Rice Genetics Newsletter, Vol. 18

Rgn18

35.	Expression of nuclear- and mitochondrial-encoded respiratory genes in rice under environmental stresses
	H. SAIKA, K. OHTSU, M. NAKAZONO, N. TSUTSUMI and A. Hirai Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan

All protein complexes in the cytochrome respiratory chain in mitochondria consist of multi-subunits. Some of these subunits are encoded by the nuclear genome, and the others are encoded by the mitochondrial genome. Thus, it is assumed that these complexes are synthesized through the coordinated expression of nuclear-encoded and mitochondrial-encoded genes. However, the regulation mechanism for this coordinated expression is unclear.

In some cases, such as rice seedlings, just after germination, the mRNA accumulation patterns are the same between nuclear- and mitochondrial-encoded respiratory genes. On the other hand, when rice seedlings were submerged, the steady-state mRNA levels of nuclear-encoded respiratory genes decreased, whereas those of mitochondrial-encoded genes remained constant for at least 24 hours (Tsuji et al. 2000). These findings indicate that the mechanisms that regulate expressions of nuclear-and mitochondrial-encoded respiratory genes may be the same or different, depending on the growth stage or growth conditions. In this report, we investigated the expressions of the genes for cytochrome c oxidase (COX2 and COX5c) and F₀F₁-ATP synthase (ATP1 and ATP2) in rice seedlings under low-temperature (4^oC), high-salt (250 mM NaCl) and drought (no water) conditions by Northern hybridization analysis using specific probes for each gene. The effect of our treatments on rice plants was checked by the expression of the genes for alternative oxidase (AOX1a) and alcohol dehydrogenase (ADH1). As shown in Figure 1B, AOX1a mRNA increased under low-temperature conditions, as was previously shown by Ito et al. (1997), and ADH1 mRNA increased under high-salt and drought

conditions, as was previously shown by Minhas and Grover (1999).

Under all conditions examined in this report, the steady-state mRNA levels of mitochondrial-encoded COX2 and ATP1 genes did not change during the 24 h period after the treatments started (Fig. 1A). However, after the treatments of low-temperature and drought conditions, the amounts of steady-state mRNA of nuclear-encoded COX5c and ATP2 genes decreased (Fig. 1A). On the other hand, under high-salt conditions, the steady-state mRNA levels of COX5c and ATP2 remained constant for 24 hours after the treatment started (Fig. 1A). These results indicate that the responses of nuclear-encoded COX5c and ATP2 genes to low-temperature and drought conditions are faster than the responses of the mitochondrial-encoded genes, COX2 and ATP1. However, these differences were not observed in the results for high-salt conditions. Thus, it is possible that the responses of nuclear-encoded respiratory genes are quicker than those of mitochondria-encoded respiratory genes to low-temperature and drought stresses, but not for salt stress. This response is similar to the response to stress caused by submergence treatment.

References

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

Minhas, D. and A. Grover, 1999. Transcript levels of genes encoding various glycolytic and fermentation enzymes change in response to abiotic stresses. Plant Sci. 146: 41-51.

Tsuji, H., M. Nakazono, D. Saisho, N. Tsutsumi and A. Hirai, 2000. Transcription levels of the nuclear-encoded respiratory genes in rice decrease by oxygen deprivation: evidence for involvement of calcium in expression of the alternative oxidase 1a gene. FEBS Letter. 471: 201-204.