Endoglucanase El from Acidothermus cellulolyticus was expressed cytosolically under control of the cauliflower mosaic virus 35S promoter in transgenic duckweed, Lemma minor 8627 without any obvious observable phenotypic effects on morphology or rate of growth. The recombinant enzyme co-migrated with the purified catalytic domain fraction of the native E1 protein on western blot analysis, revealing that the cellulose-binding domain was cleaved near or in the linker region. The duckweed-expressed enzyme was biologically active and the expression level was up to 0.24% of total soluble protein. The endoglucanase activity with carboxymethylcellulose averaged 0.2 units mg protein I extracted from fresh duckweed. The optimal temperature and pH for El enzyme activity were about 80 degrees C and pH 5, respectively. While extraction with HEPES (N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid]) buffer (pH 8) resulted in the highest recovery of total soluble proteins and El enzyme, extraction with citrate buffer (pH 4.8) at 65 degrees C enriched relative amounts of El enzyme in the extract. This study demonstrates that duckweed may offer new options for the expression of cellulolytic enzymes in transgenic plants. (C) 2006 Elsevier Ltd. All rights reserved.