DNA Levels in Dividing and Developing Plastids in Expanding Primary Leaves ofAvena sativa

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Abstract
The amounts of plastid DNA in the primary leaves of 4-d-old light- and dark-grown seedlings of Avena sativa were measured by microspectrofluorometry using the DNA-fluorochrome DAPI (4', 6-diamidino-2-phenylindole). In the light-grown primary leaves (40–45 mm long) there was a marked increase in DNA level per plastid from 10.2 to 18.5 × 10−15 g between 2.0 mm and 10 mm from the leaf base, resulting from the rate of plastid DNA synthesis being higher than the rate of plastid division. Beyond 30 mm the plastid DNA level was reduced to 14 × 10−15g due to chloroplast division rates being higher than the rate of plastid DNA synthesis, while from 20 mm plastid DNA levels were constant at 2.2 × 10−12 g per cell, which corresponds to 16000 plastome copies per cell. Observations of dark-grown leaves establish that, in Avena, light is not necessary for plastid division and the dark-grown leaf cells accumulate higher amounts of plastid DNA than light-grown leaf cells. Plastid nucleoids showed a change of distribution after completion of plastid DNA synthesis in light-grown leaves. A change in the distribution of plastid nucleoids was also observed during the greening of etioplasts of dark-grown leaves while plastid DNA level remained constant. Such changes in plastid nucleoid distribution appear to be independent of plastid DNA synthesis and correlate with the formation of grana stacks.