Piriformospora indica, a root-colonizing endophytic fungus of Sebacinales, promotes plant growth and confers resistance against biotic and abiotic stress. The fungus strongly colonizes the roots of Chinese cabbage, promotes root and shoot growth, and promotes lateral root formation. When colonized plants were exposed to polyethylene glycol to mimic drought stress, the activities of peroxidases, catalases and superoxide dismutases in the leaves were upregulated within 24h. The fungus retarded the drought-induced decline in the photosynthetic efficiency and the degradation of chlorophylls and thylakoid proteins. The expression levels of the drought-related genes DREB2A, CBL1, ANAC072 and RD29A were upregulated in the drought-stressed leaves of colonized plants. Furthermore, the CAS mRNA level for the thylakoid membrane associated Ca(2+)-sensing regulator and the amount of the CAS protein increased. We conclude that antioxidant enzyme activities, drought-related genes and CAS are three crucial targets of P. indica in Chinese cabbage leaves during the establishment of drought tolerance. P. indica-colonized Chinese cabbage provides a good model system to study root-to-shoot communication.