Abstract
Arabidopsis thaliana (Arabidopsis) increases in freezing tolerance in response to low nonfreezing temperatures, a phenomenon known as cold acclimation. The CBF regulatory pathway, which contributes to cold acclimation, includes three genes—CBF1, CBF2 and CBF3—encoding closely-related transcription factors that regulate the expression of more than 100 genes—the CBF regulon—that impart freezing tolerance. Here we compare the CBF pathways of two Arabidopsis ecotypes collected from sites in Sweden (SW) and Italy (IT). Previous studies showed that the SW ecotype was more freezing tolerant than the IT ecotype and that the IT ecotype had a nonfunctional CBF2 gene. Here we present results establishing that the difference in CBF2 alleles contributes to the difference in freezing tolerance between the two ecotypes. However, other differences in the CBF pathway as well as CBF-independent pathways contribute the large majority of the difference in freezing tolerance between the two ecotypes. The results also provided evidence that most cold-induced CBF regulon genes in both the SW and IT ecotypes are coregulated by CBF-independent pathways. Additional analysis comparing our results with those published by others examining the Col-0 accession resulted in the identification of 44 CBF regulon genes that were conserved among the three accessions suggesting that they likely have important functions in life at low temperature. The comparison further supported the conclusion that the CBF pathway can account for a large portion of the increase in freezing tolerance that occurs with cold acclimation in a given accession, but that CBF-independent pathways can also make a major contribution.
Highlights
The CBF pathway has a prominent role in cold acclimation, the process whereby plants increase in freezing tolerance in response to low temperatures [1,2,3]
Expression of the CBF regulon leads to an increase in freezing tolerance as indicated by the findings that overexpression of CBF1, CBF2, or CBF3 in transgenic plants results in an increase in freezing tolerance without exposing plants to low temperature [9, 11, 12] and that down-regulation of the CBF pathway in plants exposed to low temperature results in a decrease in freezing tolerance [13, 14]
These results suggested that differences in the CBF locus of the IT and SW ecotypes contribute to the differences in freezing tolerance and the local adaptation observed in the IT and SW ecotypes
Summary
The CBF pathway has a prominent role in cold acclimation, the process whereby plants increase in freezing tolerance in response to low temperatures [1,2,3]. Direct testing showed that the SW ecotype was more freezing tolerant than the IT ecotype and a QTL for both fitness and freezing tolerance was mapped to a site on chromosome 4 that overlapped the CBF locus (the QTL was mapped using a RIL population developed from a cross between the IT and SW ecotypes) [20]. These results suggested that differences in the CBF locus of the IT and SW ecotypes contribute to the differences in freezing tolerance and the local adaptation observed in the IT and SW ecotypes. Consistent with this suggestion was the finding that the IT ecotype had a nonfunctional cbf gene and that transformation of the SW CBF2 gene into the IT plants resulted in an increase in freezing tolerance [21]
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