title: Occurrence of the lutein-epoxide cycle in mistletoes of the Loranthaceae and Viscaceae.
abstract: The lutein-epoxide cycle (Lx cycle) is an auxiliary xanthophyll cycle known to operate only in some higher-plant species. It occurs in parallel with the common violaxanthin cycle (V cycle) and involves the same epoxidation and de-epoxidation reactions as in the V cycle. In this study, the occurrence of the Lx cycle was investigated in the two major families of mistletoe, the Loranthaceae and the Viscaceae. In an attempt to find the limiting factor(s) for the occurrence of the Lx cycle, pigment profiles of mistletoes with and without the Lx cycle were compared. The availability of lutein as a substrate for the zeaxanthin epoxidase appeared not to be critical. This was supported by the absence of the Lx cycle in the transgenic Arabidopsis plant lutOE, in which synthesis of lutein was increased at the expense of V by overexpression of epsilon-cyclase, a key enzyme for lutein synthesis. Furthermore, analysis of pigment distribution within the mistletoe thylakoids excluded the possibility of different localizations for the Lx- and V-cycle pigments. From these findings, together with previous reports on the substrate specificity of the two enzymes in the V cycle, we propose that mutation to zeaxanthin epoxidase could have resulted in altered regulation and/or substrate specificity of the enzyme that gave rise to the parallel operation of two xanthophyll cycles in some plants. The distribution pattern of Lx in the mistletoe phylogeny inferred from 18S rRNA gene sequences also suggested that the occurrence of the Lx cycle is determined genetically. Possible molecular evolutionary processes that may have led to the operation of the Lx cycle in some mistletoes are discussed.
authors.affiliation: Research School of Biological Sciences, and School of Biochemistry and Molecular Biology, Australian National University, GPO Box 475, ACT 2601 Canberra, Australia. matsubara@rsbs.anu.edu.au
authors.forename: Shizue
authors.initials: S
authors.lastname: Matsubara
authors.forename: Tomas
authors.initials: T
authors.lastname: Morosinotto
authors.forename: Roberto
authors.initials: R
authors.lastname: Bassi
authors.forename: Anna-Luise
authors.initials: AL
authors.lastname: Christian
authors.forename: Elke
authors.initials: E
authors.lastname: Fischer-Schliebs
authors.forename: Ulrich
authors.initials: U
authors.lastname: Lüttge
authors.forename: Birgit
authors.initials: B
authors.lastname: Orthen
authors.forename: Augusto C
authors.initials: AC
authors.lastname: Franco
authors.forename: Fabio R
authors.initials: FR
authors.lastname: Scarano
authors.forename: Britta
authors.initials: B
authors.lastname: Förster
authors.forename: Barry J
authors.initials: BJ
authors.lastname: Pogson
authors.forename: C Barry
authors.initials: CB
authors.lastname: Osmond
mesh_terms: D002338:Carotenoids / Q000378:metabolism; D004852:Epoxy Compounds / Q000378:metabolism*; D005843:Geography; D028183:Loranthaceae / Q000145:classification / Q000502:physiology*; D014975:Lutein / Q000378:metabolism*; D008954:Models, Biological; D028223:Pinus; D013045:Species Specificity; D028163:Viscaceae / Q000145:classification / Q000502:physiology*
chemical_list: D004852:Epoxy Compounds; D002338:Carotenoids; D014975:Lutein