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Published on January 14, 2009; 10.1104/pp.108.132621

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Received November 14, 2008
Accepted January 9, 2009

Experimental evidence for ascorbate-dependent electron transport in leaves with inactive oxygen-evolving complexes

Szilvia Z. Toth *, Jos T. Puthur , Valeria Nagy , and Gyozo Garab

Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary

* Corresponding author; email: sztoth{at}brc.hu.

Earlier we have shown that in barley leaves with heat-inactivated oxygen-evolving complexes (OEC) photosystem II (PSII) has access to a large pool of alternative electron donors (Toth et al., 2007a). 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 thaliana we show that in heat-treated leaves the rate of electron donation to PSII as well as the DCMU-sensitive electron transport towards PSI 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 TyrZ+ is involved in the electron transfer from ascorbate to PSII. These data and the similar ascorbate dependencies of the heat-treated and the Tris-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 OECs. 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 Asc, as alternative PSII electron donor, plays a physiological role in heat-stressed plants.






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