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.