BACKGROUND: Commercial conversion of lignocellulosic biomass to fermentable sugars for biofuels and chemical byproducts uses relatively expensive bulk production of biologically active cellulase enzymes, which could alternatively be achieved by using solar energy for direct production of these enzymes within feedstock crop cellulosic biomass. RESULTS: The Acidothermus cellulolyticus endo-cellulase El has been produced in transgenic maize plants. This heterologous enzyme was specifically targeted for accumulation into two sub-cellular compartments, endoplasmic reticulum (ER) or mitochondria of plant leaves and stalks. Furthermore, successful use of this maize-produced heterologous cellulase in converting cellulose into fermentable sugars for biofuels, has been confirmed. CONCLUSIONS: Green-specific expression of cellulases in maize plants can avoid public controversies associated with production of transgene products in maize seeds and/or pollen. Sub-cellular targeting of cellulases may result in better expression of transgene products because these compartments, specially ER, normally contain molecular chaperones that enhance protein folding and there the biological activity. Also, using solar energy to produce cellulases within crop cellulosic biomass can replace the costly process of cellulase production in microbial bioreactors, and therefore, save costs. (C) 2008 Society of Chemical industry