Expression of photosynthesis-related genes and their regulation by light during somatic embryogenesis in Daucus carota

To clarify the spatial and temporal pattern of gene expression for photosynthesis-associated proteins during somatic embryogenesis in Daucus carota L., the localization of mRNAs for three genes, rbcL, Lhcb and por, was examined in dark-grown and light-irradiated somatic embryos by in situ hybridization. The three mRNAs were expressed in common in the mesophyll precursor cells of light-irradiated embryos at the late torpedo and plantlet stages, but characteristic expression patterns of each photosynthesis-related gene were also observed. Expression of rbcL mRNA first occurred throughout the embryo but gradually became localized in the mesophyll precursor cells and cortex during early embryogenesis. Localization of Lhcb mRNA in the mesophyll precursor cells and shoot apical meristem became clear in the early torpedo stage. Expression of Lhcb mRNA was not affected by light during early embryogenesis, but could be induced by light in the torpedo stage, suggesting that light-inducible expression of Lhcb mRNA arises within the torpedo stage. At the late torpedo stage, clear localization of por mRNA started in mesophyll precursor cells of the cotyledon in light-irradiated embryos. Greening potency of the embryo also appeared first at this stage. Therefore, greening and initial differentiation of photosynthetic tissues during somatic embryogenesis seem to be associated with coordinated expression of mRNA for rbcL, Lhcb and por in late torpedo-shaped embryos.