Tissue-Specific Organelle DNA Degradation Mediated by DPD1 Exonuclease

Organelle DNA in plastids and mitochondria is present in multiple copies and undergoes degradation developmentally. For example, it is known that organelle DNA, detectable cytologically using DNA-fluorescent dye such as DAPI, disappears during pollen development in most angiosperms. In addition, selective degradation of organelle DNAs in male gametes soon after mating is suggested to be associated with maternal inheritance of organelle DNAs in unicellular green algae. Nevertheless, nucleases involved in such a degradation process remain unknown. Our recent study identified DPD1 nuclease which has Mg²⁺-dependent exonuclease activity in vitro and which is specifically expressed during pollen development. The discovery of DPD1 emerged from Arabidopsis mutant screening and concomitant isolation of dpd1 mutants that retain organelle DNA in mature pollen. Supporting the fact that both plastid and mitochondrial DNAs decrease in mature pollen, DPD1 is dual targeted to both organelles. DPD1 is conserved only in angiosperms: not in other photosynthetic organisms. Despite these findings, the physiological significance of organelle DNA degradation during pollen development remains unclear because dpd1 exhibits no apparent defects in pollen viability or in the maternal inheritance of organelle DNA. In fact, DPD1 appears to be highly active in the vegetative cell, which does not contribute to fertilization. On the other hand, characterization of another mutant dpd2 raises the possibility that DPD1 activity in pollen is associated with nucleotide synthesis. Based on these observations, we discuss a possible role of organelle DNA degradation mediated by DPD1.