Two NADPH:Protochlorophyllide Oxidoreductases in Barley: Evidence for the Selective Disappearance of PORA during the Light-Induced Greening of Etiolated Seedlings.

Abstract

Chlorophyll synthesis in barley is controlled by two different light-dependent NADPH:protochlorophyllide oxidoreductases, termed PORA and PORB. PORA is present abundantly in etioplasts but selectively disappears soon after the beginning of illumination. This negative light effect is mediated simultaneously at three different levels. First, the concentration of porA mRNA declines drastically during illumination of dark-grown seedlings. Second, the plastids' ability to import the precursor of PORA (pPORA) is reduced during the transition from etioplasts to chloroplasts. This effect is due to a rapid decline in the plastidic level of protochlorophyllide (Pchlide), which is required for the translocation of the pPORA. Third, PORA becomes selectively destabilized in illuminated seedlings. When illuminated, PORA-Pchlide-NADPH complexes formed in the dark photoreduce their Pchlide to Chlide and become simultaneously susceptible to attack by plastid proteases. The PORA-degrading protease activity is not detectable in etioplasts but is induced during illumination. In contrast to PORA, the second Pchlide-reducing enzyme, PORB, remains operative in both illuminated and green plants. Its translocation into plastids does not depend on its substrate, Pchlide.

Substrate-Dependent and Organ-Specific Chloroplast Protein Import in Planta

“…Unlike PORB mRNA, the accumulation of PORA mRNA is negatively regulated by light (Apel, 1981;Armstrong et al, 1995). This difference in the expression of PORA and PORB has led to the suggestion that PORA's function in etiolated seedlings might be confined to the initial phase of light-induced chloroplast development (Reinbothe et al, 1995a;Sperling et al, 1997).…”

Cell growth defect factor 1 is crucial for the plastid import of NADPH:protochlorophyllide oxidoreductase A in Arabidopsis thaliana

“…Mutagenesis and homology modeling studies have provided important insights into the catalytic mechanism of POR (7)(8)(9)(10)(11). All three POR isoforms are nuclear gene products that must be imported posttranslationally into the plastids (12,13). Components have been identified in the plastid envelope mediating this import step (14)(15)(16).…”

Photoprotective role of NADPH:protochlorophyllide oxidoreductase A

“…In angiosperms such as barley, Arabidopsis (1, 2), tobacco (3) and Amaranthus tricolor (4), and gymnosperms such as pine species (5-7), several por gene families were identified that encode highly conserved POR polypeptides. PORA represents the negatively light-regulated POR enzyme whose level drops as a result of the concerted effect of light at the levels of transcription, mRNA stability, plastid import, and protein degradation after light-induced catalysis (8)(9)(10)(11). PORB, the second POR protein identified in barley and Arabidopsis (1,12), is constitutively expressed in darkgrown, illuminated, and light-adapted plants (see ref.…”