The phytochrome (phy) family of photoreceptors regulates changes in gene expression in response to red/far-red light signals in part by physically interacting with constitutively nucleus-localized phy-interacting basic helix-loop-helix transcription factors (PIFs). Here, we show that PIF1, the member with the highest affinity for phys, is strongly sensitive to the quality and quantity of light. phyA plays a dominant role in regulating the degradation of PIF1 following initial light exposure, while phyB and phyD and possibly other phys also influence PIF1 degradation after prolonged illumination. PIF1 is rapidly phosphorylated and ubiquitinated under red and far-red light before being degraded with a half-life of ;1 to 2 min under red light. Although PIF1 interacts with phyB through a conserved active phyB binding motif, it interacts with phyA through a novel active phyA binding motif. phy interaction is necessary but not sufficient for the light-induced phosphorylation and degradation of PIF1. Domain-mapping studies reveal that the phy interaction, light-induced degradation, and transcriptional activation domains are located at the N-terminal 150-amino acid region of PIF1. Unlike PIF3, PIF1 does not interact with the two halves of either phyA or phyB separately. Moreover, overexpression of a light-stable truncated form of PIF1 causes constitutively photomorphogenic phenotypes in the dark. Taken together, these data suggest that removal of the negative regulators (e.g., PIFs) by light-induced proteolytic degradation might be sufficient to promote photomorphogenesis.

Phytochromes function as red/far-red photoreceptors in plants and are essential for light-regulated growth and development. Photomorphogenesis, the developmental program in light, is the default program in seed plants. In dark-grown seedlings, photomorphogenic growth is suppressed by the action of the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)/SUPPRESSOR OF phyA-105 (SPA) complex, which targets positive regulators of photomorphogenic growth for degradation by the proteasome. Phytochromes inhibit the COP1/SPA complex, leading to the accumulation of transcription factors promoting photomorphogenesis; yet, the mechanism by which they inactivate COP1/SPA is still unknown. Here, we show that lightactivated phytochrome A (phyA) and phytochrome B (phyB) interact with SPA1 and other SPA proteins. Fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy analyses show that SPAs and phytochromes colocalize and interact in nuclear bodies. Furthermore, light-activated phyA and phyB disrupt the interaction between COP1 and SPAs, resulting in reorganization of the COP1/SPA complex in planta. The light-induced stabilization of HFR1, a photomorphogenic factor targeted for degradation by COP1/SPA, correlates temporally with the accumulation of phyA in the nucleus and localization of phyA to nuclear bodies. Overall, these data provide a molecular mechanism for the inactivation of the COP1/ SPA complex by phyA-and phyB-mediated light perception.

Plants depend on light signals to modulate many aspects of their development and optimize their photosynthetic capacity. Phytochromes (phys), a family of photoreceptors, initiate a signal transduction pathway that alters expression of a large number of genes to induce these responses. Recently, phyA and phyB were shown to bind members of a basic helix-loop-helix family of transcription factors called phy-interacting factors (PIFs). PIF1 negatively regulates chlorophyll biosynthesis and seed germination in the dark, and lightinduced degradation of PIF1 relieves this negative regulation to promote photomorphogenesis. Here, we report that PIF1 regulates expression of a discrete set of genes in the dark, including protochlorophyllide oxidoreductase (POR), ferrochelatase (FeChII), and heme oxygenase (HO3), which are involved in controlling the chlorophyll biosynthetic pathway. Using ChIP and DNA gel-shift assays, we demonstrate that PIF1 directly binds to a G-box (CACGTG) DNA sequence element present in the PORC promoter. Moreover, in transient assays, PIF1 activates transcription of PORC in a G-boxdependent manner. These data strongly suggest that PIF1 directly and indirectly regulates key genes involved in chlorophyll biosynthesis to optimize the greening process in Arabidopsis.basic helix-loop-helix transcription factors ͉ photomorphogenesis ͉ phytochrome signaling ͉ transcriptional regulation ͉ G-box

Light signals regulate a plethora of plant responses throughout their life cycle, especially the red and far-red regions of the light spectrum perceived by the phytochrome family of photoreceptors. However, the mechanisms by which phytochromes regulate gene expression and downstream responses remain elusive. Several recent studies have unraveled the details on how phytochromes regulate photomorphogenesis. These include the identification of E3 ligases that degrade PHYTOCHROME INTERACTING FACTOR (PIF) proteins, key negative regulators, in response to light, a better view of how phytochromes inhibit another key negative regulator, CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), and an understanding of why plants evolved multiple negative regulators to repress photomorphogenesis in darkness. These advances will surely fuel future research on many unanswered questions that have intrigued plant photobiologists for decades.

Plants undergo contrasting developmental programs in dark and light. Photomorphogenesis, a light-adapted programme is repressed in the dark by the synergistic actions of CUL4 COP1-SPA E3 ubiquitin ligase and a subset of basic helix-loop-helix transcription factors called phytochrome interacting factors (PIFs). To promote photomorphogenesis, light activates the phytochrome family of sensory photoreceptors, which inhibits these repressors by poorly understood mechanisms. Here, we show that the CUL4 COP1-SPA E3 ubiquitin ligase is necessary for the light-induced degradation of PIF1 in Arabidopsis. The light-induced ubiquitylation and subsequent degradation of PIF1 is reduced in the cop1, spaQ and cul4 backgrounds. COP1, SPA1 and CUL4 preferentially form complexes with the phosphorylated forms of PIF1 in response to light. The cop1 and spaQ seeds display strong hyposensitive response to far-red light-mediated seed germination and light-regulated gene expression. These data show a mechanism by which an E3 ligase attenuates its activity by degrading its cofactor in response to light.

The ARP2/3 complex, a highly conserved nucleator of F-actin, and its activator, the SCAR complex, are essential for growth in plants and animals. In this article, we present a pathway through which roots of Arabidopsis thaliana directly perceive light to promote their elongation. The ARP2/3-SCAR complex and the maintenance of longitudinally aligned F-actin arrays are crucial components of this pathway. The involvement of the ARP2/3-SCAR complex in light-regulated root growth is supported by our finding that mutants of the SCAR complex subunit BRK1/HSPC300, or other individual subunits of the ARP2/3-SCAR complex, showed a dramatic inhibition of root elongation in the light, which mirrored reduced growth of wild-type roots in the dark. SCAR1 degradation in dark-grown wild-type roots by constitutive photomorphogenic 1 (COP1) E3 ligase and 26S proteasome accompanied the loss of longitudinal F-actin and reduced root growth. Light perceived by the root photoreceptors, cryptochrome and phytochrome, suppressed COP1-mediated SCAR1 degradation. Taken together, our data provide a biochemical explanation for light-induced promotion of root elongation by the ARP2/3-SCAR complex.

Light signals regulate plant growth and development by controlling a plethora of gene expression changes. Posttranscriptional regulation, especially pre-mRNA processing, is a key modulator of gene expression; however, the molecular mechanisms linking pre-mRNA processing and light signaling are not well understood. Here we report a protein related to the human splicing factor 45 (SPF45) named splicing factor for phytochrome signaling (SFPS), which directly interacts with the photoreceptor phytochrome B (phyB). In response to light, SFPS-RFP (red fluorescent protein) colocalizes with phyB-GFP in photobodies. loss-of-function plants are hyposensitive to red, far-red, and blue light, and flower precociously. SFPS colocalizes with U2 small nuclear ribonucleoprotein-associated factors including U2AF65B, U2A', and U2AF35A in nuclear speckles, suggesting SFPS might be involved in the 3' splice site determination. SFPS regulates pre-mRNA splicing of a large number of genes, of which many are involved in regulating light signaling, photosynthesis, and the circadian clock under both dark and light conditions. In vivo RNA immunoprecipitation (RIP) assays revealed that SFPS associates with () mRNA, a critical link between light signaling and the circadian clock. Moreover, PHYTOCHROME INTERACTING FACTORS () transcription factor genes act downstream of SFPS, as the quadruple mutant suppresses defects of mutants. Taken together, these data strongly suggest SFPS modulates light-regulated developmental processes by controlling pre-mRNA splicing of light signaling and circadian clock genes.

Ubiquitin-26S proteasome system (UPS) has been shown to play central roles in light and hormone-regulated plant growth and development. Previously, we have shown that MAX2, an F-box protein, positively regulates facets of photomorphogenic development in response to light. However, how MAX2 controls these responses is still unknown. Here, we show that MAX2 oppositely regulates GA and ABA biosynthesis to optimize seed germination in response to light. Dose-response curves showed that max2 seeds are hyposensitive to GA and hypersensitive to ABA in seed germination responses. RT-PCR assays demonstrated that the expression of GA biosynthetic genes is down-regulated, while the expression of GA catabolic genes is up-regulated in the max2 seeds compared to wild-type. Interestingly, expression of both ABA biosynthetic and catabolic genes is up-regulated in the max2 seeds compared to wild-type. Treatment with an auxin transport inhibitor, NPA, showed that increased auxin transport in max2 seedlings contributes to the long hypocotyl phenotype under light. Moreover, light-signaling phenotypes are restricted to max2, as the biosynthetic mutants in the strigolactone pathway, max1, max3, and max4, did not display any defects in seed germination and seedling de-etiolation compared to wild-type. Taken together, these data suggest that MAX2 modulates multiple hormone pathways to affect photomorphogenesis.