Enhanced production of reducing sugars from transgenic rice expressing exo-glucanase under the control of a senescence-inducible promoter

We generated transgenic rice plants that express EXG1 exo-glucanase under the control of a senescence-inducible promoter. When a GUS coding sequence was connected to a promoter region of STAY GREEN (SGR) gene of rice and introduced into rice, GUS activity was specifically observed along with senescence. When an EXG1 cDNA was connected to the SGR promoter and introduced into rice, higher cellulase activities were detected after senescence. The EXG1 transgenic plants showed enhanced enzymatic saccharification efficiencies after senescence, but no significant difference of saccharification efficiencies was observed before senescence. The saccharification efficiencies were correlated with the cellulase activities in the transgenic plants. The EXG1 transgenic plants showed neither morphological abnormality nor sterility, both of which were observed when EXG1 was constitutively overexpressed. These results indicate that expression of cell wall degrading enzymes such as cellulase by a senescence-inducible promoter is one of the ways to enhance the saccharification ability of cellulosic biomass without affecting plant growth for efficient production of biofuels.