We used rice dwarf1 (d1) mutants lacking a single-copy G␣ gene and addressed G␣'s role in disease resistance. d1 mutants exhibited a highly reduced hypersensitive response to infection by an avirulent race of rice blast. Activation of PR gene expression in the leaves of the mutants infected with rice blast was delayed for 24 h relative to the wild type. H2O2 production and PR gene expression induced by sphingolipid elicitors (SE) were strongly suppressed in d1 cell cultures. Expression of the constitutively active OsRac1, a small GTPase Rac of rice, in d1 mutants restored SE-dependent defense signaling and resistance to rice blast. G␣ mRNA was induced by an avirulent race of rice blast and SE application on the leaf. These results indicated the role of G␣ in R gene-mediated disease resistance of rice. We have proposed a model for the defense signaling of rice in which the heterotrimeric G protein functions upstream of the small GTPase OsRac1 in the early steps of signaling. Heterotrimeric G proteins, a major group of signaling molecules involved in a variety of cellular activities in mammals, are mainly responsible for various cellular responses to external signals (1). In mammals, G proteins consist of ␣, , and ␥ subunits, and at least 23 ␣, 6 , and 12 ␥ genes are known (2). In plants, a number of pharmacological studies suggested that heterotrimeric G proteins are involved in a variety of signaling, including light reception (3), hormone signaling (4), and regulation of ion channels (5). However, direct evidence to support these observations has been obtained only recently (6). Analysis of mutations in a gene encoding the G␣ subunit of rice termed dwarf1 (d1) showed that G␣ is involved in stem elongation and the determination of seed shape in rice (7, 8) and influences gibberellin signal transduction (9). More recently, in Arabidopsis, mutants in the single-copy G␣ subunit gene were shown to have reduced cell division in aerial tissues (10) and to lack regulation of the ion channel by the phytohormone abscisic acid in guard cells (11). Furthermore, involvement of G␣ in phytochromemediated light signal transduction of Arabidopsis was also demonstrated by the study of transgenic plants overexpressing the G␣ gene (12). Arabidopsis mutants lacking G were recently shown to have effects in leaf, flower, and fruit development (13). Therefore, the importance and diverse functions of heterotrimeric G proteins in the signaling of plants are recently becoming clear; however, the molecular mechanisms of G protein signaling remain to be studied.Many studies using inhibitors and agonists of heterotrimeric G proteins in several plant species have suggested that G proteins are involved in defense signaling (14-17). Particularly, changes in cytosolic Ca 2ϩ concentrations, which are often observed in elicitor-treated plant cells, are assumed to be regulated by heterotrimeric G proteins (18). However, the roles of the heterotrimeric G protein in plant defense have not been directly tested by the use of G protein mutants.In th...
The rice (Oryza sativa) lesion-mimic mutants, cell death and resistance (cdr), show spontaneous cell death on the entire leaf and exhibited significant resistance to the rice blast fungus. Our previous studies showed that CDR1 and CDR2 genes negatively regulated the phosphorylation steps leading to the activation of NADPH oxidase, which is associated with oxidative burst. To identify novel factors involved in the phosphorylation steps, the phosphorylation level of total proteins was compared between cdr mutants and wild type using two-dimensional gel electrophoresis. Here, we show that the phosphorylation level of four proteins in cdr1 was increased as compared with the wild type after calyculin A treatment. Partial amino acid sequences revealed that one of the four proteins is homologous to prohibitin (PHB), which has been shown to be associated with senescence and cell death and to function as a chaperone in the assembly of mitochondrial respiratory chain complex in yeast and mammals. Analysis of green fluorescent protein fusions indicated that rice PHB (OsPHB1) was targeted to mitochondria as found in yeast and mammals, suggesting a possibility that PHB is involved in defense response and/or programmed cell death through the mitochondrial function.Hypersensitive cell death is a major component of defense responses in plants against microbial attack and is associated with restricted pathogen (Dangl and Jones, 2001). The induction of hypersensitive cell death is often triggered by the interaction between race-specific disease resistance (R) genes of plants and corresponding avirulence (Avr) genes of microbes (Staskawicz et al., 2001). Many R genes have been isolated from various species and characterized in the past; however, molecular mechanisms of the signal transduction and regulation of the hypersensitive cell death still remain largely unknown.Induction of the hypersensitive cell death requires the expression of concerned genes and synthesis of proteins de novo (Dixon et al., 1994; Godiard et al., 1994; He et al., 1994), indicating that the hypersensitive cell death belongs to programmed cell death (PCD). Some of the basic regulatory mechanisms of PCD involved in the response to pathogens have been shown to be conserved in animals and plants (Jacobson et al., 1997; Lam et al., 2001).Apoptosis is a well-characterized form of PCD in animal cells (Jacobson et al., 1997). A variety of stimuli, including hormones, growth factors, UV irradiation, and reactive oxygen species (ROS), induce apoptotic cell death. Although the recognition of these stimuli is mediated by various receptor molecules, death signals converge on mitochondria to trigger activation of caspase and other molecules required for the execution of cell death (Green and Reed, 1998; Lam et al., 2001). Activation of the caspases requires the release of cytochrome c from mitochondria and the activation of Apaf-1 in the cytosol (Budihardjo et al., 1999). The mitochondrial permeability transition (MPT), which occurs in the inner membrane, causes rel...
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