Copy Number Variations of CBF Genes Regulating Freezing Tolerance in the Triticeae

C-repeat Binding Factor (CBF) genes are transcription factors involved in increasing the freezing tolerance of a plant. The Triticeae cereals, wheat (Triticum spp.) and barley (Hordeum vulgare) harbor at least 11 CBF genes at the FROST RESISTANCE-2 (FR-2) locus on the long arm of homoeologous group 5 chromosomes. Within each species, winter genotypes are more freezing-tolerant than spring genotypes. We recently sequenced the FR-2 locus in two winter and two spring genotypes of barley. Our data indicate that winter genotypes harbor greater copy numbers of CBF genes compared to spring types. Greater CBF copy numbers in winter genotypes result from the presence of additional CBF sequence paralogs that are absent from spring types, and due to tandem segmental duplications of CBF-containing genomic regions in winter genotypes that exist as a single copy in spring types. We further show that CBF copy number variations also occur across the winter barleys, and that increased copy numbers are associated with higher expression levels of other CBF genes at FR-2. Our data also indicate CBF copy number variations in wheat. Taken together, these data suggest that CBF copy number variation is a recurring theme in the Triticeae, and that total CBF copy number rather than a particular CBF may be the molecular basis underlying FR-2.