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Greg Armstrong Assistant Professor Department of Plant Biology 108 Botany and Zoology Bldg. 1735 Neil Ave. Columbus OH 43210 Office: (614) 292-4817 E-mail: armstrong.275@osu.edu	Back to Scientists

Research Interests:

Light is one of the most important environmental factors governing plant growth and development. Photons absorbed by the chlorophyll- and carotenoid-containing pigment-protein complexes of the photosynthetic apparatus are used for the production of chemical energy. Furthermore, specific wavelengths are monitored by a variety of regulatory chromoproteins, including phytochromes, cryptochromes and other blue and ultraviolet light photoreceptors. We are interested in understanding how plants perceive and respond to light at both the seedling and adult stages, using a spectrum of molecular, genetic, biochemical, physiological and genomic approaches.

Recent efforts have focused on visible greening as a central light-dependent event in angiosperm development. Greening occurs upon illumination because the Mg-tetrapyrroles chlorophylls a and b, together with carotenoids, are produced in large amounts in plastids and are incorporated into the membrane-bound light-harvesting antenna and reaction center complexes of photosystems I and II. Although greening reflects the establishment of photosynthesis, this process also exposes the developing seedling to the risk of photodynamic damage sensitized by chlorophyll precursors. Tetrapyrrole biosynthesis is therefore tightly regulated to prevent the accumulation of free intermediates.

Dark-grown angiosperm seedlings contain no chlorophyll because the reduction of protochlorophyllide a (Pchlide a) to chlorophyllide a requires the activity of a highly unusual, strictly light-dependent enzyme, the NADPH:Pchlide oxidoreductase (POR). POR is not only the most abundant protein in etioplast inner membranes, but also an active determinant of plastid inner membrane architecture. In etioplasts POR exists in a dark-stable ternary complex together with NADPH and Pchlide a. This complex is enzymatically active upon illumination at temperatures as low as -70°C. POR thus couples light-dependent greening and chloroplast differentiation to photomorphogenesis, while simultaneously protecting seedlings from Pchlide-induced photodynamic damage.

We are currently applying transgenic and reverse genetic approaches to obtain information about the functions, regulation and expression patterns of the three differentially light- and developmentally-regulated POR enzymes of Arabidopsis, PORA, PORB, and PORC. Whereas the expression of PORA and PORB dominates in etiolated seedlings, greened seedlings and adult plants contain almost exclusively PORB and PORC. The goals of this research area are to establish the roles of the three POR proteins throughout angiosperm ontogeny in mediating plastid development, light-dependent chlorophyll biosynthesis and protection against photooxidative damage.

Selected Publications (1995 to date):

Plastid development, chlorophyll biosynthesis and mechanisms for protection against photooxidative damage

• Su, Q., Frick, G., Armstrong, G. A., and Apel, K. (2001) POR C of Arabidopsis thaliana: A third light- and NADPH-dependent protochlorophyllide oxidoreductase that is differentially regulated by light, Plant Mol. Biol., 47, 805-813.
• Ougham, H. J., Thomas, A. M., Thomas, B. J., Frick, G. A., and Armstrong, G. A. (2001) Both light-dependent protochlorophyllide oxidoreductase A and protochlorophyllide oxidoreductase B are down-regulated in the slender mutant of barley, J. Exp. Bot., 52, 1447-1454.
• Franck, F., Sperling, U., Frick, G., Pochert, B., van Cleve, B., Apel, K., and Armstrong, G. A. (2000) Regulation of etioplast pigment-protein complexes, inner membrane architecture, and protochlorophyllide a chemical heterogeneity by light-dependent NADPH:protochlorophyllide oxidoreductases A and B, Plant Physiol., 124, 1678-1696.
• Armstrong, G. A., Apel, K., and Rüdiger, W. (2000) Does a light-harvesting protochlorophyllide a/b-binding protein complex exist? Trends Plant Sci., 5, 40-44.
• Armstrong, G. A., and Apel, K. (1998) Molecular and genetic analysis of light-dependent chlorophyll biosynthesis. Meth. Enzymol., 297, 237-244.
• Armstrong, G. A. (1998) Greening in the dark: light-independent chlorophyll biosynthesis from anoxygenic photosynthetic bacteria to gymnosperms. J. Photochem. Photobiol. B Biol., 43, 87-100.
• Sperling, U., Franck, F., van Cleve, B., Frick, G., Apel, K., and Armstrong, G. A. (1998) Etioplast differentiation in Arabidopsis: both PORA and PORB restore the prolamellar body and photoactive protochlorophyllide-F655 to the cop1 photomorphogenic mutant. Plant Cell, 10, 283-296.
• Sperling, U., van Cleve, B., Frick, G., Apel, K., and Armstrong, G. A. (1997) Overexpression of light-dependent PORA or PORB in plants depleted of endogenous POR by far-red light enhances seedling survival in white light and protects against photooxidative damage. Plant J., 12, 649-658.
• Runge, S., Sperling, U., Frick, G., Apel, K., and Armstrong, G. A. (1996) Distinct roles for light-dependent NADPH:protochlorophyllide oxidoreductases (POR) A and B during greening in higher plants. Plant J., 9, 513-523.
• Lebedev, N., van Cleve, B., Armstrong, G., and Apel, K. (1995) Chlorophyll synthesis in a deetiolated (det340) mutant of Arabidopsis without NADPH-protochlorophyllide (Pchlide) oxidoreductase (POR) A and photoactive Pchlide-F655. Plant Cell, 7, 2081-2090.
• Runge, S., van Cleve, B., Lebedev, N., Armstrong, G., and Apel, K. (1995) Isolation and classification of chlorophyll-deficient xantha mutants of Arabidopsis thaliana. Planta, 197, 490-500.
• Armstrong, G. A., Runge, S., Frick, G., Sperling, U., and Apel, K. (1995) Identification of NADPH:protochlorophyllide oxidoreductases A and B: a branched pathway for light-dependent chlorophyll biosynthesis in Arabidopsis thaliana. Plant Physiol., 108, 1505-1517.

Carotenoid biosynthesis

• Armstrong, G. A. (1999) Carotenoid genetics and biochemistry. In Cane, D. E. (ed.), Comprehensive Natural Products Chemistry, Volume 2: Isoprenoids Including Carotenoids and Steroids. Elsevier Science Ltd., Oxford, United Kingdom, pp. 321-352.
• Armstrong, G. A. (1997) Genetics of eubacterial carotenoid biosynthesis: a colorful tale. Annu. Rev. Microbiol., 51, 629-659.
• Burkhardt, P. K., Beyer, P., Wünn, J., Klöti, A., Armstrong, G. A., Schledz, M., v. Lintig, J., and Potrykus, I. (1997) Transgenic rice (Oryza sativa L.) endosperm expressing daffodil (Narcissus pseudonarcissus) phytoene synthase accumulates phytoene, a key intermediate of provitamin A biosynthesis. Plant J., 11, 1071-1078.
• Armstrong, G. A., and Hearst, J. E. (1996) Genetics and molecular biology of carotenoid pigment biosynthesis. FASEB J., 10, 228-237.
• Armstrong, G. A. (1995) Genetic analysis and regulation of carotenoid biosynthesis: structure and function of the crt genes and gene products. In Blankenship, R. E., Madigan, M. T., and Bauer, C. E. (eds.), Advances in Photosynthesis, Volume 2: Anoxygenic Photosynthetic Bacteria. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 1135-1157.
  

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