Phenomic and Transcriptomic Response of Winter Wheat to a Mild Freeze-Thaw Cycle.

With the onset of the winter season, winter wheat seedlings in the fieldare likely to be exposed to several episodes of mild freezing followedby thawing at low, positive temperatures. The impact of these freeze-thaw cycles on the ability to survive the winter was evaluated in 22 winter wheat varieties. Seedlings were cold acclimated at +4C for five weeks, then were tested for freezing tolerance with or without a mild freeze-thaw pre-freezing treatment. Plants exposed to the freeze-thaw treatment were frozen at -3C for 24 h, then allowed to thaw at +4C for 24 h, then were frozen to potentially damaging temperatures. The freeze-thaw treatment resulted in significantly greater survival of most of the varieties. Affymetrix microarray analysis revealed that hundreds of genes were upregulated by the pre-freezing treatment. During the thaw portion of the freeze-thaw treatment, most of the genes upregulated  during the freeze portion were downregulated, but many other genes were strongly upregulated. These transcription cascades revealed the cold acclimation process was stimulated by the freeze-thaw treatment and that broad-scale transcriptomic adjustment continued for at least 48 hours after thawing at +4C commenced. These results provide new insights into the cold response process and freezing tolerance of winter wheat.