Strigolactones (SLs) are a new class of carotenoid-derived phytohormones essential for developmental processes shaping plant architecture and interactions with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Despite the rapid progress in elucidating the SL biosynthetic pathway, the perception and signaling mechanisms of SL remain poorly understood. Here we show that DWARF53 (D53) acts as a repressor of SL signaling and SLs induce its degradation. We found that the rice d53 mutant, which produces an exaggerated number of tillers compared to wild type plants, is caused by a gain-of-function mutation and is insensitive to exogenous SL treatment. The D53 gene product shares predicted features with the class I Clp ATPase proteins and can form a complex with the α/β hydrolase protein DWARF14 (D14) and the F-box protein DWARF3 (D3), two previously identified signaling components potentially responsible for SL perception. We demonstrate that, in a D14- and D3-dependent manner, SLs induce D53 degradation by the proteasome and abrogate its activity in promoting axillary bud outgrowth. Our combined genetic and biochemical data reveal that D53 acts as a repressor of the SL signaling pathway, whose hormone-induced degradation represents a key molecular link between SL perception and responses.

Arabidopsis COP1 is a constitutive repressor of photomorphogenesis that interacts with photomorphogenesis-promoting factors such as HY5 to promote their proteasome-mediated degradation. SPA1 is a repressor of phytochrome A-mediated responses to far-red light. Here we report that COP1 acts as part of a large protein complex and interacts with SPA1 in a light-dependent manner. We further demonstrate the E3 ubiquitin ligase activity of COP1 on HY5 in vitro and the alteration of that activity by SPA1. Thus, the COP1-SPA1 interaction defines a critical step in coordinating COP1-mediated ubiquitination and subsequent degradation of HY5 with PHYA signaling.

Plants use light as a major source of information for optimizing growth and development. The photoreceptor phytochrome A (phyA) mediates various far-red light induced responses. Here, we show that Arabidopsis FHY3 and FAR1, which encode two proteins related to Mutator-like transposases, act together to modulate phyA signaling by directly activating the transcription of FHY1 and FHL, whose products are essential for light-induced phyA nuclear accumulation and subsequent light responses. FHY3 and FAR1 possess separable DNA-binding and transcriptional activation domains that are highly conserved in Mutator-like transposases. Further, expression of FHY3 and FAR1 is negatively regulated by phyA signaling. We propose that FHY3 and FAR1 define a novel class of transcription factors co-opted from an ancient Mutator-like transposase(s) to modulate phyA signaling homeostasis in higher plants.Plants constantly monitor their light environment in order to grow and develop optimally, using a battery of photoreceptors. Phytochromes are a family of photoreceptors that monitors the incident red (R, 600-700 nm) and far-red (FR, 700-750 nm) light wavelengths by switching reversibly between the R-absorbing, biologically inactive Pr form and the FR-absorbing, biologically active Pfr form (1,2). Upon photoactivation, phyA, the primary photoreceptor for FR light, is translocated from the cytoplasm into the nucleus to induce FR-responsive gene expression required for various photoresponses, such as seed germination, seedling deetiolation, FR-preconditioned blocking of greening, and flowering (3). Genetic studies have identified two pairs of homologous genes essential for phyA signaling: FAR1 (far-red-impaired response 1) and FHY3 (far-red elongated hypocotyl 3); FHY1 (far-red elongated hypocotyl 1) and FHL (FHY1-like) (4-7). FHY1 and FHL have been implicated in mediating the lightdependent nuclear accumulation of phyA (8,9). However, the biochemical function of FHY3 and FAR1 remains to be elucidated. FHY3 and FAR1 share extensive sequence homology with MURA, the transposase encoded by the maize Mutator element, and the predicted transposase of the maize mobile element Jittery (10,11). Both of these transposons are members of the superfamily of Mutator-like elements (MULEs) (12). Database mining and phylogenetic analysis revealed that FHY3/ FAR1-like sequences are present in various angiosperms and fall into several phylogenetic clusters intermingled with MULE transposases (13,table S1 and fig. S1). These proteins share an N-terminal C2H2-type zinc-chelating motif of the WRKY-GCM1 family, a central putative core transposase domain, and a C-terminal SWIM motif (14,15), with highly conserved predicted secondary/tertiary structures ( fig. S2 and S3). To investigate the molecular function