Natural genetic variation in acclimation capacity at sub-zero temperatures after cold acclimation at 4 degrees C in different Arabidopsis thaliana accessions.

Freezing tolerance is an important factor in the geographical distribution of plants and strongly influences crop yield. Many plants increase their freezing tolerance during exposure to low, nonfreezing temperatures (cold acclimation) and acclimation may continue at mild freezing temperatures in a process termed sub-zero acclimation. There is considerable natural variation in the cold acclimation capacity of Arabidopsis that has been used to study the molecular basis of this trait, but much less is known about the molecular basis of sub-zero acclimation. Freezing tolerance of detached leaves from the accessions C24, Columbia-0, Rschew, and Tenela was investigated using an electrolyte leakage assay. Sub-zero acclimation could be achieved by shifting plants from 4 degrees C to -3 degrees C, or by using detached leaves, either in the presence or absence of ice nucleation. The magnitude of the increase in freezing tolerance depended on both temperature and duration of sub-zero acclimation and while Columbia-0 showed no significant increase in freezing tolerance, the other three accessions increased their freezing tolerance significantly. The levels of several sugars that have been shown to be induced during cold acclimation at nonfreezing temperatures were not strongly changed during sub-zero acclimation and there was no correlation between the increases in freezing tolerance and sugar levels in the different accessions. Expression of the three cold induced CBF transcription factor genes and five of their representative target COR genes was moderately increased during sub-zero acclimation, but again there was no correlation to changes in freezing tolerance, indicating that the genetic and molecular basis of sub-zero acclimation is most likely different from that of cold acclimation at above freezing temperatures. Further studies will be needed to reveal novel signal transduction pathways and protective mechanisms important in sub-zero acclimation.