Foxtail mosaic virus: A Viral Vector for Protein Expression in Cereals

Trans-complementation of the viral movement protein mediates efficient expression of large target genes via a tobacco mosaic virus vector

The development of plant virus-based expression systems has expanded rapidly owing to their potential applications in gene functional and disease resistance research, and industrial production of pharmaceutical proteins. However, the low yield of certain proteins, especially high-molecular-mass proteins, restricts the production scale. In this study, we observed that the tobacco mosaic virus (TMV)-mediated expression of a foreign protein was correlated with the amount of the movement protein (MP) and developed a TMV-derived pAT-transMP vector system incorporating trans-complementation expression of MP. The system is capable of efficient expression of exogenous proteins, in particular those with a high molecular mass, and enables simultaneous expression of two target molecules. Furthermore, viral expression of competent CRISPR-Cas9 protein and construction of CRISPR-Cas9-mediated gene-editing system in a single pAT-transMP construct was achieved. The results demonstrated a novel role for TMV-MP in enhancing the accumulation of a foreign protein produced from the viral vector or a binary expression system. Further investigation of the mechanism underlying this role will be beneficial for optimization of plant viral vectors with broad applications.

The role of phenomics and genomics in delineating the genetic basis of complex traits in millets

Millets, comprising a diverse group of small-seeded grains, have emerged as vital crops with immense nutritional, environmental, and economic significance. The comprehension of complex traits in millets, influenced by multifaceted genetic determinants, presents a compelling challenge and opportunity in agricultural research. This review delves into the transformative roles of phenomics and genomics in deciphering these intricate genetic architectures. On the phenomics front, high-throughput platforms generate rich datasets on plant morphology, physiology, and performance in diverse environments. This data, coupled with field trials and controlled conditions, helps to interpret how the environment interacts with genetics. Genomics provides the underlying blueprint for these complex traits. Genome sequencing and genotyping technologies have illuminated the millet genome landscape, revealing diverse gene pools and evolutionary relationships. Additionally, different omics approaches unveil the intricate information of gene expression, protein function, and metabolite accumulation driving phenotypic expression. This multi-omics approach is crucial for identifying candidate genes and unfolding the intricate pathways governing complex traits. The review highlights the synergy between phenomics and genomics. Genomically informed phenotyping targets specific traits, reducing the breeding size and cost. Conversely, phenomics identifies promising germplasm for genomic analysis, prioritizing variants with superior performance. This dynamic interplay accelerates breeding programs and facilitates the development of climate-smart, nutrient-rich millet varieties and hybrids. In conclusion, this review emphasizes the crucial roles of phenomics and genomics in unlocking the genetic enigma of millets.

Major transcription factor families at the nexus of regulating abiotic stress response in millets: a comprehensive review

Millets stand out as a sustainable crop with the potential to address the issues of food insecurity and malnutrition. These small-seeded, drought-resistant cereals have adapted to survive a broad spectrum of abiotic stresses. Researchers are keen on unravelling the regulatory mechanisms that empower millets to withstand environmental adversities. The aim is to leverage these identified genetic determinants from millets for enhancing the stress tolerance of major cereal crops through genetic engineering or breeding. This review sheds light on transcription factors (TFs) that govern diverse abiotic stress responses and play role in conferring tolerance to various abiotic stresses in millets. Specifically, the molecular functions and expression patterns of investigated TFs from various families, including bHLH, bZIP, DREB, HSF, MYB, NAC, NF-Y and WRKY, are comprehensively discussed. It also explores the potential of TFs in developing stress-tolerant crops, presenting a comprehensive discussion on diverse strategies for their integration.

Comparative Analysis of the Avirulence Effectors Produced by the Fungal Stem Rust Pathogen of Wheat

Crops are constantly exposed to pathogenic microbes. Rust fungi are examples of these harmful microorganisms, which have a major economic impact on wheat production. To protect themselves from pathogens like rust fungi, plants employ a multilayered immune system that includes immunoreceptors encoded by resistance genes. Significant efforts have led to the isolation of numerous resistance genes against rust fungi in cereals, especially in wheat. However, the evolution of virulence of rust fungi hinders the durability of resistance genes as a strategy for crop protection. Rust fungi, like other biotrophic pathogens, secrete an arsenal of effectors to facilitate infection, and these are the molecules that plant immunoreceptors target for pathogen recognition and mounting defense responses. When recognized, these effector proteins are referred to as avirulence (Avr) effectors. Despite the many predicted effectors in wheat rust fungi, only five Avr genes have been identified, all from wheat stem rust. Knowledge of the Avr genes and their variation in the fungal population will inform deployment of the most appropriate wheat disease-resistance genes for breeding and farming. The review provides an overview of methodologies as well as the validation techniques that have been used to characterize Avr effectors from wheat stem rust. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

NIa-Pro of sugarcane mosaic virus targets Corn Cysteine Protease 1 (CCP1) to undermine salicylic acid-mediated defense in maize

Papain-like cysteine proteases (PLCPs) play pivotal roles in plant defense against pathogen invasions. While pathogens can secrete effectors to target and inhibit PLCP activities, the roles of PLCPs in plant-virus interactions and the mechanisms through which viruses neutralize PLCP activities remain largely uncharted. Here, we demonstrate that the expression and activity of a maize PLCP CCP1 (Corn Cysteine Protease), is upregulated following sugarcane mosaic virus (SCMV) infection. Transient silencing of CCP1 led to a reduction in PLCP activities, thereby promoting SCMV infection in maize. Furthermore, the knockdown of CCP1 resulted in diminished salicylic acid (SA) levels and suppressed expression of SA-responsive pathogenesis-related genes. This suggests that CCP1 plays a role in modulating the SA signaling pathway. Interestingly, NIa-Pro, the primary protease of SCMV, was found to interact with CCP1, subsequently inhibiting its protease activity. A specific motif within NIa-Pro termed the inhibitor motif was identified as essential for its interaction with CCP1 and the suppression of its activity. We have also discovered that the key amino acids responsible for the interaction between NIa-Pro and CCP1 are crucial for the virulence of SCMV. In conclusion, our findings offer compelling evidence that SCMV undermines maize defense mechanisms through the interaction of NIa-Pro with CCP1. Together, these findings shed a new light on the mechanism(s) controlling the arms races between virus and plant.

Multiplexed effector screening for recognition by endogenous resistance genes using positive defense reporters in wheat protoplasts

Plant resistance (R) and pathogen avirulence (Avr) gene interactions play a vital role in pathogen resistance. Efficient molecular screening tools for crops lack far behind their model organism counterparts, yet they are essential to rapidly identify agriculturally important molecular interactions that trigger host resistance. Here, we have developed a novel wheat protoplast assay that enables efficient screening of Avr/R interactions at scale. Our assay allows access to the extensive gene pool of phenotypically described R genes because it does not require the overexpression of cloned R genes. It is suitable for multiplexed Avr screening, with interactions tested in pools of up to 50 Avr candidates. We identified Avr/R-induced defense genes to create a promoter-luciferase reporter. Then, we combined this with a dual-color ratiometric reporter system that normalizes read-outs accounting for experimental variability and Avr/R-induced cell death. Moreover, we introduced a self-replicative plasmid reducing the amount of plasmid used in the assay. Our assay increases the throughput of Avr candidate screening, accelerating the study of cellular defense signaling and resistance gene identification in wheat. We anticipate that our assay will significantly accelerate Avr identification for many wheat pathogens, leading to improved genome-guided pathogen surveillance and breeding of disease-resistant crops.

Chemical-tag-based semi-annotated metabolomics facilitates gene identification and specialized metabolic pathway elucidation in wheat

The importance of metabolite modification and species-specific metabolic pathways has long been recognized. However, linking the chemical structure of metabolites to gene function in order to explore the genetic and biochemical basis of metabolism has not yet been reported in wheat (Triticum aestivum). Here, we profiled metabolic fragment enrichment in wheat leaves and consequently applied chemical-tag-based semi-annotated metabolomics in a genome-wide association study in accessions of wheat. The studies revealed that all 1,483 quantified metabolites have at least one known functional group whose modification is tailored in an enzyme-catalyzed manner and eventually allows efficient candidate gene mining. A Triticeae crop-specific flavonoid pathway and its underlying metabolic gene cluster were elucidated in further functional studies. Additionally, upon overexpressing the major effect gene of the cluster TraesCS2B01G460000 (TaOMT24), the pathway was reconstructed in rice (Oryza sativa), which lacks this pathway. The reported workflow represents an efficient and unbiased approach for gene mining using forward genetics in hexaploid wheat. The resultant candidate gene list contains vast molecular resources for decoding the genetic architecture of complex traits and identifying valuable breeding targets and will ultimately aid in achieving wheat crop improvement.

Silencing ZmPP2C-A10 with a foxtail mosaic virus (FoMV) derived vector benefits maize growth and development following water limitation

Global climate change is causing more frequent and severe droughts, which can have negative impacts on plant growth and crop productivity. Under drought conditions, plants produce the hormone ABA (abscisic acid), which regulates adaptive responses, such as stomatal closure and root elongation. Plant viruses have been used in the lab to convey new traits to plants and could also be used to increase production of ABA or to enhance downstream plant drought resistance responses. In this study, foxtail mosaic virus (FoMV) was used to silence ZmPP2C-A10, a negative regulator of ABA signalling, in maize (Zea mays L.). Both silenced and control plants were exposed to an 8-day drought treatment, followed by a 30-day period of rewatering, after which indicators of drought resistance were measured. After drought treatment, we observed a nearly twofold increase in expression of a stress-mitigation gene, ZmRAB17, reduced chlorophyll fluorescence changes (indicator of stress), and increased plant biomass and development in the ZmPP2C-A10-silenced maize compared to controls. These results demonstrate that the FoMV system can be used to silence endogenous expression of ZmPP2C-A10 and increase maize tolerance to drought. This could offer a useful tool to improve crop traits and reduce yield loss during the growing season.

Development of cassava common mosaic virus-based vector for protein expression and gene editing in cassava

BACKGROUND

Plant virus vectors designed for virus-mediated protein overexpression (VOX), virus-induced gene silencing (VIGS), and genome editing (VIGE) provide rapid and cost-effective tools for functional genomics studies, biotechnology applications and genome modification in plants. We previously reported that a cassava common mosaic virus (CsCMV, genus Potexvirus)-based VIGS vector was used for rapid gene function analysis in cassava. However, there are no VOX and VIGE vectors available in cassava.

RESULTS

In this study, we developed an efficient VOX vector (CsCMV2-NC) for cassava by modifying the CsCMV-based VIGS vector. Specifically, the length of the duplicated putative subgenomic promoter (SGP1) of the CsCMV CP gene was increased to improve heterologous protein expression in cassava plants. The modified CsCMV2-NC-based VOX vector was engineered to express genes encoding green fluorescent protein (GFP), bacterial phytoene synthase (crtB), and Xanthomonas axonopodis pv. manihotis (Xam) type III effector XopAO1 for viral infection tracking, carotenoid biofortification and Xam virulence effector identification in cassava. In addition, we used CsCMV2-NC to deliver single guide RNAs (gMePDS1/2) targeting two loci of the cassava phytoene desaturase gene (MePDS) in Cas9-overexpressing transgenic cassava lines. The CsCMV-gMePDS1/2 efficiently induced deletion mutations of the targeted MePDS with the albino phenotypes in systemically infected cassava leaves.

CONCLUSIONS

Our results provide a useful tool for rapid and efficient heterologous protein expression and guide RNA delivery in cassava. This expands the potential applications of CsCMV-based vector in gene function studies, biotechnology research, and precision breeding for cassava.

Foxtail mosaic virus: A tool for gene function analysis in maize and other monocots

Many plant viruses have been engineered into vectors for use in functional genomics studies, expression of heterologous proteins, and, most recently, gene editing applications. The use of viral vectors overcomes bottlenecks associated with mutagenesis and transgenesis approaches often implemented for analysis of gene function. There are several engineered viruses that are demonstrated or suggested to be useful in maize through proof-of-concept studies. However, foxtail mosaic virus (FoMV), which has a relatively broad host range, is emerging as a particularly useful virus for gene function studies in maize and other monocot crop or weed species. A few clones of FoMV have been independently engineered, and they have different features and capabilities for virus-induced gene silencing (VIGS) and virus-mediated overexpression (VOX) of proteins. In addition, FoMV can be used to deliver functional guide RNAs in maize and other plants expressing the Cas9 protein, demonstrating its potential utility in virus-induced gene editing applications. There is a growing number of studies in which FoMV vectors are being applied for VIGS or VOX in maize and the vast majority of these are related to maize-microbe interactions. In this review, we highlight the biology and engineering of FoMV as well as its applications in maize-microbe interactions and more broadly in the context of the monocot functional genomics toolbox.

Rescue of the first alphanucleorhabdovirus entirely from cloned complementary DNA: An efficient vector for systemic expression of foreign genes in maize and insect vectors

Recent reverse genetics technologies have enabled genetic manipulation of plant negative-strand RNA virus (NSR) genomes. Here, we report construction of an infectious clone for the maize-infecting Alphanucleorhabdovirus maydis, the first efficient NSR vector for maize. The full-length infectious clone was established using agrobacterium-mediated delivery of full-length maize mosaic virus (MMV) antigenomic RNA and the viral core proteins (nucleoprotein N, phosphoprotein P, and RNA-directed RNA polymerase L) required for viral transcription and replication into Nicotiana benthamiana. Insertion of intron 2 ST-LS1 into the viral L gene increased stability of the infectious clone in Escherichia coli and Agrobacterium tumefaciens. To monitor virus infection in vivo, a green fluorescent protein (GFP) gene was inserted in between the N and P gene junctions to generate recombinant MMV-GFP. Complementary DNA (cDNA) clones of MMV-wild type (WT) and MMV-GFP replicated in single cells of agroinfiltrated N. benthamiana. Uniform systemic infection and high GFP expression were observed in maize inoculated with extracts of the infiltrated N. benthamiana leaves. Insect vectors supported virus infection when inoculated via feeding on infected maize or microinjection. Both MMV-WT and MMV-GFP were efficiently transmitted to maize by planthopper vectors. The GFP reporter gene was stable in the virus genome and expression remained high over three cycles of transmission in plants and insects. The MMV infectious clone will be a versatile tool for expression of proteins of interest in maize and cross-kingdom studies of virus replication in plant and insect hosts.

Engineering aphid transmission of foxtail mosaic virus in the presence of potyvirus helper component proteinase through coat protein modifications

Biotechnologies that use plant viruses as plant enhancement tools have shown great potential to flexibly engineer crop traits, but field applications of these technologies are still limited by efficient dissemination methods. Potyviruses can be rapidly inoculated into plants by aphid vectors due to the presence of the potyviral helper component proteinase (HC-Pro), which binds to the DAG motif of the coat protein (CP) of the virion. Previously it was determined that a naturally occurring DAG motif in the non-aphid-transmissible potexvirus, potato aucuba mosaic virus (PAMV), is functional when a potyviral HC-Pro is provided to aphids in plants. The DAG motif of PAMV was successfully transferred to the CP of another non-aphid-transmissible potexvirus, potato virus X, to convey aphid transmission capabilities in the presence of HC-Pro. Here, we demonstrate that DAG-containing segments of the CP from two different potyviruses (sugarcane mosaic virus and turnip mosaic virus), and from the previously used potexvirus, PAMV, can make the potexvirus, foxtail mosaic virus (FoMV), aphid-transmissible when fused with the FoMV CP. We show that DAG-containing FoMVs are transmissible by aphids that have prior access to HC-Pro through potyvirus-infected plants or ectopic expression of HC-Pro. The transmission efficiency of the DAG-containing FoMVs varied from less than 10 % to over 70 % depending on the length and composition of the surrounding amino acid sequences of the DAG-containing segment, as well as due to the recipient plant species. Finally, we show that the engineered aphid-transmissible FoMV is still functional as a plant enhancement resource, as endogenous host target genes were silenced in FoMV-infected plants after aphid transmission. These results suggest that aphid transmission could be engineered into non-aphid-transmissible plant enhancement viral resources to facilitate their field applications.

The Potyviral Protein 6K2 from Turnip Mosaic Virus Increases Plant Resilience to Drought

Virus infection can increase drought tolerance of infected plants compared with noninfected plants; however, the mechanisms mediating virus-induced drought tolerance remain unclear. In this study, we demonstrate turnip mosaic virus (TuMV) infection increases Arabidopsis thaliana survival under drought compared with uninfected plants. To determine if specific TuMV proteins mediate drought tolerance, we cloned the coding sequence for each of the major viral proteins and generated transgenic A. thaliana that constitutively express each protein. Three TuMV proteins, 6K1, 6K2, and NIa-Pro, enhanced drought tolerance of A. thaliana when expressed constitutively in plants compared with controls. While in the control plant, transcripts related to abscisic acid (ABA) biosynthesis and ABA levels were induced under drought, there were no changes in ABA or related transcripts in plants expressing 6K2 under drought compared with well-watered conditions. Expression of 6K2 also conveyed drought tolerance in another host plant, Nicotiana benthamiana, when expressed using a virus overexpression construct. In contrast to ABA, 6K2 expression enhanced salicylic acid (SA) accumulation in both Arabidopsis and N. benthamiana. These results suggest 6K2-induced drought tolerance is mediated through increased SA levels and SA-dependent induction of plant secondary metabolites, osmolytes, and antioxidants that convey drought tolerance. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Harnessing plant viruses in the metagenomics era: from the development of infectious clones to applications

Recent metagenomic studies which focused on virus characterization in the entire plant environment have revealed a remarkable viral diversity in plants. The exponential discovery of viruses also requires the concomitant implementation of high-throughput methods to perform their functional characterization. Despite several limitations, the development of viral infectious clones remains a method of choice to understand virus biology, their role in the phytobiome, and plant resilience. Here, we review the latest approaches for efficient characterization of plant viruses and technical advances built on high-throughput sequencing and synthetic biology to streamline assembly of viral infectious clones. We then discuss the applications of plant viral vectors in fundamental and applied plant research as well as their technical and regulatory limitations, and we propose strategies for their safer field applications.

The function of the phytoplasma effector SWP12 depends on the properties of two key amino acids

Phytoplasmas are insect-borne bacterial pathogens capable of secreting effectors into host cells and interfering with host plant defense response processes. Previous studies have found that the Candidatus Phytoplasma tritici effector SWP12 binds to and destabilizes the wheat transcription factor TaWRKY74, increasing wheat susceptibility to phytoplasmas. Here, we used a Nicotiana benthamiana transient expression system to identify two key functional sites of SWP12 and screened a series of truncated mutants and amino acid substitution mutants to determine whether they inhibit Bax-induced cell death. Using a subcellular localization assay and online structure analysis websites, we found that structure rather than intracellular localization probably affects the function of SWP12. D33A and P85H are two inactive substitution mutants, neither of which interacts with TaWRKY74, and P85H does not inhibit Bax-induced cell death, suppress flg22-triggered reactive oxygen species (ROS) bursts, degrade TaWRKY74, or promote phytoplasma accumulation. D33A can weakly suppress Bax-induced cell death and flg22-triggered ROS bursts and degrade a portion of TaWRKY74 and weakly promote phytoplasma accumulation. S53L, CPP, and EPWB are three SWP12 homolog proteins from other phytoplasmas. Sequence analysis revealed that D33 was conserved in these proteins, and they exhibited the same polarity at P85. Transient expression in N. benthamiana showed that these proteins could inhibit Bax-induced cell death and suppress ROS bursts. Our findings clarified that P85 and D33 of SWP12 play critical and minor roles, respectively, in suppressing the plant defense response and that they play a preliminary role in determining the functions of homologous proteins.

Combined pangenomics and transcriptomics reveals core and redundant virulence processes in a rapidly evolving fungal plant pathogen

BACKGROUND

Studying genomic variation in rapidly evolving pathogens potentially enables identification of genes supporting their "core biology", being present, functional and expressed by all strains or "flexible biology", varying between strains. Genes supporting flexible biology may be considered to be "accessory", whilst the "core" gene set is likely to be important for common features of a pathogen species biology, including virulence on all host genotypes. The wheat-pathogenic fungus Zymoseptoria tritici represents one of the most rapidly evolving threats to global food security and was the focus of this study.

RESULTS

We constructed a pangenome of 18 European field isolates, with 12 also subjected to RNAseq transcription profiling during infection. Combining this data, we predicted a "core" gene set comprising 9807 sequences which were (1) present in all isolates, (2) lacking inactivating polymorphisms and (3) expressed by all isolates. A large accessory genome, consisting of 45% of the total genes, was also defined. We classified genetic and genomic polymorphism at both chromosomal and individual gene scales. Proteins required for essential functions including virulence had lower-than average sequence variability amongst core genes. Both core and accessory genomes encoded many small, secreted candidate effector proteins that likely interact with plant immunity. Viral vector-mediated transient in planta overexpression of 88 candidates failed to identify any which induced leaf necrosis characteristic of disease. However, functional complementation of a non-pathogenic deletion mutant lacking five core genes demonstrated that full virulence was restored by re-introduction of the single gene exhibiting least sequence polymorphism and highest expression.

CONCLUSIONS

These data support the combined use of pangenomics and transcriptomics for defining genes which represent core, and potentially exploitable, weaknesses in rapidly evolving pathogens.

Magnaporthe oryzae pathotype Triticum (MoT) can act as a heterologous expression system for fungal effectors with high transcript abundance in wheat

Plant pathogens deliver effector proteins to reprogramme a host plants circuitry, supporting their own growth and development, whilst thwarting defence responses. A subset of these effectors are termed avirulence factors (Avr) and can be recognised by corresponding host resistance (R) proteins, creating a strong evolutionary pressure on pathogen Avr effectors that favours their modification/deletion to evade the immune response. Hence, identifying Avr effectors and tracking their allele frequencies in a population is critical for understanding the loss of host recognition. However, the current systems available to confirm Avr effector function, particularly for obligate biotrophic fungi, remain limited and challenging. Here, we explored the utility of the genetically tractable wheat blast pathogen Magnaporthe oryzae pathotype Triticum (MoT) as a suitable heterologous expression system in wheat. Using the recently confirmed wheat stem rust pathogen (Puccina graminis f. sp. tritici) avirulence effector AvrSr50 as a proof-of-concept, we found that delivery of AvrSr50 via MoT could elicit a visible Sr50-dependant cell death phenotype. However, activation of Sr50-mediated cell death correlated with a high transgene copy number and transcript abundance in MoT transformants. This illustrates that MoT can act as an effective heterologous delivery system for fungal effectors from distantly related fungal species, but only when enough transgene copies and/or transcript abundance is achieved.

Characterization of a foxtail mosaic virus vector for gene silencing and analysis of innate immune responses in Sorghum bicolor

Sorghum is vulnerable to many biotic and abiotic stresses, which cause considerable yield losses globally. Efforts to genetically characterize beneficial sorghum traits, including disease resistance, plant architecture, and tolerance to abiotic stresses, are ongoing. One challenge faced by sorghum researchers is its recalcitrance to transformation, which has slowed gene validation efforts and utilization for cultivar development. Here, we characterize the use of a foxtail mosaic virus (FoMV) vector for virus-induced gene silencing (VIGS) by targeting two previously tested marker genes: phytoene desaturase (PDS) and ubiquitin (Ub). We additionally demonstrate VIGS of a subgroup of receptor-like cytoplasmic kinases (RLCKs) and report the role of these genes as positive regulators of early defence signalling. Silencing of subgroup 8 RLCKs also resulted in higher susceptibility to the bacterial pathogens Pseudomonas syringae pv. syringae (B728a) and Xanthomonas vasicola pv. holcicola, demonstrating the role of these genes in host defence against bacterial pathogens. Together, this work highlights the utility of FoMV-induced gene silencing in the characterization of genes mediating defence responses in sorghum. Moreover, FoMV was able to systemically infect six diverse sorghum genotypes with high efficiency at optimal temperatures for sorghum growth and therefore could be extrapolated to study additional traits of economic importance.

Caveats of Using Bacterial Type Three Secretion Assays for Validating Fungal Avirulence Effectors in Wheat

Functional characterization of effector proteins of fungal obligate biotrophic pathogens, especially confirmation of avirulence (Avr) properties, has been notoriously difficult, due to the experimental intractability of many of these organisms. Previous studies in wheat have shown promising data suggesting the type III secretion system (T3SS) of bacteria may be a suitable surrogate for delivery and detection of Avr properties of fungal effectors. However, these delivery systems were tested in the absence of confirmed Avr effectors. Here, we tested two previously described T3SS-mediated delivery systems for their suitability when delivering two confirmed Avr effectors from two fungal pathogens of wheat, Puccinia graminis f. sp. tritici and Magnaporthe oryzae pathotype tritici. We showed that both effectors (AvrSr50 and AvrRmg8) were unable to elicit a hypersensitive response on wheat seedlings with the corresponding resistance gene when expressed by the Pseudomonas fluorescens "Effector to Host Analyser" (EtHAn) system. Furthermore, we found the utility of Burkholderia glumae for screening Avr phenotypes is severely limited, as the wild-type strain elicits nonhost cell death in multiple wheat accessions. These results provide valuable insight into the suitability of these systems for screening fungal effectors for Avr properties that may help guide further development of surrogate bacterial delivery systems in wheat. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The 'Candidatus Phytoplasma tritici' effector SWP12 degrades the transcription factor TaWRKY74 to suppress wheat resistance

'Candidatus Phytoplasma tritici' ('Ca. P. tritici') is an insect-borne obligate pathogen that infects wheat (Triticum aestivum) causing wheat blue dwarf disease, and leads to yield losses. SWP12 is a potential effector secreted by 'Ca. P. tritici' that manipulates host processes to create an environment conducive to phytoplasma colonization, but the detailed mechanism of action remains to be investigated. In this study, the expression of SWP12 weakened the basal immunity of Nicotiana benthamiana and promoted leaf colonization by Phytophthora parasitica, Sclerotinia sclerotiorum, and tobacco mild green mosaic virus. Moreover, the expression of SWP12 in wheat plants promoted phytoplasma colonization. Triticum aestivum WRKY74 and N. benthamiana WRKY17 were identified as host targets of SWP12. The expression of TaWRKY74 triggered reactive oxygen species bursts, upregulated defense-related genes, and decreased TaCRR6 transcription, leading to reductions in NADH dehydrogenase complex (NDH) activity. Expression of TaWRKY74 in wheat increased plant resistance to 'Ca. P. tritici', and silencing of TaWRKY74 enhanced plant susceptibility, which indicates that TaWRKY74 is a positive regulator of wheat resistance to 'Ca. P. tritici'. We showed that SWP12 weakens plant resistance and promotes 'Ca. P. tritici' colonization by destabilizing TaWRKY74.

Development of tobacco rattle virus-based platform for dual heterologous gene expression and CRISPR/Cas reagent delivery

A large number of viral delivery systems have been developed for characterizing functional genes and producing heterologous recombinant proteins in plants, and but most of them are unable to co-express two fusion-free foreign proteins in the whole plant for extended periods of time. In this study, we modified tobacco rattle virus (TRV) as a TRVe dual delivery vector, using the strategy of gene substitution. The reconstructed TRVe had the capability to simultaneously produce two fusion-free foreign proteins at the whole level of Nicotiana benthamiana, and maintained the genetic stability for the insert of double foreign genes. Moreover, TRVe allowed systemic expression of two foreign proteins with the total lengths up to ∼900 aa residues. In addition, Cas12a protein and crRNA were delivered by the TRVe expression system for site-directed editing of genomic DNA in N. benthamiana 16c line constitutively expressing green fluorescent protein (GFP). Taker together, the TRV-based delivery system will be a simple and powerful means to rapidly co-express two non-fused foreign proteins at the whole level and facilitate functional genomics studies in plants.

Puccinia striiformis f. sp. tritici effectors in wheat immune responses

The obligate biotrophic fungus Puccinia striiformis f. sp. tritici, which causes yellow (stripe) rust disease, is among the leading biological agents resulting in tremendous yield losses on global wheat productions per annum. The combatting strategies include, but are not limited to, fungicide applications and the development of resistant cultivars. However, evolutionary pressure drives rapid changes, especially in its "effectorome" repertoire, thus allowing pathogens to evade and breach resistance. The extracellular and intracellular effectors, predominantly secreted proteins, are tactical arsenals aiming for many defense processes of plants. Hence, the identity of the effectors and the molecular mechanisms of the interactions between the effectors and the plant immune system have long been targeted in research. The obligate biotrophic nature of P. striiformis f. sp. tritici and the challenging nature of its host, the wheat, impede research on this topic. Next-generation sequencing and novel prediction algorithms in bioinformatics, which are accompanied by in vitro and in vivo validation approaches, offer a speedy pace for the discovery of new effectors and investigations of their biological functions. Here, we briefly review recent findings exploring the roles of P. striiformis f. sp. tritici effectors together with their cellular/subcellular localizations, host responses, and interactors. The current status and the challenges will be discussed. We hope that the overall work will provide a broader view of where we stand and a reference point to compare and evaluate new findings.

Insect allies-Assessment of a viral approach to plant genome editing

The Insect Allies program of the Defense Advanced Research Projects Agency has already sparked scientific debate concerning technology assessment-related issues, among which the most prevalent is that of dual use. Apart from the issues concerning peaceful applications, the technology also provides the blueprint for a potential bioweapon. However, the combination of a virus-induced genetic modification of crop plants in the field using genetically modified insect vectors poses a greater risk than the hitherto existing use of genetically modified organisms. The technology's great depth of intervention allows a number of sources for hazard and a tendency towards high exposure, but it is also encumbered with notable deficits in knowledge. These issues call for a thorough technology assessment. This article aims to provide an initial characterization from a technology assessment perspective, focusing on potential sources of risk for this novel invasive environmental biotechnology at an early stage of research and development. Integr Environ Assess Manag 2022;18:1488-1499. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Multi-omics intervention in Setaria to dissect climate-resilient traits: Progress and prospects

Millets constitute a significant proportion of underutilized grasses and are well known for their climate resilience as well as excellent nutritional profiles. Among millets, foxtail millet (Setaria italica) and its wild relative green foxtail (S. viridis) are collectively regarded as models for studying broad-spectrum traits, including abiotic stress tolerance, C₄ photosynthesis, biofuel, and nutritional traits. Since the genome sequence release, the crop has seen an exponential increase in omics studies to dissect agronomic, nutritional, biofuel, and climate-resilience traits. These studies have provided first-hand information on the structure, organization, evolution, and expression of several genes; however, knowledge of the precise roles of such genes and their products remains elusive. Several open-access databases have also been instituted to enable advanced scientific research on these important crops. In this context, the current review enumerates the contemporary trend of research on understanding the climate resilience and other essential traits in Setaria, the knowledge gap, and how the information could be translated for the crop improvement of related millets, biofuel crops, and cereals. Also, the review provides a roadmap for studying other underutilized crop species using Setaria as a model.

Effector-mediated relocalization of a maize lipoxygenase protein triggers susceptibility to Ustilago maydis

As the gall-inducing smut fungus Ustilago maydis colonizes maize (Zea mays) plants, it secretes a complex effector blend that suppresses host defense responses, including production of reactive oxygen species (ROS) and redirects host metabolism to facilitate colonization. We show that the U. maydis effector ROS burst interfering protein 1 (Rip1), which is involved in pathogen-associated molecular pattern (PAMP)-triggered suppression of host immunity, is functionally conserved in several other monocot-infecting smut fungi. We also have identified a conserved C-terminal motif essential for Rip1-mediated PAMP-triggered suppression of the ROS burst. The maize susceptibility factor lipoxygenase 3 (Zmlox3) bound by Rip1 was relocalized to the nucleus, leading to partial suppression of the ROS burst. Relocalization was independent of its enzymatic activity, revealing a distinct function for ZmLox3. Most importantly, whereas Zmlox3 maize mutant plants showed increased resistance to U. maydis wild-type strains, rip1 deletion strains infecting the Zmlox3 mutant overcame this effect. This could indicate that Rip1-triggered host resistance depends on ZmLox3 to be suppressed and that lox3 mutation-based resistance of maize to U. maydis requires functional Rip1. Together, our results reveal that Rip1 acts in several cellular compartments to suppress immunity and that targeting of ZmLox3 by Rip1 is responsible for the suppression of Rip1-dependent reduced susceptibility of maize to U. maydis.

Impacts of RNA Mobility Signals on Virus Induced Somatic and Germline Gene Editing

Viral vectors are being engineered to deliver CRISPR/Cas9 components systemically in plants to induce somatic or heritable site-specific mutations. It is hypothesized that RNA mobility signals facilitate entry of viruses or single guide RNAs (sgRNAs) into the shoot apical meristem where germline mutations can occur. Our objective was to understand the impact of RNA mobility signals on virus-induced somatic and germline gene editing in Nicotiana benthamiana and Zea mays. Previously, we showed that foxtail mosaic virus (FoMV) expressing sgRNA induced somatic mutations in N. benthamiana and Z. mays expressing Cas9. Here, we fused RNA mobility signals to sgRNAs targeting the genes encoding either N. benthamiana phytoene desaturase (PDS) or Z. mays high affinity potassium transporter 1 (HKT1). Addition of Arabidopsis thaliana Flowering Locus T (AtFT) and A. thaliana tRNA-Isoleucine (AttRNAIle) did not improve FoMV-induced somatic editing, and neither were sufficient to facilitate germline mutations in N. benthamiana. Maize FT homologs, Centroradialus 16 (ZCN16) and ZCN19, as well as AttRNAIle were found to aid somatic editing in maize but did not enable sgRNAs delivered by FoMV to induce germline mutations. Additional viral guide RNA delivery systems were assessed for somatic and germline mutations in N. benthamiana with the intention of gaining a better understanding of the specificity of mobile signal-facilitated germline editing. Potato virus X (PVX), barley stripe mosaic virus (BSMV), and tobacco rattle virus (TRV) were included in this comparative study, and all three of these viruses delivering sgRNA were able to induce somatic and germline mutations. Unexpectedly, PVX, a potexvirus closely related to FoMV, expressing sgRNA alone induced biallelic edited progeny, indicating that mobility signals are dispensable in virus-induced germline editing. These results show that PVX, BSMV, and TRV expressing sgRNA all have an innate ability to induce mutations in the germline. Our results indicate that mobility signals alone may not be sufficient to enable virus-based delivery of sgRNAs using the viruses, FoMV, PVX, BSMV, and TRV into cell types that result in germline mutations.

A maize triacylglycerol lipase inhibits sugarcane mosaic virus infection

Triacylglycerol lipase (TGL) plays critical roles in providing energy for seed germination and plant development. However, the role of TGL in regulating plant virus infection is largely unknown. In this study, we adopted affinity purification coupled with mass spectrometry and identified that a maize (Zea mays) pathogenesis-related lipase protein Z. mays TGL (ZmTGL) interacted with helper component-proteinase (HC-Pro) of sugarcane mosaic virus (SCMV). Yeast two-hybrid, luciferase complementation imaging, and bimolecular fluorescence complementation assays confirmed that ZmTGL directly interacted with SCMV HC-Pro in vitro and in vivo. The 101-460 residues of SCMV HC-Pro were important for its interaction with ZmTGL. ZmTGL and SCMV HC-Pro co-localized at the mitochondria. Silencing of ZmTGL facilitated SCMV infection, and over-expression of ZmTGL reduced the RNA silencing suppression activity, most likely through reducing HC-Pro accumulation. Our results provided evidence that the lipase hydrolase activity of ZmTGL was associated with reducing HC-Pro accumulation, activation of salicylic acid (SA)-mediated defense response, and inhibition of SCMV infection. We show that ZmTGL inhibits SCMV infection by reducing HC-Pro accumulation and activating the SA pathway.

TOPLESS promotes plant immunity by repressing auxin signaling and is targeted by the fungal effector Naked1

In plants, the antagonism between growth and defense is hardwired by hormonal signaling. The perception of pathogen-associated molecular patterns (PAMPs) from invading microorganisms inhibits auxin signaling and plant growth. Conversely, pathogens manipulate auxin signaling to promote disease, but how this hormone inhibits immunity is not fully understood. Ustilago maydis is a maize pathogen that induces auxin signaling in its host. We characterized a U. maydis effector protein, Naked1 (Nkd1), that is translocated into the host nucleus. Through its native ethylene-responsive element binding factor-associated amphiphilic repression (EAR) motif, Nkd1 binds to the transcriptional co-repressors TOPLESS/TOPLESS-related (TPL/TPRs) and prevents the recruitment of a transcriptional repressor involved in hormonal signaling, leading to the de-repression of auxin and jasmonate signaling and thereby promoting susceptibility to (hemi)biotrophic pathogens. A moderate upregulation of auxin signaling inhibits the PAMP-triggered reactive oxygen species (ROS) burst, an early defense response. Thus, our findings establish a clear mechanism for auxin-induced pathogen susceptibility. Engineered Nkd1 variants with increased expression or increased EAR-mediated TPL/TPR binding trigger typical salicylic-acid-mediated defense reactions, leading to pathogen resistance. This implies that moderate binding of Nkd1 to TPL is a result of a balancing evolutionary selection process to enable TPL manipulation while avoiding host recognition.

WAKsing plant immunity, waning diseases

With the requirement to breed more productive crop plants in order to feed a growing global population, compounded by increasingly widespread resistance to pesticides exhibited by pathogens, plant immunity is becoming an increasingly important area of research. Of the genes that contribute to disease resistance, the wall-associated receptor-like kinases (WAKs) are increasingly shown to play a major role, in addition to their contribution to plant growth and development or tolerance to abiotic stresses. Being transmembrane proteins, WAKs form a central pillar of a plant cell's ability to monitor and interact with the extracellular environment. Found in both dicots and monocots, WAKs have been implicated in defence against pathogens with diverse lifestyles and contribute to plant immunity in a variety of ways. Whilst some act as cell surface-localized immune receptors recognizing either pathogen- or plant-derived invasion molecules (e.g. effectors or damage-associated molecular patterns, respectively), others promote innate immunity through cell wall modification and strengthening, thus limiting pathogen intrusion. The ability of some WAKs to provide both durable resistance against pathogens and other agronomic benefits makes this gene family important targets in the development of future crop ideotypes and important to a greater understanding of the complexity and robustness of plant immunity.

Virus-Mediated Protein Overexpression (VOX) in Monocots to Identify and Functionally Characterize Fungal Effectors

One of the important armories that pathogens utilize to successfully colonize the plants is small secreted effector proteins, which could perform a variety of functions from suppression of plant innate immunity to manipulation of plant physiology in favor of the disease. Plants, on the other hand, evolved disease resistance genes that recognize some of the effectors or avirulence (Avr) proteins. Both, identification of the Avr proteins and understanding of the mechanisms of action of other effectors, are important areas of research in the molecular plant-pathogen interactions field as this knowledge is critical for the development of new effective pathogen control measures. To enable functional analysis of the effectors, it is desirable to be able to overexpress them readily in the host plants. Here we describe detailed experimental protocols for transient effector overexpression in wheat and other monocots using binary Barley stripe mosaic virus (BSMV)- and Foxtail mosaic virus (FoMV)-derived vectors. This functional genomics tool, better known as VOX (Virus-mediated protein OvereXpression), is rapid and relatively simple and inexpensive.

A Versatile Expression Platform in Insects and Cereals Based on a Cytorhabdovirus

In recent years, plant virus-based vectors have been widely applied to express heterologous proteins for genomic studies and commercial production. Among these versatile RNA viral vectors, the barley yellow striate mosaic virus (BYSMV)-based expression vector system has outstanding capability to express large and multiple heterologous proteins. Here we describe a detailed protocol for expression of heterologous proteins using BYSMV expression systems in monocot plants and insects.

Genetics and Genomics Interventions for Promoting Millets as Functional Foods

Several crops, including millets with immense nutritional and therapeutic values, were once a part of our regular diet. However, due to domestication and selection pressures, many of them have become marginally cultivated crops confined to a particular region, race, or locality. Millets are a perfect example of neglected species that have the potential to address both food and nutritional insecurities prevalent among the ever-growing global population. Starvation and malnutrition contribute to a large number of health-related issues, being the main reason behind the occurrence of most of the severe diseases worldwide. These constraints are repeatedly disturbing both the social and economic health of global society. Naturally, millets are rich in minerals, nutrients, and bioactive compounds, and these crops are less dependent on synthetic fertilizers, systemic irrigation, and pest/weed control. Given this, the review emphasizes the nutritional values, health benefits, processing techniques, and genomic advancements of millets. In addition, it proposes a roadmap for enhancing the utility and commercialization of millets.

Transient protein expression systems in plants and their applications

The production of recombinant proteins is important in academic research to identify protein functions. Moreover, recombinant enzymes are used in the food and chemical industries, and high-quality proteins are required for diagnostic, therapeutic, and pharmaceutical applications. Though many recombinant proteins are produced by microbial or mammalian cell-based expression systems, plants have been promoted as alternative, cost-effective, scalable, safe, and sustainable expression systems. The development and improvement of transient expression systems have significantly reduced the period of protein production and increased the yield of recombinant proteins in plants. In this review, we consider the importance of plant-based expression systems for recombinant protein production and as genetic engineering tools.

Application of Plant Viruses in Biotechnology, Medicine, and Human Health

Plant-based nanotechnology programs using virus-like particles (VLPs) and virus nanoparticles (VNPs) are emerging platforms that are increasingly used for a variety of applications in biotechnology and medicine. Tobacco mosaic virus (TMV) and potato virus X (PVX), by virtue of having high aspect ratios, make ideal platforms for drug delivery. TMV and PVX both possess rod-shaped structures and single-stranded RNA genomes encapsidated by their respective capsid proteins and have shown great promise as drug delivery systems. Cowpea mosaic virus (CPMV) has an icosahedral structure, and thus brings unique benefits as a nanoparticle. The uses of these three plant viruses as either nanostructures or expression vectors for high value pharmaceutical proteins such as vaccines and antibodies are discussed extensively in the following review. In addition, the potential uses of geminiviruses in medical biotechnology are explored. The uses of these expression vectors in plant biotechnology applications are also discussed. Finally, in this review, we project future prospects for plant viruses in the fields of medicine, human health, prophylaxis, and therapy of human diseases.

A cassava common mosaic virus vector for virus-induced gene silencing in cassava

BACKGROUND

Cassava is an important crop for food security and industry in the least-developed and developing countries. The completion of the cassava genome sequence and identification of large numbers of candidate genes by next-generation sequencing provide extensive resources for cassava molecular breeding and increase the need for rapid and efficient gene function analysis systems in cassava. Several plant virus-induced gene silencing (VIGS) systems have been developed as reverse genetic tools for rapid gene function analysis in cassava. However, these VIGS vectors could cause severe viral symptoms or inefficient gene silencing.

RESULTS

In this study, we constructed agroinfection-compatible infectious cDNA clones of cassava common mosaic virus isolate CM (CsCMV-CM, genus Potexvirus, family Alphaflexiviridae) that causes systemic infection with mild symptoms in cassava. CsCMV-CM was then modified to a viral vector carrying the Nimble cloning frame, which facilitates the rapid and high-throughput cloning of silencing fragments into the viral genome. The CsCMV-based vector successfully silenced phytoene desaturase (PDS) and magnesium chelatase subunit I (ChlI) in different cassava varieties and Nicotiana benthamiana. The silencing of the ChlI gene could persist for more than two months.

CONCLUSIONS

This CsCMV-based VIGS system provides a new tool for rapid and efficient gene function studies in cassava.

Creation and judicious application of a wheat resistance gene atlas

Disease-resistance (R) gene cloning in wheat (Triticum aestivum) has been accelerated by the recent surge of genomic resources, facilitated by advances in sequencing technologies and bioinformatics. However, with the challenges of population growth and climate change, it is vital not only to clone and functionally characterize a few handfuls of R genes, but also to do so at a scale that would facilitate the breeding and deployment of crops that can recognize the wide range of pathogen effectors that threaten agroecosystems. Pathogen populations are continually changing, and breeders must have tools and resources available to rapidly respond to those changes if we are to safeguard our daily bread. To meet this challenge, we propose the creation of a wheat R-gene atlas by an international community of researchers and breeders. The atlas would consist of an online directory from which sources of resistance could be identified and deployed to achieve more durable resistance to the major wheat pathogens, such as wheat rusts, blotch diseases, powdery mildew, and wheat blast. We present a costed proposal detailing how the interacting molecular components governing disease resistance could be captured from both the host and the pathogen through biparental mapping, mutational genomics, and whole-genome association genetics. We explore options for the configuration and genotyping of diversity panels of hexaploid and tetraploid wheat, as well as their wild relatives and major pathogens, and discuss how the atlas could inform a dynamic, durable approach to R-gene deployment. Set against the current magnitude of wheat yield losses worldwide, recently estimated at 21%, this endeavor presents one route for bringing R genes from the lab to the field at a considerable speed and quantity.

The Pleiades are a cluster of fungal effectors that inhibit host defenses

Biotrophic plant pathogens secrete effector proteins to manipulate the host physiology. Effectors suppress defenses and induce an environment favorable to disease development. Sequence-based prediction of effector function is impeded by their rapid evolution rate. In the maize pathogen Ustilago maydis, effector-coding genes frequently organize in clusters. Here we describe the functional characterization of the pleiades, a cluster of ten effector genes, by analyzing the micro- and macroscopic phenotype of the cluster deletion and expressing these proteins in planta. Deletion of the pleiades leads to strongly impaired virulence and accumulation of reactive oxygen species (ROS) in infected tissue. Eight of the Pleiades suppress the production of ROS upon perception of pathogen associated molecular patterns (PAMPs). Although functionally redundant, the Pleiades target different host components. The paralogs Taygeta1 and Merope1 suppress ROS production in either the cytoplasm or nucleus, respectively. Merope1 targets and promotes the auto-ubiquitination activity of RFI2, a conserved family of E3 ligases that regulates the production of PAMP-triggered ROS burst in plants.

Advantage of Nanotechnology-Based Genome Editing System and Its Application in Crop Improvement

Agriculture is an important source of human food. However, current agricultural practices need modernizing and strengthening to fulfill the increasing food requirements of the growing worldwide population. Genome editing (GE) technology has been used to produce plants with improved yields and nutritional value as well as with higher resilience to herbicides, insects, and diseases. Several GE tools have been developed recently, including clustered regularly interspaced short palindromic repeats (CRISPR) with nucleases, a customizable and successful method. The main steps of the GE process involve introducing transgenes or CRISPR into plants via specific gene delivery systems. However, GE tools have certain limitations, including time-consuming and complicated protocols, potential tissue damage, DNA incorporation in the host genome, and low transformation efficiency. To overcome these issues, nanotechnology has emerged as a groundbreaking and modern technique. Nanoparticle-mediated gene delivery is superior to conventional biomolecular approaches because it enhances the transformation efficiency for both temporal (transient) and permanent (stable) genetic modifications in various plant species. However, with the discoveries of various advanced technologies, certain challenges in developing a short-term breeding strategy in plants remain. Thus, in this review, nanobased delivery systems and plant genetic engineering challenges are discussed in detail. Moreover, we have suggested an effective method to hasten crop improvement programs by combining current technologies, such as speed breeding and CRISPR/Cas, with nanotechnology. The overall aim of this review is to provide a detailed overview of nanotechnology-based CRISPR techniques for plant transformation and suggest applications for possible crop enhancement.

Simultaneous gene expression and multi-gene silencing in Zea mays using maize dwarf mosaic virus

BACKGROUND

Maize dwarf mosaic virus (MDMV), a member of the genus Potyvirus, infects maize and is non-persistently transmitted by aphids. Several plant viruses have been developed as tools for gene expression and gene silencing in plants. The capacity of MDMV for both gene expression and gene silencing were examined.

RESULTS

Infectious clones of an Ohio isolate of MDMV, MDMV OH5, were obtained, and engineered for gene expression only, and for simultaneous marker gene expression and virus-induced gene silencing (VIGS) of three endogenous maize target genes. Single gene expression in single insertion constructs and simultaneous expression of green fluorescent protein (GFP) and silencing of three maize genes in a double insertion construct was demonstrated. Constructs with GFP inserted in the N-terminus of HCPro were more stable than those with insertion at the N-terminus of CP in our study. Unexpectedly, the construct with two insertion sites also retained insertions at a higher rate than single-insertion constructs. Engineered MDMV expression and VIGS constructs were transmissible by aphids (Rhopalosiphum padi).

CONCLUSIONS

These results demonstrate that MDMV-based vector can be used as a tool for simultaneous gene expression and multi-gene silencing in maize.

A novel pathogenicity determinant hijacks maize catalase 1 to enhance viral multiplication and infection

Pathogens have evolved various strategies to overcome host immunity for successful infection. Maize chlorotic mottle virus (MCMV) can cause lethal necrosis in maize (Zea mays) when it coinfects with a virus in the Potyviridae family. However, the MCMV pathogenicity determinant remains largely unknown. Here we show that the P31 protein of MCMV is important for viral accumulation and essential for symptom development. Ectopic expression of P31 using foxtail mosaic virus or potato virus X induced necrosis in systemically infected maize or Nicotiana benthamiana leaves. Maize catalases (CATs) were shown to interact with P31 in yeast and in planta. P31 accumulation was elevated through its interaction with ZmCAT1. P31 attenuated the expression of salicylic acid (SA)-responsive pathogenesis-related (PR) genes by inhibiting catalase activity during MCMV infection. In addition, silencing of ZmCATs using a brome mosaic virus-based gene silencing vector facilitated MCMV RNA and coat protein accumulation. This study reveals an important role for MCMV P31 in counteracting host defence and inducing systemic chlorosis and necrosis. Our results have implications for understanding the mechanisms in defence and counter-defence during infection of plants by various pathogens.

Ustilago maydis effector Jsi1 interacts with Topless corepressor, hijacking plant jasmonate/ethylene signaling

Ustilago maydis is the causal agent of maize smut disease. During the colonization process, the fungus secretes effector proteins that suppress immune responses and redirect the host metabolism in favor of the pathogen. As effectors play a critical role during plant colonization, their identification and functional characterization are essential to understanding biotrophy and disease. Using biochemical, molecular, and transcriptomic techniques, we performed a functional characterization of the U. maydis effector Jasmonate/Ethylene signaling inducer 1 (Jsi1). Jsi1 interacts with several members of the plant corepressor family Topless/Topless related (TPL/TPR). Jsi1 expression in Zea mays and Arabidopsis thaliana leads to transcriptional induction of the ethylene response factor (ERF) branch of the jasmonate/ethylene (JA/ET) signaling pathway. In A. thaliana, activation of the ERF branch leads to biotrophic susceptibility. Jsi1 likely activates the ERF branch via an EAR (ET-responsive element binding-factor-associated amphiphilic repression) motif, which resembles EAR motifs from plant ERF transcription factors, that interacts with TPL/TPR proteins. EAR-motif-containing effector candidates were identified from different fungal species, including Magnaporthe oryzae, Sporisorium scitamineum, and Sporisorium reilianum. Interaction between plant TPL proteins and these effector candidates from biotrophic and hemibiotrophic fungi indicates the convergent evolution of effectors modulating the TPL/TPR corepressor hub.

Oligonucleotide abundance biases aid design of a type IIS synthetic genomics framework with plant virome capacity

Synthetic genomics-driven dematerialization of genetic resources facilitates flexible hypothesis testing and rapid product development. Biological sequences have compositional biases, which, I reasoned, could be exploited for engineering of enhanced synthetic genomics systems. In proof-of-concept assays reported herein, the abundance of random oligonucleotides in viral genomic components was analyzed and used for the rational design of a synthetic genomics framework with plant virome capacity (SynViP). Type IIS endonucleases with low abundance in the plant virome, as well as Golden Gate and No See'm principles were combined with DNA chemical synthesis for seamless viral clone assembly by one-step digestion-ligation. The framework described does not require subcloning steps, is insensitive to insert terminal sequences, and was used with linear and circular DNA molecules. Based on a digital template, DNA fragments were chemically synthesized and assembled by one-step cloning to yield a scar-free infectious clone of a plant virus suitable for Agrobacterium-mediated delivery. SynViP allowed rescue of a genuine virus without biological material, and has the potential to greatly accelerate biological characterization and engineering of plant viruses as well as derived biotechnological tools. Finally, computational identification of compositional biases in biological sequences might become a common standard to aid scalable biosystems design and engineering.

Construction and biological characterization of an infectious full-length cDNA clone of a Chinese isolate of Wheat yellow mosaic virus

Wheat yellow mosaic virus (family Potyviridae; genus Bymovirus), is an important soil-borne virus that causes serious economic losses in wheat. In this study, we constructed infectious cDNA clones of WYMV genomic RNAs under the control of 35S or SP6 promoter for versatile usage (agroinfiltration or in vitro RNA transcription). Our results showed that an Agrobacterium-mediated inoculation system enabled WYMV to infect the leaves of Nicotiana benthamiana without causing WYMV systemic infection. However, in vitro transcripts from infectious cDNA clones using the SP6 promoter promoted WYMV systemic infection of wheat plants, which was then developed for further assays. The optimal temperature for virus multiplication and systemic infection of wheat was 8 °C. Additionally, a synergistic effect between WYMV and Chinese wheat mosaic virus (CWMV) was also detected. This is the first report of the construction of a Chinese isolate of WYMV and should facilitate the investigation of viral pathogenesis.

Transient reprogramming of crop plants for agronomic performance

The development of a new crop variety is a time-consuming and costly process due to the reliance of plant breeding on gene shuffling to introduce desired genes into elite germplasm, followed by backcrossing. Here, we propose alternative technology that transiently targets various regulatory circuits within a plant, leading to operator-specified alterations of agronomic traits, such as time of flowering, vernalization requirement, plant height or drought tolerance. We redesigned techniques of gene delivery, amplification and expression around RNA viral transfection methods that can be implemented on an industrial scale and with many crop plants. The process does not involve genetic modification of the plant genome and is thus limited to a single plant generation, is broadly applicable, fast, tunable and versatile, and can be used throughout much of the crop cultivation cycle. The RNA-based reprogramming may be especially useful in plant pathogen pandemics but also for commercial seed production and for rapid adaptation of orphan crops.

Development and optimization of a pepino mosaic virus-based vector for rapid expression of heterologous proteins in plants

Plant-virus-derived vectors are versatile tools with multiple applications in agricultural and medical biotechnology. In this study, we developed pepino mosaic virus (PepMV) (family Alphaflexiviridae; genus Potexvirus) into a vector for heterologous protein expression in plants. PepMV was initially cloned in a step-wise manner, fully sequenced and the full-length infectious clone was tested for infectivity in Nicotiana benthamiana. Initial infectious clones resulted in poor replication of PepMV and lack of systemic movement. Mutations in the viral sequence affected systemic infection. Two suspected mutations were altered to restore systemic infectivity. PepMV infection was apparent as early as 4 days post agroinfiltration (dpa) inoculation in N. benthamiana. A multiple cloning site was inserted into the PepMV genome for introduction and expression of foreign genes. Several modifications to the wild-type vector were made, such as a replacing the native subgenomic promoter (SGP) with a heterologous SGP, and introduction of translational enhancers and terminators, to improve heterologous expression of the foreign gene-of-interest. GFP was used as a reporter for monitoring virus infection and protein production. Strong GFP expression was observed as early as 4 dpa with a translational enhancer. The PepMV-based vector produces rapid expression of the foreign gene in comparison to two other potexvirus-based vectors. GFP production was monitored over time and optimal protein production was recorded between 5 and 7 dpa. GFP protein levels reached up to 4% and decreased to 0.5% total soluble protein at 7 and 14 dpa, respectively. Future studies will evaluate this virus-based vector for large-scale production of pharmaceutical compounds. KEY POINTS: • A pepino mosaic virus isolate was developed into a plant-based expression vector. • Expression levels of the heterologous protein were comparable or exceeded previously developed viral vectors. • Protein levels in plants were highest between 5 and 7 days and decreased gradually.

Plant-Based Vaccines: The Way Ahead?

Severe virus outbreaks are occurring more often and spreading faster and further than ever. Preparedness plans based on lessons learned from past epidemics can guide behavioral and pharmacological interventions to contain and treat emergent diseases. Although conventional biologics production systems can meet the pharmaceutical needs of a community at homeostasis, the COVID-19 pandemic has created an abrupt rise in demand for vaccines and therapeutics that highlight the gaps in this supply chain's ability to quickly develop and produce biologics in emergency situations given a short lead time. Considering the projected requirements for COVID-19 vaccines and the necessity for expedited large scale manufacture the capabilities of current biologics production systems should be surveyed to determine their applicability to pandemic preparedness. Plant-based biologics production systems have progressed to a state of commercial viability in the past 30 years with the capacity for production of complex, glycosylated, "mammalian compatible" molecules in a system with comparatively low production costs, high scalability, and production flexibility. Continued research drives the expansion of plant virus-based tools for harnessing the full production capacity from the plant biomass in transient systems. Here, we present an overview of vaccine production systems with a focus on plant-based production systems and their potential role as "first responders" in emergency pandemic situations.

Proteinaceous effector discovery and characterization in filamentous plant pathogens

The complicated interplay of plant-pathogen interactions occurs on multiple levels as pathogens evolve to constantly evade the immune responses of their hosts. Many economically important crops fall victim to filamentous pathogens that produce small proteins called effectors to manipulate the host and aid infection/colonization. Understanding the effector repertoires of pathogens is facilitating an increased understanding of the molecular mechanisms underlying virulence as well as guiding the development of disease control strategies. The purpose of this review is to give a chronological perspective on the evolution of the methodologies used in effector discovery from physical isolation and in silico predictions, to functional characterization of the effectors of filamentous plant pathogens and identification of their host targets.

UhAVR1, an HR-Triggering Avirulence Effector of Ustilago hordei, Is Secreted via the ER-Golgi Pathway, Localizes to the Cytosol of Barley Cells during in Planta-Expression, and Contributes to Virulence Early in Infection

The basidiomycete Ustilago hordei causes covered smut disease of barley and oats. Virulence effectors promoting infection and supporting pathogen lifestyle have been described for this fungus. Genetically, six avirulence genes are known and one codes for UhAVR1, the only proven avirulence effector identified in smuts to date that triggers complete immunity in barley cultivars carrying resistance gene Ruh1. A prerequisite for resistance breeding is understanding the host targets and molecular function of UhAVR1. Analysis of this effector upon natural infection of barley coleoptiles using teliospores showed that UhAVR1 is expressed during the early stages of fungal infection where it leads to HR triggering in resistant cultivars or performs its virulence function in susceptible cultivars. Fungal secretion of UhAVR1 is directed by its signal peptide and occurs via the BrefeldinA-sensitive ER-Golgi pathway in cell culture away from its host. Transient in planta expression of UhAVR1 in barley and a nonhost, Nicotiana benthamiana, supports a cytosolic localization. Delivery of UhAVR1 via foxtail mosaic virus or Pseudomonas species in both barley and N. benthamiana reveals a role in suppressing components common to both plant systems of Effector- and Pattern-Triggered Immunity, including necrosis triggered by Agrobacterium-delivered cell death inducers.

Brachypodium and plant viruses: entwined tools for discovery

In just a decade, Brachypodium distachyon (Brachypodium) has fulfilled its initial promise as a key tool for realizing new strategies for understanding host and pathogen biology during virus infections of the Poaceae. For this Tansley Insight, I have identified four areas - from the laboratory to the field - that may be particularly fruitful to explore, with a particular focus on Brachypodium-virus infections. These focus areas include: mechanisms of RNA modification of host plants and viruses; coevolution of virus-host interactions; viruses as tools of discovery; and how to explicate the complex outcomes during multivirus infections. Here, I broadly frame our current knowledge of Brachypodium-virus interactions and how these findings may inform virus studies of grasses in the laboratory, field and natural settings.

Virus-Induced Gene Silencing in Poaceae Using a Foxtail Mosaic Virus Vector

Virus-induced gene silencing (VIGS) is a powerful tool for rapidly knocking down the expression of plant genes to elucidate functional genomics. We have established a VIGS vector for monocot plants derived from Foxtail mosaic virus (FoMV), a positive-sense single-stranded RNA virus. For silencing a targeted gene, plant gene fragment was inserted into the vector between open reading frame 4 (ORF4) and ORF5 under the control of a duplicated coat protein promoter. Plants of different monocot species were infected by mechanical inoculation with sap from FoMV derivative-infected Chenopodium quinoa leaves. Gene silencing was typically observed within 2-3 weeks after inoculation. In this chapter, we describe the detailed protocol for silencing a target gene in various Poaceae plants by using FoMV-based vectors.