Differential Expression of Flowering Genes between Rapid- and Slow-Cycling Brassica rapa

Flowering time is a very important agronomic trait in Brassica crops and regulation of the time is one of major factor in the breeding program. To understand the control of flowering time in Brassica rapa, we have carried out Br300K microarray with two contrasting Brassica inbred lines, Rapid Cycling B. rapa (RCBr) as rapid cycling type and B. rapa ssp. pekinensis inbred line Chiifu as slow flowering phenotype. Reproductive process-related genes were specifically expressed in RCBr, whereas environmental stimuli-responsive genes in Chiifu. Flowering stimulating genes, such as BrFT and BrSOC1, were preferentially expressed in RCBr, while flowering repressing genes, such as BrFLC and BrMAF4, expressed in Chiifu. Several paralogues present in B. rapa, BrFLCs and BrCOLs, were expressed with paralog-specific pattern depending on flowering phenotypes: i.e., BrFLC1 and BrFLC2, major floral repressors, were expressed in Chiifu, BrFLCL/BrFLC5 in RCBr and BrFLC3 in both plants. The expression of several flowering repressing genes was gradually decreased in RCBr growth, but increased in Chiifu growth. However, the expression of genes involved in photoperiodic flowering was no difference between these two plants under LD and SD conditions, indicating photoperiodic pathway is not major factor to distinguish fast vs. slow flowering in B. rapa. The mechanism underlined in the rapid or fast flowering of RCBr would be further elucidated in association with the controlling mechanism of its short life span.