title: An intact light harvesting complex I antenna system is required for complete state transitions in Arabidopsis.
abstract: Efficient photosynthesis depends on maintaining balance between the rate of light-driven electron transport occurring in photosystem I (PSI) and photosystem II (PSII), located in the chloroplast thylakoid membranes. Balance is achieved through a process of 'state transitions' that increases energy transfer towards PSI when PSII is overexcited (state II), and towards PSII when PSI is overexcited (state I). This is achieved through redox control of the phosphorylation state of light-harvesting antenna complex II (LHCII). PSI is served by both LHCII and four light-harvesting antenna complex I (LHCI) subunits, Lhca1, 2, 3 and 4. Here we demonstrate that despite unchanged levels of LHCII phosphorylation, absence of specific Lhca subunits reduces state transitions in Arabidopsis. The severest phenotype-observed in a mutant lacking Lhca4 (ΔLhca4)-displayed a 69% reduction compared with the wild type. Yet, surprisingly, the amounts of the PSI-LHCI-LHCII supercomplex isolated by blue native polyacrylamide gel electrophoresis (BN-PAGE) from digitonin-solubilized thylakoids were similar in the wild type and ΔLhca mutants. Fluorescence excitation spectroscopy revealed that in the wild type this PSI-LHCI-LHCII supercomplex is supplemented by energy transfer from additional LHCII trimers in state II, whose binding is sensitive to digitonin, and which are absent in ΔLhca4. The grana margins of the thylakoid membrane were found to be the primary site of interaction between this 'extra' LHCII and the PSI-LHCI-LHCII supercomplex in state II. The results suggest that the LHCI complexes mediate energetic interactions between LHCII and PSI in the intact membrane.
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK.
authors.forename: Samuel L
authors.initials: SL
authors.lastname: Benson
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
authors.forename: Pratheesh
authors.initials: P
authors.lastname: Maheswaran
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
authors.forename: Maxwell A
authors.initials: MA
authors.lastname: Ware
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK.
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Horton
authors.affiliation: Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå SE-901 87, Sweden.
authors.forename: Stefan
authors.initials: S
authors.lastname: Jansson
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Ruban
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK.
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Johnson