The multiple roles of light-harvesting chlorophyll a/b-protein complexes define structure and optimize function of Arabidopsis chloroplasts: A study using two chlorophyll b-less mutants

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Abstract

The multiple roles of light-harvesting chlorophyll a/b-protein complexes in the structure and function of Arabidopsis chloroplasts were investigated using two chlorophyll b-less mutants grown under metal halide lamps with a significant far-red component. In ch1-3, all six light-harvesting proteins of photosystem (PS) II were greatly decreased; in ch1-3lhcb5, Lhcb5 was completely absent while the other five proteins were further decreased. The thylakoids of ch1-3 were less negatively-charged than the wild type, and those of ch1-3lhcb5 were even less so. Despite the expected weaker electrostatic repulsion, however, thylakoids in leaves of the mutants were not well stacked, an effect we attribute to lower van der Waals attraction, lower electrostatic attraction between opposite charges, and the absence or instability of PSII supercomplexes and peripheral light-harvesting trimers. The quantum yield of oxygen evolution in leaves decreased from 0.109 (wild type) to 0.087 (ch1-3) and 0.081 (ch1-3lhcb5) O2 (photon absorbed) 1; we attribute this decrease to an excessive spillover from PSII to PSI, a limited PSII antenna, and increased light-independent thermal dissipation in PSII in the mutants. Destabilization of the donor side of PSII, indicated by slower electron donation to the redox-active tyrosine YZradical dot in ch1-3, probably enhanced PSII susceptibility to photoinactivation, increased the non-functional PSII complexes in vivo, and further inactivated PSII complexes in vitro. The evolution of chlorophyll b-containing chloroplasts seems to fine-tune oxygenic photosynthesis.

Abbreviations

9-AA
9-aminoacridine
β-DM
n-dodecyl-β-d-maltoside
BN-PAGE
blue-native polyacrylamide gel electrophoresis
BSA
bovine serum albumin
Car
carotenoid
Chl
chlorophyll
CP
chlorophyll-binding protein
DCMU
3-(3,4-dichlorophenyl)-1,1-dimethyl urea
EDTA
ethylenediaminetetraacetic acid
EPR
electron paramagnetic resonance
HEPES
N-(2-hydroxyethyl) piperazine-N′-(2-ethanesulfonic acid)
LHCI and LHCII
light-harvesting chlorophyll-binding protein complexes of PSI and PSII, respectively
LHCIIb
major trimeric LHCII
Lut
lutein
NPQ
non-photochemical quenching
P700
photoactive Chl of the PSI reaction centre
PSI and PSII
photosystems I and II, respectively
PpBQ
phenyl-p-benzoquinone
Psb
prefix for a PSII subunit
SDS
sodium dodecyl sulfate
Vio
violaxanthin
YD and YZ
redox-active tyrosines D and Z in PSII, respectively

Keywords

Chlorophyll b deficiency
Grana
Light-harvesting complex
Photosystem I
Photosystem II
Thylakoid stacking

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