Root zone-specific localization of AMTs determines ammonium transport pathways and nitrogen allocation to shoots.

RESEARCH ARTICLE Root zone–specific localization of AMTs determines ammonium transport pathways and nitrogen allocation to shoots Fengying Duan1¤, Ricardo F. H. Giehl1, Niko Geldner2, David E. Salt3, Nicolaus von Wirén ID1* a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 Molecular Plant Nutrition, Leibniz Institute of Plant Genetics and Crop Plant Research, Corrensstr, Gatersleben, Germany, 2 Department of Plant Molecular Biology, Biophore, UNIL-Sorge, University of Lausanne, Lausanne, Switzerland, 3 Centre for Plant Integrative Biology, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, United Kingdom ¤ Current address: Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing, China * . Abstract OPEN ACCESS Citation: Duan F, Giehl RFH, Geldner N, Salt DE, von Wirén N (2018) Root zone–specific localization of AMTs determines ammonium transport pathways and nitrogen allocation to shoots. PLoS Biol 16(10): e2006024. https://doi.org/10.1371/ journal.pbio.2006024 Academic Editor: Ottoline Leyser, University of Cambridge Sainsbury Laboratory, United Kingdom of Great Britain and Northern Ireland Received: March 13, 2018 Accepted: October 2, 2018 Published: October 24, 2018 Copyright: © 2018 Duan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: Deutsche Forschungsgemeinschaft www.dfg.de (grant number WI1728/18-1). ERACAPS Programme. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. In plants, nutrient provision of shoots depends on the uptake and transport of nutrients across the root tissue to the vascular system. Nutrient delivery to the vasculature is mediated via the apoplastic transport pathway (ATP), which uses the free space in the cell walls and is controlled by apoplastic barriers and nutrient transporters at the endodermis, or via the symplastic transport pathway (STP). However, the relative importance of these transport routes remains elusive. Here, we show that the STP, mediated by the epidermal ammonium transporter 1;3 (AMT1;3), dominates the radial movement of ammonium across the root tissue when external ammonium is low, whereas apoplastic transport controlled by AMT1;2 at the endodermis prevails at high external ammonium. Then, AMT1;2 favors nitrogen (N) allocation to the shoot, revealing a major importance of the ATP for nutrient partitioning to shoots. When an endodermal bypass was introduced by abolishing Casparian strip (CS) formation, apoplastic ammonium transport decreased. By contrast, symplastic transport was increased, indicating synergism between the STP and the endodermal bypass. We further establish that the formation of apoplastic barriers alters the cell type–specific localization of AMTs and determines STP and ATP contributions. These results show how radial transport pathways vary along the longitudinal gradient of the root axis and contribute to nutrient partitioning between roots and shoots. Author summary Radial transport of nutrients from the soil to the vascular system of plant roots occurs via the symplastic transport pathway (STP) and apoplastic transport pathway (ATP). Nutrients move along the STP when crossing the plasma membrane of outer cells and moving to xylem through the cytoplasmic continuum formed by plasmodesmata. Nutrients following the ATP, in turn, initially move passively through the extracellular space but are eventually taken up by endodermal cells, in which Casparian strips (CSs) prevent further PLOS Biology | https://doi.org/10.1371/journal.pbio.2006024 October 24, 2018 1 / 22 Dissecting apoplastic and symplastic transport pathways for ammonium in roots Abbreviations: 15NH4+, 15N-labeled ammonium; ABA, abscisic acid; AMT, ammonium transporter; ATP, apoplastic transport pathway; Ca2+, calcium ion; CIF, CS integrity factor; CO2, carbon dioxide; CS, Casparian strip; DW, dry weight; EB, endodermal bypass; ESB1, enhanced suberin 1; GFP, green fluorescent protein; GSO1, GASSHO1; ICP-MS, inductively coupled plasma mass spectrometry; K+, potassium ion; MEP, methylamine permease; MES, 2-(N-morpholino) ethanesulfonic acid; MYB36, myb domain protein 36; N, nitrogen; NH4+, ammonium; NO3-, nitrate; one-half MS, half-strength Murashige and Skoog basal salt mixture; PA, piperonylic acid; PI, propidium iodide; Rh-type, Rhesus-type; SGN3, schengen 3; Sr2+, strontium ion; STP, symplastic transport pathway; tko, amt1;1amt1;2amt1;3; WT, wild-type. apoplastic movement. We assessed the contribution of these transport pathways to radial transport in roots and nutrient provision to shoots by expressing cell type–specific ammonium transporters in a CS-defective mutant. Our study reveals that i) symplastic transport is more efficient at low external ammonium supply; ii) when endodermal cells become sealed by the deposition of suberin lamellae, the expression of ammonium transporters shifts to cortical cells; and iii) apoplastic transport depends on a functional apoplastic barrier at the endodermis, favoring nitrogen (N) partitioning to shoots at high external ammonium. Introduction A major function of plant roots is the uptake and subsequent translocation of nutrients from soil to above-ground plant organs. To reach the shoot, nutrients need first to be transported radially across the root tissue before entering the xylem for root-to-shoot translocation. Once nutrients cross the plasma membrane of root epidermal cells, they enter the symplastic pathway, on which they move through the cytoplasmic continuum via plasmodesmata from cell to cell until they arrive in the xylem [1]. Nutrients may also enter the free space and cell walls of epidermal and cortical cells and move passively along the apoplastic route, which ultimately becomes blocked by the Casparian strip (CS) at the endodermis [2], where lignin depositions in anticlinal walls form a physical barrier to prevent an endodermal bypass [3]. This barrier prevents further inward movement in the apoplast. To progress further, nutrients must enter endodermal cells via membrane proteins, thereby completing the apoplastic transport pathway (ATP). As both pathways require a membrane transporter–mediated step, we refer here to the ATP and the symplastic transport pathway (STP). In basal root zones, endodermal cells become suberized, i.e., coated at the inner cell walls with aliphatic polymers, which form another apoplastic barrier, preventing access of nutrients to the plasma membrane [4,5]. Endodermal bypass, i.e., unhindered radial movement through cell w (...truncated)