Behavior of Chloroplast Nucleus during Chloroplast Development and Degeneration in Chlamydomonas reinhardii

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Abstract
Changes in morphology of chloroplast nuclei (cp-nuclei), total cp-DNA content, number of cp-nuclei, oxygen-evolution activity and chlorophyll (a and b) content were examined during the degeneration and development of chloroplasts, using Chlamydomonas reinhardii cells which had been incubated on solid medium for various periods. Under 4′-6-diamidino-2-phenylindole (DAPI) epifluorescence microscopy, each cell that had been incubated for 7 days had one cell nucleus, one cup-shaped chloroplast and about 10 small, dispersed cp-nuclei in the chloroplast. One day after incubation of these cells on fresh medium, the cell volume and cp-nuclei increased in size 2-3 fold, but rapidly decreased in size after cell division. After about 7 days of incubation, cells ceased to divide and cp-nuclei began to associate with each other. At about 20 days they formed a ring-shaped structure surrounding the pyrenoid, followed by condensation into one cp-nuclear particle near the pyrenoid. When 41-day-old cells, having only one cp-nucleus, were reinoculated on fresh solid medium, the cp-nucleus increased in size 2–3 fold, divided into several cp-nuclear particles and then dispersed into the chloroplast, forming a bead-like structure, before cell division. From microscopic fluorometry, a 4-fold increase in total cp-DNA content per chloroplast, without an increase in the number of cp-nuclear particles per chloroplast, occurred one day after the start of the experiment and one day after reinoculation of 41-day-old cells onto fresh medium. The process of condensation of dispersed cp-nuclear particles into one cp-nucleus during degeneration of the chloroplast was not accompanied by any change in total cp-DNA content per chloroplast. A large peak of oxygen-evolution (0.6–0.9 pmoles/cell/hour) was seen one day after inoculation and reinoculation of the cells. The chlorophyll content (a+b) was high (1.2–2.2 pg/cell) during the first week of incubation, after which it gradually decreased.