Genomics of cold acclimation and freezing tolerance in different Brassicaceae species

Freezing stress is a major environmental constraint for crop production. Temperate plants acquire freezing tolerance in response to a period of low non-freezing temperatures, a process known as cold acclimation, which is associated with numerous cellular changes including gene expression and metabolic changes. The ability to cold acclimate and the attainment of maximum freezing tolerance varies among plant species. Thalspi arvense (2n = 14), a wild relative of the cultivated crop Brassica napus (2n = 38) and the dicot model Arabidopsis thaliana (2n = 10), survives Canadian Prairie winters with temperature lows of -40°C, when its relatives cannot, suggesting that the mechanisms of freezing tolerance in T. arvense differ from those of A. thaliana and B. napus. This study was designed to understand the cold acclimation response of T. arvense, and to compare it with that of A. thaliana and B. napus where possible. Freezing tolerance, soluble sugars and malate were examined during cold acclimation. Within 3 wk of cold acclimation, T. arvense acquired a freezing tolerance of -16.8°C compared to -9.1°C for A. thaliana and -12°C for B. napus. The three crucifers accumulated soluble sugars and malate during cold acclimation; the comparative levels of which corresponded with the degree of freezing tolerance acquired by each species. Microarray analysis performed to identify cold responsive genes with potential roles in freezing tolerance of T. arvense identified 590 genes, about 50% of which were similar to those reported in A. thaliana while the remaining genes appeared to be specific to the low temperature response of T. arvense. Biological roles of two of the cold responsive genes (ERF4 and a SRC2-like gene) were examined through phenotypic characterisation of knockout mutants and over-expression lines in A. thaliana. ERF4 appeared to confer freezing, salt and osmotic stress sensitive phenotypes. SRC2 decreased sensitivity to ABA and increased sensitivity to salt and osmotic stresses suggesting it also negatively regulates responses to ABA and to these stresses. Characterisation of additional genes unique to T. arvense and those, showing desparate expression patterns between T. arvense and A. thaliana would help identify key freezing tolerant genes which could be eventually utilized to develop freezing tolerant canola crops.