Journal Article

The extreme halophyte <i>Salicornia veneta</i> is depleted of the extrinsic PsbQ and PsbP proteins of the oxygen-evolving complex without loss of functional activity

Cristina Pagliano, Nicoletta La Rocca, Flora Andreucci, Zsuzsanna Deák, Imre Vass, Nicoletta Rascio and Roberto Barbato

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 103, issue 3, pages 505-515
Published in print February 2009 | ISSN: 0305-7364
Published online November 2008 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcn234
The extreme halophyte Salicornia veneta is depleted of the extrinsic PsbQ and PsbP proteins of the oxygen-evolving complex without loss of functional activity

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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.

Keywords: Photosystem II; PsbQ; PsbP; halophytes; Salicornia veneta

Journal Article.  7856 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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