Transcriptome profiling of yellow leafy head development during the heading stage in Chinese cabbage (Brassica rapa subsp. pekinensis).

The yellow leafy head of Brassica rapa is known to be tasty and nutritional. Therefore, the heading stage of leaf development is critical to realize high yield and economic benefits. A widely planted commercial cultivar of B. rapa ('Qiubao', deep yellow leafy head) was used to conduct transcriptome analysis. The results showed that the yellowing of the inner leaf was likely induced by the predominant β-carotene biosynthesis pathway due to the upregulated gene geranylgeranyl diphosphate and phytoene synthase, and the downregulated gene CrtL-e, NCED4 and DWARF-27. Some genes related to chlorophyll synthesis were also found to be downregulated, such as nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase and protochlorophyllide reductase A. Transcript profiling also revealed strong changes in expression levels of hormonal genes related to auxin, cytokinin, ethylene, abscisic acid, gibberellin and brassinosteroids, suggesting the crucial role that hormones play in heading stage. Examination of carbohydrate and sucrose metabolism pathways revealed that sucrose biosynthesis is probably regulated by 6-phosphofructokinase and sucrose synthase 1 (SUS1/SuSy1) branch, instead of the sucrose-phosphate synthase branch. Several cold-response genes were induced in the late-heading stage, but the results suggest that the common C-repeat binding factor responsive pathway may not be involved in cold adaption. We also identified a series of upregulated transcription factors-AP2/ERF, MYB, bHLH, NAC and WRKY were found to be predominant. The transcripts analysis provides a preliminary genetic resource to unravel key genes and molecular mechanisms responsible for leafy head development in B. rapa, therefore, improving leafy head quality and yield through genetic means in future.