Antenna complexes protect Photosystem I from photoinhibition.
BACKGROUND
Photosystems are composed of two moieties, a reaction center and a peripheral antenna system. In photosynthetic eukaryotes the latter system is composed of proteins belonging to Lhc family. An increasing set of evidences demonstrated how these polypeptides play a relevant physiological function in both light harvesting and photoprotection. Despite the sequence similarity between antenna proteins associated with the two Photosystems, present knowledge on their physiological role is mostly limited to complexes associated to Photosystem II.


RESULTS
In this work we analyzed the physiological role of Photosystem I antenna system in Arabidopsis thaliana both in vivo and in vitro. Plants depleted in individual antenna polypeptides showed a reduced capacity for photoprotection and an increased production of reactive oxygen species upon high light exposure. In vitro experiments on isolated complexes confirmed that depletion of antenna proteins reduced the resistance of isolated Photosystem I particles to high light and that the antenna is effective in photoprotection only upon the interaction with the core complex.


CONCLUSIONS
We show that antenna proteins play a dual role in Arabidopsis thaliana Photosystem I photoprotection: first, a Photosystem I with an intact antenna system is more resistant to high light because of a reduced production of reactive oxygen species and, second, antenna chlorophyll-proteins are the first target of high light damages. When photoprotection mechanisms become insufficient, the antenna chlorophyll proteins act as fuses: LHCI chlorophylls are degraded while the reaction center photochemical activity is maintained. Differences with respect to photoprotection strategy in Photosystem II, where the reaction center is the first target of photoinhibition, are discussed.
Alboresi
Alessandro
A
Laboratoire de Génétique et Biophysique des Plantes - UMR 6191 CEA-CNRS-Université de la Méditerranée, Marseille, France. alessandro.alboresi@univr.it
Ballottari
Matteo
M
Hienerwadel
Rainer
R
Giacometti
Giorgio M
GM
Morosinotto
Tomas
T
D017360:Arabidopsis / Q000235:genetics / Q000502:physiology / Q000528:radiation effects*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D061146:Chlorophyll Binding Proteins; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000502:physiology*; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D017382:Reactive Oxygen Species / Q000378:metabolism
D029681:Arabidopsis Proteins; D061146:Chlorophyll Binding Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D017382:Reactive Oxygen Species