Rice Genetics IV - IRRI books - International Rice Research Institute

y cells separating during tissue development. Our study focuses on the lysigenousgas spaces produced in the primary roots of rice. During the normal course of development,many cells in the cortex collapse to form aerenchyma.In maize, ethylene has been implicated in signaling cell death during the lysigenousformation of aerenchyma in adventitious roots subjected to hypoxia (Drew et al 1979).According to Jackson (1994), a limited amount of oxygen outside the root may favorthe development of an anoxic stele core, where the absence of oxygen can be expectedto increase 1-aminocyclopropane-1-carboxylic acid (ACC) synthase levels,leading to the synthesis of additional ACC. This ACC would then enter the encirclinghypoxic cortex, where the small amount of oxygen present (Armstrong and Armstrong1994) is sufficient to allow oxidative conversion of ACC to ethylene by ACC oxidase.In rice, however, Jackson et al (1985) reported that aerenchyma formation in adventitiousroots was not controlled by ethylene or a low partial pressure of oxygen.On the other hand, Das and Jat (1977) reported that flooded conditions enhancedaerenchyma formation in the root cortex in some rice cultivars. Furthermore, Justinand Armstrong (1991) demonstrated that ethylene can promote aerenchyma formationin adventitious roots of rice, and showed that cultivar-specific differences explainthe discrepancies.Gas-filled spaces in rice arise from the separation of cell walls from adjacent cellsso that the radial walls from the collapsing cells aggregate together, forming “forks,”leaving a large gas-filled space or lacuna between them (Clark and Harris 1981). In anelectron-microscope study, Webb and Jackson (1986) proposed that the processes ofaerenchyma formation differ in rice and maize for the order in which events occur. Inmaize roots, cellular collapse was preceded by the loss of tonoplast integrity (Campbelland Drew 1983). In rice, however, the middle lamella degenerated, followed by cellwall disintegration and the loss of tonoplast integrity (Webb and Jackson 1986). Althoughthere has been some controversy about the process of aerenchyma formationin rice and maize, it is now generally agreed that the aerenchyma of rice roots israpidly formed as an integral part of ordinary root development (Webb and Armstrong1983, Jackson et al 1985). The detailed mechanisms of aerenchyma formation are notwell understood.Our investigation (Kawai et al 1998, Samarajeewa et al 1999) contributes to amechanistic understanding of the events leading to cortical cell death, which eventuallyresults in the formation of aerenchyma in the primary roots of rice. The mostimportant finding is that cell death began at a specific cell position. The first cells tocollapse were located at the center of the cortical tissues and cell collapse never beganat the peripheral cortical cells. The locations of cells undergoing lysis appeared precise.This indicates the existence of a targeting mechanism for initiating the first celldeath. Cells in this position were characterized by being shorter but larger in radialdiameter than other cortical cells. These features were generated by the nonuniformexpansion of cortical cells in each position. The cells of position 5 tended to expandradially more than tangentially, unlike peripheral cortical cells.The development of aerenchyma is often associated with the enlargement of corticalcells. Schussler and Longstreth (1996) also reported that the cells undergoing366 Uchimiya et al