Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/5/2006
Publication Date: 9/12/2006
Citation: Herman, E.M., Rotter, K., Premakumar, R., Elwinger, G., Bae, H., Ehler, L.L., Chen, S., Livingston, D.P. 2006. Additional Freeze Hardiness in Wheat Acquired by Exposure to -3 degree C is Associated with Extensive Physiological, Morphological, and Molecular Changes. Journal of Experimental Botany. 57(14):3601-3618. Interpretive Summary: Plants adapt to overwinter by gradually modifying their cellular composition to prepare to resist the effects of subfreezing temperature. Plant biogeography maps such as the USDA plant hardiness map of North America is largely a map of freezing tolerance of plants. This process occurs at a cold, but above freezing temperature. The genetics, cell biology, or molecular biology of this process has been extensively investigated. Breeders and agronomists have long known that further treatment of previously cold adapted plants with brief exposure to subfreezing temperature improves the freezing tolerance of plants by several degrees C to the maximum freezing tolerance. Whether this additional freezing tolerance is due to biological processes has been debated. In the present manuscript we have used cellular and molecular assays to show that wheat plants induced to acquire additional freezing tolerance by exposure to subfreezing temperature results in broad changes in physiology and composition. These experiments are significant in that they are the first comprehensive demonstration that enhanced freezing processes induced by subzero temperature has broad biological effects. The results obtained have broad applications from biogeography, to plant breeding, to basic plant biology as well as implications in climate change. Potential users of this information include scientists, plant breeders and anyone interested in the biology that underlies USDA plant hardiness map.
Technical Abstract: Cold-acclimated plants acquire an additional 3-5 degrees C of freezing tolerance in LT50 tests when exposed to –3 degrees C for 12-18 h before freezing injury occurs. The -3 degrees C treatment replicates frost that can occur in the days or weeks leading to over-wintering by freezing tolerant plants. This additional freezing tolerance is called subzero-acclimation (SZA) to differentiate it from cold-acclimation (CA) that is acquired at above-freezing temperatures. Using wheat as a model, we have obtained results indicating that SZA is accompanied by changes in physiology, cellular structure, transcriptome and proteome. Using a variety of assays, including DNA arrays, RT-PCR, 2D gels with mass spectroscopy identification of proteins, and electron microscopy, changes were observed to occur as a consequence of SZA and the acquisition of added freezing tolerance. In contrast to CA, SZA induced the movement of intracellular water to extracellular space. Many unknown and stress-related genes were up-regulated by SZA including some with obvious roles in SZA. Many genes related to photosynthesis and plastids were down-regulated. Protein changes resulting from SZA often appeared to be a loss of rather than an appearance of new proteins. From a cytological perspective, SZA resulted in alterations of organelle structure including the golgi. The results indicate that the enhanced freezing tolerance of SZA is correlated with a wide diversity of changes, indicating that the additional freezing tolerance is the result of complex biological processes.