An economical method for the conversion of lignocellulosic biomass is to use plants as bioreactors for cellulases production. Two bacterial thermostable cellulases (E2 and E3) and a E3-E2 fusion form were expressed in tobacco, driven by a double 35S promoter and 5' TEV-UTL. The enzymes were targeted to the apoplast and cytosol via 5' signal peptides and 3' retention signal peptides, respectively, and all showed functional activities. All transgenic plants exhibited normal growth compared to wild type. Transgenic plants that expressed apoplast-localized E2 had the highest average activity, about 1.5 and 3 times higher than those expressed ER-localized and cytosolic E2, respectively. Effect of subcellular compartment localization was due primarily to post-transcriptional modification, since mRNA abundances were similar despite the range of cellulase activities obtained. The recombinant cellulases exhibited good thermostability below 65A degrees C. After storing for 3 days at -20 and 28A degrees C, the enzymes lost nearly 20 and 80% of activity, respectively. The results suggested a potential application for heterologous expression of cellulases in plant for biomass conversion.