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Seed Viability and Ageing Defined by Glutathione Redox Potential

The glutathione disulphide / glutathione (GSSH / GSH) redox couple

This project is one of a series in the theme 'Science of Seed Ageing'.

Dried collections of seeds for agricultural and conservation purposes approach six million globally. The quality of seed lots is conventionally assessed, using a plethora of germination tests, requiring the input of millions of pounds for staff and other resources each year. Hence, it is desirable to develop a rapid diagnostic test for seed quality. However, the elucidation of factors that contribute to viability loss is still compromised by the lack of a universal measure that quantifies ‘stress’.

In ‘healthy’ tissue, control of factors that induce oxidative stress is supported by certain enzymes and molecular antioxidants such as glutathione (γ glutamyl-cysteinyl‑glycine). Glutathione is a ubiquitous, probably ancient, molecule with numerous functions in sulphur metabolism and stress signalling⁽¹⁾. It is the major cellular thiol antioxidant and redox buffer⁽²⁾.

Numerous attempts have been made to use antioxidants as stress markers. Using antioxidant concentrations alone has limitations, because they often show a Gaussian response to stress^(3,4), with an initial increase followed by a breakdown, making the interpretation of a single measurement ambiguous. Correlations of the redox state of antioxidants with the physiological state of plant tissue are more satisfactory. To accurately define the redox state of a concentration-dependent redox couple such as the glutathione disulphide / glutathione (GSSG/2GSH) couple, its half-cell reduction potential (E_GSSG/2GSH) and its reducing capacity must be known. A recently proposed correlation between E_GSSG/2GSH and the physiological state of human cells⁽²⁾ encouraged us to investigate the precise relationship between plant cell viability and E_GSSG/2GSH. Various species were studied in the following plant / fungal families: Asteraceae, Apocynaceae, Fabaceae, Fagaceae, Meliaceae, Myrothamnaceae, Oleaceae, Phyllanthaceae, Pinaceae, Poaceae, Pottiaceae, Rhodomelaceae, Sapotaceae, Solanaceae / Lecanoraceae, Lobariaceae and Peltigeraceae.

We first showed in four species subjected to stress treatments, that viability decreased by 50% when E_GSSG/2GSH increased to -180 to -160 mV. We then re-analysed data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E_GSSG/2GSH exceeds -160 mV. E_GSSG/2GSH is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate and produce a new plant, or if it will die⁽¹⁾.

(1) Kranner, I., Birtic, S.,Anderson, K.M. and Pritchard, H.W. (2006) Glutathione half-cell reduction potential: A universal stress marker and modulator of programmed cell death? Free Radical Biology and Medicine 40, 2155-2165.

(2) Schafer, F.Q., Buettner, G.R. (2001) Redox environment of the cell as viewed through the redox state of the glutathione disulphide/ glutathione couple. Free Radical Biology and Medicine 30, 1191-1212.

(3) Kranner, I., Cram, W.J., Zorn, M., Wornik, S., Yoshimura, I., Stabentheiner, E. and Pfeifhofer, H.W. (2005) Antioxidants and photoprotection in a lichen as compared with its isolated symbiotic partners. PNAS 102, 3141-3146.

(4) Kranner, I., Beckett, R.P., Wornick, S., Zorn, M. and Pfeifhofer, H.W. (2002) Revival of a resurrection plant correlates with its antioxidant status. The Plant Journal 31, 13-24.

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