Role of Glutathione and Low-Molecular-Weight Thiols in Seed Survival
Seed ageing and viability can be defined by the half-cell reduction potentials of glutathione and other low-molecular-weight thiols
The glutathione disulphide / glutathione (GSSH / GSH) redox couple
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.
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. It is the major cellular thiol antioxidant and redox buffer.
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, 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 correlation between E GSSG/2GSH and the physiological state of human cells (Schafer and Buettner 2001, Free Radical Biology and Medicine) encouraged us to investigate the precise relationship between plant cell viability and E GSSG/2GSH.
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 therefore appears to be a universal marker of plant cell viability that allows us to predict whether a seed will live, germinate and produce a new plant, or if it will die.
Since 2006, the concept has been expanded to include other low-molecular-weight thiols, which are generally present in seeds, although at lower concentrations than glutathione. These include cysteine, cysteinyl-glycine and gamma-glutamyl cysteine. Their individual half-cell reduction potentials were mathematically combined to define a thiol-disulphide based redox environment (E thiol-disulphide). Loss of seed viability correlated with a shift in E thiol-disulphide towards more positive values, with a LD50 value of -0.90 ± 0.093 mV M (mean ± SD). The mathematical definition of E thiol-disulphide is envisaged as a step towards the definition of the overall cellular redox environment, which will need to include all known redox-couples.
The outputs of the project are being published in peer-reviewed journals. Three PhD students and several college-based sandwich course students, work experience students and visiting scientists have been trained in the framework of this project and have contributed to the publication output.
Project partners and collaborators
Instituto Nacional de Pesquisas da Amazônia (INPA), Manaus
University of Sofia
Université Paris VI
Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben
Australian Research Council
Key papers published since 2006
Gundel, P.E., Hamilton, C.E., Seal, C.E., Helander, M., Martínez-Ghersa, M.A., Ghersa C.M., Vázquez de Aldana, B.R., Zabalgogeazcoa, I., Saikkonen, K. (2012). Antioxidants in Festuca rubra L. seeds affected by the fungal symbiont Epichloë festucae. Symbiosis: 1-8.
Zagorchev, L., Seal, C.E., Kranner, I., Odjakova, M. (2012). Redox state of low-molecular-weight thiols and disulphides during somatic embryogenesis of salt-treated suspension cultures of Dactylis glomerata L. Free Radical Research 46: 656-664.
Birtic, S., Colville, L., Pritchard, H.W., Pearce, S.R., Kranner, I. (2011). Mathematically combined half-cell reduction potentials of low-molecular-weight thiols as markers of seed ageing. Free Radical Research 45: 1093-1102.
Colville, L., Kranner, I. (2011). Desiccation tolerant plants as model systems to study redox regulation of protein thiols. Plant Growth Regulation 62: 241-255.
Cruz de Carvalho, M.H., Brunet, J., Bazin, J., Kranner, I., d’Arcy-Lametaa, A., Zuily-Fodila, Y., Contour-Ansela, D. (2010). Homoglutathione synthetase and glutathione synthetase in drought-stressed cowpea leaves: expression patterns and accumulation of low-molecular-weight thiols. Journal of Plant Physiology 167: 480-487.
Seal, C.E., Zammit, R., Scott, P., Kranner, I. (2010). Glutathione half-cell reduction potential and α-Tocopherol as viability markers during the prolonged storage of Suaeda maritima seeds. Seed Science Research 20: 47-53.
Kranner, I., Birtic, S.,Anderson, K.M. & 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.
Conferences and workshops
Since 2006, six oral and poster contributions have been presented at four international conferences, including:
COST Action FA0901 first scientific meeting (Naples, Italy, 2010).
9th ISSS meeting on seed science (Olstyn, Poland, 2008)
5th international desiccation tolerance workshop, (Drakensberg, South Africa, 2007).