Plants are promising candidates as bioreactors for the production of oral recombinant proteins in the biopharmaceutical industry. As an initial step toward provision of an oral vaccine against the severe acute respiratory syndrome coronavirus (SARS-CoV), we have expressed a partial spike (S) protein of SARS-CoV in the cytosol of nuclear-transformed plants and in the chloroplasts of plastid-transformed plants. In the construction of both nuclear and plastid transformation vectors, a 2-kilobase nucleotide sequence encoding amino acids 1-658 of the SARS-CoV spike protein (S1) was modified with nucleotide changes, but not amino acid changes, to optimize codon usage for expression in plants. To investigate the subcellular localization of S1 during transient expression in tobacco leaves, a translational fusion consisting of S1 and the green fluorescent protein (GFP) was generated. Following agroinfiltration of tobacco leaves, analysis by laser confocal scanning microscopy revealed that the S1:GFP fusion protein was localized to the cytosol. In stable transgenic tobacco plants and lettuce plants generated by Agrobacterium-mediated transformation, tobacco and lettuce leaves were observed to express the S1 at high levels from the Cauliflower Mosaic Virus 35S promoter with Northern blot analysis. When the S1 was expressed in transplastomic tobacco, S1 messenger RNA and its corresponding protein were detected on Northern and Western blot analyses, respectively. Our results demonstrate the feasibility of producing S1 in nuclear- and chloroplast-transformed plants, indicating its potential in subsequent development of a plant-derived and safe oral recombinant subunit vaccine against the SARS-CoV in edible plants.