Rice Genetics IV - IRRI books - International Rice Research Institute

(Erwee and Goodwin 1983). The symplast is known to be important in ion and metabolitetransport, especially where apoplastic barriers are found, as in the root cortex(Clarkson and Robards 1975). Morphological investigations have provided evidencethat symplasmic transport is mediated by specialized trans-cell wall structures calledplasmodesmata.The highly selective death in the specific cortical cells is reminiscent of programmedcell death in animal cells. In animals, apoptosis has been known as an activecontrol mechanism that is important in developmental and pathological processes(Yuan and Horvits 1990). Our knowledge of programmed cell death in plants lagsbehind that of animal systems. Programmed cell death is thought to occur during theautolysis of xylem vessels (Fukuda and Komamine 1982), during sex determination(Delong et al 1993), and in other processes. Mittler and Lam (1995) demonstrated inpea that the nuclei of vessel elements undergoing programmed cell death containedfragmented nuclear DNA. This finding may be evidence of the activation of a DNAdegradationmechanism prior to the final disruption of the nucleolus that occurs duringautolysis in this differentiation process. The dad1 gene, known as a suppressor ofprogrammed cell death in Caenorhabditis elegans and mammals, has been isolatedduring petal senescence of pea (Orzáez and Granell 1997), and Tanaka et al (1997)showed that the rice dad gene functions as a suppressor of cell death in animal cells.We also isolated a rice gene similar to the Bax inhibitor in yeast (Kawai et al 1999).These reports show that at least some characteristics may be shared between animaland plant programmed cell death. Even in collapsing cells, we observed that somecells contained nuclei. Thus, further studies of DNA degradation are needed.In relation to the mechanism of plant cell death, it is interesting to speculate thatthe sequential spread of cell death may be due to H 2 O 2 produced as an oxidative burst,since a high dose of H 2 O 2 induces cell death in higher plants (Levine et al 1994). Itwould be interesting to investigate further the interaction of these signals in conjunctionwith ethylene, which has been shown to stimulate aerenchyma formation.Figure 1 presents a proposed model of the processes leading to cortical cell deathin rice roots. After germination, cell division regulates the number of cells in theregion where the root meristem is located. Longitudinal cell elongation as well ashorizontal cell expansion follow. After these events, cell acidosis and tonoplast breakdownoccur in the mid cortex, followed by cell death, which coincides with movementof injected probes through plasmodesmata in a radial direction. The specificstep involved in the first cell death in aerenchyma formation of rice roots cannot beidentified unambiguously based on the present evidence, but the program leading totonoplast disruption and first cell death might begin in response to developmentalsignals, which may include endogenous ethylene. Berg et al (1995) demonstrated thatthe fate of cells in the root meristem is under positional control. It is interesting thatthe fates of both meristematic cells and those undergoing successive cell death inplant roots are under strict spatial-related regulation. Further investigation is necessaryto clarify the process of aerenchyma formation in the root cortex.368 Uchimiya et al