The thermostable endo-1,4-beta-glucanase (E1) from Acidothermus cellulolyticus, is a useful enzyme for commercial hydrolysis of cellulose into glucose. A codon-optimized synthetic gene encoding this enzyme was transformed into rice (Oryza sativa L ssp. japonica) under the control of the rice seed storage protein Gt1 promoter. The transgenic line C19 was identified as the one with the highest endoglucanase activity among the total of 36 independent transgenic lines obtained. The cellulase activity in the C19 seeds was estimated at about 830 U/g of dried seeds using CMC as substrate. The enzymes produced in the seeds had an optimum pH of 5.0 and optimum temperature of 80 degrees C, which is similar to the enzymes produced by the native bacterium host. This study demonstrates that the transgenic rice seeds could be used as a bioreactor for production of enzymes for cellulosic biomass conversion. (C) 2012 Elsevier Inc. All rights reserved.