AOBPreview published online on November 25, 2008
Annals of Botany, doi:10.1093/aob/mcn234
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The extreme halophyte Salicornia veneta is depleted of the extrinsic PsbQ and PsbP proteins of the oxygen-evolving complex without loss of functional activity
1 Dipartimento di Scienze dell'Ambiente e della Vita, Università del Piemonte Orientale, via Bellini 25/G, 15100 Alessandria, Italy
2 Dipartimento di Biologia, Università di Padova, via Bassi 58/B, 35131 Padova, Italy
3 Institute of Plant Biology, Biological Research Center, Temesvári krt. 62, H-6726 Szeged, Hungary
* For correspondence. E-mail roberto.barbato{at}mfn.unipmn.it
Received: 30 July 2008 Returned for revision: 2 September 2008 Accepted: 27 October 2008
Background and Aims: Photosystem II of oxygenic organisms is a multi-subunit protein complex made up of at least 20 subunits and requires Ca2+ and Cl– as essential co-factors. While most subunits form the catalytic core responsible for water oxidation, PsbO, PsbP and PsbQ form an extrinsic domain exposed to the luminal side of the membrane. In vitro studies have shown that these subunits have a role in modulating the function of Cl– and Ca2+, but their role(s) in vivo remains to be elucidated, as the relationships between ion concentrations and extrinsic polypeptides are not clear. With the aim of understanding these relationships, the photosynthetic apparatus of the extreme halophyte Salicornia veneta has been compared with that of spinach. Compared to glycophytes, halophytes have a different ionic composition, which could be expected to modulate the role of extrinsic polypeptides.
Methods: Structure and function of in vivo and in vitro PSII in S. veneta were investigated and compared to spinach. Light and electron microscopy, oxygen evolution, gel electrophoresis, immunoblotting, DNA sequencing, RT–PCR and time-resolved chlorophyll fluorescence were used.
Key Results: Thylakoids of S. veneta did not contain PsbQ protein and its mRNA was absent. When compared to spinach, PsbP was partly depleted (30 %), as was its mRNA. All other thylakoid subunits were present in similar amounts in both species. PSII electron transfer was not affected. Fluorescence was strongly quenched upon irradiation of plants with high light, and relaxed only after prolonged dark incubation. Quenching of fluorescence was not linked to degradation of D1 protein.
Conclusions: In S. veneta the PsbQ protein is not necessary for photosynthesis in vivo. As the amount of PsbP is sub-stoichiometric with other PSII subunits, this protein too is largely dispensable from a catalytic standpoint. One possibility is that PsbP acts as an assembly factor for PSII.
Key words: Photosystem II, PsbQ, PsbP, halophytes, Salicornia veneta
These authors contributed equally to this work.