Arabidopsis FHY3 defines a key phytochrome A signaling component directly interacting with its homologous partner FAR1

In Arabidopsis, phytochrome A (phyA) is the primary photoreceptor mediating various plant responses to far‐red (FR) light. Here we show that phyA signaling involves a combinatorial action of downstream intermediates, which controls overlapping yet distinctive sets of FR responses. FHY3 is a prominent phyA signaling intermediate sharing structural similarity to FAR1, a previously identified phyA signaling component. The fhy3 and far1 mutants display similar yet distinctive defects in phyA signaling; however, overexpression of either FHY3 or FAR1 suppresses the mutant phenotype of both genes. Moreover, overexpression of partial fragments of FHY3 can cause a dominant‐negative interference phenotype on phyA signaling that is stronger than those of the fhy3 or far1 null mutants. Further, we demonstrate that FHY3 and FAR1 are capable of homo‐ and hetero‐interaction. Our data indicate that FHY3, together with FAR1, defines a key module in a signaling network underlying phyA‐mediated FR light responses.