We analyze here, by noninvasive electrophysiology, local and systemic plant responses in the interaction of barley (Hordeum vulgare L.) with the root-colonizing basidiomycete Piriformospora indica. In the short term (seconds, minutes), a constant flow of P. indica chlamydospores along primary roots altered surface pH characteristics; whereas the root-hair zone transiently alkalized-a typical elicitor response-the elongation zone acidified, indicative of enhanced H(+) extrusion and plasma membrane H(+) ATPase stimulation. Eight to 10 min after treating roots with chlamydospores, the apoplastic pH of leaves began to acidify, which contrasts with observations of an alkalinization response to various stressors and microbe-associated molecular patterns (MAMPs). In the long term (days), plants with P. indica-colonized roots responded to inoculation with the leaf-pathogenic powdery mildew fungus Blumeria graminis f. sp. hordei with a leaf apoplastic pH increase of about 2, while the leaf apoplast of noncolonized barley responded to B. graminis f. sp. hordei merely with a pH increase of 0.8. The strong apoplastic pH response is reminiscent of B. graminis f. sp. hordei-triggered pH shifts in resistance gene-mediated resistant barley leaves or upon treatment with a chemical resistance inducer. In contrast, the MAMP N-acetylchito-octaose did not induce resistance to B. graminis f. sp. hordei and did not trigger the primed apoplastic pH shift. We speculate that the primed pH increase is indicative of and supports the potentiated systemic response to B. graminis f. sp. hordei-induced by P. indica in barley.