The objective of this investigation was to dissect the events leading to cortical cell death, which eventually resulted in gas-space formation in plant roots. Investigation focused upon the spatial and temporal development of cortical cell death in rice roots. The most significant finding is that cell death is initiated at a specific cell position. The first cells to collapse were located at the center of the cortical tissues surrounding the stele. Cells in this position were characterized by shorter length, and a larger radial diameter, than other cortical cells. It appears that cells destined to collapse expand in a radial direction before dying.
It has been reported that anoxia causes cytoplasmic acidosis in maize root tips30 and hypoxia stimulates the formation of aerenchyma in plant roots.31,32 However, the roots in our study were neither hypoxic or anoxic; staining with neutral red in the central cell position (position 5 and neighboring cells) may indicate cytoplasmic acidification resulting from loss of tonoplast integrity and diffusion of H+ from the vacuole.
Once cell collapse occurred, neighboring cells were systematically destroyed in a radial direction in cortical parenchyma tissues. As was noted, before cell death occurred, expansion of the cell in the radial direction was apparent. This may be due to relaxation of cell wall matrix and development of plasmodesmal connections in the radial direction to enable the transmission of molecules, which may be related to cell death. The results of microinjection indicate that cells that were not stained with Evans blue showed a molecular exclusion limit between 9.3-19.6 kDa. We did not observe any leakage of fluorescent substances in such cells. With regard to the direction of movement, 67% of cells showed radial transfer with F-dextran 9.3 kDa. These results indicate that the direction of cell to cell movement of large molecules (9.3 kDa) in cortical cells is mainly radial, which coincided with the direction of cell death. The leakage of substrate in half of the cells stained with Evans blue may indicate that these cells are in a late stage of cell death. In mesophyll cells, the exclusion limit for movement has been shown to be below 1 kDa.33 We noted that cortical cells of roots permit plasmodesmal movement up to 9.3 kDa, suggesting the possible existence of macromolecular trafficking through plas-modesmata in root cortical cells.
Cell-specific acidosis and membrane breakdown appear to coincide with cortical cell death. Since high doses of H2O2 induce cell death in higher plants,34 it is interesting to speculate that systemic spread of cell death may be due to H2O2 produced by oxidative burst. It would be interesting to further investigate the interaction of these signals in conjunction with ethylene, which has been known to stimulate aerenchyma formation.
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