We isolated and characterized two rice nuclear genes, OsSIG2A and OsSIG2B, encoding the putative sigma-factor of the plastid RNA polymerase. Deduced protein sequences predicted a plastid-localizing signal in the N-terminus and subsequent polypeptides similar to known SIG2 proteins. Gene expression analysis revealed that the OsSIG2A transcript is more abundant than the OsSIG2B transcript in all tissues tested and that both rice SIG2s are expressed from earlier stages of leaf development than that in the case of OsSIG1. These results indicate differential expression of SIG genes in leaf morphogenesis, suggesting the existence of tissue- and stage-specific functions of SIG proteins for transcriptional regulation of chloroplast genes in plant development.