Oxidative Stress and Death Phenomena
Research into the mechanisms of seed death caused by oxidative damage and programmed cell death during ageing of orthodox seeds or desiccation of recalcitrant seeds will improve our understanding of why seeds die during storage and enable better prediction of seed lifespan.
The 'free-radical theory of ageing', proposed by Denham Harman in the mid-1950s, predicts that the continuous formation of reactive oxygen species (ROS) throughout the life of an organism leads to cumulative damage, ageing and death. This applies to all organisms, and the seeds of higher plants are no exception. The desiccation tolerance of ‘orthodox seeds’ allows them to survive dehydration to very low intracellular water contents which minimises metabolic activity and ROS formation. Therefore, orthodox seeds live longer than most other organisms. However, even when stored at optimal conditions in seed banks, seeds are not immortal, and evidence is accumulating that the seeds of some species have intrinsically short life-spans. In this project, ROS-induced damage as a cause of seed death is being studied.
In addition to viability loss arising as a result of an accumulation of oxidative damage, seed cells can also undergo programmed cell death (PCD), which is the mechanism by which multi-cellular organisms eliminate unwanted cells. While ~1,500 papers on PCD in humans and animals are published per year, knowledge of the biochemical pathways that contribute to PCD in plants is rather limited.
Our recent results show that PCD occurs in seeds when environmental stress becomes detrimental. Thus, the seed eliminates cells that are damaged beyond repair. This will benefit the seed if only a few cells are damaged by ageing (in orthodox seeds) or desiccation (in recalcitrant seeds), but if too many cells undergo PCD, the whole seed will eventually die.
'DNA laddering' is a classic hallmark of the final, or execution phase, of PCD, in which DNA is cleaved into intra-nucleosomal fragments. Such fragments appear with increasing intensity as seeds lose viability, suggesting that death upon seed ageing during dry storage (in orthodox seeds) or during desiccation (in recalcitrant seeds) follows a controlled pattern of PCD. Based on our results and previous publications on human and animal systems, we have proposed a model of the biochemical pathways of PCD. In support of these findings, we observed, with collaborators in Beijing using terminal deoxynucleotide transferase-mediated DUTP nick end labelling (TUNEL) analysis, that PCD occurs during ageing of elm seeds. Moreover, we have determined the in situ localisation of ROS production by confocal laser scanning microscopy during the ageing process, both temporally and spatially. The role of ROS in PCD-associated events is now under investigation on a cell by cell basis.
Overall, we endeavour to achieve a synthesis of the biochemical pathways of ROS-provoked seed death and programmed cell death.
The outputs of the project are being published in peer-reviewed journals. Three PhD students and several college-based sandwich students and work experience students have been trained in the framework of this project and have contributed to the publication output.
Project Partners and Collaborators
Centre National de Semences Forestières, Ouagadougou
Germplasm Bank of Wild Species, Chinese Academy of Sciences, Kunming
Graduate University of the Chinese Academy of Sciences, Beijing
Beijing Forestry University
University of Bielefeld
University of Freiburg
National Genebank of Kenya, Kenyan Agricultural Research Institute, Nairobi
Russian Academy of Sciences, Kazan
University of KwaZulu-Natal, Pietermaritzburg
University of Salamanca
Food and Environment Research Agency, York
University College London
University of Sussex
Defra grant ZZ0105
Annex 1. Simplified scheme of mechanisms that contribute to cell death upon seed ageing and stress treatments.
Key papers published
Hu, D., Ma, G., Wang, Q., Yao, J.H., Wang, Y., Pritchard, H.W., Wang, X.F. (2012). Spatial and temporal nature of reactive oxygen species production and programmed cell death in elm (Ulmus pumila L.) seeds during controlled deterioration. Plant Cell and Environment 35: 2045-2059 (IF 5.22).
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 (9): 1093-1102 (IF 2.88).
Kranner, I., Chen, H.Y., Pritchard, H.W., Pearce, S.R., Birtic, S. (2011). Seed ageing correlates with inter-nucleosomal DNA fragmentation and loss of RNA integrity. Plant Growth Regulation 63: 63-72 (IF 1.60).
Long, R.L., Kranner, I., Steadman, K.J., Panetta, F.D., Birtic, S., Adkins, S.W. (2011). Wet-dry cycling extends seed persistence by re-instating antioxidant capacity. Plant and Soil 338: 511-519. (IF 2.73).
Seal, C.E., Zammit, R., Scott, P., Nyamongo, D.O., Daws, M.I., Kranner, I. (2010). Glutathione half-cell reduction potential as a seed viability marker of the potential oilseed crop Vernonia galamensis. Industrial Crops and Products 32 (3): 687-691. (IF 2.47).
Roach, T., Beckett, R.P., Minibayeva, F.V., Colville, L., Whitaker, C., Chen, H.Y., Bailly, C., Kranner, I. (2010). Extracellular superoxide production, viability and redox poise in response to desiccation in recalcitrant Castanea sativa seeds. Plant Cell and Environment 33 (1): 59-75 (IF 5.22).
Kranner, I., Birtic, S., Anderson, K.A., 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 (IF 5.42).
Conferences and workshops
Ten oral and poster contributions, three of which were plenary lectures, have been presented at seven international conferences, including:
29th ISTA Congress, (Köln, Germany, 2010); winner of the ISTA Seed Symposium Award 2010 presented by Manuela Nagel
9th ISSS meeting on seed science (Olstyn, Poland, 2008)
5th international desiccation tolerance workshop, 14 - 21 Jan 2007, Drakensberg, South Africa.