Global transcriptome analysis reveals circadian regulation of key pathways in plant growth and development

Unlocking Nature's Rhythms: Insights into Secondary Metabolite Modulation by the Circadian Clock

Plants, like many other living organisms, have an internal timekeeper, the circadian clock, which allows them to anticipate photoperiod rhythms and environmental stimuli to optimally adjust plant growth, development, and fitness. These fine-tuned processes depend on the interaction between environmental signals and the internal interactive metabolic network regulated by the circadian clock. Although primary metabolites have received significant attention, the impact of the circadian clock on secondary metabolites remains less explored. Transcriptome analyses revealed that many genes involved in secondary metabolite biosynthesis exhibit diurnal expression patterns, potentially enhancing stress tolerance. Understanding the interaction mechanisms between the circadian clock and secondary metabolites, including plant defense mechanisms against stress, may facilitate the development of stress-resilient crops and enhance targeted management practices that integrate circadian agricultural strategies, particularly in the face of climate change. In this review, we will delve into the molecular mechanisms underlying circadian rhythms of phenolic compounds, terpenoids, and N-containing compounds.

Timely symbiosis: circadian control of legume-rhizobia symbiosis

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialised polyploid cells within root nodules. This results in a mutualistic relationship whereby the plant host receives fixed nitrogen from the bacteria in exchange for dicarboxylic acids. This plant-microbe interaction requires the regulation of multiple metabolic and physiological processes in both the host and symbiont in order to achieve highly efficient symbiosis. Recent studies have showed that the success of symbiosis is influenced by the circadian clock of the plant host. Medicago and soybean plants with altered clock mechanisms showed compromised nodulation and reduced plant growth. Furthermore, transcriptomic analyses revealed that multiple genes with key roles in recruitment of rhizobia to plant roots, infection and nodule development were under circadian control, suggesting that appropriate timing of expression of these genes may be important for nodulation. There is also evidence for rhythmic gene expression of key nitrogen fixation genes in the rhizobium symbiont, and temporal coordination between nitrogen fixation in the bacterial symbiont and nitrogen assimilation in the plant host may be important for successful symbiosis. Understanding of how circadian regulation impacts on nodule establishment and function will identify key plant-rhizobial connections and regulators that could be targeted to increase the efficiency of this relationship.

The circadian clock and thermal regulation in plants: novel insights into the role of positive circadian clock regulators in temperature responses

The impact of rising global temperatures on crop yields is a serious concern, and the development of heat-resistant crop varieties is crucial for mitigating the effects of climate change on agriculture. To achieve this, a better understanding of the molecular basis of the thermal responses of plants is necessary. The circadian clock plays a central role in modulating plant biology in synchrony with environmental changes, including temperature fluctuations. Recent studies have uncovered the role of transcriptional activators of the core circadian network in plant temperature responses. This expert view highlights key novel findings regarding the role of the RVE and LNK gene families in controlling gene expression patterns and plant growth under different temperature conditions, ranging from regular diurnal oscillations to extreme stress temperatures. These findings reinforce the essential role of the circadian clock in plant adaptation to changing temperatures and provide a basis for future studies on crop improvement.

Promoting student interest in plant biology through an inquiry-based module exploring plant circadian rhythm, gene expression, and defense against insects

We present a weeklong curricular module for high school biology students that promotes knowledge of phytohormones, the circadian clock, and the Central Dogma. The module, which relies on easily accessible items and requires minimal space, integrates a hands-on experiment that guides students through replicating research examining circadian entrainment in postharvest cabbage from groceries. This work found that plants have cyclical, circadian expression of genes that produce phytohormones, and that such cyclical expression influences herbivory by caterpillars. Such cyclical patterns were found in plants both in situ and in postharvest cabbage. This work thus provides an ideal platform to shape student conceptions of circadian rhythms, gene expression, and plant herbivory by having students use light timers to entrain postharvest cabbage to alternating light and dark cycles and then measuring herbivory in these plants. The results should replicate previous work and demonstrate less herbivory when both plant and caterpillar are entrained to the same light and dark cycles since the expression of phytohormones involved in plant defense will be greatest when caterpillars are active. The module then concludes with a discussion of gene regulation and how this influences phytohormones. This module was field tested at four public schools, reaching over 600 students, and we present data demonstrating that the module led to learning gains and likely increases in interest in plant biology and self-efficacy.

Deoxyxylulose 5-Phosphate Synthase Does Not Play a Major Role in Regulating the Methylerythritol 4-Phosphate Pathway in Poplar

The plastidic 2-C-methylerythritol 4-phosphate (MEP) pathway supplies the precursors of a large variety of essential plant isoprenoids, but its regulation is still not well understood. Using metabolic control analysis (MCA), we examined the first enzyme of this pathway, 1-deoxyxylulose 5-phosphate synthase (DXS), in multiple grey poplar (Populus × canescens) lines modified in their DXS activity. Single leaves were dynamically labeled with 13CO2 in an illuminated, climate-controlled gas exchange cuvette coupled to a proton transfer reaction mass spectrometer, and the carbon flux through the MEP pathway was calculated. Carbon was rapidly assimilated into MEP pathway intermediates and labeled both the isoprene released and the IDP+DMADP pool by up to 90%. DXS activity was increased by 25% in lines overexpressing the DXS gene and reduced by 50% in RNA interference lines, while the carbon flux in the MEP pathway was 25-35% greater in overexpressing lines and unchanged in RNA interference lines. Isoprene emission was also not altered in these different genetic backgrounds. By correlating absolute flux to DXS activity under different conditions of light and temperature, the flux control coefficient was found to be low. Among isoprenoid end products, isoprene itself was unchanged in DXS transgenic lines, but the levels of the chlorophylls and most carotenoids measured were 20-30% less in RNA interference lines than in overexpression lines. Our data thus demonstrate that DXS in the isoprene-emitting grey poplar plays only a minor part in controlling flux through the MEP pathway.

The Cissus quadrangularis genome reveals its adaptive features in an arid habitat

Cissus quadrangularis is a tetraploid species belonging to the Vitaceae family and is known for the Crassulacean acid metabolism (CAM) pathway in the succulent stem, while the leaves perform C3 photosynthesis. Here, we report a high-quality genome of C. quadrangularis comprising a total size of 679.2 Mb which was phased into two subgenomes. Genome annotation identified 51 857 protein-coding genes, while approximately 47.75% of the genome was composed of repetitive sequences. Gene expression ratios of two subgenomes demonstrated that the sub-A genome as the dominant subgenome played a vital role during the drought tolerance. Genome divergence analysis suggests that the tetraploidization event occurred around 8.9 million years ago. Transcriptome data revealed that pathways related to cutin, suberine, and wax metabolism were enriched in the stem during drought treatment, suggesting that these genes contributed to the drought adaption. Additionally, a subset of CAM-related genes displayed diurnal expression patterns in the succulent stems but not in leaves, indicating that stem-biased expression of existing genes contributed to the CAM evolution. Our findings provide insights into the mechanisms of drought adaptation and photosynthesis transition in plants.

Combining PSII photochemistry and hydraulics improves predictions of photosynthesis and water use from mild to lethal drought

Rising temperatures and increases in drought negatively impact the efficiency and sustainability of both agricultural and forest ecosystems. Although hydraulic limitations on photosynthesis have been extensively studied, a solid understanding of the links between whole plant hydraulics and photosynthetic processes at the cellular level under changing environmental conditions is still missing, hampering our predictive power for plant mortality. Here, we examined plant hydraulic traits and CO2 assimilation rate under progressive water limitation by implementing Photosystem II (PSII) dynamics with a whole plant process model (TREES). The photosynthetic responses to plant water status were parameterized based on measurements of chlorophyll a fluorescence, gas exchange and water potential for Brassica rapa (R500) grown in a greenhouse under fully watered to lethal drought conditions. The updated model significantly improved predictions of photosynthesis, stomatal conductance and leaf water potential. TREES with PSII knowledge predicted a larger hydraulic safety margin and a decrease in percent loss of conductivity. TREES predicted a slower decrease in leaf water potential, which agreed with measurements. Our results highlight the pressing need for incorporating PSII drought photochemistry into current process models to capture cross-scale plant water dynamics from cell to whole plant level.

Protocols for in vitro reconstitution of the cyanobacterial circadian clock

Circadian clocks are intracellular systems that orchestrate metabolic processes in anticipation of sunrise and sunset by providing an internal representation of local time. Because the ~24-h metabolic rhythms they produce are important to health across diverse life forms there is growing interest in their mechanisms. However, mechanistic studies are challenging in vivo due to the complex, that is, poorly defined, milieu of live cells. Recently, we reconstituted the intact circadian clock of cyanobacteria in vitro. It oscillates autonomously and remains phase coherent for many days with a fluorescence-based readout that enables real-time observation of individual clock proteins and promoter DNA simultaneously under defined conditions without user intervention. We found that reproducibility of the reactions required strict adherence to the quality of each recombinant clock protein purified from Escherichia coli. Here, we provide protocols for preparing in vitro clock samples so that other labs can ask questions about how changing environments, like temperature, metabolites, and protein levels are reflected in the core oscillator and propagated to regulation of transcription, providing deeper mechanistic insights into clock biology.

The interplay between the circadian clock and abiotic stress responses mediated by ABF3 and CCA1/LHY

Climate change is a global concern for all life on our planet, including humans and plants. Plants' growth and development are significantly affected by abiotic stresses, including adverse temperature, inadequate or excess water availability, nutrient deficiency, and salinity. The circadian clock is a master regulator of numerous developmental and metabolic processes in plants. In an effort to identify new clock-related genes and outputs through bioinformatic analysis, we have revealed that CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) play a crucial role in regulating a wide range of abiotic stress responses and target ABSCISIC ACID RESPONSIVE ELEMENTS-BINDING FACTOR3 (ABF3), a key transcription factor in the plant hormone Abscisic acid (ABA)-signaling pathway. Specifically, we found that CCA1 and LHY regulate the expression of ABF3 under diel conditions, as well as seed germination under salinity. Conversely, ABF3 controls the expression of core clock genes and orchestrates the circadian period in a stress-responsive manner. ABF3 delivers the stress signal to the central oscillator by binding to the promoter of CCA1 and LHY. Overall, our study uncovers the reciprocal regulation between ABF3 and CCA1/LHY and molecular mechanisms underlying the interaction between the circadian clock and abiotic stress. This finding may aid in developing molecular and genetic solutions for plants to survive and thrive in the face of climate change.

Time-dependent regulation of respiration is widespread across plant evolution

Establishing the temperature dependence of respiration is critical for accurate predictions of the global carbon cycle under climate change. Diurnal temperature fluctuations, or changes in substrate availability, lead to variations in leaf respiration. Additionally, recent studies hint that the thermal sensitivity of respiration could be time-dependent. However, the role for endogenous processes, independent from substrate availability, as drivers of temporal changes in the sensitivity of respiration to temperature across phylogenies has not yet been addressed. Here, we examined the diurnal variation in the response of respiration to temperatures (R-T relationship) for different lycophyte, fern, gymnosperm and angiosperm species. We tested whether time-dependent changes in the R-T relationship would impact leaf level respiration modelling. We hypothesized that interactions between endogenous processes, like the circadian clock, and leaf respiration would be independent from changes in substrate availability. Overall, we observed a time-dependent sensitivity in the R-T relationship across phylogenies, independent of temperature, that affected modelling parameters. These results are compatible with circadian gating of respiration, but further studies should analyse the possible involvement of the clock. Our results indicate time-dependent regulation of respiration might be widespread across phylogenies, and that endogenous regulation of respiration is likely affecting leaf-level respiration fluxes.

The Roles of Circadian Clock Genes in Plant Temperature Stress Responses

Plants monitor day length and memorize changes in temperature signals throughout the day, creating circadian rhythms that support the timely control of physiological and metabolic processes. The DEHYDRATION-RESPONSE ELEMENT-BINDING PROTEIN 1/C-REPEAT BINDING FACTOR (DREB1/CBF) transcription factors are known as master regulators for the acquisition of cold stress tolerance, whereas PHYTOCHROME INTERACTING FACTOR 4 (PIF4) is involved in plant adaptation to heat stress through thermomorphogenesis. Recent studies have shown that circadian clock genes control plant responses to temperature. Temperature-responsive transcriptomes show a diurnal cycle and peak expression levels at specific times of throughout the day. Circadian clock genes play essential roles in allowing plants to maintain homeostasis by accommodating temperature changes within the normal temperature range or by altering protein properties and morphogenesis at the cellular level for plant survival and growth under temperature stress conditions. Recent studies revealed that the central oscillator genes CIRCADIAN CLOCK ASSOCIATED 1/LATE ELONGATED HYPOCOTYL (CCA1/LHY) and PSEUDO-RESPONSE REGULATOR5/7/9 (PRR5/7/9), as well as the EVENING COMPLEX (EC) genes REVEILLE4/REVEILLE8 (REV4/REV8), were involved in the DREB1 pathway of the cold signaling transcription factor and regulated the thermomorphogenesis gene PIF4. Further studies showed that another central oscillator, TIMING OF CAB EXPRESSION 1 (TOC1), and the regulatory protein ZEITLUPE (ZTL) are also involved. These studies led to attempts to utilize circadian clock genes for the acquisition of temperature-stress resistance in crops. In this review, we highlight circadian rhythm regulation and the clock genes involved in plant responses to temperature changes, as well as strategies for plant survival in a rapidly changing global climate.

Differential Response of MYB Transcription Factor Gene Transcripts to Circadian Rhythm in Tea Plants (Camellia sinensis)

The circadian clock refers to the formation of a certain rule in the long-term evolution of an organism, which is an invisible 'clock' in the body of an organism. As one of the largest TF families in higher plants, the MYB transcription factor is involved in plant growth and development. MYB is also inextricably correlated with the circadian rhythm. In this study, the transcriptome data of the tea plant 'Baiyeyihao' were measured at a photoperiod interval of 4 h (24 h). A total of 25,306 unigenes were obtained, including 14,615 unigenes that were annotated across 20 functional categories within the GO classification. Additionally, 10,443 single-gene clusters were annotated to 11 sublevels of metabolic pathways using KEGG. Based on the results of gene annotation and differential gene transcript analysis, 22 genes encoding MYB transcription factors were identified. The G10 group in the phylogenetic tree had 13 members, of which 5 were related to the circadian rhythm, accounting for 39%. The G1, G2, G8, G9, G15, G16, G18, G19, G20, G21 and G23 groups had no members associated with the circadian rhythm. Among the 22 differentially expressed MYB transcription factors, 3 members of LHY, RVE1 and RVE8 were core circadian rhythm genes belonging to the G10, G12 and G10 groups, respectively. Real-time fluorescence quantitative PCR was used to detect and validate the expression of the gene transcripts encoding MYB transcription factors associated with the circadian rhythm.

A Luciferase Imaging-Based Assay for Studying Temperature Compensation of the Circadian Clock

The pace of circadian rhythms remains relatively unchanged across a physiologically relevant range of temperatures, a phenomenon known as temperature compensation. Temperature compensation is a defining characteristic of circadian rhythms, ensuring that clock-regulated processes occur at approximately the same time of day across a wide range of conditions. Despite the identification of several genes involved in the regulation of temperature compensation, the molecular mechanisms underlying this process are still not well understood. High-throughput assays of circadian period are essential for the investigation of temperature compensation. In this chapter, we present a luciferase imaging-based method that enables robust and accurate examination of temperature compensation in the plant circadian clock.

Loss of Diel Circadian Clock Gene Cycling Is a Part of Grape Berry Ripening

Diel cycles of gene expression are thought to adapt plants to 24-h changes in environmental conditions. The circadian clock contributes to this process, but less is known about circadian programs in developing reproductive organs. While model plants and controlled conditions have contributed greatly to our knowledge of circadian clock function, there is a need to better understand its role in crop plants under field conditions with fluctuating light and temperature. In this study, we investigated changes in the circadian clock during the development of grape berries of Vitis vinifera L. We found that the transcripts of circadian clock homologs had high-amplitude oscillations prior to, but not during, ripening. As ripening progressed, the amplitude and rhythmicity of the diel oscillations decreased until most transcripts tested had no significant fluctuation over the 24-h cycle. Despite this loss of rhythmicity, the majority of circadian clock genes investigated were expressed at or near their abundance at the nadir of their pre-ripening oscillation although the berries remained transcriptionally active. From this, it can be concluded that cycling of the canonical circadian clock appears unnecessary for berry ripening. Our data suggest that changes in circadian clock dynamics during reproductive organ development may have important functional consequences.

Improving rice eating and cooking quality by enhancing endogenous expression of a nitrogen-dependent floral regulator

Improving rice eating and cooking quality (ECQ) is one of the primary tasks in rice production to meet the rising demands of consumers. However, improving grain ECQ without compromising yield faces a great challenge under varied nitrogen (N) supplies. Here, we report the approach to upgrade rice ECQ by native promoter-controlled high expression of a key N-dependent floral and circadian clock regulator Nhd1. The amplification of endogenous Nhd1 abundance alters rice heading date but does not affect the entire length of growth duration, N use efficiency and grain yield under both low and sufficient N conditions. Enhanced expression of Nhd1 reduces amylose content, pasting temperature and protein content while increasing gel consistence in grains. Metabolome and transcriptome analyses revealed that increased expression of Nhd1 mainly regulates the metabolism of carbohydrates and amino acids in the grain filling stage. Moreover, expression level of Nhd1 shows a positive relationship with grain ECQ in some local main cultivars. Thus, intensifying endogenous abundance of Nhd1 is a promising strategy to upgrade grain ECQ in rice production.

Multiomics Analysis Reveals the Involvement of JsLHY in Controlling Aroma Production in Jasmine Flowers

The Jasminum sambac flower is famous for its rich fragrance. However, our knowledge of the regulatory network for its aroma formation remains largely unknown and therefore needs further study. To this end, an integrated analysis of the volatilomics and transcriptomics of jasmine flowers at different flowering stages was performed. The results revealed many candidate transcription factors (TFs) may be involved in regulating the aroma formation of jasmine, among which the MYB-related TF LATE ELONGATED HYPOCOTYL (JsLHY) was identified as a hub gene. Using the DNA affinity purification sequencing method, dual-luciferase reporter, and yeast one-hybrid assays, we demonstrate that JsLHY can bind the gene promoter regions of six aroma-related structural genes (JsBEAT1, JsTPS34, JsCNL6, JsBPBT, JsAAAT5, and Js4CL7) and directly promote their expression. In addition, suppressing JsLHY expression decreased both the expression of JsLHY-bound genes and the content of related VOCs. The present study reveals how JsLHY participates in jasmine aroma formation.

Circadian regulation of metabolism across photosynthetic organisms

Circadian regulation produces a biological measure of time within cells. The daily cycle in the availability of light for photosynthesis causes dramatic changes in biochemical processes in photosynthetic organisms, with the circadian clock having crucial roles in adaptation to these fluctuating conditions. Correct alignment between the circadian clock and environmental day-night cycles maximizes plant productivity through its regulation of metabolism. Therefore, the processes that integrate circadian regulation with metabolism are key to understanding how the circadian clock contributes to plant productivity. This forms an important part of exploiting knowledge of circadian regulation to enhance sustainable crop production. Here, we examine the roles of circadian regulation in metabolic processes in source and sink organ structures of Arabidopsis. We also evaluate possible roles for circadian regulation in root exudation processes that deposit carbon into the soil, and the nature of the rhythmic interactions between plants and their associated microbial communities. Finally, we examine shared and differing aspects of the circadian regulation of metabolism between Arabidopsis and other model photosynthetic organisms, and between circadian control of metabolism in photosynthetic and non-photosynthetic organisms. This synthesis identifies a variety of future research topics, including a focus on metabolic processes that underlie biotic interactions within ecosystems.

Quantitative Time-Course Analysis of Osmotic and Salt Stress in Arabidopsis thaliana Using Short Gradient Multi-CV FAIMSpro BoxCar DIA

A major limitation when undertaking quantitative proteomic time-course experimentation is the tradeoff between depth-of-analysis and speed-of-analysis. In high complexity and high dynamic range sample types, such as plant extracts, balance between resolution and time is especially apparent. To address this, we evaluate multiple compensation voltage (CV) high field asymmetric waveform ion mobility spectrometry (FAIMSpro) settings using the latest label-free single-shot Orbitrap-based DIA acquisition workflows for their ability to deeply quantify the Arabidopsis thaliana seedling proteome. Using a BoxCarDIA acquisition workflow with a -30 -50 -70 CV FAIMSpro setting, we were able to consistently quantify >5000 Arabidopsis seedling proteins over a 21-min gradient, facilitating the analysis of ∼42 samples per day. Utilizing this acquisition approach, we then quantified proteome-level changes occurring in Arabidopsis seedling shoots and roots over 24 h of salt and osmotic stress, to identify early and late stress response proteins and reveal stress response overlaps. Here, we successfully quantify >6400 shoot and >8500 root protein groups, respectively, quantifying nearly ∼9700 unique protein groups in total across the study. Collectively, we pioneer a short gradient, multi-CV FAIMSpro BoxCarDIA acquisition workflow that represents an exciting new analysis approach for undertaking quantitative proteomic time-course experimentation in plants.

Diurnal gene oscillations modulated by RNA metabolism in tomato

Diurnal rhythms are known to regulate the expression of a large number of genes, coordinating plant growth and development with diel changes in light and temperature. However, the impact of RNA metabolism on rhythmic gene oscillations in plant is not yet fully understood. To address this question, we performed transcriptome and degradome profiling on tomato leaves at 6 time points during one 24 h cycle, using RNA-seq and genome-wide mapping of uncapped and cleavage transcripts (GMUCT). Time-series profiling of RNA-seq revealed 9342 diurnal-oscillated genes, which were enriched in various metabolic processes. To quantify the general level of RNA degradation for each gene, we utilized the Proportion Uncapped (PU) metric, which represents the GMUCT/RNA-seq ratio. Oscillated PU analysis revealed that 3885 genes were regulated by rhythmic RNA degradation. The RNA decay of these diurnal genes was highly coordinated with mRNA downregulation during oscillation, highlighting the critical role of internal transcription-degradation balance in rhythmic gene oscillation. Furthermore, we identified 2190 genes undergoing co-translational RNA decay (CTRD) with 5' phosphate read ends enriched at the boundary of ribosomes stalling at translational termination sites. Interestingly, diurnal-changed mRNAs with large amplitudes tended to be co-translationally decay, suggesting that CTRD contributed to the rapid turnover of diurnal mRNAs. Finally, we also identified several genes, whose miRNA cleavage efficiency oscillated in a diurnal manner. Taken together, these findings uncovered the vital functions of RNA metabolism, including rhythmic RNA degradation, CTRD, and miRNA cleavage, in modulating the diurnal mRNA oscillations during diel change at post-transcriptional level in tomato.

Time of day and genotype sensitivity adjust molecular responses to temperature stress in sorghum

Sorghum is one of the four major C4 crops that are considered to be tolerant to environmental extremes. Sorghum shows distinct growth responses to temperature stress depending on the sensitivity of the genetic background. About half of the transcripts in sorghum exhibit diurnal rhythmic expressions emphasizing significant coordination with the environment. However, an understanding of how molecular dynamics contribute to genotype-specific stress responses in the context of the time of day is not known. We examined whether temperature stress and the time of day impact the gene expression dynamics in thermo-sensitive and thermo-tolerant sorghum genotypes. We found that time of day is highly influencing the temperature stress responses, which can be explained by the rhythmic expression of most thermo-responsive genes. This effect is more pronounced in thermo-tolerant genotypes, suggesting a stronger regulation of gene expression by the time of day and/or by the circadian clock. Genotypic differences were mostly observed on average gene expression levels, which may be responsible for contrasting sensitivities to temperature stress in tolerant versus susceptible sorghum varieties. We also identified groups of genes altered by temperature stress in a time-of-day and genotype-specific manner. These include transcriptional regulators and several members of the Ca2+ -binding EF-hand protein family. We hypothesize that expression variation of these genes between genotypes along with time-of-day independent regulation may contribute to genotype-specific fine-tuning of thermo-responsive pathways. These findings offer a new opportunity to selectively target specific genes in efforts to develop climate-resilient crops based on their time-of-day and genotype variation responses to temperature stress.

Linking New Alleles at the Oscillator Loci to Flowering and Expansion of Asian Rice

The central oscillator is believed to be the key mechanism by which plants adapt to new environments. However, impacts from hybridization, the natural environment, and human selection have rarely been assessed on the oscillator of a crop. Here, from clearly identified alleles at oscillator loci (OsCCA1/LHY, OsPRR95, OsPRR37, OsPRR59, and OsPRR1) in ten diverse genomes of Oryza sativa, additional accessions, and functional analysis, we show that rice's oscillator was rebuilt primarily by new alleles from recombining parental sequences and subsequent 5' or/and coding mutations. New alleles may exhibit altered transcript levels from that of a parental allele and are transcribed variably among genetic backgrounds and natural environments in RIL lines. Plants carrying more expressed OsCCA1_a and less transcribed OsPRR1_e flower early in the paddy field. 5' mutations are instrumental in varied transcription, as shown by EMSA tests on one deletion at the 5' region of highly transcribed OsPRR1_a. Compared to relatively balanced mutations at oscillator loci of Arabidopsis thaliana, 5' mutations of OsPRR37 (and OsCCA1 to a less degree) were under negative selection while those of OsPRR1 alleles were under strong positive selection. Together, range expansion of Asian rice can be elucidated by human selection on OsPRR1 alleles via local flowering time-yield relationships.

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.

Acclimation of circadian rhythms in woodland strawberries (Fragaria vesca L.) to Arctic and mid-latitude photoperiods

BACKGROUND

Though many abiotic factors are constantly changing, the photoperiod is a predictable factor that enables plants to time many physiological responses. This timing is regulated by the circadian clock, yet little is known about how the clock adapts to the differences in photoperiod between mid-latitudes and high latitudes. The primary objective of this study was to compare how clock gene expression is modified in four woodland strawberry (Fragaria vesca L.) accessions originating from two different populations in Italy (IT1: Tenno, Italy, 45°N, IT4: Salorno, Italy, 46°N) and two in Northern Norway (NOR2: Alta, Norway, 69°N, NOR13: Indre Nordnes, Norway 69°N) when grown under simulated daylength conditions of an Arctic or mid-latitude photoperiod. The second objective was to investigate whether population origin or the difference in photoperiod influenced phytohormone accumulation.

RESULTS

The Arctic photoperiod induced lower expression in IT4 and NOR13 for six clock genes (FvLHY, FvRVE8, FvPRR9, FvPRR7, FvPRR5, and FvLUX), in IT1 for three genes (FvLHY, FvPRR9, and FvPRR5) and in NOR2 for one gene (FvPRR9). Free-running rhythms for FvLHY in IT1 and IT4 were higher after the Arctic photoperiod, while the free-running rhythm for FvLUX in IT4 was higher after the mid-latitude photoperiod. IT1 showed significantly higher expression of FvLHY and FvPRR9 than all other accessions, as well as significantly higher expression of the circadian regulated phytohormone, abscisic acid (ABA), but low levels of salicylic acid (SA). NOR13 had significantly higher expression of FvRVE8, FvTOC1, and FvLUX than all other accessions. NOR2 had extremely low levels of auxin (IAA) and high levels of the jasmonate catabolite, hydroxyjasmonic acid (OH-JA).

CONCLUSIONS

Our study shows that circadian rhythms in Fragaria vesca are driven by both the experienced photoperiod and genetic factors, while phytohormone levels are primarily determined by specific accessions' genetic factors rather than the experienced photoperiod.

SlSAMS1 enhances salt tolerance through regulation DNA methylation of SlGI in tomato

S-adenosylmethionine (SAM), which is synthesized from methionine and ATP catalyzed by S-adenosylmethionine synthetase (SAMS), is an important methyl donor in plants. SAMS and DNA methylation play an important role in the plant response to abiotic stresses. Previous studies have shown that SAMS improves salt tolerance in tomato plants, but it is not clear whether the DNA methylation pathway mediates SAMS-induced salt tolerance. This study confirmed that SlSAMS1-overexpressing plants exhibited improved salt tolerance. Through whole-genome bisulfite sequencing (WGBS) and transcriptome sequencing (RNA-seq) analysis, the study screened the circadian rhythm pathway and identified the gene SlGI in this pathway, which was regulated by SlSAMS1. The gene body region of SlGI, the core gene of the circadian rhythm pathway, was hypermethylated in SlSAMS1-overexpressing plants, and its expression level was significantly increased. Furthermore, the SlGI-overexpressing plants showed higher salt tolerance, less reduction in plant height and fresh weight, lower electrolyte leakage, malondialdehyde and H2O2 content, and higher antioxidant enzyme activity compared to wild type plants. Therefore, SlSAMS1-overexpressing plants regulated significant changes in CHG-type methylation sites of the SlGI gene body and its expression levels, leading to an enhanced salt tolerance of tomato plants.

Multiple light signaling pathways control solar tracking in sunflowers

Sunflowers are famous for their ability to track the sun throughout the day and then reorient at night to face east the following morning. This occurs by differential growth patterns, with the east sides of stems growing more during the day and the west sides of stems growing more at night. This process, termed heliotropism, is generally believed to be a specialized form of phototropism; however, the underlying mechanism is unknown. To better understand heliotropism, we compared gene expression patterns in plants undergoing phototropism in a controlled environment and in plants initiating and maintaining heliotropic growth in the field. We found the expected transcriptome signatures of phototropin-mediated phototropism in sunflower stems bending towards monochromatic blue light. Surprisingly, the expression patterns of these phototropism-regulated genes are quite different in heliotropic plants. Most genes rapidly induced during phototropism display only minor differences in expression across solar tracking stems. However, some genes that are both rapidly induced during phototropism and are implicated in growth responses to foliar shade are rapidly induced on the west sides of stems at the onset of heliotropism, suggesting a possible role for red light photoreceptors in solar tracking. To test the involvement of different photoreceptor signaling pathways in heliotropism, we modulated the light environment of plants initiating solar tracking. We found that depletion of either red and far-red light or blue light did not hinder the initiation or maintenance of heliotropism in the field. Together, our results suggest that the transcriptional regulation of heliotropism is distinct from phototropin-mediated phototropism and likely involves inputs from multiple light signaling pathways.

Genome-wide circadian gating of a cold temperature response in bread wheat

Circadian rhythms coordinate the responses of organisms with their daily fluctuating environments, by establishing a temporal program of gene expression. This schedules aspects of metabolism, physiology, development and behaviour according to the time of day. Circadian regulation in plants is extremely pervasive, and is important because it underpins both productivity and seasonal reproduction. Circadian regulation extends to the control of environmental responses through a regulatory process known as circadian gating. Circadian gating is the process whereby the circadian clock regulates the response to an environmental cue, such that the magnitude of response to an identical cue varies according to the time of day of the cue. Here, we show that there is genome-wide circadian gating of responses to cold temperatures in plants. By using bread wheat as an experimental model, we establish that circadian gating is crucial to the programs of gene expression that underlie the environmental responses of a crop of major socioeconomic importance. Furthermore, we identify that circadian gating of cold temperature responses are distributed unevenly across the three wheat subgenomes, which might reflect the geographical origins of the ancestors of modern wheat.

Characterizing the circadian connectome of Ocimum tenuiflorum using an integrated network theoretic framework

Across the three domains of life, circadian clock is known to regulate vital physiological processes, like, growth, development, defence etc. by anticipating environmental cues. In this work, we report an integrated network theoretic methodology comprising of random walk with restart and graphlet degree vectors to characterize genome wide core circadian clock and clock associated raw candidate proteins in a plant for which protein interaction information is available. As a case study, we have implemented this framework in Ocimum tenuiflorum (Tulsi); one of the most valuable medicinal plants that has been utilized since ancient times in the management of a large number of diseases. For that, 24 core clock (CC) proteins were mined in 56 template plant genomes to build their hidden Markov models (HMMs). These HMMs were then used to identify 24 core clock proteins in O. tenuiflorum. The local topology of the interologous Tulsi protein interaction network was explored to predict the CC associated raw candidate proteins. Statistical and biological significance of the raw candidates was determined using permutation and enrichment tests. A total of 66 putative CC associated proteins were identified and their functional annotation was performed.

Systems-level proteomics and metabolomics reveals the diel molecular landscape of diverse kale cultivars

Kale is a group of diverse Brassicaceae species that are nutritious leafy greens consumed for their abundance of vitamins and micronutrients. Typified by their curly, serrated and/or wavy leaves, kale varieties have been primarily defined based on their leaf morphology and geographic origin, despite having complex genetic backgrounds. Kale is a very promising crop for vertical farming due to its high nutritional content; however, being a non-model organism, foundational, systems-level analyses of kale are lacking. Previous studies in kale have shown that time-of-day harvesting can affect its nutritional composition. Therefore, to gain a systems-level diel understanding of kale across its wide-ranging and diverse genetic landscape, we selected nine publicly available and commercially grown kale cultivars for growth under near-sunlight LED light conditions ideal for vertical farming. We then analyzed changes in morphology, growth and nutrition using a combination of plant phenotyping, proteomics and metabolomics. As the diel molecular activities of plants drive their daily growth and development, ultimately determining their productivity as a crop, we harvested kale leaf tissue at both end-of-day (ED) and end-of-night (EN) time-points for all molecular analyses. Our results reveal that diel proteome and metabolome signatures divide the selected kale cultivars into two groups defined by their amino acid and sugar content, along with significant proteome differences involving carbon and nitrogen metabolism, mRNA splicing, protein translation and light harvesting. Together, our multi-cultivar, multi-omic analysis provides new insights into the molecular underpinnings of the diel growth and development landscape of kale, advancing our fundamental understanding of this nutritious leafy green super-food for horticulture/vertical farming applications.

Molecular Mapping to Discover Reliable Salinity-Resilient QTLs from the Novel Landrace Akundi in Two Bi-Parental Populations Using SNP-Based Genome-Wide Analysis in Rice

Achieving high-yield potential is always the ultimate objective of any breeding program. However, various abiotic stresses such as salinity, drought, cold, flood, and heat hampered rice productivity tremendously. Salinity is one of the most important abiotic stresses that adversely affect rice grain yield. The present investigation was undertaken to dissect new genetic loci, which are responsible for salt tolerance at the early seedling stage in rice. A bi-parental mapping population (F_2:3) was developed from the cross between BRRI dhan28/Akundi, where BRRI dhan28 (BR28) is a salt-sensitive irrigated (boro) rice mega variety and Akundi is a highly salinity-tolerant Bangladeshi origin indica rice landrace that is utilized as a donor parent. We report reliable and stable QTLs for salt tolerance from a common donor (Akundi) irrespective of two different genetic backgrounds (BRRI dhan49/Akundi and BRRI dhan28/Akundi). A robust 1k-Rice Custom Amplicon (1k-RiCA) SNP marker genotyping platform was used for genome-wide analysis of this bi-parental population. After eliminating markers with high segregation distortion, 886 polymorphic SNPs built a genetic linkage map covering 1526.5 cM of whole rice genome with an average SNP density of 1.72 cM for the 12 genetic linkage groups. A total of 12 QTLs for nine different salt tolerance-related traits were identified using QGene and inclusive composite interval mapping of additive and dominant QTL (ICIM-ADD) under salt stress on seven different chromosomes. All of these 12 new QTLs were found to be unique, as no other map from the previous study has reported these QTLs in the similar chromosomal location and found them different from extensively studied Saltol, SKC1, OsSalT, and salT locus. Twenty-eight significant digenic/epistatic interactions were identified between chromosomal regions linked to or unlinked to QTLs. Akundi acts like a new alternate donor source of salt tolerance except for other usually known donors such as Nona Bokra, Pokkali, Capsule, and Hasawi used in salt tolerance genetic analysis and breeding programs worldwide, including Bangladesh. Integration of the seven novel, reliable, stable, and background independent salinity-resilient QTLs (qSES1, qSL1, qRL1, qSUR1, qSL8, qK8, qK1) reported in this investigation will expedite the cultivar development that is highly tolerant to salt stress.

Genome-wide mapping of DNase I hypersensitive sites in pineapple leaves

Pineapple [Ananas comosus (L.) Merr.] is the most economically important crop possessing crassulacean acid metabolism (CAM) photosynthesis which has a higher water use efficiency by control of nocturnal opening and diurnal closure of stomata. To provide novel insights into the diel regulatory landscape in pineapple leaves, we performed genome-wide mapping of DNase I hypersensitive sites (DHSs) in pineapple leaves at day (2a.m.) and night (10a.m.) using a simplified DNase-seq method. As a result, totally 33340 and 28753 DHSs were found in green-tip tissue, and 29597 and 40068 were identified in white-base tissue at 2a.m. and 10a.m., respectively. We observed that majority of the pineapple genes occupied less than two DHSs with length shorter than 1 kb, and the promotor DHSs showed a proximal trend to the transcription start site (>77% promotor DHSs within 1 kb). In addition, more intergenic DHSs were identified around transcription factors or transcription co-regulators (TFs/TCs) than other functional genes, indicating complex regulatory contexts around TFs/TCs. Through combined analysis of tissue preferential DHSs and genes, we respectively found 839 and 888 coordinately changed genes in green-tip at 2a.m. and 10a.m. (AcG2 and AcG10). Furthermore, AcG2-specific, AcG10-specific and common accessible DHSs were dissected from the total photosynthetic preferential DHSs, and the regulatory networks indicated dynamic regulations with multiple cis-regulatory elements occurred to genes preferentially expressed in photosynthetic tissues. Interestingly, binding motifs of several cycling TFs were identified in the DHSs of key CAM genes, revealing a circadian regulation to CAM coordinately diurnal expression. Our results provide a chromatin regulatory landscape in pineapple leaves during the day and night. This will provide important information to assist with deciphering the circadian regulation of CAM photosynthesis.

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.

Multi-omics analyses reveal the crosstalk between the circadian clock and the response to herbicide application in Oryza sativa

Plants have evolved circadian clock systems that enable biological processes to occur in tandem with periodic changes in the environment. However, it is largely unknown whether crosstalk occurs between the circadian clock and the response to herbicide in rice. We identified 19 conserved rhythmic metabolites which were response to pesticide application and their metabolic abundance peaked mainly at ZT2 or ZT14-ZT18. We found a series of glyphosate, s-Metolachlor, fenclorim, metcamifen and GA3 response genes were expressed following stable circadian rhythms. In order to determine the patterns of their temporal expression, co-expression network analysis was done on 10,467 genes that were periodically expressed throughout a 24-hour period. Next, we identified 4,031 potential direct target genes of OsCCA1 in using DAP-seq data for OsCCA1. Of these, 339, 22, 53, 53 and 63 genes showed a response to glyphosate, s-Metolachlor, fenclorim, metcamifen and GA3 application, respectively. And they were mainly phased from dusk to midnight. Interestingly, we identified significant OsCCA1 binding peaks in the promoter regions of four herbicide resistance genes, including OsCYP81A12, OsCYP81E22, OsCYP76C2, and OsCYP76C4. Finally, we found that herbicide application could affects the expression of some of the central oscillator genes of the rice circadian clock. Here, we used multi-omics data to reveal the crosstalk between the circadian clock and herbicide response processes at the epigenomics, transcriptome, and metabolome levels in rice. This work will serve as a theoretical guide for identifying rhythmic herbicide targets, leading to the creation of new herbicides or the breeding of crops resistant to herbicides.

Constitutive expression of AtSINA2 from Arabidopsis improves grain yield, seed oil and drought tolerance in transgenic soybean

The SEVEN IN Absentia (SINA), a typical member of the RING E3 ligase family, plays a crucial role in plant growth, development and response to abiotic stress. However, its biological functions in oil crops are still unknown. Previously, we reported that overexpression of AtSINA2 in Arabidopsis positively regulated the drought tolerance of transgenic plants. In this work, we demonstrate that ectopic expression of AtSINA2 in soybean improved the shoot growth, grain yield, drought tolerance and seed oil content in transgenic plants. Compared to wild type, transgenic soybean produced greater shoot biomass and grain yield, and showed improved seed oil and drought tolerance. Physiological analyses exhibited that the increased drought tolerance of transgenic plants was accompanied with a higher chlorophyll content, and a lower malondialdehyde accumulation and water loss during drought stress. Further transcriptomic analyses revealed that the expressions of genes related to plant growth, flowering and stress response were up- or down-regulated in transgenic soybean under both normal and drought stress conditions. Our findings imply that AtSINA2 improved both agricultural production and drought tolerance, and it can be used as a candidate gene for the genetic engineering of new soybean cultivars with improved grain yield and drought resistance.

Role of long non-coding RNA in regulatory network response to Candidatus Liberibacter asiaticus in citrus

Long non-coding RNAs (lncRNAs) serve as crucial regulators in plant response to various diseases, while none have been systematically identified and characterized in response to citrus Huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus (CLas) bacteria. Here, we comprehensively investigated the transcriptional and regulatory dynamics of the lncRNAs in response to CLas. Samples were collected from leaf midribs of CLas- and mock-inoculated HLB-tolerant rough lemon (Citrus jambhiri) and HLB-sensitive sweet orange (C. sinensis) at week 0, 7, 17, and 34 following inoculation using CLas+ budwood of three biological replicates in the greenhouse. A total of 8,742 lncRNAs, including 2,529 novel lncRNAs, were identified from RNA-seq data with rRNA-removed from strand-specific libraries. Genomic variation analyses of conserved lncRNAs from 38 citrus accessions showed that 26 single nucleotide polymorphisms (SNPs) were significantly correlated with HLB. In addition, lncRNA-mRNA weighted gene co-expression network analysis (WGCNA) showed a significant module correlated with CLas-inoculation in rough lemon. Notably, the most significant LNC_28805 and multiple co-expressed genes related to plant defense in the module were targeted by miRNA5021, suggesting that LNC28805 might compete with endogenous miR5021 to maintain the homeostasis of immune gene expression levels. Candidate WRKY33 and SYP121 genes targeted by miRNA5021 were identified as two key hub genes interacting with bacteria pathogen response genes based on the prediction of protein-protein interaction (PPI) network. These two genes were also found within HLB-associated QTL in linkage group 6. Overall, our findings provide a reference for a better understanding of the role of lncRNAs involved in citrus HLB regulation.

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.

Limited water stress modulates expression of circadian clock genes in Brachypodium distachyon roots

Organisms have evolved a circadian clock for the precise timing of their biological processes. Studies primarily on model dicots have shown the complexity of the inner timekeeper responsible for maintaining circadian oscillation in plants and have highlighted that circadian regulation is more than relevant to a wide range of biological processes, especially organ development and timing of flowering. Contribution of the circadian clock to overall plant fitness and yield has also long been known. Nevertheless, the organ- and species-specific functions of the circadian clock and its relation to stress adaptation have only recently been identified. Here we report transcriptional changes of core clock genes of the model monocot Brachypodium distachyon under three different light regimes (18:6 light:dark, 24:0 light and 0:24 dark) in response to mild drought stress in roots and green plant parts. Comparative monitoring of core clock gene expression in roots and green plant parts has shown that both phase and amplitude of expression in the roots of Brachypodium plants differ markedly from those in the green plant parts, even under well-watered conditions. Moreover, circadian clock genes responded to water depletion differently in root and shoot. These results suggest an organ-specific form and functions of the circadian clock in Brachypodium roots.

Three Novel Adenylate Cyclase Genes Show Significant Biological Functions in Plant

Adenylate cyclase is the key enzyme solely synthesizing cAMP which participates in cell metabolism regulations and functions as an intracellular second messenger. However, the biological functions of plant ACs have not been elucidated clearly for their poor conservative sequences and low detectable cAMP. We performed a systematic study of plant ACs by using Chinese jujube, whose fruit exhibits the highest cAMP content among plants. Three novel ACs were identified from Chinese jujube, and two types of methods including in vitro and in vivo were used to certificate ZjAC1-3 which can catalyze the conversion of ATP into cAMP. The biological functions of significant accelerations of seed germination, root growth, and flowering were found via overexpression of these AC genes in Arabidopsis, and these functions of ACs were further demonstrated by treating the AC-overexpressing transgenic lines and wild type Arabidopsis with bithionol and dibutyryl-cAMP. At last, transcriptome data revealed that the underlying mechanism of the biological functions of ACs might be regulation of the key genes involved in the circadian rhythm pathway and the hormone signal transduction pathway. This research established a foundation for further investigating plant AC genes and provided strong evidence for cAMP serving as a signaling molecule in plants.

Burning questions for a warming and changing world: 15 unknowns in plant abiotic stress

We present unresolved questions in plant abiotic stress biology as posed by 15 research groups with expertise spanning eco-physiology to cell and molecular biology. Common themes of these questions include the need to better understand how plants detect water availability, temperature, salinity, and rising carbon dioxide (CO2) levels; how environmental signals interface with endogenous signaling and development (e.g. circadian clock and flowering time); and how this integrated signaling controls downstream responses (e.g. stomatal regulation, proline metabolism, and growth versus defense balance). The plasma membrane comes up frequently as a site of key signaling and transport events (e.g. mechanosensing and lipid-derived signaling, aquaporins). Adaptation to water extremes and rising CO2 affects hydraulic architecture and transpiration, as well as root and shoot growth and morphology, in ways not fully understood. Environmental adaptation involves tradeoffs that limit ecological distribution and crop resilience in the face of changing and increasingly unpredictable environments. Exploration of plant diversity within and among species can help us know which of these tradeoffs represent fundamental limits and which ones can be circumvented by bringing new trait combinations together. Better defining what constitutes beneficial stress resistance in different contexts and making connections between genes and phenotypes, and between laboratory and field observations, are overarching challenges.

CRISPR/Cas9-mediated AtGATA25 mutant represents a novel model for regulating hypocotyl elongation in Arabidopsis thaliana

BACKGROUND

Plants have evolved to adapt to the ever-changing environments through various morphological changes. An organism anticipates and responds to changes in its environment via the circadian clock, an endogenous oscillator lasting approximately 24 h. The circadian clock regulates various physiological processes, such as hypocotyl elongation in Arabidopsis thaliana. Phytochrome interacting factor 4 (PIF4), a member of the bHLH protein family, plays a vital hub role in light signaling pathways and temperature-mediated growth response mechanisms. PIF4 is controlled by the circadian clock and interacts with several factors. However, the components that regulate PIF4 transcription and activity are not clearly understood.

METHODS AND RESULTS

Here, we showed that the Arabidopsis thaliana GATA25 (AtGATA25) transcription factor plays a fundamental role in promoting hypocotyl elongation by positively regulating the expression of PIF4. This was confirmed to in the loss-of-function mutant of AtGATA25 via CRISPR/Cas9-mediated gene editing, which inhibits hypocotyl elongation and decreases the expression of PIF4. In contrast, the overexpression of AtGATA25 in transgenic plants resulted in increased expression of PIF4 and enhanced hypocotyl elongation. To better understand AtGATA25-mediated PIF4 transcriptional regulation, we analyzed the promoter region of the target gene PIF4 and characterized the role of GATA25 through transcriptional activation analysis.

CONCLUSION

Our findings suggest a novel role of the AtGATA25 transcription factor in hypocotyl elongation.

Transcriptional regulation of the raffinose family oligosaccharides pathway in Sorghum bicolor reveals potential roles in leaf sucrose transport and stem sucrose accumulation

Bioenergy sorghum hybrids are being developed with enhanced drought tolerance and high levels of stem sugars. Raffinose family oligosaccharides (RFOs) contribute to plant environmental stress tolerance, sugar storage, transport, and signaling. To better understand the role of RFOs in sorghum, genes involved in myo-inositol and RFO metabolism were identified and relative transcript abundance analyzed during development. Genes involved in RFO biosynthesis (SbMIPS1, SbInsPase, SbGolS1, SbRS) were more highly expressed in leaves compared to stems and roots, with peak expression early in the morning in leaves. SbGolS, SbRS, SbAGA1 and SbAGA2 were also expressed at high levels in the leaf collar and leaf sheath. In leaf blades, genes involved in myo-inositol biosynthesis (SbMIPS1, SbInsPase) were expressed in bundle sheath cells, whereas genes involved in galactinol and raffinose synthesis (SbGolS1, SbRS) were expressed in mesophyll cells. Furthermore, SbAGA1 and SbAGA2, genes that encode neutral-alkaline alpha-galactosidases that hydrolyze raffinose, were differentially expressed in minor vein bundle sheath cells and major vein and mid-rib vascular and xylem parenchyma. This suggests that raffinose synthesized from sucrose and galactinol in mesophyll cells diffuses into vascular bundles where hydrolysis releases sucrose for long distance phloem transport. Increased expression (>20-fold) of SbAGA1 and SbAGA2 in stem storage pith parenchyma of sweet sorghum between floral initiation and grain maturity, and higher expression in sweet sorghum compared to grain sorghum, indicates these genes may play a key role in non-structural carbohydrate accumulation in stems.

Phytochromes and Their Role in Diurnal Variations of ROS Metabolism and Plant Proteome

Plants are sessile organisms forced to adapt to environmental variations recurring in a day-night cycle. Extensive research has uncovered the transcriptional control of plants' inner clock and has revealed at least some part of the intricate and elaborate regulatory mechanisms that govern plant diel responses and provide adaptation to the ever-changing environment. Here, we analyzed the proteome of the Arabidopsis thaliana mutant genotypes collected in the middle of the day and the middle of the night, including four mutants in the phytochrome (phyA, phyB, phyC, and phyD) and the circadian clock protein LHY. Our approach provided a novel insight into the diel regulations, identifying 640 significant changes in the night-day protein abundance. The comparison with previous studies confirmed that a large portion of identified proteins was a known target of diurnal regulation. However, more than 300 were novel oscillations hidden under standard growth chamber conditions or not manifested in the wild type. Our results indicated a prominent role for ROS metabolism and phytohormone cytokinin in the observed regulations, and the consecutive analyses confirmed that. The cytokinin signaling significantly increased at night, and in the mutants, the hydrogen peroxide content was lower, and the night-day variation seemed to be lost in the phyD genotype. Furthermore, regulations in the lhy and phyB mutants were partially similar to those found in the catalase mutant cat2, indicating shared ROS-mediated signaling pathways. Our data also shed light on the role of the relatively poorly characterized Phytochrome D, pointing to its connection to glutathione metabolism and the regulation of glutathione S-transferases.

Comparative transcriptome analyses revealed key genes involved in high amylopectin biosynthesis in wheat

High amylopectin starch is an important modified starch for food processing industries. Despite a thorough understanding of starch biosynthesis pathway, the regulatory mechanism responsible for amylopectin biosynthesis is not well explored. The present study utilized transcriptome sequencing approach to understand the molecular basis of high amylopectin content in three high amylopectin mutant wheat lines ('TAC 6', 'TAC 358', and 'TAC 846') along with parent variety 'C 306'. Differential scanning calorimetry (DSC) of high amylopectin starch identified a high thermal transition temperature and scanning electron microscopy (SEM) revealed more spherical starch granules in mutant lines compared to parent variety. A set of 4455 differentially expressed genes (DEGs) were identified at two-fold compared to the parent variety in high amylopectin wheat mutants. At ten-fold, 279 genes, including two starch branching genes (SBEIIa and SBEIIb), were up-regulated and only 30 genes, including the starch debranching enzyme (DBE), were down-regulated. Among the genes, different isoforms of sucrose non-fermenting-1-related protein kinase-1 (TaSnRK1α2-3B and TaSnRK1α2-3D) and its regulatory subunit, sucrose non-fermenting-4 (SNF-4-2A, SNF-4-2B, and SNF-4-5D), were found to be highly up-regulated. Further, expression of the DEGs related to starch biosynthesis pathway and TaSnRK1α2 and SNF-4 was performed using qRT-PCR. High expression of TaSnRK1α2, SNF-4, and SBEII isoforms suggests their probable role in high amylopectin starch biosynthesis in grain endosperm.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03364-3.

A crosstalk of circadian clock and alternative splicing under abiotic stresses in the plants

The circadian clock is an internal time-keeping mechanism that synchronizes the physiological adaptation of an organism to its surroundings based on day and night transition in a period of 24 h, suggesting the circadian clock provides fitness by adjusting environmental constrains. The circadian clock is driven by positive and negative elements that regulate transcriptionally and post-transcriptionally. Alternative splicing (AS) is a crucial transcriptional regulator capable of generating large numbers of mRNA transcripts from limited numbers of genes, leading to proteome diversity, which is involved in circadian to deal with abiotic stresses. Over the past decade, AS and circadian control have been suggested to coordinately regulate plant performance under fluctuating environmental conditions. However, only a few reports have reported the regulatory mechanism of this complex crosstalk. Based on the emerging evidence, this review elaborates on the existing links between circadian and AS in response to abiotic stresses, suggesting an uncovered regulatory network among circadian, AS, and abiotic stresses. Therefore, the rhythmically expressed splicing factors and core clock oscillators fill the role of temporal regulators participating in improving plant growth, development, and increasing plant tolerance against abiotic stresses.

Circadian Redox Rhythm in Plant-Fungal Pathogen Interactions

Significance: Circadian-controlled cellular growth, differentiation, and metabolism are mainly achieved by a classical transcriptional-translational feedback loop (TTFL), as revealed by investigations in animals, plants, and fungi. Recent Advances: Recently, reactive oxygen species (ROS) have been reported as part of a cellular network synchronizing nontranscriptional oscillators with established TTFL components, adding complexity to regulatory mechanisms of circadian rhythm. Both circadian rhythm and ROS homeostasis have a great impact on plant immunity as well as fungal pathogenicity, therefore interconnections of these two factors are implicit in plant-fungus interactions. Critical Issues: In this review, we aim to summarize the recent advances in circadian-controlled ROS homeostasis, or ROS-modulated circadian clock, in plant-fungus pathosystems, particularly using the rice (Oryza sativa) blast fungus (Magnaporthe oryzae) pathosystem as an example. Understanding of such bidirectional interaction between the circadian timekeeping machinery and ROS homeostasis/signaling would provide a theoretical basis for developing disease control strategies for important plants/crops. Future Directions: Questions remain unanswered about the detailed mechanisms underlying circadian regulation of redox homeostasis in M. oryzae, and the consequent fungal differentiation and death in a time-of-day manner. We believe that the rice-M. oryzae pathobiosystem would provide an excellent platform for investigating such issues in circadian-ROS interconnections in a plant-fungus interaction context. Antioxid. Redox Signal. 37, 726-738.

Growth-limiting drought stress induces time-of-day-dependent transcriptome and physiological responses in hybrid poplar

Drought stress negatively impacts the health of long-lived trees. Understanding the genetic mechanisms that underpin response to drought stress is requisite for selecting or enhancing climate change resilience. We aimed to determine how hybrid poplars respond to prolonged and uniform exposure to drought; how responses to moderate and more severe growth-limiting drought stresses differed; and how drought responses change throughout the day. We established hybrid poplar trees (Populus × 'Okanese') from unrooted stem cutting with abundant soil moisture for 6 weeks. We then withheld water to establish well-watered, moderate and severe growth-limiting drought conditions. These conditions were maintained for 3 weeks during which growth was monitored. We then measured photosynthetic rates and transcriptomes of leaves that had developed during the drought treatments at two times of day. The moderate and severe drought treatments elicited distinct changes in growth and development, photosynthetic rates and global transcriptome profiles. Notably, the time of day of sampling produced the strongest effect in the transcriptome data. The moderate drought treatment elicited global transcriptome changes that were intermediate to the severe and well-watered treatments in the early evening but did not elicit a strong drought response in the morning. Stable drought conditions that are sufficient to limit plant growth elicit distinct transcriptional profiles depending on the degree of water limitation and on the time of day at which they are measured. There appears to be a limited number of genes and functional gene categories that are responsive to all of the tested drought conditions in this study emphasizing the complex nature of drought regulation in long-lived trees.

Variations in Circadian Clock Organization & Function: A Journey from Ancient to Recent

Circadian clock components exhibit structural variations in different plant systems, and functional variations during various abiotic stresses. These variations bear relevance for plant fitness and could be important evolutionarily. All organisms on earth have the innate ability to measure time as diurnal rhythms that occur due to the earth's rotations in a 24-h cycle. Circadian oscillations arising from the circadian clock abide by its fundamental properties of periodicity, entrainment, temperature compensation, and oscillator mechanism, which is central to its function. Despite the fact that a myriad of research in Arabidopsis thaliana illuminated many detailed aspects of the circadian clock, many more variations in clock components' organizations and functions remain to get deciphered. These variations are crucial for sustainability and adaptation in different plant systems in the varied environmental conditions in which they grow. Together with these variations, circadian clock functions differ drastically even during various abiotic and biotic stress conditions. The present review discusses variations in the organization of clock components and their role in different plant systems and abiotic stresses. We briefly introduce the clock components, entrainment, and rhythmicity, followed by the variants of the circadian clock in different plant types, starting from lower non-flowering plants, marine plants, dicots to the monocot crop plants. Furthermore, we discuss the interaction of the circadian clock with components of various abiotic stress pathways, such as temperature, light, water stress, salinity, and nutrient deficiency with implications for the reprogramming during these stresses. We also update on recent advances in clock regulations due to post-transcriptional, post-translation, non-coding, and micro-RNAs. Finally, we end this review by summarizing the points of applicability, a remark on the future perspectives, and the experiments that could clear major enigmas in this area of research.

Spatially specific mechanisms and functions of the plant circadian clock

Like many organisms, plants have evolved a genetic network, the circadian clock, to coordinate processes with day/night cycles. In plants, the clock is a pervasive regulator of development and modulates many aspects of physiology. Clock-regulated processes range from the correct timing of growth and cell division to interactions with the root microbiome. Recently developed techniques, such as single-cell time-lapse microscopy and single-cell RNA-seq, are beginning to revolutionize our understanding of this clock regulation, revealing a surprising degree of organ, tissue, and cell-type specificity. In this review, we highlight recent advances in our spatial view of the clock across the plant, both in terms of how it is regulated and how it regulates a diversity of output processes. We outline how understanding these spatially specific functions will help reveal the range of ways that the clock provides a fitness benefit for the plant.

Evolution of circadian clocks along the green lineage

Circadian clocks govern temporal programs in the green lineage (Chloroplastida) as they do in other photosynthetic pro- and eukaryotes, bacteria, fungi, animals, and humans. Their physiological properties, including entrainment, phase responses, and temperature compensation, are well conserved. The involvement of transcriptional/translational feedback loops in the oscillatory machinery and reversible phosphorylation events are also maintained. Circadian clocks control a large variety of output rhythms in green algae and terrestrial plants, adjusting their metabolism and behavior to the day-night cycle. The angiosperm Arabidopsis (Arabidopsis thaliana) represents a well-studied circadian clock model. Several molecular components of its oscillatory machinery are conserved in other Chloroplastida, but their functions may differ. Conserved clock components include at least one member of the CIRCADIAN CLOCK ASSOCIATED1/REVEILLE and one of the PSEUDO RESPONSE REGULATOR family. The Arabidopsis evening complex members EARLY FLOWERING3 (ELF3), ELF4, and LUX ARRHYTHMO are found in the moss Physcomitrium patens and in the liverwort Marchantia polymorpha. In the flagellate chlorophyte alga Chlamydomonas reinhardtii, only homologs of ELF4 and LUX (named RHYTHM OF CHLOROPLAST ROC75) are present. Temporal ROC75 expression in C. reinhardtii is opposite to that of the angiosperm LUX, suggesting different clock mechanisms. In the picoalga Ostreococcus tauri, both ELF genes are missing, suggesting that it has a progenitor circadian "green" clock. Clock-relevant photoreceptors and thermosensors vary within the green lineage, except for the CRYPTOCHROMEs, whose variety and functions may differ. More genetically tractable models of Chloroplastida are needed to draw final conclusions about the gradual evolution of circadian clocks within the green lineage.

Rice CIRCADIAN CLOCK ASSOCIATED 1 transcriptionally regulates ABA signaling to confer multiple abiotic stress tolerance

The circadian clock facilitates the survival and reproduction of crop plants under harsh environmental conditions such as drought and osmotic and salinity stresses, mainly by reprogramming the endogenous transcriptional landscape. Nevertheless, the genome-wide roles of core clock components in rice (Oryza sativa L.) abiotic stress tolerance are largely uncharacterized. Here, we report that CIRCADIAN CLOCK ASSOCIATED1 (OsCCA1), a vital clock component in rice, is required for tolerance to salinity, osmotic, and drought stresses. DNA affinity purification sequencing coupled with transcriptome analysis identified 692 direct transcriptional target genes of OsCCA1. Among them, the genes involved in abscisic acid (ABA) signaling, including group A protein phosphatase 2C genes and basic region and leucine zipper 46 (OsbZIP46), were substantially enriched. Moreover, OsCCA1 could directly bind the promoters of OsPP108 and OsbZIP46 to activate their expression. Consistently, oscca1 null mutants generated via genome editing displayed enhanced sensitivities to ABA signaling. Together, our findings illustrate that OsCCA1 confers multiple abiotic stress tolerance likely by orchestrating ABA signaling, which links the circadian clock with ABA signaling in rice.

Multi-omic analysis shows REVEILLE clock genes are involved in carbohydrate metabolism and proteasome function

Plants are able to sense changes in their light environments, such as the onset of day and night, as well as anticipate these changes in order to adapt and survive. Central to this ability is the plant circadian clock, a molecular circuit that precisely orchestrates plant cell processes over the course of a day. REVEILLE (RVE) proteins are recently discovered members of the plant circadian circuitry that activate the evening complex and PSEUDO-RESPONSE REGULATOR genes to maintain regular circadian oscillation. The RVE8 protein and its two homologs, RVE 4 and 6 in Arabidopsis (Arabidopsis thaliana), have been shown to limit the length of the circadian period, with rve 4 6 8 triple-knockout plants possessing an elongated period along with increased leaf surface area, biomass, cell size, and delayed flowering relative to wild-type Col-0 plants. Here, using a multi-omics approach consisting of phenomics, transcriptomics, proteomics, and metabolomics we draw new connections between RVE8-like proteins and a number of core plant cell processes. In particular, we reveal that loss of RVE8-like proteins results in altered carbohydrate, organic acid, and lipid metabolism, including a starch excess phenotype at dawn. We further demonstrate that rve 4 6 8 plants have lower levels of 20S proteasome subunits and possess significantly reduced proteasome activity, potentially explaining the increase in cell-size observed in RVE8-like mutants. Overall, this robust, multi-omic dataset provides substantial insight into the far-reaching impact RVE8-like proteins have on the diel plant cell environment.