Identification and molecular characterization of FKF1 and GI homologous genes in soybean

In Arabidopsis, FKF1 (FLAVIN BINDING, KELCH REPEAT, F-BOX1) and GI (GIGANTEA) play important roles in flowering pathway through regulating daytime CO (CONSTANS) expression, and such a function is conserved across plants studied. But related reports are limited for soybean. In this study, we cloned FKF1 and GI homologs in soybean, and named as GmFKF1, GmFKF2, GmGI1, GmGI2, and GmGI3, respectively. GmGI1 had two alternative splicing forms, GmGI1α and GmGI1β. GmFKF1/2 transcripts were diurnally regulated, with a peak at zeitgeber time 12 (ZT12) in long days and at ZT10 in short days. The diurnal phases between GmGIs transcript levels greatly differed. GmGI2 expression was regulated by both the circadian clock and photoperiod. But the rhythmic phases of GmGI1 and GmGI3 expression levels were mainly conferred by long days. GmFKFs shared similar spatio-temporal expression profiles with GmGIs in all of the tissue/organs in different developmental stages in both LD and SD. Both GmFKF and GmGI proteins were targeted to the nucleus. Yeast two hybrid assays showed GmFKF1/GmFKF2 interacted with GmGI1/GmGI2/GmCDF1 (CYCLING DOF FACTOR CDF1 homolog in soybean); and the LOV (Light, Oxygen, or Voltage) domain in GmFKF1/GmFKF2 played an important role in these interactions. N-terminus of GmGI2 was sufficient to mediate its interaction with GmCDF1. Interestingly, N-terminus not full of GmGI3 interacted with GmFKF1/GmFKF2/GmCDF1. Ectopic over-expression of the GmFKF1 or GmFKF2 in Arabidopsis enhanced flowering in SD. Collectively, GmFKF and GmGI in soybean had conserved functional domains at DNA sequence level, but specific characters at function level with their homologs in other plants.