title: Inhibition of lycopene cyclase results in accumulation of chlorophyll precursors.
abstract: Free porphyrins and their magnesium complexes, including chlorophylls, are potent photo-sensitizers. Plants usually accumulate these compounds bound to proteins together with protective compounds like carotenoids. Besides their protective role, carotenoids can play a structural role in these complexes. To analyze the effect of impaired carotenogenesis on plastid membranes we applied to barley seedlings the bleaching herbicide 2-(4-chlorophenylthio)triethylamine (CPTA) as a specific inhibitor for the cyclization of lycopene. To avoid interference with photo-oxidation, the essential experiments were performed on seedlings grown in darkness. While the amount of total carotenoids decreased, we found accumulation of more 6-carotene than lycopene in darkness clearly showing that CPTA inhibits the lycopene beta-cyclase more effectively than the lycopene epsilon-cyclase. The CPTA treatment resulted in accumulation of non-photoactive protochlorophyllide a; the amount of photoactive protochlorophyllide and NADPH:protochlorophyllide oxidoreductase remained constant. Further, the level of Mg protophorphyrin and its monomethyl ester increased to an extent similar to that obtained by application of 5-aminolevulinic acid (ALA). The perturbation of the ultrastructure of etioplast inner membranes, observed after CPTA-treatment, was not found after ALA-treatment; this excluded the accumulated tetrapyrroles as responsible for the perturbation. By contrast, the down-regulation of Lhcb and RbcS genes found after CPTA-treatment was compatible with the presumed role of Mg protophorphyrin as "plastid signal" for regulation of nuclear gene expression. Possible mechanisms for enhancement of tetrapyrrole accumulation by non-cyclic carotenoids are discussed.
authors.affiliation: Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35131 Padova, Italy.
authors.forename: Nicoletta
authors.initials: N
authors.lastname: La Rocca
authors.forename: Nicoletta
authors.initials: N
authors.lastname: Rascio
authors.forename: Ulrike
authors.initials: U
authors.lastname: Oster
authors.forename: Wolfhart
authors.initials: W
authors.lastname: Rüdiger
mesh_terms: D002338:Carotenoids / Q000096:biosynthesis*; D002734:Chlorophyll / Q000096:biosynthesis; D003624:Darkness; D005021:Ethylamines; D001467:Hordeum / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure; D019753:Intramolecular Lyases / Q000378:metabolism*; D018087:Plastids / Q000648:ultrastructure*; D011166:Porphyrins / Q000096:biosynthesis*; D036226:Seedlings / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D005021:Ethylamines; D011166:Porphyrins; D002734:Chlorophyll; C007143:2-(4-chlorophenylthio)triethylamine; D002338:Carotenoids; D019753:Intramolecular Lyases; C070925:lycopene cyclase-isomerase