[The starchy endosperm denucleation by a process of programmed cell death during rice grain development]

The cellular ultrastructural morphology, grain filling ratio and activity of related enzymes during denucleation development stage in starchy endosperm cell of an Indica rice variety Zhongxian 8836 were observed and analyzed. The endosperm cellularization was completed at about the 3rd day after flowering (DAF). At 5 DAF a few endosperm cells initiated denucleation development. At 13 DAF almost all the starchy endosperm cells completed their denucleation. The nuclear degradation is the first stage of programmed cell death (PCD) in starchy endosperm. It occurred unsynchronously among different starchy endosperm cells. The nuclear degradation of starchy endosperm cell during denucleation development stage showed not only morphological features commonly observed in animal and plant PCD, but also some unique characteristics. Mitochondria degeneration was observed along with nuclear degradation, indicating there were interrelations between the two processes. Enzymes related to PCD, such as super-oxide dismutases (SOD) and catalase (CAT), as well as enzymes related to starch synthesis, such as ADP-glucose pyriphosphorylase (AGP), soluble starch synthase (SSS) and starch branching enzyme or Q-enzyme, showed very high activity during the denucleation development stage. Maximum grain filling rate and grain weight increase were also achieved in the denucleation developing stage of most starch endosperm cells. The denucleated cell remained alive in the developing endosperm, keeping its normal metabolisms, the synthesis and accumulation of starch and storage proteins. However, enzyme activities and grain filling rate were apparently dropped to a lower level after denucleation. The starchy endosperm cell finally completed its PCD process when it was completely filled with reserves. Evan's blue staining indicated that cell death occurred unsynchronously among the starchy endosperm cells with initiation points randomly distributed in the endosperm tissue.