1a. Objectives (from AD-416)
Determine the extent of variation in genetic control of freezing tolerance in wheat with the purpose of identifying wheat lines with different and new combinations of genes that confer freezing tolerance. Define the role of phospholipid-related genes in cold acclimation and freezing tolerance.
1b. Approach (from AD-416)
The overall approach is to use artificial freezing of cold-acclimated winter wheat plants, with temperature measurements taken every two minutes in the crown zone of the plants, to precisely describe the components of the freezing process that are injurious to the plants. Pharmacological agents that enhance or inhibit phospholipase enzyme activity will be used in whole-plant assays to assess their impact on cold acclimation and freezing tolerance; concomitant microarray analysis will be used to define the genes involved.
3. Progress Report
Very little is known of adaptive changes that may occur in wheat plants as they freeze to potentially damaging temperatures. The regulation of gene expression in the crowns of wheat plants as they were slowly frozen from zero to -12 degrees C was studied. Expression of more than 400 genes changed significantly during the freezing process; genes upregulated outnumbered those downregulated by about a 9:1 ratio. As the temperature was lowered from -10 to -12 degrees C, five genes were significantly upregulated, while no genes were downregulated. Some of the genes were expressed differently in a very freezing tolerant wheat variety compared to a less freezing tolerant variety. These results indicated that wheat plants actively adjust gene expression during the freezing process, revealing a previously unknown aspect of freezing tolerance. The observed differences among varieties in their ability to respond to the sub-freezing temperatures suggested there is genetic variation for this ability that may be exploited to develop new varieties with improved freezing tolerance. We demonstrated that wheat plants actively adjust expression levels of hundreds of genes as they freeze to potentially damaging temperatures, but whether these changes translate into improved freezing tolerance was unknown. We tested the impact of a mild freeze-thaw cycle on the freezing tolerance of 22 winter wheat varieties. Plants frozen at -3 degrees C for 24 hours, then thawed at +3 C for 24 hours were significantly more tolerant of subsequent freezing to potentially damaging temperatures than plants that had not been exposed to the freeze-thaw cycle. This finding indicated that wheat plants respond to the freeze-thaw exposure with processes that result in improved freezing tolerance. Selecting plants with greater ability to respond to the freeze-thaw cycle may lead to new varieties with improved freezing tolerance.
1. Freezing tolerance in wheat. Very little was known of how wheat plants respond to the onset of sub freezing temperatures. ARS researchers at Pullman, WA measured the impact of a mild freeze-thaw cycle on the ability of wheat plants to survive subsequent exposure to potentially damaging temperatures, and measured changes in expression of thousands of genes as the temperature was lowered. Plants that had been exposed to a freeze-thaw cycle survived subsequent freezing to potentially damaging temperatures significantly more often than plants that had not been exposed to a freeze- thaw cycle. Over 300 genes significantly increased expression as the temperature was lowered. These results revealed a complex, previously unknown mechanism of response to the onset of freezing temperatures. Selection of plant lines that are better able to exploit the advantage conferred by the freeze- thaw cycle may lead to new varieties with improved freezing tolerance.
Skinner, D.Z. 2009. Post-acclimation transcriptome adjustment is a major factor in freezing tolerance of winter wheat. Funct Integr Genomics. 9:513–523. DOI 10.1007/s10142-009-0126-y