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The roots of most extant plants are able to become engaged in an interaction with a small group of fungi of the fungal order Glomales (Glomeromycota). This interaction—arbuscular mycorrhizal (AM) symbiosis—is the evolutionary precursor of most other mutualistic root-microbe associations. The molecular analysis of this interaction can elucidate basic principles regarding such associations. This review summarizes our present knowledge about cellular and molecular aspects of AM. Emphasis is placed on morphological changes in colonized cells, transfer of nutrients between both interacting partners, and plant defence responses. Similarities to and differences from other associations of plant and microorganisms are highlighted regarding defence reactions and signal perception.
Publikation
The identification and quantification of cyclohexenone glycoside derivatives from the model legume Lotus japonicus revealed far higher levels than expected according to the stoichiometric relation to another, already determined carotenoid cleavage product, i.e., mycorradicin. Mycorradicin is responsible for the yellow coloration of many arbuscular mycorrhizal (AM) roots and is usually esterified in a complex way to other compounds. After liberation from such complexes it has been detected in AM roots of many, but not of all plants examined. The non-stoichiometric occurrence of this compound compared with other carotenoid cleavage products suggested that carotenoid biosynthesis might be activated upon mycorrhization even in plant species without detectable levels of mycorradicin. This assumption has been supported by inhibition of a key enzyme of carotenoid biosynthesis (phytoene desaturase) and quantification of the accumulating enzymic substrate (phytoene). Our observations suggest that the activation of carotenoid biosynthesis in AM roots is a general phenomenon and that quantification of mycorradicin is not always a good indicator for this activation.
Publikation
We investigated the accumulation of reactive oxygen species (ROS) in arbuscular mycorrhizal (AM) roots from Medicago truncatula, Zea mays and Nicotiana tabacum using three independent staining techniques. Colonized root cortical cells and the symbiotic fungal partner were observed to be involved in the production of ROS. Extraradical hyphae and spores from Glomus intraradices accumulated small levels of ROS within their cell wall and produced ROS within the cytoplasm in response to stress. Within AM roots, we observed a certain correlation of arbuscular senescence and H2O2 accumulation after staining by diaminobenzidine (DAB) and a more general accumulation of ROS close to fungal structures when using dihydrorhodamine 123 (DHR 123) for staining. According to electron microscopical analysis of AM roots from Z. mays after staining by CeCl3, intracellular accumulation of H2O2 was observed in the plant cytoplasm close to intact and collapsing fungal structures, whereas intercellular H2O2 was located on the surface of fungal hyphae. These characteristics of ROS accumulation in AM roots suggest similarities to ROS accumulation during the senescence of legume root nodules.
Publikation
Colonization of root cortical cells by arbuscular mycorrhizal fungi leads to marked cytological changes of plastids and mitochondria. Plastids in particular are forming tubular extensions partially connecting individual organelles in a network-like way. These cytological changes correspond to an increased need for plastid and mitochondrial products during establishment and functioning of the symbiosis. The analysis of metabolite and transcript levels in mycorrhizal and nonmycorrhizal roots from Medicago truncatula revealed concomitant changes regarding a number of metabolic pathways. Our results indicate the activation of the mitochondrial tricarboxylic acid cycle and of plastid biosynthetic pathways producing fatty acids, amino acids, and apocarotenoids. These observations provide a general overview of structural and metabolic changes of plastids and mitochondria during colonization of root cortical cells by arbuscular mycorrhizal fungi.