Multiplication and Differentiation of Plastid Nucleoids during Development of Chloroplasts and Etioplasts from Proplastids in Triticum aestivum

The morphological changes of plastid nucleoids (pt nucleoids) in the shoot apex and along the axis of the leaf blade in Triticum aestivum L. cv. Asakaze were followed with fluorescence microscopy after staining with 4′6-diamidino-2-phenylindole (DAPI) and quantified with supersensitive microspectrophotometry. Proplastids in the shoot apex contained 1–10 spherical pt nucleoids. These pt nucleoids changed to a row of spherical and cup-shaped pt nucleoids in sausage-shaped plastids at the leaf base in both dark and light conditions, in which active cell division was observed. These structures have a higher copy number of plastid DNA (pt DNA) (72–78 copies) compared to proplastids in the shoot apex (32–45 copies) and, therefore, may reflect that active pt DNA synthesis is in progression. In the dark, the cup-shaped pt nucleoids in the spherical etioplasts, which originated from the sausage-shaped plastids, grew to form ring-shaped pt nucleoids. Each ring-shaped pt nucleoid is sub-divided into smaller pt nucleoids. Under continuous illumination, similar morphological changes of pt nucleoids occurred except for distribution of small pt nucleoids into young chloroplasts as well as in mature chloroplasts. However, pt nucleotids of leucoplasts in epidermal and vascular bundle sheath cells did not show conspicuous changes along the axis of the leaf blade. The significance of these observations is discussed in relation to plastid differentiation and to the plastid division cycle.