Sucrose control of translation mediated by an upstream open reading frame-encoded peptide

Ribosome decision graphs for the representation of eukaryotic RNA translation complexity

The application of ribosome profiling has revealed an unexpected abundance of translation in addition to that responsible for the synthesis of previously annotated protein-coding regions. Multiple short sequences have been found to be translated within single RNA molecules, within both annotated protein-coding and noncoding regions. The biological significance of this translation is a matter of intensive investigation. However, current schematic or annotation-based representations of mRNA translation generally do not account for the apparent multitude of translated regions within the same molecules. They also do not take into account the stochasticity of the process that allows alternative translations of the same RNA molecules by different ribosomes. There is a need for formal representations of mRNA complexity that would enable the analysis of quantitative information on translation and more accurate models for predicting the phenotypic effects of genetic variants affecting translation. To address this, we developed a conceptually novel abstraction that we term ribosome decision graphs (RDGs). RDGs represent translation as multiple ribosome paths through untranslated and translated mRNA segments. We termed the latter "translons." Nondeterministic events, such as initiation, reinitiation, selenocysteine insertion, or ribosomal frameshifting, are then represented as branching points. This representation allows for an adequate representation of eukaryotic translation complexity and focuses on locations critical for translation regulation. We show how RDGs can be used for depicting translated regions and for analyzing genetic variation and quantitative genome-wide data on translation for characterization of regulatory modulators of translation.

Genome-Wide Analysis of C/S1-bZIP Subfamilies in Populus tomentosa and Unraveling the Role of PtobZIP55/21 in Response to Low Energy

C/S1 basic leucine zipper (bZIP) transcription factors are essential for plant survival under energy deficiency. However, studies on the responses of C/S1-bZIPs to low energy in woody plants have not yet been reported. In this study, members of C/S1-bZIP subfamilies in Populus tomentosa were systematically analyzed using bioinformatic approaches. Four C-bZIPs and 10 S1-bZIPs were identified, and their protein properties, phylogenetic relationships, gene structures, conserved motifs, and uORFs were systematically investigated. In yeast two-hybrid assays, direct physical interactions between C-bZIP and S1-bZIP members were observed, highlighting their potential functional synergy. Moreover, expression profile analyses revealed that low energy induced transcription levels of most C/S1-bZIP members, with bZIP55 and bZIP21 (a homolog of bZIP55) exhibiting particularly significant upregulation. When the expression of bZIP55 and bZIP21 was co-suppressed using artificial microRNA mediated gene silencing in transgenic poplars, root growth was promoted. Further analyses revealed that bZIP55/21 negatively regulated the root development of P. tomentosa in response to low energy. These findings provide insights into the molecular mechanisms by which C/S1-bZIPs regulate poplar growth and development in response to energy deprivation.

What, where, and how: Regulation of translation and the translational landscape in plants

Translation is a crucial step in gene expression and plays a vital role in regulating various aspects of plant development and environmental responses. It is a dynamic and complex program that involves interactions between mRNAs, transfer RNAs, and the ribosome machinery through both cis- and trans-regulation while integrating internal and external signals. Translational control can act in a global (transcriptome-wide) or mRNA-specific manner. Recent advances in genome-wide techniques, particularly ribosome profiling and proteomics, have led to numerous exciting discoveries in both global and mRNA-specific translation. In this review, we aim to provide a "primer" that introduces readers to this fascinating yet complex cellular process and provide a big picture of how essential components connect within the network. We begin with an overview of mRNA translation, followed by a discussion of the experimental approaches and recent findings in the field, focusing on unannotated translation events and translational control through cis-regulatory elements on mRNAs and trans-acting factors, as well as signaling networks through 3 conserved translational regulators TOR, SnRK1, and GCN2. Finally, we briefly touch on the spatial regulation of mRNAs in translational control. Here, we focus on cytosolic mRNAs; translation in organelles and viruses is not covered in this review.

Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane

Sugars transported from leaves (source) to stems (sink) energize cell growth, elongation, and maintenance. which are regulated by a variety of genes. This review reflects progress and prospects in the regulatory mechanism for maximum sucrose accumulation, including the role of sucrose metabolizing enzymes, sugar transporters and the elucidation of post-transcriptional control of sucrose-induced regulation of translation (SIRT) in the accumulation of sucrose. The current review suggests that SIRT is emerging as a significant mechanism controlling Scbzip44 activities in response to endogenous sugar signals (via the negative feedback mechanism). Sucrose-controlled upstream open reading frame (SC-uORF) exists at the 5' leader region of Scbzip44's main ORF, which inhibits sucrose accumulation through post-transcriptional regulatory mechanisms. Sucrose transporters (SWEET1a/4a/4b/13c, TST, SUT1, SUT4 and SUT5) are crucial for sucrose translocation from source to sink. Particularly, SWEET13c was found to be a major contributor to the efflux in the transportation of stems. Tonoplast sugar transporters (TSTs), which import sucrose into the vacuole, suggest their tissue-specific role from source to sink. Sucrose cleavage has generally been linked with invertase isozymes, whereas sucrose synthase (SuSy)-catalyzed metabolism has been associated with biosynthetic processes such as UDP-Glc, cellulose, hemicellulose and other polymers. However, other two key sucrose-metabolizing enzymes, such as sucrose-6-phosphate phosphohydrolase (S6PP) and sucrose phosphate synthase (SPS) isoforms, have been linked with sucrose biosynthesis. These findings suggest that manipulation of genes, such as overexpression of SPS genes and sucrose transporter genes, silencing of the SC-uORF of Scbzip44 (removing the 5' leader region of the main ORF that is called SIRT-Insensitive) and downregulation of the invertase genes, may lead to maximum sucrose accumulation. This review provides an overview of sugarcane sucrose-regulating systems and baseline information for the development of cultivars with higher sucrose accumulation.

Improved super-resolution ribosome profiling reveals prevalent translation of upstream ORFs and small ORFs in Arabidopsis

A crucial step in functional genomics is identifying actively translated ORFs and linking them to biological functions. The challenge lies in identifying short ORFs, as their identification is greatly influenced by data quality and depth. Here, we improved the coverage of super-resolution Ribo-seq in Arabidopsis (Arabidopsis thaliana), revealing uncharacterized translation events for nuclear, chloroplastic, and mitochondrial genes. Assisted by a transcriptome assembly, we identified 7,751 unconventional translation events, comprising 6,996 upstream ORFs (uORFs) and 209 downstream ORFs on annotated protein-coding genes, as well as 546 ORFs in presumed noncoding RNAs. Proteomic data confirmed the production of stable proteins from some of these unannotated translation events. We present evidence of active translation from primary transcripts of trans-acting small interfering RNAs (TAS1-4) and microRNAs (pri-MIR163 and pri-MIR169) and periodic ribosome stalling supporting cotranslational decay. Additionally, we developed a method for identifying extremely short uORFs, including 370 minimum uORFs (AUG-stop), and 2,921 tiny uORFs (2 to 10 amino acids) and 681 uORFs that overlap with each other. Remarkably, these short uORFs exhibit strong translational repression as do longer uORFs. We also systematically discovered 594 uORFs regulated by alternative splicing, suggesting widespread isoform-specific translational control. Finally, these prevalent uORFs are associated with numerous important pathways. In summary, our improved Arabidopsis translational landscape provides valuable resources to study gene expression regulation.

OsbZIP38/87-mediated activation of OsHXK7 improves the viability of rice cells under hypoxic conditions

Maintenance of energy metabolism is critical for rice (Oryza sativa) tolerance under submerged cultivation. Here, OsHXK7 was the most actively induced hexokinase gene in the embryos of hypoxically germinating rice seeds. Suspension-cultured cells established from seeds of T-DNA null mutants for the OsHXK7 locus did not regrow after 3-d-hypoxic stress and showed increased susceptibility to low-oxygen stress-in terms of viability-and decreased alcoholic fermentation activities compared to those of the wild-type. The promoter element containing the TGACG-motif, a well-known target site for the basic leucine zipper (bZIP) transcription factors, was responsible for sugar regulation of the OsHXK7 promoter activity. Systematic screening of the OsbZIP genes showing the similar expression patterns to that of OsHXK7 in the transcriptomic datasets produced two bZIP genes, OsbZIP38 and 87, belonging to the S1 bZIP subfamily as the candidate for the activator for this gene expression. Gain- and loss-of-function experiments through transient expression assays have demonstrated that these two bZIP proteins are indeed involved in the induction of OsHXK7 expression under starvation or low-energy conditions. Our finding suggests that C/S1 bZIP network-mediated hypoxic deregulation of sugar-responsive genes may work in concert for the molecular adaptation of rice cells to submergence.

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.

Ribosome Decision Graphs for the Representation of Eukaryotic RNA Translation Complexity

The application of ribosome profiling has revealed an unexpected abundance of translation in addition to that responsible for the synthesis of previously annotated protein-coding regions. Multiple short sequences have been found to be translated within single RNA molecules, both within annotated protein-coding and non-coding regions. The biological significance of this translation is a matter of intensive investigation. However, current schematic or annotation-based representations of mRNA translation generally do not account for the apparent multitude of translated regions within the same molecules. They also do not take into account the stochasticity of the process that allows alternative translations of the same RNA molecules by different ribosomes. There is a need for formal representations of mRNA complexity that would enable the analysis of quantitative information on translation and more accurate models for predicting the phenotypic effects of genetic variants affecting translation. To address this, we developed a conceptually novel abstraction that we term Ribosome Decision Graphs (RDGs). RDGs represent translation as multiple ribosome paths through untranslated and translated mRNA segments. We termed the later 'translons'. Non-deterministic events, such as initiation, re-initiation, selenocysteine insertion or ribosomal frameshifting are then represented as branching points. This representation allows for an adequate representation of eukaryotic translation complexity and focuses on locations critical for translation regulation. We show how RDGs can be used for depicting translated regions, analysis of genetic variation and quantitative genome-wide data on translation for characterisation of regulatory modulators of translation.

Strigolactones repress nodule development and senescence in pea

Strigolactones are a class of phytohormones that are involved in many different plant developmental processes, including the rhizobium-legume nodule symbiosis. Although both positive and negative effects of strigolactones on the number of nodules have been reported, the influence of strigolactones on nodule development is still unknown. Here, by means of the ramosus (rms) mutants of Pisum sativum (pea) cv Terese, we investigated the impact of strigolactone biosynthesis (rms1 and rms5) and signaling (rms3 and rms4) mutants on nodule growth. The rms mutants had more red, that is, functional, and larger nodules than the wild-type plants. Additionally, the increased nitrogen fixation and senescence zones with consequently reduced meristematic and infection zones indicated that the rms nodules developed faster than the wild-type nodules. An enhanced expression of the nodule zone-specific molecular markers for meristem activity and senescence supported the enlarged, fast maturing nodules. Interestingly, the master nodulation regulator, NODULE INCEPTION, NIN, was strongly induced in nodules of all rms mutants but not prior to inoculation. Determination of sugar levels with both bulk and spatial metabolomics in roots and nodules, respectively, hints at slightly increased malic acid levels early during nodule primordia formation and reduced sugar levels at later stages, possibly the consequence of an increased carbon usage of the enlarged nodules, contributing to the enhanced senescence. Taken together, these results suggest that strigolactones regulate the development of nodules, which is probably mediated through NIN, and available plant sugars.

Interaction between sugar transport and plant development

Endogenous programs and constant interaction with the environment regulate the development of the plant organism and its individual organs. Sugars are necessary building blocks for plant and organ growth and at the same time act as critical integrators of the metabolic state into the developmental program. There is a growing recognition that the specific type of sugar and its subcellular or tissue distribution is sensed and translated to developmental responses. Therefore, the transport of sugars across membranes is a key process in adapting plant organ properties and overall development to the nutritional state of the plant. In this review, we discuss how plants exploit various sugar transporters to signal growth responses, for example, to control the development of sink organs such as roots or fruits. We highlight which sugar transporters are involved in root and shoot growth and branching, how intracellular sugar allocation can regulate senescence, and, for example, control fruit development. We link the important transport processes to downstream signaling cascades and elucidate the factors responsible for the integration of sugar signaling and plant hormone responses.

The translational landscape of bread wheat during grain development

The dynamics of gene expression in crop grains has typically been investigated at the transcriptional level. However, this approach neglects translational regulation, a widespread mechanism that rapidly modulates gene expression to increase the plasticity of organisms. Here, we performed ribosome profiling and polysome profiling to obtain a comprehensive translatome data set of developing bread wheat (Triticum aestivum) grains. We further investigated the genome-wide translational dynamics during grain development, revealing that the translation of many functional genes is modulated in a stage-specific manner. The unbalanced translation between subgenomes is pervasive, which increases the expression flexibility of allohexaploid wheat. In addition, we uncovered widespread previously unannotated translation events, including upstream open reading frames (uORFs), downstream open reading frames (dORFs), and open reading frames (ORFs) in long noncoding RNAs, and characterized the temporal expression dynamics of small ORFs. We demonstrated that uORFs act as cis-regulatory elements that can repress or even enhance the translation of mRNAs. Gene translation may be combinatorially modulated by uORFs, dORFs, and microRNAs. In summary, our study presents a translatomic resource that provides a comprehensive and detailed overview of the translational regulation in developing bread wheat grains. This resource will facilitate future crop improvements for optimal yield and quality.

GhbZIP30-GhCCCH17 module accelerates corm dormancy release by reducing endogenous ABA under cold storage in Gladiolus

Gladiolus hybridus is one of the most popular flowers worldwide. However, its corm dormancy characteristic largely limits its off-season production. Long-term cold treatment (LT), which increases sugar content and reduces abscisic acid (ABA), is an efficient approach to accelerate corm dormancy release (CDR). Here, we identified a GhbZIP30-GhCCCH17 module that mediates the antagonism between sugars and ABA during CDR. We showed that sugars promoted CDR by reducing ABA levels in Gladiolus. Our data demonstrated that GhbZIP30 transcription factor directly binds the GhCCCH17 zinc finger promoter and activates its transcription, confirmed by yeast one-hybrid, dual-luciferase (Dual-LUC), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) and electrophoretic mobility shift assay (EMSA). GhCCCH17 is a transcriptional activator, and its nuclear localisation is altered by glucose and cytokinin treatments. Both GhbZIP30 and GhCCCH17 positively respond to LT, sugars, and cytokinin treatments. Silencing GhbZIP30 or GhCCCH17 resulted in delayed CDR by regulating ABA metabolic genes, while their overexpression promoted CDR. Taken together, we propose that the GhbZIP30-GhCCCH17 module is involved in cold- and glucose-induced CDR by regulating ABA metabolic genes.

A model worker: Multifaceted modulation of AUXIN RESPONSE FACTOR3 orchestrates plant reproductive phases

The key phytohormone auxin is involved in practically every aspect of plant growth and development. Auxin regulates these processes by controlling gene expression through functionally distinct AUXIN RESPONSE FACTORs (ARFs). As a noncanonical ARF, ARF3/ETTIN (ETT) mediates auxin responses to orchestrate multiple developmental processes during the reproductive phase. The arf3 mutation has pleiotropic effects on reproductive development, causing abnormalities in meristem homeostasis, floral determinacy, phyllotaxy, floral organ patterning, gynoecium morphogenesis, ovule development, and self-incompatibility. The importance of ARF3 is also reflected in its precise regulation at the transcriptional, posttranscriptional, translational, and epigenetic levels. Recent studies have shown that ARF3 controls dynamic shoot apical meristem (SAM) maintenance in a non-cell autonomous manner. Here, we summarize the hierarchical regulatory mechanisms by which ARF3 is regulated and the diverse roles of ARF3 regulating developmental processes during the reproductive phase.

Disrupting Sc-uORFs of a transcription factor bZIP1 using CRISPR/Cas9 enhances sugar and amino acid contents in tomato (Solanum lycopersicum)

Induced mutations in the SC-uORF of the tomato transcription factor gene SlbZIP1 by the CRISPR/Cas9 system led to the high accumulation of sugar and amino acid contents in tomato fruits. Tomato (Solanum lycopersicum) is one of the most popular and consumed vegetable crops in the world. Among important traits for tomato improvement such as yield, biotic and abiotic resistances, appearance, post-harvest shelf life and fruit quality, the last one seems to face more challenges because of its genetic and biochemical complexities. In this study, a dual-gRNAs CRISPR/Cas9 system was developed to induce targeted mutations in uORF regions of the SlbZIP1, a gene involved in the sucrose-induced repression of translation (SIRT) mechanism. Different induced mutations in the SlbZIP1-uORF region were identified at the T0 generation, stably transferred to the offspring, and no mutation was found at potential off-target sites. The induced mutations in the SlbZIP1-uORF region affected the transcription of SlbZIP1 and related genes in sugar and amino acid biosynthesis. Fruit component analysis showed significant increases in soluble solid, sugar and total amino acid contents in all SlbZIP1-uORF mutant lines. The accumulation of sour-tasting amino acids, including aspartic and glutamic acids, raised from 77 to 144%, while the accumulation of sweet-tasting amino acids such as alanine, glycine, proline, serine, and threonine increased from 14 to 107% in the mutant plants. Importantly, the potential SlbZIP1-uORF mutant lines with desirable fruit traits and no impaired effect on plant phenotype, growth and development were identified under the growth chamber condition. Our result indicates the potential utility of the CRISPR/Cas9 system for fruit quality improvement in tomato and other important crops.

Cellular differentiation, hormonal gradient, and molecular alternation between the division zone and the elongation zone of bamboo internodes

Bamboo differentiates a cell division zone (DZ) and a cell elongation zone (EZ) to promote internode elongation during rapid growth. However, the biological mechanisms underlying this sectioned growth behavior are still unknown. Using histological, physiological, and genomic data, we found that the cell wall and other subcellular organelles such as chloroplasts are more developed in the EZ. Abundant hydrogen peroxide accumulated in the pith cells of the EZ, and stomata formed completely in the EZ. In contrast, most cells in the DZ were in an undifferentiated state with wrinkled cell walls and dense cytoplasm. Hormone detection revealed that the levels of gibberellin, auxin, cytokinin, and brassinosteroid were higher in the DZ than in the EZ. However, the levels of salicylic acid and jasmonic acid were higher in the EZ than in the DZ. Transcriptome analysis with qRT-PCR quantification revealed that the transcripts for cell division and primary metabolism had higher expression in the DZ, whereas the genes for photosynthesis, cell wall growth, and secondary metabolism were dramatically upregulated in the EZ. Overexpression of a MYB transcription factor, BmMYB83, promotes cell wall lignification in transgenic plants. BmMYB83 is specifically expressed in cells that may have lignin deposits, such as protoxylem vessels and fiber cells. Our results indicate that hormone gradient and transcriptome reprogramming, as well as specific expression of key genes such as BmMYB83, may lead to differentiation of cell growth in the bamboo internode.

Noise reduction by upstream open reading frames

Gene expression is prone to burst production, making it a highly noisy process that requires additional controls. Upstream open reading frames (uORFs) are widely present in the 5' leader sequences of 30-50% of eukaryotic messenger RNAs^1-3. The translation of uORFs can repress the translation efficiency of the downstream main coding sequences. Whether the low translation efficiency leads to a different variation, or noise, in gene expression has not been investigated, nor has the direct biological impact of uORF-repressed translation. Here we show that uORFs achieve low but precise protein production in plant cells, possibly by reducing the protein production rate. We also demonstrate that, by buffering a stable TIMING OF CAB EXPRESSION 1 (TOC1) protein production level, uORFs contribute to the robust operation of the plant circadian clock. Our results provide both an action model and the biological impact of uORFs in translational control to mitigate transcriptional noise for precise protein production.

Plants utilise ancient conserved peptide upstream open reading frames in stress-responsive translational regulation

The regulation of protein synthesis plays an important role in the growth and development of all organisms. Upstream open reading frames (uORFs) are commonly found in eukaryotic messenger RNA transcripts and typically attenuate the translation of associated downstream main ORFs (mORFs). Conserved peptide uORFs (CPuORFs) are a rare subset of uORFs, some of which have been shown to conditionally regulate translation by ribosome stalling. Here, we show that Arabidopsis CPuORF19, CPuORF46 and CPuORF47, which are ancient in origin, regulate translation of any downstream ORF, in response to the agriculturally significant environmental signals, heat stress and water limitation. Consequently, these CPuORFs represent a versatile toolkit for inducible gene expression with broad applications. Finally, we note that different classes of CPuORFs may operate during distinct phases of translation, which has implications for the bioengineering of these regulatory factors.

Gene expression profiling reveals transcription factor networks and subgenome bias during Brassica napus seed development

We profiled the global gene expression landscape across the reproductive lifecycle of Brassica napus. Comparative analysis of this nascent amphidiploid revealed the contribution of each subgenome to plant reproduction. Whole-genome transcription factor networks identified BZIP11 as a transcriptional regulator of early B. napus seed development. Knockdown of BZIP11 using RNA interference resulted in a similar reduction in gene activity of predicted gene targets, and a reproductive-lethal phenotype. Global mRNA profiling revealed lower accumulation of Cⁿ subgenome transcripts relative to the Aⁿ subgenome. Subgenome-specific transcription factor networks identified distinct transcription factor families enriched in each of the Aⁿ and Cⁿ subgenomes early in seed development. Analysis of laser-microdissected seed subregions further reveal subgenome expression dynamics in the embryo, endosperm and seed coat of early stage seeds. Transcription factors predicted to be regulators encoded by the Aⁿ subgenome are expressed primarily in the seed coat, whereas regulators encoded by the Cⁿ subgenome were expressed primarily in the embryo. Data suggest subgenome bias are characteristic features of the B. napus seed throughout development, and that such bias might not be universal across the embryo, endosperm and seed coat of the developing seed. Transcriptional networks spanning both the Aⁿ and Cⁿ genomes of the whole B. napus seed can identify valuable targets for seed development research and that -omics level approaches to studying gene regulation in B. napus can benefit from both broad and high-resolution analyses.

S1-bZIP Transcription Factors Play Important Roles in the Regulation of Fruit Quality and Stress Response

Sugar metabolism not only determines fruit sweetness and quality but also acts as signaling molecules to substantially connect with other primary metabolic processes and, therefore, modulates plant growth and development, fruit ripening, and stress response. The basic region/leucine zipper motif (bZIP) transcription factor family is ubiquitous in eukaryotes and plays a diverse array of biological functions in plants. Among the bZIP family members, the smallest bZIP subgroup, S1-bZIP, is a unique one, due to the conserved upstream open reading frames (uORFs) in the 5' leader region of their mRNA. The translated small peptides from these uORFs are suggested to mediate Sucrose-Induced Repression of Translation (SIRT), an important mechanism to maintain sucrose homeostasis in plants. Here, we review recent research on the evolution, sequence features, and biological functions of this bZIP subgroup. S1-bZIPs play important roles in fruit quality, abiotic and biotic stress responses, plant growth and development, and other metabolite biosynthesis by acting as signaling hubs through dimerization with the subgroup C-bZIPs and other cofactors like SnRK1 to coordinate the expression of downstream genes. Direction for further research and genetic engineering of S1-bZIPs in plants is suggested for the improvement of quality and safety traits of fruit.

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.

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.

Translational regulation in pathogenic and beneficial plant-microbe interactions

Plants are surrounded by a vast diversity of microorganisms. Limiting pathogenic microorganisms is crucial for plant survival. On the other hand, the interaction of plants with beneficial microorganisms promotes their growth or allows them to overcome nutrient deficiencies. Balancing the number and nature of these interactions is crucial for plant growth and development, and thus, for crop productivity in agriculture. Plants use sophisticated mechanisms to recognize pathogenic and beneficial microorganisms and genetic programs related to immunity or symbiosis. Although most research has focused on characterizing changes in the transcriptome during plant-microbe interactions, the application of techniques such as Translating Ribosome Affinity Purification (TRAP) and Ribosome profiling allowed examining the dynamic association of RNAs to the translational machinery, highlighting the importance of the translational level of control of gene expression in both pathogenic and beneficial interactions. These studies revealed that the transcriptional and the translational responses are not always correlated, and that translational control operates at cell-specific level. In addition, translational control is governed by cis-elements present in the 5'mRNA leader of regulated mRNAs, e.g. upstream open reading frames (uORFs) and sequence-specific motifs. In this review, we summarize and discuss the recent advances made in the field of translational control during pathogenic and beneficial plant-microbe interactions.

Dissection of a rice OsMac1 mRNA 5' UTR to uncover regulatory elements that are responsible for its efficient translation

The untranslated regions (UTRs) of mRNAs are involved in many posttranscriptional regulatory pathways. The rice OsMac1 mRNA has three splicing variants of the 5' UTR (UTRa, UTRb, and UTRc), which include a CU-rich region and three upstream open reading frames (uORFs). UTRc contains an additional 38-nt sequence, termed sp38, which acts as a strong translational enhancer of the downstream ORF; reporter analysis revealed translational efficiencies >15-fold higher with UTRc than with the other splice variants. Mutation analysis of UTRc demonstrated that an optimal sequence length of sp38, rather than its nucleotide sequence is essential for UTRc to promote efficient translation. In addition, the 5' 100 nucleotides of CU-rich region contribute to UTRc translational enhancement. Strikingly, three uORFs did not reveal their inhibitory potential within the full-length leader, whereas deletion of the 5' leader fragment preceding the leader region with uORFs nearly abolished translation. Computational prediction of UTRc structural motifs revealed stem-loop structures, termed SL1-SL4, and two regions, A and B, involved in putative intramolecular interactions. Our data suggest that SL4 binding to Region-A and base pairing between Region-B and the UTRc 3'end are critically required for translational enhancement. Since UTRc is not capable of internal initiation, we presume that the three-dimensional leader structures can allow translation of the leader downstream ORF, likely allowing the bypass of uORFs.

Global analysis of boron-induced ribosome stalling reveals its effects on translation termination and unique regulation by AUG-stops in Arabidopsis shoots

We previously reported that ribosome stalling at AUG-stop sequences in the 5'-untranslated region plays a critical role in regulating the expression of Arabidopsis thaliana NIP5;1, which encodes a boron uptake transporter, in response to boron conditions in media. This ribosome stalling is triggered specifically by boric acid, but the mechanisms are unknown. Although upstream open reading frames (uORFs) are known in many cases to regulate translation through peptides encoded by the uORF, AUG-stop stalling does not involve any peptide synthesis. The unique feature of AUG-stops - that termination follows immediately after initiation - suggests a possible effect of boron on the translational process itself. However, the generality of AUG-stop-mediated translational regulation and the effect of boron on translation at the genome scale are not clear. Here, we conducted a ribosome profiling analysis to reveal the genome-wide regulation of translation in response to boron conditions in A. thaliana shoots. We identified hundreds of translationally regulated genes that function in various biological processes. Under high-boron conditions, transcripts with reduced translation efficiency were rich in uORFs, highlighting the importance of uORF-mediated translational regulation. We found 673 uORFs that had more frequent ribosome association. Moreover, transcripts that were translationally downregulated under high-boron conditions were rich in minimum uORFs (AUG-stops), suggesting that AUG-stops play a global role in the boron response. Metagene analysis revealed that boron increased the ribosome occupancy of stop codons, indicating that this element is involved in global translational termination processes.

Advances and perspectives in discovery and functional analysis of small secreted proteins in plants

Small secreted proteins (SSPs) are less than 250 amino acids in length and are actively transported out of cells through conventional protein secretion pathways or unconventional protein secretion pathways. In plants, SSPs have been found to play important roles in various processes, including plant growth and development, plant response to abiotic and biotic stresses, and beneficial plant-microbe interactions. Over the past 10 years, substantial progress has been made in the identification and functional characterization of SSPs in several plant species relevant to agriculture, bioenergy, and horticulture. Yet, there are potentially a lot of SSPs that have not been discovered in plant genomes, which is largely due to limitations of existing computational algorithms. Recent advances in genomics, transcriptomics, and proteomics research, as well as the development of new computational algorithms based on machine learning, provide unprecedented capabilities for genome-wide discovery of novel SSPs in plants. In this review, we summarize known SSPs and their functions in various plant species. Then we provide an update on the computational and experimental approaches that can be used to discover new SSPs. Finally, we discuss strategies for elucidating the biological functions of SSPs in plants.

Narrow Leaf21, encoding ribosomal protein RPS3A, controls leaf development in rice

Leaf morphology influences photosynthesis, transpiration, and ultimately crop yield. However, the molecular mechanism of leaf development is still not fully understood. Here, we identified and characterized the narrow leaf21 (nal21) mutant in rice (Oryza sativa), showing a significant reduction in leaf width, leaf length and plant height, and increased tiller number. Microscopic observation revealed defects in the vascular system and reduced epidermal cell size and number in the nal21 leaf blade. Map-based cloning revealed that NAL21 encodes a ribosomal small subunit protein RPS3A. Ribosome-targeting antibiotics resistance assay and ribosome profiling showed a significant reduction in the free 40S ribosome subunit in the nal21 mutant. The nal21 mutant showed aberrant auxin responses in which multiple auxin response factors (ARFs) harboring upstream open-reading frames (uORFs) in their 5'-untranslated region were repressed at the translational level. The WUSCHEL-related homeobox 3A (OsWOX3A) gene, a key transcription factor involved in leaf blade lateral outgrowth, is also under the translational regulation by RPS3A. Transformation with modified OsARF11, OsARF16, and OsWOX3A genomic DNA (gDNA) lacking uORFs rescued the narrow leaf phenotype of nal21 to a better extent than transformation with their native gDNA, implying that RPS3A could regulate translation of ARFs and WOX3A through uORFs. Our results demonstrate that proper translational regulation of key factors involved in leaf development is essential to maintain normal leaf morphology.

Sugar and Nitrate Sensing: A Multi-Billion-Year Story

Sugars and nitrate play a major role in providing carbon and nitrogen in plants. Understanding how plants sense these nutrients is crucial, most notably for crop improvement. The mechanisms underlying sugar and nitrate sensing are complex and involve moonlighting proteins such as the nitrate transporter NRT1.1/NFP6.3 or the glycolytic enzyme HXK1. Major components of nutrient signaling, such as SnRK1, TOR, and HXK1, are relatively well conserved across eukaryotes, and the diversification of components such as the NRT1 family and the SWEET sugar transporters correlates with plant terrestrialization. In plants, Tre6P plays a hormone-like role in plant development. In addition, nutrient signaling has evolved to interact with the more recent hormone signaling, allowing fine-tuning of physiological and developmental responses.

Sugar modulation of anaerobic-response networks in maize root tips

Sugar supply is a key component of hypoxia tolerance and acclimation in plants. However, a striking gap remains in our understanding of mechanisms governing sugar impacts on low-oxygen responses. Here, we used a maize (Zea mays) root-tip system for precise control of sugar and oxygen levels. We compared responses to oxygen (21 and 0.2%) in the presence of abundant versus limited glucose supplies (2.0 and 0.2%). Low-oxygen reconfigured the transcriptome with glucose deprivation enhancing the speed and magnitude of gene induction for core anaerobic proteins (ANPs). Sugar supply also altered profiles of hypoxia-responsive genes carrying G4 motifs (sources of regulatory quadruplex structures), revealing a fast, sugar-independent class followed more slowly by feast-or-famine-regulated G4 genes. Metabolite analysis showed that endogenous sugar levels were maintained by exogenous glucose under aerobic conditions and demonstrated a prominent capacity for sucrose re-synthesis that was undetectable under hypoxia. Glucose abundance had distinctive impacts on co-expression networks associated with ANPs, altering network partners and aiding persistence of interacting networks under prolonged hypoxia. Among the ANP networks, two highly interconnected clusters of genes formed around Pyruvate decarboxylase 3 and Glyceraldehyde-3-phosphate dehydrogenase 4. Genes in these clusters shared a small set of cis-regulatory elements, two of which typified glucose induction. Collective results demonstrate specific, previously unrecognized roles of sugars in low-oxygen responses, extending from accelerated onset of initial adaptive phases by starvation stress to maintenance and modulation of co-expression relationships by carbohydrate availability.

Revisiting sORFs: overcoming challenges to identify and characterize functional microproteins

Short ORFs (sORFs), that is, occurrences of a start and stop codon within 100 codons or less, can be found in organisms of all domains of life, outnumbering annotated protein-coding ORFs by orders of magnitude. Even though functional proteins smaller than 100 amino acids are known, the coding potential of sORFs has often been overlooked, as it is not trivial to predict and test for functionality within the large number of sORFs. Recent advances in ribosome profiling and mass spectrometry approaches, together with refined bioinformatic predictions, have enabled a huge leap forward in this field and identified thousands of likely coding sORFs. A relatively low number of small proteins or microproteins produced from these sORFs have been characterized so far on the molecular, structural, and/or mechanistic level. These however display versatile and, in some cases, essential cellular functions, allowing for the exciting possibility that many more, previously unknown small proteins might be encoded in the genome, waiting to be discovered. This review will give an overview of the steadily growing microprotein field, focusing on eukaryotic small proteins. We will discuss emerging themes in the molecular action of microproteins, as well as advances and challenges in microprotein identification and characterization.

Insights Into Translatomics in the Nervous System

Most neurological disorders are caused by abnormal gene translation. Generally, dysregulation of elements involved in the translational process disrupts homeostasis in neurons and neuroglia. Better understanding of how the gene translation process occurs requires detailed analysis of transcriptomic and proteomic profile data. However, a lack of strictly direct correlations between mRNA and protein levels limits translational investigation by combining transcriptomic and proteomic profiling. The much better correlation between proteins and translated mRNAs than total mRNAs in abundance and insufficiently sensitive proteomics approach promote the requirement of advances in translatomics technology. Translatomics which capture and sequence the mRNAs associated with ribosomes has been effective in identifying translational changes by genetics or projections, ribosome stalling, local translation, and transcript isoforms in the nervous system. Here, we place emphasis on the main three translatomics methods currently used to profile mRNAs attached to ribosome-nascent chain complex (RNC-mRNA). Their prominent applications in neurological diseases including glioma, neuropathic pain, depression, fragile X syndrome (FXS), neurodegenerative disorders are outlined. The content reviewed here expands our understanding on the contributions of aberrant translation to neurological disease development.

Sucrose signaling in higher plants

Sucrose controls various developmental and metabolic processes in plants. In this review, we evaluate whether sucrose could be a preferred signaling molecule that controls processes like carbohydrate metabolism, accumulation of storage proteins, sucrose transport, anthocyanin accumulation, and floral induction. We summarize putative sucrose-dependent signaling pathways. Sucrose, but not other sugars, stimulates the genes that encode ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase I, and UDP-glucose pyrophosphorylase in several species. The class-1 patatin promoter is induced under high sucrose conditions in potato (Solanum tuberosum). Exogenous sucrose reduces the loading of sucrose to the phloem by inhibiting the expression of the sucrose transporter and its protein activity in sugar beet (Beta vulgaris). Sucrose also influences a wide range of growth processes, including cell division, ribosome synthesis, cotyledon development, far-red light signaling, and tuber development. Floral induction is promoted by sucrose in several species. The molecular mechanisms by which sucrose functions as a signal are largely unknown. Sucrose enhances the expression of transcription factors such as AtWRKY20 and MYB75, which function upstream of the sucrose-responsive genes. Sucrose controls the expression of AtbZIP11 at the post-transcriptional level by the peptide encoded by uORF2. Sucrose levels affect translation of a group of mRNAs in Arabidopsis. Sucrose increases the activity of AGPase by posttranslational redox-modification. Sucrose interrupts the interaction between sucrose transporter SUT4 and cytochrome b5. In addition, the SNF-related protein kinase-1 appears to be involved in sucrose-dependent pathways by controlling sucrose synthase (SUS4) expression.

Translational gene regulation in plants: A green new deal

The molecular machinery for protein synthesis is profoundly similar between plants and other eukaryotes. Mechanisms of translational gene regulation are embedded into the broader network of RNA-level processes including RNA quality control and RNA turnover. However, over eons of their separate history, plants acquired new components, dropped others, and generally evolved an alternate way of making the parts list of protein synthesis work. Research over the past 5 years has unveiled how plants utilize translational control to defend themselves against viruses, regulate translation in response to metabolites, and reversibly adjust translation to a wide variety of environmental parameters. Moreover, during seed and pollen development plants make use of RNA granules and other translational controls to underpin developmental transitions between quiescent and metabolically active stages. The economics of resource allocation over the daily light-dark cycle also include controls over cellular protein synthesis. Important new insights into translational control on cytosolic ribosomes continue to emerge from studies of translational control mechanisms in viruses. Finally, sketches of coherent signaling pathways that connect external stimuli with a translational response are emerging, anchored in part around TOR and GCN2 kinase signaling networks. These again reveal some mechanisms that are familiar and others that are different from other eukaryotes, motivating deeper studies on translational control in plants. This article is categorized under: Translation > Translation Regulation RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Ribosome profiling reveals the effects of nitrogen application translational regulation of yield recovery after abrupt drought-flood alternation in rice

Abrupt drought-flood alternation is a frequent meteorological disaster during the summer in Southern China. The study of physiological and translation mechanisms of rice yield recovery after abrupt drought-flood alternation has great potential benefits in field production. Our results showed that yield recovery upon nitrogen (N) application after abrupt drought-flood alternation was due to the increase in effective panicle numbers per plant. The N application resulted in the regulation of physiological and biochemical as well as growth development processes, which led to a rapid growth recovery effect after abrupt drought-flood alternation stress in rice. Using ribosome profiling combined with RNA sequencing (RNA-seq) technology, the interactions between transcription and translation for N application after abrupt drought-flood alternation were analyzed. It was found that a small proportion of response genes were shared at the transcriptional and translational levels, that is, 14% of the expressed genes were upregulated and 6.6% downregulated. Further analysis revealed that the translation efficiency (TE) of the genes was influenced by their sequence characteristics, including their GC content, coding sequence length and normalized minimal free energy. Compared with the number of untranslated upstream open reading frames (uORFs), the increased number of translated uORFs promoted the improvement of TE. The TE of the uORFs for N application was lower than the control without N application after abrupt drought-flood alternation. This study characterizes the translational regulatory pattern in response to N application after abrupt drought-flood alternation stress.

Fine-tuning sugar content in strawberry

Fine-tuning quantitative traits for continuous subtle phenotypes is highly advantageous. We engineer the highly conserved upstream open reading frame (uORF) of FvebZIPs1.1 in strawberry (Fragaria vesca), using base editor A3A-PBE. Seven novel alleles are generated. Sugar content of the homozygous T1 mutant lines is 33.9-83.6% higher than that of the wild-type. We also recover a series of transgene-free mutants with 35 novel genotypes containing a continuum of sugar content. All the novel genotypes could be immediately fixed in subsequent generations by asexual reproduction. Genome editing coupled with asexual reproduction offers tremendous opportunities for quantitative trait improvement.

Conserved Upstream Open Reading Frame Nascent Peptides That Control Translation

Cells utilize transcriptional and posttranscriptional mechanisms to alter gene expression in response to environmental cues. Gene-specific controls, including changing the translation of specific messenger RNAs (mRNAs), provide a rapid means to respond precisely to different conditions. Upstream open reading frames (uORFs) are known to control the translation of mRNAs. Recent studies in bacteria and eukaryotes have revealed the functions of evolutionarily conserved uORF-encoded peptides. Some of these uORF-encoded nascent peptides enable responses to specific metabolites to modulate the translation of their mRNAs by stalling ribosomes and through ribosome stalling may also modulate the level of their mRNAs. In this review, we highlight several examples of conserved uORF nascent peptides that stall ribosomes to regulate gene expression in response to specific metabolites in bacteria, fungi, mammals, and plants.

uORF Shuffling Fine-Tunes Gene Expression at a Deep Level of the Process

Upstream open reading frames (uORFs) are present in the 5’ leader sequences (or 5’ untranslated regions) upstream of the protein-coding main ORFs (mORFs) in eukaryotic polycistronic mRNA. It is well known that a uORF negatively affects translation of the mORF. Emerging ribosome profiling approaches have revealed that uORFs themselves, as well as downstream mORFs, can be translated. However, it has also been revealed that plants can fine-tune gene expression by modulating uORF-mediated regulation in some situations. This article reviews several proposed mechanisms that enable genes to escape from uORF-mediated negative regulation and gives insight into the application of uORF-mediated regulation for precisely controlling gene expression.

Comprehensive genome-wide identification of angiosperm upstream ORFs with peptide sequences conserved in various taxonomic ranges using a novel pipeline, ESUCA

Background

Upstream open reading frames (uORFs) in the 5′-untranslated regions (5′-UTRs) of certain eukaryotic mRNAs encode evolutionarily conserved functional peptides, such as cis-acting regulatory peptides that control translation of downstream main ORFs (mORFs). For genome-wide searches for uORFs with conserved peptide sequences (CPuORFs), comparative genomic studies have been conducted, in which uORF sequences were compared between selected species. To increase chances of identifying CPuORFs, we previously developed an approach in which uORF sequences were compared using BLAST between Arabidopsis and any other plant species with available transcript sequence databases. If this approach is applied to multiple plant species belonging to phylogenetically distant clades, it is expected to further comprehensively identify CPuORFs conserved in various plant lineages, including those conserved among relatively small taxonomic groups.

Results

To efficiently compare uORF sequences among many species and efficiently identify CPuORFs conserved in various taxonomic lineages, we developed a novel pipeline, ESUCA. We applied ESUCA to the genomes of five angiosperm species, which belong to phylogenetically distant clades, and selected CPuORFs conserved among at least three different orders. Through these analyses, we identified 89 novel CPuORF families. As expected, ESUCA analysis of each of the five angiosperm genomes identified many CPuORFs that were not identified from ESUCA analyses of the other four species. However, unexpectedly, these CPuORFs include those conserved across wide taxonomic ranges, indicating that the approach used here is useful not only for comprehensive identification of narrowly conserved CPuORFs but also for that of widely conserved CPuORFs. Examination of the effects of 11 selected CPuORFs on mORF translation revealed that CPuORFs conserved only in relatively narrow taxonomic ranges can have sequence-dependent regulatory effects, suggesting that most of the identified CPuORFs are conserved because of functional constraints of their encoded peptides.

Conclusions

This study demonstrates that ESUCA is capable of efficiently identifying CPuORFs likely to be conserved because of the functional importance of their encoded peptides. Furthermore, our data show that the approach in which uORF sequences from multiple species are compared with those of many other species, using ESUCA, is highly effective in comprehensively identifying CPuORFs conserved in various taxonomic ranges.

Translational Inhibition of α-Neurexin 2

Neurexins are extensively investigated presynaptic cell-adhesion molecules which play important roles in transmitting signals and processing information at synapses that connect neurons into a vast network of cellular communications. Synaptic transmission of information is a fast and dynamic process which relies on rapid and tight regulation of synaptic protein expression. However, the mechanism underlying those regulation is still not fully understood. Therefore, we explore how the expression of NRXN2α, one of encoding genes for neurexins, is regulated at the translational level. NRXN2α transcript has a long and conserved 5'-untranslated region (5'UTR) suggestive of the rapid regulation of protein expression at the translational level. We first demonstrate that the 5'UTR has negative effects on the expression of the NRXN2α and find a critical subregion responsible for the major inhibitory function. Then we identify a particular secondary structure of G-quadruplex in the 5'UTR. Moreover, we find that the synergistic roles of G-quadruplex and upstream AUGs are responsible for most of NRXN2α-5'UTR inhibitory effects. In conclusion, we uncovered 5' UTR of neurexin2 potentially inhibits neurexin2 translation by multiple mechanisms. In addition, this study underscores the importance of direct protein quantitation in experiments rather than using mRNA as an indirect estimate of protein expression.

Seed-Stored mRNAs that Are Specifically Associated to Monosomes Are Translationally Regulated during Germination

The life cycle of many organisms includes a quiescent stage, such as bacterial or fungal spores, insect larvae, or plant seeds. Common to these stages is their low water content and high survivability during harsh conditions. Upon rehydration, organisms need to reactivate metabolism and protein synthesis. Plant seeds contain many mRNAs that are transcribed during seed development. Translation of these mRNAs occurs during early seed germination, even before the requirement of transcription. Therefore, stored mRNAs are postulated to be important for germination. How these mRNAs are stored and protected during long-term storage is unknown. The aim of this study was to investigate how mRNAs are stored in dry seeds and whether they are indeed translated during seed germination. We investigated seed polysome profiles and the mRNAs and protein complexes that are associated with these ribosomes in seeds of the model organism Arabidopsis (Arabidopsis thaliana). We showed that most stored mRNAs are associated with monosomes in dry seeds; therefore, we focus on monosomes in this study. Seed ribosome complexes are associated with mRNA-binding proteins, stress granule, and P-body proteins, which suggests regulated packing of seed mRNAs. Interestingly, ∼17% of the mRNAs that are specifically associated with monosomes are translationally up-regulated during seed germination. These mRNAs are transcribed during seed maturation, suggesting a role for this developmental stage in determining the translational fate of mRNAs during early germination.

Metabolite Control of Translation by Conserved Peptide uORFs: The Ribosome as a Metabolite Multisensor

Ribosomes translate the mRNA code into protein, and this process can be controlled by metabolites that bind to the translating ribosome in interaction with the nascent protein.

Genome-wide identification and characterization of the GDP-L-galactose phosphorylase gene family in bread wheat

BACKGROUND

Ascorbate is a powerful antioxidant in plants and an essential micronutrient for humans. The GDP-L-galactose phosphorylase (GGP) gene encodes the rate-limiting enzyme of the L-galactose pathway-the dominant ascorbate biosynthetic pathway in plants-and is a promising gene candidate for increasing ascorbate in crops. In addition to transcriptional regulation, GGP production is regulated at the translational level through an upstream open reading frame (uORF) in the long 5'-untranslated region (5'UTR). The GGP genes have yet to be identified in bread wheat (Triticum aestivum L.), one of the most important food grain sources for humans.

RESULTS

Bread wheat chromosomal groups 4 and 5 were found to each contain three homoeologous TaGGP genes on the A, B, and D subgenomes (TaGGP2-A/B/D and TaGGP1-A/B/D, respectively) and a highly conserved uORF was present in the long 5'UTR of all six genes. Phylogenetic analyses demonstrated that the TaGGP genes separate into two distinct groups and identified a duplication event of the GGP gene in the ancestor of the Brachypodium/Triticeae lineage. A microsynteny analysis revealed that the TaGGP1 and TaGGP2 subchromosomal regions have no shared synteny suggesting that TaGGP2 may have been duplicated via a transposable element. The two groups of TaGGP genes have distinct expression patterns with the TaGGP1 homoeologs broadly expressed across different tissues and developmental stages and the TaGGP2 homoeologs highly expressed in anthers. Transient transformation of the TaGGP coding sequences in Nicotiana benthamiana leaf tissue increased ascorbate concentrations more than five-fold, confirming their functional role in ascorbate biosynthesis in planta.

CONCLUSIONS

We have identified six TaGGP genes in the bread wheat genome, each with a highly conserved uORF. Phylogenetic and microsynteny analyses highlight that a transposable element may have been responsible for the duplication and specialized expression of GGP2 in anthers in the Brachypodium/Triticeae lineage. Transient transformation of the TaGGP coding sequences in N. benthamiana demonstrated their activity in planta. The six TaGGP genes and uORFs identified in this study provide a valuable genetic resource for increasing ascorbate concentrations in bread wheat.

HISTONE DEACETYLASE 9 stimulates auxin-dependent thermomorphogenesis in Arabidopsis thaliana by mediating H2A.Z depletion

Many plant species respond to unfavorable high ambient temperatures by adjusting their vegetative body plan to facilitate cooling. This process is known as thermomorphogenesis and is induced by the phytohormone auxin. Here, we demonstrate that the chromatin-modifying enzyme HISTONE DEACETYLASE 9 (HDA9) mediates thermomorphogenesis but does not interfere with hypocotyl elongation during shade avoidance. HDA9 is stabilized in response to high temperature and mediates histone deacetylation at the YUCCA8 locus, a rate-limiting enzyme in auxin biosynthesis, at warm temperatures. We show that HDA9 permits net eviction of the H2A.Z histone variant from nucleosomes associated with YUCCA8, allowing binding and transcriptional activation by PHYTOCHROME INTERACTING FACTOR 4, followed by auxin accumulation and thermomorphogenesis.

Culm transcriptome sequencing of Badila (Saccharum officinarum L.) and analysis of major genes involved in sucrose accumulation

Sugarcane is an important sugar and energy crop worldwide. It utilises highly efficient C4 photosynthesis and accumulates sucrose in its culms. The sucrose content in sugarcane culms is a quantitative trait controlled by multiple genes. The regulatory mechanism underlying the maximum sucrose level in sugarcane culms remains unclear. We used transcriptome sequences to identify the potential regulatory genes involved in sucrose accumulation in Saccarum officinarum L. cv. Badila. The sucrose accumulating internodes at the elongation and mature growth stage and the immature internodes with low sucrose content at the mature stage were used for RNA sequencing. The obtained differentially expressed genes (DEGs) related to sucrose accumulation were analysed. Results showed that the transcripts encoding invertase (beta-fructofuranosidase, EC: 3.2.1.26) which catalyses sucrose hydrolysis and 6-phosphofructokinase (PFK, EC: 2.7.1.11), a key glycolysis regulatory enzyme, were downregulated in the high sucrose accumulation internodes. The transcripts encoding key enzymes for ABA, gibberellin and ethylene synthesis were also downregulated during sucrose accumulation. Furthermore, regulated protein kinase, transcription factor and sugar transporter genes were also obtained. This research can clarify the molecular regulation network of sucrose accumulation in sugarcane.

Novel pipeline identifies new upstream ORFs and non-AUG initiating main ORFs with conserved amino acid sequences in the 5' leader of mRNAs in Arabidopsis thaliana

Eukaryotic mRNAs contain a 5' leader sequence preceding the main open reading frame (mORF) and, depending on the species, 20%-50% of eukaryotic mRNAs harbor an upstream ORF (uORF) in the 5' leader. An unknown fraction of these uORFs encode sequence conserved peptides (conserved peptide uORFs, CPuORFs). Experimentally validated CPuORFs demonstrated to regulate the translation of downstream mORFs often do so in a metabolite concentration-dependent manner. Previous research has shown that most CPuORFs possess a start codon context suboptimal for translation initiation, which turns out to be favorable for translational regulation. The suboptimal initiation context may even include non-AUG start codons, which makes CPuORFs hard to predict. For this reason, we developed a novel pipeline to identify CPuORFs unbiased of start codon using well-annotated sequence data from 31 eudicot plant species and rice. Our new pipeline was able to identify 29 novel Arabidopsis thaliana (Arabidopsis) CPuORFs, conserved across a wide variety of eudicot species of which 15 do not initiate with an AUG start codon. In addition to CPuORFs, the pipeline was able to find 14 conserved coding regions directly upstream and in frame with the mORF, which likely initiate translation on a non-AUG start codon. Altogether, our pipeline identified highly conserved coding regions in the 5' leaders of Arabidopsis transcripts, including in genes with proven functional importance such as LHY, a key regulator of the circadian clock, and the RAPTOR1 subunit of the target of rapamycin (TOR) kinase.

The Arabidopsis bZIP transcription factor family-an update

The basic (region) leucine zippers (bZIPs) are evolutionarily conserved transcription factors in eukaryotic organisms. Here, we have updated the classification of the Arabidopsis thaliana bZIP-family, comprising 78 members, which have been assorted into 13 groups. Arabidopsis bZIPs are involved in a plethora of functions related to plant development, environmental signalling and stress response. Based on the classification, we have highlighted functional and regulatory aspects of selected well-studied bZIPs, which may serve as prototypic examples for the particular groups.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

TOR signaling in plants: conservation and innovation

Target of rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in both plants and animals, despite their distinct developmental programs and survival strategies. Indeed, TOR integrates nutrient, energy, hormone, growth factor and environmental inputs to control proliferation, growth and metabolism in diverse multicellular organisms. Here, we compare the molecular composition, upstream regulators and downstream signaling relays of TOR complexes in plants and animals. We also explore and discuss the pivotal functions of TOR signaling in basic cellular processes, such as translation, cell division and stem/progenitor cell regulation during plant development.

Craniofrontonasal Syndrome Caused by Introduction of a Novel uATG in the 5'UTR of EFNB1

Craniofrontonasal syndrome (CFNS) is an X-linked disorder caused by EFNB1 mutations in which females are more severely affected than males. Severe male phenotypes are associated with mosaicism, supporting cellular interference for sex bias in this disease. Although many variants have been found in the coding region of EFNB1, only 2 pathogenic variants have been identified in the same nucleotide in 5'UTR, disrupting the stop codon of an upstream open reading frame (uORF). uORFs are known to be part of a wide range of post-transcriptional regulation processes, and just recently, their association with human diseases has come to light. In the present study, we analyzed EFNB1 in a female patient with typical features of CFNS. We identified a variant, located at c.-411, creating a new upstream ATG (uATG) in the 5'UTR of EFNB1, which is predicted to alter an existing uORF. Dual-luciferase reporter assays showed significant reduction in protein translation, but no difference in the mRNA levels. Our study demonstrates, for the first time, the regulatory impact of uATG formation on EFNB1 levels and suggests that this should be the target region in molecular diagnosis of CFNS cases without pathogenic variants in the coding and splice sites regions of EFNB1.

Boron-Dependent Translational Suppression of the Borate Exporter BOR1 Contributes to the Avoidance of Boron Toxicity

Boron (B) is an essential element for plants; however, as high B concentrations are toxic, B transport must be tightly regulated. BOR1 is a borate exporter in Arabidopsis (Arabidopsis thaliana) that facilitates B translocation into shoots under B deficiency conditions. When the B supply is sufficient, BOR1 expression is down-regulated by selective degradation of BOR1 protein, while additional BOR1 regulatory mechanisms are proposed to exist. In this study, we identified a novel B-dependent BOR1 translational suppression mechanism. In vivo and in vitro reporter assays demonstrated that BOR1 translation was reduced in a B-dependent manner and that the 5'-untranslated region was both necessary and sufficient for this process. Mutational analysis revealed that multiple upstream open reading frames in the 5'-untranslated region were required for BOR1 translational suppression, and this process depended on the efficiency of translational reinitiation at the BOR1 open reading frame after translation of the upstream open reading frames. To understand the physiological significance of BOR1 regulation, we characterized transgenic plants defective in either one or both of the BOR1 regulation mechanisms. BOR1 translational suppression was induced at higher B concentrations than those triggering BOR1 degradation. Plants lacking both regulation mechanisms exhibited more severe shoot growth reduction under high-B conditions than did plants lacking BOR1 degradation alone, thus demonstrating the importance of BOR1 translational suppression. This study demonstrates that two mechanisms of posttranscriptional BOR1 regulation, each induced under different B concentrations, contribute to the avoidance of B toxicity in plants.