Transcriptomic Response of Medicago lupulina MlS-1 Roots to Arbuscular Mycorrhizal Fungi with Contrasting Symbiotic Efficiency

Arbuscular mycorrhiza (AM) plays a crucial role in agroecosystems. The molecular mechanisms by which host plants regulate symbiosis efficiency remain poorly understood. This study compared root transcriptomic responses of mycotrophic Medicago lupulina MlS-1 to effective Rhizophagus irregularis RCAM00320 versus ineffective Glomus sp. 129.1Te under phosphorus deficiency. RNA-Seq analysis revealed that the effective symbiont induced a large-scale, coordinated transcriptional reprogramming, characterized by targeted upregulation of symbiotic phosphate and ammonium transporters, strong induction of genes for lipid mobilization (the triacylglycerol lipase gene LIP) and the structural protein gene (Oleosin) for lipid droplet stabilization, to support fungal nutrition, and a strategy of controlled cell wall remodeling that minimized the release of immunogenic damage-associated molecular patterns. In contrast, the ineffective strain triggered only a fragmented, low-amplitude stress response, accompanied by hormonal deregulation and activation of defense-related genes, suggesting plant mechanisms to restrict resources to non-functional symbionts. The identified gene set (potential AM efficiency markers) has candidate markers with potential diagnostic value for screening AM inoculants and breeding mycotrophic legumes.