An in vitro system with micropropagated oaks (Quercus robur) and the ectomycorrhizal fungus Piloderma croceum, which is characterized by a delayed mycorrhiza formation, was used to identify plant transcripts upregulated in the premycorrhizal phase.

Complementary DNA (cDNA) populations of uninoculated roots and fungal mycelium were subtracted from a cDNA population of inoculated roots. Differential expression was confirmed by reverse Northern and 50 clones for different polypeptides were found to be up-regulated. Twenty-nine clones were investigated in more detail.

For approximately half of the cDNA fragments no homologies could be identified in databases. The residual fragments code for polypeptides with homologies to known proteins involved in signal perception and transmission, stress responses, metabolism and growth.

Since many of the identified genes have not yet been described in the context of symbiotic events, their potential roles during early phases of the recognition process are discussed.