Insights into the Mechanism Underlying the Symbiosis between Seagrass and a Lulworthiaceae Fungus.

Various fungi have been identified in seagrass compartments; however, the exact nature of their interactions with these marine flowering plants remains largely uncharacterized. The partnership between seagrasses and Lulworthiaceae fungi represents a typical and compelling example of such an underwater association. Here, we combine UHPLC-MS/MS, transcriptomics, and plant growth assays to validate the putative symbiosis between the seagrass Halophila ovalis and the Lulworthiaceae fungus Halophilomyces hongkongensis, and elucidate the underlying mechanism of this relationship. We confirmed that H. hongkongensis produces the phytohormone indole-3-acetic acid (IAA). Through transcriptomic analysis, we proposed its IAA biosynthesis pathways. All identified IAA biosynthesis genes were upregulated in seagrass roots/rhizomes compared to rhizosphere sediments, with the tryptophan aminotransferase gene (indole-3-pyruvic acid pathway) exhibiting a significant increase. Furthermore, H. hongkongensis was confirmed as a plant growth-promoting fungus; its culture filtrates promoted Arabidopsis thaliana shoot and lateral root/root hair growth, an effect strongly correlated with IAA production and highly likely in seagrass. Beyond IAA-specific analyses, its upregulated genes were significantly enriched in pathways such as tryptophan metabolism, starch/sucrose metabolism, and DNA replication. Collectively, these results indicate that H. hongkongensis establishes a growth-promoting symbiosis with H. ovalis by upregulating its IAA biosynthesis genes and secreting IAA; in return, the host provides carbohydrates that sustain fungal metabolism and support active DNA replication. This study provides the first mechanistic verification of a seagrass-Lulworthiaceae symbiosis, significantly advancing our understanding of marine plant-fungal interactions. It also demonstrates the first IAA-linked plant growth-promoting capacity of a member from the cryptic marine fungal family Lulworthiaceae.