Role of Glutathione and Low-Molecular-Weight Thiols in Seed Survival
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.
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.
Key papers published since 2011
Nagel, M., Kranner, I., Neumann, K., Rolletschek, H., Seal, C.E., Colville, L., Fernández-Marín, B., Börner, A. (In Press) Genome-wide association mapping and biochemical markers reveal that seed ageing and longevity are intricately affected by genetic background and developmental and environmental conditions in barley. Plant, Cell and Environment.
Gundel, P.E., Sorzoli, N., Ueno, A.C., Ghersa, C.M., Seal, C.E., Bastías, D.A., Martínez-Ghersa, M.A. (2015) Impact of ozone on the viability and antioxidant content of grass seeds is affected by a vertically transmitted symbiotic fungus. Environmental and Experimental Botany 113: 40-46.
Zagorchev, L., Seal, C.E., Kranner, I., Odjakova, M. (2014) A central role for thiols in plant tolerance to abiotic stress. International Journal of Molecular Sciences 14: 7405-7432.
Chen, H., Osuna, D., Colville, L., Lorenzo, O., Graeber, K., Kuster, H., Leubner-Metzger, G., Kranner, I. (2013) Transcriptome-wide mapping of pea seed ageing reveals a pivotal role for genes related to oxidative stress and programmed cell death. PLOS ONE 8 (10): e7847. doi:10.1371/journal.pone.0078471
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
Project partners and collaborators
Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA)
University of Innsbruck
University of Sofia
Kunming Institute of Botany
Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben
University of Padova
Royal Holloway University of London
University of Sussex