JXB Advance Access originally published online on November 28, 2003
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Journal of Experimental Botany, Vol. 55, No. 394, pp. 145-146, January 1, 2004
© 2004 Oxford University Press
GENE NOTE |
The rice pyruvate decarboxylase 3 gene, which lacks introns, is transcribed in mature pollen*
Received 17 July 2003; Accepted 7 October 2003
1 Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo,Tokyo 113-8657, Japan
2 Research Institute of Flower Biotechnology, Northeast Forestry University, Harbin 150040, China
3 Asian Natural Environmental Science Center, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-8657, Japan
* The nucleotide sequence data reported here were deposited in DDBJ under the accession number AB111050. These authors contributed equally to this work. Present address: School of Agriculture, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, Aichi 468-8502, Japan. 
To whom correspondence should be addressed. Fax: +81 3 5841 5183. E-mail: anakazo{at}mail.ecc.u-tokyo.ac.jp
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Abstract |
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The rice pyruvate decarboxylase 3 gene (PDC3), which has no introns, was previously postulated to be a pseudogene because no PDC3 mRNA had been detected, even under anaerobic conditions. However, in this study, it was found that rice PDC3 transcripts accumulated in panicles after heading. Within anthers obtained from the panicles, PDC3 was shown to be transcribed in mature pollen by in situ hybridization. These results suggest that the rice PDC3 is a functional gene. Its product may play a role in aerobic alcoholic fermentation in mature pollen.
Key words: Alcoholic fermentation, intron-less gene, Oryza sativa, pollen, pyruvate decarboxylase.
Alcoholic fermentation consists of two reactions, in which pyruvate is first decarboxylated to acetaldehyde by pyruvate decarboxylase (PDC), and then acetaldehyde is converted to ethanol by alcohol dehydrogenase (ADH). In plants, this pathway occurs at low levels under aerobic conditions, and is strongly induced under anaerobic conditions (Peschke and Sachs, 1993; Sachs et al., 1996; Vartapetian and Jackson, 1997). An exception is alcoholic fermentation during pollen development, in which the pathway increases even under aerobic conditions (Bucher et al., 1995; Tadege and Kuhlemeier, 1997). The products of aerobic fermentation (acetaldehyde and ethanol) are reported to be metabolized in a pathway that bypasses mitochondrial pyruvate dehydrogenase (Tadege et al., 1999). The bypass contributes to respiration and lipid biosynthesis in tobacco pollen (Tadege et al., 1999; Mellema et al., 2002).
Rice PDC is encoded by at least four PDC genes (Hossain et al., 1996; Rivoal et al., 1997). Among the four PDC genes that have been identified (PDC1–PDC4), cDNA clones encoding PDC1, PDC2 and PDC4 have been isolated and their expressions of the three PDC genes have been shown to be induced under anaerobic conditions (Hossain et al., 1994a; Huq et al., 1995; Rivoal et al., 1997). A genomic clone of PDC3 was isolated from rice (cv. IR54) and its nucleotide sequence was determined (Hossain et al., 1994b). The rice PDC1 and PDC2 genes consist of six exons and five introns (Hossain et al., 1996; Huq et al., 1999), whereas the rice PDC3 gene lacks introns (Hossain et al., 1994b). Furthermore, no transcript of PDC3 was found even under anaerobic conditions by northern hybridization. Therefore, PDC3 was suggested to be a pseudogene (Hossain et al., 1994b). However, organ-specific expression of rice PDC3, which would be a more sensitive test, was not examined. In the present study, PDC3 transcripts were sought in various organs of rice (cv. Nipponbare) using northern hybridization and were detected in panicles after heading, but not in other organs or tissues (Fig. 1A). On the other hand, PDC1 mRNA was found in roots of light-grown seedlings, and PDC2 mRNA was observed in all the organs examined, although its expression was highest in young panicles (Fig. 1A).
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Anthers in panicles after heading contain mature pollen. Because mature pollen is a site of alcoholic fermentation (Tadege and Kuhlemeier, 1997; Mellema et al., 2002), it may accumulate PDC3 mRNA. To examine this possibility, in situ hybridization of PDC3 transcripts was performed using anthers obtained from panicles after heading, according to the method of Ishiwatari et al. (2000) except that FAA (5% formalin, 45% ethanol and 5% acetic acid) was used as a fixative. As expected, the antisense PDC3 probe, but not the sense probe, specifically hybridized to mature pollen (Fig. 1B).
A search for PDC3 homologues in the rice EST clone database found one clone, E3273, which was assumed to encode PDC3. The EST clone was constructed from mRNA extracted from panicles at the flowering stage of rice (cv. Nipponbare). This stage perfectly corresponds to the stage at which PDC3 mRNA was detected by northern hybridization (Fig. 1A). The 1967 bp insert of the E3273 clone was completely sequenced (accession number AB111050 [GenBank] ). The clone contained a complete open reading frame encoding a polypeptide of 587 amino acid residues. The predicted amino acid sequence was 94% identical to that of PDC3 reported previously (Hossain et al., 1994b; data not shown). It was concluded that this PDC gene is PDC3 because it lacks introns and its chromosomal location corresponds to the location of PDC3 mapped by Huq et al. (1999) (data not shown). The difference in the deduced amino acid sequences is probably due to a difference in cultivars [Japonica rice cv. Nipponbare (this study) and Indica rice cv. IR54 (Hossain et al., 1994b)].
These results indicate that the rice PDC3 gene is not a pseudogene but a functional gene and that PDC3 may play a role in alcoholic fermentation in mature pollen.
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Acknowledgements |
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The authors express their appreciation to Hiroyuki Tsuji for his technical assistance. This work was partly supported by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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References |
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