) and a hypha, penetrating the root surface. A longitudinal section through a root cortex cell inside the root tissue, hosting an arbuscule. (B) TEM image of a subcellular region of a sectioned host cell showing a part of an arbuscule, with a trunk and several branches in different sizes distributed in the plant cytoplasm and between plant cell organelles. In the TEM image, the arbuscule branches appear darker than the plant cell cytoplasm and can have different shapes depending on whether they were cut longitudinally or transversally. In this section, the trunk has been sectioned longitudinally, whereas most thick and fine branches appear as round shapes as they have been cut transversally. Collapsing arbuscule branches can be recognized as squeezed fungal structures with linear shapes. Scale bar: 2 µm; (C) TEM image of an arbuscular branch cross-section and (D) corresponding schematic drawing. The arbuscule is surrounded by a plant-derived PAM and PAS and a layer of plant cell cytoplasm. The slightly wavy appearance of the PAM is caused by minimal shrinkage of structures within chemical fixation. The arbuscule cell wall and the fungal plasma membrane envelopes the cytoplasm of R. irregularis and appears electron dense. The fungal cytoplasm contains a nucleus and mitochondria with electron-dense substructures and several bright vacuolar spaces. Scale bar: 0.5 µm. (E) Close-up of panel (C) with details of the plant-microbe interface layers (arrows) that are in close contact to each other. Scale bar: 250 nm.
Fig 3.
Ultrastructure of a haustorium of the downy mildew-causing oomycete Hyaloperonospora arabidopsidis in Arabidopsis thaliana.
(A) Schematic drawing showing from bottom to top an A. thaliana plant, a magnification of a leaf surface colonized by H. arabidopsidis, and a longitudinal section of a palisade mesophyll leaf cell hosting a haustorium (HA). (B) TEM image of a cross-section of an oomycete intercellular hypha in the plant intercellular space and an oomycete haustorium inside the plant mesophyll cell surrounded by plant cytoplasm and a chloroplast. Scale bar: 5 µm; (C) TEM image of a haustorium in cross section and (D) corresponding schematic drawing. The haustorium is surrounded by a layer of plant cell cytoplasm, a plant-derived EHM, and the extrahaustorial matrix (EHMX). The EHMX, with a strong electron-dense appearance, is a carbohydrate-rich, gel-like layer, which lies between the haustorium and the EHM and mediates molecular exchange across this interface. The cytoplasm is surrounded by the plasma membrane and by the oomycete cell wall, a brighter, more electron-lucent appearing layer. The haustorial cytoplasm contains several bright vacuolar spaces, a nucleus, as well as mitochondria with small substructures of the cristae. The haustorium cell wall and the plasma membrane envelope the cytoplasm of the oomycete. Scale bar: 1 µm. (E) Close-up image of (C) with details of the plant-microbe interface layers (arrows), that are in direct contact to each other. Scale bar: 500 nm.