Plant and Cell Physiology, 2003, Vol. 44, No. 10 963-974
© 2003 Oxford University Press
Functional Analysis of Isoforms of NADPH:Protochlorophyllide Oxidoreductase (POR), PORB and PORC, in Arabidopsis thaliana
1 Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501 Japan
2 Plant Functions Laboratory, RIKEN, Wako, 351-0198 Japan
3 Division of Developmental Biology, National Institute for Basic Biology, Okazaki, 444-8585 Japan
4 Research Institute for Biological Sciences, Okayama, 716-1241 Japan
5 Kazusa DNA Research Institute, Yana 1532-3, Kisarazu, 292-0812 Japan
6 Mitsui Plant Biotechnology Research Institute (disbanded in March 1999), Tsukuba, 305-0047 Japan
7 Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657 Japan
NADPH:protochlorophyllide oxidoreductase (POR) catalyzes the light-dependent reduction of protochlorophyllide. To elucidate the physiological function of three differentially regulated POR isoforms (PORA, PORB and PORC) in Arabidopsis thaliana, we isolated T-DNA tagged null mutants of porB and porC. The mature seedlings of the mutants had normal photosynthetic competencies, showing that PORB and PORC are interchangeable and functionally redundant in developed plants. In etiolated seedlings, only porB showed a reduction in the photoactive protochlorophyllide and the size of prolamellar bodies (PLBs), indicating that PORB, as well as PORA, functioned in PLB assembly and photoactive protochlorophyllide formation in etiolated seedlings. When illuminated, the etiolated porB seedling was able to green to a similar extent as the wild type, whereas the greening was significantly reduced under low light conditions. During greening, high light irradiation increased the level of PORC protein, and the greening of porC was repressed under high light conditions. The porB, but not porC, etiolated seedling was more sensitive to the far-red block of greening than the wild type, which is caused by depletion of endogenous POR proteins resulting in photo-oxidative damage. These results suggest that, at the onset of greening, PLBs are important for efficient capture of light energy for photoconversion under various light conditions, and PORC, which is induced by high light irradiation, contributes to photoprotection during greening of the etiolated seedlings.
8 Corresponding author: E-mail, tmasuda{at}bio.titech.ac.jp; Fax, +81-45-924-5823.
9 Current address: Section of Plant Biology, Division of Biological Sciences, University of California, Davis, CA 95616, U.S.A.
10 Current address: Department of Cellular Mechanisms and Functions, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-8601 Japan.
11 Current address: Boyce Thompson Institute, Cornell University, Tower Road, Ithaca, NY 14853-1901, U.S.A.
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