Abstract
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.
Highlights
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
We showed that when the oxygen evolution in barley (Hordeum vulgare) leaves was inhibited by a short heat pulse (50°C, 40 s in a water bath), PSII was supplied by electrons from a large pool of alternative donors (Toth et al, 2007a)
In order to clarify the putative role of Asc as an alternative electron donor, first we compared wildtype and Asc-deficient mutant Arabidopsis plants
Summary
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. 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(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. Based on in vitro data, obtained on thylakoids with inactivated OECs (Katoh and San Pietro, 1967; Yamashita and Butler, 1968; Mano et al, 2004), we suggested that this alternative electron donor of PSII was ascorbate (Asc; Toth et al, 2007a). It has been proposed that the lumenal Asc plays a significant role in vivo by maintaining the violaxanthin deepoxidase activity and by supporting the electron transport in reaction center complexes with inactive
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