*2.4. Transfer Rate of Organellar DNA to Nucleus*

The frequency of organellar DNA transfer is positively correlated with the number of mitochondria or plastids in the cell [55–57]. Two independent laboratories established a similar screen experimental system to measure the rate of nuclear DNA transferred from plastids; this system was based on transplastomic lines containing a selectable marker gene, whose transfer to the nuclear can confer antibiotic resistance of the regenerated or outcrossed progenies [10,12]. De novo NUPT formation occurred once in approximately 16,000 pollen grains [10] or once in every five million somatic cells screened [12]; this result suggests the high frequency of DNA transfer from the chloroplast to the nucleus. Such high frequency of organellar DNA transfer to the nucleus was also observed in yeast, in which the transfer rate of the mitochondrial DNA to the nucleus is approximately 2 <sup>×</sup> 10−<sup>5</sup> per cell per generation [58]. Abiotic stresses, such as subtle heat or cold treatment, can even increase the transfer frequency [55,59]. It is noteworthy that in these experiments, the estimated frequency of DNA transfer from the organelle to the nucleus may be fewer than in reality, because the screen system only focused on the transfer of the selectable marker gene from the organelle genome; those events where the transferred organellar genome fragment did not contain the selective gene may have been missed.

The frequency of plastid-to-nucleus DNA transfer differs markedly among diverse tissues [10,12,60]. The transfer frequency in gametophytic tissue is higher than that in somatic cells. Specifically, the transfer frequency is much higher in male germlines (1 per 11,000 pollen grains) than that in female germlines (1 stable transposition in 273,000 ovules) [60]. The elevated frequency examined within male germline may be caused by the fact that the chloroplast DNA within pollens are more likely to degrade, which was examined in a number of plant species [61].
