pEAQ: versatile expression vectors for easy and quick transient expression of heterologous proteins in plants

Characterization of a longan pericarp browning related peroxidase with a focus on its role in proanthocyanidin and lignin polymerization

The longan pericarp turns brown dramatically after harvesting, but the mechanism is not well understood. In this work, two peroxidases were purified from longan pericarp and found to be identical to the class III peroxidases PRX53-2 and PRX53-3. In vitro, PRX53-2/3 catalyzed the browning of several pericarp abundant proanthocyanidin and lignin monomers, such as (-)-epicatechin (EC), (+)-catechin (CT) and coniferyl alcohol (ConA). PRX53-2 was upregulated and highly-expressed, while PRX53-3 was expressed at low levels after harvesting; thus, PRX53-2 was considered a browning-related gene. The reaction with both proanthocyanidin and lignin presented a greater degree of brown coloration compared to the single substrate reactions. Several procyanidins isomers, EC-ConA and CT-ConA were detected in the double-substrate reaction. These results not only demonstrate that the effects of PRX53-2 on proanthocyanidin and lignin polymerization may be crucial for longan pericarp browning, but also help in developing new strategies or preservatives to delay pericarp browning.

Distinctive acidity in citrus fruit is linked to loss of proanthocyanidin biosynthesis

The distinctive acidity of citrus fruit is determined by a regulatory complex of MYB and bHLH transcription factors together with a WDR protein (MBW complex) which operates in the unique juice vesicles of the fruit. We describe a mutation affecting the MYB protein, named Nicole, in sweet orange and identify its target genes that determine hyperacidification, specifically. We propose that the acidity, typical of citrus fruits, was the result of a loss of the ability of Nicole to activate the gene encoding anthocyanidin reductase, an enzyme essential for the synthesis of proanthocyanidins, which are absent in citrus fruit.

A gene cluster for polyamine transport and modification improves salt tolerance in tomato

Polyamines act as protective compounds directly protecting plants from stress-related damage, while also acting as signaling molecules to participate in serious abiotic stresses. However, the molecular mechanisms underlying these effects are poorly understood. Here, we utilized metabolome genome-wide association study to investigate the polyamine content of wild and cultivated tomato accessions, and we discovered a new gene cluster that drove polyamine content during tomato domestication. The gene cluster contains two polyphenol oxidases (SlPPOE and SlPPOF), two BAHD acyltransferases (SlAT4 and SlAT5), a coumaroyl-CoA ligase (Sl4CL6), and a polyamine uptake transporter (SlPUT3). SlPUT3 mediates polyamine uptake and transport, while the five other genes are involved in polyamine modification. Further salt tolerance assays demonstrated that SlPPOE, SlPPOF, and SlAT5 overexpression lines showed greater phenolamide accumulation and salt tolerance as compared with wild-type (WT). Meanwhile, the exogenous application of Spm to SlPUT3-OE lines displayed salt tolerance compared with WT, while having the opposite effect in slput3 lines, confirms that the polyamine and phenolamide can play a protective role by alleviating cell damage. SlPUT3 interacted with SlPIP2;4, a H2O2 transport protein, to maintain H2O2 homeostasis. Polyamine-derived H2O2 linked Spm to stress responses, suggesting that Spm signaling activates stress response pathways. Collectively, our finding reveals that the H2O2-polyamine-phenolamide module coordinately enhanced tomato salt stress tolerance and provide a foundation for tomato stress-resistance breeding.

LlbHLH87 interacts with LlSPT to modulate thermotolerance via activation of LlHSFA2 and LlEIN3 in lily

Basic helix-loop-helix (bHLH) proteins comprise one of the largest families of transcription factors in plants, which play roles in plant development, secondary metabolism, and the response to biotic/abiotic stresses. However, the roles of bHLH proteins in thermotolerance are largely unknown. Herein, we identified a heat-inducible member of the bHLH family in lily (Lilium longiflorum), named LlbHLH87, which plays a role in thermotolerance. LlbHLH87 was rapidly induced by transient heat stress, and its encoded protein was localized to the nucleus, exhibiting transactivation activity in both yeast and plant cells. Overexpression of LlbHLH87 in Arabidopsis enhanced basal thermotolerance, while silencing of LlbHLH87 in lily reduced basal thermotolerance. Further analysis showed that LlbHLH87 bound to the promoters of HEAT STRESS TRANSCRIPTION FACTOR A2 (LlHSFA2) and ETHYLENE-INSENSITIVE 3 (LlEIN3) to directly activate their expression. In addition, LlbHLH87 interacted with itself and with SPATULA (LlSPT) protein. LlSPT was activated by extended heat stress and its protein competed for the homologous interaction of LlbHLH87, which reduced the transactivation ability of LlbHLH87 for target genes. Compared with that observed under LlbHLH87 overexpression alone, co-overexpression of LlbHLH87 and LlSPT reduced the basal thermotolerance of lily to sudden heat shock, but improved its thermosensitivity to prolonged heat stress treatment. Overall, our data demonstrated that LlbHLH87 regulates thermotolerance via activation of LlEIN3 and LlHSFA2, along with an antagonistic interaction with LlSPT.

A developmental gradient reveals biosynthetic pathways to eukaryotic toxins in monocot geophytes

Numerous eukaryotic toxins that accumulate in geophytic plants are valuable in the clinic, yet their biosynthetic pathways have remained elusive. A notable example is the >150 Amaryllidaceae alkaloids (AmAs), including galantamine, an FDA-approved treatment for Alzheimer's disease. We show that while AmAs accumulate to high levels in many daffodil tissues, biosynthesis is localized to nascent, growing tissue at the leaf base. A similar trend is found in the production of steroidal alkaloids (e.g., cyclopamine) in corn lily. This model of active biosynthesis enabled the elucidation of a complete set of biosynthetic genes that can be used to produce AmAs. Taken together, our work sheds light on the developmental and enzymatic logic of diverse alkaloid biosynthesis in daffodils. More broadly, it suggests a paradigm for biosynthesis regulation in monocot geophytes, where plants are protected from herbivory through active charging of newly formed cells with eukaryotic toxins that persist as above-ground tissue develops.

Single-cell RNA sequencing facilitates the elucidation of the complete biosynthesis of the antidepressant hyperforin in St. John's wort

Hyperforin is the compound responsible for the effectiveness of St. John's wort (Hypericum perforatum) as an antidepressant, but its complete biosynthetic pathway remains unknown. Gene discovery based on co-expression analysis of bulk RNA-sequencing data or genome mining failed to discover the missing steps in hyperforin biosynthesis. In this study, we sequenced the 1.54-Gb tetraploid H. perforatum genome assembled into 32 chromosomes with the scaffold N50 value of 42.44 Mb. By single-cell RNA sequencing, we identified a type of cell, "Hyper cells", wherein hyperforin biosynthesis de novo takes place in both the leaves and flowers. Through pathway reconstitution in yeast and tobacco, we identified and characterized four transmembrane prenyltransferases (HpPT1-4) that are localized at the plastid envelope and complete the hyperforin biosynthetic pathway. The hyperforin polycyclic scaffold is created by a reaction cascade involving an irregular isoprenoid coupling and a tandem cyclization. Our findings reveal how and where hyperforin is biosynthesized, enabling synthetic-biology reconstitution of the complete pathway. Thus, this study not only deepens our comprehension of specialized metabolism at the cellular level but also provides strategic guidance for elucidation of the biosynthetic pathways of other specializied metabolites in plants.

Identification and characterization of a key LcTPS in the biosynthesis of volatile monoterpenes and sesquiterpenes in Litchi fruit

Litchi (Litchi chinensis Sonn.) has a desirable sweet taste and exotic aroma, making it popular in the markets. However, the biosynthesis of aroma volatiles in litchi fruit has rarely been investigated. In this study, the content and composition of volatile compounds were determined during litchi fruit ripening. In the mature green and mature red stages of litchi, 49 and 45 volatile compounds were detected, respectively. Monoterpenes were found to be the most abundant volatile compounds in mature red fruit, and their contents significantly increased compared to green fruit, mainly including citronellol, geraniol, myrcene, and D-limonene, which contributed to the aroma in litchi fruit. By comparing the expression profiles of the genes involved in the terpene synthesis pathway during fruit development, a terpene synthesis gene (LcTPS1-2) was identified and characterized as a major player in the synthesis of monoterpenes and sesquiterpenes. A subcellular localization analysis found LcTPS1-2 to be present in the plastid and cytoplasm. The recombinant LcTPS1-2 enzyme was able to catalyze the formation of three monoterpenes, myrcene, geraniol and citral, from geranyl pyrophosphate (GPP) and to convert farnesyl diphosphate (FPP) to a sesquiterpene, caryophyllene in vitro. Transgenic Arabidopsis thaliana plants overexpressing LcTPS1-2 exclusively released one monoterpene D-limonene, and three sesquiterpenes cis-thujopsene, (E)-β-famesene and trans-β-ionone. These results indicate that LcTPS1-2 plays an important role in the production of major volatile terpenes in litchi fruit and provides a basis for future investigations of terpenoid biosynthesis in litchi and other horticultural crops.

Biosynthesis of bridged tricyclic sesquiterpenes in Inula lineariifolia

Presilphiperfolane-type sesquiterpenes represent a unique group of atypical sesquiterpenoids characterized by their distinctive tricyclic structure. They have significant potential as lead compounds for pharmaceutical and agrochemical development. Herein, we utilized a transcriptomic approach to identify a terpene synthase (TPS) gene responsible for the biosynthesis of rare presilphiperfolane-type sesquiterpenes in Inula lineariifolia, designated as IlTPS1. Through phylogenetic analysis, we have identified the evolutionary conservation of key motifs, including RR(x)8W, DDxxD, and NSE/DTE in IlTPS1, which are shared with other tricyclic sesquiterpene synthases in the TPS-a subfamily of Asteraceae plants. Subsequent biochemical characterization of recombinant IlTPS1 revealed it to be a multiproduct enzyme responsible for the synthesis of various tricyclic sesquiterpene alcohols from farnesyl diphosphate (FPP), resulting in production of seven distinct sesquiterpenes. Mass spectrometry and nuclear magnetic resonance (NMR) spectrometry identified presilphiperfolan-8β-ol and presilphiperfol-7-ene as predominant products. Furthermore, biological activity assays revealed that the products from IlTPS1 exhibited a potent antifungal activity against Nigrospora oryzae. Our study represents a significant advancement as it not only functionally identifies the first step enzyme in presilphiperfolane biosynthesis but also establishes the heterologous bioproduction of these unique sesquiterpenes.

Advances in Subcellular Accumulation Design for Recombinant Protein Production in Tobacco

Plants and their use as bioreactors for the generation of recombinant proteins have become one of the hottest topics in the field of Plant Biotechnology and Plant Synthetic Biology. Plant bioreactors offer superior engineering potential compared to other types, particularly in the realm of subcellular accumulation strategies for increasing production yield and quality. This review explores established and emerging strategies for subcellular accumulation of recombinant proteins in tobacco bioreactors, highlighting recent advancements in the field. Additionally, the review provides reference to the crucial initial step of selecting an optimal subcellular localization for the target protein, a design that substantially impacts production outcomes.

A novel mutation in non-constitutive lycopene beta cyclase (CstLcyB2a) from Crocus sativus modulates carotenoid/apocarotenoid content, biomass and stress tolerance in plants

MAIN CONCLUSION

Mutation at A126 in lycopene-β-cyclase of Crocus (CstLcyB2a) sterically hinders its binding of δ-carotene without affecting lycopene binding, thereby diverting metabolic flux towards β-carotene and apocarotenoid biosynthesis. Crocus sativus, commonly known as saffron, has emerged as an important crop for research because of its ability to synthesize unique apocarotenoids such as crocin, picrocrocin and safranal. Metabolic engineering of the carotenoid pathway can prove a beneficial strategy for enhancing the quality of saffron and making it resilient to changing climatic conditions. Here, we demonstrate that introducing a novel mutation at A126 in stigma-specific lycopene-β-cyclase of Crocus (CstLcyB2a) sterically hinders its binding of δ-carotene, but does not affect lycopene binding, thereby diverting metabolic flux towards β-carotene formation. Thus, A126L-CstLcyB2a expression in lycopene-accumulating bacterial strains resulted in enhanced production of β-carotene. Transient expression of A126L-CstLcyB2a in C. sativus stigmas enhanced biosynthesis of crocin. Its stable expression in Nicotiana tabacum enhanced β-branch carotenoids and phyto-hormones such as abscisic acid (ABA) and gibberellic acids (GA's). N. tabacum transgenic lines showed better growth performance and photosynthetic parameters including maximum quantum efficiency (Fv/Fm) and light-saturated capacity of linear electron transport. Exogenous application of hormones and their inhibitors demonstrated that a higher ratio of GA4/ABA has positive effects on biomass of wild-type and transgenic plants. Thus, these findings provide a platform for the development of new-generation crops with improved productivity, quality and stress tolerance.

Variation in a Poaceae-conserved fatty acid metabolic gene cluster controls rice yield by regulating male fertility

A wide variety of metabolic gene clusters exist in eukaryotic genomes, but fatty acid metabolic gene clusters have not been discovered. Here, combining with metabolic and phenotypic genome-wide association studies, we identify a major locus containing a six-gene fatty acid metabolic gene cluster on chromosome 3 (FGC3) that controls the cutin monomer hydroxymonoacylglycerols (HMGs) contents and rice yield, possibly through variation in the transcription of FGC3 members. We show that HMGs are sequentially synthesized in the endoplasmic reticulum by OsFAR2, OsKCS11, OsGPAT6, OsCYP704B2 and subsequently transported to the apoplast by OsABCG22 and OsLTPL82. Mutation of FGC3 members reduces HMGs, leading to defective male reproductive development and a significant decrease in yield. OsMADS6 and OsMADS17 directly regulate FGC3 and thus influence male reproduction and yield. FGC3 is conserved in Poaceae and likely formed prior to the divergence of Pharus latifolius. The eukaryotic fatty acid and plant primary metabolic gene cluster we identified show a significant impact on the origin and evolution of Poaceae and has potential for application in hybrid crop breeding.

Hijacking of N-fixing legume albumin-1 genes enables the cyclization and stabilization of defense peptides

The legume albumin-1 gene family, arising after nodulation, encodes linear a- and b-chain peptides for nutrient storage and defense. Intriguingly, in one prominent legume, Clitoria ternatea, the b-chains are replaced by domains producing ultra-stable cyclic peptides called cyclotides. The mechanism of this gene hijacking is until now unknown. Cyclotides require recruitment of ligase-type asparaginyl endopeptidases (AEPs) for maturation (cyclization), necessitating co-evolution of two gene families. Here we compare a chromosome-level C. ternatea genome with grain legumes to reveal an 8 to 40-fold expansion of the albumin-1 gene family, enabling the additional loci to undergo diversification. Iterative rounds of albumin-1 duplication and diversification create four albumin-1 enriched genomic islands encoding cyclotides, where they are physically grouped by similar pI and net charge values. We identify an ancestral hydrolytic AEP that exhibits neofunctionalization and multiple duplication events to yield two ligase-type AEPs. We propose cyclotides arise by convergence in C. ternatea where their presence enhances defense from biotic attack, thus increasing fitness compared to lineages with linear b-chains and ultimately driving the replacement of b-chains with cyclotides.

Engineered poplar for bioproduction of the triterpene squalene

Building sustainable platforms to produce biofuels and specialty chemicals has become an increasingly important strategy to supplement and replace fossil fuels and petrochemical-derived products. Terpenoids are the most diverse class of natural products that have many commercial roles as specialty chemicals. Poplar is a fast growing, biomassdense bioenergy crop with many species known to produce large amounts of the hemiterpene isoprene, suggesting an inherent capacity to produce significant quantities of other terpenes. Here we aimed to engineer poplar with optimized pathways to produce squalene, a triterpene commonly used in cosmetic oils, a potential biofuel candidate, and the precursor to the further diversified classes of triterpenoids and sterols. The squalene production pathways were either re-targeted from the cytosol to plastids or co-produced with lipid droplets in the cytosol. Squalene and lipid droplet co-production appeared to be toxic, which we hypothesize to be due to disruption of adventitious root formation, suggesting a need for tissue specific production. Plastidial squalene production enabled up to 0.63 mg/g fresh weight in leaf tissue, which also resulted in reductions in isoprene emission and photosynthesis. These results were also studied through a technoeconomic analysis, providing further insight into developing poplar as a production host.

Improving transient protein expression in agroinfiltrated Nicotiana benthamiana

Agroinfiltration of Nicotiana benthamiana is routinely used in plant science and molecular pharming to transiently express proteins of interest. Here, we discuss four phenomena that should be avoided to improve transient expression. Immune responses can be avoided by depleting immune receptors and employing pathogen-derived effectors; transcript degradation by using silencing inhibitors or RNA interference machinery mutants; endoplasmic reticulum stress by co-expressing chaperones; and protein degradation can be avoided with subcellular targeting, protease mutants and co-expressing protease inhibitors. We summarise the reported increased yields for various recombinant proteins achieved with these approaches and highlight remaining challenges to further improve the efficiency of this versatile protein expression platform.

Molecular farming for sustainable production of clinical-grade antimicrobial peptides

Antimicrobial peptides (AMPs) are emerging as next-generation therapeutics due to their broad-spectrum activity against drug-resistant bacterial strains and their ability to eradicate biofilms, modulate immune responses, exert anti-inflammatory effects and improve disease management. They are produced through solid-phase peptide synthesis or in bacterial or yeast cells. Molecular farming, i.e. the production of biologics in plants, offers a low-cost, non-toxic, scalable and simple alternative platform to produce AMPs at a sustainable cost. In this review, we discuss the advantages of molecular farming for producing clinical-grade AMPs, advances in expression and purification systems and the cost advantage for industrial-scale production. We further review how 'green' production is filling the sustainability gap, streamlining patent and regulatory approvals and enabling successful clinical translations that demonstrate the future potential of AMPs produced by molecular farming. Finally, we discuss the regulatory challenges that need to be addressed to fully realize the potential of molecular farming-based AMP production for therapeutics.

Unlocking saponin biosynthesis in soapwort

Soapwort (Saponaria officinalis) is a flowering plant from the Caryophyllaceae family with a long history of human use as a traditional source of soap. Its detergent properties are because of the production of polar compounds (saponins), of which the oleanane-based triterpenoid saponins, saponariosides A and B, are the major components. Soapwort saponins have anticancer properties and are also of interest as endosomal escape enhancers for targeted tumor therapies. Intriguingly, these saponins share common structural features with the vaccine adjuvant QS-21 and, thus, represent a potential alternative supply of saponin adjuvant precursors. Here, we sequence the S. officinalis genome and, through genome mining and combinatorial expression, identify 14 enzymes that complete the biosynthetic pathway to saponarioside B. These enzymes include a noncanonical cytosolic GH1 (glycoside hydrolase family 1) transglycosidase required for the addition of D-quinovose. Our results open avenues for accessing and engineering natural and new-to-nature pharmaceuticals, drug delivery agents and potential immunostimulants.

Antigen-functionalized turnip mosaic virus nanoparticles increase antibody sensing in saliva. A case study with SARS-CoV-2 RBD

Nanoparticles derived from plant viruses play an important role in nanomedicine due to their biocompatibility, self-assembly and easily-modifiable surface. In this study, we developed a novel platform for increasing antibody sensing using viral nanoparticles derived from turnip mosaic virus (TuMV) functionalized with SARS-CoV-2 receptor binding domain (RBD) through three different methods: chemical conjugation, gene fusion and the SpyTag/SpyCatcher technology. Even though gene fusion turned out to be unsuccessful, the other two constructs were proven to significantly increase antibody sensing when tested with saliva of patients with different infection and vaccination status to SARS-CoV-2. Our findings show the high potential of TuMV nanoparticles in the fields of diagnostics and immunodetection, being especially attractive for the development of novel antibody sensing devices.

EpMYB2 positively regulates chicoric acid biosynthesis by activating both primary and specialized metabolic genes in purple coneflower

Chicoric acid is the major active ingredient of the world-popular medicinal plant purple coneflower (Echinacea purpurea (L.) Menoch). It is recognized as the quality index of commercial hot-selling Echinacea products. While the biosynthetic pathway of chicoric acid in purple coneflower has been elucidated recently, its regulatory network remains elusive. Through co-expression and phylogenetic analysis, we found EpMYB2, a typical R2R3-type MYB transcription factor (TF) responsive to methyl jasmonate (MeJA) simulation, is a positive regulator of chicoric acid biosynthesis. In addition to directly regulating chicoric acid biosynthetic genes, EpMYB2 positively regulates genes of the upstream shikimate pathway. We also found that EpMYC2 could activate the expression of EpMYB2 by binding to its G-box site, and the EpMYC2-EpMYB2 module is involved in the MeJA-induced chicoric acid biosynthesis. Overall, we identified an MYB TF that positively regulates the biosynthesis of chicoric acid by activating both primary and specialized metabolic genes. EpMYB2 links the gap between the JA signaling pathway and chicoric acid biosynthesis. This work opens a new direction toward engineering purple coneflower with higher medicinal qualities.

Debottlenecking the L-DOPA 4,5-dioxygenase step with enhanced tyrosine supply boosts betalain production in Nicotiana benthamiana

Synthetic biology provides emerging tools to produce valuable compounds in plant hosts as sustainable chemical production platforms. However, little is known about how supply and utilization of precursors is coordinated at the interface of plant primary and specialized metabolism, limiting our ability to efficiently produce high levels of target specialized metabolites in plants. L-Tyrosine is an aromatic amino acid precursor of diverse plant natural products including betalain pigments, which are used as the major natural food red colorants and more recently a visual marker for plant transformation. Here, we studied the impact of enhanced L-tyrosine supply on the production of betalain pigments by expressing arogenate dehydrogenase (TyrA) from table beet (Beta vulgaris, BvTyrAα), which has relaxed feedback inhibition by L-tyrosine. Unexpectedly, betalain levels were reduced when BvTyrAα was coexpressed with the betalain pathway genes in Nicotiana benthamiana leaves; L-tyrosine and 3,4-dihydroxy-L-phenylalanine (L-DOPA) levels were drastically elevated but not efficiently converted to betalains. An additional expression of L-DOPA 4,5-dioxygenase (DODA), but not CYP76AD1 or cyclo-DOPA 5-O-glucosyltransferase, together with BvTyrAα and the betalain pathway, drastically enhanced betalain production, indicating that DODA is a major rate-limiting step of betalain biosynthesis in this system. Learning from this initial test and further debottlenecking the DODA step maximized betalain yield to an equivalent or higher level than that in table beet. Our data suggest that balancing between enhanced supply ("push") and effective utilization ("pull") of precursor by alleviating a bottleneck step is critical in successful plant synthetic biology to produce high levels of target compounds.

Production of therapeutic glycoproteins in glycoengineered plant: old farm for new crops

Plant-based expression systems have emerged as promising avenues for the production of recombinant N-linked glycoproteins. This review offers insights into the evolution and progress of plant glycoengineering. It delves into the distinctive features of plant-derived N-glycans, the diverse range of plant hosts employed for glycoprotein synthesis, and the advancements in glycoengineering strategies aimed at generating glycoproteins with N-glycan structures akin to those produced in mammalian cell lines. Furthermore, alternative strategies for augmenting glycoengineering efforts and the current spectrum of applications for plant-produced N-glycan recombinant proteins are examined, underscoring their potential significance in biopharmaceutical manufacturing.

Sourcing DNA parts for synthetic biology applications in plants

Transgenic approaches are now standard in plant biology research aiming to characterize gene function or improve crops. Recent advances in DNA synthesis and assembly make constructing transgenes a routine task. What remains nontrivial is the selection of the DNA parts and optimization of the transgene design. Early career researchers and seasoned molecular biologists alike often face difficult decisions on what promoter or terminator to use, what tag to include, and where to place it. This review aims to inform about the current approaches being employed to identify and characterize DNA parts with the desired functionalities and give general advice on basic construct design. Furthermore, we hope to share the excitement about new experimental and computational tools being developed in this field.

Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model. We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field. EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side-chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores, but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores. These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

The Potential of Plant-Produced Virus-like Particle Vaccines for African Horse Sickness and Other Equine Orbiviruses

African horse sickness is a devastating viral disease of equids. It is transmitted by biting midges of the genus Culicoides with mortalities reaching over 90% in naïve horses. It is endemic to sub-Saharan Africa and is seasonally endemic in many parts of southern Africa. However, outbreaks in Europe and Asia have occurred that caused significant economic issues. There are attenuated vaccines available for control of the virus but concerns regarding the safety and efficacy means that alternatives are sought. One promising alternative is the use of virus-like particles in vaccine preparations, which have the potential to be safer and more efficacious as vaccines against African horse sickness. These particles are best made in a complex, eukaryotic system, but due to technical challenges, this may cause significant economic strain on the developing countries most affected by the disease. Therefore, this review also summarises the success so far, and potential, of recombinant protein expression in plants to reduce the economic strain of production.

Development of a Plant-Expressed Subunit Vaccine against Brucellosis

Brucellosis is an important bacterial disease of livestock and the most common zoonotic disease. The current vaccines are effective but unsafe, as they result in animal abortions and are pathogenic to humans. Virus-like particles are being investigated as molecular scaffolds for foreign antigen presentation to the immune system. Here, we sought to develop a new-generation vaccine by presenting selected Brucella melitensis T cell epitopes on the surface of Orbivirus core-like particles (CLPs) and transiently expressing these chimeric particles in Nicotiana benthamiana plants. We successfully demonstrated the assembly of five chimeric CLPs in N. benthamiana plants, with each CLP presenting a different T cell epitope. The safety and protective efficacy of three of the highest-yielding CLPs was investigated in a mouse model of brucellosis. All three plant-expressed chimeric CLPs were safe when inoculated into BALB/c mice at specific antigen doses. However, only one chimeric CLP induced protection against the virulent Brucella strain challenge equivalent to the protection induced by the commercial Rev1 vaccine. Here, we have successfully shown the assembly, safety and protective efficacy of plant-expressed chimeric CLPs presenting B. melitensis T cell epitopes. This is the first step in the development of a safe and efficacious subunit vaccine against brucellosis.

TBL38 atypical homogalacturonan-acetylesterase activity and cell wall microdomain localization in Arabidopsis seed mucilage secretory cells

Plant cell walls constitute complex polysaccharidic/proteinaceous networks whose biosynthesis and dynamics implicate several cell compartments. The synthesis and remodeling of homogalacturonan pectins involve Golgi-localized methylation/acetylation and subsequent cell wall-localized demethylation/deacetylation. So far, TRICHOME BIREFRINGENCE-LIKE (TBL) family members have been described as Golgi-localized acetyltransferases targeting diverse hemicelluloses or pectins. Using seed mucilage secretory cells (MSCs) from Arabidopsis thaliana, we demonstrate the atypical localization of TBL38 restricted to a cell wall microdomain. A tbl38 mutant displays an intriguing homogalacturonan immunological phenotype in this cell wall microdomain and in an MSC surface-enriched abrasion powder. Mass spectrometry oligosaccharide profiling of this fraction reveals an increased homogalacturonan acetylation phenotype. Finally, TBL38 displays pectin acetylesterase activity in vitro. These results indicate that TBL38 is an atypical cell wall-localized TBL that displays a homogalacturonan acetylesterase activity rather than a Golgi-localized acetyltransferase activity as observed in previously studied TBLs. TBL38 function during seed development is discussed.

MYB transcription factors encoded by diversified tandem gene clusters cause varied Morella rubra fruit color

Chinese bayberry (Morella rubra) is a fruit tree with a remarkable variation in fruit color, ranging from white to dark red as determined by anthocyanin content. In dark red "Biqi" (BQ), red "Dongkui" (DK), pink "Fenhong" (FH), and white "Shuijing" (SJ), we identified an anthocyanin-related MYB transcription factor-encoding gene cluster of four members, i.e. MrMYB1.1, MrMYB1.2, MrMYB1.3, and MrMYB2. Collinear analysis revealed that the MYB tandem cluster may have occurred in a highly conserved region of many eudicot genomes. Two alleles of MrMYB1.1 were observed; MrMYB1.1-1 (MrMYB1.1n) was a full-length allele and homozygous in "BQ", MrMYB1.1-2 (MrMYB1.1d) was a nonfunctional allele with a single base deletion and homozygous in "SJ", and MrMYB1.1n/MrMYB1.1d were heterozygous in "DK" and "FH". In these four cultivars, expression of MrMYB1.1, MrMYB1.2, and MrMYB2 was enhanced during ripening. Both alleles were equally expressed in MrMYB1.1n/MrMYB1.1d heterozygous cultivars as revealed by a cleaved amplified polymorphic sequence marker. Expression of MrMYB1.3 was restricted to some dark red cultivars only. Functional characterization revealed that MrMYB1.1n and MrMYB1.3 can induce anthocyanin accumulation while MrMYB1.1d, MrMYB1.2, and MrMYB2 cannot. DNA-protein interaction assays indicated that MrMYB1.1n and MrMYB1.3 can directly bind to and activate the promoters of anthocyanin-related genes via interaction with a MYC-like basic helix-loop-helix protein MrbHLH1. We concluded that the specific genotype of MrMYB1.1 alleles, as well as the exclusive expression of MrMYB1.3 in some dark red cultivars, contributes to fruit color variation. The study provides insights into the mechanisms for regulation of plant anthocyanin accumulation by MYB tandem clusters.

A Multifunctionalized Potyvirus-Derived Nanoparticle That Targets and Internalizes into Cancer Cells

Plant viral nanoparticles (VNPs) are attractive to nanomedicine researchers because of their safety, ease of production, resistance, and straightforward functionalization. In this paper, we developed and successfully purified a VNP derived from turnip mosaic virus (TuMV), a well-known plant pathogen, that exhibits a high affinity for immunoglobulins G (IgG) thanks to its functionalization with the Z domain of staphylococcal Protein A via gene fusion. We selected cetuximab as a model IgG to demonstrate the versatility of this novel TuMV VNP by developing a fluorescent nanoplatform to mark tumoral cells from the Cal33 line of a tongue squamous cell carcinoma. Using confocal microscopy, we observed that fluorescent VNP-cetuximab bound selectively to Cal33 and was internalized, revealing the potential of this nanotool in cancer research.

Two gene clusters and their positive regulator SlMYB13 that have undergone domestication-associated negative selection control phenolamide accumulation and drought tolerance in tomato

Among plant metabolites, phenolamides, which are conjugates of hydroxycinnamic acid derivatives and polyamines, play important roles in plant adaptation to abiotic and biotic stresses. However, the molecular mechanisms underlying phenolamide metabolism and regulation as well as the effects of domestication and breeding on phenolamide diversity in tomato remain largely unclear. In this study, we performed a metabolite-based genome-wide association study and identified two biosynthetic gene clusters (BGC7 and BGC11) containing 12 genes involved in phenolamide metabolism, including four biosynthesis genes (two 4CL genes, one C3H gene, and one CPA gene), seven decoration genes (five AT genes and two UGT genes), and one transport protein gene (DTX29). Using gene co-expression network analysis we further discovered that SlMYB13 positively regulates the expression of two gene clusters, thereby promoting phenolamide accumulation. Genetic and physiological analyses showed that BGC7, BGC11 and SlMYB13 enhance drought tolerance by enhancing scavenging of reactive oxygen species and increasing abscisic acid content in tomato. Natural variation analysis suggested that BGC7, BGC11 and SlMYB13 were negatively selected during tomato domestication and improvement, leading to reduced phenolamide content and drought tolerance of cultivated tomato. Collectively, our study discovers a key mechanism of phenolamide biosynthesis and regulation in tomato and reveals that crop domestication and improvement shapes metabolic diversity to affect plant environmental adaptation.

Complete biosynthesis of the potent vaccine adjuvant QS-21

QS-21 is a potent vaccine adjuvant currently sourced by extraction from the Chilean soapbark tree. It is a key component of human vaccines for shingles, malaria, coronavirus disease 2019 and others under development. The structure of QS-21 consists of a glycosylated triterpene scaffold coupled to a complex glycosylated 18-carbon acyl chain that is critical for immunostimulant activity. We previously identified the early pathway steps needed to make the triterpene glycoside scaffold; however, the biosynthetic route to the acyl chain, which is needed for stimulation of T cell proliferation, was unknown. Here, we report the biogenic origin of the acyl chain, characterize the series of enzymes required for its synthesis and addition and reconstitute the entire 20-step pathway in tobacco, thereby demonstrating the production of QS-21 in a heterologous expression system. This advance opens up unprecedented opportunities for bioengineering of vaccine adjuvants, investigating structure-activity relationships and understanding the mechanisms by which these compounds promote the human immune response.

Guanosine-specific single-stranded ribonuclease effectors of a phytopathogenic fungus potentiate host immune responses

Plants activate immunity upon recognition of pathogen-associated molecular patterns. Although phytopathogens have evolved a set of effector proteins to counteract plant immunity, some effectors are perceived by hosts and induce immune responses. Here, we show that two secreted ribonuclease effectors, SRN1 and SRN2, encoded in a phytopathogenic fungus, Colletotrichum orbiculare, induce cell death in a signal peptide- and catalytic residue-dependent manner, when transiently expressed in Nicotiana benthamiana. The pervasive presence of SRN genes across Colletotrichum species suggested the conserved roles. Using a transient gene expression system in cucumber (Cucumis sativus), an original host of C. orbiculare, we show that SRN1 and SRN2 potentiate host pattern-triggered immunity responses. Consistent with this, C. orbiculare SRN1 and SRN2 deletion mutants exhibited increased virulence on the host. In vitro analysis revealed that SRN1 specifically cleaves single-stranded RNAs at guanosine, leaving a 3'-end phosphate. Importantly, the potentiation of C. sativus responses by SRN1 and SRN2, present in the apoplast, depends on ribonuclease catalytic residues. We propose that the pathogen-derived apoplastic guanosine-specific single-stranded endoribonucleases lead to immunity potentiation in plants.

Two NAC transcription factors regulated fruit softening through activating xyloglucan endotransglucosylase/hydrolase genes during kiwifruit ripening

Kiwifruit is a climacteric fruit that is prone to ripening and softening. Understanding molecular regulatory mechanism of kiwifruit softening, is helpful to ensure long-term storage of fruit. In the study, two NAC TFs and two XTH genes were isolated from kiwifruit. Phylogenetic tree showed that both AcNAC1 and AcNAC2 belonged to NAP subfamily, AcXTH1 belong to I subfamily, and AcXTH2 belong to III subfamily. Bioinformatics analysis predicted that AcNAC1 and AcNAC2 possessed similar three-dimensional structural, and belonged to hydrophilic proteins. AcXTH1 and AcXTH2 were hydrophilic proteins and contained signal peptides. AcXTH1 had a transmembrane structure, but AcXTH2 did not. qRT-PCR results showed that AcNAC1, AcNAC2, AcXTH1 and AcXTH2 were increased during kiwifruit ripening. Correlation analysis showed that kiwifruit softening was closely related to endotransglucosylase/hydrolase genes and NAC TFs, as well as there was also a close relationship between AcXTHs and AcNACs. Moreover, both AcNAC1 and AcNAC2 were transcriptional activators located in nucleus, which bound to and activated the promoters of AcXTH1 and AcXTH2. In shortly, we proved that the roles of NAC TFs in mediating fruit softening during kiwifruit ripening. Altogether, our results clarified that AcNAC1 and AcNAC2 were transcriptional activators, and took part in kiwifruit ripening and softening through activating endotransglucosylase/hydrolase genes, providing a new insight of fruit softening network in kiwifruit ripening.

Silencing ScGUX2 reduces xylan glucuronidation and improves biomass saccharification in sugarcane

There is an increasing need for renewable energy sources to replace part of our fossil fuel-based economy and reduce greenhouse gas emission. Sugarcane bagasse is a prominent feedstock to produce cellulosic bioethanol, but strategies are still needed to improve the cost-effective exploitation of this potential energy source. In model plants, it has been shown that GUX genes are involved in cell wall hemicellulose decoration, adding glucuronic acid substitutions on the xylan backbone. Mutation of GUX genes increases enzyme access to cell wall polysaccharides, reducing biomass recalcitrance in Arabidopsis thaliana. Here, we characterized the sugarcane GUX genes and silenced GUX2 in commercial hybrid sugarcane. The transgenic lines had no penalty in development under greenhouse conditions. The sugarcane GUX1 and GUX2 enzymes generated different patterns of xylan glucuronidation, suggesting they may differently influence the molecular interaction of xylan with cellulose and lignin. Studies using biomass without chemical or steam pretreatment showed that the cell wall polysaccharides, particularly xylan, were less recalcitrant in sugarcane with GUX2 silenced than in WT plants. Our findings suggest that manipulation of GUX in sugarcane can reduce the costs of second-generation ethanol production and enhance the contribution of biofuels to lowering the emission of greenhouse gases.

Development of Virus-like Particle Plant-Based Vaccines against Avian H5 and H9 Influenza A Viruses

Avian influenza A virus (AIV) is a significant cause of mortality in poultry, causing substantial economic loss, particularly in developing countries, and has zoonotic potential. For example, highly pathogenic avian influenza (HPAI) viruses of the H5 subtype have been circulating in Egypt for around two decades. In the last decade, H5N1 viruses of clade 2.2.1 have been succeeded by the antigenically distinct H5N8 clade 2.3.4.4b viruses. Furthermore, H9N2 viruses co-circulate with the H5N8 viruses in Egyptian poultry. It is widely recognised that effective vaccination against IAV requires a close antigenic match between the vaccine and viruses circulating in the field. Therefore, approaches to develop cost-effective vaccines that can be rapidly adapted to local virus strains are required for developing countries such as Egypt. In this project, the haemagglutinin (HA) proteins of Egyptian H5 and H9 viruses were expressed by transient transfection of plants (Nicotiana benthamiana). The formation of virus-like particles (VLPs) was confirmed by transmission electron microscopy. Mice were immunised with four doses of either H5 or H9 VLPs with adjuvant. Antibody and cellular immune responses were measured against the corresponding recombinant protein using ELISA and enzyme-linked immunosorbent assay (ELISpot), respectively. Chickens were immunised with one dose of H5 VLPs, eliciting HA-specific antibodies measured by ELISA and a pseudotyped virus neutralisation test using a heterologous H5 HA. In conclusion, plant-based VLP vaccines have potential for producing an effective vaccine candidate within a short time at a relatively low cost.

Reconstitution of early paclitaxel biosynthetic network

Paclitaxel is an anticancer therapeutic produced by the yew tree. Over the last two decades, a significant bottleneck in the reconstitution of early paclitaxel biosynthesis has been the propensity of heterologously expressed pathway cytochromes P450, including taxadiene 5α-hydroxylase (T5αH), to form multiple products. Here, we structurally characterize four new products of T5αH, many of which appear to be over-oxidation of the primary mono-oxidized products. By tuning the promoter strength for T5αH expression in Nicotiana plants, we observe decreased levels of these proposed byproducts with a concomitant increase in the accumulation of taxadien-5α-ol, the paclitaxel precursor, by three-fold. This enables the reconstitution of a six step biosynthetic pathway, which we further show may function as a metabolic network. Our result demonstrates that six previously characterized Taxus genes can coordinatively produce key paclitaxel intermediates and serves as a crucial platform for the discovery of the remaining biosynthetic genes.

Protective efficacy of a plant-produced beta variant rSARS-CoV-2 VLP vaccine in golden Syrian hamsters

In the quest for heightened protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants, we engineered a prototype vaccine utilizing the plant expression system of Nicotiana benthamiana, to produce a recombinant SARS-CoV-2 virus-like particle (VLP) vaccine presenting the S-protein from the Beta (B.1.351) variant of concern (VOC). This innovative vaccine, formulated with either a squalene oil-in-water emulsion or a synthetic CpG oligodeoxynucleotide adjuvant, demonstrated efficacy in a golden Syrian Hamster challenge model. The Beta VLP vaccine induced a robust humoral immune response, with serum exhibiting neutralization not only against SARS-CoV-2 Beta but also cross-neutralizing Delta and Omicron pseudoviruses. Protective efficacy was demonstrated, evidenced by reduced viral RNA copies and mitigated weight loss and lung damage compared to controls. This compelling data instills confidence in the creation of a versatile platform for the local manufacturing of potential pan-sarbecovirus vaccines, against evolving viral threats.

Enhanced binding and inhibition of SARS-CoV-2 by a plant-derived ACE2 protein containing a fused mu tailpiece

Infectious diseases such as Coronavirus disease 2019 (COVID-19) and Middle East respiratory syndrome (MERS) present an increasingly persistent crisis in many parts of the world. COVID-19 is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The angiotensin-converting enzyme 2 (ACE2) is a crucial cellular receptor for SARS-CoV-2 infection. Inhibition of the interaction between SARS-CoV-2 and ACE2 has been proposed as a target for the prevention and treatment of COVID-19. We produced four recombinant plant-derived ACE2 isoforms with or without the mu tailpiece (μ-tp) of immunoglobulin M (IgM) and the KDEL endoplasmic reticulum retention motif in a plant expression system. The plant-derived ACE2 isoforms bound whole SARS-CoV-2 virus and the isolated receptor binding domains of SARS-CoV-2 Alpha, Beta, Gamma, Delta, and Omicron variants. Fusion of μ-tp and KDEL to the ACE2 protein (ACE2 μK) had enhanced binding activity with SARS-CoV-2 in comparison with unmodified ACE2 protein derived from CHO cells. Furthermore, the plant-derived ACE2 μK protein exhibited no cytotoxic effects on Vero E6 cells and effectively inhibited SARS-CoV-2 infection. The efficient and rapid scalability of plant-derived ACE2 μK protein offers potential for the development of preventive and therapeutic agents in the early response to future viral outbreaks.

Insight into neofunctionalization of 2,3-oxidosqualene cyclases in B,C-ring-opened triterpene biosynthesis in quinoa

Bioactive triterpenes feature complex fused-ring structures, primarily shaped by the first-committed enzyme, 2,3-oxidosqualene cyclases (OSCs) in plant triterpene biosynthesis. Triterpenes with B,C-ring-opened skeletons are extremely rare with unknown formation mechanisms, harbouring unchartered chemistry and biology. Here, through mining the genome of Chenopodium quinoa followed by functional characterization, we identified a stress-responsive and neofunctionalized OSC capable of generating B,C-ring-opened triterpenes, including camelliol A and B and the novel (-)-quinoxide A as wax components of the specialized epidermal bladder cells, namely the quinoxide synthase (CqQS). Protein structure analysis followed by site-directed mutagenesis identified key variable amino acid sites underlying functional interconversion between pentacyclic β-amyrin synthase (CqbAS1) and B,C-ring-opened triterpene synthase CqQS. Mutation of one key residue (N612K) in even evolutionarily distant Arabidopsis β-amyrin synthase could generate quinoxides, indicating a conserved mechanism for B,C-ring-opened triterpene formation in plants. Quantum computation combined with docking experiments further suggests that conformations of conserved W613 and F413 of CqQS might be key to selectively stabilizing intermediate carbocations towards B,C-ring-opened triterpene formation. Our findings shed light on quinoa triterpene skeletal diversity and mechanisms underlying B,C-ring-opened triterpene biosynthesis, opening avenues towards accessing their chemistry and biology and paving the way for quinoa trait engineering and quality improvement.

Plant Engineering to Enable Platforms for Sustainable Bioproduction of Terpenoids

Terpenoids represent the most diverse class of natural products, with a broad spectrum of industrial relevance including applications in green solvents, flavors and fragrances, nutraceuticals, colorants, and therapeutics. They are typically challenging to extract from their natural sources, where they occur in small amounts and mixtures of related but unwanted byproducts. Formal chemical synthesis, where established, is reliant on petrochemistry. Hence, there is great interest in developing sustainable solutions to assemble biosynthetic pathways in engineered host organisms. Metabolic engineering for chemical production has largely focused on microbial hosts, yet plants offer a sustainable production platform. In addition to containing the precursor pathways that generate the terpenoid building blocks as well as the cell structures and compartments required, or tractable localization for the enzymes involved, plants may provide a low input system to produce these chemicals using carbon dioxide and sunlight only. There have been significant recent advancements in the discovery of pathways to terpenoids of interest as well as strategies to boost yields in host plants. While part of the phytochemical field is focusing on the discovery of biosynthetic pathways, this review will focus on advancements using the pathway toolbox and toward engineering plants for the production of terpenoids. We will highlight strategies currently used to produce target products, optimization of known pathways to improve yields, compartmentalization of pathways within cells, and genetic tools developed to facilitate complex engineering of biosynthetic pathways. These advancements in Synthetic Biology are bringing engineered plant systems closer to commercially relevant hosts for the bioproduction of terpenoids.

Compartmentalized Terpenoid Production in Plants Using Agrobacterium-Mediated Transient Expression

As the field of plant synthetic biology continues to grow, Agrobacterium-mediated transient expression has become an essential method to rapidly test pathway candidate genes in a combinatorial fashion. This is especially important when elucidating and engineering more complex pathways to produce commercially relevant chemicals like many terpenoids, a widely diverse class of natural products of often industrial relevance. Agrobacterium-mediated transient expression has facilitated multiplex expression of recombinant and modified enzymes, including synthetic biology approaches to compartmentalize the biosynthesis of terpenoids subcellularly. Here, we describe methods on how to deploy Agrobacterium-mediated transient expression in Nicotiana benthamiana to rapidly develop terpenoid pathways and compartmentalize terpenoid biosynthesis within plastids, the cytosol, or at the surface of lipid droplets.

Methyltransferase-like (METTL) homologues participate in Nicotiana benthamiana antiviral responses

Methyltransferase (MTase) enzymes catalyze the addition of a methyl group to a variety of biological substrates. MTase-like (METTL) proteins are Class I MTases whose enzymatic activities contribute to the epigenetic and epitranscriptomic regulation of multiple cellular processes. N⁶-adenosine methylation (m⁶A) is a common chemical modification of eukaryotic and viral RNA whose abundance is jointly regulated by MTases and METTLs, demethylases, and m⁶A binding proteins. m⁶A affects various cellular processes including RNA degradation, post-transcriptional processing, and antiviral immunity. Here, we used Nicotiana benthamiana and plum pox virus (PPV), an RNA virus of the Potyviridae family, to investigated the roles of MTases in plant-virus interaction. RNA sequencing analysis identified MTase transcripts that are differentially expressed during PPV infection; among these, accumulation of a METTL gene was significantly downregulated. Two N. benthamiana METTL transcripts (NbMETTL1 and NbMETTL2) were cloned and further characterized. Sequence and structural analyses of the two encoded proteins identified a conserved S-adenosyl methionine (SAM) binding domain, showing they are SAM-dependent MTases phylogenetically related to human METTL16 and Arabidopsis thaliana FIONA1. Overexpression of NbMETTL1 and NbMETTL2 caused a decrease of PPV accumulation. In sum, our results indicate that METTL homologues participate in plant antiviral responses.

Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model.We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field.EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores.These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

SARS-CoV-2 spike protein S1 subunit induces potent neutralizing responses in mice and is effective against Delta and Omicron variants

SARS-CoV-2, the virus responsible for the COVID-19 pandemic, belongs to the betacoronavirus genus. This virus has a high mutation rate, which rapidly evolves into new variants with different properties, such as increased transmissibility or immune evasion. Currently, the most prevalent global SARS-CoV-2 variant is Omicron, which is more transmissible than previous variants. Current available vaccines may be less effective against some currently existing SARS-CoV-2 variants, including the Omicron variant. The S1 subunit of the spike protein of SARS-CoV-2 has been a major target for COVID-19 vaccine development. It plays a crucial role in the virus's entry into host cells and is the primary target for neutralizing antibodies. In this study, the S1 subunit of the spike protein of SARS-CoV-2 was engineered and produced at a high level in Nicotiana benthamiana plant. The expression level of the recombinant S1 protein was greater than the 0.5-g/kg fresh weight, and the purification yield was at least ~0.3 g of pure protein/kg of plant biomass, which would make a plant-produced S1 antigen an ideal vaccine candidate for commercialization. Purified, the plant-produced SARS-CoV-2 S1 protein exhibited significantly higher binding to the SARS-CoV-2 receptor, angiotensin-converting enzyme 2 (ACE2). Moreover, we also show that recombinant S1 protein/antigen-elicited antibodies can neutralize the Delta or Omicron variants. Collectively, our results demonstrate that a plant-produced S1 antigen could be a promising vaccine candidate against SARS-CoV-2 variants including Omicron.

Petal size is controlled by the MYB73/TPL/HDA19-miR159-CKX6 module regulating cytokinin catabolism in Rosa hybrida

The size of plant lateral organs is determined by well-coordinated cell proliferation and cell expansion. Here, we report that miR159, an evolutionarily conserved microRNA, plays an essential role in regulating cell division in rose (Rosa hybrida) petals by modulating cytokinin catabolism. We uncover that Cytokinin Oxidase/Dehydrogenase6 (CKX6) is a target of miR159 in petals. Knocking down miR159 levels results in the accumulation of CKX6 transcripts and earlier cytokinin clearance, leading to a shortened cell division period and smaller petals. Conversely, knocking down CKX6 causes cytokinin accumulation and a prolonged developmental cell division period, mimicking the effects of exogenous cytokinin application. MYB73, a R2R3-type MYB transcription repressor, recruits a co-repressor (TOPLESS) and a histone deacetylase (HDA19) to form a suppression complex, which regulates MIR159 expression by modulating histone H3 lysine 9 acetylation levels at the MIR159 promoter. Our work sheds light on mechanisms for ensuring the correct timing of the exit from the cell division phase and thus organ size regulation by controlling cytokinin catabolism.

Discovery of isoflavone phytoalexins in wheat reveals an alternative route to isoflavonoid biosynthesis

Isoflavones are a group of phenolic compounds mostly restricted to plants of the legume family, where they mediate important interactions with plant-associated microbes, including in defense from pathogens and in nodulation. Their well-studied health promoting attributes have made them a prime target for metabolic engineering, both for bioproduction of isoflavones as high-value molecules, and in biofortification of food crops. A key gene in their biosynthesis, isoflavone synthase, was identified in legumes over two decades ago, but little is known about formation of isoflavones outside of this family. Here we identify a specialized wheat-specific isoflavone synthase, TaCYP71F53, which catalyzes a different reaction from the leguminous isoflavone synthases, thus revealing an alternative path to isoflavonoid biosynthesis and providing a non-transgenic route for engineering isoflavone production in wheat. TaCYP71F53 forms part of a biosynthetic gene cluster that produces a naringenin-derived O-methylated isoflavone, 5-hydroxy-2',4',7-trimethoxyisoflavone, triticein. Pathogen-induced production and in vitro antimicrobial activity of triticein suggest a defense-related role for this molecule in wheat. Genomic and metabolic analyses of wheat ancestral grasses further show that the triticein gene cluster was introduced into domesticated emmer wheat through natural hybridization ~9000 years ago, and encodes a pathogen-responsive metabolic pathway that is conserved in modern bread wheat varieties.

Impact of mutations on the plant-based production of recombinant SARS-CoV-2 RBDs

Subunit vaccines based on recombinant viral antigens are valuable interventions to fight existing and evolving viruses and can be produced at large-scale in plant-based expression systems. The recombinant viral antigens are often derived from glycosylated envelope proteins of the virus and glycosylation plays an important role for the immunogenicity by shielding protein epitopes. The receptor-binding domain (RBD) of the SARS-CoV-2 spike is a principal target for vaccine development and has been produced in plants, but the yields of recombinant RBD variants were low and the role of the N-glycosylation in RBD from different SARS-CoV-2 variants of concern is less studied. Here, we investigated the expression and glycosylation of six different RBD variants transiently expressed in leaves of Nicotiana benthamiana. All of the purified RBD variants were functional in terms of receptor binding and displayed almost full N-glycan occupancy at both glycosylation sites with predominately complex N-glycans. Despite the high structural sequence conservation of the RBD variants, we detected a variation in yield which can be attributed to lower expression and differences in unintentional proteolytic processing of the C-terminal polyhistidine tag used for purification. Glycoengineering towards a human-type complex N-glycan profile with core α1,6-fucose, showed that the reactivity of the neutralizing antibody S309 differs depending on the N-glycan profile and the RBD variant.

Tandem duplication and sub-functionalization of clerodane diterpene synthase originate the blooming of clerodane diterpenoids in Scutellaria barbata

Scutellaria barbata is a traditional Chinese herb medicine and a major source of bioactive clerodane diterpenoids. However, barely clerodanes have been isolated from the closely related S. baicalensis. Here we assembled a chromosome-level genome of S. barbata and identified three class II clerodane diterpene synthases (SbarKPS1, SbarKPS2 and SbaiKPS1) from these two organisms. Using in vitro and in vivo assays, SbarKPS1 was characterized as a monofunctional (-)-kolavenyl diphosphate synthases ((-)-KPS), while SbarKPS2 and SbaiKPS1 produced major neo-cleroda-4(18),13E-dienyl diphosphate with small amount of (-)-KPP. SbarKPS1 and SbarKPS2 shared a high protein sequence identity and formed a tandem gene pair, indicating tandem duplication and sub-functionalization probably led to the evolution of monofunctional (-)-KPS in S. barbata. Additionally, SbarKPS1 and SbarKPS2 were primarily expressed in the leaves and flowers of S. barbata, which was consistent with the distribution of major clerodane diterpenoids scutebarbatine A and B. In contrast, SbaiKPS1 was barely expressed in any tissue of S. baicalensis. We further explored the downstream class I diTPS by functional characterizing of SbarKSL3 and SbarKSL4. Unfortunately, no dephosphorylated product was detected in the coupled assays with SbarKSL3/KSL4 and four class II diTPSs (SbarKPS1, SbarKPS2, SbarCPS2 and SbarCPS4) when a phosphatase inhibitor cocktail was included. Co-expression of SbarKSL3/KSL4 with class II diTPSs in yeast cells did not increase the yield of the corresponding dephosphorylated products, either. Together, these findings elucidated the involvement of two class II diTPSs in clerodane biosynthesis in S. barbata, while the class I diTPS is likely not responsible for the subsequent dephosphorylation step.

Reconstitution of Early Paclitaxel Biosynthetic Network

Paclitaxel is an anticancer therapeutic produced by the yew tree. Over the last two decades, a significant bottleneck in the reconstitution of early paclitaxel biosynthesis has been the propensity of heterologously expressed pathway cytochromes P450, including taxadiene 5α-hydroxylase (T5αH), to form multiple products. This diverts metabolic flux away from the paclitaxel precursor, taxadien-5α-ol, thus previous attempts of reconstitution have not yielded sufficient material for characterization, regardless of the heterologous host. Here, we structurally characterized four new products of T5αH, many of which appear to be over-oxidation of the primary mono-oxidized products. By tuning the promoter strength for T5αH expression, levels of these proposed byproducts decrease with a concomitant increase in the accumulation of taxadien-5α-ol by four-fold. This engineered system enabled the reconstitution of a six step biosynthetic pathway to produce isolatable 5α,10β-diacetoxy-taxadien-13α-ol. Furthermore, we showed that this pathway may function as a metabolic network rather than a linear pathway. The engineering of the paclitaxel biosynthetic network demonstrates that Taxus genes can coordinatively function for the biosynthetic production of key early stage paclitaxel intermediates and serves as a crucial platform for the discovery of the remaining biosynthetic genes.

Multiplicity of the Agrobacterium Infection of Nicotiana benthamiana for Transient DNA Delivery

Biological DNA transfer into plant cells mediated by Agrobacterium represents one of the most powerful tools for the engineering and study of plant systems. Transient expression of transfer DNA (T-DNA) in particular enables rapid testing of gene products and has been harnessed for facile combinatorial expression of multiple genes. In analogous mammalian cell-based gene expression systems, a clear sense of the multiplicity of infection (MOI) allows users to predict and control viral transfection frequencies for applications requiring single versus multiple transfection events per cell. Despite the value of Agrobacterium-mediated transient transformation of plants, MOI has not been quantified. Here, we analyze the Poisson probability distribution of the T-DNA transfer in leaf pavement cells to determine the MOI for the widely used model system Agrobacterium GV3101/Nicotiana benthamiana. These data delineate the relationship between an individual Agrobacterium strain infiltration OD600, plant cell perimeter, and leaf age, as well as plant cell coinfection rates. Our analysis establishes experimental regimes where the probability of near-simultaneous delivery of >20 unique T-DNAs to a given plant cell remains high throughout the leaf at infiltration OD600 above ∼0.2 for individual strains. In contrast, single-strain T-DNA delivery can be achieved at low strain infiltration OD600: at OD600 0.02, we observe that ∼40% of plant cells are infected, with 80% of those infected cells containing T-DNA product from just a single strain. We anticipate that these data will enable users to develop new approaches to in-leaf library development using Agrobacterium transient expression and reliable combinatorial assaying of multiple heterologous proteins in a single plant cell.

A LlMYB305-LlC3H18-LlWRKY33 module regulates thermotolerance in lily

The CCCH proteins play important roles in plant growth and development, hormone response, pathogen defense and abiotic stress tolerance. However, the knowledge of their roles in thermotolerance are scarce. Here, we identified a heat-inducible CCCH gene LlC3H18 from lily. LlC3H18 was localized in the cytoplasm and nucleus under normal conditions, while it translocated in the cytoplasmic foci and co-located with the markers of two messenger ribonucleoprotein (mRNP) granules, processing bodies (PBs) and stress granules (SGs) under heat stress conditions, and it also exhibited RNA-binding ability. In addition, LlC3H18 exhibited transactivation activity in both yeast and plant cells. In lily and Arabidopsis, overexpression of LlC3H18 damaged their thermotolerances, and silencing of LlC3H18 in lily also impaired its thermotolerance. Similarly, Arabidopsis atc3h18 mutant also showed decreased thermotolerance. These results indicated that the appropriate expression of C3H18 was crucial for establishing thermotolerance. Further analysis found that LlC3H18 directly bound to the promoter of LlWRKY33 and activated its expression. Besides, it was found that LlMYB305 acted as an upstream factor of LlC3H18 and activated its expression. In conclusion, we demonstrated that there may be a LlMYB305-LlC3H18-LlWRKY33 regulatory module in lily that is involved in the establishment of thermotolerance and finely regulates heat stress response.

A chloroplast protein atlas reveals punctate structures and spatial organization of biosynthetic pathways

Chloroplasts are eukaryotic photosynthetic organelles that drive the global carbon cycle. Despite their importance, our understanding of their protein composition, function, and spatial organization remains limited. Here, we determined the localizations of 1,034 candidate chloroplast proteins using fluorescent protein tagging in the model alga Chlamydomonas reinhardtii. The localizations provide insights into the functions of poorly characterized proteins; identify novel components of nucleoids, plastoglobules, and the pyrenoid; and reveal widespread protein targeting to multiple compartments. We discovered and further characterized cellular organizational features, including eleven chloroplast punctate structures, cytosolic crescent structures, and unexpected spatial distributions of enzymes within the chloroplast. We also used machine learning to predict the localizations of other nuclear-encoded Chlamydomonas proteins. The strains and localization atlas developed here will serve as a resource to accelerate studies of chloroplast architecture and functions.