Additional，the content“The PCD of starch endosperm is one of the special types”has been added into the reversed paper.
The PCD of starch endosperm is one of the special types
It was confirmed that the death of wheat endosperm cells was a PCD process by Evans-Blue staining, TUNEL assay, DNA gel electrophoresis, and TEM analysis. We discovery that chromatin condensation (Fig. 4E), and nuclear residual bodies formation (Fig. 4H) in the endosperm cells; consequently extensive degradation of nuclear DNA occurs, producing fragments with 180-200 bp or multimers which shows DNA laddering on gel electrophoresis (Fig. 3, lane 1-5). Evans-Blue staining shows that the starch endosperms PCD process from the 15 DPA until the end of seed maturity, and the death positions are random (Fig. 1 E, I). TUNEL-positive nuclei were first detected in the endosperm cells at 12 DPA (Fig. 2A). Then, the number of positive nuclei increased during the development of the starch endosperm (Fig. 2B).The mitochondria showed intact structures until 14-16 DPA (Fig. 4C). Until 20 DPA, most of the mitochondria degradation, showed a central hollow (Fig. 4G). The PCD of starch endosperm exhibits some differences from vacuolar cell death and necrosis. They have the same characteristics: chromatin condensation and DNA fragmentation. The similarities in starch endosperm PCD and vacuolar cell death: They maybe need a long time to finish the development process. Such as endosperm PCD need 18d in wheat (from 12 DPA (Figs. 2A and 4C) to mature (30 DPA)). The differences: Firstly, Not only the big lytic vacuole and vacuolization, but also vacuolar hydrolases were observed in the starch endosperm PCD. Secondly, the degradation of mitochondria is accompanied with nuclear degradation, but vacuolar cell death having the characteristics of the remaining mitochondria and other organelles, as well as the plasma membrane, remain morphologically intact until rupture of the tonoplast. The similarities in starch endosperm PCD and necrosis: the degradation of the mitochondrial was also observed in necrosis and starch endosperm PCD. The differences: First, necrosis is an typically acute cell death response that develops rapidly and cell corpse remains largely unprocessed；but starch endosperm PCD lasts for a long time. Second, early rupture of plasma membrance was detected in the necrosis; but this phenomenon was not observed in the starch endosperm PCD. Thus, starch endosperm PCD is a unique type of PCD which needs to be distinguished with vacuolar cell death and necrosis.