The intron of Arabidopsis thaliana polyubiquitin genes is conserved in location and is a quantitative determinant of chimeric gene expression

Arabidopsis as a model for translational research

Arabidopsis thaliana is currently the most-studied plant species on earth, with an unprecedented number of genetic, genomic, and molecular resources having been generated in this plant model. In the era of translating foundational discoveries to crops and beyond, we aimed to highlight the utility and challenges of using Arabidopsis as a reference for applied plant biology research, agricultural innovation, biotechnology, and medicine. We hope that this review will inspire the next generation of plant biologists to continue leveraging Arabidopsis as a robust and convenient experimental system to address fundamental and applied questions in biology. We aim to encourage laboratory and field scientists alike to take advantage of the vast Arabidopsis datasets, annotations, germplasm, constructs, methods, and molecular and computational tools in our pursuit to advance understanding of plant biology and help feed the world's growing population. We envision that the power of Arabidopsis-inspired biotechnologies and foundational discoveries will continue to fuel the development of resilient, high-yielding, nutritious plants for the betterment of plant and animal health and greater environmental sustainability.

GET3B is involved in chloroplast biogenesis and interacts with the thylakoidal ALB3 and ALB4 insertases

KEY MESSAGE

Proteomic, functional physiological analyses of get3b mutant plants highlight GET3B's role in chloroplast function. Genetic and interaction analyses indicate get3b and srp54 as mutual potentiators that might share terminal insertases. Protein targeting and insertion into membranes are essential for cellular organization and organelle function. The Guided Entry of Tail-anchored (GET) pathway facilitates the post-translational targeting and insertion of tail-anchored (TA) membrane proteins. Arabidopsis thaliana has four GET3 homologues, including AtGET3B and AtGET3D localized to chloroplasts. These photosynthetic organelles possess complex membrane systems, and the mechanisms underlying their protein targeting and membrane biogenesis are not fully understood. This study conducted a comprehensive proteomic analysis of get3b mutant plastids, which displayed significant alterations. Fluorometric based complex assembly as well as CO2 assimilation analyses confirmed that disruption of GET3B function displayed a significant impact on photosystem II assembly as well as carbon fixation, respectively, indicating a functional role in chloroplast biogenesis. Additionally, genetic interactions were found between GET3B and the two component STIC system, which cooperates with the cpSRP pathway which is involved in the co-translational sorting of thylakoid proteins. Further, physical interactions were observed between GET3B and the C-terminus of ALB3 and ALB4 in vitro and the full length proteins in vivo, indicating a role of GET3B in protein targeting and membrane integration within chloroplasts. These findings enhance our understanding of GET3B's involvement in stromal protein targeting and thylakoidal biogenesis.

A novel geminivirus-derived 3' flanking sequence of terminator mediates the gene expression enhancement

Exploring the new elements to re-design the expression cassette is crucial in synthetic biology. Viruses are one of the most important sources for exploring gene expression elements. In this study, we found that the DNA sequence of the SBG51 deltasatellite from the Sweet potato leaf curl virus (SPLCV) greatly enhanced the gene expression when flanked downstream of the terminator. The SBG51 sequence increased transient GFP gene expression in Nicotiana benthamiana leaves by up to ~6 times and ~10 times compared to the gene expression controlled by the UBQ10 promoter and 35S promoter alone, respectively. The increased GFP gene expression level contributed to the continuous accumulation of GFP protein and GFP fluorescence until 8 days post-inoculation (dpi). The SBG51 sequence also enhanced the gene expression in the transgenic Arabidopsis plants and maintained the spatio-temporal pattern of the FLOWERING LOCUS T (FT) and TOO MANY MOUTHS (TMM) promoters. We identified a 123 bp of AT-rich sequence containing seven "ATAAA" or "TTAAA" elements from the SBG51 DNA, which had the gene expression enhancement effect. Furthermore, the artificial synthetic sequences containing tandem repeated "ATAAA" or "TTAAA" elements were sufficient to increase the gene expression but did not alter the polyadenylation of mRNA, similar to the function of matrix attachment regions (MAR). Additionally, the compact artificial synthetic sequence also had an effect on yeast when the expression cassette was integrated into the genome. We conclude that the geminivirus deltasatellite-derived sequence and the "ATAAA"/"TTAAA" elements are powerful tools for enhancing gene expression.

Natural variation in salt-induced changes in root:shoot ratio reveals SR3G as a negative regulator of root suberization and salt resilience in Arabidopsis

Soil salinity is one of the major threats to agricultural productivity worldwide. Salt stress exposure alters root and shoots growth rates, thereby affecting overall plant performance. While past studies have extensively documented the effect of salt stress on root elongation and shoot development separately, here we take an innovative approach by examining the coordination of root and shoot growth under salt stress conditions. Utilizing a newly developed tool for quantifying the root:shoot ratio in agar-grown Arabidopsis seedlings, we found that salt stress results in a loss of coordination between root and shoot growth rates. We identify a specific gene cluster encoding domain-of-unknown-function 247 (DUF247), and characterize one of these genes as Salt Root:shoot Ratio Regulator Gene (SR3G). Further analysis elucidates the role of SR3G as a negative regulator of salt stress tolerance, revealing its function in regulating shoot growth, root suberization, and sodium accumulation. We further characterize that SR3G expression is modulated by WRKY75 transcription factor, known as a positive regulator of salt stress tolerance. Finally, we show that the salt stress sensitivity of wrky75 mutant is completely diminished when it is combined with sr3g mutation. Together, our results demonstrate that utilizing root:shoot ratio as an architectural feature leads to the discovery of a new stress resilience gene. The study's innovative approach and findings not only contribute to our understanding of plant stress tolerance mechanisms but also open new avenues for genetic and agronomic strategies to enhance crop environmental resilience.

Decoupling the pleiotropic effects of VRT-A2 during reproductive development enhances wheat grain length and weight

VEGETATIVE TO REPRODUCTIVE TRANSITION 2 (VRT-A2) is a subspecies-forming gene that confers the long-glume and large-grain traits of tetraploid Polish wheat (Triticum polonicum; AABB) and hexaploid Xinjiang rice wheat (T. petropavlovskyi; AABBDD). Transcriptional activation of VRT-A2 due to a natural sequence variation in its Intron-1 region significantly enhances grain weight but also causes some basal spikelets to fail to completely develop, thus decreasing grain number per spike and yield. This yield penalty has presented a challenge for the use of VRT-A2 in breeding high-yield wheat. Here, we report the characterization of 2 regulatory modules that fine-tune VRT-A2 expression in bread wheat (T. aestivum): (i) the APETALA2/Ethylene Responsive Factor (AP2/ERF)-type transcription factor MULTI-FLORET SPIKELET1 (TaMFS1) represses VRT-A2 expression by recruiting a transcriptional corepressor and a histone deacetylase and (ii) the STRUCTURE-SPECIFIC RECOGNITION PROTEIN 1 (TaSSRP1) facilitates VRT-A2 activation by assembling Mediator and further RNA polymerase II. Deleting TaMFS1 triggered moderate upregulation of VRT-A2 results in significantly increased grain weight without the yield penalty. Our study thus provides a feasible strategy for overcoming the tradeoffs of pleotropic genes by editing their upstream transcriptional regulators.

Widespread position-dependent transcriptional regulatory sequences in plants

Much of what we know about eukaryotic transcription stems from animals and yeast; however, plants evolved separately for over a billion years, leaving ample time for divergence in transcriptional regulation. Here we set out to elucidate fundamental properties of cis-regulatory sequences in plants. Using massively parallel reporter assays across four plant species, we demonstrate the central role of sequences downstream of the transcription start site (TSS) in transcriptional regulation. Unlike animal enhancers that are position independent, plant regulatory elements depend on their position, as altering their location relative to the TSS significantly affects transcription. We highlight the importance of the region downstream of the TSS in regulating transcription by identifying a DNA motif that is conserved across vascular plants and is sufficient to enhance gene expression in a dose-dependent manner. The identification of a large number of position-dependent enhancers points to fundamental differences in gene regulation between plants and animals.

Pupylation-Based Proximity-Tagging of FERONIA-Interacting Proteins in Arabidopsis

The plasma membrane-localized receptor kinase FERONIA (FER) plays critical roles in a remarkable variety of biological processes throughout the life cycle of Arabidopsis thaliana. Revealing the molecular connections of FER that underlie these processes starts with identifying the proteins that interact with FER. We applied pupylation-based interaction tagging (PUP-IT) to survey cellular proteins in proximity to FER, encompassing weak and transient interactions that can be difficult to capture for membrane proteins. We reproducibly identified 581, 115, and 736 specific FER-interacting protein candidates in protoplasts, seedlings, and flowers, respectively. We also confirmed 14 previously characterized FER-interacting proteins. Protoplast transient gene expression expedited the testing of new gene constructs for PUP-IT analyses and the validation of candidate proteins. We verified the proximity labeling of five selected candidates that were not previously characterized as FER-interacting proteins. The PUP-IT method could be a valuable tool to survey and validate protein-protein interactions for targets of interest in diverse subcellular compartments in plants.

Cleave and Rescue gamete killers create conditions for gene drive in plants

Gene drive elements promote the spread of linked traits and can be used to change the composition or fate of wild populations. Cleave and Rescue (ClvR) drive elements sit at a fixed chromosomal position and include a DNA sequence-modifying enzyme such as Cas9/gRNAs that disrupts endogenous versions of an essential gene and a recoded version of the essential gene resistant to cleavage. ClvR spreads by creating conditions in which those lacking ClvR die because they lack functional versions of the essential gene. Here we demonstrate the essential features of the ClvR gene drive in the plant Arabidopsis thaliana through killing of gametes that fail to inherit a ClvR that targets the essential gene YKT61. Resistant alleles, which can slow or prevent drive, were not observed. Modelling shows plant ClvRs are robust to certain failure modes and can be used to rapidly drive population modification or suppression. Possible applications are discussed.

Phylogeny-linked occurrence of ribosome stalling on the mRNAs of Arabidopsis unfolded protein response factor bZIP60 orthologs in divergent plant species

The bZIP60, XBP1 and HAC1 mRNAs encode transcription factors that mediate the unfolded protein response (UPR) in plants, animals and yeasts, respectively. Upon UPR, these mRNAs undergo unconventional cytoplasmic splicing on the endoplasmic reticulum (ER) to produce active transcription factors. Although cytoplasmic splicing is conserved, the ER targeting mechanism differs between XBP1 and HAC1. The ER targeting of HAC1 mRNA occurs before translation, whereas that of XBP1 mRNA involves a ribosome-nascent chain complex that is stalled when a hydrophobic peptide emerges from the ribosome; the corresponding mechanism is unknown for bZIP60. Here, we analyzed ribosome stalling on bZIP60 orthologs of plants. Using a cell-free translation system, we detected nascent peptide-mediated ribosome stalling during the translation elongation of the mRNAs of Arabidopsis, rice and Physcomitrium (moss) orthologs, and the termination-step stalling in the Selaginella (lycopod) ortholog, all of which occurred ∼50 amino acids downstream of a hydrophobic region. Transfection experiments showed that ribosome stalling contributes to cytoplasmic splicing in bZIP60u orthologs of Arabidopsis and Selaginella. In contrast, ribosome stalling was undetectable for liverwort, Klebsormidium (basal land plant), and green algae orthologs. This study highlights the evolutionary diversity of ribosome stalling and its contribution to ER targeting in plants.

A quantitative gibberellin signaling biosensor reveals a role for gibberellins in internode specification at the shoot apical meristem

Growth at the shoot apical meristem (SAM) is essential for shoot architecture construction. The phytohormones gibberellins (GA) play a pivotal role in coordinating plant growth, but their role in the SAM remains mostly unknown. Here, we developed a ratiometric GA signaling biosensor by engineering one of the DELLA proteins, to suppress its master regulatory function in GA transcriptional responses while preserving its degradation upon GA sensing. We demonstrate that this degradation-based biosensor accurately reports on cellular changes in GA levels and perception during development. We used this biosensor to map GA signaling activity in the SAM. We show that high GA signaling is found primarily in cells located between organ primordia that are the precursors of internodes. By gain- and loss-of-function approaches, we further demonstrate that GAs regulate cell division plane orientation to establish the typical cellular organization of internodes, thus contributing to internode specification in the SAM.

Cleave and Rescue gamete killers create conditions for gene drive in plants

Gene drive elements promote the spread of linked traits, even when their presence confers a fitness cost to carriers, and can be used to change the composition or fate of wild populations. Cleave and Rescue (ClvR) drive elements sit at a fixed chromosomal position and include a DNA sequence-modifying enzyme such as Cas9/gRNAs (the Cleaver/Toxin) that disrupts endogenous versions of an essential gene, and a recoded version of the essential gene resistant to cleavage (the Rescue/Antidote). ClvR spreads by creating conditions in which those lacking ClvR die because they lack functional versions of the essential gene. We demonstrate the essential features of ClvR gene drive in the plant Arabidopsis thaliana through killing of gametes that fail to inherit a ClvR that targets the essential gene YKT61, whose expression is required in male and female gametes for their survival. Resistant (uncleavable but functional) alleles, which can slow or prevent drive, were not observed. Modeling shows plant ClvRs are likely to be robust to certain failure modes and can be used to rapidly drive population modification or suppression. Possible applications in plant breeding, weed control, and conservation are discussed.

E and M SARS-CoV-2 membrane protein expression and enrichment with plant lipid droplets

Plants are gaining traction as a cost-effective and scalable platform for producing recombinant proteins. However, expressing integral membrane proteins in plants is challenging due to their hydrophobic nature. In our study, we used transient and stable expression systems in Nicotiana benthamiana and Camelina sativa respectively to express SARS-CoV-2 E and M integral proteins, and target them to lipid droplets (LDs). LDs offer an ideal environment for folding hydrophobic proteins and aid in their purification through flotation. We tested various protein fusions with different linkers and tags and used three dimensional structure predictions to assess their effects. E and M mostly localized in the ER in N. benthamiana leaves but E could be targeted to LDs in oil accumulating tobacco when fused with oleosin, a LD integral protein. In Camelina sativa seeds, E and M were however found associated with purified LDs. By enhancing the accumulation of E and M within LDs through oleosin, we enriched these proteins in the purified floating fraction. This strategy provides an alternative approach for efficiently producing and purifying hydrophobic pharmaceuticals and vaccines using plant systems.

A double-negative feedback loop between miR319c and JAW-TCPs establishes growth pattern in incipient leaf primordia in Arabidopsis thaliana

The microRNA miR319 and its target JAW-TCP transcription factors regulate the proliferation-to-differentiation transition of leaf pavement cells in diverse plant species. In young Arabidopsis leaf primordia, JAW-TCPs are detected towards the distal region whereas the major mRNA319-encoding gene MIR319C, is expressed at the base. Little is known about how this complementary expression pattern of MIR319C and JAW-TCPs is generated. Here, we show that MIR319C is initially expressed uniformly throughout the incipient primordia and is later abruptly down-regulated at the distal region, with concomitant distal appearance of JAW-TCPs, when leaves grow to ~100 μm long. Loss of JAW-TCPs causes distal extension of the MIR319C expression domain, whereas ectopic TCP activity restricts MIR319C more proximally. JAW-TCPs are recruited to and are capable of depositing histone H3K27me3 repressive marks on the MIR319C chromatin. JAW-TCPs fail to repress MIR319C in transgenic seedlings where the TCP-binding cis-elements on MIR319C are mutated, causing miR319 gain-of-function-like phenotype in the embryonic leaves. Based on these results, we propose a model for growth patterning in leaf primordia wherein MIR319C and JAW-TCPs repress each other and divide the uniformly growing primordia into distal differentiation zone and proximal proliferation domain.

Plant Promoters and Terminators for High-Precision Bioengineering

High-precision bioengineering and synthetic biology require fine-tuning gene expression at both transcriptional and posttranscriptional levels. Gene transcription is tightly regulated by promoters and terminators. Promoters determine the timing, tissues and cells, and levels of the expression of genes. Terminators mediate transcription termination of genes and affect mRNA levels posttranscriptionally, e.g., the 3'-end processing, stability, translation efficiency, and nuclear to cytoplasmic export of mRNAs. The promoter and terminator combination affects gene expression. In the present article, we review the function and features of plant core promoters, proximal and distal promoters, and terminators, and their effects on and benchmarking strategies for regulating gene expression.

Next-generation ABACUS biosensors reveal cellular ABA dynamics driving root growth at low aerial humidity

The plant hormone abscisic acid (ABA) accumulates under abiotic stress to recast water relations and development. To overcome a lack of high-resolution sensitive reporters, we developed ABACUS2s-next-generation Förster resonance energy transfer (FRET) biosensors for ABA with high affinity, signal-to-noise ratio and orthogonality-that reveal endogenous ABA patterns in Arabidopsis thaliana. We mapped stress-induced ABA dynamics in high resolution to reveal the cellular basis for local and systemic ABA functions. At reduced foliar humidity, root cells accumulated ABA in the elongation zone, the site of phloem-transported ABA unloading. Phloem ABA and root ABA signalling were both essential to maintain root growth at low humidity. ABA coordinates a root response to foliar stresses, enabling plants to maintain foraging of deeper soil for water uptake.

A contemporary reassessment of the enhanced transient expression system based on the tombusviral silencing suppressor protein P19

Transient transgenic expression accelerates pharming and facilitates protein studies in plants. One embodiment of the approach involves leaf infiltration of Agrobacterium strains whose T-DNA is engineered with the gene(s) of interest. However, gene expression during 'agro-infiltration' is intrinsically and universally impeded by the onset of post-transcriptional gene silencing (PTGS). Nearly 20 years ago, a simple method was developed, whereby co-expression of the tombusvirus-encoded P19 protein suppresses PTGS and thus enhances transient gene expression. Yet, how PTGS is activated and suppressed by P19 during the process has remained unclear to date. Here, we address these intertwined questions in a manner also rationalizing how vastly increased protein yields are achieved using a minimal viral replicon as a transient gene expression vector. We also explore, in side-by-side analyses, why some proteins do not accumulate to the expected high levels in the assay, despite vastly increased mRNA levels. We validate that enhanced co-expression of multiple constructs is achieved within the same transformed cells, and illustrate how the P19 system allows rapid protein purification for optimized downstream in vitro applications. Finally, we assess the suitability of the P19 system for subcellular localization studies - an originally unanticipated, yet increasingly popular application - and uncover shortcomings of this specific implement. In revisiting the P19 system using contemporary knowledge, this study sheds light onto its hitherto poorly understood mechanisms while further illustrating its versatility but also some of its limits.

Halophyte Nitraria billardieri CIPK25 promotes photosynthesis in Arabidopsis under salt stress

The calcineurin B-like (CBL)-interacting protein kinases (CIPKs), a type of plant-specific genes in the calcium signaling pathway, function in response to adverse environments. However, few halophyte derived CIPKs have been studied for their role in plant physiological and developmental adaptation during abiotic stresses, which inhibits the potential application of these genes to improve environmental adaptability of glycophytes. In this study, we constructed Nitraria billardieri CIPK25 overexpressing Arabidopsis and analyzed the seedling development under salt treatment. Our results show that Arabidopsis with NbCIPK25 expression exhibits more vigorous growth than wild type plants under salt condition. To gain insight into the molecular mechanisms underlying salt tolerance, we profiled the transcriptome of WT and transgenic plants via RNA-seq. GO and KEGG analyses revealed that upregulated genes in NbCIPK25 overexpressing seedlings under salt stress are enriched in photosynthesis related terms; Calvin-cycle genes including glyceraldehyde-3-phosphate dehydrogenases (GAPDHs) are significantly upregulated in transgenic plants, which is consistent with a decreased level of NADPH (GAPDH substrate) and increased level of NADP⁺. Accordingly, NbCIPK25 overexpressing plants exhibited more efficient photosynthesis; soluble sugar and proteins, as photosynthesis products, showed a higher accumulation in transgenic plants. These results provide molecular insight into how NbCIPK25 promotes the expression of genes involved in photosynthesis, thereby maintaining plant growth under salt stress. Our finding supports the potential application of halophyte-derived NbCIPK25 in genetic modification for better salt adaptation.

Increased rates of gene-editing events using a simplified RNAi configuration designed to reduce gene silencing

An optimal RNAi configuration that could restrict gene expression most efficiently was determined. This approach was also used to target PTGS and yielded higher rates of gene-editing events. Although it was characterized long ago, transgene silencing still strongly impairs transgene overexpression, and thus is a major barrier to plant crop gene-editing. The development of strategies that could prevent transgene silencing is therefore essential to the success of gene editing assays. Transgene silencing occurs via the RNA silencing process, which regulates the expression of essential genes and protects the plant from viral infections. The RNA silencing machinery thereby controls central biological processes such as growth, development, genome integrity, and stress resistance. RNA silencing is typically induced by aberrant RNA, that may lack 5' or 3' processing, or may consist in double-stranded or hairpin RNA, and involves DICER and ARGONAUTE family proteins. In this study, RNAi inducing constructs were designed in eleven different configurations and were evaluated for their capacity to induce silencing in Nicotiana spp. using transient and stable transformation assays. Using reporter genes, it was found that the overexpression of a hairpin consisting of a forward tandem inverted repeat that started with an ATG and that was not followed downstream by a transcription terminator, could downregulate gene expression most potently. Furthermore, using this method, the downregulation of the NtSGS3 gene caused a significant increase in transgene expression both in transient and stable transformation assays. This SGS3 silencing approach was also employed in gene-editing assays and caused higher rates of gene-editing events. Taken together, these findings suggested the optimal genetic configuration to cause RNA silencing and showed that this strategy may be used to restrict PTGS during gene-editing experiments.

Halophyte Nitraria billardieri CIPK25 mitigates salinity-induced cell damage by alleviating H2O2 accumulation

The plant-specific module of calcineurin B-like proteins (CBLs) and CBL-interacting protein kinases (CIPKs) play a crucial role in plant adaptation to different biotic and abiotic stresses in various plant species. Despite the importance of the CBL-CIPK module in regulating plant salt tolerance, few halophyte CIPK orthologs have been studied. We identified NbCIPK25 in the halophyte Nitraria billardieri as a salt-responsive gene that may improve salt tolerance in glycophytes. Sequence analyses indicated that NbCIPK25 is a typical CIPK family member with a conserved NAF motif, which contains the amino acids: asparagine, alanine, and phenylalanine. NbCIPK25 overexpression in salt-stressed transgenic Arabidopsis seedlings resulted in enhanced tolerance to salinity, a higher survival rate, longer newly grown roots, more root meristem cells, and less damaged root cells in comparison to wild-type (WT) plants. H2O2 accumulation and malondialdehyde (MDA) content were both deceased in NbCIPK25-transgenic plants under salt treatment. Furthermore, their proline content, an important factor for scavenging reactive oxygen species, accumulated at a significantly higher level. In concordance, the transcription of genes related to proline accumulation was positively regulated in transgenic plants under salt condition. Finally, we observed a stronger auxin response in salt-treated transgenic roots. These results provide evidence for NbCIPK25 improving salt tolerance by mediating scavenging of reactive oxygen species, thereby protecting cells from oxidation and maintaining plant development under salt stress. These findings suggest the potential application of salt-responsive NbCIPK25 for cultivating glycophytes with a higher salt tolerance through genetic engineering.

Building the Plant SynBio Toolbox through Combinatorial Analysis of DNA Regulatory Elements

While the installation of complex genetic circuits in microorganisms is relatively routine, the synthetic biology toolbox is severely limited in plants. Of particular concern is the absence of combinatorial analysis of regulatory elements, the long design-build-test cycles associated with transgenic plant analysis, and a lack of naming standardization for cloning parts. Here, we use previously described plant regulatory elements to design, build, and test 91 transgene cassettes for relative expression strength. Constructs were transiently transfected into Nicotiana benthamiana leaves and expression of a fluorescent reporter was measured from plant canopies, leaves, and protoplasts isolated from transfected plants. As anticipated, a dynamic level of expression was achieved from the library, ranging from near undetectable for the weakest cassette to a ∼200-fold increase for the strongest. Analysis of expression levels in plant canopies, individual leaves, and protoplasts were correlated, indicating that any of the methods could be used to evaluate regulatory elements in plants. Through this effort, a well-curated 37-member part library of plant regulatory elements was characterized, providing the necessary data to standardize construct design for precision metabolic engineering in plants.

Gene activation via Cre/lox-mediated excision in cowpea (Vigna unguiculata)

Expression of Cre recombinase by AtRps5a_pro or AtDD45_pro enabled Cre/lox-mediated recombination at an early embryonic developmental stage upon crossing, activating transgenes in the hybrid cowpea and tobacco. Genetic engineering ideally results in precise spatiotemporal control of transgene expression. To activate transgenes exclusively in a hybrid upon fertilization, we evaluated a Cre/lox-mediated gene activation system with the Cre recombinase expressed by either AtRps5a or AtDD45 promoters that showed activity in egg cells and young embryos. In crosses between Cre recombinase lines and transgenic lines harboring a lox-excision reporter cassette with ZsGreen driven by the AtUbq3 promoter after Cre/lox-mediated recombination, we observed complete excision of the lox-flanked intervening DNA sequence between the AtUbq3_pro and the ZsGreen coding sequence in F₁ progeny upon genotyping but no ZsGreen expression in F₁ seeds or seedlings. The incapability to observe ZsGreen fluorescence was attributed to the activity of the AtUbq3_pro. Strong ZsGreen expression in F₁ seeds was observed after recombination when ZsGreen was driven by the AtUbq10 promoter. Using the AtDD45_pro to express Cre resulted in more variation in recombination frequencies between transgenic lines and crosses. Regardless of the promoter used to regulate Cre, mosaic F₁ progeny were rare, suggesting gene activation at an early embryo-developmental stage. Observation of ZsGreen-expressing tobacco embryos at the globular stage from crosses with the AtRps5a_proCre lines pollinated by the AtUbq3_prolox line supported the early activation mode.

Optimization of T-DNA configuration with UBIQUITIN10 promoters and tRNA-sgRNA complexes promotes highly efficient genome editing in allotetraploid tobacco

KEY MESSAGE

Combination of UBIQUITIN10 promoter-directed CAS9 and tRNA-gRNA complexes in gene-editing assay induces 80% mutant phenotype with a knockout of the four allelic copies in the T0 generation of allotetraploid tobaccos. While gene-editing methodologies, such as CRISPR-Cas9, have been developed and successfully used in many plant species, their use remains challenging, because they most often rely on stable or transient transgene expression. Regrettably, in all plant species, transformation causes epigenetic effects such as gene silencing and variable transgene expression. Here, UBIQUITIN10 promoters from several plant species were characterized and showed their capacity to direct high levels of transgene expression in transient and stable transformation assays, which in turn was used to improve the selection process of regenerated transformants. Furthermore, we compared various sgRNAs delivery systems and showed that the combination of UBIQUITIN10 promoters and tRNA-sgRNA complexes produced 80% mutant phenotype with a complete knockout of the four allelic copies, while the remaining 20% exhibited weaker phenotype, which suggested partial allelic knockout, in the T0 generation of the allotetraploid Nicotiana tabacum. These data provide valuable information to optimize future designs of gene editing constructs for plant research and crop improvement and open the way for valuable gene editing projects in non-model Solanaceae species.

The Cell Fate Controlling CLE40 Peptide Requires CNGCs to Trigger Highly Localized Ca2+ Transients in Arabidopsis thaliana Root Meristems

Communication between plant cells and their biotic environment largely depends on the function of plasma membrane localized receptor-like kinases (RLKs). Major players in this communication within root meristems are secreted peptides, including CLAVATA3/EMBRYO SURROUNDING REGION40 (CLE40). In the distal root meristem, CLE40 acts through the RLK ARABIDOPSIS CRINKLY4 (ACR4) and the leucine-rich repeat (LRR) RLK CLAVATA1 (CLV1) to promote cell differentiation. In the proximal meristem, CLE40 signaling requires the LRR receptor-like protein CLAVATA2 (CLV2) and the membrane localized pseudokinase CORYNE (CRN) and serves to inhibit cell differentiation. The molecular components that act immediately downstream of the CLE40-activated receptors are not yet known. Here, we show that active CLE40 signaling triggers the release of intracellular Ca2+ leading to increased cytosolic Ca2+ concentration ([Ca2+]cyt) in a small subset of proximal root meristem cells. This rise in [Ca2+]cyt depends on the CYCLIC NUCLEOTIDE GATED CHANNELS (CNGCs) 6 and 9 and on CLV1. The precise function of changes in [Ca2+]cyt is not yet known but might form a central part of a fine-tuned response to CLE40 peptide that serves to integrate root meristem growth with stem cell fate decisions and initiation of lateral root primordia.

Coexpression of Fungal Cell Wall-Modifying Enzymes Reveals Their Additive Impact on Arabidopsis Resistance to the Fungal Pathogen, Botrytis cinerea

Simple Summary

In the present study, we created transgenic Arabidopsis plants overexpressing two fungal acetylesterases and a fungal feruloylesterase that acts on cell wall polysaccharides and studied their possible complementary additive effects on host defense reactions against the fungal pathogen, Botrytis cinerea. Our results showed that the Arabidopsis plants overexpressing two acetylesterases together contributed significantly higher resistance to B. cinerea in comparison with single protein expression. Conversely, coexpression of either of the acetyl esterases together with feruloylesterase compensates the latter’s negative impact on plant resistance. The results also provided evidence that combinatorial coexpression of some cell wall polysaccharide-modifying enzymes might exert an additive effect on plant immune response by constitutively priming plant defense pathways even before pathogen invasion. These findings have potential uses in protecting valuable crops against pathogens.

Abstract

The plant cell wall (CW) is an outer cell skeleton that plays an important role in plant growth and protection against both biotic and abiotic stresses. Signals and molecules produced during host–pathogen interactions have been proven to be involved in plant stress responses initiating signal pathways. Based on our previous research findings, the present study explored the possibility of additively or synergistically increasing plant stress resistance by stacking beneficial genes. In order to prove our hypothesis, we generated transgenic Arabidopsis plants constitutively overexpressing three different Aspergillus nidulans CW-modifying enzymes: a xylan acetylesterase, a rhamnogalacturonan acetylesterase and a feruloylesterase. The two acetylesterases were expressed either together or in combination with the feruloylesterase to study the effect of CW polysaccharide deacetylation and deferuloylation on Arabidopsis defense reactions against a fungal pathogen, Botrytis cinerea. The transgenic Arabidopsis plants expressing two acetylesterases together showed higher CW deacetylation and increased resistance to B. cinerea in comparison to wild-type (WT) Col-0 and plants expressing single acetylesterases. While the expression of feruloylesterase alone compromised plant resistance, coexpression of feruloylesterase together with either one of the two acetylesterases restored plant resistance to the pathogen. These CW modifications induced several defense-related genes in uninfected healthy plants, confirming their impact on plant resistance. These results demonstrated that coexpression of complementary CW-modifying enzymes in different combinations have an additive effect on plant stress response by constitutively priming the plant defense pathways. These findings might be useful for generating valuable crops with higher protections against biotic stresses.

A homolog of GuidedEntry of Tail-anchored proteins3 functions in membrane-specific protein targeting in chloroplasts of Arabidopsis

The chloroplasts and mitochondria of photosynthetic eukaryotes contain proteins that are closely related to cytosolic Guided Entry of Tail-anchored proteins3 (Get3). Get3 is a targeting factor that efficiently escorts tail-anchored (TA) proteins to the ER. Because other components of the cytosolic-targeting pathway appear to be absent in organelles, previous investigators have asserted that organellar Get3 homologs are unlikely to act as targeting factors. However, we show here both that the Arabidopsis thaliana chloroplast homolog designated as GET3B is structurally similar to cytosolic Get3 proteins and that it selectively binds a thylakoid-localized TA protein. Based on genetic interactions between a get3b mutation and mutations affecting the chloroplast signal recognition particle-targeting pathway, as well as changes in the abundance of photosynthesis-related proteins in mutant plants, we propose that GET3B acts primarily to direct proteins to the thylakoids. Furthermore, through molecular complementation experiments, we show that function of GET3B depends on its ability to hydrolyze ATP, and this is consistent with action as a targeting factor. We propose that GET3B and related organellar Get3 homologs play a role that is analogous to that of cytosolic Get3 proteins, and that GET3B acts as a targeting factor in the chloroplast stroma to deliver TA proteins in a membrane-specific manner.

A molecular toolkit for the green seaweed Ulva mutabilis

The green seaweed Ulva mutabilis is an ecologically important marine primary producer as well as a promising cash crop cultivated for multiple uses. Despite its importance, several molecular tools are still needed to better understand seaweed biology. Here, we report the development of a flexible and modular molecular cloning toolkit for the green seaweed U. mutabilis based on a Golden Gate cloning system. The toolkit presently contains 125 entry vectors, 26 destination vectors, and 107 functionally validated expression vectors. We demonstrate the importance of endogenous regulatory sequences for transgene expression and characterize three endogenous promoters suitable to drive transgene expression. We describe two vector architectures to express transgenes via two expression cassettes or a bicistronic approach. The majority of selected transformants (50%-80%) consistently give clear visual transgene expression. Furthermore, we made different marker lines for intracellular compartments after evaluating 13 transit peptides and 11 tagged endogenous Ulva genes. Our molecular toolkit enables the study of Ulva gain-of-function lines and paves the way for gene characterization and large-scale functional genomics studies in a green seaweed.

Arabidopsis Ca2+-ATPases 1, 2, and 7 in the endoplasmic reticulum contribute to growth and pollen fitness

Generating cellular Ca2+ signals requires coordinated transport activities from both Ca2+ influx and efflux pathways. In Arabidopsis (Arabidopsis thaliana), multiple efflux pathways exist, some of which involve Ca2+-pumps belonging to the Autoinhibited Ca2+-ATPase (ACA) family. Here, we show that ACA1, 2, and 7 localize to the endoplasmic reticulum (ER) and are important for plant growth and pollen fertility. While phenotypes for plants harboring single-gene knockouts (KOs) were weak or undetected, a triple KO of aca1/2/7 displayed a 2.6-fold decrease in pollen transmission efficiency, whereas inheritance through female gametes was normal. The triple KO also resulted in smaller rosettes showing a high frequency of lesions. Both vegetative and reproductive phenotypes were rescued by transgenes encoding either ACA1, 2, or 7, suggesting that all three isoforms are biochemically redundant. Lesions were suppressed by expression of a transgene encoding NahG, an enzyme that degrades salicylic acid (SA). Triple KO mutants showed elevated mRNA expression for two SA-inducible marker genes, Pathogenesis-related1 (PR1) and PR2. The aca1/2/7 lesion phenotype was similar but less severe than SA-dependent lesions associated with a double KO of vacuolar pumps aca4 and 11. Imaging of Ca2+ dynamics triggered by blue light or the pathogen elicitor flg22 revealed that aca1/2/7 mutants display Ca2+ transients with increased magnitudes and durations. Together, these results indicate that ER-localized ACAs play important roles in regulating Ca2+ signals, and that the loss of these pumps results in male fertility and vegetative growth deficiencies.

Prospects Of Non-Coding Elements In Genomic Dna Based Gene Therapy

Gene therapy has made significant development since the commencement of the first clinical trials a few decades ago and has remained a dynamic area of research regardless of obstacles such as immune response and insertional mutagenesis. Progression in various technologies like next-generation sequencing (NGS) and nanotechnology has established the importance of non-coding segments of a genome, thereby taking gene therapy to the next level. In this review, we have summarized the importance of non-coding elements, highlighting the advantages of using full-length genomic DNA loci (gDNA) compared to complementary DNA (cDNA) or minigene, currently used in gene therapy. The focus of this review is to provide an overview of the advances and the future of potential use of gDNA loci in gene therapy, expanding the therapeutic repertoire in molecular medicine.

CIPK11: a calcineurin B-like protein-interacting protein kinase from Nitraria tangutorum, confers tolerance to salt and drought in Arabidopsis

BACKGROUND

The CIPKs are a group of plant-specific Ser/Thr protein kinases acting in response to calcium signaling, which plays an important role in the physiological and developmental adaptation of plants to adverse environments. However, the functions of halophyte-derived CIPKs are still poorly understood, that limits a potential application of CIPKs from halophytes for improving the tolerance of glycophytes to abiotic stresses.

RESULTS

In this study, we characterized the NtCIPK11 gene from the halophyte Nitraria tangutorum and subsequently analyzed its role in salt and drought stress tolerance, using Arabidopsis as a transgenic model system. NtCIPK11 expression was upregulated in N. tangutorum root, stem and blade tissues after salt or drought treatment. Overexpressing NtCIPK11 in Arabidopsis improved seed germination on medium containing different levels of NaCl. Moreover, the transgenic plants grew more vigorously under salt stress and developed longer roots under salt or drought conditions than the WT plants. Furthermore, NtCIPK11 overexpression altered the transcription of genes encoding key enzymes involved in proline metabolism in Arabidopsis exposed to salinity, however, which genes showed a relatively weak expression in the transgenic Arabidopsis undergoing mannitol treatment, a situation that mimics drought stress. Besides, the proline significantly accumulated in NtCIPK11-overexpressing plants compared with WT under NaCl treatment, but that was not observed in the transgenic plants under drought stress caused by mannitol application.

CONCLUSIONS

We conclude that NtCIPK11 promotes plant growth and mitigates damage associated with salt stress by regulating the expression of genes controlling proline accumulation. These results extend our understanding on the function of halophyte-derived CIPK genes and suggest that NtCIPK11 can serve as a candidate gene for improving the salt and drought tolerance of glycophytes through genetic engineering.

TGA class II transcription factors are essential to restrict oxidative stress in response to UV-B stress in Arabidopsis

Plants possess a robust metabolic network for sensing and controlling reactive oxygen species (ROS) levels upon stress conditions. Evidence shown here supports a role for TGA class II transcription factors as critical regulators of genes controlling ROS levels in the tolerance response to UV-B stress in Arabidopsis. First, tga256 mutant plants showed reduced capacity to scavenge H2O2 and restrict oxidative damage in response to UV-B, and also to methylviologen-induced photooxidative stress. The TGA2 transgene (tga256/TGA2 plants) complemented these phenotypes. Second, RNAseq followed by clustering and Gene Ontology term analyses indicate that TGA2/5/6 positively control the UV-B-induced expression of a group of genes with oxidoreductase, glutathione transferase, and glucosyltransferase activities, such as members of the glutathione S-transferase Tau subfamily (GSTU), which encodes peroxide-scavenging enzymes. Accordingly, increased glutathione peroxidase activity triggered by UV-B was impaired in tga256 mutants. Third, the function of TGA2/5/6 as transcriptional activators of GSTU genes in the UV-B response was confirmed for GSTU7, GSTU8, and GSTU25, using quantitative reverse transcription-PCR and ChIP analyses. Fourth, expression of the GSTU7 transgene complemented the UV-B-susceptible phenotype of tga256 mutant plants. Together, this evidence indicates that TGA2/5/6 factors are key regulators of the antioxidant/detoxifying response to an abiotic stress such as UV-B light overexposure.

A Ratiometric Calcium Reporter CGf Reveals Calcium Dynamics Both in the Single Cell and Whole Plant Levels Under Heat Stress

Land plants evolved to quickly sense and adapt to temperature changes, such as hot days and cold nights. Given that calcium (Ca²⁺) signaling networks are implicated in most abiotic stress responses, heat-triggered changes in cytosolic Ca²⁺ were investigated in Arabidopsis leaves and pollen. Plants were engineered with a reporter called CGf, a ratiometric, genetically encoded Ca²⁺ reporter with an mCherry reference domain fused to an intensiometric Ca²⁺ reporter GCaMP6f. Relative changes in [Ca²⁺]_cyt were estimated based on CGf's apparent K _D around 220 nM. The ratiometric output provided an opportunity to compare Ca²⁺ dynamics between different tissues, cell types, or subcellular locations. In leaves, CGf detected heat-triggered cytosolic Ca²⁺ signals, comprised of three different signatures showing similarly rapid rates of Ca²⁺ influx followed by differing rates of efflux (50% durations ranging from 5 to 19 min). These heat-triggered Ca²⁺ signals were approximately 1.5-fold greater in magnitude than blue light-triggered signals in the same leaves. In contrast, growing pollen tubes showed two different heat-triggered responses. Exposure to heat caused tip-focused steady growth [Ca²⁺]_cyt oscillations to shift to a pattern characteristic of a growth arrest (22%), or an almost undetectable [Ca²⁺]_cyt (78%). Together, these contrasting examples of heat-triggered Ca²⁺ responses in leaves and pollen highlight the diversity of Ca²⁺ signals in plants, inviting speculations about their differing kinetic features and biological functions.

Chitin Biosynthesis Inhibition of Meloidogyne incognita by RNAi-Mediated Gene Silencing Increases Resistance to Transgenic Tobacco Plants

Meloidogyne incognita is a devastating plant parasitic nematode that causes root knot disease in a wide range of plants. In the present study, we investigated host-induced RNA interference (RNAi) gene silencing of chitin biosynthesis pathway genes (chitin synthase, glucose-6-phosphate isomerase, and trehalase) in transgenic tobacco plants. To develop an RNAi vector, ubiquitin (UBQ1) promoter was directly cloned, and to generate an RNAi construct, expression of three genes was suppressed using the GATEWAY system. Further, transgenic Nicotiana benthamiana lines expressing dsRNA for chitin synthase (CS), glucose-6-phosphate isomerase (GPI), and trehalase 1 (TH1) were generated. Quantitative PCR analysis confirmed endogenous mRNA expression of root knot nematode (RKN) and revealed that all three genes were more highly expressed in the female stage than in eggs and in the parasitic stage. In vivo, transformed roots were challenged with M. incognita. The number of eggs and root knots were significantly decreased by 60-90% in RNAi transgenic lines. As evident, root galls obtained from transgenic RNAi lines exhibited 0.01- to 0.70-fold downregulation of transcript levels of targeted genes compared with galls isolated from control plants. Furthermore, phenotypic characteristics such as female size and width were also marginally altered, while effect of egg mass per egg number in RNAi transgenic lines was reduced. These results indicate the relevance and significance of targeting chitin biosynthesis genes during the nematode lifespan. Overall, our results suggest that further developments in RNAi efficiency in commercially valued crops can be applied to employ RNAi against other plant parasitic nematodes.

Introns mediate post-transcriptional enhancement of nuclear gene expression in the green microalga Chlamydomonas reinhardtii

Efficient nuclear transgene expression in the green microalga Chlamydomonas reinhardtii is generally hindered by low transcription rates. Introns can increase transcript abundance by a process called Intron-Mediated Enhancement (IME) in this alga and has been broadly observed in other eukaryotes. However, the mechanisms of IME in microalgae are poorly understood. Here, we identified 33 native introns from highly expressed genes in C. reinhardtii selected from transcriptome studies as well as 13 non-native introns. We investigated their IME capacities and probed the mechanism of action by modification of splice sites, internal sequence motifs, and position within transgenes. Several introns were found to elicit strong IME and found to be broadly applicable in different expression constructs. We determined that IME in C. reinhardtii exclusively occurs from introns within transcribed ORFs regardless of the promoter and is not induced by traditional enhancers of transcription. Our results elucidate some mechanistic details of IME in C. reinhardtii, which are similar to those observed in higher plants yet underly distinctly different induction processes. Our findings narrow the focus of targets responsible for algal IME and provides evidence that introns are underestimated regulators of C. reinhardtii nuclear gene expression.

Although many genetic tools and basic transformation strategies exist for the model microalga Chlamydomonas reinhardtii, high-level genetic engineering with this organism is hindered by its inherent recalcitrance to foreign gene expression and limited knowledge of responsible expression regulators. In this work, we characterized the dynamics of 33 endogenous and 13 non-native introns and their effect on gene expression as artificial insertions into codon optimized transgenes. We found that introns from different origins have the capacity to increase gene expression rates. Intron-mediated enhancement was observed exclusively when these elements were placed in transcripts but not outside of transcribed mRNA regions. Insertion of different endogenous introns into coding sequences was found to positively affect expression rates through a synergy of additive transcription enhancement and exon length reduction, similar to those natively found in the C. reinhardtii genome. Our results indicate that intensive mRNA processing plays an underestimated role in the regulation of native gene expression in C. reinhardtii. In addition to internal sequence motifs, the location of artificially introduced introns greatly affected transgene expression levels. This work is highly valuable to the greater microalgal and synthetic biology research communities and contributes to broadening our understanding of eukaryotic intron-mediated enhancement.

First Come, First Served: Sui Generis Features of the First Intron

Most of the transcribed genes in eukaryotic cells are interrupted by intervening sequences called introns that are co-transcriptionally removed from nascent messenger RNA through the process of splicing. In Arabidopsis, 79% of genes contain introns and more than 60% of intron-containing genes undergo alternative splicing (AS), which ostensibly is considered to increase protein diversity as one of the intrinsic mechanisms for fitness to the varying environment or the internal developmental program. In addition, recent findings have prevailed in terms of overlooked intron functions. Here, we review recent progress in the underlying mechanisms of intron function, in particular by focusing on unique features of the first intron that is located in close proximity to the transcription start site. The distinct deposition of epigenetic marks and nucleosome density on the first intronic DNA sequence, the impact of the first intron on determining the transcription start site and elongation of its own expression (called intron-mediated enhancement, IME), translation control in 5′-UTR, and the new mechanism of the trans-acting function of the first intron in regulating gene expression at the post-transcriptional level are summarized.

Improving the genome editing efficiency of CRISPR/Cas9 in Arabidopsis and Medicago truncatula

MAIN CONCLUSION

An improved CRISPR/Cas9 system with the Arabidopsis UBQ10 promoter-driven Cas9 exhibits consistently high mutation efficiency in Arabidopsis and M. truncatula. CRISPR/Cas9 is a powerful genome editing technology that has been applied in several crop species for trait improvement due to its simplicity, versatility, and specificity. However, the mutation efficiency of CRISPR/Cas9 in Arabidopsis and M. truncatula (Mt) is still challenging and inconsistent. To analyze the functionality of the CRISPR/Cas9 system in two model dicot species, four different promoter-driven Cas9 systems to target phytoene desaturase (PDS) genes were designed. Agrobacterium-mediated transformation was used for the delivery of constructed vectors to host plants. Phenotypic and genotypic analyses revealed that the Arabidopsis UBQ10 promoter-driven Cas9 significantly improves the mutation efficiency to 95% in Arabidopsis and 70% in M. truncatula. Moreover, the UBQ10-Cas9 system yielded 11% homozygous mutants in the T1 generation in Arabidopsis. Sequencing analyses of mutation events indicated that single-nucleotide insertions are the most frequent events in Arabidopsis, whereas multi-nucleotide deletions are dominant in bi-allelic and mono-allelic homozygous mutants in M. truncatula. Taken together, the UBQ10 promoter facilitates the best improvement in the CRISPR/Cas9 efficiency in PDS gene editing, followed by the EC1.2 promoter. Consistently, the improved UBQ10-Cas9 vector highly enhanced the mutation efficiency by four-fold over the commonly used 35S promoter in both dicot species.

A simple and efficient Agrobacterium-mediated transient expression system to dissect molecular processes in Brassica rapa and Brassica napus

The family Brassicaceae is a source of important crop species, including Brassica napus (oilseed rape), Brassica oleracea, and B. rapa, that is used globally for oil production or as a food source (e.g., pak choi or turnip). However, despite advances in recent years, including genome sequencing, a lack of established tools tailored to the study of Brassica crop species has impeded efforts to understand their molecular processes in greater detail. Here, we describe the use of a simple Agrobacterium-mediated transient expression system adapted to B. rapa and B. napus that could facilitate study of molecular and biochemical events in these species. We also demonstrate the use of this method to characterize the N-degron pathway of protein degradation in B. rapa. The N-degron pathway is a subset of the ubiquitin-proteasome system and represents a mechanism through which proteins may be targeted for degradation based on the identity of their N-terminal amino acid residue. Interestingly, N-degron-mediated processes in plants have been implicated in the regulation of traits with potential agronomic importance, including the responses to pathogens and to abiotic stresses such as flooding tolerance. The stability of transiently expressed N-degron reporter proteins in B. rapa indicates that its N-degron pathway is highly conserved with that of Arabidopsis thaliana. These findings highlight the utility of Agrobacterium-mediated transient expression in B. rapa and B. napus and establish a framework to investigate the N-degron pathway and its roles in regulating agronomical traits in these species.

SIGNIFICANCE STATEMENT

We describe an Agrobacterium-mediated transient expression system applicable to Brassica crops and demonstrate its utility by identifying the destabilizing residues of the N-degron pathway in B. rapa. As the N-degron pathway functions as an integrator of environmental signals, this study could facilitate efforts to improve the robustness of Brassica crops.

Genome-Wide Identification and Characterization of UTR-Introns of Citrus sinensis

Introns exist not only in coding sequences (CDSs) but also in untranslated regions (UTRs) of a gene. Recent studies in animals and model plants such as Arabidopsis have revealed that the UTR-introns (UIs) are widely presented in most genomes and involved in regulation of gene expression or RNA stability. In the present study, we identified introns at both 5'UTRs (5UIs) and 3'UTRs (3UIs) of sweet orange genes, investigated their size and nucleotide distribution characteristics, and explored the distribution of cis-elements in the UI sequences. Functional category of genes with predicted UIs were further analyzed using GO, KEGG, and PageMan enrichment. In addition, the organ-dependent splicing and abundance of selected UI-containing genes in root, leaf, and stem were experimentally determined. Totally, we identified 825 UI- and 570 3UI-containing transcripts, corresponding to 617 and 469 genes, respectively. Among them, 74 genes contain both 5UI and 3UI. Nucleotide distribution analysis showed that 5UI distribution is biased at both ends of 5'UTR whiles 3UI distribution is biased close to the start site of 3'UTR. Cis- elements analysis revealed that 5UI and 3UI sequences were rich of promoter-enhancing related elements, indicating that they might function in regulating the expression through them. Function enrichment analysis revealed that genes containing 5UI are significantly enriched in the RNA transport pathway. While, genes containing 3UI are significantly enriched in splicesome. Notably, many pentatricopeptide repeat-containing protein genes and the disease resistance genes were identified to be 3UI-containing. RT-PCR result confirmed the existence of UIs in the eight selected gene transcripts whereas alternative splicing events were found in some of them. Meanwhile, qRT-PCR result showed that UIs were differentially expressed among organs, and significant correlation was found between some genes and their UIs, for example: The expression of VPS28 and its 3UI was significantly negative correlated. This is the first report about the UIs in sweet orange from genome-wide level, which could provide evidence for further understanding of the role of UIs in gene expression regulation.

Analyzing Subcellular Reorganization During Early Arabidopsis Embryogenesis Using Fluorescent Markers

Virtually all growth, developmental, physiological, and defense responses in plants are accompanied by reorganization of subcellular structures to enable altered cellular growth, differentiation or function. Visualizing cellular reorganization is therefore critical to understand plant biology at the cellular scale. Fluorescently labeled markers for organelles, or for cellular components are widely used in combination with confocal microscopy to visualize cellular reorganization. Early during plant embryogenesis, the precursors for all major tissues of the seedling are established, and in Arabidopsis, this entails a set of nearly invariant switches in cell division orientation and directional cell expansion. Given that these cellular reorganization events are genetically regulated and coupled to formative events in plant development, they offer a good model to understand the genetic control of cellular reorganization in plant development. Until recently, it has been challenging to visualize subcellular structures in the early Arabidopsis embryo for two reasons: embryos are deeply embedded in seed coat and fruit, and in addition, no dedicated fluorescent markers, expressed in the embryo, were available. We recently established both an imaging approach and a set of markers for the early Arabidopsis embryo. Here, we describe a detailed protocol to use these new tools in imaging cellular reorganization.

Parsley ubiquitin promoter displays higher activity than the CaMV 35S promoter and the chrysanthemum actin 2 promoter for productive, constitutive, and durable expression of a transgene in Chrysanthemum morifolium

The chrysanthemum (Chrysanthemum morifolium) is one of the most popular ornamental plants in the world. Genetic transformation is a promising tool for improving traits, editing genomes, and studying plant physiology. Promoters are vital components for efficient transformation, determining the level, location, and timing of transgene expression. The cauliflower mosaic virus (CaMV) 35S promoter is most frequently used in dicotyledonous plants but is less efficient in chrysanthemums than in tobacco or torenia plants. Previously, we used the parsley ubiquitin (PcUbi) promoter in chrysanthemums for the first time and analyzed its activity in transgenic calli. To expand the variety of constitutive promoters in chrysanthemums, we cloned the upstream region of the actin 2 (CmACT2) gene and compared its promoter activity with the 35S and PcUbi promoters in several organs, as well as its durability for long-term cultivation. The CmACT2 promoter has higher activity than the 35S promoter in calli but is less durable. The PcUbi promoter has the highest activity not only in calli but also in leaves, ray florets, and disk florets, and retains its activity after long-term cultivation. In conclusion, we have provided useful information and an additional type of promoter available for transgene expression in chrysanthemums.

Targeted insertion of large DNA sequences by homology-directed repair or non-homologous end joining in engineered tobacco BY-2 cells using designed zinc finger nucleases

Targeted integration of recombinant DNA fragments into plant genomes by DNA double-strand break (DSB) repair mechanisms has become a powerful tool for precision engineering of crops. However, many targeting platforms require the screening of many transgenic events to identify a low number of targeted events among many more random insertion events. We developed an engineered transgene integration platform (ETIP) that uses incomplete marker genes at the insertion site to enable rapid phenotypic screening and recovery of targeted events upon functional reconstitution of the marker genes. The two marker genes, encoding neomycin phosphotransferase II (nptII) and Discosoma sp. red fluorescent protein (DsRed) enable event selection on kanamycin-containing selective medium and subsequent screening for red fluorescent clones. The ETIP design allows targeted integration of donor DNA molecules either by homology-directed repair (HDR) or non-homologous end joining (NHEJ)-mediated mechanisms. Targeted donor DNA integration is facilitated by zinc finger nucleases (ZFN). The ETIP cassette was introduced into Nicotiana tabacum BY-2 suspension cells to generate target cell lines containing a single copy locus of the transgene construct. The utility of the ETIP platform has been demonstrated by targeting DNA constructs containing up to 25-kb payload. The success rate for clean targeted DNA integration was up to 21% for HDR and up to 41% for NHEJ based on the total number of calli analyzed by next-generation sequencing (NGS). The rapid generation of targeted events with large DNA constructs expands the utility of the nuclease-mediated gene addition platform both for academia and the commercial sector.

Insights into the complex role of GRAS transcription factors in the arbuscular mycorrhiza symbiosis

To improve access to limiting nutrients, the vast majority of land plants forms arbuscular mycorrhizal (AM) symbioses with Glomeromycota fungi. We show here that AM-related GRAS transcription factors from different subgroups are upregulated during a time course of mycorrhization. Based on expression studies in mutants defective in arbuscule branching (ram1-1, with a deleted MtRam1 GRAS transcription factor gene) or in the formation of functional arbuscules (pt4-2, mutated in the phosphate transporter gene MtPt4), we demonstrate that the five AM-related GRAS transcription factor genes MtGras1, MtGras4, MtGras6, MtGras7, and MtRad1 can be differentiated by their dependency on MtRAM1 and MtPT4, indicating that the network of AM-related GRAS transcription factors consists of at least two regulatory modules. One module involves the MtRAM1- and MtPT4-independent transcription factor MtGRAS4 that activates MtGras7. Another module is controlled by the MtRAM1- and MtPT4-dependent transcription factor MtGRAS1. Genome-wide expression profiles of mycorrhized MtGras1 knockdown and ram1-1 roots differ substantially, indicating different targets. Although an MtGras1 knockdown reduces transcription of AM-related GRAS transcription factor genes including MtRam1 and MtGras7, MtGras1 overexpression alone is not sufficient to activate MtGras genes. MtGras1 knockdown roots display normal fungal colonization, with a trend towards the formation of smaller arbuscules.

Introns as Gene Regulators: A Brick on the Accelerator

A picture is beginning to emerge from a variety of organisms that for a subset of genes, the most important sequences that regulate expression are situated not in the promoter but rather are located within introns in the first kilobase of transcribed sequences. The actual sequences involved are difficult to identify either by sequence comparisons or by deletion analysis because they are dispersed, additive, and poorly conserved. However, expression-controlling introns can be identified computationally in species with relatively small introns, based on genome-wide differences in oligomer composition between promoter-proximal and distal introns. The genes regulated by introns are often expressed in most tissues and are among the most highly expressed in the genome. The ability of some introns to strongly stimulate mRNA accumulation from several hundred nucleotides downstream of the transcription start site, even when the promoter has been deleted, reveals that our understanding of gene expression remains incomplete. It is unlikely that any diseases are caused by point mutations or small deletions that reduce the expression of an intron-regulated gene unless splicing is also affected. However, introns may be particularly useful in practical applications such as gene therapy because they strongly activate expression but only affect the transcription unit in which they are located.

GHB614 × T304-40 × GHB119 × COT102 Cotton: Protein Expression Analyses of Field-Grown Samples

Food and feed safety assessment is not enhanced by performing protein expression analysis on stacked trait products. The expression levels of six proteins in cotton matrices from four single cotton events and three conventionally stacked trait cotton products are reported. Three proteins were for insect control; two proteins confer herbicide tolerance; and one protein was a transformation-selectable marker. The cotton matrices were produced at three U.S., five Brazil, and two Argentina field trials. Similar protein expression was observed for all six proteins in the stacked trait products and the single events. However, when two copies of the bar gene were present in the stacked trait products, the expression level of phosphinothricin acetyl transferase herbicide tolerance was additive. Conventional breeding of genetically engineered traits does not alter the level or pattern of expression of the newly introduced proteins, except when multiple copies of the same transgene are present.

Local Auxin Biosynthesis Is a Key Regulator of Plant Development

Auxin is a major phytohormone that controls numerous aspects of plant development and coordinates plant responses to the environment. Morphogenic gradients of auxin govern cell fate decisions and underlie plant phenotypic plasticity. Polar auxin transport plays a central role in auxin maxima generation. The discovery of the exquisite spatiotemporal expression patterns of auxin biosynthesis genes of the WEI8/TAR and YUC families suggested that local auxin production may contribute to the formation of auxin maxima. Herein, we systematically addressed the role of local auxin biosynthesis in plant development and responses to the stress phytohormone ethylene by manipulating spatiotemporal patterns of WEI8. Our study revealed that local auxin biosynthesis and transport act synergistically and are individually dispensable for root meristem maintenance. In contrast, flower fertility and root responses to ethylene require local auxin production that cannot be fully compensated for by transport in the generation of morphogenic auxin maxima.

The Tetrazole Analogue of the Auxin Indole-3-acetic Acid Binds Preferentially to TIR1 and Not AFB5

Indole-3-acetic acid (auxin) is considered one of the cardinal hormones in plant growth and development. It regulates a wide range of processes throughout the plant. Synthetic auxins exploit the auxin-signaling pathway and are valuable as herbicidal agrochemicals. Currently, despite a diversity of chemical scaffolds all synthetic auxins have a carboxylic acid as the active core group. By applying bio-isosteric replacement we discovered that indole-3-tetrazole was active by surface plasmon resonance spectrometry, showing that the tetrazole could initiate assembly of the Transport Inhibitor Resistant 1 (TIR1) auxin coreceptor complex. We then tested the tetrazole's efficacy in a range of whole plant physiological assays and in protoplast reporter assays, which all confirmed auxin activity, albeit rather weak. We then tested indole-3-tetrazole against the AFB5 homologue of TIR1, finding that binding was selective against TIR1, absent with AFB5. The kinetics of binding to TIR1 are contrasted to those for the herbicide picloram, which shows the opposite receptor preference, as it binds to AFB5 with far greater affinity than to TIR1. The basis of the preference of indole-3-tetrazole for TIR1 was revealed to be a single residue substitution using molecular docking, and assays using tir1 and afb5 mutant lines confirmed selectivity in vivo. Given the potential that a TIR1-selective auxin might have for unmasking receptor-specific actions, we followed a rational design, lead optimization campaign, and a set of chlorinated indole-3-tetrazoles was synthesized. Improved affinity for TIR1 and the preference for binding to TIR1 was maintained for 4- and 6-chloroindole-3-tetrazoles, coupled with improved efficacy in vivo. This work expands the range of auxin chemistry for the design of receptor-selective synthetic auxins.

Genes for seed longevity in barley identified by genomic analysis on near isogenic lines

Genes controlling differences in seed longevity between 2 barley (Hordeum vulgare) accessions were identified by combining quantitative genetics "omics" technologies in near isogenic lines (NILs). The NILs were derived from crosses between the spring barley landraces L94 from Ethiopia and Cebada Capa from Argentina. A combined transcriptome and proteome analysis on mature, nonaged seeds of the 2 parental lines and the L94 NILs by RNA-sequencing and total seed proteomic profiling identified the UDP-glycosyltransferase MLOC_11661.1 as candidate gene for the quantitative trait loci on 2H, and the NADP-dependent malic enzyme (NADP-ME) MLOC_35785.1 as possible downstream target gene. To validate these candidates, they were expressed in Arabidopsis under the control of constitutive promoters to attempt complementing the T-DNA knockout line nadp-me1. Both the NADP-ME MLOC_35785.1 and the UDP-glycosyltransferase MLOC_11661.1 were able to rescue the nadp-me1 seed longevity phenotype. In the case of the UDP-glycosyltransferase, with high accumulation in NILs, only the coding sequence of Cebada Capa had a rescue effect.

Contrasting duplication patterns reflect functional diversities of ubiquitin and ubiquitin-like protein modifiers in plants

Ubiquitin (Ub) and Ub-like proteins, collectively forming the ubiquiton family, regulate nearly all aspects of cellular processes via post-translational modifications. Studies devoted to specific members suggested a large expansion of this family in plants; however, a lack of systematic analysis hinders the comparison of individual members at both evolutionary history and functional divergence levels, which may provide new insight into biological functions. In this work, we first retrieved a total of 5856 members of 17 known ubiquiton subfamilies in 50 plant genomes by searching both prior annotations and missing loci in each genome. We then applied this list to analyze the duplication history of major ubiquiton subfamilies in plants. We show that autophagy-related protein 8 (ATG8), membrane-anchored Ub-fold (MUB), small Ub-like modifier (SUMO) and Ub loci encode 88% of the plant ubiquiton family. Although whole genome duplications (WGDs) significantly expanded the family, we discovered contrasting duplication patterns both in species and in subfamilies. Within the family, the ATG8 and MUB members were primarily duplicated through WGDs, whereas a significant number of Ub and SUMO loci were generated through retroposition and tandem duplications, respectively. Although Ub coding regions are highly conserved in plants, promoter activity analysis demonstrated lineage-specific expression patterns of polyUb genes in Oryza sativa (rice) and Arabidopsis, confirming their retroposition origin. Based on the theory of dosage balance constraints, our study suggests that ubiquiton members duplicated through WGDs play crucial roles in plants, and that the regulatory pathways involving ATG8 and MUB are more conserved than those controlled by Ub and SUMO.

Evaluation of synthetic promoters in Physcomitrella patens

Securing a molecular toolbox including diverse promoters is essential for genome engineering. However, native promoters have limitations such as the available number or the length of the promoter. In this work, three short synthetic promoters were characterized by using the yellow fluorescent protein Venus. All of the tested promoters were active and showed higher mRNA expression than housekeeping gene PpAct7, and similar protein expression level to the AtUBQ10 promoter. This study shows that few cis-regulatory elements are enough to establish a strong promoter for continuous expression of genes in plants. Along with this, the study enhance the number of available promotors to be used in P. patens. It also demonstrates the potential to construct multiple non-native promoters on demand, which would aid to resolve one bottleneck in multiple pathway expression in P. patens and other plants.

Engineering Introns to Express RNA Guides for Cas9- and Cpf1-Mediated Multiplex Genome Editing

The clustered regularly interspaced short palindromic repeat (CRISPR) system has emerged as the revolutionary platform for DNA targeting. This system uses a site-specific RNA guide to direct a CRISPR effector (e.g., Cas9 and Cpf1) to a DNA target. Here, we elaborate a general strategy to simultaneously express multiple guide RNAs (gRNA) and CRISPR RNAs (crRNA) from introns of Cas9 and Cpf1. This method utilizes the endogenous tRNA processing system or crRNA processing activity of Cpf1 to cleave the spliced intron that contains tRNA-gRNA polycistron or crRNA-crRNA array. We demonstrated that the tRNA-gRNA intron is able to fuse with Cas9 as one gene. Such a hybrid gene could be expressed using one polymerase II promoter, and exhibited high efficiency and robustness in simultaneously targeting multiple sites. We also implemented this strategy in Cpf1-mediated genome editing using intronic tRNA-crRNA and crRNA-crRNA arrays. Interestingly, hybrid genes containing Cpf1 and intronic crRNA array exhibited remarkably increased efficiency compared with the conventional Cpf1 vectors. Taken together, this study presents a method to express CRISPR reagents from one hybrid gene to increase genome-editing efficiency and capacity. Owing to its simplicity and versatility, this method could be broadly used to develop sophisticated CRISPR tools in eukaryotes.

Plant Translation Initiation Complex eIFiso4F Directs Pokeweed Antiviral Protein to Selectively Depurinate Uncapped Tobacco Etch Virus RNA

Pokeweed antiviral protein (PAP) is a ribosome inactivating protein (RIP) that depurinates the sarcin/ricin loop (SRL) of rRNA, inhibiting protein synthesis. PAP depurinates viral RNA, and in doing so, lowers the infectivity of many plant viruses. The mechanism by which PAP accesses uncapped viral RNA is not known, impeding scientists from developing effective antiviral agents for the prevention of the diseases caused by uncapped RNA viruses. Kinetic rates of PAP interacting with tobacco etch virus (TEV) RNA, in the presence and absence of eIFiso4F, were examined, addressing how the eIF affects selective PAP targeting and depurination of the uncapped viral RNA. PAP-eIFs copurification assay and fluorescence resonance energy transfer demonstrate that PAP forms a ternary complex with the eIFiso4G and eIFiso4E, directing the depurination of uncapped viral RNA. eIFiso4F selectively targets PAP to depurinate TEV RNA by increasing PAP's specificity constant for uncapped viral RNA 12-fold, when compared to the depurination of an oligonucleotide RNA that mimics the SRL of large rRNA, and cellular capped luciferase mRNA. This explains how PAP is able to lower infectivity of pokeweed viruses, while preserving its own ribosomes and cellular RNA from depurination: PAP utilizes cellular eIFiso4F in a novel strategy to target uncapped viral RNA. It may be possible to modulate and utilize these PAP-eIFs interactions for their public health benefit; by repurposing them to selectively target PAP to depurinate uncapped viral RNA, many plant and animal diseases caused by these viruses could be alleviated.