Expression of the gene encoding the alternative oxidase is induced by treatment of rice with the bleaching herbicide norflurazon

53. Expression of the gene encoding the alternative oxidase is induced by treatment of rice with the bleaching herbicide norflurazon D. Saisho, M. Nakazono, N. Tsutsumi and A. Hirai Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657 Japan There are two respiratory pathways, the cytochrome pathway and the alternative pathway, in mitochondria of higher plants. Electron flow via the cytochrome pathway is coupled to AlP synthesis and its terminal oxidase is cytochrome c oxidase (COX), whereas electron transport via the alternative pathway is not coupled to production of AlP and its terminal oxidase is alternative oxidase (AOX) (McIntosh 1994). We previously reported that expression of the AOX gene, which is encoded in the nuclear genome, is induced by low temperature in rice (Ito et al. 1997) and by addition of the respiratory inhibitor antimycin A in Arabidopsis thaliana (Saisho et al. 1997). These findings suggest that, by reduction of engagement of the cytochrome pathway, one or more signals is transmitted from the mitochondria to the nucleus and leads to an increase in the expression of the nuclear- encoded AOX gene. It is known that respiration is closely associated with photosynthesis in plants (Krömer 1995). This suggests that mitochondria and plastids communicate with each other during development. It is of interest to examine whether the expression of respiratory genes is regulated by the states of plastids in plants. The herbicide norflurazon blocks the biogenesis of plastids and causes the bleaching of plants under the light. It was reported that the expression of nuclear genes (e.g., CAB and RbcS) related to photosynthesis is suppressed by treatment with norflurazon, leading to the proposal that expression of these genes responds to some signals from plastids to the nucleus (Susek and Chory 1992). To understand the interaction between mitochondria and plastids, we investigated the effects of norfiurazon on the expression of rice respiratory genes (the nuclear-encoded OsAOXla and OsCOX5c genes and the mitochondrial-encoded OsCOX1 gene). Rice (cv Nipponbare) was grown at 28°C for 6 days in the presence of 0, 2 and 5 uM norflurazon. We isolated total RNA from seedlings and performed a Northern blot hybridization. As shown in Figure 1, the amount of OsAOXla transcripts dramatically increased as a result of treatment with norfiurazon. In contrast, the steady-state levels of OsCOX5c and OsCOX1 mRNAs were the same in the norfiurazon-treated plants as those in the plants without treatment. These results suggest that expression of UsA OXia, but not OsCOX5c and OsCOX1, is regulated by some signals from plastids. This raises the possibility of an interaction between plastids and mitochondria. In fact, in the chioroplast mutator(chm) of Arabidopsis thaliana, rearrangement of the mitochondrial genome affects the structure of plastids, indicating a close interaction between mitochondria and plastids (Sakamoto et al. 1996). A similar signal transduction between the two organelles might operate in rice as a result of treatment with norflurazon. Induction of the expression of UsA OXia by treatment with norflurazon appears to be associated with a signal transduction from plastids to the nucleus. Nevertheless, the mechanism responsible for the induction is unclear. It is known that reactive oxygen species (ROS) are generated in plastids by treatment with norfiurazon under the light (Susek and Chory 1992). Recently, it has been reported that AOX lowers ROS production in plant cells (Maxwell et al. 1999). Therefore, it is possible that ROS generated in plastids by norfiurazon treatment may be one of the signal molecules that induces the expression of mitochondrial respiratory gene, UsA OXia. Further investigations will be needed to understand the mechanisms regulating the expression of UsA OXia gene in the norfiurazontreated rice plants. References Ito, Y., D. Saisho, M. Nakazono, N. Tsutsumi and A. Hirai, 1997. Transcript levels of tandem-arranged alternafive oxidase genes in rice are increased by low temperature. Gene 203: 121-129. Kromer, S., 1995. Respiration during photosynthesis. Annu. Rev. Plant PhysioL Plant MoL BioL 46: 45-70. Mcintosh, L., 1994. Molecular biology of the alternative oxidase. Plant PhysioL 105: 781-786. Maxwell, D.P., Y. Wang and L McIntosh, 1999. The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells. Proc. NatL Acad. Sic. USA 96: 8271-8276. Saisho, D., E Nambara, S. Naito, N. Tsutsumi, A. Hirai and M. Nakazono, 1997. Characterization of the gene family for alternative oxidase from Arabidopsis thaliana. Plant MoL Biol. 35: 585-596. Sakamoto, W., H. Kondo, M. Murata and F. Motoyoshi, 1996. Altered mitochondrial gene expression in a maternal distorted leaf mutant of Arabidopsis induced by chioroplast mutator. Plant Cell 8: 1377-1390. Susek, R.E. and J. Chory, 1992. A tale of two genomes: role of a chioroplast signal in coordinating nuclear and plastid genome expression. Aust. J. Plant Physiol. 19: 387-399.

53. Expression of the gene encoding the alternative oxidase is induced by treatment of rice with the bleaching herbicide norflurazon
D. Saisho, M. Nakazono, N. Tsutsumi and A. Hirai

Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657 Japan

There are two respiratory pathways, the cytochrome pathway and the alternative pathway, in mitochondria of higher plants. Electron flow via the cytochrome pathway is coupled to AlP synthesis and its terminal oxidase is cytochrome c oxidase (COX), whereas electron transport via the alternative pathway is not coupled to production of AlP and its terminal oxidase is alternative oxidase (AOX) (McIntosh 1994). We previously reported that expression of the AOX gene, which is encoded in the nuclear genome, is induced by low temperature in rice (Ito et al. 1997) and by addition of the respiratory inhibitor antimycin A in Arabidopsis thaliana (Saisho et al. 1997). These findings suggest that, by reduction of engagement of the cytochrome pathway, one or more signals is transmitted from the mitochondria to the nucleus and leads to an increase in the expression of the nuclear- encoded AOX gene.

It is known that respiration is closely associated with photosynthesis in plants (Krömer 1995). This suggests that mitochondria and plastids communicate with each other during development. It is of interest to examine whether the expression of respiratory genes is regulated by the states of plastids in plants. The herbicide norflurazon blocks the biogenesis of plastids and causes the bleaching of plants under the light. It was reported that the expression of nuclear genes (e.g., CAB and RbcS) related to photosynthesis is suppressed by treatment with norflurazon, leading to the proposal that expression of these genes responds to some signals from plastids to the nucleus (Susek and Chory 1992). To understand the interaction between mitochondria and plastids, we investigated the effects of norfiurazon on the expression of rice respiratory genes (the nuclear-encoded OsAOXla and OsCOX5c genes and the mitochondrial-encoded OsCOX1 gene). Rice (cv Nipponbare) was grown at 28°C for 6 days in the presence of 0, 2 and 5 uM norflurazon. We isolated total RNA from seedlings and performed a Northern blot hybridization. As shown in Figure 1, the amount of OsAOXla transcripts dramatically increased as a result of treatment with norfiurazon. In contrast, the steady-state levels of OsCOX5c and OsCOX1 mRNAs were the same in the norfiurazon-treated plants as those in the plants without treatment. These results suggest that expression of UsA OXia, but not OsCOX5c and OsCOX1, is regulated by some signals from plastids. This raises the possibility of an interaction between plastids and mitochondria. In fact, in the chioroplast mutator(chm) of Arabidopsis thaliana, rearrangement of the mitochondrial genome affects the structure of plastids, indicating a close interaction between mitochondria and plastids (Sakamoto et al. 1996). A similar signal transduction between the two organelles might operate in rice as a result of treatment with norflurazon.

Induction of the expression of UsA OXia by treatment with norflurazon appears to be associated with a signal transduction from plastids to the nucleus. Nevertheless, the mechanism responsible for the induction is unclear. It is known that reactive oxygen species (ROS) are generated in plastids by treatment with norfiurazon under the light (Susek and Chory 1992). Recently, it has been reported that AOX lowers ROS production in plant cells (Maxwell et al. 1999). Therefore, it is possible that ROS generated in plastids by norfiurazon treatment may be one of the signal molecules that induces the expression of mitochondrial respiratory gene, UsA OXia. Further investigations will be needed to understand the mechanisms regulating the expression of UsA OXia gene in the norfiurazontreated rice plants.

References

Ito, Y., D. Saisho, M. Nakazono, N. Tsutsumi and A. Hirai, 1997. Transcript levels of tandem-arranged alternafive oxidase genes in rice are increased by low temperature. Gene 203: 121-129.

Kromer, S., 1995. Respiration during photosynthesis. Annu. Rev. Plant PhysioL Plant MoL BioL 46: 45-70.

Mcintosh, L., 1994. Molecular biology of the alternative oxidase. Plant PhysioL 105: 781-786.

Maxwell, D.P., Y. Wang and L McIntosh, 1999. The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells. Proc. NatL Acad. Sic. USA 96: 8271-8276.

Saisho, D., E Nambara, S. Naito, N. Tsutsumi, A. Hirai and M. Nakazono, 1997. Characterization of the gene family for alternative oxidase from Arabidopsis thaliana. Plant MoL Biol. 35: 585-596.

Sakamoto, W., H. Kondo, M. Murata and F. Motoyoshi, 1996. Altered mitochondrial gene expression in a maternal distorted leaf mutant of Arabidopsis induced by chioroplast mutator. Plant Cell 8: 1377-1390.

Susek, R.E. and J. Chory, 1992. A tale of two genomes: role of a chioroplast signal in coordinating nuclear and plastid genome expression. Aust. J. Plant Physiol. 19: 387-399.