Genome-wide analysis of PRR gene family uncovers their roles in circadian rhythmic changes and response to drought stress in Gossypium hirsutum L

BACKGROUND

The circadian clock not only participates in regulating various stages of plant growth, development and metabolism, but confers plant environmental adaptability to stress such as drought. Pseudo-Response Regulators (PRRs) are important component of the central oscillator (the core of circadian clock) and play a significant role in plant photoperiod pathway. However, no systematical study about this gene family has been performed in cotton.

METHODS

PRR genes were identified in diploid and tetraploid cotton using bioinformatics methods to investigate their homology, duplication and evolution relationship. Differential gene expression, KEGG enrichment analysis and qRT-PCR were conducted to analyze PRR gene expression patterns under diurnal changes and their response to drought stress.

RESULTS

A total of 44 PRR family members were identified in four Gossypium species, with 16 in G. hirsutum, 10 in G. raimondii, and nine in G. barbadense as well as in G. arboreum. Phylogenetic analysis indicated that PRR proteins were divided into five subfamilies and whole genome duplication or segmental duplication contributed to the expansion of Gossypium PRR gene family. Gene structure analysis revealed that members in the same clade are similar, and multiple cis-elements related to light and drought stress response were enriched in the promoters of GhPRR genes. qRT-PCR results showed that GhPRR genes transcripts presented four expression peaks (6 h, 9 h, 12 h, 15 h) during 24 h and form obvious rhythmic expression trend. Transcriptome data with PEG treatment, along with qRT-PCR verification suggested that members of clade III (GhPRR5a, b, d) and clade V (GhPRR3a and GhPRR3c) may be involved in drought response. This study provides an insight into understanding the function of PRR genes in circadian rhythm and in response to drought stress in cotton.

3,4-Dibromo-7-Azaindole Modulates Arabidopsis Circadian Clock by Inhibiting Casein Kinase 1 Activity

The circadian clock is a timekeeping system for regulation of numerous biological daily rhythms. One characteristic of the circadian clock is that period length remains relatively constant in spite of environmental fluctuations, such as temperature change. Here, using the curated collection of in-house small molecule chemical library (ITbM chemical library), we show that small molecule 3,4-dibromo-7-azaindole (B-AZ) lengthened the circadian period of Arabidopsis thaliana (Arabidopsis). B-AZ has not previously been reported to have any biological and biochemical activities. Target identification can elucidate the mode of action of small molecules, but we were unable to make a molecular probe of B-AZ for target identification. Instead, we performed other analysis, gene expression profiling that potentially reveals mode of action of molecules. Short-term treatment of B-AZ decreased the expression of four dawn- and morning-phased clock-associated genes, CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1), LATE ELONGATED HYPOCOTYL (LHY), PSEUDO-RESPONSE REGULATOR 9 (PRR9) and PRR7. Consistently, amounts of PRR5 and TIMING OF CAB EXPRESSION 1 (TOC1) proteins, transcriptional repressors of CCA1, LHY, PRR9 and PRR7 were increased upon B-AZ treatment. B-AZ inhibited Casein Kinase 1 family (CK1) that phosphorylates PRR5 and TOC1 for targeted degradation. A docking study and molecular dynamics simulation suggested that B-AZ interacts with the ATP-binding pocket of human CK1 delta, whose amino acid sequences are highly similar to those of Arabidopsis CK1. B-AZ-induced period-lengthening effect was attenuated in prr5 toc1 mutants. Collectively, this study provides a novel and simple structure CK1 inhibitor that modulates circadian clock via accumulation of PRR5 and TOC1.

Expression analysis of four pseudo-response regulator (PRR) genes in Chrysanthemum morifolium under different photoperiods

Genes encoding pseudo-response regulator (PRR) proteins play significant roles in plant circadian clocks. In this study, four genes related to flowering time were isolated from Chrysanthemum morifolium. Phylogenetic analysis showed that they are highly homologous to the counterparts of PRRs of Helianthus annuus and named as CmPRR2, CmPRR7, CmPRR37, and CmPRR73. Conserved motifs prediction indicated that most of the closely related members in the phylogenetic tree share common protein sequence motifs, suggesting functional similarities among the PRR proteins within the same subtree. In order to explore functions of the genes, we selected two Chrysanthemum varieties for comparison; that is, a short-day sensitive Zijiao and a short-day insensitive Aoyunbaixue. Compared to Aoyunbaixue, Zijiao needs 13 more days to complete the flower bud differentiation. Evidence from spatio-temporal gene expression patterns demonstrated that the CmPRRs are highly expressed in flower and stem tissues, with a growing trend across the Chrysanthemum developmental process. In addition, we also characterized the CmPRRs expression patterns and found that CmPRRs can maintain their circadian oscillation features to some extent under different photoperiod treatment conditions. These lines of evidence indicated that the four CmPRRs undergo circadian oscillation and possibly play roles in regulating the flowering time of C. morifolium.

Casein kinases I and 2α phosphorylate oryza sativa pseudo-response regulator 37 (OsPRR37) in photoperiodic flowering in rice

Flowering time (or heading date) is controlled by intrinsic genetic programs in response to environmental cues, such as photoperiod and temperature. Rice, a facultative short-day (SD) plant, flowers early in SD and late in long-day (LD) conditions. Casein kinases (CKs) generally act as positive regulators in many signaling pathways in plants. In rice, Heading date 6 (Hd6) and Hd16 encode CK2α and CKI, respectively, and mainly function to delay flowering time. Additionally, the major LD-dependent floral repressors Hd2/Oryza sativa Pseudo-Response Regulator 37 (OsPRR37; hereafter PRR37) and Ghd7 also confer strong photoperiod sensitivity. In floral induction, Hd16 acts upstream of Ghd7 and CKI interacts with and phosphorylates Ghd7. In addition, Hd6 and Hd16 also act upstream of Hd2. However, whether CKI and CK2α directly regulate the function of PRR37 remains unclear. Here, we use in vitro pull-down and in vivo bimolecular fluorescence complementation assays to show that CKI and CK2α interact with PRR37. We further use in vitro kinase assays to show that CKI and CK2α phosphorylate different regions of PRR37. Our results indicate that direct posttranslational modification of PRR37 mediates the genetic interactions between these two protein kinases and PRR37. The significance of CK-mediated phosphorylation for PRR37 and Ghd7 function is discussed.

PHYTOCHROME C is an essential light receptor for photoperiodic flowering in the temperate grass, Brachypodium distachyon

We show that in the temperate grass, Brachypodium distachyon, PHYTOCHROME C (PHYC), is necessary for photoperiodic flowering. In loss-of-function phyC mutants, flowering is extremely delayed in inductive photoperiods. PHYC was identified as the causative locus by utilizing a mapping by sequencing pipeline (Cloudmap) optimized for identification of induced mutations in Brachypodium. In phyC mutants the expression of Brachypodium homologs of key flowering time genes in the photoperiod pathway such as GIGANTEA (GI), PHOTOPERIOD 1 (PPD1/PRR37), CONSTANS (CO), and florigen/FT are greatly attenuated. PHYC also controls the day-length dependence of leaf size as the effect of day length on leaf size is abolished in phyC mutants. The control of genes upstream of florigen production by PHYC was likely to have been a key feature of the evolution of a long-day flowering response in temperate pooid grasses.

Natural variation in OsPRR37 regulates heading date and contributes to rice cultivation at a wide range of latitudes

Heading date and photoperiod sensitivity are fundamental traits that determine rice adaptation to a wide range of geographic environments. By quantitative trait locus (QTL) mapping and candidate gene analysis using whole-genome re-sequencing, we found that Oryza sativa Pseudo-Response Regulator37 (OsPRR37; hereafter PRR37) is responsible for the Early heading7-2 (EH7-2)/Heading date2 (Hd2) QTL which was identified from a cross of late-heading rice 'Milyang23 (M23)' and early-heading rice 'H143'. H143 contains a missense mutation of an invariantly conserved amino acid in the CCT (CONSTANS, CO-like, and TOC1) domain of PRR37 protein. In the world rice collection, different types of nonfunctional PRR37 alleles were found in many European and Asian rice cultivars. Notably, the japonica varieties harboring nonfunctional alleles of both Ghd7/Hd4 and PRR37/Hd2 flower extremely early under natural long-day conditions, and are adapted to the northernmost regions of rice cultivation, up to 53° N latitude. Genetic analysis revealed that the effects of PRR37 and Ghd7 alleles on heading date are additive, and PRR37 down-regulates Hd3a expression to suppress flowering under long-day conditions. Our results demonstrate that natural variations in PRR37/Hd2 and Ghd7/Hd4 have contributed to the expansion of rice cultivation to temperate and cooler regions.

OsELF3 is involved in circadian clock regulation for promoting flowering under long-day conditions in rice

Heading date is a critical trait that determines cropping seasons and regional adaptability in rice (Oryza sativa). Research efforts during the last decade have identified some important photoperiod pathway genes that are conserved between Arabidopsis and rice. In this study, we identified a novel gene, Oryza sativa ELF3 (OsELF3), which is a putative homolog of the ELF3 gene in Arabidopsis thaliana. OsELF3 was required for the control of heading date under long-day conditions. Its Tos17-tagging mutants exhibited a delayed heading date phenotype only under long-day, but not short-day, conditions. OsELF3 was highly expressed in leaf blades, and the OsELF3 protein was localized in the nucleolus. An obvious diurnal rhythm of OsELF3 transcript level was observed, with a trough in the early day and a peak in the late night in wild-type plants. However, this expression pattern was disrupted in oself3 mutants. Further investigations showed that the expression of OsGI and Ghd7 was up-regulated in the oself3 mutant, indicating that OsELF3 acts as a negative regulator upstream of OsGI and Ghd7 in the flowering-time control under long-day conditions. The rhythmic expression of circadian clock-related genes, including some OsPRR members, was obviously affected in oself3 mutants. Our results indicated that OsELF3 acts as a floral activator in the long-day photoperiodic pathway via its crosstalk with the circadian clock in rice.

Inferring transcriptional gene regulation network of starch metabolism in Arabidopsis thaliana leaves using graphical Gaussian model

BACKGROUND

Starch serves as a temporal storage of carbohydrates in plant leaves during day/night cycles. To study transcriptional regulatory modules of this dynamic metabolic process, we conducted gene regulation network analysis based on small-sample inference of graphical Gaussian model (GGM).

RESULTS

Time-series significant analysis was applied for Arabidopsis leaf transcriptome data to obtain a set of genes that are highly regulated under a diurnal cycle. A total of 1,480 diurnally regulated genes included 21 starch metabolic enzymes, 6 clock-associated genes, and 106 transcription factors (TF). A starch-clock-TF gene regulation network comprising 117 nodes and 266 edges was constructed by GGM from these 133 significant genes that are potentially related to the diurnal control of starch metabolism. From this network, we found that β-amylase 3 (b-amy3: At4g17090), which participates in starch degradation in chloroplast, is the most frequently connected gene (a hub gene). The robustness of gene-to-gene regulatory network was further analyzed by TF binding site prediction and by evaluating global co-expression of TFs and target starch metabolic enzymes. As a result, two TFs, indeterminate domain 5 (AtIDD5: At2g02070) and constans-like (COL: At2g21320), were identified as positive regulators of starch synthase 4 (SS4: At4g18240). The inference model of AtIDD5-dependent positive regulation of SS4 gene expression was experimentally supported by decreased SS4 mRNA accumulation in Atidd5 mutant plants during the light period of both short and long day conditions. COL was also shown to positively control SS4 mRNA accumulation. Furthermore, the knockout of AtIDD5 and COL led to deformation of chloroplast and its contained starch granules. This deformity also affected the number of starch granules per chloroplast, which increased significantly in both knockout mutant lines.

CONCLUSIONS

In this study, we utilized a systematic approach of microarray analysis to discover the transcriptional regulatory network of starch metabolism in Arabidopsis leaves. With this inference method, the starch regulatory network of Arabidopsis was found to be strongly associated with clock genes and TFs, of which AtIDD5 and COL were evidenced to control SS4 gene expression and starch granule formation in chloroplasts.

Coincident light and clock regulation of pseudoresponse regulator protein 37 (PRR37) controls photoperiodic flowering in sorghum

Optimal flowering time is critical to the success of modern agriculture. Sorghum is a short-day tropical species that exhibits substantial photoperiod sensitivity and delayed flowering in long days. Genotypes with reduced photoperiod sensitivity enabled sorghum's utilization as a grain crop in temperate zones worldwide. In the present study, Ma(1), the major repressor of sorghum flowering in long days, was identified as the pseudoresponse regulator protein 37 (PRR37) through positional cloning and analysis of SbPRR37 alleles that modulate flowering time in grain and energy sorghum. Several allelic variants of SbPRR37 were identified in early flowering grain sorghum germplasm that contain unique loss-of-function mutations. We show that in long days SbPRR37 activates expression of the floral inhibitor CONSTANS and represses expression of the floral activators Early Heading Date 1, FLOWERING LOCUS T, Zea mays CENTRORADIALIS 8, and floral induction. Expression of SbPRR37 is light dependent and regulated by the circadian clock, with peaks of RNA abundance in the morning and evening in long days. In short days, the evening-phase expression of SbPRR37 does not occur due to darkness, allowing sorghum to flower in this photoperiod. This study provides insight into an external coincidence mechanism of photoperiodic regulation of flowering time mediated by PRR37 in the short-day grass sorghum and identifies important alleles of SbPRR37 that are critical for the utilization of this tropical grass in temperate zone grain and bioenergy production.

The circadian oscillator gene GIGANTEA mediates a long-term response of the Arabidopsis thaliana circadian clock to sucrose

Circadian clocks are 24-h timing devices that phase cellular responses; coordinate growth, physiology, and metabolism; and anticipate the day-night cycle. Here we report sensitivity of the Arabidopsis thaliana circadian oscillator to sucrose, providing evidence that plant metabolism can regulate circadian function. We found that the Arabidopsis circadian system is particularly sensitive to sucrose in the dark. These data suggest that there is a feedback between the molecular components that comprise the circadian oscillator and plant metabolism, with the circadian clock both regulating and being regulated by metabolism. We used also simulations within a three-loop mathematical model of the Arabidopsis circadian oscillator to identify components of the circadian clock sensitive to sucrose. The mathematical studies identified GIGANTEA (GI) as being associated with sucrose sensing. Experimental validation of this prediction demonstrated that GI is required for the full response of the circadian clock to sucrose. We demonstrate that GI acts as part of the sucrose-signaling network and propose this role permits metabolic input into circadian timing in Arabidopsis.

Quantitative analysis of regulatory flexibility under changing environmental conditions

The circadian clock controls 24-h rhythms in many biological processes, allowing appropriate timing of biological rhythms relative to dawn and dusk. Known clock circuits include multiple, interlocked feedback loops. Theory suggested that multiple loops contribute the flexibility for molecular rhythms to track multiple phases of the external cycle. Clear dawn- and dusk-tracking rhythms illustrate the flexibility of timing in Ipomoea nil. Molecular clock components in Arabidopsis thaliana showed complex, photoperiod-dependent regulation, which was analysed by comparison with three contrasting models. A simple, quantitative measure, Dusk Sensitivity, was introduced to compare the behaviour of clock models with varying loop complexity. Evening-expressed clock genes showed photoperiod-dependent dusk sensitivity, as predicted by the three-loop model, whereas the one- and two-loop models tracked dawn and dusk, respectively. Output genes for starch degradation achieved dusk-tracking expression through light regulation, rather than a dusk-tracking rhythm. Model analysis predicted which biochemical processes could be manipulated to extend dusk tracking. Our results reveal how an operating principle of biological regulators applies specifically to the plant circadian clock.

Does the core circadian clock in the moss Physcomitrella patens (Bryophyta) comprise a single loop?

BACKGROUND

The endogenous circadian clock allows the organism to synchronize processes both to daily and seasonal changes. In plants, many metabolic processes such as photosynthesis, as well as photoperiodic responses, are under the control of a circadian clock. Comparative studies with the moss Physcomitrella patens provide the opportunity to study many aspects of land plant evolution. Here we present a comparative overview of clock-associated components and the circadian network in the moss P. patens.

RESULTS

The moss P. patens has a set of conserved circadian core components that share genetic relationship and gene expression patterns with clock genes of vascular plants. These genes include Myb-like transcription factors PpCCA1a and PpCCA1b, pseudo-response regulators PpPRR1-4, and regulatory elements PpELF3, PpLUX and possibly PpELF4. However, the moss lacks homologs of AtTOC1, AtGI and the AtZTL-family of genes, which can be found in all vascular plants studied here. These three genes constitute essential components of two of the three integrated feed-back loops in the current model of the Arabidopsis circadian clock mechanism. Consequently, our results suggest instead a single loop circadian clock in the moss. Possibly as a result of this, temperature compensation of core clock gene expression appears to be decreased in P. patens.

CONCLUSIONS

This study is the first comparative overview of the circadian clock mechanism in a basal land plant, the moss P. patens. Our results indicate that the moss clock mechanism may represent an ancestral state in contrast to the more complex and partly duplicated structure of subsequent land plants. These findings may provide insights into the understanding of the evolution of circadian network topology.

PSEUDO-RESPONSE REGULATORS 9, 7, and 5 are transcriptional repressors in the Arabidopsis circadian clock

An interlocking transcriptional-translational feedback loop of clock-associated genes is thought to be the central oscillator of the circadian clock in plants. TIMING OF CAB EXPRESSION1 (also called PSEUDO-RESPONSE REGULATOR1 [PRR1]) and two MYB transcription factors, CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), play pivotal roles in the loop. Genetic studies have suggested that PRR9, PRR7, and PRR5 also act within or close to the loop; however, their molecular functions remain unknown. Here, we demonstrate that PRR9, PRR7, and PRR5 act as transcriptional repressors of CCA1 and LHY. PRR9, PRR7, and PRR5 each suppress CCA1 and LHY promoter activities and confer transcriptional repressor activity to a heterologous DNA binding protein in a transient reporter assay. Using a glucocorticoid-induced PRR5-GR (glucorticoid receptor) construct, we found that PRR5 directly downregulates CCA1 and LHY expression. Furthermore, PRR9, PRR7, and PRR5 associate with the CCA1 and LHY promoters in vivo, coincident with the timing of decreased CCA1 and LHY expression. These results suggest that the repressor activities of PRR9, PRR7, and PRR5 on the CCA1 and LHY promoter regions constitute the molecular mechanism that accounts for the role of these proteins in the feedback loop of the circadian clock.

Time for circadian rhythms: plants get synchronized

Most organisms adjust their physiology and metabolism in synchronization with the diurnal and seasonal time by using an endogenous mechanism known as circadian clock. In plants, light and temperature signals interact with the circadian system to regulate the circadian rhythmicity of physiological and developmental processes including flowering time. Recent studies in Arabidopsis thaliana now reveal that the circadian clock orchestrates not only the expression of protein coding genes but also the rhythmic oscillation of introns, intergenic regions, and noncoding RNAs. Furthermore, recent evidence showing the existence of different oscillators at separate parts of the plant has placed the spotlight on the diverse mechanisms and communicating channels that regulate circadian synchronization in plants.