FAR-RED ELONGATED HYPOCOTYL1 and FHY1-LIKE associate with the Arabidopsis transcription factors LAF1 and HFR1 to transmit phytochrome A signals for inhibition of hypocotyl elongation.

Among the five phytochromes in Arabidopsis thaliana, phytochrome A (phyA) plays a major role in seedling deetiolation. Mutant analyses have identified more than 10 positive components acting downstream of phyA to inhibit hypocotyl elongation. However, their sites of action and their hierarchical relationships are poorly understood. Here, we investigated the genetic and molecular relationship between two homologous proteins, FAR-RED ELONGATED HYPOCOTYL1 (FHY1) and FHY1-LIKE (FHL), and two transcription factors, LONG AFTER FAR-RED LIGHT1 (LAF1) and LONG HYPOCOTYL IN FAR-RED1 (HFR1). Analyses of double and triple mutants showed that LAF1, a myb factor, and HFR1, a basic helix-loop-helix factor, independently transmit phyA signals downstream of FHY1 and FHL. Coimmunoprecipitation experiments showed that phyA, FHY1, FHL, LAF1, and HFR1 are components of protein complexes in vivo. In vitro pull-down assays demonstrated direct interactions between partner proteins with the N-terminal region of FHY1, as well as that of FHL, interacting with the LAF1 N-terminal portion and the HFR1 C-terminal region. These results suggest that, in addition to assisting phyA nuclear accumulation, FHY1 and FHL are required to assemble photoreceptor/transcription factor complexes for phyA signaling.

Phytochrome nuclear body: an emerging model to study interphase nuclear dynamics and signaling.

In higher plants, one of the most striking effects of light at the cellular level is the formation of phytochrome nuclear bodies (PNBs). In Arabidopsis, two types of PNBs have been described: a transient type of PNBs (tPNBs), containing both phytochrome A and phytochrome B, observed during the dark-to-light transition and a relatively photo-stable type of phytochrome B-containing PNBs (sPNBs) under continuous light. Despite the separation of the cell-biological observations of PNBs from the traditional model of light signaling elucidated by genetic and biochemical approaches, a growing body of evidence indicates that PNBs are intimately involved in phytochrome signaling. Both positive and negative light signaling components have been colocalized to PNBs, which provides direct evidence bridging PNBs and phytochrome signaling. In particular, the sPNB serves as an excellent tractable marker for early phytochrome signaling events, and thus provides a remarkable genetic system to investigate the mechanistic connection between interphase subnuclear dynamics and cell signaling.