Plasma-activated water promotes and finely tunes arbuscular mycorrhizal symbiosis in Lotus japonicus

Background: Plasma-activated water (PAW) is a recently developed cutting-edge technology that is increasingly gaining interest for its applications in medicine, food industry and agriculture. In plant biology, PAW has been shown to enhance seed germination, plant growth, and plant resilience against biotic and abiotic stresses. Despite increasing knowledge of the beneficial effects exerted by PAW on plants, little information is currently available about how this emerging technology may affect mutualistic plant-microbe interactions in the rhizosphere. Results: In this work we investigated the impact of irrigation with PAW, generated by a plasma torch, on arbuscular mycorrhizal (AM) symbiosis. Roots of the model legume Lotus japonicus expressing the bioluminescent Ca2+ reporter aequorin responded to treatment with PAW 5' (obtained by 5 min water exposure to plasma) with the immediate induction of cytosolic and nuclear Ca2+ signals, indicating that Ca2+-mediated signalling is one of the earliest cellular responses to PAW. The long-lasting elevations in intracellular Ca2+ levels were not found to alter cell viability. Quantitative analyses of AM fungal accommodation in the host plant roots along with phosphate accumulation in leaves, as well as chemical analysis of N, C, S in shoots, showed that treatments with PAW play a modulatory role on plant AM symbiotic performance, in a manner dependent on the time interval of water exposure to the plasma and on the duration of plant treatment with PAW. In particular, irrigation with PAW 5' increased fungal colonization after 4 weeks, leading to a significant increase in leaf phosphate content after 7 weeks. Conclusions: Our findings reveal that PAW enhances AM symbiosis by facilitating early fungal accommodation in roots and subsequently increasing phosphate content in leaves at later stages. A better understanding of the mechanisms underlying the effects of PAW on the plant microbiome may drive research towards a fine-tuning of this novel green technology to maximize its beneficial effects in the context of a more sustainable agriculture.