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      Strigolactones Stimulate Arbuscular Mycorrhizal Fungi by Activating Mitochondria


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          The association of arbuscular mycorrhizal (AM) fungi with plant roots is the oldest and ecologically most important symbiotic relationship between higher plants and microorganisms, yet the mechanism by which these fungi detect the presence of a plant host is poorly understood. Previous studies have shown that roots secrete a branching factor (BF) that strongly stimulates branching of hyphae during germination of the spores of AM fungi. In the BF of Lotus, a strigolactone was found to be the active molecule. Strigolactones are known as germination stimulants of the parasitic plants Striga and Orobanche. In this paper, we show that the BF of a monocotyledonous plant, Sorghum, also contains a strigolactone. Strigolactones strongly and rapidly stimulated cell proliferation of the AM fungus Gigaspora rosea at concentrations as low as 10 −13 M. This effect was not found with other sesquiterperne lactones known as germination stimulants of parasitic weeds. Within 1 h of treatment, the density of mitochondria in the fungal cells increased, and their shape and movement changed dramatically. Strigolactones stimulated spore germination of two other phylogenetically distant AM fungi, Glomus intraradices and Gl. claroideum. This was also associated with a rapid increase of mitochondrial density and respiration as shown with Gl. intraradices. We conclude that strigolactones are important rhizospheric plant signals involved in stimulating both the pre-symbiotic growth of AM fungi and the germination of parasitic plants.


          Strigolactones from Sorghum promote growth of multiple arbuscular mycorrhizal fungi, underscoring the importance of these molecules in the symbiotic relationship between plant roots and the rhizosphere.

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          Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi.

          Arbuscular mycorrhizal (AM) fungi form mutualistic, symbiotic associations with the roots of more than 80% of land plants. The fungi are incapable of completing their life cycle in the absence of a host root. Their spores can germinate and grow in the absence of a host, but their hyphal growth is very limited. Little is known about the molecular mechanisms that govern signalling and recognition between AM fungi and their host plants. In one of the first stages of host recognition, the hyphae of AM fungi show extensive branching in the vicinity of host roots before formation of the appressorium, the structure used to penetrate the plant root. Host roots are known to release signalling molecules that trigger hyphal branching, but these branching factors have not been isolated. Here we have isolated a branching factor from the root exudates of Lotus japonicus and used spectroscopic analysis and chemical synthesis to identify it as a strigolactone, 5-deoxy-strigol. Strigolactones are a group of sesquiterpene lactones, previously isolated as seed-germination stimulants for the parasitic weeds Striga and Orobanche. The natural strigolactones 5-deoxy-strigol, sorgolactone and strigol, and a synthetic analogue, GR24, induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora margarita at very low concentrations.
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            Root exudation and rhizosphere biology.

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              The strigolactone germination stimulants of the plant-parasitic Striga and Orobanche spp. are derived from the carotenoid pathway.

              The seeds of parasitic plants of the genera Striga and Orobanche will only germinate after induction by a chemical signal exuded from the roots of their host. Up to now, several of these germination stimulants have been isolated and identified in the root exudates of a series of host plants of both Orobanche and Striga spp. In most cases, the compounds were shown to be isoprenoid and belong to one chemical class, collectively called the strigolactones, and suggested by many authors to be sesquiterpene lactones. However, this classification was never proven; hence, the biosynthetic pathways of the germination stimulants are unknown. We have used carotenoid mutants of maize (Zea mays) and inhibitors of isoprenoid pathways on maize, cowpea (Vigna unguiculata), and sorghum (Sorghum bicolor) and assessed the effects on the root exudate-induced germination of Striga hermonthica and Orobanche crenata. Here, we show that for these three host and two parasitic plant species, the strigolactone germination stimulants are derived from the carotenoid pathway. Furthermore, we hypothesize how the germination stimulants are formed. We also discuss this finding as an explanation for some phenomena that have been observed for the host-parasitic plant interaction, such as the effect of mycorrhiza on S. hermonthica infestation.

                Author and article information

                Role: Academic Editor
                PLoS Biol
                PLoS Biology
                Public Library of Science (San Francisco, USA )
                July 2006
                27 June 2006
                : 4
                : 7
                : e226
                [1] 1UMR 5546, Pôle de Biotechnologies Végétales, Castanet-Tolosan, France
                [2] 2Laboratoire Biotechnologie-Bioprocédés, L'Institut National des Sciences Appliquées, Toulouse, France
                [3] 3IFR40, Pôle de Biotechnologies Végétales, Castanet-Tolosan, France
                [4] 4UMR 5546, Centre National de la Recherche Scientifique–Université Toulouse III, Pôle de Biotechnologies Végétales, Castanet-Tolosan, France
                The Salk Institute for Biological Studies United States of America
                Copyright: © 2006 Besserer 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.
                : 14 October 2005
                : 4 May 2006
                Research Article
                Plant Science
                Yeast and Fungi

                Life sciences
                Life sciences


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