Integrate QTL Mapping and Transcription Profiles Reveal Candidate Genes Regulating Flowering Time in Brassica napus.

Flowering at the proper time is an important part of acclimation to the ambient environment and season and maximizes the plant yield. To reveal the genetic architecture and molecular regulation of flowering time in oilseed rape (Brassica napus), we performed an RNA-seq analysis of the two parents after vernalization at low temperature and combined this with quantitative trait loci (QTL) mapping in an F2 population. A genetic linkage map that included 1,017 markers merged into 268 bins and covered 793.53 cM was constructed. Two QTLs associated with flowering time were detected in the F2 population. qFTA06 was the major QTL in the 7.06 Mb interval on chromosome A06 and accounted for 19.3% of the phenotypic variation. qFTC08 was located on chromosome C06 and accounted for 8.6% of the phenotypic variation. RNA-seq analysis revealed 4,626 differentially expressed genes (DEGs) between two parents during vernalization. Integration between QTL mapping and RNA-seq analysis revealed six candidate genes involved in the regulation of flowering time through the circadian clock/photoperiod, auxin and ABA hormone signal, and cold signal transduction and vernalization pathways. These results provide insights into the molecular genetic architecture of flowering time in B. napus.

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.