A Hyphal‐Mycorrhizal Framework for Interpreting Sulfur Transport and Assimilation‐Related Transcriptional Responses Between AMF Extraradical Hyphae and Mycorrhizal Roots in Soybean

ABSTRACT

Sulfur (S) is an essential macronutrient that plays a critical role in plant growth and development. Under intensive agricultural management, declining sulfur availability in cold‐region black soils has become a major constraint on sulfur acquisition in soybean. Although arbuscular mycorrhizal fungi (AMF) are known to enhance sulfur uptake through symbiosis, the molecular mechanisms underlying AMF‐mediated sulfur transport and assimilation remain poorly understood. Here, we investigated molecular responses associated with AMF‐mediated sulfur acquisition, transport and assimilation in soybean, with particular emphasis on coordinated processes between extraradical hyphae and mycorrhizal roots. A compartmented pot culture system was employed to assess biological traits and transcriptomic profiles of Funneliformis mosseae ( F. mosseae ) hyphae and soybean mycorrhizal roots. In the extraradical hyphae compartment, sulfur supply strongly induced the expression of F. mosseae SULTR2;1 and SULTR4;1 homologs, as well as assimilation‐related genes sat and cysH . In the mycorrhizal root compartment, key sulfur transporter genes ( SULTR3;5 , SULTR4;2 ) and assimilation genes ( cysC , cysH ) were significantly upregulated. Integrated analyses revealed coordinated expression patterns of genes related to sulfur transport and assimilation between the F. mosseae extraradical hyphal compartment and the mycorrhizal root compartment, which were associated with a 65.4% increase in soybean root sulfur content. Consistently, the abundances of key sulfur assimilation enzymes (ATPS, APR and APSK) increased in mycorrhizal roots, accompanied by enhanced sulfur assimilation responses and cysteine accumulation. Collectively, this study provides insights into the molecular basis of AMF‐mediated sulfur acquisition, transport and assimilation, and highlights the coordinated responses between fungal hyphae and mycorrhizal roots within the hyphal‐mycorrhizal symbiotic system, thereby providing a theoretical basis for improving sulfur nutrition in soybean.