In vivo study of developmental programmed cell death using the lace plant (Aponogeton madagascariensis; Aponogetonaceae) leaf model system.

Programmed cell death (PCD) is required for many morphological changes, but in plants it has been studied in much less detail than in animals. The unique structure and physiology of the lace plant (Aponogeton madagascariensis) is well suited for the in vivo study of developmental PCD. Live streaming video and quantitative analysis, coupled with transmission electron microscopy, were used to better understand the PCD sequence, with an emphasis on the chloroplasts. Dividing, dumbbell-shaped chloroplasts persisted until the late stages of PCD. However, the average size and number of chloroplasts, and the starch granules associated with them, declined steadily in a manner reminiscent of leaf senescence, but distinct from PCD described in the Zinnia tracheary element system. Remaining chloroplasts often formed a ring around the nucleus. Transvacuolar strands, which appeared to be associated with chloroplast transport, first increased and then decreased. Mitochondrial streaming ceased abruptly during the late stages of PCD, apparently due to tonoplast rupture. This rupture occurred shortly before the rapid degradation of the nucleus and plasma membrane collapse, in a manner also reminiscent of the Zinnia model. The presence of numerous objects in the vacuoles suggests increased macro-autophagy before cell death. These objects were rarely observed in cells not undergoing PCD.