The promoter for the maize C-4 pyruvate,orthophosphate dikinase gene directs cell- and tissue-specific transcription in transgenic maize plants

The pyruvate, orthophosphate dikinase (PPDK) gene coding the chloroplast en zyme involved in C-4 photosynthesis has a dual promoter system, The first p romoter is responsible for the transcription of a larger transcript and its product is targeted to the chloroplast (hence, it is designated as C4Pdk p romoter) while the second promoter is responsible for the transcription of a smaller transcript and its product remains in the cytosol. In this study, chimeric maize C4Pdk promoter (0.9 or 1.5 kb)-beta-glucuronidase or lucife rase fusion genes were introduced into maize plants by Agrobacterium-mediat ed transformation. The cell- and tissue-specificities of the maize C4Pdk pr omoter in the transgenic maize plants were examined by histochemical and en zymic activity analyses of the reporters in different photosynthetic cells and tissues. The results showed that the reporter proteins are almost exclu sively localized in leaf mesophyll cells, Among the tissues tested, leaf bl ade had the highest reporter activities with sheath exhibiting about 10% of the activities in blade. Husk, stem, tassel and root had no or very little reporter activities. Taken together, these results suggest that the maize C4Pdk promoter is specifically transcribed in the mesophyll cells of leaf b lade and to a much less extent in the mesophyll cells of sheath, but not in leaf bundle sheath cells or other tissues. Furthermore, the 0.9 kb maize C 4Pdk promoter sequences appear to contain the necessary cis-acting elements for its cell- and organ-specific expression.