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Front. Plant Sci. | doi: 10.3389/fpls.2018.00255

Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family

  • 1Cell & Metabolic Biology, Leibniz-Institut für Pflanzenbiochemie (IPB), Germany
  • 2Helmholtz-Zentrum für Umweltforschung (UFZ), Germany
  • 3Institut für Biologie II, Fakultät für Biologie, Albert-Ludwigs-Universität Freiburg, Germany
  • 4Wageningen Plant Research, WUR, Netherlands

Strigolactones are apocarotenoid phytohormones synthesized from carotenoid precursors. They are produced most abundantly in roots for exudation into the rhizosphere to cope with mineral nutrient starvation through support of root symbionts. Abscisic acid (ABA) is another apocarotenoid phytohormone synthesized in roots, which is involved in responses to abiotic stress. Typically low carotenoid levels in roots raise the issue of precursor supply for the biosynthesis of these two apocarotenoids in this organ. Increased ABA levels upon abiotic stress in Poaceae roots are known to be supported by a particular isoform of PHYTOENE SYNTHASE (PSY), catalyzing the rate-limiting step in carotenogenesis. Here we report on novel PSY3 isogenes from Medicago truncatula (MtPSY3) and Solanum lycopersicum (SlPSY3) strongly expressed exclusively upon root interaction with symbiotic arbuscular mycorrhizal (AM) fungi and moderately in response to phosphate starvation. They belong to a widespread clade of conserved PSYs restricted to dicots (dPSY3) distinct from the Poaceae-PSY3s involved in ABA formation. An ancient origin of dPSY3s and a potential co-evolution with the AM symbiosis is discussed in the context of PSY evolution. Knockdown of MtPSY3 in hairy roots of M. truncatula strongly reduced strigolactone and AM-induced C13 alpha-ionol/C14 mycorradicin apocarotenoids. Inhibition of the reaction subsequent to phytoene synthesis revealed strongly elevated levels of phytoene indicating induced flux through the carotenoid pathway in roots upon mycorrhization. dPSY3 isogenes are coregulated with upstream isogenes and downstream carotenoid cleavage steps towards strigolactones (D27, CCD7, CCD8) suggesting a combined carotenoid/apocarotenoid pathway, which provides “just in time”-delivery of precursors for apocarotenoid formation.

Keywords: Carotenoids, apocarotenoids, strigolactones, Symbiosis, arbuscular mycorrhiza, Medicago truncatula, Solanum lycopersicum

Received: 27 Nov 2017; Accepted: 12 Feb 2018.

Edited by:

Joachim Kopka, Max Planck Institute of Molecular Plant Physiology (MPG), Germany

Reviewed by:

Jonathan Gershenzon, Max Planck Institute for Chemical Ecology (MPG), Germany
Eloise Foo, University of Tasmania, Australia  

Copyright: © 2018 Stauder, Welsch, Camagna, Kohlen, Balcke, Tissier and Walter. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Michael H. Walter, Leibniz-Institut für Pflanzenbiochemie (IPB), Cell & Metabolic Biology, Weinberg 3, Halle (Saale), D - 06120, Germany, mhwalter@ipb-halle.de