Overexpression of COL9, a CONSTANS-LIKE gene, delays flowering by reducing expression of CO and FT in Arabidopsis thaliana

CONSTANS (CO) is an important floral regulator in the photoperiod pathway, integrating the circadian clock and light signal into a control for flowering time. It is known that CO promotes flowering in Arabidopsis under long-day conditions. CONSTANS-LIKE 9 (COL9) is a member of the CONSTANS-LIKE gene family, encoding a nuclear protein. The expression of COL9 is regulated by the circadian clock in the photoperiod pathway and is detected in various organs. Unexpectedly, overexpression of COL9 in transgenic Arabidopsis resulted in delayed flowering, while co-suppression lines and a transferred DNA (T-DNA) knockout line showed earlier flowering under long-day conditions. Overexpression of COL9 did not enhance the late-flowering phenotype in a co mutant background. Double overexpressors produced by overexpression of CO in COL9 transgenic lines showed an early flowering phenotype similar to single CO overexpressors. The pattern of oscillation of a number of circadian-associated genes remained unchanged in the COL9 transgenic lines. Compared with wild-type plants, the abundance of CO and FLOWERING LOCUS T (FT) mRNA was reduced in the COL9 overexpression lines. Our results indicate that COL9 is involved in regulation of flowering time by repressing the expression of CO, concomitantly reducing the expression of FT and delaying floral transition.