Expression patterns of HDA9, HDA6 and FLD in Chinese cabbage (Brassica rapa L. ssp. pekinensis) under different photoperiods and their protein interactions

Histone deacetylases (HDACs) are important regulators that participate in plant growth and development. HISTONE DEACETYLASE 9 (HDA9), HISTONE DEACETYLASE 6 (HDA6) and FLOWERING LOCUS D (FLD) are reported to be involved in flowering regulation, stress response and other regulation in Arabidopsis, but whether and how there is a protein relationship between them in Chinese cabbage (Brassica rapa L. ssp. pekinensis) is still unknown. In this study, BrFLD, two members of BrHDA6, and BrHDA9 were cloned from Brassica rapa and ubiquitously expressed in roots, stems, cauline leaves, flower buds and opening flowers. Those genes exhibited their highest transcript levels in cauline leaves and lowest levels in stems or flower buds. BrFLD and two BrHDA6s were highly induced by long-day photoperiod, but BrHDA9 was highly induced by short-day. The genes are diel regulated and have different patterns across the photoperiods. Yeast two-hybrid assays and glutathione S-transferase (GST) pull-down assays revealed that the BrFLD protein interacts with the two BrHAD6 proteins but not with the BrHDA9 protein, and no interaction was detected between BrHDA9 and the two BrHDA6s. Further research showed that the SWIRM domain of BrFLD and the C-domain of BrHDA6 are the key domains involved in these interactions. This result suggested that HDAC genes are regulated by photoperiod and participate in autonomous pathway control flowering time through integration with FLD or regulate flowering independently of FLD.