QTL mapping for cold tolerance and higher overwintering survival rate in winter rapeseed (Brassica napus).

The cold tolerance of winter rapeseed (Brassica napus) cultivars is critically important for winter survival and yield formation in northern China. Few studies have examined the genetic mechanism underlying the overwintering survival of B. napus. Here, an F2 population including 174 lines and an F2:3 population including 174 lines were generated to identify the quantitative trait loci (QTLs) related to the cold tolerance of B. napus. A genetic linkage map including 1,017 markers merged into 268 bins covering 793.53 cM was constructed. A total of 16 QTLs for two cold-tolerance indicators related to overwintering success were detected among the two populations. These QTLs were responsible for explaining 0.97%-12.74% of the phenotypic variation. Two major QTLs, qOWRTA07 and qOWRLA07, explaining more than 10% of the phenotypic variation were identified in overlapping regions, and we suspected that these two QTLs might represent the same QTL mapped between the two bins, c07b004 and c07b005, corresponding to the physical interval from 21.4 M to 23.4 M on chromosome A07. One gene, BnaA07G0198300ZS, contained the candidate region for overwintering rate (OWR). RT-qPCR analysis showed that the expression of this gene significantly differed between the two parents (NST57 and CY12), and its expression was higher in NST57 than in CY12. This gene may be involved in the cold-response during overwintering period of B. napus. These results are important for the molecular breeding to improve the cold tolerance and overwintering success of winter oilseed rape.

Overexpression of BrAFP1 gene from winter rapeseed (Brassica rapa) confers cold tolerance in Arabidopsis.

Antifreeze proteins (AFPs) can bind to ice crystals and restrain the formation of larger crystals, a strategy vital to the survival of plants in freezing environments. The BrAFP1 from winter rapeseed cultivars 'Longyou 7' with high cold tolerance was cloned and overexpressed in Arabidopsis. BrAFP1 was localized in the cytoplasm and nucleus. Under cold stress, SOD activity and free proline content were higher, MDA content and relative conductivity were lower in transgenic lines than those in wide-type Arabidopsis. Frostbite of transgenic plants was minimized, whereas frostbite of the Arabidopsis afp1 mutant was severe. Transition of the amino acid at position 17 of BrAFP1 was related to the increased winter survival of the rapeseed cultivar. Cultivars with higher survival rates had a predilection for tyrosine, not tryptophan, at the 17th site in the amino sequence of BrAFP1. Transcription of BrAFP1 was induced more rapidly, and the expression of the gene was also higher, in Longyou 7 than that in Tianyou 4 under cold stress. Overall, the high expression of BrAPF1 confers more cold-tolerance in Longyou 7.