Post-acclimation transcriptome adjustment is a major factor in freezing tolerance of winter wheat

Cold-acclimated winter wheat plants were slowly frozen to -10 degrees C, and then the temperature was either maintained at -10 degrees C or was lowered further to -12 degrees C. Expression levels of a total of 423 genes were significantly altered in these treatments; genes upregulated outnumbered those downregulated by about a 9:1 ratio. Sixty-eight genes were upregulated at least fivefold in all freezing treatments; 17 of these 68 encoded transcription factors including C-repeat binding factor (Cbf), WRKY, or other Zn-finger proteins, indicating strong upregulation of genes involved in transcription regulation. Sixteen of the 68 highly upregulated genes encoded kinases, phosphatases, calcium trafficking-related proteins, or glycosyltransferases, indicating upregulation of genes involved in signal transduction. Six genes encoding chlorophyll a/b binding-like proteins were upregulated uniquely in response to the -12 degrees C treatment, suggesting a protective role of pigment-binding proteins in freezing stress response. Most genes responded similarly in the very freezing tolerant cultivar Norstar and in the moderately freezing tolerant Tiber, but some genes responded in opposite fashion in the two cultivars. These results showed that wheat crowns actively adapt as the temperature declines to potentially damaging levels, and genetic variation for this ability exists among cultivars.