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Research topics

We are currently focusing our efforts on researching  molecular mechanisms of plant defense to attack by pathogens and insects. If you would like to know more information on our research, please click topics listed below.

Mechanism of virus resistance

Mechanism of rice blast resistance

Mechanism of the wound response

Chemical control of plant diseases

Gene recombination technology

1. Mechanism of virus resistance

One of the induced disease resistance responses that plants use to defend themselves against pathogens is regulated by plant resistance (R) genes whose products directly or indirectly recognize products of the corresponding pathogen avirulence (Avr ) genes. The absence of either an R gene or the corresponding Avr gene can allow the pathogen to spread beyond the initial point of infection, resulting in the development of systemic disease.In many plant-pathogen interactions, host recognition of Avr gene products triggers rapid and localized cell death at the site of pathogen invasion, known as a hypersensitive response (HR), usually resulting in the formation of necrotic lesions. HR is characterized by a set of defense responses, such as the generation of reactive oxygen species, accumulation of antimicrobial compounds, and induction of defense-related genes. HR is thought to limit pathogens to the initial site of infection, thereby inhibiting their multiplication and spread.

We have been mainly using the tobacco-Tobacco mosaic virus (TMV) interaction as a model system for studying R -mediated plant disease resistance. Previous studies have suggested that WIPK and SIPK, tobacco mitogen-activated protein kinases (MAPKs), are positively involved in both local resistance to TMV multiplication and systemic viral resistance, possibly by regulating HR cell death. However, we recently found that WIPK and SIPK function to negatively regulate local resistance to TMV multiplication and positively regulate systemic viral resistance (Kobayashi et al. 2010).

Selected publications
Kobayashi et al. (2010) Silencing of WIPK and SIPK mitogen-activated  protein kinases reduces Tobacco mosaic virus accumulation but permits systemic viral spread in tobacco possessing the N resistance gene. Mol. Plant-Microbe Interact. 23:1032-1041.

Kobayashi et al. (2010) Analyses of the cis-regulatory regions responsible for the transcriptional activation of the N resistance gene by Tobacco mosaic virus. J. Phytopathol. 158: 826-828.

Takabatake et al. (2007) MAP kinases function at the downstream of HSP90 and upstream of mitochondria in TMV-resistance gene N-mediated hypersensitive cell death. Plant Cell Physiol. 48: 498-510.

Seo et al. (2000) Reduced levels of chloroplast FtsH protein in tobacco mosaic virus-infected tobacco leaves accelerate the hypersensitive reaction. Plant Cell 12: 917-932.

Mitsuhara et al. (1999) Animal cell-death suppressors Bcl-xL and Ced-9 inhibit cell death in tobacco plants. Curr. Biol. 9: 775-778.

Ohtsubo et al. (1999) Ethylene promotes the necrotic lesion formation and basic PR gene expression in TMV-infected tobacco. Plant Cell Physiol. 40: 808-817.

2. Mechanism of rice blast resistance

Blast disease caused by the plant fungus Magnaporthe oryzae is one of the most serious diseases in rice. We have been studying the molecular mechanism of blast disease resistance. Rice plants carrying the Pi-i resistance gene to blast fungus restrict invaded fungus in infected tissue via HR. We previously indicated that a rapid and transient accumulation of the plant hormone ethylene occurs during Pi-i-mediated HR, and an inhibotor for ethylene biosynthesis, but not for ethylene signaling, compromises Pi-i-mediated blast resistance (Iwai et al., 2006). These results suggested that ethylene biosynthesis is important for blast resistance. However, in the last step of ethylene biosynthesis, cyanide is known to be produced in stoichiometrically equivalent amounts with ethylene. Therefore, the individual roles of ethylene and cyanide in rice blast resistance remain unevaluated. Recently, we found that Pi-i-mediated resistance was compromised in transgenic rice lines, in which ethylene biosynthetic enzyme genes were silenced and then ethylene production was inhibited (Seo et al., 2011). The compromised resistance in transgenic lines was recovered by exogenously applying cyanide but not ethylene. Cyanide inhibited the growth of blast fungus in vitro and in planta. These results suggest that cyanide, whose production is triggered by HR in infected tissue, contributes to the resistance in rice plants via restriction of fungal growth.

Selected publications
Seo et al. (2011) Cyanide, a co-product of plant hormone ethylene biosynthesis, contributes to the resistance of rice to blast fungus. Plant Physiol. 155: 502-514.

Hasegawa et al. (2010) Phytoalexin accumulation in the interaction between rice and the blast fungus. Mol. Plant-Microbe Interact. 23: 1000-1011.

Mitsuhara et al. (2008) Characteristic expression of twelve rice PR1 family genes in response to pathogen infection, wounding, and defense-related signal compounds. Mol. Genet. Genomics 279: 415-427.

Sasaki et al. (2007) Characterization of two rice peroxidase promoters that respond to blast fungus-infection. Mol. Genet. Genomics 278: 709-722.

Iwai et al. (2007) Probenazole-induced accumulation of salicylic acid confers resistance to Magnaporthe grisea in adult rice plants. Plant Cell Physiol. 48: 915-924.

Iwai et al. (2006) Contribution of ethylene biosynthesis for resistance to blast fungus infection in young rice plants. Plant Physiol. 142: 1202-1215.

3. Mechanism of the wound response 

Wounding caused by physical injury and herbivore or insect attack is one of the most severe environmental stresses that plants encounter during thier life cycle. Among the plant cell responses to external stimuli, wound response is thought to be one of the most rapid. In addition, the response to wonding is systemic. Therefore, the response of plants to wounding is thought to be a good model system for studying signal transduction in plants. We have been studying the mechanism of signal transduction in the wound response of tobacco and rice plants. We have identified WIPK, a tobacco MAPK, as a factor involved in signal transduction of the wound response and WRK, a tobacco receptor-like protein kinase, as a possible receptor for the stimulation by wounding.

Selected publications
Hiraga et al. (2009) Involvement of two rice ETHYLENE INSENSITIVE3-LIKE genes in wound signaling. Mol. Gen. Genomics 282: 517-529.

Sasaki et al. (2007) Two novel AP2/ERF domain proteins interact with cis-element VWRE for wound-induced expression of the Tobacco tpoxN1 gene. Plant J. 50: 1079-1092.

Seo et al. (2007) The mitogen-activated protein kinases WIPK and SIPK regulate the levels of jasmonic and salicylic acids in wounded tobacco plants. Plant J. 49: 899-909.

Takabatake et al. (2006) Involvement of wound-induced receptor-like protein kinase in wound signal transduction in tobacco plants. Plant J. 47: 249-257.

Katou et al. (2005) Catalytic activation of plant MAPK phosphatase NtMKP1 by its physiological substrate SIPK, but not by calmodulins. J. Biol. Chem. 280: 39569-39581.

Seo et al. (1999) Jasmonate-based wound signal transduction requires activation of WIPK, a tobacco mitogen-activated protein kinase. Plant Cell 11: 289-298.

Seo et al. (1995) Tobacco MAP kinase: A possible mediator in wound signal transduction pathways. Science 270: 1988-1992.

4. Chemical control of plant diseases

Various techniques, such as fungicides, insecticides, resistance cultivars, biological control, and cultural practices, have been developed to control plant diseases. Among these techniques, using the ability of plants to induce resistance to a disease is thought to be the most effective method for controlling the disease. Plants respond to infection with pathogens by producing a large number of low-molecular-weight substances that induces defense responses such as expression of defense-related genes and accumulation of antibiotics. These defense-related natural substances produced by plants would be expected to become good materials for development of agrobiochemicals with low environmental impact that protect crops from diseases. We have been trying to explore such natural substances.

Seleceted publication
Seo et al. (2003) A diterpene as an endogenous signal for the activation of defense responses to tobacco mosaic virus infection and wounding in tobacco. Plant Cell 15: 863-873.

5. Gene recombination technology


Gene recombination technology is necessary to study gene function. We have been working toward development of new gene recombination technologies for studying functions of plant genes and their practical application. A high-expression binary vector (see Figure) we developed has been widely using by many researchers. Using new technologies we developed, we have also made transgenic plants conferring resistance to pathogens.

Selected publications
Mitsuhara et al. (2006) Genetic studies of transgenic rice plants overproducing an antibacterial peptide show that a high level of transgene expression did not cause inferior effects on host plants. Plant Biotechnol. 23: 63-69.

Iwai et al. (2002) Enhanced resistance to seed-transmitted bacterial diseases in transgenic rice plants overproducing an oat cell-wall-bound thionin. Mol Plant-Microbe Interact. 15: 515-521.

Fukuoka et al. (2000) Agrobacterium-mediated transformation of monocot and dicot plants using the NCR promoter derived from soybean chlorotic mottle virus. Plant Cell Rep. 19: 815-820.

Mitsuhara et al. (2000) Induced expression of sarcotoxin IA enhanced host resistance against both bacterial and fungal pathogens in transgenic tobacco. Mol. Plant-Microbe Interact. 13: 860-868.

Ohshima et al. (1999) Enhanced resistance to bacterial diseases in transgenic tobacco leaf overexpressing sarcotoxin IA, a bactericidal peptide of insect. J. Biochem. 125: 431-435.

Mitsuhara et al. (1996) Efficient promoter cassettes for enhanced expression of foreign genes in dicotyledonous and monocotyledonous plants. Plant Cell Physiol. 37: 49-59.

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