Characterization of iron transporters in arbuscular mycorrhiza and their impact on symbiotic functioning

Abstract

Iron (Fe) is a critical micronutrient for the growth and survival of most organisms, playing an important structural role in proteins and as an enzyme cofactor. Despite its abundance in nature, Fe is often not available to plants, particularly in alkaline soils, since it exists mostly in its oxidized state, Fe (III). To prevent chlorosis and poor development, plants have evolved strategies to acquire Fe from the rhizosphere. Non-grasses plants use a Strategy I, which involves a plasma membrane H+-ATPase to acidify the rhizosphere and solubilize Fe, a ferric reductase (FRO1) to reduce Fe (III) to Fe (II), and the Fe (II) transporter (IRT) for uptake across the plasma membrane. Grasses, on the other hand, employ a Strategy II, which includes the production of phytosiderophores (PS) to chelate Fe (III) and oligopeptide transporters YS1 or YS1-like to transport the PS-Fe chelates into root cells. The establishment of beneficial associations with soil microorganisms is another strategy evolved by plants to cope with Fe deficiency. Arbuscular mycorrhizal (AM) fungi, belonging to the subphylum Glomeromycotina, are among the most prominent microorganisms that contribute to plant nutrition. These fungi form a mutualistic symbiosis with most terrestrial plant species. They colonize biotrophically the root cortex and develop an extensive network of extraradical hyphae in the soil that overgrows the soil surrounding the roots. In return for the carbon compounds provided by the plants, AM fungi deliver to the plant the nutrients they take up beyond the nutrient depletion zones developed around the roots. It is well established that AM fungi can help plants to acquire low mobility nutrients, such as phosphorus, nitrogen, zinc, copper and Fe. Besides enhancing nutrient uptake to their host plants, AM fungi provide increased tolerance against biotic and abiotic stresses. AM fungi play a crucial role in modulating plant metal acquisition over a wide range of soil metal concentrations, as they increase plant metal acquisition in soils deficient in these elements but reduce metal uptake in contaminated soils. The importance of the AM symbiosis for plant development in both Fe-deficient and Fecontaminated soils has been established. However, little is known about the mechanisms of Fe transport and homeostasis in AM. Within this PhD thesis to get further insights into the mechanisms of Fe homeostasis in AM, we employed a multidisciplinary approach combining in silico, physiological and molecular tools. We used the model AM fungus Rhizophagus irregularis DAOM197198 v2.0 and A1, A4, A5, B3 and C2 v1.0, which can be easily grown in in vitro monoxenic cultures and in vivo whole plant bidimensional experimental systems. For the studies on the plant side we used Solanum lycopersicum, an economically important crop that has been used as a model plant for studying Fe homeostasis in Strategy I plants.