Orchestrated transcription of key pathways in Arabidopsis by the circadian clock

Periodicity and Spectral Composition of Light in the Regulation of Hypocotyl Elongation of Sunflower Seedlings

This study presents the hypocotyl elongation of sunflower seedlings germinated under different light conditions. Elongation was rhythmic under diurnal (LD) photoperiods but uniform (arrhythmic) under free-running conditions of white light (LL) or darkness (DD). On the sixth day after the onset of germination, seedlings were entrained in all diurnal photoperiods. Their hypocotyl elongation was dual, showing different kinetics in daytime and nighttime periods. The daytime elongation peak was around midday and 1–2 h after dusk in the nighttime. Plantlets compensated for the differences in the daytime and nighttime durations and exhibited similar overall elongation rates, centered around the uniform elongation in LL conditions. Thus, plants from diurnal photoperiods and LL could be grouped together as white-light treatments that suppressed hypocotyl elongation. Hypocotyl elongation was significantly higher under DD than under white-light photoperiods. In continuous monochromatic blue, yellow, green, or red light, hypocotyl elongation was also uniform and very high. The treatments with monochromatic light and DD had similar overall elongation rates; thus, they could be grouped together. Compared with white light, monochromatic light promoted hypocotyl elongation. Suppression of hypocotyl elongation and rhythmicity reappeared in some combination with two or more monochromatic light colors. The presence of red light was obligatory for this suppression. Plantlets entrained in diurnal photoperiods readily slipped from rhythmic into uniform elongation if they encountered any kind of free-running conditions. These transitions occurred whenever the anticipated duration of daytime or nighttime was extended more than expected, or when plantlets were exposed to constant monochromatic light. This study revealed significant differences in the development of sunflower plantlets illuminated with monochromatic or white light.

Orchestrated translation specializes dinoflagellate metabolism three times per day

Many cells specialize for different metabolic tasks at different times over their normal ZT cycle by changes in gene expression. However, in most cases, circadian gene expression has been assessed at the mRNA accumulation level, which may not faithfully reflect protein synthesis rates. Here, we use ribosome profiling in the dinoflagellate Lingulodinium polyedra to identify thousands of transcripts showing coordinated translation. All of the components in carbon fixation are concurrently regulated at ZT0, predicting the known rhythm of carbon fixation, and many enzymes involved in DNA replication are concurrently regulated at ZT12, also predicting the known rhythm in this process. Most of the enzymes in glycolysis and the TCA cycle are also regulated together, suggesting rhythms in these processes as well. Surprisingly, a third cluster of transcripts show peak translation at approximately ZT16, and these transcripts encode enzymes involved in transcription, translation, and amino acid biosynthesis. The latter has physiological consequences, as measured free amino acid levels increase at night and thus represent a previously undocumented rhythm in this model. Our results suggest that ribosome profiling may be a more accurate predictor of changed metabolic state than transcriptomics.

Temporally Regulated Plant-Nematode Gene Networks Implicate Metabolic Pathways

Root-knot nematodes (RKN) (Meloidogyne spp.) constantly communicate with their host to establish and maintain specialized feeding cells. They likely regulate this interaction by monitoring host biology. As plant host biology is influenced by light and gene expression varies correspondingly, RKN gene transcription and biology likely follow similar patterns. We profiled RKN transcripts over a period of 24 h and identified approximately 1,000 differentially expressed genes (DEG) in nematode and model host Medicago truncatula, with the majority of DEG occurring in the middle of the dark period. Many of the plant DEG are involved in defense-response pathways, while the nematode DEG are involved in establishing infection, suggesting a strong host-nematode interaction occurring during the dark. To identify interacting genes, we developed a plant-nematode gene network based on DEG signals. The phenylpropanoid pathway was identified as a significant plant-nematode interacting pathway, representing four of 33 genes in the network. We further examined if this pathway interacts similarly in another host, tomato, by quantifying phenolic and flavonoid compounds produced by this pathway. Phenolic compounds showed a significant increase in production during the day in uninoculated plants as compared with during the night. However, during the dark period, there was an increase in flavonoid content in infected plants when compared with uninfected controls, indicating potential host defense mechanisms active during the height of nematode activity at night. This study elucidated cross-species interacting pathways that could be targeted to develop novel management strategies to these important pests.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Keeping time in the dark: Potato diel and circadian rhythmic gene expression reveals tissue-specific circadian clocks

The circadian clock is an internal molecular oscillator and coordinates numerous physiological processes through regulation of molecular pathways. Tissue-specific clocks connected by mobile signals have previously been found to run at different speeds in Arabidopsis thaliana tissues. However, tissue variation in circadian clocks in crop species is unknown. In this study, leaf and tuber global gene expression in cultivated potato under cycling and constant environmental conditions was profiled. In addition, we used a circadian-regulated luciferase reporter construct to study tuber gene expression rhythms. Diel and circadian expression patterns were present among 17.9% and 5.6% of the expressed genes in the tuber. Over 500 genes displayed differential tissue specific diel phases. Intriguingly, few core circadian clock genes had circadian expression patterns, while all such genes were circadian rhythmic in cultivated tomato leaves. Furthermore, robust diel and circadian transcriptional rhythms were observed among detached tubers. Our results suggest alternative regulatory mechanisms and/or clock composition is present in potato, as well as the presence of tissue-specific independent circadian clocks. We have provided the first evidence of a functional circadian clock in below-ground storage organs, holding important implications for other storage root and tuberous crops.

The intersection between circadian and heat-responsive regulatory networks controls plant responses to increasing temperatures

Increasing temperatures impact plant biochemistry, but the effects can be highly variable. Both external and internal factors modulate how plants respond to rising temperatures. One such factor is the time of day or season the temperature increase occurs. This timing significantly affects plant responses to higher temperatures altering the signaling networks and affecting tolerance levels. Increasing overlaps between circadian signaling and high temperature responses have been identified that could explain this sensitivity to the timing of heat stress. ELF3, a circadian clock component, functions as a thermosensor. ELF3 regulates thermoresponsive hypocotyl elongation in part through its cellular localization. The temperature sensitivity of ELF3 depends on the length of a polyglutamine region, explaining how plant temperature responses vary between species. However, the intersection between the circadian system and increased temperature stress responses is pervasive and extends beyond this overlap in thermosensing. Here, we review the network responses to increased temperatures, heat stress, and the impacts on the mechanisms of gene expression from transcription to translation, highlighting the intersections between the elevated temperature and heat stress response pathways and circadian signaling, focusing on the role of ELF3 as a thermosensor.

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.

Dual Role for FHY3 in Light Input to the Clock

The red-light regulated transcription factors FHY3 and FAR1 form a key point of light input to the plant circadian clock in positively regulating expression of genes within the central clock. However, the fhy3 mutant shows an additional red light-specific disruption of rhythmicity which is inconsistent with this role. Here we demonstrate that only fhy3 and not far1 mutants show this red specific disruption of rhythmicity. We examined the differences in rhythmic transcriptome in red versus white light and reveal differences in patterns of rhythmicity among the central clock proteins suggestive of a change in emphasis within the central mechanism of the clock, changes which underlie the red specificity of the fhy3 mutant. In particular, changes in enrichment of promoter elements were consistent with a key role for the HY5 transcription factor, a known integrator of the ratio of red to blue light in regulation of the clock. Examination of differences in the rhythmic transcriptome in the fhy3 mutant in red light identified specific disruption of the CCA1-regulated ELF3 and LUX central clock genes, while the CCA1 target TBS element, TGGGCC, was enriched among genes that became arrhythmic. Coupled with the known interaction of FHY3 but not FAR1 with CCA1 we propose that the red-specific circadian phenotype of fhy3 may involve disruption of the previously demonstrated moderation of CCA1 activity by FHY3 rather than a disruption of its own transcriptional regulatory activity. Together, this evidence suggests a conditional redundancy between FHY3 and HY5 in the integration of red and blue light input to the clock in order to enable a plasticity in response to light and optimise plant adaptation. Furthermore, our evidence also suggests changes in CCA1 activity between red and white light transcriptomes. This, together with the documented interaction of HY5 with CCA1, leads us to propose a model whereby this integration of red and blue signals may at least partly occur via direct FHY3 and HY5 interaction with CCA1 leading to moderation of CCA1 activity.

Family Members Additively Repress the Ectopic Expression of BASIC PENTACYSTEINE3 to Prevent Disorders in Arabidopsis Circadian Vegetative Development

BARLEY B-RECOMBINANT/BASIC PENTACYSTEINE (BBR/BPC) family members are plant-specific GAGA-motif binding factors (GAFs) controlling multiple developmental processes of growth and propagation. BPCs recruit histone remodeling factors for transcriptional repression of downstream targets. It has been revealed that BPCs have an overlapping and antagonistic relationship in regulating development. In this study, we showed disturbances interfering with the homeostasis of BPC expressions impede growth and development. The ectopic expression of BPC3 results in the daily growth defect shown by higher-order bpc mutants. Oscillations of multiple circadian clock genes are phase-delayed in the quadruple mutant of bpc1 bpc2 bpc4 bpc6 (bpc1,2,4,6). By introducing the overexpression of BPC3 into wild-type Arabidopsis, we found that BPC3 is a repressor participating in its repression and repressing multiple regulators essential to the circadian clock. However, the induction of BPC3 overexpression did not fully replicate clock defects shown by the quadruple mutant, indicating that in addition to the BPC3 antagonization, BPC members also cofunction in the circadian clock regulation. A leaf edge defect similar to that shown by bpc1,2,4,6 is also observed under BPC3 induction, accompanied by repression of a subset of TCPs required for the edge formation. This proves that BPC3 is a repressor that must be confined during the vegetative phase. Our findings demonstrate that BPCs form a meticulous repressor network for restricting their repressive functions to molecular mechanisms controlling plant growth and development.

Expression Level Dominance and Homeolog Expression Bias Upon Cold Stress in the F1 Hybrid Between the Invasive Sphagneticola trilobata and the Native S. calendulacea in South China, and Implications for Its Invasiveness

The role of hybridization is significant in biological invasion, and thermotolerance is a trait critical to range expansions. The South American Sphagneticola trilobata is now widespread in South China, threatening the native S. calendulacea by competition and hybridization. Furthermore, upon formation, their F1 hybrid can quickly replace both parents. In this study, the three taxa were used as a model to investigate the consequences of hybridization on cold tolerance, particularly the effect of subgenome dominance in the hybrid. Upon chilling treatments, physiological responses and transcriptome profiles were compared across different temperature points to understand their differential responses to cold. While both parents showed divergent responses, the hybrid’s responses showed an overall resemblance to S. calendulacea, but the contribution of homeolog expression bias to cold stress was not readily evident in the F1 hybrid possibly due to inherent bias that comes with the sampling location. Our findings provided insights into the role of gene expression in differential cold tolerance, and further contribute to predicting the invasive potential of other hybrids between S. trilobata and its congeners around the world.

Mutations that alter Arabidopsis flavonoid metabolism affect the circadian clock

Flavonoids are a well-known class of specialized metabolites that play key roles in plant development, reproduction, and survival. Flavonoids are also of considerable interest from the perspective of human health, as both phytonutrients and pharmaceuticals. RNA sequencing analysis of an Arabidopsis null allele for chalcone synthase (CHS), which catalyzes the first step in flavonoid metabolism, has uncovered evidence that these compounds influence the expression of genes associated with the plant circadian clock. Analysis of promoter-luciferase constructs further showed that the transcriptional activity of CCA1 and TOC1, two key clock genes, is altered in CHS-deficient seedlings across the day/night cycle. Similar findings for a mutant line lacking flavonoid 3'-hydroxylase (F3'H) activity, and thus able to synthesize mono- but not dihydroxylated B-ring flavonoids, suggests that the latter are at least partially responsible; this was further supported by the ability of quercetin to enhance CCA1 promoter activity in wild-type and CHS-deficient seedlings. The effects of flavonoids on circadian function were also reflected in photosynthetic activity, with chlorophyll cycling abolished in CHS- and F3'H-deficient plants. Remarkably, the same phenotype was exhibited by plants with artificially high flavonoid levels, indicating that neither the antioxidant potential nor the light-screening properties of flavonoids contribute to optimal clock function, as has recently also been demonstrated in animal systems. Collectively, the current experiments point to a previously unknown connection between flavonoids and circadian cycling in plants and open the way to better understanding of the molecular basis of flavonoid action.

Functional characterization of genes with daily expression patterns in common wheat

Our findings suggest most wheat biological processes are under the control of the daily expressed genes. Plant circadian rhythms represent daily changes in the activity of various processes, which are based on changes in the levels of gene expression and protein synthesis. In wheat, some key components of plant circadian clock have been identified, but there is little data on the daily expression and interactions of these genes. To study the common wheat daily transcriptome, RNA sequencing was performed. Using these data, genes expressed in daily pattern and the metabolic pathways controlled by them were identified: responses to stimuli and nutrients, transport, photoperiodism, photomorphogenesis, synthesis and degradation of different metabolites, and regulation of the processes of RNA synthesis. It was shown that a significant part of the transcriptome can vary greatly daily. Five expression patterns were identified. They were characterized by peaks at different time points and described the genes underlying these patterns. The analysis of the enrichment of gene ontology terms with various patterns allowed us to describe the main metabolic pathways in each group. Wheat homologs of the genes related to circadian clock in Arabidopsis were identified. Most of them were represented by three homoeologous genes expressed uniformly. Comparison of their expression patterns demonstrated a shift in the expression peaks for some core and accessory genes; the majority of wheat circadian genes were expressed in accordance with Arabidopsis homologs. This may indicate a similar functional role of these genes in wheat.

Phosphorylation of MdERF17 by MdMPK4 promotes apple fruit peel degreening during light/dark transitions

As apple fruits (Malus domestica) mature, they accumulate anthocyanins concomitantly with losing chlorophyll (Chl); however, the molecular pathways and events that coordinate Chl degradation and fruit coloration have not been elucidated. We showed previously that the transcription factor ETHYLENE RESPONSE FACTOR17 (MdERF17) modulates Chl degradation in apple fruit peels and that variation in the pattern of MdERF17 serine (Ser) residues is responsible for differences in its transcriptional regulatory activity. Here, we report that MdERF17 interacts with and is phosphorylated by MAP KINASE4 (MdMPK4-14G). Phosphorylation of MdERF17 at residue Thr67 by MdMPK4-14G is necessary for its transcriptional regulatory activity and its regulation of Chl degradation. We also show that MdERF17 mutants with different numbers of Ser repeat insertions exhibit altered phosphorylation profiles, with more repeats increasing its interaction with MdMPK4. MdMPK4-14G can be activated by exposure to darkness and is involved in the dark-induced degreening of fruit peels. We also demonstrate that greater phosphorylation of MdERF17 by MdMPK4-14G is responsible for the regulation of Chl degradation during light/dark transitions. Overall, our findings reveal the mechanism by which MdMPK4 controls fruit peel coloration.

Role of Circadian Rhythms in Major Plant Metabolic and Signaling Pathways

Plants require an endogenous regulatory network and mechanism to cope with diurnal environmental changes and compensate for their sessile nature. Plants use the circadian clock to anticipate diurnal changes. Circadian rhythm predicts a 24-h cycle with 16 h of light and 8 h of darkness in response to abiotic and biotic factors as well as the appropriate temperature. For a plant's fitness, proper growth, and development, these rhythms synchronize the diurnal photoperiodic changes. Input pathway, central oscillator, and output pathway are the three components that make up the endogenous clock. There are also transcriptional and translational feedback loops (TTFLs) in the clock, which are dependent on the results of gene expression. Several physiological processes, such as stress acclimatization, hormone signaling, morphogenesis, carbon metabolism, and defense response, are currently being investigated for their interactions with the circadian clock using phenotypic, genomic, and metabolic studies. This review examines the role of circadian rhythms in the regulation of plant metabolic pathways, such as photosynthesis and carbon metabolism, as well as developmental and degenerative processes, such as flowering and senescence. Furthermore, we summarized signaling pathways related to circadian rhythms, such as defense response and gene regulatory pathways.

Plant circadian clock control of Medicago truncatula nodulation via regulation of nodule cysteine-rich peptides

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialized polyploid cells within root nodules, which undergo tightly regulated metabolic activity. By carrying out expression analysis of transcripts over time in Medicago truncatula nodules, we found that the circadian clock enables coordinated control of metabolic and regulatory processes linked to nitrogen fixation. This involves the circadian clock-associated transcription factor LATE ELONGATED HYPOCOTYL (LHY), with lhy mutants being affected in nodulation. Rhythmic transcripts in root nodules include a subset of nodule-specific cysteine-rich peptides (NCRs) that have the LHY-bound conserved evening element in their promoters. Until now, studies have suggested that NCRs act to regulate bacteroid differentiation and keep the rhizobial population in check. However, these conclusions came from the study of a few members of this very large gene family that has complex diversified spatio-temporal expression. We suggest that rhythmic expression of NCRs may be important for temporal coordination of bacterial activity with the rhythms of the plant host, in order to ensure optimal symbiosis.

Transcriptome Profiling in the Marine Red Alga Neopyropia yezoensis Under Light/Dark Cycle

Many organisms are subjected to a daily cycle of light and darkness, which significantly influences metabolic and physiological processes. In the present study, Neopyropia yezoensis, one of the major cultivated seaweeds used in "nori," was harvested in the morning and evening during light/dark treatments to investigate daily changes in gene expression using RNA-sequencing. A high abundance of transcripts in the morning includes the genes associated with carbon-nitrogen assimilations, polyunsaturated fatty acid, and starch synthesis. In contrast, the upregulation of a subset of the genes associated with the pentose phosphate pathway, cell cycle, and DNA replication at evening is necessary for the tight control of light-sensitive processes, such as DNA replication. Additionally, a high abundance of transcripts at dusk encoding asparaginase and glutamate dehydrogenase imply that regulation of asparagine catabolism and tricarboxylic acid cycle possibly contributes to supply nitrogen and carbon, respectively, for growth during the dark. In addition, genes encoding cryptochrome/photolyase family and histone modification proteins were identified as potential key players for regulating diurnal rhythmic genes.

Circadian clock in plants: Linking timing to fitness

Endogenous circadian clock integrates cyclic signals of environment and daily and seasonal behaviors of organisms to achieve spatiotemporal synchronization, which greatly improves genetic diversity and fitness of species. This review addresses recent studies on the plant circadian system in the field of chronobiology, covering topics on molecular mechanisms, internal and external Zeitgebers, and hierarchical regulation of physiological outputs. The architecture of the circadian clock involves the autoregulatory transcriptional feedback loops, post-translational modifications of core oscillators, and epigenetic modifications of DNA and histones. Here, light, temperature, humidity, and internal elemental nutrients are summarized to illustrate the sensitivity of the circadian clock to timing cues. In addition, the circadian clock runs cell-autonomously, driving independent circadian rhythms in various tissues. The core oscillators responds to each other with biochemical factors including calcium ions, mineral nutrients, photosynthetic products, and hormones. We describe clock components sequentially expressed during a 24-h day that regulate rhythmic growth, aging, immune response, and resistance to biotic and abiotic stresses. Notably, more data have suggested the circadian clock links chrono-culture to key agronomic traits in crops.

Regulatory Role of Circadian Clocks on ABA Production and Signaling, Stomatal Responses, and Water-Use Efficiency under Water-Deficit Conditions

Plants deploy molecular, physiological, and anatomical adaptations to cope with long-term water-deficit exposure, and some of these processes are controlled by circadian clocks. Circadian clocks are endogenous timekeepers that autonomously modulate biological systems over the course of the day–night cycle. Plants’ responses to water deficiency vary with the time of the day. Opening and closing of stomata, which control water loss from plants, have diurnal responses based on the humidity level in the rhizosphere and the air surrounding the leaves. Abscisic acid (ABA), the main phytohormone modulating the stomatal response to water availability, is regulated by circadian clocks. The molecular mechanism of the plant’s circadian clock for regulating stress responses is composed not only of transcriptional but also posttranscriptional regulatory networks. Despite the importance of regulatory impact of circadian clock systems on ABA production and signaling, which is reflected in stomatal responses and as a consequence influences the drought tolerance response of the plants, the interrelationship between circadian clock, ABA homeostasis, and signaling and water-deficit responses has to date not been clearly described. In this review, we hypothesized that the circadian clock through ABA directs plants to modulate their responses and feedback mechanisms to ensure survival and to enhance their fitness under drought conditions. Different regulatory pathways and challenges in circadian-based rhythms and the possible adaptive advantage through them are also discussed.

Light and Plant Growth Regulators on In Vitro Proliferation

Plant tissue cultures depend entirely upon artificial light sources for illumination. The illumination should provide light in the appropriate regions of the electromagnetic spectrum for photomorphogenic responses and photosynthetic metabolism. Controlling light quality, irradiances and photoperiod enables the production of plants with desired characteristics. Moreover, significant money savings may be achieved using both more appropriate and less consuming energy lamps. In this review, the attention will be focused on the effects of light characteristics and plant growth regulators on shoot proliferation, the main process in in vitro propagation. The effects of the light spectrum on the balance of endogenous growth regulators will also be presented. For each light spectrum, the effects on proliferation but also on plantlet quality, i.e., shoot length, fresh and dry weight and photosynthesis, have been also analyzed. Even if a huge amount of literature is available on the effects of light on in vitro proliferation, the results are often conflicting. In fact, a lot of exogenous and endogenous factors, but also the lack of a common protocol, make it difficult to choose the most effective light spectrum for each of the large number of species. However, some general issues derived from the analysis of the literature are discussed.

In Silico Characterization and Expression Analysis of GIGANTEA Genes in Potato

GIGANTEA (GI) genes are ubiquitous in the plant kingdom and are involved in diverse processes from flowering during stress responses to tuberization; the latter occurs in potato (Solanum tuberosum L.). GI genes have a diurnal cycle of expression; however, no details on the regulation of GI gene expression in potato have been reported thus far. The aim of our work was the analysis of the GI promoter sequence and studying GI expression in different organs and under abiotic stress conditions in potato. Two GI genes homologous to Arabidopsis GI located on chromosomes 4 and 12 (StGI.04 and StGI.12) were identified in the genome-sequenced potato S. phureja. The GI promoter regions of the commercial potato cultivar ‘Désirée’ were cloned and found to be almost identical to the S. phureja GI promoter sequence. More than ten TF families binding to the GI promoters were predicted. EVENING ELEMENT and ABSCISIC ACID RESPONSE ELEMENT LIKE elements related to circadian regulation and a binding site for POTATO HOMEOBOX 20 presumably involved in tuber initiation were detected in both GI promoters. However, the locations of these elements and several other cis-acting regulatory elements as well as the organ-specific expression and responses of the genes to abiotic stresses and abscisic acid were different. Thus, we presume that the function of StGI.04 and StGI.12 are at least partially different. This study lays foundation for further investigation of the roles of GI genes in potato.

The two clock proteins CCA1 and LHY activate VIN3 transcription during vernalization through the vernalization-responsive cis-element

Vernalization, a long-term cold-mediated acquisition of flowering competence, is critically regulated by VERNALIZATION INSENSITIVE 3 (VIN3), a gene induced by vernalization in Arabidopsis. Although the function of VIN3 has been extensively studied, how VIN3 expression itself is upregulated by long-term cold is not well understood. In this study, we identified a vernalization-responsive cis-element in the VIN3 promoter, VREVIN3, composed of a G-box and an evening element (EE). Mutations in either the G-box or the EE prevented VIN3 expression from being fully induced upon vernalization, leading to defects in the vernalization response. We determined that the core clock proteins CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and LATE-ELONGATED HYPOCOTYL (LHY) associate with the EE of VREVIN3, both in vitro and in vivo. In a cca1 lhy double mutant background harboring a functional FRIGIDA allele, long-term cold-mediated VIN3 induction and acceleration of flowering were impaired, especially under mild cold conditions such as at 12°C. During prolonged cold exposure, oscillations of CCA1/LHY transcripts were altered, while CCA1 abundance increased at dusk, coinciding with the diurnal peak of VIN3 transcripts. We propose that modulation of the clock proteins CCA1 and LHY participates in the systems involved in sensing long-term cold for the activation of VIN3 transcription.

The circadian clock ticks in plant stress responses

The circadian clock, a time-keeping mechanism, drives nearly 24-h self-sustaining rhythms at the physiological, cellular, and molecular levels, keeping them synchronized with the cyclic changes of environmental signals. The plant clock is sensitive to external and internal stress signals that act as timing cues to influence the circadian rhythms through input pathways of the circadian clock system. In order to cope with environmental stresses, many core oscillators are involved in defense while maintaining daily growth in various ways. Recent studies have shown that a hierarchical multi-oscillator network orchestrates the defense through rhythmic accumulation of gene transcripts, alternative splicing of mRNA precursors, modification and turnover of proteins, subcellular localization, stimuli-induced phase separation, and long-distance transport of proteins. This review summarizes the essential role of circadian core oscillators in response to stresses in Arabidopsis thaliana and crops, including daily and seasonal abiotic stresses (low or high temperature, drought, high salinity, and nutrition deficiency) and biotic stresses (pathogens and herbivorous insects). By integrating time-keeping mechanisms, circadian rhythms and stress resistance, we provide a temporal perspective for scientists to better understand plant environmental adaptation and breed high-quality crop germplasm for agricultural production.

Effects of different photoperiods on flower opening, flower closing and circadian expression of clock-related genes in Iris domestica and I. dichotoma

The circadian clock can entrain to forced light-dark cycles by adjusting the phases and periods of flower opening and closing in ephemeral flowers. The responses of circadian rhythms to the same light conditions differ from species. However, the differences in internal genetic mechanisms underlying the different responses between species remain unclear. Iris domestica and I. dichotoma have ephemeral flowers and significantly divergent flower opening and closing times. The effects of different photoperiods (continuous darkness, 4L20D, 8L16D, 12L12D, 16L8D, 20L4D and continuous white light) on flower opening and closing, and expression patterns of seven genes (CRYPTOCHROME 1, PHYTOCHROME B, LATE ELONGATED HYPOCOTYL, PSEUDO RESPONSE REGULATOR 95, PHYTOCHROME INTERACTING FACTOR 4-like, SMUX AUXIN UP RNA 64-like and senescence-associated gene 39-like) involved in the circadian regulation of flower opening and closing were compared between I. domestica and I. dichotoma. Flower opening and closing in the two species exhibited circadian rhythms under continuous darkness (DD), but showed arrhythmia in continuous white light (LL). In the two species, keeping robust rhythms, strong synchronicity, rapid progressions of flower opening and closing and reaching full opening stage required a dark period longer than 4 h. In light-dark cycles with dark periods longer than 4 h, flower opening and closing times of the two species delayed with the delay of dawn, and the degree to which flower opening time varies with the time of dawn was greater in I. dichotoma than in I. domestica. The arrhythmia of flower opening and closing under 20L4D and LL would result from the arrhythmic output signals rather than arrhythmia of oscillators and photoreceptors. The different responses of the two species to the change of photoperiods would be caused by the transcriptional differences of genes in the output pathway of circadian clock system rather than in the input pathway or oscillators.

Enhancement of Arabidopsis growth by non-24 hour day-night cycles

Plant yield can be increased by matching the internal circadian rhythms with the external light and dark cycle (circadian resonance). The circadian resonance reported in the past was analyzed under light-dark cycles with 20-, 24-, or 28-hr periods; however, the mechanism for circadian resonance is still debatable due to the experimental time schedules in previous studies being few in number and widely separated. By analyzing the yield of Arabidopsis thaliana grown under eight different external light and dark periods, we found that the yield increased when the external cycle was 22 and 26 hr instead of 24 hr. Time course RNA-seq analysis determined that seedling circadian clock genes had a free-running period of 22 ± 1 hr. Furthermore, a group of genes with 25- to 26-hr period rhythms were also observed in the seedlings with a 22- ± 1-hr period as their circadian clock. We propose that resonance that occurred by matching the expression time of a group of genes with the 25- to 26-hr cycle and providing an external day-night cycle of 25 to 26 hr was one factor that caused the yield increase.

The SEEL motif and members of the MYB-related REVEILLE transcription factor family are important for the expression of LORELEI in the synergid cells of the Arabidopsis female gametophyte

Synergid cells in the micropylar end of the female gametophyte are required for critical cell-cell signaling interactions between the pollen tube and the ovule that precede double fertilization and seed formation in flowering plants. LORELEI (LRE) encodes a putative GPI-anchored protein that is expressed primarily in the synergid cells, and together with FERONIA, a receptor-like kinase, it controls pollen tube reception by the receptive synergid cell. Still, how LRE expression is controlled in synergid cells remains poorly characterized. We identified candidate cis-regulatory elements enriched in LRE and other synergid cell-expressed genes. One of the candidate motifs ('TAATATCT') in the LRE promoter was an uncharacterized variant of the Evening Element motif that we named as the Short Evening Element-like (SEEL) motif. Deletion or point mutations in the SEEL motif of the LRE promoter resulted in decreased reporter expression in synergid cells, demonstrating that the SEEL motif is important for expression of LRE in synergid cells. Additionally, we found that LRE expression is decreased in the loss of function mutants of REVEILLE (RVE) transcription factors, which are clock genes known to bind the SEEL and other closely related motifs. We propose that RVE transcription factors regulate LRE expression in synergid cells by binding to the SEEL motif in the LRE promoter. Identification of cis-regulatory elements and transcription factors involved in the expression of LRE will serve as a foundation to characterize the gene regulatory networks in synergid cells.

Plant Responses Underlying Timely Specialized Metabolites Induction of Brassica Crops

A large subset of plant stress-signaling pathways, including those related with chemical defense production, exhibit diurnal or circadian oscillations. However the extent to which diurnal or circadian time influences the stress mediated accumulation of plant specialized metabolites remains largely unknown. Because plant responses to physical stress (e.g., wounding) is considered a common component of mounting a response against a broad range of environmental stresses, including herbivory, we have utilized mechanical wounding as the stress stimulus to determine the direct contribution of time of day on the induced defenses of Brassica crops. We analyzed glucosinolates (GSLs) from leaves of broccoli (Brassica oleracea) and turnip greens (Brassica rapa) following exposure to mechanical wounding at dawn (ZT0), mid-day (ZT4), and dusk (ZT8). Several GSLs differentially accumulated and their changes depended upon the time of day at wounding was performed. This response varied considerably between species. In a parallel experiment, we investigated whether diurnal activation of Brassica phytochemicals in response to wounding might prime plants against herbivore attack. Results showed that maximal response of plant chemical defense against larvae of the generalist pest Mamestra brassicae occurred at ZT0 in broccoli and ZT8 in turnip greens. Metabolome analysis for global trends of time dependent compounds showed that sulfur-containing phytochemicals, GSL hydrolysis products, auxin-signaling components, and other metabolites activators of plant disease resistance (nicotinamide and pipecolate) had important contributions to the responses of M. brassicae feeding behavior in broccoli at morning. Overall, the findings in this study highlight a significant role for time of day in the wound stress responsive metabolome, which can in turn affect plant-herbivore interactions.

The biology of time: dynamic responses of cell types to developmental, circadian and environmental cues

As sessile organisms, plants are finely tuned to respond dynamically to developmental, circadian and environmental cues. Genome-wide studies investigating these types of cues have uncovered the intrinsically different ways they can impact gene expression over time. Recent advances in single-cell sequencing and time-based bioinformatic algorithms are now beginning to reveal the dynamics of these time-based responses within individual cells and plant tissues. Here, we review what these techniques have revealed about the spatiotemporal nature of gene regulation, paying particular attention to the three distinct ways in which plant tissues are time sensitive. (i) First, we discuss how studying plant cell identity can reveal developmental trajectories hidden in pseudotime. (ii) Next, we present evidence that indicates that plant cell types keep their own local time through tissue-specific regulation of the circadian clock. (iii) Finally, we review what determines the speed of environmental signaling responses, and how they can be contingent on developmental and circadian time. By these means, this review sheds light on how these different scales of time-based responses can act with tissue and cell-type specificity to elicit changes in whole plant systems.

Fifteen compelling open questions in plant cell biology

As scientists, we are at least as excited about the open questions-the things we do not know-as the discoveries. Here, we asked 15 experts to describe the most compelling open questions in plant cell biology. These are their questions: How are organelle identity, domains, and boundaries maintained under the continuous flux of vesicle trafficking and membrane remodeling? Is the plant cortical microtubule cytoskeleton a mechanosensory apparatus? How are the cellular pathways of cell wall synthesis, assembly, modification, and integrity sensing linked in plants? Why do plasmodesmata open and close? Is there retrograde signaling from vacuoles to the nucleus? How do root cells accommodate fungal endosymbionts? What is the role of cell edges in plant morphogenesis? How is the cell division site determined? What are the emergent effects of polyploidy on the biology of the cell, and how are any such "rules" conditioned by cell type? Can mechanical forces trigger new cell fates in plants? How does a single differentiated somatic cell reprogram and gain pluripotency? How does polarity develop de-novo in isolated plant cells? What is the spectrum of cellular functions for membraneless organelles and intrinsically disordered proteins? How do plants deal with internal noise? How does order emerge in cells and propagate to organs and organisms from complex dynamical processes? We hope you find the discussions of these questions thought provoking and inspiring.

Photoprotection contributes to freezing tolerance as revealed by RNA-seq profiling of rhododendron leaves during cold acclimation and deacclimation over time

Cold acclimation (CA) and deacclimation (DA), which are often accompanied by changes in freezing tolerance (FT), carbohydrates and hormones, are crucial for winter survival, especially under global warming. Plants with weak CA and premature DA caused by warm winters and/or unseasonal warm spells can be easily injured by adverse reactions to cold. Thus, understanding the molecular mechanisms of FT is imperative. In this study, we used high-throughput RNA-seq to profile the CA and DA of leaves of overwintering Rhododendron "Miyo-no-Sakae" over time; these leaves do not undergo dormancy but do undergo photoprotection during CA, and they do not grow during DA. Using Mfuzz and weighted gene coexpression network analysis, we identified specific transcriptional characteristics in each phase of CA and DA and proposed networks involving coexpressed genes and physiological traits. In particular, we discovered that the circadian rhythm is critical for obtaining the strongest FT, and high expression of circadian rhythm-related genes might be linked to sugar accumulation during winter. Furthermore, evergreen leaves exhibited robust photoprotection during winter, as revealed by high values of nonphotochemical quenching, high expression of transcripts annotated as "early light-induced proteins", loss of granum stacks and destacking of thylakoids, all of which were alleviated during DA. The strong requirement of photoprotection could be the reason for decreased abscisic acid (ABA) and jasmonic acid (JA) contents during CA, and decreases in ABA and JA contents may contribute to decreases in lignin content. Our data suggest that the molecular mechanisms of FT in overwintering leaves are unique, which may be due to the high requirements for photoprotection during winter.

Systems scale characterization of circadian rhythm pathway in Camellia sinensis

Tea (Camellia sinensis) is among the most valuable commercial crops being a non-alcoholic beverage having antioxidant properties. Like in other plants, circadian oscillator in tea modulates several biological processes according to earth's revolution dependent variations in environmental cues like light and temperature. In the present study, we report genome wide identification and characterization of circadian oscillator (CO) proteins in tea. We first mined the genes (24, in total) involved in circadian rhythm pathway in the 56 plant species having available genomic information and then built their hidden Markov models (HMMs). Using these HMMs, 24 proteins were identified in tea and were further assessed for their functional annotation. Expression analysis of all these 24 CO proteins was then performed in 3 abiotic (A) and 3 biotic conditions (B) stress conditions and co-expressed as well as differentially expressed genes in the selected 6 stress conditions were elaborated. A methodology to identify the differentially expressed genes in specific types of stresses (A or B) is proposed and novel markers among CO proteins are presented. By mapping the identified CO proteins against the recently reported genome wide interologous protein-protein interaction network of tea (TeaGPIN), an interaction sub-network of tea CO proteins (TeaCO-PIN) is developed and analysed. Out of 24 CO proteins, structures of 4 proteins could be successfully predicted and validated using consensus of three structure prediction algorithms and their stability was further assessed using molecular dynamic simulations at 100 ns. Phylogenetic analysis of these proteins is performed to examine their molecular evolution.

How Stress Affects Your Budget-Stress Impacts on Starch Metabolism

Starch is a polysaccharide that is stored to be used in different timescales. Transitory starch is used during nighttime when photosynthesis is unavailable. Long-term starch is stored to support vegetative or reproductive growth, reproduction, or stress responses. Starch is not just a reserve of energy for most plants but also has many other roles, such as promoting rapid stomatal opening, making osmoprotectants, cryoprotectants, scavengers of free radicals and signals, and reverting embolised vessels. Biotic and abiotic stress vary according to their nature, strength, duration, developmental stage of the plant, time of the day, and how gradually they develop. The impact of stress on starch metabolism depends on many factors: how the stress impacts the rate of photosynthesis, the affected organs, how the stress impacts carbon allocation, and the energy requirements involved in response to stress. Under abiotic stresses, starch degradation is usually activated, but starch accumulation may also be observed when growth is inhibited more than photosynthesis. Under biotic stresses, starch is usually accumulated, but the molecular mechanisms involved are largely unknown. In this mini-review, we explore what has been learned about starch metabolism and plant stress responses and discuss the current obstacles to fully understanding their interactions.

Evaluating the Effects of the Circadian Clock and Time of Day on Plant Gravitropic Responses

Circadian rhythms are regular oscillations of an organism's physiology with a period of approximately 24 h. In the model plant Arabidopsis thaliana, circadian rhythms regulate a suite of physiological processes, including transcription, photosynthesis, growth, and flowering. The circadian clock and external rhythmic factors have extensive control of the underlying biochemistry and physiology. Therefore, it is critical to consider the time of day when performing gravitropism experiments, even if the circadian clock is not a focus of study. We describe the critical factors and methods to be considered and methods to investigate the possible circadian regulation of gravitropic responses.

Agrobacterium-Mediated Seedling Transformation to Measure Circadian Rhythms in Arabidopsis

Circadian clocks are endogenous timing mechanisms that allow an organism to adapt cellular processes in anticipation of predictable changes in the environment. Luciferase reporters are well utilized as an effective, nondestructive method to measure circadian rhythms of promoter activity in Arabidopsis. Obtaining stable transgenic reporter lines can be laborious. Here, we report a protocol for Agrobacterium-mediated seedling transformation tailored for plant circadian studies. We show that period estimates generated from wild-type and clock-mutant seedlings transformed with circadian luciferase reporters are similar to rhythms obtained from equivalent stable transgenic seedlings. These experiments demonstrate the versatility and robustness of the protocol for testing new constructs or quickly assessing circadian effects in any genotype of interest.

GUN4 Affects the Circadian Clock and Seedlings Adaptation to Changing Light Conditions

The chloroplast is a key organelle for photosynthesis and perceiving environmental information. GENOME UNCOUPLED 4 (GUN4) has been shown to be required for the regulation of both chlorophyll synthesis, reactive oxygen species (ROS) homeostasis and plastid retrograde signaling. In this study, we found that growth of the gun4 mutant was significantly improved under medium strong light (200 μmol photons m⁻²s⁻¹) compared to normal light (100 μmol photons m⁻²s⁻¹), in marked contrast to wild-type (WT). Further analysis revealed that GUN4 interacts with SIGNAL RECOGNITION PARTICLE 54 KDA SUBUNIT (SRP43) and SRP54. RNA-seq analysis indicated that the expression of genes for light signaling and the circadian clock is altered in gun4 compared with (WT). qPCR analysis confirmed that the expression of the clock genes CLOCK-RELATED 1 (CCA1), LATE ELONGATION HYPOCOTYL (LHY), TIMING OF CAB EXPRESSION 1 (TOC1) and PSEUDO RESPONSE REGULATOR 7 (PRR7) is significantly changed in the gun4 and srp54 mutants under normal and medium strong light conditions. These results suggest that GUN4 may coordinate the adaptation of plants to changing light conditions by regulating the biological clock, although it is not clear whether the effect is direct or indirect.

Daily Regulation of Key Metabolic Pathways in Two Seagrasses Under Natural Light Conditions

The circadian clock is an endogenous time-keeping mechanism that enables organisms to adapt to external environmental cycles. It produces rhythms of plant metabolism and physiology, and interacts with signaling pathways controlling daily and seasonal environmental responses through gene expression regulation. Downstream metabolic outputs, such as photosynthesis and sugar metabolism, besides being affected by the clock, can also contribute to the circadian timing itself. In marine plants, studies of circadian rhythms are still way behind in respect to terrestrial species, which strongly limits the understanding of how they coordinate their physiology and energetic metabolism with environmental signals at sea. Here, we provided a first description of daily timing of key core clock components and clock output pathways in two seagrass species, Cymodocea nodosa and Zostera marina (order Alismatales), co-occurring at the same geographic location, thus exposed to identical natural variations in photoperiod. Large differences were observed between species in the daily timing of accumulation of transcripts related to key metabolic pathways, such as photosynthesis and sucrose synthesis/transport, highlighting the importance of intrinsic biological, and likely ecological attributes of the species in determining the periodicity of functions. The two species exhibited a differential sensitivity to light-to-dark and dark-to-light transition times and could adopt different growth timing based on a differential strategy of resource allocation and mobilization throughout the day, possibly coordinated by the circadian clock. This behavior could potentially derive from divergent evolutionary adaptations of the species to their bio-geographical range of distributions.

Alternative polyadenylated mRNAs behave as asynchronous rhythmic transcription in Arabidopsis

Alternative polyadenylation (APA) is a widespread post-transcriptional modification method that changes the 3' ends of transcripts by altering poly(A) site usage. However, the longitudinal transcriptomic 3' end profile and its mechanism of action are poorly understood. We applied diurnal time-course poly(A) tag sequencing (PAT-seq) for Arabidopsis and identified 3284 genes that generated both rhythmic and arrhythmic transcripts. These two classes of transcripts appear to exhibit dramatic differences in expression and translation activisty. The asynchronized transcripts derived by APA are embedded with different poly(A) signals, especially for rhythmic transcripts, which contain higher AAUAAA and UGUA signal proportions. The Pol II occupancy maximum is reached upstream of rhythmic poly(A) sites, while it is present directly at arrhythmic poly(A) sites. Integrating H3K9ac and H3K4me3 time-course data analyses revealed that transcriptional activation of histone markers may be involved in the differentiation of rhythmic and arrhythmic APA transcripts. These results implicate an interplay between histone modification and RNA 3'-end processing, shedding light on the mechanism of transcription rhythm and alternative polyadenylation.

Daily dynamics of intermediate metabolite profiles lead to time-dependent phenylethanoid glycosides production in Scrophularia striata during the day/night cycle

Phenylethanoid glycosides (PhGs) are important medicinal compounds found in Scrophularia striata, one of the plant species native to Iran. Since almost all aspects of plant life are controlled by night/light cycle, studying its relationship to valuable plant metabolites production will help us to determine the right time for their extraction. Therefore, the aim of this investigation is to figure out whether the diel light oscillations control PhGs production and how it relates to daily changes in upstream metabolic reactions and circadian clock in S. striata. For this, daily rhythms of metabolic pathways were examined every 4 h during a day/night cycle in 3 groups of control (16 h light/8 h dark), continuous light and darkness. The results showed that acteoside and echinacoside levels in each group peaked during the night and day, respectively. Thus, the PhGs production follows a rhythmic behavior in S. striata, which is probably controlled by circadian clock. Also, the levels of photosynthetic pigments, carbohydrates, amino acids, phenolic acids, phytohormones and phenylalanine ammonia-lyase (PAL) and tyrosine ammonia-lyase (TAL) enzyme activities varied diel in a similar or different way among study groups. The observations revealed that light/dark cycle controls the carbon and energy flow from light reception to the production and consumption of starch, biosynthesis of phenylalanine, tyrosine, cinnamic acid and coumaric acid, activation of hormonal signaling pathways and enzymes involved in phenylpropanoid pathway. Overall, it can be concluded that PhGs accumulation time-dependent patterns is likely due to daily fluctuations in upstream metabolic reactions induced by light/dark cycle.

Field microenvironments regulate crop diel transcript and metabolite rhythms

Most research in plant chronobiology has been done in laboratory conditions. However, laboratories usually fail to mimic natural conditions and their slight fluctuations, highlighting or obfuscating rhythmicity. High-density crops, such as sugarcane (Saccharum hybrid), generate field microenvironments with specific light and temperature regimes resulting from mutual shading. We measured the metabolic and transcriptional rhythms in the leaves of 4-month-old (4 mo) and 9 mo field-grown sugarcane. Most of the assayed rhythms in 9 mo sugarcane peaked >1 h later than in 4 mo sugarcane, including rhythms of the circadian clock gene, LATE ELONGATED HYPOCOTYL (LHY). We hypothesized that older sugarcane perceives dawn later than younger sugarcane as a consequence of self-shading. As a test, we measured LHY rhythms in plants on the east and the west sides of a field. We also tested if a wooden wall built between lines of sugarcane plants changed their rhythms. The LHY peak was delayed in the plants in the west of the field or beyond the wall; both shaded at dawn. We conclude that plants in the same field may have different phases resulting from field microenvironments, impacting important agronomical traits, such as flowering time, stalk weight and number.

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.

Experimental Design for Time-Series RNA-Seq Analysis of Gene Expression and Alternative Splicing

RNA-sequencing (RNA-seq) is currently the method of choice for analysis of differential gene expression. To fully exploit the wealth of data generated from genome-wide transcriptomic approaches, the initial design of the experiment is of paramount importance. Biological rhythms in nature are pervasive and are driven by endogenous gene networks collectively known as circadian clocks. Measuring circadian gene expression requires time-course experiments which take into account time-of-day factors influencing variability in expression levels. We describe here an approach for characterizing diurnal changes in expression and alternative splicing for plants undergoing cooling. The method uses inexpensive everyday laboratory equipment and utilizes an RNA-seq application (3D RNA-seq) that can handle complex experimental designs and requires little or no prior bioinformatics expertise.

Does biological rhythm transmit from plants to rhizosphere microbes?

Plant physiological and metabolic processes are modulated by rhythmic gene expression in a large part. Meanwhile, plants are also regulated by rhizosphere microorganisms, which are fed by root exudates and provide beneficial functions to their plant host. Whether the biorhythms in plants would transfer to the rhizosphere microbial community is still uncertain and their intricate connection remains poorly understood. Here, we investigated the role of the Arabidopsis circadian clock in shaping the rhizosphere microbial community using wild-type plants and clock mutants (cca1-1 and toc1-101) with transcriptomic, metabolomic and 16S rRNA gene sequencing analysis throughout a 24-h period. Deficiencies of the central circadian clock led to abnormal diurnal rhythms for thousands of expressed genes and dozens of root exudates. The bacterial community failed to follow obvious patterns in the 24-h period, and there was lack of coordination with plant growth in the clock mutants. Our results suggest that the robust rhythmicity of genes and root exudation due to circadian clock in plants is an important driving force for the positive succession of rhizosphere communities, which will feedback on plant development.

XAP5 CIRCADIAN TIMEKEEPER Affects Both DNA Damage Responses and Immune Signaling in Arabidopsis

Numerous links have been reported between immune response and DNA damage repair pathways in both plants and animals but the precise nature of the relationship between these fundamental processes is not entirely clear. Here, we report that XAP5 CIRCADIAN TIMEKEEPER (XCT), a protein highly conserved across eukaryotes, acts as a negative regulator of immunity in Arabidopsis thaliana and plays a positive role in responses to DNA damaging radiation. We find xct mutants have enhanced resistance to infection by a virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000, and are hyper-responsive to the defense-activating hormone salicylic acid (SA) when compared to wild-type. Unlike most mutants with constitutive effector-triggered immunity (ETI), xct plants do not have increased levels of SA and retain enhanced immunity at elevated temperatures. Genetic analysis indicates XCT acts independently of NONEXPRESSOR OF PATHOGENESIS RELATED GENES1 (NPR1), which encodes a known SA receptor. Since DNA damage has been reported to potentiate immune responses, we next investigated the DNA damage response in our mutants. We found xct seedlings to be hypersensitive to UV-C and γ radiation and deficient in phosphorylation of the histone variant H2A.X, one of the earliest known responses to DNA damage. These data demonstrate that loss of XCT causes a defect in an early step of the DNA damage response pathway. Together, our data suggest that alterations in DNA damage response pathways may underlie the enhanced immunity seen in xct mutants.

CBF transcription factors involved in the cold response of Camellia japonica (Naidong)

CBFs belong to the ERF subfamily of the AP2 supergene family and often play an important role in the cold acclimation of temperate plants. However, the role of CBFs in Camellia japonica (Naidong), the only Camellia japonica population found in the temperate zones of China, remains unclear. It is very important to study the genetic composition of C. japonica (Naidong) to adapt to low temperature for Camellia species. Using full-length transcriptome data, we identified four CjCBF genes that respond to cold stress and analyzed their evolutionary relationships, domains, and expression patterns. The phylogeny of CBFs of 19 angiosperms divided the genes into three categories, and the four CjCBFs belong to a small subcluster. The strong response of CjCBF1 to cold treatment and its sustained high level of expression indicated that it plays an important role in the process of cold acclimation. A yeast two-hybrid assay revealed an interaction between CjCBF1, CjCBF2, and CjCBF5, and subcellular localization confirmed this finding. The expression of CjCBFs was tissue-specific: CBF1 was mainly expressed in leaves, and CBF3 was mainly expressed in stem. The responses of the four CjCBFs to drought and high temperature and the effect of light were also characterized. Our study provides new insight into the role of CBFs in the cold response in C. japonica (Naidong).

The Arabidopsis circadian clock protein PRR5 interacts with and stimulates ABI5 to modulate abscisic acid signaling during seed germination

Seed germination and postgerminative growth require the precise coordination of multiple intrinsic and environmental signals. The phytohormone abscisic acid (ABA) suppresses these processes in Arabidopsis thaliana and the circadian clock contributes to the regulation of ABA signaling. However, the molecular mechanism underlying circadian clock-mediated ABA signaling remains largely unknown. Here, we found that the core circadian clock proteins PSEUDO-RESPONSE REGULATOR5 (PRR5) and PRR7 physically associate with ABSCISIC ACID-INSENSITIVE5 (ABI5), a crucial transcription factor of ABA signaling. PRR5 and PRR7 positively modulate ABA signaling redundantly during seed germination. Disrupting PRR5 and PRR7 simultaneously rendered germinating seeds hyposensitive to ABA, whereas the overexpression of PRR5 enhanced ABA signaling to inhibit seed germination. Consistent with this, the expression of several ABA-responsive genes is upregulated by PRR proteins. Genetic analysis demonstrated that PRR5 promotes ABA signaling mainly dependently on ABI5. Further mechanistic investigation revealed that PRR5 stimulates the transcriptional function of ABI5 without affecting its stability. Collectively, our results indicate that these PRR proteins function synergistically with ABI5 to activate ABA responses during seed germination, thus providing a mechanistic understanding of how ABA signaling and the circadian clock are directly integrated through a transcriptional complex involving ABI5 and central circadian clock components.

Cut the noise or couple up: Coordinating circadian and synthetic clocks

Circadian clocks are important to much of life on Earth and are of inherent interest to humanity, implicated in fields ranging from agriculture and ecology to developmental biology and medicine. New techniques show that it is not simply the presence of clocks, but coordination between them that is critical for complex physiological processes across the kingdoms of life. Recent years have also seen impressive advances in synthetic biology to the point where parallels can be drawn between synthetic biological and circadian oscillators. This review will emphasize theoretical and experimental studies that have revealed a fascinating dichotomy of coupling and heterogeneity among circadian clocks. We will also consolidate the fields of chronobiology and synthetic biology, discussing key design principles of their respective oscillators.

Light exposure mediates circadian rhythms of rhizosphere microbial communities

Microbial community circadian rhythms have a broad influence on host health and even though light-induced environmental fluctuations could regulate microbial communities, the contribution of light to the circadian rhythms of rhizosphere microbial communities has received little attention. To address this gap, we monitored diel changes in the microbial communities in rice (Oryza sativa L.) rhizosphere soil under light-dark and constant dark regimes, identifying microbes with circadian rhythms caused by light exposure and microbial circadian clocks, respectively. While rhizosphere microbial communities displayed circadian rhythms under light-dark and constant dark regimes, taxa possessing circadian rhythms under the two conditions were dissimilar. Light exposure concealed microbial circadian clocks as a regulatory driver, leading to fewer ecological niches in light versus dark communities. These findings disentangle regulation mechanisms for circadian rhythms in the rice rhizosphere microbial communities and highlight the role of light-induced regulation of rhizosphere microbial communities.

The circadian-controlled PIF8-BBX28 module regulates petal senescence in rose flowers by governing mitochondrial ROS homeostasis at night

Reactive oxygen species (ROS) are unstable reactive molecules that are toxic to cells. Regulation of ROS homeostasis is crucial to protect cells from dysfunction, senescence, and death. In plant leaves, ROS are mainly generated from chloroplasts and are tightly temporally restricted by the circadian clock. However, little is known about how ROS homeostasis is regulated in nonphotosynthetic organs, such as petals. Here, we showed that hydrogen peroxide (H2O2) levels exhibit typical circadian rhythmicity in rose (Rosa hybrida) petals, consistent with the measured respiratory rate. RNA-seq and functional screening identified a B-box gene, RhBBX28, whose expression was associated with H2O2 rhythms. Silencing RhBBX28 accelerated flower senescence and promoted H2O2 accumulation at night in petals, while overexpression of RhBBX28 had the opposite effects. RhBBX28 influenced the expression of various genes related to respiratory metabolism, including the TCA cycle and glycolysis, and directly repressed the expression of SUCCINATE DEHYDROGENASE 1, which plays a central role in mitochondrial ROS (mtROS) homeostasis. We also found that PHYTOCHROME-INTERACTING FACTOR8 (RhPIF8) could activate RhBBX28 expression to control H2O2 levels in petals and thus flower senescence. Our results indicate that the circadian-controlled RhPIF8-RhBBX28 module is a critical player that controls flower senescence by governing mtROS homeostasis in rose.

Global Survey of Alternative Splicing in Rice by Direct RNA Sequencing During Reproductive Development: Landscape and Genetic Regulation

Alternative splicing is a widespread phenomenon, which generates multiple isoforms of the gene product. Reproductive development is the key process for crop production. Although numerous forms of alternative splicing have been identified in model plants, large-scale study of alternative splicing dynamics during reproductive development in rice has not been conducted. Here, we investigated alternative splicing of reproductive development of young panicles (YP), unfertilized florets (UF) and fertilized florets (F) in rice using direct RNA sequencing, small RNA sequencing, and degradome sequencing. We identified a total of 35,317 alternative splicing (AS) events, among which 67.2% splicing events were identified as novel alternative splicing events. Intron retention (IR) was the most abundant alternative splicing subtype. Splicing factors that differentially expressed and alternatively spliced could result in global alternative splicing. Global analysis of miRNAs-targets prediction revealed that alternative spliced transcripts affected miRNAs' targets during development. Degradome sequencing detected only 6.8% of the differentially alternative splicing transcripts, suggesting a productive transcripts generation during development. In addition, alternative splicing isoforms of Co-like, a transcription factor, interacted with Casein kinase 1-like protein HD1 (CKI) examined in luciferase assay, which could modulate normal male-floral organs development and flowering time. These results reveal that alternative splicing is intensely associated with developmental stages, and a high complexity of gene regulation.

Reactive oxygen species homeostasis and circadian rhythms in plants

Elucidation of the molecular mechanisms by which plants sense and respond to environmental stimuli that influence their growth and yield is a prerequisite for understanding the adaptation of plants to climate change. Plants are sessile organisms and one important factor for their successful acclimation is the temporal coordination of the 24 h daily cycles and the stress response. The crosstalk between second messengers, such as Ca2+, reactive oxygen species (ROS), and hormones is a fundamental aspect in plant adaptation and survival under environmental stresses. In this sense, the circadian clock, in conjunction with Ca2+- and hormone-signalling pathways, appears to act as an important mechanism controlling plant adaptation to stress. The relationship between the circadian clock and ROS-generating and ROS-scavenging mechanisms is still not fully understood, especially at the post-transcriptional level and in stress situations in which ROS levels increase and changes in cell redox state occur. In this review, we summarize the information regarding the relationship between the circadian clock and the ROS homeostasis network. We pay special attention not only to the transcriptional regulation of ROS-generating and ROS-scavenging enzymes, but also to the few studies that have been performed at the biochemical level and those conducted under stress conditions.

Interpreting machine learning models to investigate circadian regulation and facilitate exploration of clock function

The circadian clock is an internal molecular 24-h timer that is critical to life on Earth. We describe a series of artificial intelligence (AI)– and machine learning (ML)–based approaches that enable more cost-effective analysis and insight into circadian regulation and function. Throughout the manuscript, we illuminate what is inside the ML “black box” via explanation or interpretation of predictive ML models. Using this interpretation of our models, we derive biological insights into why a prediction was made, alongside accurate predictions. Most innovatively, we use only DNA sequence features for accurate circadian gene expression prediction. Using explainable AI, we define possible, responsible regulatory elements as we make these predictions; this critically requires no prior knowledge of regulatory elements.

The circadian clock is an important adaptation to life on Earth. Here, we use machine learning to predict complex, temporal, and circadian gene expression patterns in Arabidopsis. Most significantly, we classify circadian genes using DNA sequence features generated de novo from public, genomic resources, facilitating downstream application of our methods with no experimental work or prior knowledge needed. We use local model explanation that is transcript specific to rank DNA sequence features, providing a detailed profile of the potential circadian regulatory mechanisms for each transcript. Furthermore, we can discriminate the temporal phase of transcript expression using the local, explanation-derived, and ranked DNA sequence features, revealing hidden subclasses within the circadian class. Model interpretation/explanation provides the backbone of our methodological advances, giving insight into biological processes and experimental design. Next, we use model interpretation to optimize sampling strategies when we predict circadian transcripts using reduced numbers of transcriptomic timepoints. Finally, we predict the circadian time from a single, transcriptomic timepoint, deriving marker transcripts that are most impactful for accurate prediction; this could facilitate the identification of altered clock function from existing datasets.