On the fate of plastid DNA molecules during leaf development: response to the Golczyk et al. Commentary.

In their commentary, Golczyk et al. (2014) state that the level of plastid DNA (ptDNA) remains approximately constant during mature, aging, and senescent stages of leaf development, in apparent contrast with our conclusion of ptDNA decline from previous studies. However, the ptDNA decline that we reported was found during proplastid-to-chloroplast development, before the mature-to-senescent leaf stages. We found little change in ptDNA after chloroplast maturation. Furthermore, we argue that their data fail to demonstrate constancy in ptDNA during development: Most of their data are not quantitative and demonstrate neither constancy nor change in ptDNA content. We also show that our inability to detect 4#,6-diamidino-2-phenylindole (DAPI)– stainable DNA in some plastids is not due to “artifact-prone methods,” as suggested by Golczyk et al. In our opinion, the fundamental issues concern whether ptDNA content declines during leaf development, the magnitude of that decline, and by extension, the functional relevance of such a decline. Our conclusions for three plant species with respect to changes in ptDNA content during leaf development are summarized in Figure 1, based on four methods that provide quantitative data on ptDNA mass per plastid and per cell and on molecular structure. The only quantitative data provided by Golczyk et al. (2014) are based on amounts of DNA between two closely spaced quantitative PCR (qPCR) primers and thus depend on the unvalidated assumption that such tiny DNA fragments represent genome-sized ptDNAmolecules. We conclude that an accurate assessment of ptDNA must consider not only ptDNA quantity but also molecular quality, a parameter not addressed by Golczyk et al. In our opinion, the manner of data presentation is the principal difference leading to our opposing conclusions.