PREFERENTIAL DEGRADATION OF PLASTID DNA WITH PRESERVATION OF MITOCHONDRIAL-DNA IN THE SPERM CELLS OF PELARGONIUM-ZONALE DURING POLLEN DEVELOPMENT

In the present study, we studied changes in organellar DNA in the sper m cells of maturing pollen of Pelargonium zonale, a plant typical to e xhibit biparental inheritance, by fluorescence microscopy after staini ng with 4',6-diamidino-2-phenylindole (DAPI) and by immunogold electro n microscopy using anti-DNA antibody. Fluorescence intensities of DAPI -stained plastid nuclei in generative and sperm cells at various devel opmental stages were quantified with a video-intensified microscope ph oton counting system (VIMPCS). Results indicated that the amount of DN A per plastid in generative cells increased gradually during pollen de velopment and reached a maximum value (about 70 T per plastid; 1 T rep resents the amount of DNA in a particle of T4 phage) in young sperm ce lls at 5 days before flowering. However, the DNA content of plastids w as subsequently reduced to about 20% of the maximum value on the day o f flowering. Moreover, the DNA content of the plastid further decrease d to 4% of the maximum value when pollen grains were cultured for 6 h in germination medium. In contrast, the amount of DNA per mitochondrio n did not decrease significantly around the flowering day. Similar res ults were also obtained by immunogold electron microscopy using anti-D NA antibody. The density of gold particles on plastids decreased durin g pollen maturation whereas labelling density on mitochondria remained relatively constant. The number of plastids and mitochondria per gene rative cell or per pair of sperm cells did not change significantly, i ndicating that the segregation of DNA by plastid division was not resp onsible for the decrease in the amount of DNA per plastid. These resul ts indicate that the plastid DNA is preferentially degraded, but the m itochondrial DNA is preserved, in the sperm cells of P. zonale. While the plastid DNA of the sperm cells decreased before fertilization, it was also suggested that the low DNA contents that remain in the plasti ds of the sperm cells are enough to account for the biparental inherit ance of prastids in P. zonale.