The OsGEX2 Gene Promoter Confers Sperm Cell Expression in Transgenic Rice

Expression control elements (i.e., promoters) are crucial components required for the genetic engineering of plants, but relatively few well-characterized organ-specific promoters are available. We have characterized the rice Gamete Expressed 2 (OsGEX2) gene promoter in transgenic rice plants. The OsGEX2 gene (Os09g25650) is homologous to Arabidopsis GEX2, a gene that exhibits expression specifically in the gametic cells of Arabidopsis pollen. The OsGEX2 gene transcript was only detected in rice pollen (or mixed tissues containing pollen) and not found in other organs or tissues. Transgenic rice plants containing an OsGEX2 promoter fused to a GUSPlus reporter gene displayed cell type-specific beta-glucuronidase enzyme activity localized within the sperm cells of mature rice pollen. This expression pattern was clearly distinct from the uniform reporter gene activity observed in the pollen vegetative cell of transgenic rice carrying the rice Pollen Specific 1 promoter. The sperm cell reporter gene activity in OsGEX2-GUSPlus transgenic plants correlated well with the native gene transcript levels, suggesting that the cis-regulatory elements necessary for this specificity are present in the 1.9-kb OsGEX2 promoter fragment. The OsGEX2 promoter with its cell type-specific expression will be a useful tool for precisely controlling sperm cell gene expression in rice and potentially other cereal crop plants.