Genome-wide analysis of genes associated with bolting in heading type chinese cabbage

Abstract

Bolting or flowering time affects productivity of the leafy vegetable Chinese cabbage (Brassica rapa ssp. pekinensis) and also the time required for its breeding programs. Understanding the bolting process at the molecular level and identifying the genes involved will provide valuable tools for genetic engineering and development of functional markers. To achieve these goals, microarray analyses using either outer leaves or core tissues of completely headed Chinese cabbage were performed at three developmental stages, and the findings were confirmed using RT-PCR. A large number of genes were specifically or preferentially expressed in each tissue: photosynthesis and abiotic signal responding genes were expressed in outer leaves, while genes involved in hormone responses, flower development, and histone modification were expressed in core tissues. Genes promoting bolting, such as BrPIF4, BrPIF5, and BrCOLs, were highly expressed in outer leaves, a signal-perceiving tissue, whereas floral repressors, such as BrFLCs, BrFRL, and BrMAF1s, were predominantly expressed in core tissues, with levels of expression decreasing in later stages. Although the B. rapa genome contains three BrFT paralogs, only BrFT1 was expressed. BrFD and BrFE, genes related to the transport of BrFTs, showed consistent expression. The late bolting phenotype of Huissen, the F1 variety examined in this study, may result from expression of repressors of flowering, such as BrGASA5, BrTFL1, BrMAF1, BrFLCs, and BrKNAT1, in core tissues. Coordinated regulation of several floral promoters and repressors appears to be necessary for control of the bolting process or flowering time in B. rapa.