Preferential retention of circadian clock genes during diploidization following whole genome triplication in Brassica rapa

Diurnal oscillations of epigenetic modifications are associated with variation in rhythmic expression of homoeologous genes in Brassica napus

BACKGROUND

Epigenetic modifications that exhibit circadian oscillations also promote circadian oscillations of gene expression. Brassica napus is a heterozygous polyploid species that has undergone distant hybridization and genome doubling events and has a young and distinct species origin. Studies incorporating circadian rhythm analysis of epigenetic modifications can offer new insights into differences in diurnal oscillation behavior among subgenomes and the regulation of diverse expressions of homologous gene rhythms in biological clocks.

RESULTS

In this study, we created a high-resolution and multioscillatory gene expression dataset, active histone modification (H3K4me3, H3K9ac), and RNAPII recruitment in Brassica napus. We also conducted the pioneering characterization of the diurnal rhythm of transcription and epigenetic modifications in an allopolyploid species. We compared the evolution of diurnal rhythms between subgenomes and observed that the Cn subgenome had higher diurnal oscillation activity in both transcription and active histone modifications than the An subgenome. Compared to the A subgenome in Brassica rapa, the An subgenome of Brassica napus displayed significant changes in diurnal oscillation characteristics of transcription. Homologous gene pairs exhibited a higher proportion of diurnal oscillation in transcription than subgenome-specific genes, attributed to higher chromatin accessibility and abundance of active epigenetic modification types. We found that the diurnal expression of homologous genes displayed diversity, and the redundancy of the circadian system resulted in extensive changes in the diurnal rhythm characteristics of clock genes after distant hybridization and genome duplication events. Epigenetic modifications influenced the differences in the diurnal rhythm of homologous gene expression, and the diurnal oscillation of homologous gene expression was affected by the combination of multiple histone modifications.

CONCLUSIONS

Herein, we presented, for the first time, a characterization of the diurnal rhythm characteristics of gene expression and its epigenetic modifications in an allopolyploid species. Our discoveries shed light on the epigenetic factors responsible for the diurnal oscillation activity imbalance between subgenomes and homologous genes' rhythmic expression differences. The comprehensive time-series dataset we generated for gene expression and epigenetic modifications provides a valuable resource for future investigations into the regulatory mechanisms of protein-coding genes in Brassica napus.

Genome-Wide Identification of BrCAX Genes and Functional Analysis of BrCAX1 Involved in Ca2+ Transport and Ca2+ Deficiency-Induced Tip-Burn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Calcium (Ca2+) plays essential roles in plant growth and development. Ca2+ deficiency causes a physiological disorder of tip-burn in Brassiceae crops and is involved in the regulation of cellular Ca2+ homeostasis. Although the functions of Ca2+/H+ exchanger antiporters (CAXs) in mediating transmembrane transport of Ca2+ have been extensively characterized in multiple plant species, the potential roles of BrCAX genes remain unclear in Chinese cabbage. In this study, eight genes of the BrCAX family were genome-widely identified in Chinese cabbage. These BrCAX proteins contained conserved Na_Ca_ex domain and belonged to five members of the CAX family. Molecular evolutionary analysis and sequence alignment revealed the evolutionary conservation of BrCAX family genes. Expression profiling demonstrated that eight BrCAX genes exhibited differential expression in different tissues and under heat stress. Furthermore, Ca2+ deficiency treatment induced the typical symptoms of tip-burn in Chinese cabbage seedlings and a significant decrease in total Ca2+ content in both roots and leaves. The expression changes in BrCAX genes were related to the response to Ca2+ deficiency-induced tip-burn of Chinese cabbage. Specially, BrCAX1-1 and BrCAX1-2 genes were highly expressed gene members of the BrCAX family in the leaves and were significantly differentially expressed under Ca2+ deficiency stress. Moreover, overexpression of BrCAX1-1 and BrCAX1-2 genes in yeast and Chinese cabbage cotyledons exhibited a higher Ca2+ tolerance, indicating the Ca2+ transport capacity of BrCAX1-1 and BrCAX1-2. In addition, suppression expression of BrCAX1-1 and BrCAX1-2 genes reduced cytosolic Ca2+ levels in the root tips of Chinese cabbage. These results provide references for functional studies of BrCAX genes and to investigate the regulatory mechanisms underlying Ca2+ deficiency disorder in Brassiceae vegetables.

Differences in pseudogene evolution contributed to the contrasting flavors of turnip and Chiifu, two Brassica rapa subspecies

Pseudogenes are important resources for investigation of genome evolution and genomic diversity because they are nonfunctional but have regulatory effects that influence plant adaptation and diversification. However, few systematic comparative analyses of pseudogenes in closely related species have been conducted. Here, we present a turnip (Brassica rapa ssp. rapa) genome sequence and characterize pseudogenes among diploid Brassica species/subspecies. The results revealed that the number of pseudogenes was greatest in Brassica oleracea (CC genome), followed by B. rapa (AA genome) and then Brassica nigra (BB genome), implying that pseudogene differences emerged after species differentiation. In Brassica AA genomes, pseudogenes were distributed asymmetrically on chromosomes because of numerous chromosomal insertions/rearrangements, which contributed to the diversity among subspecies. Pseudogene differences among subspecies were reflected in the flavor-related glucosinolate (GSL) pathway. Specifically, turnip had the highest content of pungent substances, probably because of expansion of the methylthioalkylmalate synthase-encoding gene family in turnips; these genes were converted into pseudogenes in B. rapa ssp. pekinensis (Chiifu). RNA interference-based silencing of the gene encoding 2-oxoglutarate-dependent dioxygenase 2, which is also associated with flavor and anticancer substances in the GSL pathway, resulted in increased abundance of anticancer compounds and decreased pungency of turnip and Chiifu. These findings revealed that pseudogene differences between turnip and Chiifu influenced the evolution of flavor-associated GSL metabolism-related genes, ultimately resulting in the different flavors of turnip and Chiifu.

Sinapis genomes provide insights into whole-genome triplication and divergence patterns within tribe Brassiceae

Sinapis alba and Sinapis arvensis are mustard crops within the Brassiceae tribe of the Brassicaceae family, and represent an important genetic resource for crop improvement. We performed the de novo assembly of Brassica nigra, S. alba, and S. arvensis, and conducted comparative genomics to investigate the pattern of genomic evolution since an ancient whole-genome triplication event. Both Sinapis species retained evidence of the Brassiceae whole-genome triplication approximately 20.5 million years ago (Mya), with subgenome dominance observed in gene density, gene expression, and selective constraint. While S. alba diverged from the ancestor of Brassica and Raphanus at approximately 12.5 Mya, the divergence time of S. arvensis and B. nigra was approximately 6.5 Mya. S. arvensis and B. nigra had greater collinearity compared with their relationship to either Brassica rapa or Brassica oleracea. Two chromosomes of S. alba (Sal03 and Sal08) were completely collinear with two ancestral chromosomes proposed in the Ancestral Crucifer Karyotype (ACK) genomic block model, the first time this has been observed in the Brassiceae. These results are consistent with S. alba representing a relatively ancient lineage of the species evolved from the common ancestor of tribe Brassiceae, and suggest that the phylogeny of the Brassica and Sinapis genera requires some revision. Our study provides new insights into the genome evolution and phylogenetic relationships of Brassiceae and provides genomic information for genetic improvement of these plants.

Circadian regulation of the transcriptome in a complex polyploid crop

The circadian clock is a finely balanced timekeeping mechanism that coordinates programmes of gene expression. It is currently unknown how the clock regulates expression of homoeologous genes in polyploids. Here, we generate a high-resolution time-course dataset to investigate the circadian balance between sets of 3 homoeologous genes (triads) from hexaploid bread wheat. We find a large proportion of circadian triads exhibit imbalanced rhythmic expression patterns, with no specific subgenome favoured. In wheat, period lengths of rhythmic transcripts are found to be longer and have a higher level of variance than in other plant species. Expression of transcripts associated with circadian controlled biological processes is largely conserved between wheat and Arabidopsis; however, striking differences are seen in agriculturally critical processes such as starch metabolism. Together, this work highlights the ongoing selection for balance versus diversification in circadian homoeologs and identifies clock-controlled pathways that might provide important targets for future wheat breeding.

Core circadian clock and light signaling genes brought into genetic linkage across the green lineage

The circadian clock is conserved at both the level of transcriptional networks as well as core genes in plants, ensuring that biological processes are phased to the correct time of day. In the model plant Arabidopsis (Arabidopsis thaliana), the core circadian SHAQKYF-type-MYB (sMYB) genes CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and REVEILLE (RVE4) show genetic linkage with PSEUDO-RESPONSE REGULATOR 9 (PRR9) and PRR7, respectively. Leveraging chromosome-resolved plant genomes and syntenic ortholog analysis enabled tracing this genetic linkage back to Amborella trichopoda, a sister lineage to the angiosperm, and identifying an additional evolutionarily conserved genetic linkage in light signaling genes. The LHY/CCA1-PRR5/9, RVE4/8-PRR3/7, and PIF3-PHYA genetic linkages emerged in the bryophyte lineage and progressively moved within several genes of each other across an array of angiosperm families representing distinct whole-genome duplication and fractionation events. Soybean (Glycine max) maintained all but two genetic linkages, and expression analysis revealed the PIF3-PHYA linkage overlapping with the E4 maturity group locus was the only pair to robustly cycle with an evening phase, in contrast to the sMYB-PRR morning and midday phase. While most monocots maintain the genetic linkages, they have been lost in the economically important grasses (Poaceae), such as maize (Zea mays), where the genes have been fractionated to separate chromosomes and presence/absence variation results in the segregation of PRR7 paralogs across heterotic groups. The environmental robustness model is put forward, suggesting that evolutionarily conserved genetic linkages ensure superior microhabitat pollinator synchrony, while wide-hybrids or unlinking the genes, as seen in the grasses, result in heterosis, adaptation, and colonization of new ecological niches.

Identification and Characterization of PSEUDO-RESPONSE REGULATOR (PRR) 1a and 1b Genes by CRISPR/Cas9-Targeted Mutagenesis in Chinese Cabbage (Brassica rapa L.)

The CRISPR/Cas9 site-directed gene-editing system offers great advantages for identifying gene function and crop improvement. The circadian clock measures and conveys day length information to control rhythmic hypocotyl growth in photoperiodic conditions, to achieve optimal fitness, but operates through largely unknown mechanisms. Here, we generated core circadian clock evening components, Brassica rapa PSEUDO-RESPONSE REGULATOR (BrPRR) 1a, 1b, and 1ab (both 1a and 1b double knockout) mutants, using CRISPR/Cas9 genome editing in Chinese cabbage, where 9–16 genetic edited lines of each mutant were obtained. The targeted deep sequencing showed that each mutant had 2–4 different mutation types at the target sites in the BrPRR1a and BrPRR1b genes. To identify the functions of BrPRR1a and 1b genes, hypocotyl length, and mRNA and protein levels of core circadian clock morning components, BrCCA1 (CIRCADIAN CLOCK-ASSOCIATED 1) and BrLHY (LATE ELONGATED HYPOCOTYL) a and b were examined under light/dark cycles and continuous light conditions. The BrPRR1a and 1ab double mutants showed longer hypocotyls, lower core circadian clock morning component mRNA and protein levels, and a shorter circadian rhythm than wildtype (WT). On the other hand, the BrPRR1b mutant was not significantly different from WT. These results suggested that two paralogous genes may not be associated with the same regulatory function in Chinese cabbage. Taken together, our results demonstrated that CRISPR/Cas9 is an efficient tool for achieving targeted genome modifications and elucidating the biological functions of circadian clock genes in B. rapa, for both breeding and improvement.

Genome-Wide Identification and Characterization of PRR Gene Family and their Diurnal Rhythmic Expression Profile in Maize

As essential components of the circadian clock, the pseudo-response regulator (PRR) gene family plays critical roles in plant photoperiod pathway. In this study, we performed a genome-wide identification and a systematic analysis of the PRR gene family in maize. Nine ZmPRRs were identified, and the gene structure, conserved motif, evolution relationship, and expression pattern of ZmPRRs were analyzed comprehensively. Phylogenetic analysis indicated that the nine ZmPRR genes were divided into three groups, except for ZmPRR73, two of which were highly homologous to each of the AtPRR or OsPRR quintet members. Promoter cis-element analysis of ZmPRRs demonstrated that they might be involved in multiple signaling transduction pathways, such as light response, biological or abiotic stress response, and hormone response. qRT-PCR analysis revealed that the levels of ZmPRRs transcripts varied considerably and exhibited a diurnal rhythmic oscillation expression pattern in the given 24-h period under both SD and LD conditions, which indicated that the level of transcription of ZmPRRs expression is subjected to a circadian rhythm and modulated by light and the circadian clock. The present study will provide an insight into further exploring the biological function and regulatory mechanism of ZmPRR genes in circadian rhythm and response to photoperiod in maize.

The BrGI Circadian Clock Gene Is Involved in the Regulation of Glucosinolates in Chinese Cabbage

Circadian clocks integrate environmental cues with endogenous signals to coordinate physiological outputs. Clock genes in plants are involved in many physiological and developmental processes, such as photosynthesis, stomata opening, stem elongation, light signaling, and floral induction. Many Brassicaceae family plants, including Chinese cabbage (Brassica rapa ssp. pekinensis), produce a unique glucosinolate (GSL) secondary metabolite, which enhances plant protection, facilitates the design of functional foods, and has potential medical applications (e.g., as antidiabetic and anticancer agents). The levels of GSLs change diurnally, suggesting a connection to the circadian clock system. We investigated whether circadian clock genes affect the biosynthesis of GSLs in Brassica rapa using RNAi-mediated suppressed transgenic Brassica rapa GIGENTEA homolog (BrGI knockdown; hereafter GK1) Chinese cabbage. GIGANTEA plays an important role in the plant circadian clock system and is related to various developmental and metabolic processes. Using a validated GK1 transgenic line, we performed RNA sequencing and high-performance liquid chromatography analyses. The transcript levels of many GSL pathway genes were significantly altered in GK1 transgenic plants. In addition, GSL contents were substantially reduced in GK1 transgenic plants. We report that the BrGI circadian clock gene is required for the biosynthesis of GSLs in Chinese cabbage plants.

Abiotic stress through time

Throughout plant evolution the circadian clock has expanded into a complex signaling network, coordinating physiological and metabolic processes with the environment. Early land plants faced new environmental pressures that required energy-demanding stress responses. Integrating abiotic stress response into the circadian system provides control over daily energy expenditure. Here, we describe the evolution of the circadian clock in plants and the limited, yet compelling, evidence for conserved regulation of abiotic stress. The need to introduce abiotic stress tolerance into current crops has expanded research into wild accessions and revealed extensive variation in circadian clock parameters across monocot and eudicot species. We argue that research into the ancestral links between the clock and abiotic stress will benefit crop improvement efforts.

Circadian Clock Components Offer Targets for Crop Domestication and Improvement

During plant domestication and improvement, farmers select for alleles present in wild species that improve performance in new selective environments associated with cultivation and use. The selected alleles become enriched and other alleles depleted in elite cultivars. One important aspect of crop improvement is expansion of the geographic area suitable for cultivation; this frequently includes growth at higher or lower latitudes, requiring the plant to adapt to novel photoperiodic environments. Many crops exhibit photoperiodic control of flowering and altered photoperiodic sensitivity is commonly required for optimal performance at novel latitudes. Alleles of a number of circadian clock genes have been selected for their effects on photoperiodic flowering in multiple crops. The circadian clock coordinates many additional aspects of plant growth, metabolism and physiology, including responses to abiotic and biotic stresses. Many of these clock-regulated processes contribute to plant performance. Examples of selection for altered clock function in tomato demonstrate that with domestication, the phasing of the clock is delayed with respect to the light–dark cycle and the period is lengthened; this modified clock is associated with increased chlorophyll content in long days. These and other data suggest the circadian clock is an attractive target during breeding for crop improvement.

Genome and time-of-day transcriptome of Wolffia australiana link morphological minimization with gene loss and less growth control

Rootless plants in the genus Wolffia are some of the fastest growing known plants on Earth. Wolffia have a reduced body plan, primarily multiplying through a budding type of asexual reproduction. Here, we generated draft reference genomes for Wolffia australiana (Benth.) Hartog & Plas, which has the smallest genome size in the genus at 357 Mb and has a reduced set of predicted protein-coding genes at about 15,000. Comparison between multiple high-quality draft genome sequences from W. australiana clones confirmed loss of several hundred genes that are highly conserved among flowering plants, including genes involved in root developmental and light signaling pathways. Wolffia has also lost most of the conserved nucleotide-binding leucine-rich repeat (NLR) genes that are known to be involved in innate immunity, as well as those involved in terpene biosynthesis, while having a significant overrepresentation of genes in the sphingolipid pathways that may signify an alternative defense system. Diurnal expression analysis revealed that only 13% of Wolffia genes are expressed in a time-of-day (TOD) fashion, which is less than the typical ∼40% found in several model plants under the same condition. In contrast to the model plants Arabidopsis and rice, many of the pathways associated with multicellular and developmental processes are not under TOD control in W. australiana, where genes that cycle the conditions tested predominantly have carbon processing and chloroplast-related functions. The Wolffia genome and TOD expression data set thus provide insight into the interplay between a streamlined plant body plan and optimized growth.

Expansion of the circadian transcriptome in Brassica rapa and genome-wide diversification of paralog expression patterns

An important challenge of crop improvement strategies is assigning function to paralogs in polyploid crops. Here we describe the circadian transcriptome in the polyploid crop Brassica rapa. Strikingly, almost three-quarters of the expressed genes exhibited circadian rhythmicity. Genetic redundancy resulting from whole genome duplication is thought to facilitate evolutionary change through sub- and neo-functionalization among paralogous gene pairs. We observed genome-wide expansion of the circadian expression phase among retained paralogous pairs. Using gene regulatory network models, we compared transcription factor targets between B. rapa and Arabidopsis circadian networks to reveal evidence for divergence between B. rapa paralogs that may be driven in part by variation in conserved non-coding sequences (CNS). Additionally, differential drought response among retained paralogous pairs suggests further functional diversification. These findings support the rapid expansion and divergence of the transcriptional network in a polyploid crop and offer a new approach for assessing paralog activity at the transcript level.

Interspecific analysis of diurnal gene regulation in panicoid grasses identifies known and novel regulatory motifs

Background

The circadian clock drives endogenous 24-h rhythms that allow organisms to adapt and prepare for predictable and repeated changes in their environment throughout the day-night (diurnal) cycle. Many components of the circadian clock in Arabidopsis thaliana have been functionally characterized, but comparatively little is known about circadian clocks in grass species including major crops like maize and sorghum.

Results

Comparative research based on protein homology and diurnal gene expression patterns suggests the function of some predicted clock components in grasses is conserved with their Arabidopsis counterparts, while others have diverged in function. Our analysis of diurnal gene expression in three panicoid grasses sorghum, maize, and foxtail millet revealed conserved and divergent evolution of expression for core circadian clock genes and for the overall transcriptome. We find that several classes of core circadian clock genes in these grasses differ in copy number compared to Arabidopsis, but mostly exhibit conservation of both protein sequence and diurnal expression pattern with the notable exception of maize paralogous genes. We predict conserved cis-regulatory motifs shared between maize, sorghum, and foxtail millet through identification of diurnal co-expression clusters for a subset of 27,196 orthologous syntenic genes. In this analysis, a Cochran–Mantel–Haenszel based method to control for background variation identified significant enrichment for both expected and novel 6–8 nucleotide motifs in the promoter regions of genes with shared diurnal regulation predicted to function in common physiological activities.

Conclusions

This study illustrates the divergence and conservation of circadian clocks and diurnal regulatory networks across syntenic orthologous genes in panacoid grass species. Further, conserved local regulatory sequences contribute to the architecture of these diurnal regulatory networks that produce conserved patterns of diurnal gene expression.

Gene body demethylation increases expression and is associated with self-pruning during grape genome duplication

A colchicine-induced autotetraploid grapevine exhibiting potentially valuable agronomic traits for grape production and breeding, including self-pruning, was identified. This study investigated DNA methylation variation and its role in gene expression during self-pruning in the autotetraploid grapevine. We used RNA-Seq to estimate differentially expressed genes between diploid and autotetraploid grapevine shoot tips. The genes showing increases in the autotetraploid were mainly related to stress response pathways, whereas those showing decreases in the autotetraploid were related to biological metabolism and biosynthesis. Whole-genome bisulfite sequencing was performed to produce single-base methylomes for the diploid and autotetraploid grapevines. Comparison between the methylomes revealed that they were conserved in CG and CHG contexts. In the autotetraploid grapevine, hypodifferentially methylated regions (DMRs) and hyper-DMRs in the gene body increased or decreased gene expression, respectively. Our results indicated that a hypo-DMR in the ACO1 gene body increased its expression and might promote self-pruning. This study reports that hypo-DMRs in the gene body increase gene expression in plants and reveals the mechanism underlying the changes in the modifications affecting gene expression during genome duplication. Overall, our results provide valuable information for understanding the relationships between DNA methylation, gene expression, and autotetraploid breeding in grape.

Transcriptional Structure of Petunia Clock in Leaves and Petals

The plant circadian clock coordinates environmental signals with internal processes including secondary metabolism, growth, flowering, and volatile emission. Plant tissues are specialized in different functions, and petals conceal the sexual organs while attracting pollinators. Here we analyzed the transcriptional structure of the petunia (Petunia x hybrida) circadian clock in leaves and petals. We recorded the expression of 13 clock genes in petunia under light:dark (LD) and constant darkness (DD). Under light:dark conditions, clock genes reached maximum expression during the light phase in leaves and the dark period in petals. Under free running conditions of constant darkness, maximum expression was delayed, especially in petals. Interestingly, the rhythmic expression pattern of PhLHY persisted in leaves and petals in LD and DD. Gene expression variability differed among leaves and petals, time of day and photoperiod. The transcriptional noise was higher especially in leaves under constant darkness. We found that PhPRR7, PhPRR5, and PhGI paralogs showed changes in gene structure including exon number and deletions of CCT domain of the PRR family. Our results revealed that petunia petals presented a specialized clock.

The Clock Gene TOC1 in Shoots, Not Roots, Determines Fitness of Nicotiana attenuata under Drought

The highly conserved core circadian clock component TIMING OF CAB EXPRESSION1 (TOC1) contextualizes environmental stress responses in plants, for example by gating abscisic acid signaling and suppressing thermoresponsive growth. Selective interaction of TOC1 with PHYTOCHROME B under far-red-enriched light suggests a connection between circadian gating of light responses and sensitivity to ABA, an important regulator of growth and stress responses, including under drought. However, the fitness consequences of TOC1 function, particularly in the root, are poorly understood. Here, we used the desert annual, Nicotiana attenuata, to investigate the function of TOC1 in shoots and roots for maintaining fitness under drought, in both field and glasshouse experiments. Despite marked decreases in leaf water loss, TOC1-deficient lines failed to maintain fitness in response to drought stress as measured by total seed capsule production. Restoring TOC1 transcript levels in shoots via micrografting was sufficient to restore wild-type drought responses under field conditions. Microarrays identified a coexpression module in leaves strongly linking red and far-red light signaling to drought responses in a TOC1-dependent manner, but experiments with phytochrome-deficient lines revealed that the effects of TOC1 deficiency under drought cannot be attributed to changes in red/far-red light perception alone. Taken together, these results elucidate the sophisticated, tissue-dependent role of the circadian clock in maintaining fitness in the face of long-term abiotic stresses such as drought.

Evolution and conservation of polycomb repressive complex 1 core components and putative associated factors in the green lineage

Background

Polycomb group (PcG) proteins play important roles in animal and plant development and stress response. Polycomb repressive complex 1 (PRC1) and PRC2 are the key epigenetic regulators of gene expression, and are involved in almost all developmental stages. PRC1 catalyzes H2A monoubiquitination resulting in transcriptional silencing or activation. The PRC1 components in the green lineage were identified and evolution and conservation was analyzed by bioinformatics techniques. RING Finger Protein 1 (RING1), B lymphoma Mo-MLV insertion region 1 homolog (BMI1), Like Heterochromatin Protein 1 (LHP1) and Embryonic Flower 1 (EMF1) are the PRC1 core components and Vernalization 1 (VRN1), VP1/ABI3-Like 1/2/3 (VAL1/2/3), Alfin-like 1–7 (AL1–7), Inhibitor of growth 1/2 (ING1/2), and Early Bolting in Short Days (EBS) / Short Life (SHL) are the associated factors.

Results

Each PRC1 subunit possesses special domain organizations, such as RING and the ring finger and WD40-associated ubiquitin-like (RAWUL) domains for RING1 and BMI1, chromatin organization modifier (CHROMO) and chromo shadow (ChSh) domains for LHP1, one or two B3 DNA binding domain(s) for VRN1, B3 and zf-CW domains for VAL1/2/3, Alfin and Plant HomeoDomain (PHD) domains for AL1–7, ING and PHD domains for ING1/2, Bromoadjacent homology (BAT) and PHD domains for EBS/SHL. Six new motifs are uncovered in EMF1.

The PRC1 core components RING1 and BMI1, and the associated factors VAL1/2/3, AL1–7, ING1/2, and EBS/SHL exist from alga to higher plants, whereas LHP1 only occurs in higher plants. EMF1 and VRN1 are present only in eudicots. PRC1 components undergo duplication in the plant evolution. Most of plants carry the homologous core component LHP1, the associated factor EMF1, and several homologs in RING1, BMI1, VRN1, AL1–7, ING1/2/3, and EBS/SHL. Cabbage, cotton, poplar, orange and maize often exhibit more gene copies than other species. Domain organization analysis shows that duplicated gene functions may be of diverse.

Conclusions

The PRC1 core components RING1 and BMI1, and the associated factors VAL1/2/3, AL1–7, ING1/2, and EBS/SHL originate from algae. The core component LHP1 is from moss and the associated factors EMF1 and VRN1 are from dicotyledon. PRC1 components are of functional redundancy and diversity in evolution.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5905-9) contains supplementary material, which is available to authorized users.

The Plant Circadian Oscillator

It has been nearly 300 years since the first scientific demonstration of a self-sustaining circadian clock in plants. It has become clear that plants are richly rhythmic, and many aspects of plant biology, including photosynthetic light harvesting and carbon assimilation, resistance to abiotic stresses, pathogens, and pests, photoperiodic flower induction, petal movement, and floral fragrance emission, exhibit circadian rhythmicity in one or more plant species. Much experimental effort, primarily, but not exclusively in Arabidopsis thaliana, has been expended to characterize and understand the plant circadian oscillator, which has been revealed to be a highly complex network of interlocked transcriptional feedback loops. In addition, the plant circadian oscillator has employed a panoply of post-transcriptional regulatory mechanisms, including alternative splicing, adjustable rates of translation, and regulated protein activity and stability. This review focuses on our present understanding of the regulatory network that comprises the plant circadian oscillator. The complexity of this oscillatory network facilitates the maintenance of robust rhythmicity in response to environmental extremes and permits nuanced control of multiple clock outputs. Consistent with this view, the clock is emerging as a target of domestication and presents multiple targets for targeted breeding to improve crop performance.

Transcriptome Analysis of Diurnal Gene Expression in Chinese Cabbage

Plants have developed timing mechanisms that enable them to maintain synchrony with daily environmental events. These timing mechanisms, i.e., circadian clocks, include transcriptional/translational feedback loops that drive 24 h transcriptional rhythms, which underlie oscillations in protein abundance, thus mediating circadian rhythms of behavior, physiology, and metabolism. Circadian clock genes have been investigated in the diploid model plant Arabidopsis thaliana. Crop plants with polyploid genomes—such as Brassica species—have multiple copies of some clock-related genes. Over the last decade, numerous studies have been aimed at identifying and understanding the function of paralogous genes with conserved sequences, or those that diverged during evolution. Brassica rapa’s triplicate genomes retain sequence-level collinearity with Arabidopsis. In this study, we used RNA sequencing (RNAseq) to profile the diurnal transcriptome of Brassica rapa seedlings. We identified candidate paralogs of circadian clock-related genes and assessed their expression levels. These genes and their related traits that modulate the diurnal rhythm of gene expression contribute to the adaptation of crop cultivars. Our findings will contribute to the mechanistic study of circadian clock regulation inherent in polyploidy genome crops, which differ from those of model plants, and thus will be useful for future breeding studies using clock genes.

Translating Flowering Time From Arabidopsis thaliana to Brassicaceae and Asteraceae Crop Species

Flowering and seed set are essential for plant species to survive, hence plants need to adapt to highly variable environments to flower in the most favorable conditions. Endogenous cues such as plant age and hormones coordinate with the environmental cues like temperature and day length to determine optimal time for the transition from vegetative to reproductive growth. In a breeding context, controlling flowering time would help to speed up the production of new hybrids and produce high yield throughout the year. The flowering time genetic network is extensively studied in the plant model species Arabidopsis thaliana, however this knowledge is still limited in most crops. This article reviews evidence of conservation and divergence of flowering time regulation in A. thaliana with its related crop species in the Brassicaceae and with more distant vegetable crops within the Asteraceae family. Despite the overall conservation of most flowering time pathways in these families, many genes controlling this trait remain elusive, and the function of most Arabidopsis homologs in these crops are yet to be determined. However, the knowledge gathered so far in both model and crop species can be already exploited in vegetable crop breeding for flowering time control.

Conservation and Diversification of Circadian Rhythmicity Between a Model Crassulacean Acid Metabolism Plant Kalanchoë fedtschenkoi and a Model C3 Photosynthesis Plant Arabidopsis thaliana

Crassulacean acid metabolism (CAM) improves photosynthetic efficiency under limited water availability relative to C3 photosynthesis. It is widely accepted that CAM plants have evolved from C3 plants and it is hypothesized that CAM is under the control of the internal circadian clock. However, the role that the circadian clock plays in the evolution of CAM is not well understood. To identify the molecular basis of circadian control over CAM evolution, rhythmic gene sets were identified in a CAM model plant species (Kalanchoë fedtschenkoi) and a C3 model plant species (Arabidopsis thaliana) through analysis of diel time-course gene expression data using multiple periodicity detection algorithms. Based on protein sequences, ortholog groups were constructed containing genes from each of these two species. The ortholog groups were categorized into five gene sets based on conservation and diversification of rhythmic gene expression. Interestingly, minimal functional overlap was observed when comparing the rhythmic gene sets of each species. Specifcally, metabolic processes were enriched in the gene set under circadian control in K. fedtschenkoi and numerous genes were found to have retained or gained rhythmic expression in K. fedtsechenkoi. Additonally, several rhythmic orthologs, including CAM-related orthologs, displayed phase shifts between species. Results of this analysis point to several mechanisms by which the circadian clock plays a role in the evolution of CAM. These genes provide a set of testable hypotheses for future experiments.

Spatio-temporal expression dynamics differ between homologues of flowering time genes in the allopolyploid Brassica napus

Polyploidy is a recurrent feature of eukaryotic evolution and has been linked to increases in complexity, adaptive radiation and speciation. Within angiosperms such events have occurred repeatedly in many plant lineages. Here we investigate the retention and spatio-temporal expression dynamics of duplicated genes predicted to regulate the floral transition in Brassica napus (oilseed rape, OSR). We show that flowering time genes are preferentially retained relative to other genes in the OSR genome. Using a transcriptome time series in two tissues (leaf and shoot apex) across development we show that 67% of these retained flowering time genes are expressed. Furthermore, between 64% (leaf) and 74% (shoot apex) of the retained gene homologues show diverged expression patterns relative to each other across development, suggesting neo- or subfunctionalization. A case study of homologues of the shoot meristem identity gene TFL1 reveals differences in cis-regulatory elements that could explain this divergence. Such differences in the expression dynamics of duplicated genes highlight the challenges involved in translating gene regulatory networks from diploid model systems to more complex polyploid crop species.

Decoys Untangle Complicated Redundancy and Reveal Targets of Circadian Clock F-Box Proteins

Eukaryotic circadian clocks utilize the ubiquitin proteasome system to precisely degrade clock proteins. In plants, the F-box-type E3 ubiquitin ligases ZEITLUPE (ZTL), FLAVIN-BINDING, KELCH REPEAT, F-BOX1 (FKF1), and LOV KELCH PROTEIN2 (LKP2) regulate clock period and couple the clock to photoperiodic flowering in response to end-of-day light conditions. To better understand their functions, we expressed decoy ZTL, FKF1, and LKP2 proteins that associate with target proteins but are unable to ubiquitylate their targets in Arabidopsis (Arabidopsis thaliana). These dominant-negative forms of the proteins inhibit the ubiquitylation of target proteins and allow for the study of ubiquitylation-independent and -dependent functions of ZTL, FKF1, and LKP2. We demonstrate the effects of expressing ZTL, FKF1, and LKP2 decoys on the circadian clock and flowering time. Furthermore, the decoy E3 ligases trap substrate interactions, and using immunoprecipitation-mass spectrometry, we identify interacting partners. We focus studies on the clock transcription factor CCA1 HIKING EXPEDITION (CHE) and show that ZTL interacts directly with CHE and can mediate CHE ubiquitylation. We also demonstrate that CHE protein is degraded in the dark and that degradation is reduced in a ztl mutant plant, showing that CHE is a bona fide ZTL target protein. This work increases our understanding of the genetic and biochemical roles for ZTL, FKF1, and LKP2 and also demonstrates an effective methodology for studying complicated genetic redundancy among E3 ubiquitin ligases.

Genome-Wide Identification, Classification, and Expression Divergence of Glutathione-Transferase Family in Brassica rapa under Multiple Hormone Treatments

The GSTs is one of the most important multifunctional protein families which has been playing a crucial role in the different aspects of plant growth. This extensive study about GSTs may establish a solid foundation for the brief functional analysis of BraGSTs in future. In this study, a total of 75 genes were identified in B. rapa. Phylogenetic analysis characterized them into eight different subclasses, while Tau and Phi subclasses were the most numerous. The exon-intron structure and the motif composition of BraGSTs were exhibited accordingly to their subclasses. Notably, we also investigated 15 tandem paralogous pairs of genes, which highlighted that all the pairs were purifying in nature as their synonymous values were lower than 1.00. Duplication analysis indicated that about 45.33% of genes mainly occurred through tandem duplication in B. rapa. Predominately, the tandem cluster of genes in subclass Tau was greater than the other subclasses. Furthermore, among eight multiple hormonal treatments (ABA, GA, BR, ETH, IAA, IBA, NPA, and JA), most number of BraGSTs was activated by NPA, BR, and ABA treatments. This analysis has provided comprehensive information about GSTs family which may assist in elucidating their exact functions in B. rapa.

Genome-Wide Identification, Molecular Evolution, and Expression Profiling Analysis of Pectin Methylesterase Inhibitor Genes in Brassica campestris ssp. chinensis

Pectin methylesterase inhibitor genes (PMEIs) are a large multigene family and play crucial roles in cell wall modifications in plant growth and development. Here, a comprehensive analysis of the PMEI gene family in Brassicacampestris, an important leaf vegetable, was performed. We identified 100 BrassicacampestrisPMEI genes (BcPMEIs), among which 96 BcPMEIs were unevenly distributed on 10 chromosomes and nine tandem arrays containing 20 BcPMEIs were found. We also detected 80 pairs of syntenic PMEI orthologs. These findings indicated that whole-genome triplication (WGT) and tandem duplication (TD) were the main mechanisms accounting for the current number of BcPMEIs. In evolution, BcPMEIs were retained preferentially and biasedly, consistent with the gene balance hypothesis and two-step theory, respectively. The molecular evolution analysis of BcPMEIs manifested that they evolved through purifying selection and the divergence time is in accordance with the WGT data of B. campestris. To obtain the functional information of BcPMEIs, the expression patterns in five tissues and the cis-elements distributed in promoter regions were investigated. This work can provide a better understanding of the molecular evolution and biological function of PMEIs in B. campestris.

Diel pattern of circadian clock and storage protein gene expression in leaves and during seed filling in cowpea (Vigna unguiculata)

BACKGROUND

Cowpea (Vigna unguiculata) is an important source of protein supply for animal and human nutrition. The major storage globulins VICILIN and LEGUMIN (LEG) are synthesized from several genes including LEGA, LEGB, LEGJ and CVC (CONVICILIN). The current hypothesis is that the plant circadian core clock genes are conserved in a wide array of species and that primary metabolism is to a large extent controlled by the plant circadian clock. Our aim was to investigate a possible link between gene expression of storage proteins and the circadian clock.

RESULTS

We identified cowpea orthologues of the core clock genes VunLHY, VunTOC1, VunGI and VunELF3, the protein storage genes VunLEG, VunLEGJ, and VunCVC as well as nine candidate reference genes used in RT-PCR. ELONGATION FACTOR 1-A (ELF1A) resulted the most suitable reference gene. The clock genes VunELF3, VunGI, VunTOC1 and VunLHY showed a rhythmic expression profile in leaves with a typical evening/night and morning/midday phased expression. The diel patterns were not completely robust and only VungGI and VungELF3 retained a rhythmic pattern under free running conditions of darkness. Under field conditions, rhythmicity and phasing apparently faded during early pod and seed development and was regained in ripening pods for VunTOC1 and VunLHY. Mature seeds showed a rhythmic expression of VunGI resembling leaf tissue under controlled growth chamber conditions. Comparing time windows during developmental stages we found that VunCVC and VunLEG were significantly down regulated during the night in mature pods as compared to intermediate ripe pods, while changes in seeds were non-significant due to high variance. The rhythmic expression under field conditions was lost under growth chamber conditions.

CONCLUSIONS

The core clock gene network is conserved in cowpea leaves showing a robust diel expression pattern except VunELF3 under growth chamber conditions. There appears to be a clock transcriptional reprogramming in pods and seeds compared to leaves. Storage protein deposition may be circadian regulated under field conditions but the strong environmental signals are not met under artificial growth conditions. Diel expression pattern in field conditions may result in better usage of energy for protein storage.

Genome-wide identification, characterization, and evolutionary analysis of flowering genes in radish (Raphanus sativus L.)

BACKGROUND

Radish (Raphanus sativus L.) belongs to the family Brassicaceae, and is an economically important root crop grown worldwide. Flowering is necessary for plant propagation, but it is also an important agronomic trait influencing R. sativus fleshy taproot yield and quality in the case of an imbalance between vegetative and reproductive growth. There is currently a lack of detailed information regarding the pathways regulating the flowering genes or their evolution in R. sativus. The release of the R. sativus genome sequence provides an opportunity to identify and characterize the flowering genes using a comparative genomics approach.

RESULTS

We identified 254 R. sativus flowering genes based on sequence similarities and analyses of syntenic regions. The genes were unevenly distributed on the various chromosomes. Furthermore, we discovered the existence of R. sativus core function genes in the flowering regulatory network, which revealed that basic flowering pathways are relatively conserved between Arabidopsis thaliana and R. sativus. Additional comparisons with Brassica oleracea and Brassica rapa indicated that the retained flowering genes differed among species after genome triplication events. The R. sativus flowering genes were preferentially retained, especially those associated with gibberellin signaling and metabolism. Moreover, analyses of selection pressures suggested that the genes in vernalization and autonomous pathways were more variable than the genes in other R. sativus flowering pathways.

CONCLUSIONS

Our results revealed that the core flowering genes are conserved between R. sativus and A. thaliana to a certain extent. Moreover, the copy number variation and functional differentiation of the homologous genes in R. sativus increased the complexity of the flowering regulatory networks after genome polyploidization. Our study provides an integrated framework for the R. sativus flowering pathways and insights into the evolutionary relationships between R. sativus flowering genes and the genes from A. thaliana and close relatives.

Cross-species complementation reveals conserved functions for EARLY FLOWERING 3 between monocots and dicots

Plant responses to the environment are shaped by external stimuli and internal signaling pathways. In both the model plant Arabidopsis thaliana (Arabidopsis) and crop species, circadian clock factors are critical for growth, flowering, and circadian rhythms. Outside of Arabidopsis, however, little is known about the molecular function of clock gene products. Therefore, we sought to compare the function of Brachypodium distachyon (Brachypodium) and Setaria viridis (Setaria) orthologs of EARLY FLOWERING 3, a key clock gene in Arabidopsis. To identify both cycling genes and putative ELF3 functional orthologs in Setaria, a circadian RNA-seq dataset and online query tool (Diel Explorer) were generated to explore expression profiles of Setaria genes under circadian conditions. The function of ELF3 orthologs from Arabidopsis, Brachypodium, and Setaria was tested for complementation of an elf3 mutation in Arabidopsis. We find that both monocot orthologs were capable of rescuing hypocotyl elongation, flowering time, and arrhythmic clock phenotypes. Using affinity purification and mass spectrometry, our data indicate that BdELF3 and SvELF3 could be integrated into similar complexes in vivo as AtELF3. Thus, we find that, despite 180 million years of separation, BdELF3 and SvELF3 can functionally complement loss of ELF3 at the molecular and physiological level.

A novel role of the soybean clock gene LUX ARRHYTHMO in male reproductive development

The evening complex of ELF4-ELF3-LUX proteins is an integral component of a plant circadian clock. LUX ARRHYTHMO (LUX) is one of the key components of the evening complex, and that play a key role in circadian rhythms and flowering. Here, we report that diverged soybean LUX has the additional role in male reproductive development. We studied diurnal and circadian rhythms of soybean LUX (GmLUXa, GmLUXb, and GmLUXc) using qRT-PCR, and show its nuclear localisation by particle bombardment. Yeast-two hybrid (Y2H) studies indicate that both GmLUXb and GmLUXc form an evening complex with GmELF4b and GmELF3a, respectively. Ectopic expression of GmLUXb in Arabidopsis lux mutants can complement functions of AtLUX, whereas GmLUXc generates novel phenotypes of serrated leaves, stunted plants, shortened anther filament, and low seed set. Overall, our results suggest that the LUX gene has diverged in soybean where GmLUXb and GmLUXc share the role to control flowering time, but GmLUXc has evolved to regulate anther filament growth and seed set by regulating the Gibberellin hormone biosynthesis pathway.

Diurnal Cycling Transcription Factors of Pineapple Revealed by Genome-Wide Annotation and Global Transcriptomic Analysis

Circadian clock provides fitness advantage by coordinating internal metabolic and physiological processes to external cyclic environments. Core clock components exhibit daily rhythmic changes in gene expression, and the majority of them are transcription factors (TFs) and transcription coregulators (TCs). We annotated 1,398 TFs from 67 TF families and 80 TCs from 20 TC families in pineapple, and analyzed their tissue-specific and diurnal expression patterns. Approximately 42% of TFs and 45% of TCs displayed diel rhythmic expression, including 177 TF/TCs cycling only in the nonphotosynthetic leaf tissue, 247 cycling only in the photosynthetic leaf tissue, and 201 cycling in both. We identified 68 TF/TCs whose cycling expression was tightly coupled between the photosynthetic and nonphotosynthetic leaf tissues. These TF/TCs likely coordinate key biological processes in pineapple as we demonstrated that this group is enriched in homologous genes that form the core circadian clock in Arabidopsis and includes a STOP1 homolog. Two lines of evidence support the important role of the STOP1 homolog in regulating CAM photosynthesis in pineapple. First, STOP1 responds to acidic pH and regulates a malate channel in multiple plant species. Second, the cycling expression pattern of the pineapple STOP1 and the diurnal pattern of malate accumulation in pineapple leaf are correlated. We further examined duplicate-gene retention and loss in major known circadian genes and refined their evolutionary relationships between pineapple and other plants. Significant variations in duplicate-gene retention and loss were observed for most clock genes in both monocots and dicots.

Genome-wide Analysis and Expression Divergence of the Trihelix family in Brassica Rapa: Insight into the Evolutionary Patterns in Plants

Trihelix gene family is an important transcription factor (TF) family involved in plants' growth and development. This extensive study of trihelix genes from Arabidopsis thaliana to Brassica rapa could shed light on the evolution in plants and support crop breeding. In this study, a total of 52 trihelix genes were identified in B.rapa. Whole-genome annotation, molecular-evolution and gene-expression analyses of all known trihelix genes were conducted. By statistics of the number of trihelix genes in each species, we found the expansion of trihelix gene family started with angiosperm evolution. And SIP1 was more preferentially retained than other subgroups (GT-1, GT-2, GT_γ, SH4), consistent with the gene dosage hypothesis. Then we investigated the evolutionary patterns, footprints and conservation of trihelix genes in selected plants. The putative trihelix proteins were highly conserved, but their expression patterns varied. Half of these genes were highly expressed in all the selected organs but some showed tissue-specific expression patterns. Furthermore, among six abiotic stresses (Cold, Heat, PEG, NaCl, ABA and GA), most trihelix genes were activated by salt and ABA treatment. In summary, the phylogenetic, evolution and expression analyses of trihelix gene family in B.rapa establish a solid foundation for future comprehensive functional analysis of BraTHs.

Using RNA-Seq for Genomic Scaffold Placement, Correcting Assemblies, and Genetic Map Creation in a Common Brassica rapa Mapping Population

Brassica rapa is a model species for agronomic, ecological, evolutionary, and translational studies. Here, we describe high-density SNP discovery and genetic map construction for a B. rapa recombinant inbred line (RIL) population derived from field collected RNA sequencing (RNA-Seq) data. This high-density genotype data enables the detection and correction of putative genome misassemblies and accurate assignment of scaffold sequences to their likely genomic locations. These assembly improvements represent 7.1-8.0% of the annotated B. rapa genome. We demonstrate how using this new resource leads to a significant improvement for QTL analysis over the current low-density genetic map. Improvements are achieved by the increased mapping resolution and by having known genomic coordinates to anchor the markers for candidate gene discovery. These new molecular resources and improvements in the genome annotation will benefit the Brassicaceae genomics community and may help guide other communities in fine-tuning genome annotations.

Statistical analysis of fractionation resistance by functional category and expression

Background

The current literature establishes the importance of gene functional category and expression in promoting or suppressing duplicate gene loss after whole genome doubling in plants, a process known as fractionation. Inspired by studies that have reported gene expression to be the dominating factor in preventing duplicate gene loss, we analyzed the relative effect of functional category and expression.

Methods

We use multivariate methods to study data sets on gene retention, function and expression in rosids and asterids to estimate effects and assess their interaction.

Results

Our results suggest that the effect on duplicate gene retention fractionation by functional category and expression are independent and have no statistical interaction.

Conclusion

In plants, functional category is the more dominant factor in explaining duplicate gene loss.

Natural variation in CIRCADIAN CLOCK ASSOCIATED 1 is associated with flowering time in Brassica rapa

Flowering time is a very important agronomic trait and the development of molecular markers associated with this trait can facilitate crop breeding. CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), a core oscillator component of circadian rhythms that affect metabolic pathways in plants, has been implicated in flowering time control in species of Brassica. CCA1 gene sequences from three Brassica rapa inbred lines, showing either early flowering or late flowering phenotypes, were analyzed and a high level of sequence variation was identified, especially within the fourth intron. Using this information, three PCR primer sets were designed and tested using various inbred lines of B. rapa. The usage of InDel markers was further validated by evaluation of flowering time and high resolution melting (HRM) analysis. Both methods, PCR and HRM, validated the use of newly developed markers. Additional sequence analyses of Brassica plants with diploid (AA, BB, or CC) and allotetraploid genomes further confirmed a large number of sequence polymorphisms in the CCA1 gene, including insertions/deletions in the fourth intron. Our results demonstrated that sequence variations in CCA1 can be used to develop valuable trait-related molecular markers for Brassica crop breeding.

Into the Evening: Complex Interactions in the Arabidopsis Circadian Clock

In Arabidopsis thaliana an assembly of proteins named the evening complex (EC) has been established as an essential component of the circadian clock with conserved functions in regulating plant growth and development. Recent studies identifying EC-regulated genes and EC-interacting proteins have expanded our understanding of EC function. In this review we focus on new progress uncovering how the EC contributes to the circadian network through the integration of environmental inputs and the direct regulation of key clock genes. We also summarize new findings of how the EC directly regulates clock outputs, such as photoperiodic and thermoresponsive growth, and provide new perspectives on future experiments to address unsolved questions related to the EC.

Reduction of GIGANTEA expression in transgenic Brassica rapa enhances salt tolerance

Here we report the enhancement of tolerance to salt stress in Brassica rapa (Chinese cabbage) through the RNAi-mediated reduction of GIGANTEA ( GI ) expression. Circadian clocks integrate environmental signals with internal cues to coordinate diverse physiological outputs. The GIGANTEA (GI) gene was first discovered due to its important contribution to photoperiodic flowering and has since been shown to be a critical component of the plant circadian clock and to contribute to multiple environmental stress responses. We show that the GI gene in Brassica rapa (BrGI) is similar to Arabidopsis GI in terms of both expression pattern and function. BrGI functionally rescued the late-flowering phenotype of the Arabidopsis gi-201 loss-of-function mutant. RNAi-mediated suppression of GI expression in Arabidopsis Col-0 and in the Chinese cabbage, B. rapa DH03, increased tolerance to salt stress. Our results demonstrate that the molecular functions of GI described in Arabidopsis are conserved in B. rapa and suggest that manipulation of gene expression through RNAi and transgenic overexpression could enhance tolerance to abiotic stresses and thus improve agricultural crop production.

Into the Evening: Complex Interactions in the Arabidopsis Circadian Clock

In Arabidopsis thaliana an assembly of proteins named the evening complex (EC) has been established as an essential component of the circadian clock with conserved functions in regulating plant growth and development. Recent studies identifying EC-regulated genes and EC-interacting proteins have expanded our understanding of EC function. In this review we focus on new progress uncovering how the EC contributes to the circadian network through the integration of environmental inputs and the direct regulation of key clock genes. We also summarize new findings of how the EC directly regulates clock outputs, such as photoperiodic and thermoresponsive growth, and provide new perspectives on future experiments to address unsolved questions related to the EC.

Comprehensive analysis of the polygalacturonase and pectin methylesterase genes in Brassica rapa shed light on their different evolutionary patterns

Pectins are fundamental polysaccharides in the plant primary cell wall. Polygalacturonases (PGs) and pectin methylesterases (PMEs), major components of the pectin remodeling and disassembly network, are involved in cell separation processes during many stages of plant development. A comprehensive study of these genes in plants could shed light on the evolution patterns of their structural development. In this study, we conducted whole-genome annotation, molecular evolution and gene expression analyses of PGs and PMEs in Brassica rapa and 8 other plant species. A total of 100 PGs and 110 PMEs were identified in B. rapa; they primarily diverged from 12–18 MYA and PMEs were retained more than PGs. Along with another 305 PGs and 348 PMEs in the 8 species, two different expansion or evolution types were discovered: a new branch of class A PGs appeared after the split of gymnosperms and angiosperms, which led to the rapid expansion of PGs; the pro domain was obtained or lost in the proPMEs through comprehensive analyses among PME genes. In addition, the PGs and PMEs exhibit diverged expression patterns. These findings will lead to novel insight regarding functional divergence and conservation in the gene families and provide more support for molecular evolution analyses.

Selection during crop diversification involves correlated evolution of the circadian clock and ecophysiological traits in Brassica rapa

Crop selection often leads to dramatic morphological diversification, in which allocation to the harvestable component increases. Shifts in allocation are predicted to impact (as well as rely on) physiological traits; yet, little is known about the evolution of gas exchange and related anatomical features during crop diversification. In Brassica rapa, we tested for physiological differentiation among three crop morphotypes (leaf, turnip, and oilseed) and for correlated evolution of circadian, gas exchange, and phenological traits. We also examined internal and surficial leaf anatomical features and biochemical limits to photosynthesis. Crop types differed in gas exchange; oilseed varieties had higher net carbon assimilation and stomatal conductance relative to vegetable types. Phylogenetically independent contrasts indicated correlated evolution between circadian traits and both gas exchange and biomass accumulation; shifts to shorter circadian period (closer to 24 h) between phylogenetic nodes are associated with higher stomatal conductance, lower photosynthetic rate (when CO2 supply is factored out), and lower biomass accumulation. Crop type differences in gas exchange are also associated with stomatal density, epidermal thickness, numbers of palisade layers, and biochemical limits to photosynthesis. Brassica crop diversification involves correlated evolution of circadian and physiological traits, which is potentially relevant to understanding mechanistic targets for crop improvement.

Retention, Molecular Evolution, and Expression Divergence of the Auxin/Indole Acetic Acid and Auxin Response Factor Gene Families in Brassica Rapa Shed Light on Their Evolution Patterns in Plants

Auxin/indole acetic acids (Aux/IAAs) and auxin response factors (ARFs), major components of the Aux signaling network, are involved in many developmental processes in plants. Investigating their evolution will provide new sight on the relationship between the molecular evolution of these genes and the increasing morphotypes of plants. We constructed comparative analyses of the retention, structure, expansion, and expression patterns of Aux/IAAs and ARFs in Brassica rapa and their evolution in eight other plant species, including algae, bryophytes, lycophytes, and angiosperms. All 33 of the ARFs, including 1 ARF-like (AL) (a type of ARF-like protein) and 53 Aux/IAAs, were identified in the B. rapa genome. The genes mainly diverged approximately 13 Ma. After the split, no Aux/IAA was completely lost, and they were more preferentially retained than ARFs. In land plants, compared with ARFs, which increased in stability, Aux/IAAs expanded more rapidly and were under more relaxed selective pressure. Moreover, BraIAAs were expressed in a more tissue-specific fashion than BraARFs and demonstrated functional diversification during gene duplication under different treatments, which enhanced the cooperative interaction of homologs to help plants adapt to complex environments. In addition, ALs existed widely and had a closer relationship with ARFs, suggesting that ALs might be the initial structure of ARFs. Our results suggest that the rapid expansion and preferential retention of Aux/IAAs are likely paralleled by the increasingly complex morphotypes in Brassicas and even in land plants. Meanwhile, the data support the hypothesis that the PB1 domain plays a key role in the origin of both Aux/IAAs and ARFs.

Dissecting the complex molecular evolution and expression of polygalacturonase gene family in Brassica rapa ssp. chinensis

Polygalacturonases (PGs) participate in pectin disassembly of cell wall and belong to one of the largest hydrolase families in plants. In this study, we identified 99 PG genes in Brassica rapa. Comprehensive analysis of phylogeny, gene structures, physico-chemical properties and coding sequence evolution demonstrated that plant PGs should be classified into seven divergent clades and each clade's members had specific sequence and structure characteristics, and/or were under specific selection pressures. Genomic distribution and retention rate analysis implied duplication events and biased retention contributed to PG family's expansion. Promoter divergence analysis using "shared motif method" revealed a significant correlation between regulatory and coding sequence evolution of PGs, and proved Clades A and E were of ancient origin. Quantitative real-time PCR analysis showed that expression patterns of PGs displayed group specificities in B. rapa. Particularly, nearly half of PG family members, especially those of Clades C, D and F, closely relates to reproductive development. Most duplicates showed similar expression profiles, suggesting dosage constraints accounted for preservation after duplication. Promoter-GUS assay further indicated PGs' extensive roles and possible redundancy during reproductive development. This work can provide a scientific classification of plant PGs, dissect the internal relationships between their evolution and expressions, and promote functional researches.

Diversification and evolution of the SDG gene family in Brassica rapa after the whole genome triplication

Histone lysine methylation, controlled by the SET Domain Group (SDG) gene family, is part of the histone code that regulates chromatin function and epigenetic control of gene expression. Analyzing the SDG gene family in Brassica rapa for their gene structure, domain architecture, subcellular localization, rate of molecular evolution and gene expression pattern revealed common occurrences of subfunctionalization and neofunctionalization in BrSDGs. In comparison with Arabidopsis thaliana, the BrSDG gene family was found to be more divergent than AtSDGs, which might partly explain the rich variety of morphotypes in B. rapa. In addition, a new evolutionary pattern of the four main groups of SDGs was presented, in which the Trx group and the SUVR subgroup evolved faster than the E(z), Ash groups and the SUVH subgroup. These differences in evolutionary rate among the four main groups of SDGs are perhaps due to the complexity and variability of the regions that bind with biomacromolecules, which guide SDGs to their target loci.

Divergence in Enzymatic Activities in the Soybean GST Supergene Family Provides New Insight into the Evolutionary Dynamics of Whole-Genome Duplicates

Whole-genome duplication (WGD), or polyploidy, is a major force in plant genome evolution. A duplicate of all genes is present in the genome immediately following a WGD event. However, the evolutionary mechanisms responsible for the loss of, or retention and subsequent functional divergence of polyploidy-derived duplicates remain largely unknown. In this study we reconstructed the evolutionary history of the glutathione S-transferase (GST) gene family from the soybean genome, and identified 72 GST duplicated gene pairs formed by a recent Glycine-specific WGD event occurring approximately 13 Ma. We found that 72% of duplicated GST gene pairs experienced gene losses or pseudogenization, whereas 28% of GST gene pairs have been retained in the soybean genome. The GST pseudogenes were under relaxed selective constraints, whereas functional GSTs were subject to strong purifying selection. Plant GST genes play important roles in stress tolerance and detoxification metabolism. By examining the gene expression responses to abiotic stresses and enzymatic properties of the ancestral and current proteins, we found that polyploidy-derived GST duplicates show the divergence in enzymatic activities. Through site-directed mutagenesis of ancestral proteins, this study revealed that nonsynonymous substitutions of key amino acid sites play an important role in the divergence of enzymatic functions of polyploidy-derived GST duplicates. These findings provide new insights into the evolutionary and functional dynamics of polyploidy-derived duplicate genes.

Modeling development and quantitative trait mapping reveal independent genetic modules for leaf size and shape

Improved predictions of fitness and yield may be obtained by characterizing the genetic controls and environmental dependencies of organismal ontogeny. Elucidating the shape of growth curves may reveal novel genetic controls that single-time-point (STP) analyses do not because, in theory, infinite numbers of growth curves can result in the same final measurement. We measured leaf lengths and widths in Brassica rapa recombinant inbred lines (RILs) throughout ontogeny. We modeled leaf growth and allometry as function valued traits (FVT), and examined genetic correlations between these traits and aspects of phenology, physiology, circadian rhythms and fitness. We used RNA-seq to construct a SNP linkage map and mapped trait quantitative trait loci (QTL). We found genetic trade-offs between leaf size and growth rate FVT and uncovered differences in genotypic and QTL correlations involving FVT vs STPs. We identified leaf shape (allometry) as a genetic module independent of length and width and identified selection on FVT parameters of development. Leaf shape is associated with venation features that affect desiccation resistance. The genetic independence of leaf shape from other leaf traits may therefore enable crop optimization in leaf shape without negative effects on traits such as size, growth rate, duration or gas exchange.

Comprehensive analysis of the flowering genes in Chinese cabbage and examination of evolutionary pattern of CO-like genes in plant kingdom

In plants, flowering is the most important transition from vegetative to reproductive growth. The flowering patterns of monocots and eudicots are distinctly different, but few studies have described the evolutionary patterns of the flowering genes in them. In this study, we analysed the evolutionary pattern, duplication and expression level of these genes. The main results were as follows: (i) characterization of flowering genes in monocots and eudicots, including the identification of family-specific, orthologous and collinear genes; (ii) full characterization of CONSTANS-like genes in Brassica rapa (BraCOL genes), the key flowering genes; (iii) exploration of the evolution of COL genes in plant kingdom and construction of the evolutionary pattern of COL genes; (iv) comparative analysis of CO and FT genes between Brassicaceae and Grass, which identified several family-specific amino acids, and revealed that CO and FT protein structures were similar in B. rapa and Arabidopsis but different in rice; and (v) expression analysis of photoperiod pathway-related genes in B. rapa under different photoperiod treatments by RT-qPCR. This analysis will provide resources for understanding the flowering mechanisms and evolutionary pattern of COL genes. In addition, this genome-wide comparative study of COL genes may also provide clues for evolution of other flowering genes.

Circadian Clock Genes Universally Control Key Agricultural Traits

Circadian clocks are endogenous timers that enable plants to synchronize biological processes with daily and seasonal environmental conditions in order to allocate resources during the most beneficial times of day and year. The circadian clock regulates a number of central plant activities, including growth, development, and reproduction, primarily through controlling a substantial proportion of transcriptional activity and protein function. This review examines the roles that alleles of circadian clock genes have played in domestication and improvement of crop plants. The focus here is on three groups of circadian clock genes essential to clock function in Arabidopsis thaliana: PSEUDO-RESPONSE REGULATORs, GIGANTEA, and the evening complex genes early flowering 3, early flowering 4, and lux arrhythmo. homologous genes from each group underlie quantitative trait loci that have beneficial influences on key agricultural traits, especially flowering time but also yield, biomass, and biennial growth habit. Emerging insights into circadian clock regulation of other fundamental plant processes, including responses to abiotic and biotic stresses, are discussed to highlight promising avenues for further crop improvement.

Allelic polymorphism of GIGANTEA is responsible for naturally occurring variation in circadian period in Brassica rapa

GIGANTEA (GI) was originally identified by a late-flowering mutant in Arabidopsis, but subsequently has been shown to act in circadian period determination, light inhibition of hypocotyl elongation, and responses to multiple abiotic stresses, including tolerance to high salt and cold (freezing) temperature. Genetic mapping and analysis of families of heterogeneous inbred lines showed that natural variation in GI is responsible for a major quantitative trait locus in circadian period in Brassica rapa. We confirmed this conclusion by transgenic rescue of an Arabidopsis gi-201 loss of function mutant. The two B. rapa GI alleles each fully rescued the delayed flowering of Arabidopsis gi-201 but showed differential rescue of perturbations in red light inhibition of hypocotyl elongation and altered cold and salt tolerance. The B. rapa R500 GI allele, which failed to rescue the hypocotyl and abiotic stress phenotypes, disrupted circadian period determination in Arabidopsis. Analysis of chimeric B. rapa GI alleles identified the causal nucleotide polymorphism, which results in an amino acid substitution (S264A) between the two GI proteins. This polymorphism underlies variation in circadian period, cold and salt tolerance, and red light inhibition of hypocotyl elongation. Loss-of-function mutations of B. rapa GI confer delayed flowering, perturbed circadian rhythms in leaf movement, and increased freezing and increased salt tolerance, consistent with effects of similar mutations in Arabidopsis. Collectively, these data suggest that allelic variation of GI-and possibly of clock genes in general-offers an attractive target for molecular breeding for enhanced stress tolerance and potentially for improved crop yield.

Evolutionary relationships among barley and Arabidopsis core circadian clock and clock-associated genes

The circadian clock regulates a multitude of plant developmental and metabolic processes. In crop species, it contributes significantly to plant performance and productivity and to the adaptation and geographical range over which crops can be grown. To understand the clock in barley and how it relates to the components in the Arabidopsis thaliana clock, we have performed a systematic analysis of core circadian clock and clock-associated genes in barley, Arabidopsis and another eight species including tomato, potato, a range of monocotyledonous species and the moss, Physcomitrella patens. We have identified orthologues and paralogues of Arabidopsis genes which are conserved in all species, monocot/dicot differences, species-specific differences and variation in gene copy number (e.g. gene duplications among the various species). We propose that the common ancestor of barley and Arabidopsis had two-thirds of the key clock components identified in Arabidopsis prior to the separation of the monocot/dicot groups. After this separation, multiple independent gene duplication events took place in both monocot and dicot ancestors.