Longevity of preserved germplasm: The temperature dependency of aging reactions in glassy matrices of dried fern spores

Authors: BALLESTEROS, DANIEL - Royal Botanical Gardens; Hill, Lisa; Lynch, Ryan; PENCE, VALERIE - Cincinnati Zoo & Botanical Garden; PRITCHARD, HUGH - Royal Botanical Gardens; Walters, Christina

Submitted to: Plant and Cell Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2018
Publication Date: 11/6/2018
Citation: Ballesteros, D., Hill, L.M., Lynch, R.T., Pence, V., Pritchard, H., Walters, C.T. 2018. Longevity of preserved germplasm: The temperature dependency of aging reactions in glassy matrices of dried fern spores. Plant and Cell Physiology. 60(2):376-392. https://doi.org/10.1093/pcp/pcy217.
DOI: https://doi.org/10.1093/pcp/pcy217

Interpretive Summary: We often think that the temperatures of liquid nitrogen are so cold that all molecular movement is stopped and so chemical reactivity comes to a halt. We can show, however, that molecular motion occurs at these extremely low temperatures and this motion allows aging to continue, albeit at much slower rates than at higher temperatures. Motion of lipids were detected by measuring crystallization rate. Motion within the cytoplasm is inferred as short distance, rapid vibrations and rotations, which replace long range diffusive motion at liquid nitrogen temperatures. We use fern spores in the study because their simple single cell structure simplifies the study. Moreover, spores tend to age faster than seeds, so we only had to wait 12 years to see changes during cryogenic storage. The work draws upon research in the pharmaceutical literature to explain expiration dates for medications. Our work shows that we can predict how fast germplasm will age during long-term storage.

Technical Abstract: This study explores temperature dependency of aging rate in dry cells over a broad temperature range encompassing the fluid to solid transition (Tg) and well below. Spores from diverse species of ferns were stored at temperatures ranging from +45C to ~-176C (vapor phase above liquid nitrogen), and viability was monitored periodically for up to 12 years. Accompanying measurements using differential scanning calorimetry (DSC) provide insights into structural changes that occur such as Tg, between +45 and -20C (depending on moisture), and triacylglycerol (TAG) crystallization, between -5 and -35C (depending on species). We detected aging even at cryogenic temperatures, which we consider analogous to unscheduled degradation of pharmaceuticals stored well below Tg and caused by a shift in the nature of molecular motions that dominate chemical reactivity. We occasionally observed faster aging of spores stored in the freezer compared to the refrigerator, and linked this with mobility and crystallization within TAG, which likely influences molecular motion of dried cytoplasm in a narrow temperature range. Temperature dependency of longevity was remarkably similar among diverse fern spores, despite widely disparate aging rates. This provides a powerful tool to predict deterioration of germplasm preserved in the solid state. Future work will increase our understanding of molecular organization and composition contributing to differences in longevity.