Studies on the interaction of the symbiotic, root-colonizing endophytic fungus Piriformospora indica with the model plant Arabidopsis showed general growth-promoting effects. We propose to investigate the colonization of the fungus P.indica on host plants and the molecular mechanisms underlying the improved performance resulting from the interaction. In particular : 1) Impact on salt tolerance and K+ nutrition of rice and arabidopsis plants and the role of fungal and plant Na+ and K+ transporters. 2) Transcriptomic analysis of plant fungal interaction under different stress conditions and identification of key regulatory factors; and 3) Study of the transcriptional regulation of the plant ion transporters under the symbiosis. The proposed research will transfer the obtained data into relevant crops (Oryza sativa, potato and tomato) and used in proof-of-concept experiments.