Lettuce (Lactuca sativa L.) is an annual autogamous diploid plant belonging to Asteraceae and amongst the top ten most profitable crop worldwide due to its nutritional value. Nevertheless, extensive cultivation of lettuce has led to major concerns, especially in fertiliser exploitation both in soil and extra soil system, which damages the ecosystem and has an increasing cost for the farmers. In this context, arbuscular mycorrhizal (AM) symbioses could help reducing the impact of fertilisers, improving plant nutrition, particularly phosphorus (P), and contributing to growing healthier plants for human consumption. In our work, we compared lettuce plants (cv. Salinas) grown in soilless culture with optimal P (P-PO4-) concentration in the nutrient solution (HiP/-AM), with sub-optimal P nutrition (LoP/-AM) and, finally, with plants grown with sub-optimal P concentration and inoculated with the AM fungus Funneliformis mossae before transplanting (LoP/+AM). Overall, in comparison with LoP/-AM plants, 3,057 genes were differentially regulated by mycorrhizal symbiosis (-LoP/+AM) and 2,606 genes were induced by optimal phosphate nutrition (HiP/-AM). Interestingly, the pattern of differentially expressed genes for both effects showed major differences suggesting an extensive regulation of genes related to photosynthesis, solute transport, metabolism of phytohormones, redox homeostasis, and transcriptional regulation. Concerning metabolic pathways, mycorrhizal plants boosted the activation of genes involved in phenylpropanoids, carotenoids, and vitamins production. In conclusion, transcriptomic data strongly support the potential of AM symbionts to act as biofertilisers for lettuce soilless cultivation with beneficial effects on both plant growth and leaf content of health promoting phytochemicals through genetic pathways largely different from those activated in plants grown with optimal P supply.