Programmed cell death has been observed in various developmental processes in higher plants. Such events are characteristic of xylem development,25 tapetum cell degradation to sustain pollen development,26 sexual organ formation27 and other processes. The presence of air-filled spaces known as aerenchyma in numerous plant species is considered to be an important anatomical adaptive feature necessary for their survival under flooded conditions. Ample evidence has been presented to show that this system provides a diffusion path of low resistance for the transportation of oxygen from aerial plant parts, roots or rhizomes, thus enabling the plants to grow in a waterlogged, O2-deficient environment.13 Thus, plants with highly developed root porosity show better oxygen transport, which contributes to their survival under flooded conditions. The formation of porosity in roots under flooded conditions is common in a number of plant species well adapted to waterlogged conditions.16 Under waterlogged conditions, many plant species develop aerenchyma in their roots. For instance, in maize roots air space development was caused by endogenous as well as exogenously applied ethylene.28 Gas-filled spaces in rice arise from the death of certain cortical cells, and from separation of cell walls from adjacent cells so that the radial walls from the collapsing cells aggregate, forming "forks" leaving a large gas-filled or lacunal space between them. The cell collapse in rice roots thus occurs selectively and is regulated by a naturally programmed degenerative processes.14
We demonstrate here that the initiation of cell death in cortical cells of rice roots is characterized by spatial acidification and membrane leakage in specific cell positions. Anatomical characterization of rice cortical cells and the possible existence of macromo-
lecular trafficking through plasmodesmal connections are also described. Most importantly, a degradation pattern of cortical cells occurred in the cell file, where molecular movement to radial direction was dominant.29
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