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
A range of subzero temperature regimes was investigated for their impact on the ability of winter wheat plants to survive freezing episodes. A set of 48 temperature regimes was found to result in the complete range of survival (0-100%) of each of 24 wheat lines investigated. Data analysis is underway. Results are being used to select wheat lines to cross to combine apparently disparate sources of freezing tolerance into a single wheat line. A laboratory method of assessing the impact of applied chemicals on the ability of wheat plants to withstand freezing was further developed. This method is being used to evaluate the impact on freezing tolerance of chemicals that influence hormone signaling, and chemicals that influence phospholipid composition. The main impact of these chemicals appears to be on the ability to acclimate to cold temperatures. Chemicals that enhance, and chemicals that inhibit the ability to cold-acclimate have been identified. This information will be used to further our understanding of the metabolic processes involved in cold acclimation and freezing tolerance. A genome-wide study of gene expression changes in wheat plants undergoing freezing to potentially damaging temperatures was completed. A previously unknown precise level of gene expression control was revealed. Over 400 genes significantly increased expression as plants were cooled to-10 degrees, then a small number of stress-response related genes continued to increase expression as the plants were cooled further.
1. Genetic variation of freezing tolerance in winter wheat. The inheritance of the ability of wheat plants to survive freezing under variable levels of severity is poorly understood. Twenty-four wheat lines were evaluated by ARS scientists in Pullman, WA for freezing tolerance under a series of freezing conditions. Early results indicated that 48 temperature regimes resulted in the complete range of survival (0-100%). Replicated trials of all 24 wheat lines were completed under each of the 48 conditions. Results showed that some varieties survived best under one set of conditions, but different varieties survived best under a different set of conditions indicating that multiple mechanisms contribute to freezing survival, and that different wheat varieties make use of some of these mechanisms more effectively than others. It may be possible to genetically combine the ability to use these different mechanisms into one wheat variety, thereby improving freezing tolerance.
2. Severity of freezing stress determines which genes are involved in hardiness of winter wheat. Changes in gene expression levels as wheat plants are frozen had not been investigated previously. Genome-wide technology was used by ARS scientists in Pullman, WA to evaluate the expression of over 50,000 genes in wheat plants as they were chilled to potentially damaging temperatures. Over 400 genes were found to significantly change expression as the plants were cooled to -10 degrees. A small number of genes were found to increase expression as the plants were either held at -10 or were further cooled to -12 degrees. These results revealed for the first time that specific genes respond in a very precise manner to differing levels of freezing stress. This information provides a new avenue of approach for targeting specific genes for the improvement of freezing stress response.Skinner, D.Z. 2009. Phospholipase A2 activity during cold acclimation of wheat. Crop Sci. 49:323-328. DOI:10.2135/cropsci2008.06.0359