The Role of the CBF Regulon in Cold Tolerance and Cold Acclimation

The CBF regulon is present even in chilling-sensitive species such as tomato, and tomato CBF genes can trigger cold acclimation when expressed in Arabidopsis thaliana. Thus, the responses activated by CBFs appear to vary between species that are able to cold acclimate (e.g. Arabidopsis) and those that cannot (e.g. tomato). In addition, however, differences in CBF expression can be linked to natural variations in freezing tolerance (Fig. 4.23).

Fig. 4.23. Correlation between habitat temperature, acclimated freezing tolerance a and gene expression b in particular, of the CBF regulon c-e of Arabidopsis thaliana accessions from different climates. The temperature data for the accessions are the average minimum temperature during January for winter annuals and April for summer annuals, obtained from the nearest weather station. Freezing tolerance is given as the temperature at which 50% electrolyte leakage occurred from detached leaves (LT₅₀) of plants. For cold acclimation the plants were subjected to a temperature of 4 °C for 14 days. The data are means from 4-6 independent experiments.

The acclimation capacity b was measured as the difference between the non-acclimated and acclimated LT₅₀ of detached leaves. It correlates nearly linearly with the extent of changes in gene expression. Changes were classified as significant when the gene-wise false discovery rate-corrected F test and t test p values were both less than 0.05.

c-e Expression of the CBF transcription factor genes CBF1-3 in cold-acclimated plants. Scatterplots of GCRMA (guanine cytosine robust multi-array analysis) on a log2 scale, where a doubling of expression intensity corresponds to a unit of 1. Expression estimates are shown against the LT₅₀ of detached leaves from the nine accessions after 14-day cold acclimation. The data are means ± standard errors (SEs) from three independent experiments (Hannah et al. 2006)

The higher sensitivity to freezing temperatures of A. thaliana accessions from North Africa and Southern Europe (Fig. 4.23) correlates with lower CBF expression and concomitantly lower COR gene expression relative to accessions from Norther and Eastern Europe. Comparative sequencing of CBF genes from these accessions indicated relaxed selection on these genes in accessions from habitats that have a much lower incidence of frost events (Zhen and Ungerer 2008). However, not for all CBF genes expression can be directly linked with cold tolerance or cold acclimation (Fig. 4.23e).