Trehalose-6-phosphate signaling regulates thermoresponsive hypocotyl growth in Arabidopsis thaliana

Functions of sucrose and trehalose 6-phosphate in controlling plant development

Plants exhibit enormous plasticity in regulating their architecture to be able to adapt to a constantly changing environment and carry out vital functions such as photosynthesis, anchoring, and nutrient uptake. Phytohormones play a role in regulating these responses, but sugar signalling mechanisms are also crucial. Sucrose is not only an important source of carbon and energy fuelling plant growth, but it also functions as a signalling molecule that influences various developmental processes. Trehalose 6-phosphate (Tre6P), a sucrose-specific signalling metabolite, is emerging as an important regulator in plant metabolism and development. Key players involved in sucrose and Tre6P signalling pathways, including MAX2, SnRK1, bZIP11, and TOR, have been implicated in processes such as flowering, branching, and root growth. We will summarize our current knowledge of how these pathways shape shoot and root architecture and highlight how sucrose and Tre6P signalling are integrated with known signalling networks in shaping plant growth.

The trehalose 6-phosphate phosphatase family in plants

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signalling metabolite linking plant growth and development to carbon metabolism. While recent work has focused predominantly on the enzymes that produce Tre6P, little is known about the proteins that catalyse its degradation, the trehalose 6-phosphate phosphatases (TPPs). Often occurring in large protein families, TPPs exhibit cell-, tissue- and developmental stage-specific expression patterns, suggesting important regulatory functions in controlling local levels of Tre6P and trehalose as well as Tre6P signalling. Furthermore, growing evidence through gene expression studies and transgenic approaches shows that TPPs play an important role in integrating environmental signals with plant metabolism. This review highlights the large diversity of TPP isoforms in model and crop plants and identifies how modulating Tre6P metabolism in certain cell types, tissues, and at different developmental stages may promote stress tolerance, resilience and increased crop yield.

Natural variation of warm temperature-induced raffinose accumulation identifies TREHALOSE-6-PHOSPHATE SYNTHASE 1 as a modulator of thermotolerance

High-temperature stress limits plant growth and reproduction. Exposure to high temperature, however, also elicits a physiological response, which protects plants from the damage evoked by heat. This response involves a partial reconfiguration of the metabolome including the accumulation of the trisaccharide raffinose. In this study, we explored the intraspecific variation of warm temperature-induced raffinose accumulation as a metabolic marker for temperature responsiveness with the aim to identify genes that contribute to thermotolerance. By combining raffinose measurements in 250 Arabidopsis thaliana accessions following a mild heat treatment with genome-wide association studies, we identified five genomic regions that were associated with the observed trait variation. Subsequent functional analyses confirmed a causal relationship between TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) and warm temperature-dependent raffinose synthesis. Moreover, complementation of the tps1-1 null mutant with functionally distinct TPS1 isoforms differentially affected carbohydrate metabolism under more severe heat stress. While higher TPS1 activity was associated with reduced endogenous sucrose levels and thermotolerance, disruption of trehalose 6-phosphate signalling resulted in higher accumulation of transitory starch and sucrose and was associated with enhanced heat resistance. Taken together, our findings suggest a role of trehalose 6-phosphate in thermotolerance, most likely through its regulatory function in carbon partitioning and sucrose homoeostasis.

Sailing in complex nutrient signaling networks: Where I am, where to go, and how to go?

To ensure survival and promote growth, sessile plants have developed intricate internal signaling networks tailored in diverse cells and organs with both shared and specialized functions that respond to various internal and external cues. A fascinating question arises: how can a plant cell or organ diagnose the spatial and temporal information it is experiencing to know "where I am," and then is able to make the accurate specific responses to decide "where to go" and "how to go," despite the absence of neuronal systems found in mammals. Drawing inspiration from recent comprehensive investigations into diverse nutrient signaling pathways in plants, this review focuses on the interactive nutrient signaling networks mediated by various nutrient sensors and transducers. We assess and illustrate examples of how cells and organs exhibit specific responses to changing spatial and temporal information within these interactive plant nutrient networks. In addition, we elucidate the underlying mechanisms by which plants employ posttranslational modification codes to integrate different upstream nutrient signals, thereby conferring response specificities to the signaling hub proteins. Furthermore, we discuss recent breakthrough studies that demonstrate the potential of modulating nutrient sensing and signaling as promising strategies to enhance crop yield, even with reduced fertilizer application.

PYRIDOX(AM)INE 5'-PHOSPHATE OXIDASE3 of Arabidopsis thaliana maintains carbon/nitrogen balance in distinct environmental conditions

The identification of factors that regulate C/N utilization in plants can make a substantial contribution to optimization of plant health. Here, we explored the contribution of pyridox(am)ine 5'-phosphate oxidase3 (PDX3), which regulates vitamin B6 homeostasis, in Arabidopsis (Arabidopsis thaliana). Firstly, N fertilization regimes showed that ammonium application rescues the leaf morphological phenotype of pdx3 mutant lines but masks the metabolite perturbance resulting from impairment in utilizing soil nitrate as a source of N. Without fertilization, pdx3 lines suffered a C/N imbalance and accumulated nitrogenous compounds. Surprisingly, exploration of photorespiration as a source of endogenous N driving this metabolic imbalance, by incubation under high CO2, further exacerbated the pdx3 growth phenotype. Interestingly, the amino acid serine, critical for growth and N management, alleviated the growth phenotype of pdx3 plants under high CO2, likely due to the requirement of pyridoxal 5'-phosphate for the phosphorylated pathway of serine biosynthesis under this condition. Triggering of thermomorphogenesis by growth of plants at 28 °C (instead of 22 °C) did not appear to require PDX3 function, and we observed that the consequent drive toward C metabolism counters the C/N imbalance in pdx3. Further, pdx3 lines suffered a salicylic acid-induced defense response, probing of which unraveled that it is a protective strategy mediated by nonexpressor of pathogenesis related1 (NPR1) and improves fitness. Overall, the study demonstrates the importance of vitamin B6 homeostasis as managed by the salvage pathway enzyme PDX3 to growth in diverse environments with varying nutrient availability and insight into how plants reprogram their metabolism under such conditions.

In vivo protein kinase activity of SnRK1 fluctuates in Arabidopsis rosettes during light-dark cycles

Sucrose-nonfermenting 1 (SNF1)-related kinase 1 (SnRK1) is a central hub in carbon and energy signaling in plants, and is orthologous with SNF1 in yeast and the AMP-activated protein kinase (AMPK) in animals. Previous studies of SnRK1 relied on in vitro activity assays or monitoring of putative marker gene expression. Neither approach gives unambiguous information about in vivo SnRK1 activity. We have monitored in vivo SnRK1 activity using Arabidopsis (Arabidopsis thaliana) reporter lines that express a chimeric polypeptide with an SNF1/SnRK1/AMPK-specific phosphorylation site. We investigated responses during an equinoctial diel cycle and after perturbing this cycle. As expected, in vivo SnRK1 activity rose toward the end of the night and rose even further when the night was extended. Unexpectedly, although sugars rose after dawn, SnRK1 activity did not decline until about 12 h into the light period. The sucrose signal metabolite, trehalose 6-phosphate (Tre6P), has been shown to inhibit SnRK1 in vitro. We introduced the SnRK1 reporter into lines that harbored an inducible trehalose-6-phosphate synthase construct. Elevated Tre6P decreased in vivo SnRK1 activity in the light period, but not at the end of the night. Reporter polypeptide phosphorylation was sometimes negatively correlated with Tre6P, but a stronger and more widespread negative correlation was observed with glucose-6-phosphate. We propose that SnRK1 operates within a network that controls carbon utilization and maintains diel sugar homeostasis, that SnRK1 activity is regulated in a context-dependent manner by Tre6P, probably interacting with further inputs including hexose phosphates and the circadian clock, and that SnRK1 signaling is modulated by factors that act downstream of SnRK1.

ZTL regulates thermomorphogenesis through TOC1 and PRR5

Plants adapt to high temperature stresses through thermomorphogenesis, a process that includes stem elongation and hyponastic leaf growth. Thermomorphogenesis is gated by the circadian clock through two evening-expressed clock components, TIMING OF CAB EXPRESSION1 (TOC1) and PSEUDO-RESPONSE REGULATORS5 (PRR5). These proteins directly interact with and inhibit PHYTOCHROME INTERACTING FACTOR4 (PIF4), a basic helix-loop-helix transcription factor that promotes thermoresponsive growth. PIF4-mediated thermoresponsive growth is positively regulated by ZEITLUPE (ZTL), a central clock component, but the molecular mechanisms underlying this are poorly understood. Here, we show that ZTL regulates thermoresponsive growth through TOC1 and PRR5. Genetic analyses reveal that ZTL regulates PIF4 activity as well as PIF4 expression. In Arabidopsis thaliana, ztl mutants exhibit highly accumulated TOC1 and PRR5 and unresponsive expression of PIF4 target genes under exposure to high temperatures. Mutations in TOC1 and PRR5 restore thermoactivation of PIF4 target genes and thermoresponsive growth in ztl mutants. We also show that the molecular chaperone heat-shock protein 90 promotes thermoresponsive growth through the ZTL-TOC1/PRR5 signaling module. Further, we show that ZTL protein stability is increased at high temperatures. Taken together, our results demonstrate that ZTL-mediated degradation of TOC1 and PRR5 enhances the sensitivity of hypocotyl growth to high temperatures.

CYCLIN-DEPENDENT KINASE 8 positively regulates oil synthesis by activating WRINKLED1 transcription

CYCLIN-DEPENDENT KINASE 8 (CDK8), a component of the kinase module of the Mediator complex in Arabidopsis, is involved in many processes, including flowering, plant defense, drought, and energy stress responses. Here, we investigated cdk8 mutants and CDK8-overexpressing lines to evaluate whether CDK8 also plays a role in regulating lipid synthesis, an energy-demanding anabolism. Quantitative lipid analysis demonstrated significant reductions in lipid synthesis rates and lipid accumulation in developing siliques and seedlings of cdk8, and conversely, elevated lipid contents in wild-type seed overexpressing CDK8. Transactivation assays show that CDK8 is necessary for maximal transactivation of the master seed oil activator WRINKLED1 (WRI1) by the seed maturation transcription factor ABSCISIC ACID INSENSITIVE3, supporting a direct regulatory role of CDK8 in oil synthesis. Thermophoretic studies show GEMINIVIRUS REP INTERACTING KINASE1, an activating kinase of KIN10 (a catalytic subunit of SUCROSE NON-FERMENTING1-RELATED KINASE1), physically interacts with CDK8, resulting in its phosphorylation and degradation in the presence of KIN10. This work defines a mechanism whereby, once activated, KIN10 downregulates WRI1 expression and suppresses lipid synthesis via promoting the degradation of CDK8. The KIN10-CDK8-dependent regulation of lipid synthesis described herein is additional to our previously reported KIN10-dependent phosphorylation and degradation of WRI1.

Genome-Wide Characterization of Trehalose-6-Phosphate Synthase Gene Family of Brassica napus and Potential Links with Agronomic Traits

Trehalose and trehalose-6 phosphate played important roles in floral organ development, embryonic development, cell morphogenesis, and signal transduction under abiotic stress. However, little is known about the trehalose-6-phosphate synthase (TPS) gene family in Brassica napus. In this study, in total, 26 TPS genes in B. napus (BnTPS genes) were identified and classified into two groups. In each group, the BnTPS genes showed relatively conserved gene structures. The protein-protein interaction (PPI) network and enrichment analysis indicated that BnTPS genes were involved in the glycolysis/gluconeogenesis, fructose and mannose metabolism, galactose metabolism, pentose phosphate pathway, carbohydrate transmembrane transport, trehalose-phosphatase activity, etc. The expression of BnTPS genes varied greatly across different tissues, while most of the BnTPS genes showed a considerable improvement in expression under different abiotic stresses, indicating that BnTPS genes were significantly responsive to the abiotic treatments. In addition, the association mapping analysis revealed that eight BnTPS genes were potential regulators of particular agronomic traits. Among them, the gene BnTPS23 was significantly associated with the primary flowering time (PFT), full flowering time (FFT1), and final flowering time (FFT2), suggesting that BnTPS genes may play an important role in regulating key agronomic traits in B. napus. In summary, our research provides a better understanding of BnTPS genes, facilitates the breeding of superior B. napus varieties, and paves the way for future functional studies.

Compartmentalization, a key mechanism controlling the multitasking role of the SnRK1 complex

SNF1-related protein kinase 1 (SnRK1), the plant ortholog of mammalian AMP-activated protein kinase/fungal (yeast) Sucrose Non-Fermenting 1 (AMPK/SNF1), plays a central role in metabolic responses to reduced energy levels in response to nutritional and environmental stresses. SnRK1 functions as a heterotrimeric complex composed of a catalytic α- and regulatory β- and βγ-subunits. SnRK1 is a multitasking protein involved in regulating various cellular functions, including growth, autophagy, stress response, stomatal development, pollen maturation, hormone signaling, and gene expression. However, little is known about the mechanism whereby SnRK1 ensures differential execution of downstream functions. Compartmentalization has been recently proposed as a new key mechanism for regulating SnRK1 signaling in response to stimuli. In this review, we discuss the multitasking role of SnRK1 signaling associated with different subcellular compartments.

Growing at the right time: interconnecting the TOR pathway with photoperiod and circadian regulation

Plants can adjust their growth to specific times of the day and season. Different photoperiods result in distinct growth patterns, which correlate with specific carbon-partitioning strategies in source (leaves) and sink (roots) organs. Therefore, external cues such as light, day length, and temperature need to be integrated with intracellular processes controlling overall carbon availability and anabolism. The target of rapamycin (TOR) pathway is a signalling hub where environmental signals, circadian information, and metabolic processes converge to regulate plant growth. TOR complex mutants display altered patterns of root growth and starch levels. Moreover, depletion of TOR or reduction in cellular energy levels affect the pace of the clock by extending the period length, suggesting that this pathway could participate in circadian metabolic entrainment. However, this seems to be a mutual interaction, since the TOR pathway components are also under circadian regulation. These results strengthen the role of this signalling pathway as a master sensor of metabolic status, integrating day length and circadian cues to control anabolic processes in the cell, thus promoting plant growth and development. Expanding this knowledge from Arabidopsis thaliana to crops will improve our understanding of the molecular links connecting environmental perception and growth regulation under field conditions.

SAUR63 stimulates cell growth at the plasma membrane

In plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H⁺-ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H⁺-ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H⁺-ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth.

Plant organs reach their final shape and size after substantial cell expansion. Proton pumps at the plasma membrane promote cell expansion by acidifying the cell wall to loosen it, and by increasing electrochemical potential across the plasma membrane for solute uptake that maintains intracellular turgor. Plasma-membrane-associated proteins tightly regulate proton pump activity, in order for organs to grow to an appropriate extent. We have studied requirements for activity of one such regulatory protein in the model plant Arabidopsis called SAUR63. This protein is made rapidly in response to plant growth hormones, and it increases proton pump activity to promote organ growth. These activities depend on its binding to anionic lipids in the plasma membrane, and forced plasma membrane association of SAUR63 can increase growth. Many proteins in the same family are found within Arabidopsis and in all land plants, and likely differ in their affinity for the plasma membrane or in other properties. Further studies of other family members may show how such proteins regulate growth under diverse physiological contexts.

EARLY FLOWERING 3 represses the nighttime growth response to sucrose in Arabidopsis

Plant growth depends on the supply of carbohydrates produced by photosynthesis. Exogenously applied sucrose promotes the growth of the hypocotyl in Arabidopsis thaliana seedlings grown under short days. Whether this effect of sucrose is stronger under the environmental conditions where the light input for photosynthesis is limiting remains unknown. We characterised the effects of exogenous sucrose on hypocotyl growth rates under light compared to simulated shade, during different portions of the daily cycle. The strongest effects of exogenous sucrose occurred under shade and during the night; i.e., the conditions where there is reduced or no photosynthesis. Conversely, a faster hypocotyl growth rate, predicted to enhance the demand of carbohydrates, did not associate to a stronger sucrose effect. The early flowering 3 (elf3) mutation strongly enhanced the impact of sucrose on hypocotyl growth during the night of a white-light day. This effect occurred under short, but not under long days. The addition of sucrose enhanced the fluorescence intensity of ELF3 nuclear speckles. The elf3 mutant showed increased abundance of PHYTOCHROME INTERACTING FACTOR4 (PIF4), which is a transcription factor required for a full response to sucrose. Sucrose increased PIF4 protein abundance by post-transcriptional mechanisms. Under shade, elf3 showed enhanced daytime and reduced nighttime effects of sucrose. We conclude that ELF3 modifies the responsivity to sucrose according to the time of the daily cycle and the prevailing light or shade conditions.

Arabidopsis HISTONE DEACETYLASE 9 Stimulates Hypocotyl Cell Elongation by Repressing GIGANTEA Expression Under Short Day Photoperiod

Developmental plasticity contributes to plant adaptation and fitness in a given condition. Hypocotyl elongation is under the tight control of complex genetic networks encompassing light, circadian, and photoperiod signaling. In this study, we demonstrate that HISTONE DEACETYLASE 9 (HDA9) mediates day length-dependent hypocotyl cell elongation. HDA9 binds to the GIGANTEA (GI) locus involved in photoperiodic hypocotyl elongation. The short day (SD)-accumulated HDA9 protein promotes histone H3 deacetylation at the GI locus during the dark period, promoting hypocotyl elongation. Consistently, HDA9-deficient mutants display reduced hypocotyl length, along with an increase in GI gene expression, only under SD conditions. Taken together, our study reveals the genetic basis of day length-dependent cell elongation in plants.

The Transcriptome and Metabolome Reveal the Potential Mechanism of Lodging Resistance in Intergeneric Hybrids between Brassica napus and Capsella bursa-pastoris

Lodging is one of the main reasons for the reduction in seed yield and is the limitation of mechanized harvesting in B. napus. The dissection of the regulatory mechanism of lodging resistance is an important goal in B. napus. In this study, the lodging resistant B. napus line, YG689, derived from the hybridization between B. napus cv. Zhongyou 821 (ZY821) and Capsella bursa-pastoris, was used to dissect the regulation mechanism of hard stem formation by integrating anatomical structure, transcriptome and metabolome analyses. It was shown that the lignocellulose content of YG689 is higher than that of ZY821, and some differentially expressed genes (DEGs) involved in the lignocellulose synthesis pathway were revealed by transcriptome analyses. Meanwhile, GC–TOF–MS and UPLC–QTOF–MS identified 40, 54, and 31 differential metabolites in the bolting stage, first flower stage, and the final flower stage. The differential accumulation of these metabolites might be associated with the lignocellulose biosynthesis in B. napus. Finally, some important genes that regulate the metabolic pathway of lignocellulose biosynthesis, such as BnaA02g18920D, BnaA10g15590D, BnaC05g48040D, and NewGene_216 were identified in B. napus through the combination of transcriptomics and metabolomics data. The present results explored the potential regulatory mechanism of lignocellulose biosynthesis, which provided a new clue for the breeding of B. napus with lodging resistance in the future.

Dynamic epigenetic modifications in plant sugar signal transduction

In eukaryotes, dynamic chromatin states are closely related to changes in gene expression. Epigenetic modifications help plants adapt to their ever-changing environment by modulating gene expression via covalent modification at specific sites on DNA or histones. Sugars provide energy, but also function as signaling molecules to control plant growth and development. Various epigenetic modifications participate in sensing and transmitting sugar signals. Here we summarize recent progress in uncovering the epigenetic mechanisms involved in sugar signal transduction, including histone acetylation and deacetylation, histone methylation and demethylation, and DNA methylation. We also highlight changes in chromatin marks when crosstalk occurs between sugar signaling and the light, temperature, and phytohormone signaling pathways, and describe potential questions and approaches for future research.

Sugars and the speed of life-Metabolic signals that determine plant growth, development and death

Plant growth and development depend on the availability of carbohydrates synthesised in photosynthesis (source activity) and utilisation of these carbohydrates for growth (sink activity). External conditions, such as temperature, nutrient availability and stress, can affect source as well as sink activity. Optimal utilisation of resources is under circadian clock control. This molecular timekeeper ensures that growth responses are adjusted to different photoperiod and temperature settings by modulating starch accumulation and degradation accordingly. For example, during the night, starch degradation is required to provide sugars for growth. Under favourable growth conditions, high sugar availability stimulates growth and development, resulting in an overall accelerated life cycle of annual plants. Key signalling components include trehalose-6-phosphate (Tre6P), which reflects sucrose availability and stimulates growth and branching when the conditions are favourable. Under sink limitation, Tre6P does, however, inhibit night-time starch degradation. Tre6P interacts with Sucrose-non-fermenting1-Related Kinase1 (SnRK1), a protein kinase that inhibits growth under starvation and stress conditions and delays development (including flowering and senescence). Tre6P inhibits SnRK1 activity, but SnRK1 increases the Tre6P to sucrose ratio under favourable conditions. Alongside Tre6P, Target of Rapamycin (TOR) stimulates processes such as protein synthesis and growth when sugar availability is high. In annual plants, an accelerated life cycle results in early leaf and plant senescence, thus shortening the lifespan. While the availability of carbohydrates in the form of sucrose and other sugars also plays an important role in seasonal life cycle events (phenology) of perennial plants, the sugar signalling pathways in perennials are less well understood.

Role of sugar and auxin crosstalk in plant growth and development

Under the natural environment, nutrient signals interact with phytohormones to coordinate and reprogram plant growth and survival. Sugars are important molecules that control almost all morphological and physiological processes in plants, ranging from seed germination to senescence. In addition to their functions as energy resources, osmoregulation, storage molecules, and structural components, sugars function as signaling molecules and interact with various plant signaling pathways, such as hormones, stress, and light to modulate growth and development according to fluctuating environmental conditions. Auxin, being an important phytohormone, is associated with almost all stages of the plant's life cycle and also plays a vital role in response to the dynamic environment for better growth and survival. In the previous years, substantial progress has been made that showed a range of common responses mediated by sugars and auxin signaling. This review discusses how sugar signaling affects auxin at various levels from its biosynthesis to perception and downstream gene activation. On the same note, the review also highlights the role of auxin signaling in fine-tuning sugar metabolism and carbon partitioning. Furthermore, we discussed the crosstalk between the two signaling machineries in the regulation of various biological processes, such as gene expression, cell cycle, development, root system architecture, and shoot growth. In conclusion, the review emphasized the role of sugar and auxin crosstalk in the regulation of several agriculturally important traits. Thus, engineering of sugar and auxin signaling pathways could potentially provide new avenues to manipulate for agricultural purposes.

SNF1-related protein kinase 1: the many-faced signaling hub regulating developmental plasticity in plants

The Snf1-related protein kinase 1 (SnRK1) is the plant homolog of the heterotrimeric AMP-activated protein kinase/sucrose non-fermenting 1 (AMPK/Snf1), which works as a major regulator of growth under nutrient-limiting conditions in eukaryotes. Along with its conserved role as a master regulator of sugar starvation responses, SnRK1 is involved in controlling the developmental plasticity and resilience under diverse environmental conditions in plants. In this review, through mining and analyzing the interactome and phosphoproteome data of SnRK1, we are highlighting its role in fundamental cellular processes such as gene regulation, protein synthesis, primary metabolism, protein trafficking, nutrient homeostasis, and autophagy. Along with the well-characterized molecular interaction in SnRK1 signaling, our analysis highlights several unchartered regions of SnRK1 signaling in plants such as its possible communication with chromatin remodelers, histone modifiers, and inositol phosphate signaling. We also discuss potential reciprocal interactions of SnRK1 signaling with other signaling pathways and cellular processes, which could be involved in maintaining flexibility and homeostasis under different environmental conditions. Overall, this review provides a comprehensive overview of the SnRK1 signaling network in plants and suggests many novel directions for future research.

Dynamic Nutrient Signaling Networks in Plants

Nutrients are vital to life through intertwined sensing, signaling, and metabolic processes. Emerging research focuses on how distinct nutrient signaling networks integrate and coordinate gene expression, metabolism, growth, and survival. We review the multifaceted roles of sugars, nitrate, and phosphate as essential plant nutrients in controlling complex molecular and cellular mechanisms of dynamic signaling networks. Key advances in central sugar and energy signaling mechanisms mediated by the evolutionarily conserved master regulators HEXOKINASE1 (HXK1), TARGET OF RAPAMYCIN (TOR), and SNF1-RELATED PROTEIN KINASE1 (SNRK1) are discussed. Significant progress in primary nitrate sensing, calcium signaling, transcriptome analysis, and root-shoot communication to shape plant biomass and architecture are elaborated. Discoveries on intracellular and extracellular phosphate signaling and the intimate connections with nitrate and sugar signaling are examined. This review highlights the dynamic nutrient, energy, growth, and stress signaling networks that orchestrate systemwide transcriptional, translational, and metabolic reprogramming, modulate growth and developmental programs, and respond to environmental cues. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

External and Internal Reshaping of Plant Thermomorphogenesis

Plants dynamically adapt to changing temperatures to ensure propagation and reproductive success, among which morphogenic responses to warm temperatures have been extensively studied in recent years. As readily inferred from the cyclic co-oscillations of environmental cues in nature, plant thermomorphogenesis is coordinately reshaped by various external conditions. Accumulating evidence supports that internal and developmental cues also contribute to harmonizing thermomorphogenic responses. The external and internal reshaping of thermomorphogenesis is facilitated by versatile temperature sensing and interorgan communication processes, circadian and photoperiodic gating of thermomorphogenic behaviors, and their metabolic coordination. Here, we discuss recent advances in plant thermal responses with focus on the diel and seasonal reshaping of thermomorphogenesis and briefly explore its application to developing climate-smart crops.

Guard cells control hypocotyl elongation through HXK1, HY5, and PIF4

The hypocotyls of germinating seedlings elongate in a search for light to enable autotrophic sugar production. Upon exposure to light, photoreceptors that are activated by blue and red light halt elongation by preventing the degradation of the hypocotyl-elongation inhibitor HY5 and by inhibiting the activity of the elongation-promoting transcription factors PIFs. The question of how sugar affects hypocotyl elongation and which cell types stimulate and stop that elongation remains unresolved. We found that overexpression of a sugar sensor, Arabidopsis hexokinase 1 (HXK1), in guard cells promotes hypocotyl elongation under white and blue light through PIF4. Furthermore, expression of PIF4 in guard cells is sufficient to promote hypocotyl elongation in the light, while expression of HY5 in guard cells is sufficient to inhibit the elongation of the hy5 mutant and the elongation stimulated by HXK1. HY5 exits the guard cells and inhibits hypocotyl elongation, but is degraded in the dark. We also show that the inhibition of hypocotyl elongation by guard cells' HY5 involves auto-activation of HY5 expression in other tissues. It appears that guard cells are capable of coordinating hypocotyl elongation and that sugar and HXK1 have the opposite effect of light on hypocotyl elongation, converging at PIF4.

Genome-wide identification and expression analysis of the trehalose-6-phosphate synthase (TPS) gene family in cucumber (Cucumis sativus L.)

Trehalose-6-phosphate synthase (TPS) is significant in the growth, development and stress resistance of plants. We identified the cucumber TPS family and its physicochemical properties, domains, gene structures, evolutionary relationships, gene locations, cis-acting elements, conserved motifs, and expression patterns using bioinformatics. Our results uncovered seven CsTPS genes in the cucumber genome and named CsTPS1-CsTPS7 according to their locations in the chromosomes. Seven CsTPS genes were randomly distributed in six cucumber chromosomes. Domain analysis showed that the TPS and TPP domains exist in all CsTPSs, and an additional hydrolase-3 domain exist in CsTPS3, CsTPS5 and CsTPS6. Phylogenetic analysis showed that TPS proteins from Arabidopsis, rice, soybean, and cucumber were divided into two subfamilies (Class I and Class II) and they were further divided into seven subgroups. TPS proteins from Arabidopsis and cucumber were grouped together, suggesting a close evolutionary relationship. Gene structure analysis indicated that most Class I genes contained 16-17 introns, while Class II genes (except CsTPS7) had two introns. Motif analysis showed that Class II genes had 10 complete conserved motifs, while Class I genes lacked motif 8 and motif 9. Furthermore, CsTPS genes possessed numerous cis-acting elements related to stress, hormone, and light response in the promoter regions. GO analysis indicated multiple functions for the CsTPS proteins. Expression analysis of CsTPS genes in different tissues found that they were expressed in roots, stems and leaves, with the highest expression levels in roots. The expression analysis of CsTPSs under different treatments showed that CsTPS genes may participate in the response to abiotic stress, plant hormones and sugar treatments.

Plant thermotropism: an underexplored thermal engagement and avoidance strategy

Various strategies evolved in plants to adjust the position of organs relative to the prevailing temperature condition, which allows optimal plant growth and performance. Such responses are classically separated into nastic and tropic responses. During plant thermotropic responses, organs move towards (engage) or away (avoid) from a directional temperature cue. Despite thermotropism being a classic botanical concept, the underlying ecological function and molecular and biophysical mechanisms remain poorly understood to this day. This contrasts to the relatively well-studied thermonastic movements (hyponasty) of e.g., rosette leaves. In this review, we provide an update on the current knowledge on plant thermotropisms and propose directions for future research and application.

Trimethylamine N-oxide is a new plant molecule that promotes abiotic stress tolerance

Trimethylamine N-oxide (TMAO) is a well-known naturally occurring osmolyte in animals that counteracts the effect of different denaturants related to environmental stress and has recently been associated with severe human chronic diseases. In plants, however, the presence of TMAO has not yet been reported. In this study, we demonstrate that plants contain endogenous levels of TMAO, that it is synthesized by flavin-containing monooxygenases, and that its levels increase in response to abiotic stress conditions. In addition, our results reveal that TMAO operates as a protective osmolyte in plants, promoting appropriate protein folding and as an activator of abiotic stress-induced gene expression. Consistent with these functions, we show that TMAO enhances plant adaptation to low temperatures, drought, and high salt. We have thus uncovered a previously unidentified plant molecule that positively regulates abiotic stress tolerance.

Phytochromes control metabolic flux, and their action at the seedling stage determines adult plant biomass

Phytochrome photoreceptors are known to regulate plastic growth responses to vegetation shade. However, recent reports also suggest an important role for phytochromes in carbon resource management, metabolism, and growth. Here, we use 13CO2 labelling patterns in multiallele phy mutants to investigate the role of phytochrome in the control of metabolic fluxes. We also combine quantitative data of 13C incorporation into protein and cell wall polymers, gas exchange measurements, and system modelling to investigate why biomass is decreased in adult multiallele phy mutants. Phytochrome influences the synthesis of stress metabolites such as raffinose and proline, and the accumulation of sugars, possibly through regulating vacuolar sugar transport. Remarkably, despite their modified metabolism and vastly altered architecture, growth rates in adult phy mutants resemble those of wild-type plants. Our results point to delayed seedling growth and smaller cotyledon size as the cause of the adult-stage phy mutant biomass defect. Our data signify a role for phytochrome in metabolic stress physiology and carbon partitioning, and illustrate that phytochrome action at the seedling stage sets the trajectory for adult biomass production.

Molecular and functional dissection of EARLY-FLOWERING 3 (ELF3) and ELF4 in Arabidopsis

The circadian clock is an endogenous timekeeper system that generates biological rhythms of approximately 24 h in most organisms. EARLY FLOWERING 3 (ELF3) and ELF4 were initially identified as negative regulators of flowering time in Arabidopsis thaliana. They were then found to play crucial roles in the circadian clock by integrating environmental light and ambient temperature signals and transmitting them to the central oscillator, thereby regulating various downstream cellular and physiological processes. At dusk, ELF3 acts as a scaffold, interacting with ELF4 and the transcription factor LUX ARRHYTHMO (PHYTOCLOCK1) to form an EVENING COMPLEX (EC). This complex represses the transcription of multiple circadian clock-related genes, thus inhibiting hypocotyl elongation and flowering. Subsequent studies have expanded knowledge about the regulatory roles of the EC in thermomorphogenesis and shade responses. In addition, ELF3 and ELF4 also form multiple complexes with other proteins including chromatin remodeling factors, histone deacetylase, and transcription factors, thus enabling the transcriptional repression of diverse targets. In this review, we summarize the recent advances in elucidating the regulatory mechanisms of ELF3 and ELF4 in plants and discuss directions for future research on ELF3 and ELF4.

Overexpression of BBX18 Promotes Thermomorphogenesis Through the PRR5-PIF4 Pathway

Thermomorphogenesis is the morphological response of plants to an elevation in the ambient temperature, which is mediated by the bHLH transcription factor PIF4. The evening-expressed clock component, PRR5, directly represses the expression of PIF4 mRNA. Additionally, PRR5 interacts with PIF4 protein and represses its transactivation activity, which in turn suppresses the thermoresponsive growth in the evening. Here, we found that the B-box zinc finger protein, BBX18, interacts with PRR5 through the B-Box2 domain. Deletion of the B-Box2 domain abolished the functions of BBX18, including the stimulation of PIF4 mRNA expression and hypocotyl growth. Overexpression of BBX18, and not of B-Box2-deleted BBX18, restored the expression of thermoresponsive genes in the evening. We further show that BBX18 prevents PRR5 from inhibiting PIF4-mediated high temperature responses. Taken together, our results suggest that BBX18 regulates thermoresponsive growth through the PRR5-PIF4 pathway.

SnRK1 and trehalose 6-phosphate - two ancient pathways converge to regulate plant metabolism and growth

SUCROSE-NON-FERMENTING1-RELATED KINASE1 (SnRK1) belongs to a family of protein kinases that originated in the earliest eukaryotes and plays a central role in energy and metabolic homeostasis. Trehalose 6-phosphate (Tre6P) is the intermediate of trehalose biosynthesis, and has even more ancient roots, being found in all three domains of life - Archaea, Bacteria and Eukarya. In plants, the function of SnRK1 has diverged from its orthologues in fungi and animals, evolving new roles in signalling of nutrient status and abiotic stress. Tre6P has also acquired a novel function in plants as a signal and homeostatic regulator of sucrose, the dominant sugar in plant metabolism. These two ancient pathways have converged in a unique way in plants, enabling them to coordinate their metabolism, growth, and development with their environment, which is essential for their autotrophic and sessile lifestyle.

Sugar Signaling and Post-transcriptional Regulation in Plants: An Overlooked or an Emerging Topic?

Plants are autotrophic organisms that self-produce sugars through photosynthesis. These sugars serve as an energy source, carbon skeletons, and signaling entities throughout plants' life. Post-transcriptional regulation of gene expression plays an important role in various sugar-related processes. In cells, it is regulated by many factors, such as RNA-binding proteins (RBPs), microRNAs, the spliceosome, etc. To date, most of the investigations into sugar-related gene expression have been focused on the transcriptional level in plants, while only a few studies have been conducted on post-transcriptional mechanisms. The present review provides an overview of the relationships between sugar and post-transcriptional regulation in plants. It addresses the relationships between sugar signaling and RBPs, microRNAs, and mRNA stability. These new items insights will help to reach a comprehensive understanding of the diversity of sugar signaling regulatory networks, and open onto new investigations into the relevance of these regulations for plant growth and development.