A unique form of cell death occurring in meristematic root tips of completely submerged maize seedlings

In maize (Zea mays L.) seedlings subjected to complete submergence, meristematic cells of primary root tips underwent a series of ultrastructural changes. The changes in the cytoplasm preceded those in the nucleus. The former included cytoplasmic vacuolation, proliferation of the endoplasmic reticulum, which was mostly adjacent to the plasma membrane, and appearance of membrane-bound bodies containing intact organelles, as well as cytoplasmic degradation and disruption of the plasma membrane; the latter included nuclear shrinkage and degradation. TUNEL-positive results indicating the occurrence of DNA strand breaks were detected 12 h after submergence, while the ultrastructural changes in the nucleus were apparent 24 h after submergence. No DNA laddering or chromatin condensation occurred in the death process of the meristematic root tip cells. It has been suggested that apoptosis and necrosis just represent extreme ends of a wide range of possible morphological and biochemical death, and various forms of programmed cell death (PCD) have been identified in plant cells so far. According to previous knowledge about cell death and our data, we conclude that the death of the meristematic root tip cells may be a unique form of cell death, in part resembling PCD and in part resembling necrosis. Root tip death may provide an adaptive advantage to maize seedlings to anoxic stress.