The F box protein AFR is a positive regulator of phytochrome A-mediated light signaling

Light is an important environmental cue to plants, and much of their physiology is influenced by light [1-3]. The light signals that drive these responses are perceived by photoreceptors including the red/far-red responsive phytochromes (phyA-E) [4]. In addition to direct effects, light also exerts its influence by modifying the rhythms generated by the circadian clock [11 9 5,6]. In Arabidopsis thaliana, the molecular makeup of the interface between the central clock and its input/output pathways is not fully defined, but a major point of control is likely to be protein turnover mediated by the ubiquitin/26S proteasome system [7]. To identify additional constituents of this interface, stable double-stranded RNA interference (RNAi) was used to reduce mRNA levels of rhythmically expressed candidate genes encoding putative components of E3 ubiquitin ligases (i.e., F box and RING finger proteins), followed by screening of the transgenic plants for circadian and light signaling defects. RNAi lines with diminished expression of the novel gene ATTENUATED FAR-RED RESPONSE (AFR) display phenotypes consistent with impaired phyA-mediated light signaling. Furthermore, AFR is a true SCIF E3 ubiquitin ligase component. SCFAFR is expected to mediate the turnover of a repressor of phyA signaling, possibly to prepare the plant to receive light signals at dawn.