The DIURNAL project: DIURNAL and circadian expression profiling, model-based pattern matching, and promoter analysis

Natural variation in LONELY GUY-Like 1 regulates rice grain weight under warmer night conditions

Global nighttime temperatures are rising at twice the rate of daytime temperatures and pose a challenge for rice (Oryza sativa) production. High nighttime temperature (HNT) stress affects rice yield by reducing grain weight, size, and fertility. Although the genes associated with these yield parameters have been identified and characterized under normal temperatures, the genetic basis of grain weight regulation under HNT stress remains less explored. We examined the natural variation for rice single grain weight (SGW) under HNT stress imposed during grain development. A genome-wide association analysis identified several loci associated with grain weight under HNT stress. A locus, SGW1, specific to HNT conditions resolved to LONELY GUY-Like 1 (LOGL1), which encodes a putative cytokinin-activation enzyme. We demonstrated that LOGL1 contributes to allelic variation at SGW1. Accessions with lower LOGL1 transcript abundance had higher grain weight under HNT. This was supported by the higher grain weight of logl1-mutants relative to the wild type under HNT. Compared to logl1-mutants, LOGL1 over-expressers showed increased sensitivity to HNT. We showed that LOGL1 regulates the thiamin biosynthesis pathway, which is under circadian regulation, which in turn is likely perturbed by HNT stress. These findings provide a genetic source to enhance rice adaptation to warming night temperatures and improve our mechanistic understanding of HNT stress tolerance pathways.

A circadian clock output functions independently of phyB to sustain daytime PIF3 degradation

The circadian clock is an endogenous oscillator, and its importance lies in its ability to impart rhythmicity on downstream biological processes, or outputs. Our knowledge of output regulation, however, is often limited to an understanding of transcriptional connections between the clock and outputs. For instance, the clock is linked to plant growth through the gating of photoreceptors via rhythmic transcription of the nodal growth regulators, PHYTOCHROME-INTERACTING FACTORs (PIFs), but the clock's role in PIF protein stability is less clear. Here, we identified a clock-regulated, F-box type E3 ubiquitin ligase, CLOCK-REGULATED F-BOX WITH A LONG HYPOCOTYL 1 (CFH1), that specifically interacts with and degrades PIF3 during the daytime. Additionally, genetic evidence indicates that CFH1 functions primarily in monochromatic red light, yet CFH1 confers PIF3 degradation independent of the prominent red-light photoreceptor phytochrome B (phyB). This work reveals a clock-mediated growth regulation mechanism in which circadian expression of CFH1 promotes sustained, daytime PIF3 degradation in parallel with phyB signaling.

Appropriate induction of TOC1 ensures optimal MYB44 expression in ABA signaling and stress response in Arabidopsis

Plants possess the remarkable ability to integrate the circadian clock with various signalling pathways, enabling them to quickly detect and react to both external and internal stress signals. However, the interplay between the circadian clock and biological processes in orchestrating responses to environmental stresses remains poorly understood. TOC1, a core component of the plant circadian clock, plays a vital role in maintaining circadian rhythmicity and participating in plant defences. Here, our study reveals a direct interaction between TOC1 and the promoter region of MYB44, a key gene involved in plant defence. TOC1 rhythmically represses MYB44 expression, thereby ensuring elevated MYB44 expression at dawn to help the plant in coping with lowest temperatures during diurnal cycles. Additionally, both TOC1 and MYB44 can be induced by cold stress in an Abscisic acid (ABA)-dependent and independent manner. TOC1 demonstrates a rapid induction in response to lower temperatures compared to ABA treatment, suggesting timely flexible regulation of TOC1-MYB44 regulatory module by the circadian clock in ensuring a proper response to diverse stresses and maintaining a balance between normal physiological processes and energy-consuming stress responses. Our study elucidates the role of TOC1 in effectively modulating expression of MYB44, providing insights into the regulatory network connecting the circadian clock, ABA signalling, and stress-responsive genes.

Arabidopsis seeds altered in the circadian clock protein TOC1 are characterized by higher level of linolenic acid

The circadian clock plays a critical role in regulating plant physiology and metabolism. However, the way in which the clock impacts the regulation of lipid biosynthesis in seeds is partially understood. In the present study, we characterized the seed fatty acid (FA) and glycerolipid (GL) compositions of pseudo-response regulator mutants. Among these mutants, toc1 (timing of cab expression 1) exhibited the most significant differences compared to control plants. These included an increase in total FA content, characterized by elevated levels of linolenic acid (18:3) along with a reduction in linoleic acid (18:2). Furthermore, our findings revealed that toc1 developing seeds showed increased expression of genes related to FA metabolism. Our results show a connection between TOC1 and lipid metabolism in Arabidopsis seeds.

PIF transcriptional regulators are required for rhythmic stomatal movements

Stomata govern the gaseous exchange between the leaf and the external atmosphere, and their function is essential for photosynthesis and the global carbon and oxygen cycles. Rhythmic stomata movements in daily dark/light cycles prevent water loss at night and allow CO2 uptake during the day. How the actors involved are transcriptionally regulated and how this might contribute to rhythmicity is largely unknown. Here, we show that morning stomata opening depends on the previous night period. The transcription factors PHYTOCHROME-INTERACTING FACTORS (PIFs) accumulate at the end of the night and directly induce the guard cell-specific K+ channel KAT1. Remarkably, PIFs and KAT1 are required for blue light-induced stomata opening. Together, our data establish a molecular framework for daily rhythmic stomatal movements under well-watered conditions, whereby PIFs are required for accumulation of KAT1 at night, which upon activation by blue light in the morning leads to the K+ intake driving stomata opening.

Spatiotemporal oxygen dynamics in young leaves reveal cyclic hypoxia in plants

Oxygen is essential for plant growth and development. Hypoxia occurs in plants due to limited oxygen availability following adverse environmental conditions as well in hypoxic niches in otherwise normoxic environments. However, the existence and functional integration of spatiotemporal oxygen dynamics with plant development remains unknown. In animal systems dynamic fluctuations in oxygen availability are known as cyclic hypoxia. In this study, we demonstrate that cyclic fluctuations in internal oxygen levels occur in young emerging leaves of Arabidopsis plants. Cyclic hypoxia in plants is based on a mechanism requiring the ETHYLENE RESPONSE FACTORS type VII (ERFVII) that are central components of the oxygen-sensing machinery in plants. The ERFVII-dependent mechanism allows precise adjustment of leaf growth in response to carbon status and oxygen availability within plant cells. This study thus establishes a functional connection between internal spatiotemporal oxygen dynamics and developmental processes of plants.

Core clock genes adjust growth cessation time to day-night switches in poplar

Poplar trees use photoperiod as a precise seasonal indicator, synchronizing plant phenology with the environment. Daylength cue determines FLOWERING LOCUS T 2 (FT2) daily expression, crucial for shoot apex development and establishment of the annual growing period. However, limited evidence exists for the molecular factors controlling FT2 transcription and the conservation with the photoperiodic control of Arabidopsis flowering. We demonstrate that FT2 expression mediates growth cessation response quantitatively, and we provide a minimal data-driven model linking core clock genes to FT2 daily levels. GIGANTEA (GI) emerges as a critical inducer of the FT2 activation window, time-bound by TIMING OF CAB EXPRESSION (TOC1) and LATE ELONGATED HYPOCOTYL (LHY2) repressions. CRISPR/Cas9 loss-of-function lines validate these roles, identifying TOC1 as a long-sought FT2 repressor. Additionally, model simulations predict that FT2 downregulation upon daylength shortening results from a progressive narrowing of this activation window, driven by the phase shift observed in the preceding clock genes. This circadian-mediated mechanism enables poplar to exploit FT2 levels as an accurate daylength-meter.

Unlocking allelic variation in circadian clock genes to develop environmentally robust and productive crops

MAIN CONCLUSION

Molecular mechanisms of biological rhythms provide opportunities to harness functional allelic diversity in core (and trait- or stress-responsive) oscillator networks to develop more climate-resilient and productive germplasm. The circadian clock senses light and temperature in day-night cycles to drive biological rhythms. The clock integrates endogenous signals and exogenous stimuli to coordinate diverse physiological processes. Advances in high-throughput non-invasive assays, use of forward- and inverse-genetic approaches, and powerful algorithms are allowing quantitation of variation and detection of genes associated with circadian dynamics. Circadian rhythms and phytohormone pathways in response to endogenous and exogenous cues have been well documented the model plant Arabidopsis. Novel allelic variation associated with circadian rhythms facilitates adaptation and range expansion, and may provide additional opportunity to tailor climate-resilient crops. The circadian phase and period can determine adaptation to environments, while the robustness in the circadian amplitude can enhance resilience to environmental changes. Circadian rhythms in plants are tightly controlled by multiple and interlocked transcriptional-translational feedback loops involving morning (CCA1, LHY), mid-day (PRR9, PRR7, PRR5), and evening (TOC1, ELF3, ELF4, LUX) genes that maintain the plant circadian clock ticking. Significant progress has been made to unravel the functions of circadian rhythms and clock genes that regulate traits, via interaction with phytohormones and trait-responsive genes, in diverse crops. Altered circadian rhythms and clock genes may contribute to hybrid vigor as shown in Arabidopsis, maize, and rice. Modifying circadian rhythms via transgenesis or genome-editing may provide additional opportunities to develop crops with better buffering capacity to environmental stresses. Models that involve clock gene‒phytohormone‒trait interactions can provide novel insights to orchestrate circadian rhythms and modulate clock genes to facilitate breeding of all season crops.

Plants distinguish different photoperiods to independently control seasonal flowering and growth

Plants measure daylength (photoperiod) to regulate seasonal growth and flowering. Photoperiodic flowering has been well studied, but less is known about photoperiodic growth. By using a mutant with defects in photoperiodic growth, we identified a seasonal growth regulation pathway that functions in long days in parallel to the canonical long-day photoperiod flowering mechanism. This is achieved by using distinct mechanisms to detect different photoperiods: The flowering pathway measures photoperiod as the duration of light intensity, whereas the growth pathway measures photoperiod as the duration of photosynthetic activity (photosynthetic period). Plants can then independently control expression of genes required for flowering or growth. This demonstrates that seasonal flowering and growth are dissociable, allowing them to be coordinated independently across seasons.

Environmental F actors coordinate circadian clock function and rhythm to regulate plant development

Changes in the external environment necessitate plant growth plasticity, with environmental signals such as light, temperature, and humidity regulating growth and development. The plant circadian clock is a biological time keeper that can be "reset" to adjust internal time to changes in the external environment. Exploring the regulatory mechanisms behind plant acclimation to environmental factors is important for understanding how plant growth and development are shaped and for boosting agricultural production. In this review, we summarize recent insights into the coordinated regulation of plant growth and development by environmental signals and the circadian clock, further discussing the potential of this knowledge.

The eINTACT method for studying nuclear changes in host plant cells targeted by bacterial effectors in native infection contexts

Type-III effector proteins are major virulence determinants that most gram-negative bacteria inject into host cells to manipulate cellular processes for infection. Because effector-targeted cells are embedded and underrepresented in infected plant tissues, it is technically challenging to isolate them for focused studies of effector-induced cellular changes. This protocol describes a novel technique, effector-inducible isolation of nuclei tagged in specific cell types (eINTACT), for isolating biotin-labeled nuclei from Arabidopsis plant cells that have received Xanthomonas bacterial effectors by using streptavidin-coated magnetic beads. This protocol is an extension of the existing Nature Protocols Protocol of the INTACT method for the affinity-based purification of nuclei of specific cell types in the context of developmental biology. In a phytopathology scenario, our protocol addresses how to obtain eINTACT transgenic lines and compatible bacterial mutants, verify the eINTACT system and purify nuclei of bacterial effector-recipient cells from infected tissues. Differential analyses of purified nuclei from plants infected by bacteria expressing the effector of interest and those from plants infected by effector-deletion bacterial mutants will reveal the effector-dependent nuclear changes in targeted host cells. Provided that the eINTACT system is available, the infection experiment takes 5 d, and the procedures, from collecting bacteria-infected leaves to obtaining nuclei of effector-targeted cells, can be completed in 4 h. eINTACT is a unique method for isolating high-quality nuclei from bacterial effector-targeted host cells in native infection contexts. This method is adaptable to study the functions of type-III effectors from numerous gram-negative bacteria in host plants that are amenable to transformation.

The Circadian Clock Coordinates the Tradeoff between Adaptation to Abiotic Stresses and Yield in Crops

Plants have evolved a circadian clock to adapt to ever-changing diel and seasonal environmental conditions. The circadian clock is generally considered an internal system that has evolved to adapt to cyclic environmental cues, especially diel light and temperature changes, which is essential for higher plants as they are sessile organisms. This system receives environmental signals as input pathways which are integrated by circadian core oscillators to synchronize numerous output pathways, such as photosynthesis, the abiotic stress response, metabolism, and development. Extreme temperatures, salinity, and drought stresses cause huge crop losses worldwide, imposing severe pressure on areas of agricultural land. In crop production, the circadian system plays a significant role in determining flowering time and responding to external abiotic stresses. Extensive studies over the last two decades have revealed that the circadian clock can help balance the tradeoff between crop yield-related agronomic traits and adaptation to stress. Herein, we focus on summarizing how the circadian clock coordinates abiotic stress responses and crop yield. We also propose that there might be an urgent need to better utilize circadian biology in the future design of crop breeding to achieve high yields under stress conditions.

The circadian clock regulates PIF3 protein stability in parallel to red light

The circadian clock is an endogenous oscillator, but its importance lies in its ability to impart rhythmicity on downstream biological processes or outputs. Focus has been placed on understanding the core transcription factors of the circadian clock and how they connect to outputs through regulated gene transcription. However, far less is known about posttranslational mechanisms that tether clocks to output processes through protein regulation. Here, we identify a protein degradation mechanism that tethers the clock to photomorphogenic growth. By performing a reverse genetic screen, we identify a clock-regulated F-box type E3 ubiquitin ligase, CLOCK-REGULATED F-BOX WITH A LONG HYPOCOTYL 1 ( CFH1 ), that controls hypocotyl length. We then show that CFH1 functions in parallel to red light signaling to target the transcription factor PIF3 for degradation. This work demonstrates that the circadian clock is tethered to photomorphogenesis through the ubiquitin proteasome system and that PIF3 protein stability acts as a hub to integrate information from multiple environmental signals.

Dynamic growth QTL action in diverse light environments: characterization of light regime-specific and stable QTL in Arabidopsis

Plant growth is a complex process affected by a multitude of genetic and environmental factors and their interactions. To identify genetic factors influencing plant performance under different environmental conditions, vegetative growth was assessed in Arabidopsis thaliana cultivated under constant or fluctuating light intensities, using high-throughput phenotyping and genome-wide association studies. Daily automated non-invasive phenotyping of a collection of 382 Arabidopsis accessions provided growth data during developmental progression under different light regimes at high temporal resolution. Quantitative trait loci (QTL) for projected leaf area, relative growth rate, and PSII operating efficiency detected under the two light regimes were predominantly condition-specific and displayed distinct temporal activity patterns, with active phases ranging from 2 d to 9 d. Eighteen protein-coding genes and one miRNA gene were identified as potential candidate genes at 10 QTL regions consistently found under both light regimes. Expression patterns of three candidate genes affecting projected leaf area were analysed in time-series experiments in accessions with contrasting vegetative leaf growth. These observations highlight the importance of considering both environmental and temporal patterns of QTL/allele actions and emphasize the need for detailed time-resolved analyses under diverse well-defined environmental conditions to effectively unravel the complex and stage-specific contributions of genes affecting plant growth processes.

Systematic characterization of photoperiodic gene expression patterns reveals diverse seasonal transcriptional systems in Arabidopsis

Photoperiod is an annual cue measured by biological systems to align growth and reproduction with the seasons. In plants, photoperiodic flowering has been intensively studied for over 100 years, but we lack a complete picture of the transcriptional networks and cellular processes that are photoperiodic. We performed a transcriptomics experiment on Arabidopsis plants grown in 3 different photoperiods and found that thousands of genes show photoperiodic alteration in gene expression. Gene clustering, daily expression integral calculations, and cis-element analysis then separate photoperiodic genes into co-expression subgroups that display 19 diverse seasonal expression patterns, opening the possibility that many photoperiod measurement systems work in parallel in Arabidopsis. Then, functional enrichment analysis predicts co-expression of important cellular pathways. To test these predictions, we generated a comprehensive catalog of genes in the phenylpropanoid biosynthesis pathway, overlaid gene expression data, and demonstrated that photoperiod intersects with 2 major phenylpropanoid pathways differentially, controlling flavonoids but not lignin. Finally, we describe the development of a new app that visualizes photoperiod transcriptomic data for the wider community.

Improved chilling tolerance in glasshouse-grown potted sweet basil by end-of-production, short-duration supplementary far red light

Sweet basil is a popular culinary herb used in many cuisines around the world and is widely grown commercially for retail as a live potted plant. However, basil is easily damaged by temperatures below 12 °C meaning plants must be transported from the grower to the retailer in a warm transport chain, adding considerable commercial cost in temperate countries. Improvement of chilling tolerance has been demonstrated in post-harvest crops such as tomato fruits and, indeed, fresh cut basil, by manipulation of the red:far red ratio of light provided to plants throughout the photoperiod and for a significant duration of the growing process in controlled environment chambers. We tested the effectiveness of periodic short-duration end-of-production supplementary far red light treatments designed for use with basil plants grown in a large scale commercial glasshouse for the live potted basil market. Four days of periodic, midday supplementary far red light given at end of production induced robust tolerance to 24 h of 4 °C cold treatment, resulting in greatly reduced visual damage, and reduced physiological markers of chilling injury including electrolyte leakage and reactive oxygen species accumulation. Antioxidant levels were also maintained at higher levels in live potted basil following this cold treatment. RNAseq-based analysis of gene expression changes associated with this response pointed to increased conversion of starch to soluble raffinose family oligosaccharide sugars; increased biosynthesis of anthocyanins and selected amino acids; inactivation of gibberellin signaling; and reduced expression of fatty acid desaturases, all previously associated with increased chilling tolerance in plants. Our findings offer an efficient, non-invasive approach to induce chilling tolerance in potted basil which is suitable for application in a large-scale commercial glasshouse.

Chilling stress drives organ-specific transcriptional cascades and dampens diurnal oscillation in tomato

Improving chilling tolerance in cold-sensitive crops, e.g. tomato, requires knowledge of the early molecular response to low temperature in these under-studied species. To elucidate early responding processes and regulators, we captured the transcriptional response at 30 minutes and 3 hours in the shoots and at 3 hours in the roots of tomato post-chilling from 24°C to 4°C. We used a pre-treatment control and a concurrent ambient temperature control to reveal that majority of the differential expression between cold and ambient conditions is due to severely compressed oscillation of a large set of diurnally regulated genes in both the shoots and roots. This compression happens within 30 minutes of chilling, lasts for the duration of cold treatment, and is relieved within 3 hours of return to ambient temperatures. Our study also shows that the canonical ICE1/CAMTA-to-CBF cold response pathway is active in the shoots, but not in the roots. Chilling stress induces synthesis of known cryoprotectants (trehalose and polyamines), in a CBF-independent manner, and induction of multiple genes encoding proteins of photosystems I and II. This study provides nuanced insights into the organ-specific response in a chilling sensitive plant, as well as the genes influenced by an interaction of chilling response and the circadian clock.

OsTOC1 plays dual roles in the regulation of plant circadian clock by functioning as a direct transcription activator or repressor

Plant clock function relies on precise timing of gene expression through complex regulatory networks consisting of activators and repressors at the core of oscillators. Although TIMING OF CAB EXPRESSION 1 (TOC1) has been recognized as a repressor involved in shaping oscillations and regulating clock-driven processes, its potential to directly activate gene expression remains unclear. In this study, we find that OsTOC1 primarily acts as a transcriptional repressor for core clock components, including OsLHY and OsGI. Here, we show that OsTOC1 possesses the ability to directly activate the expression of circadian target genes. Through binding to the promoters of OsTGAL3a/b, transient activation of OsTOC1 induces the expression of OsTGAL3a/b, indicating its role as an activator contributing to pathogen resistance. Moreover, TOC1 participates in regulating multiple yield-related traits in rice. These findings suggest that TOC1's function as a transcriptional repressor is not inherent, providing flexibility to circadian regulations, particularly in outputs.

New Horizons in Plant Photoperiodism

Photoperiod-measuring mechanisms allow organisms to anticipate seasonal changes to align reproduction and growth with appropriate times of the year. This review provides historical and modern context to studies of plant photoperiodism. We describe how studies of photoperiodic flowering in plants led to the first theoretical models of photoperiod-measuring mechanisms in any organism. We discuss how more recent molecular genetic studies in Arabidopsis and rice have revisited these concepts. We then discuss how photoperiod transcriptomics provides new lessons about photoperiodic gene regulatory networks and the discovery of noncanonical photoperiod-measuring systems housed in metabolic networks of plants. This leads to an examination of nonflowering developmental processes controlled by photoperiod, including metabolism and growth. Finally, we highlight the importance of understanding photoperiodism in the context of climate change, delving into the rapid latitudinal migration of plant species and the potential role of photoperiod-measuring systems in generating photic barriers during migration.

Submergence Stress Alters the Expression of Clock Genes and Configures New Zeniths and Expression of Outputs in Brachypodium distachyon

Plant networks of oscillating genes coordinate internal processes with external cues, contributing to increased fitness. We hypothesized that the response to submergence stress may dynamically change during different times of the day. In this work, we determined the transcriptome (RNA sequencing) of the model monocotyledonous plant, Brachypodium distachyon, during a day of submergence stress, low light, and normal growth. Two ecotypes of differential tolerance, Bd21 (sensitive) and Bd21-3 (tolerant), were included. We submerged 15-day-old plants under a long-day diurnal cycle (16 h light/8 h dark) and collected samples after 8 h of submergence at ZT0 (dawn), ZT8 (midday), ZT16 (dusk), ZT20 (midnight), and ZT24 (dawn). Rhythmic processes were enriched both with up- and down-regulated genes, and clustering highlighted that the morning and daytime oscillator components (PRRs) show peak expression in the night, and a decrease in the amplitude of the clock genes (GI, LHY, RVE) was observed. Outputs included photosynthesis-related genes losing their known rhythmic expression. Up-regulated genes included oscillating suppressors of growth, hormone-related genes with new late zeniths (e.g., JAZ1, ZEP), and mitochondrial and carbohydrate signaling genes with shifted zeniths. The results highlighted genes up-regulated in the tolerant ecotype such as METALLOTHIONEIN3 and ATPase INHIBITOR FACTOR. Finally, we show by luciferase assays that Arabidopsis thaliana clock genes are also altered by submergence changing their amplitude and phase. This study can guide the research of chronocultural strategies and diurnal-associated tolerance mechanisms.

COLD REGULATED GENE 27 and 28 Antagonize the Transcriptional Activity of the RVE8/LNK1/LNK2 Circadian Complex

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

Parallel mechanisms detect different photoperiods to independently control seasonal flowering and growth in plants

For nearly 100 years, we have known that both growth and flowering in plants are seasonally regulated by the length of the day (photoperiod). Intense research focus and powerful genetic tools have propelled studies of photoperiodic flowering, but far less is known about photoperiodic growth, in part because tools were lacking. Here, using a new genetic tool that visually reports on photoperiodic growth, we identified a seasonal growth regulation pathway, from photoperiod detection to gene expression. Surprisingly, this pathway functions in long days but is distinct from the canonical long day photoperiod flowering mechanism. This is possible because the two mechanisms detect the photoperiod in different ways: flowering relies on measuring photoperiod by directly detecting duration of light intensity while the identified growth pathway relies on measuring photosynthetic period indirectly by detecting the duration of photosynthetic metabolite production. In turn, the two pathways then control expression of genes required for flowering or growth independently. Finally, our tools allow us to show that these two types of photoperiods, and their measurement systems, are dissociable. Our results constitute a new view of seasonal timekeeping in plants by showing that two parallel mechanisms measure different photoperiods to control plant growth and flowering, allowing these processes to be coordinated independently across seasons.

Current Insights into m6A RNA Methylation and Its Emerging Role in Plant Circadian Clock

N6-adenosine methylation (m⁶A) is a prevalent form of RNA modification found in the expressed transcripts of many eukaryotic organisms. Moreover, m⁶A methylation is a dynamic and reversible process that requires the functioning of various proteins and their complexes that are evolutionarily conserved between species and include methylases, demethylases, and m⁶A-binding proteins. Over the past decade, the m⁶A methylation process in plants has been extensively studied and the understanding thereof has drastically increased, although the regulatory function of some components relies on information derived from animal systems. Notably, m⁶A has been found to be involved in a variety of factors in RNA processing, such as RNA stability, alternative polyadenylation, and miRNA regulation. The circadian clock in plants is a molecular timekeeping system that regulates the daily and rhythmic activity of many cellular and physiological processes in response to environmental changes such as the day-night cycle. The circadian clock regulates the rhythmic expression of genes through post-transcriptional regulation of mRNA. Recently, m⁶A methylation has emerged as an additional layer of post-transcriptional regulation that is necessary for the proper functioning of the plant circadian clock. In this review, we have compiled and summarized recent insights into the molecular mechanisms behind m⁶A modification and its various roles in the regulation of RNA. We discuss the potential role of m⁶A modification in regulating the plant circadian clock and outline potential future directions for the study of mRNA methylation in plants. A deeper understanding of the mechanism of m⁶A RNA regulation and its role in plant circadian clocks will contribute to a greater understanding of the plant circadian clock.

Isolation of Nuclei Tagged in Specific Cell Types (INTACT) in Arabidopsis

Many functionally distinct plant tissues have relatively low numbers of cells that are embedded within complex tissues. For example, the shoot apical meristem (SAM) consists of a small population of pluripotent stem cells surrounded by developing leaves and/or flowers at the growing tip of the plant. It is technically challenging to collect enough high-quality SAM samples for molecular analyses. Isolation of Nuclei Tagged in specific Cell Types (INTACT) is an easily reproducible method that allows the enrichment of biotin-tagged cell-type-specific nuclei from the total nuclei pool using biotin-streptavidin affinity purification. Here, we provide a detailed INTACT protocol for isolating nuclei from the Arabidopsis SAM. One can also adapt this protocol to isolate nuclei from other tissues and cell types for investigating tissue/cell-type-specific transcriptome and epigenome and their changes during developmental programs at a high spatiotemporal resolution. Furthermore, due to its low cost and simple procedures, INTACT can be conducted in any standard molecular laboratory.

Time of Day Analysis over a Field Grown Developmental Time Course in Rice

Plants integrate time of day (TOD) information over an entire season to ensure optimal growth, flowering time, and grain fill. However, most TOD expression studies have focused on a limited number of combinations of daylength and temperature under laboratory conditions. Here, an Oryza sativa (rice) expression study that followed TOD expression in the field over an entire growing season was re-analyzed. Similar to Arabidopsis thaliana, almost all rice genes have a TOD-specific expression over the developmental time course. As has been suggested in other grasses, thermocycles were a stronger cue for TOD expression than the photocycles over the growing season. All the core circadian clock genes display consistent TOD expression over the season with the interesting exception that the two grass paralogs of EARLY FLOWERING 3 (ELF3) display a distinct phasing based on the interaction between thermo- and photo-cycles. The dataset also revealed how specific pathways are modulated to distinct TOD over the season consistent with the changing biology. The data presented here provide a resource for researchers to study how TOD expression changes under natural conditions over a developmental time course, which will guide approaches to engineer more resilient and prolific crops.

The 5'-3' mRNA Decay Pathway Modulates the Plant Circadian Network in Arabidopsis

Circadian rhythms enable organisms to anticipate and adjust their physiology to periodic environmental changes. These rhythms are controlled by biological clocks that consist of a set of clock genes that regulate each other's expression. Circadian oscillations in messenger RNA (mRNA) levels require the regulation of mRNA production and degradation. While transcription factors controlling clock function have been well characterized from cyanobacteria to humans, the role of factors controlling mRNA decay is largely unknown. Here, we show that mutations in SM-LIKE PROTEIN 1 (LSM1) and exoribonucleases 4 (XRN4), components of the 5'-3' mRNA decay pathway, alter clock function in Arabidopsis. We found that lsm1 and xrn4 mutants display long-period phenotypes for clock gene expression. In xrn4, these circadian defects were associated with changes in circadian phases of expression, but not overall mRNA levels, of several core-clock genes. We then used noninvasive transcriptome-wide mRNA stability analysis to identify genes and pathways regulated by XRN4. Among genes affected in the xrn4 mutant at the transcriptional and posttranscriptional level, we found an enrichment in genes involved in auxin, ethylene and drought recovery. Large effects were not observed for canonical core-clock genes, although the mRNAs of several auxiliary clock genes that control the pace of the clock were stabilized in xrn4 mutants. Our results establish that the 5'-3' mRNA decay pathway constitutes a novel posttranscriptional regulatory layer of the circadian gene network, which probably acts through a combination of small effects on mRNA stability of several auxiliary and some core-clock genes.

Genome-scale analysis of Arabidopsis splicing-related protein kinase families reveals roles in abiotic stress adaptation

Nearly 60 - 80 % of intron-containing plant genes undergo alternative splicing in response to either stress or plant developmental cues. RNA splicing is performed by a large ribonucleoprotein complex called the spliceosome in conjunction with associated subunits such as serine arginine (SR) proteins, all of which undergo extensive phosphorylation. In plants, there are three main protein kinase families suggested to phosphorylate core spliceosome subunits and related splicing factors based on orthology to human splicing-related kinases: the SERINE/ARGININE PROTEIN KINASES (SRPK), ARABIDOPSIS FUS3 COMPLEMENT (AFC), and Pre-mRNA PROCESSING FACTOR 4 (PRP4K) protein kinases. To better define the conservation and role(s) of these kinases in plants, we performed a genome-scale analysis of the three families across photosynthetic eukaryotes, followed by extensive transcriptomic and bioinformatic analysis of all Arabidopsis thaliana SRPK, AFC, and PRP4K protein kinases to elucidate their biological functions. Unexpectedly, this revealed the existence of SRPK and AFC phylogenetic groups with distinct promoter elements and patterns of transcriptional response to abiotic stress, while PRP4Ks possess no phylogenetic sub-divisions, suggestive of functional redundancy. We also reveal splicing-related kinase families are both diel and photoperiod regulated, implicating different orthologs as discrete time-of-day RNA splicing regulators. This foundational work establishes a number of new hypotheses regarding how reversible spliceosome phosphorylation contributes to both diel plant cell regulation and abiotic stress adaptation in plants.

CAST-R: An application to visualize circadian and heat stress-responsive genes in plants

The circadian clock helps organisms to anticipate and coordinate gene regulatory responses to changes in environmental stimuli. Under stresses, both time of day and the circadian clock closely control the magnitude of plant responses. The identification of clock-regulated genes is, therefore, important when studying the influence of environmental factors. Here, we present CAST-R (Circadian And heat STress-Responsive), a "Shiny" application that allows users to identify and visualize circadian and heat stress-responsive genes in plants. More specifically, users can generate and export profiles and heatmaps representing transcript abundance of a single or of multiple Arabidopsis (Arabidopsis thaliana) genes over a 24-h time course, in response to heat stress and during recovery following the stress. The application also takes advantage of published Arabidopsis chromatin immunoprecipitation-sequencing datasets to visualize the connections between clock proteins and their targets in an interactive network. In addition, CAST-R offers the possibility to perform phase (i.e. timing of expression) enrichment analyses for rhythmic datasets from any species, within and beyond plants. This functionality combines statistical analyses and graphical representations to identify significantly over- and underrepresented phases within a subset of genes. Lastly, profiles of transcript abundance can be visualized from multiple circadian datasets generated in Arabidopsis, Brassica rapa, barley (Hordeum vulgare), and rice (Oryza sativa). In summary, CAST-R is a user-friendly interface that allows the rapid identification of circadian and stress-responsive genes through multiple modules of visualization. We anticipate that this tool will make it easier for users to obtain temporal and dynamic information on genes of interest that links plant responses to environmental signals.

Circadian autonomy and rhythmic precision of the Arabidopsis female reproductive organ

The plant circadian clock regulates essential biological processes including flowering time or petal movement. However, little is known about how the clock functions in flowers. Here, we identified the circadian components and transcriptional networks contributing to the generation of rhythms in pistils, the female reproductive organ. When detached from the rest of the flower, pistils sustain highly precise rhythms, indicating organ-specific circadian autonomy. Analyses of clock mutants and chromatin immunoprecipitation assays showed distinct expression patterns and specific regulatory functions for clock activators and repressors in pistils. Genetic interaction studies also suggested a hierarchy of the repressing activities that provide robustness and precision to the pistil clock. Globally, the circadian function in pistils primarily governs responses to environmental stimuli and photosynthesis and controls pistil growth and seed weight and production. Understanding the circadian intricacies in reproductive organs may prove useful for optimizing plant reproduction and productivity.

A genome for Cissus illustrates features underlying its evolutionary success in dry savannas

Cissus is the largest genus in Vitaceae and is mainly distributed in the tropics and subtropics. Crassulacean acid metabolism (CAM), a photosynthetic adaptation to the occurrence of succulent leaves or stems, indicates that convergent evolution occurred in response to drought stress during species radiation. Here we provide the chromosomal level assembly of Cissus rotundifolia (an endemic species in Eastern Africa) and a genome-wide comparison with grape to understand genome divergence within an ancient eudicot family. Extensive transcriptome data were produced to illustrate the genetics underpinning C. rotundifolia's ecological adaption to seasonal aridity. The modern karyotype and smaller genome of C. rotundifolia (n = 12, 350.69 Mb/1C), which lack further whole-genome duplication, were mainly derived from gross chromosomal rearrangements such as fusions and segmental duplications, and were sculpted by a very recent burst of retrotransposon activity. Bias in local gene amplification contributed to its remarkable functional divergence from grape, and the specific proliferated genes associated with abiotic and biotic responses (e.g. HSP-20, NBS-LRR) enabled C. rotundifolia to survive in a hostile environment. Reorganization of existing enzymes of CAM characterized as diurnal expression patterns of relevant genes further confer the ability to thrive in dry savannas.

Clock-Controlled and Cold-Induced CYCLING DOF FACTOR6 Alters Growth and Development in Arabidopsis

The circadian clock represents a critical regulatory network, which allows plants to anticipate environmental changes as inputs and promote plant survival by regulating various physiological outputs. Here, we examine the function of the clock-regulated transcription factor, CYCLING DOF FACTOR 6 (CDF6), during cold stress in Arabidopsis thaliana. We found that the clock gates CDF6 transcript accumulation in the vasculature during cold stress. CDF6 mis-expression results in an altered flowering phenotype during both ambient and cold stress. A genome-wide transcriptome analysis links CDF6 to genes associated with flowering and seed germination during cold and ambient temperatures, respectively. Analysis of key floral regulators indicates that CDF6 alters flowering during cold stress by repressing photoperiodic flowering components, FLOWERING LOCUS T (FT), CONSTANS (CO), and BROTHER OF FT (BFT). Gene ontology enrichment further suggests that CDF6 regulates circadian and developmental-associated genes. These results provide insights into how the clock-controlled CDF6 modulates plant development during moderate cold stress.

RhythmicDB: A Database of Predicted Multi-Frequency Rhythmic Transcripts

The physiology and behavior of living organisms are featured by time-related variations driven by molecular clockworks that arose during evolution stochastically and heterogeneously. Over the years, several high-throughput experiments were performed to evaluate time-dependent gene expression in different cell types across several species and experimental conditions. Here, these were retrieved, manually curated, and analyzed by two software packages, BioCycle and MetaCycle, to infer circadian or ultradian transcripts across different species. These transcripts were stored in RhythmicDB and made publically available.

Arabidopsis Circadian Clock Repress Phytochrome a Signaling

The plants' internal circadian clock can strongly influence phytochrome signaling in response to the changes in the external light environment. Phytochrome A (phyA) is the photoreceptor that mediates various far-red (FR) light responses. phyA signaling is modulated by FHY3 and FAR1, which directly activate the transcription of FHY1 and FHL, whose products are essential for light-induced phyA nuclear accumulation and subsequent light responses. However, the mechanisms by which the clock regulates phyA signaling are poorly understood. Here, we discovered that FHY1 expression is diurnally regulated, peaking in the middle of the day. Two Arabidopsis core clock components, CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and TIMING OF CAB EXPRESSION1 (TOC1), repress FHY3/FAR1-mediated FHY1/FHL activation. Consistently, the specific expression pattern of FHY1 under diurnal conditions is altered in cca1-1, toc1-101, CCA1, and TOC1 overexpression plants. Furthermore, far-red induced gene expression and particularly nuclear accumulation of phyA are compromised in TOC1 and CCA1 overexpression seedlings. Our results therefore revealed a previously unidentified FHY1 expression pattern in diurnal cycles, which is negatively regulated by CCA1 and TOC1.

Phosphorylation of RNA Polymerase II by CDKC;2 Maintains the Arabidopsis Circadian Clock Period

The circadian clock is an internal timekeeping system that governs about 24 h biological rhythms of a broad range of developmental and metabolic activities. The clocks in eukaryotes are thought to rely on lineage-specific transcriptional-translational feedback loops. However, the mechanisms underlying the basic transcriptional regulation events for clock function have not yet been fully explored. Here, through a combination of chemical biology and genetic approaches, we demonstrate that phosphorylation of RNA polymerase II by CYCLIN DEPENDENT KINASE C; 2 (CDKC;2) is required for maintaining the circadian period in Arabidopsis. Chemical screening identified BML-259, the inhibitor of mammalian CDK2/CDK5, as a compound lengthening the circadian period of Arabidopsis. Short-term BML-259 treatment resulted in decreased expression of most clock-associated genes. Development of a chemical probe followed by affinity proteomics revealed that BML-259 binds to CDKC;2. Loss-of-function mutations of cdkc;2 caused a long period phenotype. In vitro experiments demonstrated that the CDKC;2 immunocomplex phosphorylates the C-terminal domain of RNA polymerase II, and BML-259 inhibits this phosphorylation. Collectively, this study suggests that transcriptional activity maintained by CDKC;2 is required for proper period length, which is an essential feature of the circadian clock in Arabidopsis.

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.

Conservation of dynamic characteristics of transcriptional regulatory elements in periodic biological processes

Background

Cell and circadian cycles control a large fraction of cell and organismal physiology by regulating large periodic transcriptional programs that encompass anywhere from 15 to 80% of the genome despite performing distinct functions. In each case, these large periodic transcriptional programs are controlled by gene regulatory networks (GRNs), and it has been shown through genetics and chromosome mapping approaches in model systems that at the core of these GRNs are small sets of genes that drive the transcript dynamics of the GRNs. However, it is unlikely that we have identified all of these core genes, even in model organisms. Moreover, large periodic transcriptional programs controlling a variety of processes certainly exist in important non-model organisms where genetic approaches to identifying networks are expensive, time-consuming, or intractable. Ideally, the core network components could be identified using data-driven approaches on the transcriptome dynamics data already available.

Results

This study shows that a unified set of quantified dynamic features of high-throughput time series gene expression data are more prominent in the core transcriptional regulators of cell and circadian cycles than in their outputs, in multiple organism, even in the presence of external periodic stimuli. Additionally, we observe that the power to discriminate between core and non-core genes is largely insensitive to the particular choice of quantification of these features.

Conclusions

There are practical applications of the approach presented in this study for network inference, since the result is a ranking of genes that is enriched for core regulatory elements driving a periodic phenotype. In this way, the method provides a prioritization of follow-up genetic experiments. Furthermore, these findings reveal something unexpected—that there are shared dynamic features of the transcript abundance of core components of unrelated GRNs that control disparate periodic phenotypes.

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.

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.

A spatial model of the plant circadian clock reveals design principles for coordinated timing

Individual plant cells possess a genetic network, the circadian clock, that times internal processes to the day-night cycle. Mathematical models of the clock are typically either "whole-plant" that ignore tissue or cell type-specific clock behavior, or "phase-only" that do not include molecular components. To address the complex spatial coordination observed in experiments, here we implemented a clock network model on a template of a seedling. In our model, the sensitivity to light varies across the plant, and cells communicate their timing via local or long-distance sharing of clock components, causing their rhythms to couple. We found that both varied light sensitivity and long-distance coupling could generate period differences between organs, while local coupling was required to generate the spatial waves of clock gene expression observed experimentally. We then examined our model under noisy light-dark cycles and found that local coupling minimized timing errors caused by the noise while allowing each plant region to maintain a different clock phase. Thus, local sensitivity to environmental inputs combined with local coupling enables flexible yet robust circadian timing.

Day and night isotope labelling reveal metabolic pathway specific regulation of protein synthesis rates in Arabidopsis

Plants have a diurnal separation of metabolic fluxes and a need for differential maintenance of protein machinery in the day and night. To directly assess the output of the translation process and to estimate the ATP investment involved, the individual rates of protein synthesis and degradation of hundreds of different proteins need to be measured simultaneously. We quantified protein synthesis and degradation through pulse labelling with heavy hydrogen in Arabidopsis thaliana rosettes to allow such an assessment of ATP investment in leaf proteome homeostasis on a gene-by-gene basis. Light-harvesting complex proteins were synthesised and degraded much faster in the day (approximately 10:1), while carbon metabolism and vesicle trafficking components were translated at similar rates day or night. Few leaf proteins changed in abundance between the day and the night despite reduced protein synthesis rates at night, indicating that protein degradation rates are tightly coordinated. The data reveal how the pausing of photosystem synthesis and degradation at night allows the redirection of a decreased energy budget to a selective night-time maintenance schedule.

Timing to grow: roles of clock in thermomorphogenesis

Plants coordinate their growth and developmental programs with changes in temperature. This process is termed thermomorphogenesis. The underlying molecular mechanisms have begun to emerge in these nonstressful responses to adjustments in prevailing temperature. The circadian clock is an internal timekeeper that ensures growth, development, and fitness across a wide range of environmental conditions and it responds to thermal changes. Here, we highlight how the circadian clock gates thermoresponsive hypocotyl growth in plants, with an emphasis on different action mode of evening complex (EC) in thermomorphogenesis. We also discuss the biochemical and molecular mechanisms of EC in transducing temperature signals to the key integrator PIF4. This provides future perspectives on unanswered questions on EC-associated thermomorphogenesis.

Underwater CAM photosynthesis elucidated by Isoetes genome

To conserve water in arid environments, numerous plant lineages have independently evolved Crassulacean Acid Metabolism (CAM). Interestingly, Isoetes, an aquatic lycophyte, can also perform CAM as an adaptation to low CO2 availability underwater. However, little is known about the evolution of CAM in aquatic plants and the lack of genomic data has hindered comparison between aquatic and terrestrial CAM. Here, we investigate underwater CAM in Isoetes taiwanensis by generating a high-quality genome assembly and RNA-seq time course. Despite broad similarities between CAM in Isoetes and terrestrial angiosperms, we identify several key differences. Notably, Isoetes may have recruited the lesser-known 'bacterial-type' PEPC, along with the 'plant-type' exclusively used in other CAM and C4 plants for carboxylation of PEP. Furthermore, we find that circadian control of key CAM pathway genes has diverged considerably in Isoetes relative to flowering plants. This suggests the existence of more evolutionary paths to CAM than previously recognized.

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.

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.

A metabolic daylength measurement system mediates winter photoperiodism in plants

Plants have served as a preeminent study system for photoperiodism due to their propensity to flower in concordance with the seasons. A nearly singular focus on understanding photoperiodic flowering has prevented the discovery of other photoperiod measuring systems necessary for vegetative health. Here, we use bioinformatics to identify photoperiod-induced genes in Arabidopsis. We show that one, PP2-A13, is expressed exclusively in, and required for, plant fitness in short, winter-like photoperiods. We create a real-time photoperiod reporter, using the PP2-A13 promoter driving luciferase, and show that photoperiodic regulation is independent of the canonical CO/FT mechanism for photoperiodic flowering. We then reveal that photosynthesis combines with circadian-clock-controlled starch production to regulate cellular sucrose levels to control photoperiodic expression of PP2-A13. This work demonstrates the existence of a photoperiod measuring system housed in the metabolic network of plants that functions to control seasonal cellular health.

Inference of Gene Regulatory Network Uncovers the Linkage between Circadian Clock and Crassulacean Acid Metabolism in Kalanchoë fedtschenkoi

The circadian clock drives time-specific gene expression, enabling biological processes to be temporally controlled. Plants that conduct crassulacean acid metabolism (CAM) photosynthesis represent an interesting case of circadian regulation of gene expression as stomatal movement is temporally inverted relative to stomatal movement in C3 plants. The mechanisms behind how the circadian clock enabled physiological differences at the molecular level is not well understood. Recently, the rescheduling of gene expression was reported as a mechanism to explain how CAM evolved from C3. Therefore, we investigated whether core circadian clock genes in CAM plants were re-phased during evolution, or whether networks of phase-specific genes were simply re-wired to different core clock genes. We identified candidate core clock genes based on gene expression features and then applied the Local Edge Machine (LEM) algorithm to infer regulatory relationships between this new set of core candidates and known core clock genes in Kalanchoë fedtschenkoi. We further inferred stomata-related gene targets for known and candidate core clock genes and constructed a gene regulatory network for core clock and stomata-related genes. Our results provide new insight into the mechanism of circadian control of CAM-related genes in K. fedtschenkoi, facilitating the engineering of CAM machinery into non-CAM plants for sustainable crop production in water-limited environments.

Time of the day prioritizes the pool of translating mRNAs in response to heat stress

The circadian clock helps organisms to anticipate and coordinate gene regulatory responses to changes in environmental stimuli. Under growth limiting temperatures, the time of the day modulates the accumulation of polyadenylated mRNAs. In response to heat stress, plants will conserve energy and selectively translate mRNAs. How the clock and/or the time of the day regulates polyadenylated mRNAs bound by ribosomes in response to heat stress is unknown. In-depth analysis of Arabidopsis thaliana translating mRNAs found that the time of the day gates the response of approximately one-third of the circadian-regulated heat-responsive translatome. Specifically, the time of the day and heat stress interact to prioritize the pool of mRNAs in cue to be translated. For a subset of mRNAs, we observed a stronger gated response during the day, and preferentially before the peak of expression. We propose previously overlooked transcription factors (TFs) as regulatory nodes and show that the clock plays a role in the temperature response for select TFs. When the stress was removed, the redefined priorities for translation recovered within 1 h, though slower recovery was observed for abiotic stress regulators. Through hierarchical network connections between clock genes and prioritized TFs, our work provides a framework to target key nodes underlying heat stress tolerance throughout the day.

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.

Time Will Tell: Intercellular Communication in the Plant Clock

Multicellular organisms have evolved local and long-distance signaling mechanisms to synchronize development and response to stimuli among a complex network of cells, tissues, and organs. Biological timekeeping is one such activity that is suggested to be coordinated within an organism to anticipate and respond to daily and seasonal patterns in the environment. New research into the plant clock suggests circadian rhythms are communicated between cells and across long distances. However, further clarity is required on the nature of the signaling molecules and the mechanisms underlying signal translocation. Here we summarize the roles and properties of tissue-specific circadian rhythms, discuss the evidence for local and long-distance clock communication, and evaluate the potential signaling molecules and transport mechanisms involved in this system.