Initial proplastid-to-chloroplast differentiation in the developing vegetative shoot apical meristem of Arabidopsis

In dicot plants, the process by which undifferentiated plastids, termed proplastids, differentiate into mature functional chloroplasts begins in the shoot apical meristem (SAM) and young leaf primordia, and continues along leaf development. In this work, we followed initial chloroplast biogenesis in cells of the SAM in Arabidopsis, during the early stages of germination, using chlorophyll fluorescence as a marker. We found that cells bound to form the SAM in the mature seed embryo lack chlorophyll, while plastids in the rest of the embryo cells are somewhat developed. The initial appearance of chlorophyll in the SAM occurred two days after the onset of germination, was not expedited by higher light intensities, and required a light period of between five to ten hours within these two days. In addition, we found that biogenesis of chloroplasts occurred only in the upper layer of the SAM, as opposed to the two central subtending cell layers. This pattern was maintained, mirroring the developmental status of plastids in the mature vegetative SAM. The work also presents another model for studying proplastid-to-chloroplast development, in which differentiation can be followed in SAM cells in a defined time-wise fashion.