Programmed cell death is responsible for replaceable bud senescence in chestnut (Castanea mollissima BL.)

In the chestnut “replaceable bud” cultivar ‘Tima zhenzhu’, the auxiliary bud formed on the fruiting branch dies after fruiting, giving rise to a morphology more suitable than the wild type’s for intensive cultivation and heightened production. Here, we show that many of the hallmarks of programmed cell death (PCD) occur during the senescence of the replaceable bud, including DNA degradation, a high ratio of PCD cells and the breakdown of cell ultrastructure. The time course of the senescence was followed by sampling the developing bud from 20 to 40 days after flowering. In cv. ‘Tima zhenzhu’, DNA degradation was detectable prior to any visible sign of bud senescence, while it did not occur in the wild type (cv. ‘Dabanhong’). The ratio of PCD cells (as determined by flow cytometry) rose over the sampling period and was consistently higher in cv. ‘Tima zhenzhu’ than in cv. ‘Dabanhong’. After staining the bud cell nuclei with propidium iodide, it was clear that both their chromatin content and overall size fell over the sampling period in cv. ‘Tima zhenzhu’ while in cv. ‘Dabanhong’, no such decrease occurred. Other characteristics of PCD were noted in cv. ‘Tima zhenzhu’’s bud cells, including chromatin condensation, tonoplast invagination and DNA cleavage. We conclude that the replaceable bud senescence phenomenon is driven by PCD. The manipulation of this trait may have potential for remodeling the pattern of development of the fruit-bearing branches of chestnut.