Dependence of arbuscular-mycorrhizal fungi on their plant host for palmitic acid synthesis

Abstract

Lipids are the major form of carbon storage in arbuscular-mycorrhizal fungi. We studied fatty acid synthesis by Glomus intraradices and Gigaspora rosea. [(14)C]Acetate and [(14)C]sucrose were incorporated into a synthetic culture medium to test fatty acid synthetic ability in germinating spores (G. intraradices and G. rosea), mycorrhized carrot roots, and extraradical fungal mycelium (G. intraradices). Germinating spores and extraradical hyphae could not synthesize 16-carbon fatty acids but could elongate and desaturate fatty acids already present. The growth stimulation of germinating spores by root exudates did not stimulate fatty acid synthesis. 16-Carbon fatty acids (16:0 and 16:1) were synthesized only by the fungi in the mycorrhized roots. Our data strongly suggest that the fatty acid synthase activity of arbuscular-mycorrhizal fungi is expressed exclusively in the intraradical mycelium and indicate that fatty acid metabolism may play a major role in the obligate biotrophism of arbuscular-mycorrhizal fungi.

FIG. 1.

Experimental system for labeling mycorrhized roots or extraradical hyphae with [1-¹⁴C]acetate or [U-¹⁴C]sucrose. Mycorrhized roots are grown in one compartment (root compartment), while the majority of the extraradical fungal mycelium grows in the second compartment (fungal compartment).

FIG. 2.

HPLC chromatograms (A₂₄₂) and [1-¹⁴C]acetate-labeled radioactivity measurements (CPM) of fatty acids from germinating spores. G. intraradices (A) and G. rosea (B) are shown. Solid lines denote the fatty acid profile, and dotted lines denote the radioactivity profile. The experiment was repeated twice and gave similar results.

FIG. 3.

HPLC chromatograms (A₂₄₂) and [1-¹⁴C]acetate-labeled radioactivity measurements (CPM) of fatty acids in compartmented petri dishes. (A) Fatty acid analysis of the fungal compartment (labeling in the fungal compartment). (B) Fatty acid analysis of the fungal compartment (labeling in root compartment). (C) Fatty acid analysis of the root compartment (labeling in the root compartment). (D) Fatty acid analysis of noncolonized roots (labeling in root compartment). Solid lines denote the fatty acid profile, and dotted lines denote the radioactivity profile. The experiment was repeated three times and gave similar results.

FIG. 4.

HPLC chromatograms (A₂₄₂) and radioactivity measurements (CPM) of fatty acids labeled with [U-¹⁴C]sucrose in the root compartment. (A) Fungal compartment (G. intraradices); (B) root compartment. Solid lines denote the fatty acid profile, and dotted lines denote the radioactivity profile. The experiment was repeated three times and gave similar results.

FIG. 5.

Proposed pathway for the synthesis of fatty acids by the extraradical and intraradical mycelium of arbuscular-mycorrhizal fungi when supplied with acetate or sucrose. Fatty acid synthase activity never occurs in the extraradical hyphae.