Molecular genetic mechanism of OsFKF1 in flowering time in rice

Abstract

Flowering time (or heading date) is a critical trait that determines seasonal and regional adaptation in plants. Many important regulators in photoperiodic flowering pathway are conserved between Arabidopsis thaliana (At) and rice (Oryza sativa

Os), however some are diversified in their functions in the regulation of flowering time. In the facultative long-day (LD) plant Arabidopsis thaliana, FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1), a blue-light receptor, regulates photoperiodic flowering through the transcriptional and post-translational regulation of CONSTANS (CO) under inductive LD conditions. By contrast, the facultative short day (SD) plant rice (Oryza sativa) flowers early under inductive SD and late under non-inductive LD conditions, and the regulatory function of OsFKF1, an ortholog of Arabidopsis FKF1, remains elusive. In this study, we characterized the T-DNA insertion osfkf1 knockout mutants which showed a late flowering phenotype under SD, LD, and natural LD conditions. Transcriptional analysis revealed that OsFKF1 up-regulates expression of the floral activator Early heading date 2 (Ehd2) and down-regulates expression of the floral repressor Grain number, plant height and heading date 7 (Ghd7) under both SD and LD conditions. Consequently, the expression levels of potent floral inducers Early heading date 1 (Ehd1), Heading date 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1), downstream of Ehd2 and Ghd7, decrease in osfkf1 mutants. Notably, we found that OsFKF1 can up-regulate Ehd1 expression in a blue light-dependent manner, without modulating the expression of Ehd2 and Ghd7. In contrast to the LD-specific floral activator AtFKF1, OsFKF1 likely acts as an autonomous floral activator because it promotes flowering independent of photoperiod, probably via its distinct roles in controlling expression of rice-specific genes including Ehd2, Ghd7, and Ehd1. In contast, OsFKF1 also interacts with OsGI and OsCDF1 (also termed OsDof12) like Arabidopsis FKF1, which interacts with GIGANTEA (GI) and CYCLING DOF FACTOR 1 (CDF1). Taken together, our data indicate that OsFKF1 is a floral inducer in rice and we have identified similar and distinct roles of FKF1 in Arabidopsis and rice.