[Development of molecular markers linked to the resistant QTL for downy mildew in Brassica rapa L. ssp. pekinensis].

Downy mildew, caused by the oomycete Hyaloperonospora parasitica Constant. (Pers. ex Fr.), is one of the most severe diseases in Chinese cabbage, leading to reduction of yield and quality of the harvested products. Therefore, identifying molecular markers linked to the major QTL for downy mildew resistance will be helpful in breeding resistant varieties of Chinese cabbage. Here, one highly susceptible line 91-112, one highly resistant line T12-19, and the derived DH population were employed to develop linked molecular markers for the major QTL, BrDW, for downy mildew. With BLAST and IMap analysis, the RAPD marker K14-1030 linked to BrDW was anchored on KBrB058M10 (on Contig214). On the basis of the BAC and BAC-end sequences around KBrB058M10, a set of PCR primers were designed, and the methods of restriction analysis and HRM analysis were used to develop molecular makers. Finally, five polymorphism markers were developed, containing one Indel marker named Brb062-Indel230, three CAPS markers named Brb094-DraⅠ787, Brb094-AatⅡ666 and Brb043-BglⅡ715, and one SNP marker named Brh019-SNP137. In addition, one SSR marker from Unigene sequence homologous with KBrB058M10 (known as bru1209) was developed. The map distances between the six markers and RAPD marker K14-1030 were 4.3 cM, 1.7 cM, 5.9 cM, 5.9 cM, 4.6 cM, and 0.8 cM, respectively. The percentage of accuracy in selecting for downy mildew-resistant lines from the DH population were 69.7%, 70.9%, 72.4%, 72.4%, 58.3%, and 74.2%. These markers could be used in marker assisted selection to improve downy mildew resistance in Chinese cabbage.

[cDNA-AFLP analysis on transcripts associated with bolting in Brassica rapa L. ssp. pekinensis].

Premature bolting, caused by low temperature in spring and summer cultivation in low land and high land respectively, leads to reduction of the yield and quality of the harvested products in Chinese cabbage. Therefore, exploring genes involved in vernalization response is important to the improvement of Chinese cabbage varieties. Here, one extremely early bolting line (DH-54) and one extremely late bolting line (DH-43) were employed, and the cDNA-AFLP approach was used to identify key components involved in the low-temperature required vernalization response. Of 256 primer recombinations screened, a total of 191 differential expressed transcript-derived fragments (TDFs) were identified, and 82 TDFs were sequenced. BLAST and alignments showed that 52 candidate TDFs shared high levels of similarity with genes of known function, 22 TDFs of unknown function and 8 novel ESTs. The TDFs of known function were involved in genes encoding enzymes working in metabolism, proteins related to stress and defense, signal transduction, and transcription regulation, etc.