Evidence of programmed cell death in maize suspension cultures

Abstract

Programmed cell death (PCD) is an active cell death process involved in the selective elimination of unwanted cells, and it is found throughout animal and plant kingdoms. The term apoptosis usually refers to a morphological type often observed in PCD that involves nuclear shrinkage and fragmentation, cellular shrinkage, DNA fragmentation, membrane budding, the formation of apoptotic bodies, and digestion by macrophages. It is distinct from necrosis, the latter being cell death as a response to severe trauma, and is not genetically based. The goal of this research is to determine if there is evidence of apoptosis in maize (Zea mays L., cv. Black Mexican Sweet) suspension cultures. Maize suspension cultures were chosen for this research because of the vast availability of information on maize. They also have the advantage of being comprised of small aggregates of cells, which are more amenable to manipulation than whole plants due to the lack of complex tissue organization. The objectives of this study were twofold. First, examine growth characteristics of maize suspension cultures treated with exogenous ethylene and/or hypoxia. Second, determine whether cell death occurs in maize suspension cultures and whether actual cell death follows the pattern of PCD or necrosis. The data indicated that cells with exogenous ethylene and/ or hypoxia treatments altered growth characteristics in suspension cultures, including growth depression, reduction of oxygen uptake, induction of CO₂ production, ethylene production and ADH activity. Evan's blue staining was used to assess cell viability and morphology of cell death. A further investigation was conducted to determine whether the cell condensation and shrinkage that occurred during cell death induced by ethylene and/or hypoxia treatments would be similar to the cell collapse characteristic of apoptosis in animal cells. DNA fragmentation and cytochrome c release were examined to determine whether cell death approximates more closely to necrosis or to PCD. The results of this research indicated that cell death might not be activated by ethylene and/or hypoxia treatments, and cell death occurred in maize suspension cultures might not have the same mechanism as PDC found in the animal paradigm.