Measurement of Phytochrome B Thermal Reversion Rates In Vivo.

Thermal reversion of phytochromes is the light-independent but strongly temperature-dependent relaxation of the light-activated Pfr form of phytochromes back into the inactive Pr ground state. The thermal reversion rates of different phytochromes vary considerably. For phytochrome B (phyB), thermal reversion represents a critical parameter affecting phyB activity as it reduces the active phyB Pfr pool, accelerated by increasing temperatures. Phytochromes are dimers existing in three different states: Pfr-Pfr homodimer, Pfr-Pr heterodimer, and Pr-Pr homodimer. Consequently, thermal reversion occurs in two steps, with Pfr-Pfr to Pfr-Pr reversion being much slower than reversion from Pfr-Pr to Pr-Pr. To measure thermal reversion in vivo, the relative proportion of Pfr in relation to the total amount of phytochrome (Ptot) must be determined in living samples. This is accomplished by in vivo spectroscopy utilizing dual wavelength ratiospectrophotometers, optimized for assaying phytochromes in highly scattering plant material. The method is depending on the photoreversibility of phytochromes displaying light-induced absorbance changes in response to actinic irradiation. In this chapter, we describe the experimental design and explain step-by-step the calculations necessary to determine the thermal reversion rates of phyB in vivo, taking into account phytochrome dimerization.

A DELAY OF GERMINATION 1 (DOG1)-like protein regulates spore germination in the moss Physcomitrium patens.

DELAY OF GERMINATION 1 is a key regulator of dormancy in flowering plants before seed germination. Bryophytes develop haploid spores with an analogous function to seeds. Here, we investigate whether DOG1 function during germination is conserved between bryophytes and flowering plants and analyse the underlying mechanism of DOG1 action in the moss Physcomitrium patens. Phylogenetic and in silico expression analyses were performed to identify and characterise DOG1 domain-containing genes in P. patens. Germination assays were performed to characterise a Ppdog1-like1 mutant, and replacement with AtDOG1 was carried out. Yeast two-hybrid assays were used to test the interaction of the PpDOG1-like protein with DELLA proteins from P. patens and A. thaliana. P. patens possesses nine DOG1 domain-containing genes. The DOG1-like protein PpDOG1-L1 (Pp3c3_9650) interacts with PpDELLAa and PpDELLAb and the A. thaliana DELLA protein AtRGA in yeast. Protein truncations revealed the DOG1 domain as necessary and sufficient for interaction with PpDELLA proteins. Spores of Ppdog1-l1 mutant germinate faster than wild type, but replacement with AtDOG1 reverses this effect. Our data demonstrate a role for the PpDOG1-LIKE1 protein in moss spore germination, possibly alongside PpDELLAs. This suggests a conserved DOG1 domain function in germination, albeit with differential adaptation of regulatory networks in seed and spore germination.

EID1 promotes the response to canopy shade in Arabidopsis thaliana by repressing the action of phytochrome A.

The phytochrome (phy) system enables plants to adapt to canopy shade. By sensing the reduction of the red:far-red light ratio in shade, phyA and phyB trigger downstream signalling cascades which eventually lead to enhanced elongation growth. In this study, we show that the F-box protein EID1 takes on an essential function within the shade avoidance response in Arabidopsis thaliana by repressing phyA action and thereby allowing seedlings to elongate in shade. Thus, altering EID1 activity provides a means to adapt the shade response without affecting phyB action and could have played a role in the evolution of shade tolerance.

Phytochromes transmit photoperiod information via the evening complex in Brachypodium.

BACKGROUND: Daylength is a key seasonal cue for animals and plants. In cereals, photoperiodic responses are a major adaptive trait, and alleles of clock genes such as PHOTOPERIOD1 (PPD1) and EARLY FLOWERING3 (ELF3) have been selected for in adapting barley and wheat to northern latitudes. How monocot plants sense photoperiod and integrate this information into growth and development is not well understood. RESULTS: We find that phytochrome C (PHYC) is essential for flowering in Brachypodium distachyon. Conversely, ELF3 acts as a floral repressor and elf3 mutants display a constitutive long day phenotype and transcriptome. We find that ELF3 and PHYC occur in a common complex. ELF3 associates with the promoters of a number of conserved regulators of flowering, including PPD1 and VRN1. Consistent with observations in barley, we are able to show that PPD1 overexpression accelerates flowering in short days and is necessary for rapid flowering in response to long days. PHYC is in the active Pfr state at the end of the day, but we observe it undergoes dark reversion over the course of the night. CONCLUSIONS: We propose that PHYC acts as a molecular timer and communicates information on night-length to the circadian clock via ELF3.

Phytochrome higher order mutants reveal a complex set of light responses in the moss Physcomitrium patens.

Phytochromes are photoreceptors enabling plants to respond to various light conditions. Independent gene duplications resulted in small phytochrome families in mosses, ferns and seed plants. This phytochrome diversity is hypothesised to be critical for sensing and adapting to different light conditions, but experimental evidence for this idea is lacking for mosses and ferns. The moss model species Physcomitrium patens contains seven phytochromes grouped into three clades, PHY1/3, PHY2/4 and PHY5. Here, we used CRISPR/Cas9-generated single and higher order mutants to investigate their role in light regulation of protonema and gametophore growth, protonema branching and induction of gametophores. We found both specific and partially overlapping roles for the three phytochrome clades in regulating these responses in different light conditions. PHY1/3 clade phytochromes act as primary far-red light receptors, while PHY5 clade phytochromes are the primary red light receptors. PHY2/4 clade phytochromes have functions in both red and far-red light. We also observed that PHY1/3 and PHY2/4 clade phytochromes promote gametophore growth in simulated canopy shade and also play a role in blue light. Similar to seed plants, gene duplications in the phytochrome lineage in mosses were followed by functional diversification into red and far-red light-sensing phytochromes.

COLD REGULATED GENE 27 and 28 antagonize the transcriptional activity of the RVE8/LNK1/LNK2 circadian complex.

Many molecular and physiological processes in plants occur at a specific time of day. These daily rhythms are coordinated in part by the circadian clock, a timekeeper that uses daylength and temperature to maintain rhythms of ∼24 h in various clock-regulated phenotypes. The circadian MYB-like transcription factor REVEILLE 8 (RVE8) interacts with its transcriptional coactivators NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED 1 (LNK1) and LNK2 to promote the expression of evening-phased clock genes and cold tolerance factors. While genetic approaches have commonly been used to discover connections within the clock and between clock elements and other pathways, here, we used affinity purification coupled with mass spectrometry (APMS) to identify time-of-day-specific protein interactors of the RVE8-LNK1/LNK2 complex in Arabidopsis (Arabidopsis thaliana). Among the interactors of RVE8/LNK1/LNK2 were COLD-REGULATED GENE 27 (COR27) and COR28, which coprecipitated in an evening-specific manner. In addition to COR27 and COR28, we found an enrichment of temperature-related interactors that led us to establish a previously uncharacterized role for LNK1 and LNK2 in temperature entrainment of the clock. We established that RVE8, LNK1, and either COR27 or COR28 form a tripartite complex in yeast (Saccharomyces cerevisiae) and that the effect of this interaction in planta serves to antagonize transcriptional activation of RVE8 target genes, potentially through mediating RVE8 protein degradation in the evening. Together, these results illustrate how a proteomic approach can be used to identify time-of-day-specific protein interactions. Discovery of the RVE8-LNK-COR protein complex indicates a previously unknown regulatory mechanism for circadian and temperature signaling pathways.

Phytochromes and flowering: legumes do it another way.

Phytochromes have a crucial role in the regulation of flowering in many plants, but the underlying molecular mechanisms vary among species. Recently, Lin et al. described a unique phytochrome A (phyA)-controlled photoperiodic flowering pathway in soybean (Glycine max), revealing a novel mechanism for photoperiodic regulation of flowering.

The phytochrome interacting proteins ERF55 and ERF58 repress light-induced seed germination in Arabidopsis thaliana.

Seed germination is a critical step in the life cycle of plants controlled by the phytohormones abscisic acid (ABA) and gibberellin (GA), and by phytochromes, an important class of photoreceptors in plants. Here we show that light-dependent germination is enhanced in mutants deficient in the AP2/ERF transcription factors ERF55 and ERF58. Light-activated phytochromes repress ERF55/ERF58 expression and directly bind ERF55/ERF58 to displace them from the promoter of PIF1 and SOM, genes encoding transcriptional regulators that prevent the completion of germination. The same mechanism controls the expression of genes that encode ABA or GA metabolic enzymes to decrease levels of ABA and possibly increase levels of GA. Interestingly, ERF55 and ERF58 are themselves under transcriptional control of ABA and GA, suggesting that they are part of a self-reinforcing signalling loop which controls the completion of germination. Overall, we identified a role of ERF55/ERF58 in phytochrome-mediated regulation of germination completion.

Phytochrome A Mediates the Disassembly of Processing Bodies in Far-Red Light.

Phytochromes are red- and far-red light receptors that control the growth and development of plants, enabling them to respond adequately to changing light conditions. It has been shown that halted mRNAs stored in RNA granules called processing bodies are released upon light perception and contribute to the adaptation to the light environment. However, the photophysiological background of this process is largely unknown. We found that light of different wavelengths can trigger the disassembly of processing bodies in a dose- and time-dependent manner. We show that phytochromes control this process in red- and far-red light and that cytoplasmic phytochrome A is sufficient and necessary for the far-red light-induced disassembly of processing bodies. This adds a novel, unexpected cytoplasmic function to the processes controlled by phytochrome A. Overall, our findings suggest a role of phytochromes in the control of translationally halted mRNAs that are stored in processing bodies. We expect our findings to facilitate understanding of how light and environmental cues control the assembly and disassembly of processing bodies, which could have broader implications for the regulation of non-membranous organelles in general.

Uncovering a novel function of the CCR4-NOT complex in phytochrome A-mediated light signalling in plants.

Phytochromes are photoreceptors regulating growth and development in plants. Using the model plant Arabidopsis, we identified a novel signalling pathway downstream of the far-red light-sensing phytochrome, phyA, that depends on the highly conserved CCR4-NOT complex. CCR4-NOT is integral to RNA metabolism in yeast and animals, but its function in plants is largely unknown. NOT9B, an Arabidopsis homologue of human CNOT9, is a component of the CCR4-NOT complex, and acts as negative regulator of phyA-specific light signalling when bound to NOT1, the scaffold protein of the complex. Light-activated phyA interacts with and displaces NOT9B from NOT1, suggesting a potential mechanism for light signalling through CCR4-NOT. ARGONAUTE 1 and proteins involved in splicing associate with NOT9B and we show that NOT9B is required for specific phyA-dependent alternative splicing events. Furthermore, association with nuclear localised ARGONAUTE 1 raises the possibility that NOT9B and CCR4-NOT are involved in phyA-modulated gene expression.

Place a seedling on a windowsill, and soon you will notice the fragile stem bending towards the glass to soak in the sun and optimize its growth. Plants can 'sense' light thanks to specialized photoreceptor molecules: for instance, the phytochrome A is responsible for detecting weak and 'far-red' light from the very edge of the visible spectrum. Once the phytochrome has been activated, this message is relayed to the rest of the plant through an intricate process that requires other molecules. The CCR4-NOT protein complex is vital for all plants, animals and fungi, suggesting that it was already present in early life forms. Here, Schwenk et al. examine whether CCR4-NOT could have acquired a new role in plants to help them respond to far-red light. Scanning the genetic information of the plant model Arabidopsis thaliana revealed that the gene encoding the NOT9 subunit of CCR4-NOT had been duplicated in plants during evolution. NOT9B, the protein that the new copy codes for, has a docking site that can attach to both phytochrome A and CCR4-NOT. When NOT9B binds phytochrome A, it is released from the CCR4-NOT complex: this could trigger a cascade of reactions that ultimately changes how A. thaliana responds to far-red light. Plants that had not enough or too much NOT9B were respectively more or less responsive to that type of light, showing that the duplication of the gene coding for this subunit had helped plants respond to certain types of light. The findings by Schwenk et al. illustrate how existing structures can be repurposed during evolution to carry new roles. They also provide a deeper understanding of how plants optimize their growth, a useful piece of information in a world where most people rely on crops as their main source of nutrients.

Phytochrome B and PCH1 protein dynamics store night temperature information.

Plants experience temperature fluctuations during the course of the daily cycle, and although stem growth responds rapidly to these changes we largely ignore whether there is a short-term memory of previous conditions. Here we show that nighttime temperatures affect the growth of the hypocotyl of Arabidopsis thaliana seedlings not only during the night but also during the subsequent photoperiod. Active phytochrome B (phyB) represses nighttime growth and warm temperatures reduce active phyB via thermal reversion. The function of PHOTOPERIODIC CONTROL OF HYPOCOTYL1 (PCH1) is to stabilise active phyB in nuclear bodies but, surprisingly, warmth reduces PCH1 gene expression and PCH1 stability. When phyB was active at the beginning of the night, warm night temperatures enhanced the levels of nuclear phyB and reduced hypocotyl growth rate during the following day. However, when end-of-day far-red light minimised phyB activity, warm night temperatures reduced the levels of nuclear phyB and enhanced the hypocotyl growth rate during the following day. This complex growth pattern was absent in the phyB mutant. We propose that temperature-induced changes in the levels of PCH1 and in the size of the physiologically relevant nuclear pool of phyB amplify the impact of phyB-mediated temperature sensing.

COLD REGULATED 27 and 28 are targets of CONSTITUTIVELY PHOTOMORPHOGENIC 1 and negatively affect phytochrome B signalling.

Phytochromes are red/far-red light receptors in plants involved in the regulation of growth and development. Phytochromes can sense the light environment and contribute to measuring day length; thereby, they allow plants to respond and adapt to changes in the ambient environment. Two well-characterized signalling pathways act downstream of phytochromes and link light perception to the regulation of gene expression. The CONSTITUTIVELY PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA-105 (COP1/SPA) E3 ubiquitin ligase complex and the PHYTOCHROME INTERACTING FACTORs (PIFs) are key components of these pathways and repress light responses in the dark. In light-grown seedlings, phytochromes inhibit COP1/SPA and PIF activity and thereby promote light signalling. In a yeast-two-hybrid screen for proteins binding to light-activated phytochromes, we identified COLD-REGULATED GENE 27 (COR27). COR27 and its homologue COR28 bind to phyA and phyB, the two primary phytochromes in seed plants. COR27 and COR28 have been described previously with regard to a function in the regulation of freezing tolerance, flowering and the circadian clock. Here, we show that COR27 and COR28 repress early seedling development in blue, far-red and in particular red light. COR27 and COR28 contain a conserved Val-Pro (VP)-peptide motif, which mediates binding to the COP1/SPA complex. COR27 and COR28 are targeted for degradation by COP1/SPA and mutant versions with a VP to AA amino acid substitution in the VP-peptide motif are stabilized. Overall, our data suggest that COR27 and COR28 accumulate in light but act as negative regulators of light signalling during early seedling development, thereby preventing an exaggerated response to light.

PHYTOCHROME INTERACTING FACTORs in the moss Physcomitrella patens regulate light-controlled gene expression.

Phytochromes are red and far-red light receptors in plants that control growth and development in response to changes in the environment. Light-activated phytochromes enter the nucleus and act on a set of downstream signalling components to regulate gene expression. PHYTOCHROME INTERACTING FACTORs (PIFs) belong to the basic helix-loop-helix family of transcription factors and directly bind to light-activated phytochromes. Potential homologues of PIFs have been identified in ferns, bryophytes and streptophyte algae, and it has been shown that the potential PIF homologues from Physcomitrella patens, PIF1 to PIF4, have PIF function when expressed in Arabidopsis. However, their function in Physcomitrella is still unknown. Seed plant PIFs bind to G-box-containing promoters and, therefore, we searched the Physcomitrella genome for genes that contain G-boxes in their promoter. Here, we show that Physcomitrella PIFs activate these promoters in a G-box-dependent manner, suggesting that they could be direct PIF targets. Furthermore, we generated Physcomitrella pif1, pif2, pif3 and pif4 knock out mutant lines and quantified the expression of potential PIF direct target genes. The expression of these genes was generally reduced in pif mutants compared to the wildtype, but for several genes, the relative induction upon a short light treatment was higher in pif mutants than the wildtype. In contrast, expression of these genes was strongly repressed in continuous light, and pif mutants showed partial downregulation of these genes in the dark. Thus, the overall function of PIFs in light-regulated gene expression might be an ancient property of PIFs.

PCH1 and PCHL Directly Interact with PIF1, Promote Its Degradation, and Inhibit Its Transcriptional Function during Photomorphogenesis.

PHOTOPERIODIC CONTROL OF HYPOCOTYL 1 (PCH1) and PCH1-LIKE (PCHL) were shown to directly bind to phytochrome B (phyB) and suppress phyB thermal reversion, resulting in plants with dramatically enhanced light sensitivity. Here, we show that PCH1 and PCHL also positively regulate various light responses, including seed germination, hypocotyl gravitropism, and chlorophyll biosynthesis, by physically interacting with PHYTOCHROME INTERACTING FACTOR 1 (PIF1) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1). PCH1 and PCHL interact with PIF1 both in the dark and light, and regulate PIF1 abundance. Moreover, PCH1 and PCHL facilitate the physical interaction between phyB and PIF1 in vivo to promote the light-induced degradation of PIF1. PCH1 and PCHL also inhibit the DNA-binding ability of PIF1 to negatively regulate the expressions of PIF1 target genes. In addition, PCH1 and PCHL interact with COP1 and undergo degradation through the 26S proteasome pathway in the dark. Consistently, pch1 suppresses cop1 phenotype in darkness. Collectively, our study reveals a novel mechanism by which PCH1 and PCHL regulate diverse light responses not only by stabilizing phyB Pfr form but also by directly interacting with PIF1 and COP1, providing a molecular understanding of the control of hypocotyl growth by these proteins.

Differential phosphorylation of the N-terminal extension regulates phytochrome B signaling.

Phytochrome B (phyB) is an excellent light quality and quantity sensor that can detect subtle changes in the light environment. The relative amounts of the biologically active photoreceptor (phyB Pfr) are determined by the light conditions and light independent thermal relaxation of Pfr into the inactive phyB Pr, termed thermal reversion. Little is known about the regulation of thermal reversion and how it affects plants' light sensitivity. In this study we identified several serine/threonine residues on the N-terminal extension (NTE) of Arabidopsis thaliana phyB that are differentially phosphorylated in response to light and temperature, and examined transgenic plants expressing nonphosphorylatable and phosphomimic phyB mutants. The NTE of phyB is essential for thermal stability of the Pfr form, and phosphorylation of S86 particularly enhances the thermal reversion rate of the phyB Pfr-Pr heterodimer in vivo. We demonstrate that S86 phosphorylation is especially critical for phyB signaling compared with phosphorylation of the more N-terminal residues. Interestingly, S86 phosphorylation is reduced in light, paralleled by a progressive Pfr stabilization under prolonged irradiation. By investigating other phytochromes (phyD and phyE) we provide evidence that acceleration of thermal reversion by phosphorylation represents a general mechanism for attenuating phytochrome signaling.

Arabidopsis FHY1 and FHY1-LIKE Are Not Required for Phytochrome A Signal Transduction in the Nucleus.

Photoreceptors of the phytochrome family control a multitude of responses in plants. Phytochrome A (phyA) is essential for far-red light perception, which is important for germination and seedling establishment in strong canopy shade. Translocation of phyA from the cytosol into nucleus is a key step in far-red light signaling and requires FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). FHY1/FHL bind to phyA downstream signaling components. Therefore, it has been suggested that FHY1/FHL also have a function in assembling phyA transcription factor complexes in the nucleus. Yet, in this study, we show that constitutively nuclear-localized phyA is active in the absence of FHY1 and FHL. Furthermore, an artificial FHY1, consisting of an SV40 NLS, a phyA binding site, and a YFP tag as spacer between them, complements the fhy1-3 fhl-1 double mutant. These findings show that FHY1 and FHL are not required for phyA downstream signaling in the nucleus. However, we found that lines expressing phyA-NLS-YFP are hypersensitive to red and far-red light and that slightly increased levels of constitutively nuclear-localized phyA result in photomorphogenic development in the dark. Thus, restricting phyA to the cytosol and inducing nuclear transport in light by interaction with FHY1/FHL might be important to suppress photomorphogenesis in the dark.