Fungal and Plant Tools for the Uptake of Nutrients in Arbuscular Mycorrhizas

Abstract

This is the author's final version of the contribution published as: One of the most ancient and successful strategies which plants adopted for nutrient acquisition is the formation of symbiotic association with Arbuscular Mycorrhizal (AM) fungi. The AM fungi can grow extensively in the soil thanks to an extended hyphal network and, therefore, reach unexplored soil niches, resulting in a high efficient mining for nutrients that are not accessible to the plant roots.On the other hand, the plant provides the fungus with up to 20% of its assimilated carbon, allowing the fungus to conclude its life cycle with the production of new spores. The AM symbiosis has been the default situation for plants since their appearance on Earth with the exception of those plants that lost this capacity during their evolution, such as Arabidopsis thaliana and plants belonging to the Brassicales order (Bravo et al., 2015).The key structure of the nutrient exchange between the two symbiotic partners is a highly branched hypha, so-called arbuscule for its similarities to a tiny tree, completely surrounded by the invagination of the plant plasma membrane. The last is called perifungal membrane and it is enriched by plant nutrient transporters, which are induced by the AM fungal presence. Comparing fossils from 450 mya arbuscules (Remy et al., 1994) with confocal microscope pictures of the same structure from nowadays plants is sufficient to realize how well conserved have been the arbuscules, suggesting that mechanism controlling the fungal morphogenesis inside a plant cell have been maintained along the evolution, irrespectively of the plant identity (Bonfante and Genre, 2008). Despite the high level of conservation of the arbuscule across time, it is worth to remember that it constitutes an ephemeral structure lasting for around 5 days within a single plant cortical cell (Kobae and Hata, 2010). The mechanisms underlying the collapse of the arbuscule are still to be disentangled: a possible weekly checkpoint assures that the symbiosis is still efficient to the partners. It is known that, among different partners, a fine-tuned molecular mechanism regulates the symbiosis, while the exchange of nutrients is a major tool to assess it, as described by the biological market theory (Kiers et al., 2011). A detailed knowledge and research on nutrient uptake within AM symbiosis will have therefore two possible outputs: better understanding the impact of AM fungi on plant nutrition, improving their use in a more sustainable agriculture, and revealing which molecular drivers had led and controlled the establishment of this highly conserved and long-lasting relationship.In this chapter, we will provide a detailed description of our current knowledge about fungal and plant molecular mechanisms involved in nutrient uptake within an Arbuscular Mycorrhizal symbiosis. In particular, three major macronutrients (nitrogen, phosphorous and sulfur) will be discussed with a special focus on old and new paradigms, future perspectives and open questions.The i...

Interactions of Mycorrhiza and Protists in the Rhizosphere Systemically Alter Microbial Community Composition, Plant Shoot-to-Root Ratio and Within-Root System Nitrogen Allocation

“…In contrast to AMF, protists are known to increase N availability to plants by releasing significant amounts of NH + 4 from consumed microbial biomass, a mechanism known as microbial loop (Clarholm, 1985;Bonkowski and Clarholm, 2012). Such changes in nitrogen form can feed back on mycorrhizal development (Giovannetti et al, 2017;Bukovská et al, 2018;Mollavali et al, 2018). Corresponding to the Nlimiting experimental conditions, the increase in total plant N uptake (+22%) in the 0-P treatment corresponded to a 24% increased plant biomass as compared to the control (0-0).…”

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

“…This is reflected in preferences for NH 4 + in most plant and fungal species (Nygren et al ., ; Lilleskov et al ., ). Both fungi of different functional guilds and plants possess multiple NH 4 + and NO 3 − and amino acid transporters to acquire simple N compounds (Casieri et al ., ; Giovannetti et al ., ; Nehls & Plassard, ). Goodale () estimated that N uptake of EcM trees may exceed that of AM trees by 50% in temperate forests.…”

Interaction between Humic Substances and Plant Hormones for Phosphorous Acquisition

“…In non-inoculated plants, P is directly absorbed by nutrient transporters located in the root epidermis and cortex. When plants are inoculated with AMF, P is absorbed by hyphae/mycelia via nutrient transporters and subsequently translocated to fungal structures located inside the cortical cells of the roots [152,156]. These intracellular structures are called arbuscules and formed by hyphae differentiation.…”