Plants interact with a wide variety of fungi in a mutualistic, parasitic or neutral way. The associations formed depend on the exchange of nutrients and signalling molecules between the partners. The latter include a diverse set of protein classes, of which some have been demonstrated to be functionally implicated in defence, nutrient uptake and establishing a symbiotic relationship. Here, we have analysed the secretomes of the mutualistic, root-endophytic fungus Piriformospora indica and Arabidopsis thaliana when cultivated alone or in a co-culture. More than one hundred proteins were identified as differentially secreted, including proteins associated with growth, development, abiotic and biotic stress response and mucilage. One protein found in the co-culture is PLAT1 (Polycystin, Lipoxygenase, Alpha-toxin and Triacylglycerol lipases). PLAT1 has not been associated with plant-fungal-interaction before and is known to play a role in abiotic stress responses. It co-localizes with Brassicaceae-specific endoplasmic reticulum bodies (ER bodies), which are involved in the formation of the defence compound scopolin. We observed degraded ER bodies in P. indica infected Arabidopsis roots and plat1 mutants were stronger colonised than wild tyype (WT) plants. Furthermore, PLAT1 RNA (ribonucleic acid) and scopolin levels were detected to be regulated by the fungus in a stage-specific manner.