Carbon uptake and the metabolism and transport of lipids in an arbuscular mycorrhiza

Abstract

Both the plant and the fungus benefit nutritionally in the arbuscular mycorrhizal symbiosis: The host plant enjoys enhanced mineral uptake and the fungus receives fixed carbon. In this exchange the uptake, metabolism, and translocation of carbon by the fungal partner are poorly understood. We therefore analyzed the fate of isotopically labeled substrates in an arbuscular mycorrhiza (in vitro cultures of Ri T-DNA-transformed carrot [Daucus carota] roots colonized by Glomus intraradices) using nuclear magnetic resonance spectroscopy. Labeling patterns observed in lipids and carbohydrates after substrates were supplied to the mycorrhizal roots or the extraradical mycelium indicated that: (a) 13C-labeled glucose and fructose (but not mannitol or succinate) are effectively taken up by the fungus within the root and are metabolized to yield labeled carbohydrates and lipids; (b) the extraradical mycelium does not use exogenous sugars for catabolism, storage, or transfer to the host; (c) the fungus converts sugars taken up in the root compartment into lipids that are then translocated to the extraradical mycelium (there being little or no lipid synthesis in the external mycelium); and (d) hexose in fungal tissue undergoes substantially higher fluxes through an oxidative pentose phosphate pathway than does hexose in the host plant.

Figure 1

Method of labeling and analysis of metabolism and transport in carrot roots colonized by G. intraradices. IPA, Isopropyl alcohol. Root tissue was 20 times the amount of fungal tissue found on the fungal side; the fungal tissue on the root side represents an insignificant amount of fungal hyphae and spores. The hyphae within the root represent only 1% to 2% of the tissue.

Figure 7

Proposed model for major fluxes of carbon in the fungus in the symbiotic state of AM mycorrhizae.

Figure 2

A, In vivo ¹³C-NMR spectrum of extraradical mycelium of G. intraradices prelabeled with ¹³C1-Glc from the root compartment. B, Unlabeled fungal tissue of the same age as that shown in A. C, ¹³C-NMR spectrum of isopropyl alcohol extract of G. intraradices-colonized root tissues after labeling with ¹³C1-Glc in the root compartment. D, ¹³C-NMR spectrum of isopropyl alcohol extract of unlabeled G. intraradices-colonized root tissues.

Figure 3

Glyceryl region of the ¹³C-NMR spectra of isopropyl alcohol extracts of external mycelium after ¹³C1-Glc labeling from the root side of the Petri plate (A), of external mycelium after ¹³C1-Glc labeling from the fungal compartment (B), of mycorrhizal root tissue after ¹³C1-Glc labeling from the root side (C), of mycorrhizal roots after ¹³C1-Glc labeling from the fungal compartment (D), and of unlabeled lipids from the fungal compartment of unlabeled plates showing integrated areas (controls) (E).

Figure 4

Double-bond regions of ¹³C-NMR spectra of isopropyl alcohol extracts of unlabeled nonmycorrhizal (NM) root tissue (A); unlabeled mycorrhizal (M) root tissue (B); unlabeled extraradical mycelium (C); ¹³C-labeled nonmycorrhizal root tissue (D); ¹³C-labeled mycorrhizal root tissue (E); and ¹³C-labeled extraradical mycelia (F).

Figure 5

²H-NMR spectra of isopropyl alcohol extracted lipids from fungal tissue 2 weeks after the introduction of ²H₂O into the fungal (A) or the root (B) compartment. Resonance at 7.24 ppm is the internal reference of natural-abundance ²H in the solvent (CHCl₃). The ²H lipid resonances were observed at 0.8 to 2.4 ppm.

Figure 6

Glyceryl portion of the ¹³C-NMR spectra of isopropyl alcohol extracts of extraradical mycelia labeled on the root side of the plate with ¹³C1,2-Glc (A); mycorrhizal roots labeled on the root side of the plate with ¹³C1,2-Glc (B); mycorrhizal roots labeled on the fungal side of the plate with ¹³C1,2-Glc (C); and extraradical mycelia labeled on the fungal side of the plate with ¹³C1,2-Glc (D).