Accumulation of a thermostable endo-1,4-β-D-glucanase in the apoplast of Arabidopsis thaliana leaves

Plant biomass, the most abundant renewable resource on earth, is a potential source of fermentable sugars for production of alternative transportation fuels and other chemicals. Bioconversion of plant biomass to fermentable glucose involves enzymatic hydrolysis of cellulose, a major polysaccharide constituent. Because commercially available microbial cellulases are prohibitively expensive for bioethanol processes, we have investigated the feasibility of producing these enzymes in plants as a low-cost, potentially high-volume alternative to traditional production methods. We have successfully expressed the catalytic domain of a thermostable (T-opt=81 degrees C) endo-1,4-beta-D-glucanase from the eubacterium, Acidothermus cellulolyticus, in the apoplast of tobacco BY-2 suspension cells and leaves of Arabidopsis thaliana plants. The apoplast-targeting cassette designed for this work consists of the cauliflower mosaic virus 35S promoter, the tobacco mosaic virus Omega translational enhancer, the sequence encoding the tobacco Pr1a signal peptide, and the polyadenylation signal of nopaline synthase. Recombinant E1 catalytic domain was targeted to the ER by the signal peptide and secreted into the apoplast via the default pathway. Secretion of the enzyme did not detectably affect the growth rate of transgenic BY-2 cells, although the protein was enzymatically active at elevated temperatures. Similarly, transgenic plants exhibited no abnormal phenotypes correlating with expression of the enzyme. Close agreement between independent immunochemical and activity-based assays indicates that the enzyme accumulated to concentrations up to 26% of the total soluble protein in leaves of primary A. thaliana transformants. The amount of functional endoglucanase produced illustrates that plants can accumulate very large quantities of enzyme for commercial biomass conversion.