ABSTRACT
Ectomycorrhiza development alters gene expression in the fungal and plant symbionts. The identification of a large number of genes expressed exclusively or predominantly in the symbiosis will contribute greatly to the understanding of the development of the ectomycorrhizal symbiosis. We have constructed a cDNA library of 4-day-old Eucalyptus globulus-Pisolithus tinctorius ectomycorrhiza and sequenced 850 cDNAs cloned randomly or obtained through suppression subtractive hybridization (SSH). Based on the absence of a database match, 43% of the ectomycorrhiza ESTs are coding for novel genes. At the developmental stage analysed (fungal sheath formation), the majority of the identified sequences represented 'housekeeping' proteins, i.e. proteins involved in gene/protein expression, cell-wall proteins, metabolic enzymes, and components of signalling systems. We screened arrayed cDNAs to identify symbiosis-regulated genes by using differential hybridization. Comparisons of signals from free-living partners and symbiotic tissues revealed significant differences in expression levels (differential expression ratio >2.5) for 17% of the genes analysed. No ectomycorrhiza-specific gene was detected. The results successfully demonstrate the use of the cDNA array and SSH systems as general approaches for dissecting symbiosis development, and provide the first global picture of the cellular functions operating in ectomycorrhiza.