Rice Genetics IV - IRRI books - International Rice Research Institute

lysis appeared to be larger in diameter than the other cortical cells in Sagittaria lancifoliaroots. Kawase (1974) demonstrated that cortical cells of sunflower stem treated withcellulase were enlarged radially, and some disintegrated, leading to an intercellularspace. Cell expansion depends on change in cell walls, and radial cell enlargementwas explained by Fan and Maclachlan (1996) as an induction of cellulase acting onmicrofibrils, which are predominantly oriented transversely. Cellulase activity is increasedby ethylene in plants (Horton and Osborne 1967). It has already been establishedthat aerenchyma development in flooded maize roots is associated with increasedactivity of cellulase (Horton and Osborne 1967), and that this process is promotedby ethylene (Drew et al 1979). Saab and Sachs (1996) cloned a flooding-induciblexyloglucan endo-transglycosylase (XET) gene, a cell-wall-loosening enzyme,and showed that ethylene treatment under aerobic conditions induced XET gene expression.Based on a microscopic analysis, it has been reported that the first clearly observedevent in aerenchyma formation in maize roots is tonoplast disruption (Campbelland Drew 1983). In our study, the first cells to be stained with neutral red were atposition 5 in the cortex. This result suggests that vacuoles may mature (i.e., acidosis)more rapidly in position 5 than in other positions. After this process, cytoplasmicacidification resulting in the loss of tonoplast integrity may occur by the diffusion ofH + from vacuoles. The signal that provokes vacuole disruption is unknown.Evans blue clearly stained cortical cells in position 5 first in section III. Thesecells comprise the upper range of the cells that were stained first with neutral red(section II) and comprise the lower range of the cells in which collapse began (sectionIV). Furthermore, Evans blue did not stain concave cells (i.e., collapsing cells). Theseresults indicate that cells in position 5 develop vacuoles, followed by a loss of membraneintegrity, and collapse. Several studies show that the ability of plant cells to takeup Evans blue signifies an irreversible change that leads to cell death (Gaff andOkong’O-Ogola 1971).Webb and Jackson (1986) observed the process of aerenchyma formation in riceroots by transmission and cryo-scanning electron microscopy and reported that breakdownof the cell wall preceded the loss of tonoplast integrity. This result seems tocontradict our finding. In maize, cell wall degradation is preceded by tonoplast disruptionand the dispersal of cytoplasm. Until this stage is reached, cell walls remainunaltered (Campbell and Drew 1983). Based on our data, it is difficult to determinethe sequence of the processes that occur inside of cells. However, we have shown inrice that the ability to be stained by Evans blue, which may be related to plasmamembrane disruption, precedes cell collapse.Once cell collapse occurred, the cells lost contact with neighboring (tangential)cells and were sequentially destroyed in a radial direction in cortical parenchymatissues. Using electron microscopy, van der Weele et al (1996) observed an earlierstage of cell death, which began with the loss of vacuolar solutes and the collapse ofcells in maize, and hypothesized that dying cells may release a message that initiatesthe process of cell death in neighboring cells. The movement of small moleculesbetween plant cells occurs through a cytoplasmic syncytium termed the symplastMolecular dissection of cell death in rice 367