Role Of Arbuscular Mycorrhizal Fungi In The Nitrogen Uptake Of Plants: Current Knowledge And Researc

  • Thread starter jumpincactus
  • Start date
  • Tagged users None
jumpincactus

jumpincactus

Premium Member
Supporter
11,609
438
Heres an opener, I will post the PDF for the entire paper for those of you that want to delve deeper. Awesome stuff!!!

Agronomy 2015, 5, 587-612; doi:10.3390/agronomy5040587 agronomy ISSN 2073-4395 www.mdpi.com/journal/agronomy

Review Role of Arbuscular Mycorrhizal Fungi in the Nitrogen Uptake of Plants: Current Knowledge and Research Gaps

Heike Bücking * and Arjun Kafle Biology and Microbiology Department, South Dakota State University, Brookings, SD 57007,
USA; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-605-688-5463; Fax: +1-605-688-5624. Academic Editors: Anne Krapp and Bertrand Hirel Received: 9 November 2015 / Accepted: 10 December 2015 / Published: 16 December 2015

Abstract: Arbuscular mycorrhizal (AM) fungi play an essential role for the nutrient uptake of the majority of land plants, including many important crop species. The extraradical mycelium of the fungus takes up nutrients from the soil, transfers these nutrients to the intraradical mycelium within the host root, and exchanges the nutrients against carbon from the host across a specialized plant-fungal interface. The contribution of the AM symbiosis to the phosphate nutrition has long been known, but whether AM fungi contribute similarly to the nitrogen nutrition of their host is still controversially discussed. However, there is a growing body of evidence that demonstrates that AM fungi can actively transfer nitrogen to their host, and that the host plant with its carbon supply stimulates this transport, and that the periarbuscular membrane of the host is able to facilitate the active uptake of nitrogen from the mycorrhizal interface. In this review, our current knowledge about nitrogen transport through the fungal hyphae and across the mycorrhizal interface is summarized, and we discuss the regulation of these pathways and major research gaps.


1. Introduction The arbuscular mycorrhizal (AM) symbiosis plays a key role for the nutrient uptake of more than 60% of land plants, including many important crop species such as wheat, corn, and soybean [1]. AM fungi are ubiquitous and can account for up to 50% of the microbial biomass in soils [2]. The extraradical mycelium (ERM) of the fungus acts as an extension of the root system and increases the uptake of phosphate (P), nitrogen (N), sulfur, and magnesium but also of trace elements, such as copper and zinc. In addition, AM fungi also provide non-nutritional benefits to their host plant, as they improve the resistance of plants against several abiotic (drought, salinity, heavy metals) and biotic (pathogens, herbivores) stresses [3]. AM fungi therefore play a key role in the survival and fitness of plants and act as “ecosystem engineers” of plant communities [4]. However, the benefits for the plant are not free of charge, and the plant transfers between 4% to 22% of its assimilated carbon (C) to the AM fungus [5]. It has been suggested that the substantial C costs are responsible for the variability of mycorrhizal growth responses that have been described. These growth responses can range from highly beneficial to detrimental and follow a mutualism to parasitism continuum [6–8]. AM fungi belong to the fungal phylum Glomeromycota, and are obligate biotrophs that are unable to complete their life cycle without the carbon supply from their host. This dependency on the host and the observation that many host plants suppress their mycorrhizal colonization particularly under high nutrient supply conditions has led to the overall assumption that the host plant is in control of the symbiosis [9]. However, the long co-evolution of about 400 to 450 million years for both partners in the AM symbiosis also allowed the fungus to improve its strategies to control the nutrient transport to the host despite its obligate biotrophic life cycle [10,11]. It has been suggested that carbon to nutrient exchange in the AM symbiosis is controlled by biological market dynamics and that reciprocal reward mechanisms ensure a “fair trade” between both partners in the AM symbiosis [10]. AM fungi and their plant partners form a complex network of many-to-many interactions; each host plant is colonized by communities of AM fungi and fungal individuals colonize multiple host plants simultaneously and interconnect plants by common mycorrhizal networks (CMNs). These many-to-many interactions allow both partners in the symbiosis to choose among multiple trading partners but also force both partners to compete with other partners for nutrient or carbon resources [12,13]. CMNs play a key role for the long distance transport of nutrients, water, stress chemicals and allelochemicals and allow the interconnected host plants to “communicate” with other plants within their CMN [14–18]. CMNs have also been discussed as a pathway for the transport of N from donor to recipient plants [19]. It is clear that CMNs affect the survival and fitness, behavior and competitiveness of the plants and fungi that are linked via these networks, but our current understanding about how the nutrient or infochemical allocation among plants within a CMN is controlled, is very limited [11,20]. The contribution of AM fungi to the N nutrition of their host plant is still under debate [21], and it has been suggested that higher N contents in mycorrhizal plants are just a consequence of an improved supply with P [22]. However, it is clear that AM fungi transfer N to their host, and plants are able to take up N from the mycorrhizal interface. This review summarizes our current knowledge about N uptake, metabolism, and transport in the AM symbiosis and discusses major research gaps in our understanding.

2. Mycorrhizal Roots Have Two Uptake Pathways for Nutrients
Mycorrhizal roots have two uptake pathways for nutrients: the plant uptake pathway (PP) and the mycorrhizal uptake pathway (MP; Figure 1). The PP involves the uptake of nutrients via high- or low affinity uptake transporters in the epidermis or root hairs. Particularly for nutrients with a low mobility in the soil (e.g., P), the uptake via the PP is often limited by the development of depletion zones around the roots. By contrast, the MP involves the uptake by high affinity nutrient transporters in the ERM, followed by the translocation along the hyphae to the intraradical mycelium (IRM) in the root cortex, and the uptake from the mycorrhizal interface by mycorrhiza-inducible plant uptake transporters [23]. However, a plant is simultaneously colonized by communities of AM fungi that can differ in their efficiency with which their MP contributes to the total uptake of nutrients by the plant. The uptake and transport of nutrients via both pathways and their contribution to the nutrient supply of the plant has so far primarily been studied for P [21,23–25], but both pathways also play a role in the N uptake by plants.

Here is the PDF for the entire paper/study
www.mdpi.com/2073-4395/5/4/587/pdf
 
Top Bottom