The Chloroplast Nucleus : Distribution, Number, Size, and Shape, and a Model for the Multiplication of the Chloroplast Genome during Chloroplast Development :

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The number, size, shape and distribution of chloroplast nuclei (ct-nuclei) in variety of plants, 69 species, and their changes during chloroplast development and chloroplast division, were studied with 4'6-diamidino-2-phenylindole (DAPI) epifluorescent microscopy. The proplastid in dormant seeds of Brassica juncea contained only one ovoid ct-nucleus. This ct-nucleus increased 2- to 4-fold in size during dark culture and became cup-shaped. Occasionally the ct-nucleus was located near starch grains or a prolamellar body. During greening the ct-nucleus divided into about 20 small ct-nuclei, each 0.2μm in diameter, which were distributed randomly throughout the mature chloroplast. Similar events in the ct-nucleus took place in Vicia ,faba and Pisum sativum. In most of the land plants and algae examined, the number of ct-nuclei in mature chloroplasts ranged between 8 and 40. Chloroplasts in Zygnematales, Ulotrichales and Ulvales contained larger numbers of ct-nuclei (40-300) . Most plants could be classified into five types according to differences in the shape, size and distribution of the ct-nuclei in their mature chloroplasts. The ct-nuclear division cycle of a representative type with randomly scattered ct-nuclei (SN-type), was studied in detail in Chlamydomonas reinhardii cells synchronized in a 12 hr light and 12 hr dark regimen. Results showed that the 10 ct-nuclei in a single chloroplast increased in size synchronously during enlargement of the cell and chloroplast. These ct-nuclei became dumbbell-shaped, and divided semi-synchronously just before chloroplastkinesis (chloroplast division excluding chloroplast nuclear division). After ct-nuclear division, about 40 daughter ct-nuclei were distributed equally among 4 daughter chloroplasts. Similar cyclic changes in the size and shape of ct-nuclei during the chloroplast division cycle was observed in higher plants and in algae such as Cryptomonas. Changes in ct-nuclei during chlor oplast development, and chloroplast division, as well as variations in the ct-nuclei in different species are discussed in terms of the chloroplast genome.

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