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BIOENERGETICS AND PHOTOSYNTHESIS

Experimental Evidence for Ascorbate-Dependent Electron Transport in Leaves with Inactive Oxygen-Evolving Complexes^1,[OA]

Szilvia Z. Tóth^*, Jos T. Puthur², Valéria Nagy and Gyz Garab

Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, H–6701 Szeged, Hungary

Previously, we showed that in barley (Hordeum vulgare) leaves with heat-inactivated oxygen-evolving complexes, photosystem II (PSII) has access to a large pool of alternative electron donors. Based on in vitro data, we proposed that this donor was ascorbate, yet this hypothesis has not been substantiated in vivo. In this paper, with the aid of chlorophyll a fluorescence induced by short (5-ms) light pulses and 820-nm absorbance transient measurements on wild-type and ascorbate-deficient (vtc2-1) mutant leaves of Arabidopsis (Arabidopsis thaliana), we show that in heat-treated leaves the rate of electron donation to PSII as well as the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive electron transport toward photosystem I depend on the ascorbate content of the leaves: upon ascorbate treatment, the donation half-time in the wild type and the mutant decreased from 25 to 22 ms and from 55 to 32 ms, respectively. Thermoluminescence measurements show that Tyr_Z⁺ is involved in the electron transfer from ascorbate to PSII. These data and the similar ascorbate dependencies of the heat-treated and the tris(hydroxymethyl)aminomethane-treated thylakoid membranes, with maximal donation half-times of about 16 ms, show that ascorbate is capable of supporting a sustained electron transport activity in leaves containing inactivated oxygen-evolving complexes. This alternative electron transport appears to be ubiquitous in the plant kingdom and is present in the green alga Chlamydomonas reinhardtii, and its rate depends on the physiological state of the plants and on environmental conditions. Our data suggest that ascorbate, as an alternative PSII electron donor, plays a physiological role in heat-stressed plants.

² Present address: Post Graduate and Research Department of Botany, St. Thomas College, Pala, Kottayam 686 574, Kerala, India.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Szilvia Z. Tóth (sztoth{at}brc.hu).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.132621

^* Corresponding author; e-mail sztoth{at}brc.hu.

Received November 14, 2008; accepted January 9, 2009; published January 14, 2009.

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On the Inside

Peter V. Minorsky
Plant Physiol. 2009 149: 1205-1206. [Full Text]