Plastid inheritance revisited: emerging role of organelle DNA degradation in angiosperms.

Plastids are essential organelles in angiosperms and show non-Mendelian inheritance due to their evolution as endosymbionts. In approximately 80% of angiosperms, plastids are thought to be inherited from the maternal parent, whereas other species transmit plastids biparentally. Maternal inheritance can be generally explained by the stochastic segregation of maternal plastids after fertilization because the zygote is overwhelmed by the maternal cytoplasm. In contrast, biparental inheritance shows transmission of organelles from both parents. In some species, maternal inheritance is not absolute and paternal leakage occurs at a very low frequency (~10-5). A key process controlling the inheritance mode lies in the behavior of plastids during male gametophyte (pollen) development, with accumulating evidence indicating that the plastids themselves or their DNAs are eliminated during pollen maturation or at fertilization. Cytological observations in numerous angiosperm species have revealed several critical steps that mutually influence the degree of plastid transmission quantitatively among different species. This review revisits plastid inheritance and focuses on the mechanistic viewpoint. Particularly, we focus on a recent finding demonstrating that both low temperature and plastid DNA degradation mediated by the organelle exonuclease DPD1 influence the degree of paternal leakage significantly in tobacco. Given these findings, we also highlight the emerging role of DPD1 in organelle DNA degradation.