Sulfur Transfer through an Arbuscular Mycorrhiza

Despite the importance of sulfur (S) for plant nutrition, the role of the arbuscular mycorrhizal (AM) symbiosis in S uptake has received little attention. To address this issue, S-35-labeling experiments were performed on mycorrhizas of transformed carrot (Daucus carota) roots and Glomus intraradices grown monoxenically on bicompartmental petri dishes. The uptake and transfer of (SO42-)-S-35 by the fungus and resulting S-35 partitioning into different metabolic pools in the host roots was analyzed when altering the sulfate concentration available to roots and supplying the fungal compartment with cysteine (Cys), methionine (Met), or glutathione. Additionally, the uptake, transfer, and partitioning of S-35 from the reduced S sources [S-35] Cys and [S-35] Met was determined. Sulfate was taken up by the fungus and transferred to mycorrhizal roots, increasing root S contents by 25% in a moderate (not growth-limiting) concentration of sulfate. High sulfate levels in the mycorrhizal root compartment halved the uptake of (SO42-)-S-35 from the fungal compartment. The addition of 1 mM Met, Cys, or glutathione to the fungal compartment reduced the transfer of sulfate by 26%, 45%, and 80%, respectively, over 1 month. Similar quantities of S-35 were transferred to mycorrhizal roots whether (SO42-)-S-35, [S-35] Cys, or [S-35] Met was supplied in the fungal compartment. Fungal transcripts for putative S assimilatory genes were identified, indicating the presence of the trans-sulfuration pathway. The suppression of fungal sulfate transfer in the presence of Cys coincided with a reduction in putative sulfate permease and not sulfate adenylyltransferase transcripts, suggesting a role for fungal transcriptional regulation in S transfer to the host. Atestable model is proposed describing root S acquisition through the AM symbiosis.