C-4 Gene Expression In Mesophyll And Bundle Sheath Cells

A distinguishing characteristic of C-4 plants that possess Kranz-type leaf anatomy is the presence of two photosynthetic cell types, bundle sheath (BS) and mesophyll (MP) cells, which occur in leaves and other photosynthetic organs. In mature C-4 leaves, these two cell types differentially express many genes that are required for the function of this pathway. Well-known examples include the BS cell-specific genes encoding Rubisco (plastid rbcL and nuclear RbcS) and decarboxylating malic enzymes (Me genes), and MP cell-specific genes encoding PEPC and PPdK (nuclear Ppc and Pdk genes). Recent studies have identified several other genes that show specificity to MP and BS cells. Many of the C-4 genes originated from non-photosynthetic ancestors, acquiring several new regulatory properties during their evolution to C-4 function. Within each cell type, the C-4 genes are regulated by many factors, including development, light, and photosynthetic metabolism. Genes encoding the various C-4 proteins often show changes in their patterns of expression during the development of leaves and other organs, and these patterns can vary between different genes and between different species. Such observations indicate a lack of correlation in the expression of different MP and BS-cell specific genes, supporting the conclusion that for the most part these are independently regulated. The complexity of C-4 gene expression has led investigators to incorporate a wide variety of methods in their efforts to understand this regulation at the whole plant level; these include in situ hybridization, immunolocalization, transient expression and transgenic plants, and analysis of expression at the levels of transcription, translation, and RNA stability. These investigations have provided evidence that for some C-4 genes, multiple regulatory mechanisms have become incorporated and function together in order to achieve full BS and MP cell-specificity required for the function of this highly efficient CO2 assimilation pathway.