Poster: Posttranscriptional Regulation

Abs # 1075: Posttranscriptional regulation of POR gene expression correlates with red leaf formation in Amaranthus tricolor

Presenter:	Iwamoto, Kuninori , iwamoto@biol.s.u-tokyo.ac.jp
Authors	Iwamoto, Kuninori  (A)   Sugiyama, Munetaka  (B)   Fukuda, Hiroo  (A) (C)
Affiliations:	(A): Dept. Biol. Sci., Grad. Sch. Sci., Univ. Tokyo (B): Botanical Gardens, Grad. Sch. Sci., Univ. Tokyo (C): Plant Science Center, RIKEN

Amaranthus tricolor L. tricolor cv. Earlysplendor, an ornamental amaranth, generates red leaves instead of green leaves in late summer to early autumn in response to short-day photoperiods. Red leaves are characterized by lower levels of chlorophyll accumulation rather than higher levels of red pigment (betacyanin) accumulation. In spite of the low accumulation of chlorophyll in red leaves, the metabolic activity toward the production of Mg-protoporphyrin, an intermediate in the biosynthesis pathway for chlorophyll, was detected in red leaves as well as in green leaves. RNA gel blot analysis was performed to assess the expression of genes encoding twelve enzymes involved in chlorophyll biosynthesis. Among these enzymes, red-leaf-specific reduction of gene expression was observed only for NADPH-protochlorophyllide oxidoreductase (POR), a key enzyme catalyzing a later step of chlorophyll biosynthesis. In addition, immunoblot analysis showed little accumulation of POR protein(s) in red leaves. The POR transcripts remained at a very low level throughout development of red leaves. These findings indicate that the non-activation of POR gene expression and resultant loss of chlorophyll synthesis activity play a key role in red leaf formation of amaranth. Promoter activity of upstream regions of POR genes was examined in green and red leaves using an Agrobacterium-mediated transient assay system. The POR upstream regions exhibited higher promoter activity in red leaves than in green leaves. Results of transcriptional run-on assay supported the high transcription rate of POR genes in red leaves. These data suggest that the low accumulation level of POR mRNAs in red leaves of amaranth should be attributed to posttranscriptional regulation.