Agroinfiltration reduces ABA levels and suppresses Pseudomonas syringae-elicited salicylic acid production in Nicotiana tabacum

'Candidatus Phytoplasma mali' SAP11-Like protein modulates expression of genes involved in energy production, photosynthesis, and defense in Nicotiana occidentalis leaves

BACKGROUND

'Candidatus Phytoplasma mali', the causal agent of apple proliferation disease, exerts influence on its host plant through various effector proteins, including SAP11CaPm which interacts with different TEOSINTE BRANCHED1/ CYCLOIDEA/ PROLIFERATING CELL FACTOR 1 and 2 (TCP) transcription factors. This study examines the transcriptional response of the plant upon early expression of SAP11CaPm. For that purpose, leaves of Nicotiana occidentalis H.-M. Wheeler were Agrobacterium-infiltrated to induce transient expression of SAP11CaPm and changes in the transcriptome were recorded until 5 days post infiltration.

RESULTS

The RNA-seq analysis revealed that presence of SAP11CaPm in leaves leads to downregulation of genes involved in defense response and related to photosynthetic processes, while expression of genes involved in energy production was enhanced.

CONCLUSIONS

The results indicate that early SAP11CaPm expression might be important for the colonization of the host plant since phytoplasmas lack many metabolic genes and are thus dependent on metabolites from their host plant.

In vivo surface-enhanced Raman scattering nanosensor for the real-time monitoring of multiple stress signalling molecules in plants

When under stress, plants release molecules to activate their defense system. Detecting these stress-related molecules offers the possibility to address stress conditions and prevent the development of diseases. However, detecting endogenous signalling molecules in living plants remains challenging due to low concentrations of these analytes and interference with other compounds; additionally, many methods currently used are invasive and labour-intensive. Here we show a non-destructive surface-enhanced Raman scattering (SERS)-based nanoprobe for the real-time detection of multiple stress-related endogenous molecules in living plants. The nanoprobe, which is placed in the intercellular space, is optically active in the near-infrared region (785 nm) to avoid interferences from plant autofluorescence. It consists of a Si nanosphere surrounded by a corrugated Ag shell modified by a water-soluble cationic polymer poly(diallyldimethylammonium chloride), which can interact with multiple plant signalling molecules. We measure a SERS enhancement factor of 2.9 × 10⁷ and a signal-to-noise ratio of up to 64 with an acquisition time of ~100 ms. To show quantitative multiplex detection, we adopted a binding model to interpret the SERS intensities of two different analytes bound to the SERS hot spot of the nanoprobe. Under either abiotic or biotic stress, our optical nanosensors can successfully monitor salicylic acid, extracellular adenosine triphosphate, cruciferous phytoalexin and glutathione in Nasturtium officinale, Triticum aestivum L. and Hordeum vulgare L.-all stress-related molecules indicating the possible onset of a plant disease. We believe that plasmonic nanosensor platforms can enable the early diagnosis of stress, contributing to a timely disease management of plants.

Transient proteolysis reduction of Nicotiana benthamiana-produced CAP256 broadly neutralizing antibodies using CRISPR/Cas9

The hypersensitive response is elicited by Agrobacterium infiltration of Nicotiana benthamiana, including the induction and accumulation of pathogenesis-related proteins, such as proteases. This includes the induction of the expression of several cysteine proteases from the C1 (papain-like cysteine protease) and C13 (legumain-like cysteine protease) families. This study demonstrates the role of cysteine proteases: NbVPE-1a, NbVPE-1b, and NbCysP6 in the proteolytic degradation of Nicotiana benthamiana (glycosylation mutant ΔXTFT)-produced anti-human immunodeficiency virus broadly neutralizing antibody, CAP256-VRC26.25. Three putative cysteine protease cleavage sites were identified in the fragment crystallizable region. We further demonstrate the transient coexpression of CAP256-VRC26.25 with CRISPR/Cas9-mediated genome editing vectors targeting the NbVPE-1a, NbVPE-1b, and NbCysP6 genes which resulted in a decrease in CAP256-VRC26.25 degradation. No differences in structural features were observed between the human embryonic kidney 293 (HEK293)-produced and ΔXTFT broadly neutralizing antibodies produced with and without the coexpression of genome-editing vectors. Furthermore, despite the presence of proteolytically degraded fragments of plant-produced CAP256-VRC26.25 without the coexpression of genome editing vectors, no influence on the in vitro functional activity was detected. Collectively, we demonstrate an innovative in planta strategy for improving the quality of the CAP256 antibodies through the transient expression of the CRISPR/Cas9 vectors.

Impact of Pseudomonas sp. SVB-B33 on Stress- and Cell Wall-Related Genes in Roots and Leaves of Hemp under Salinity

Salinity is a type of abiotic stress that negatively affects plant growth and development. Textile hemp (Cannabis sativa L.) is an important multi-purpose crop that shows sensitivity to salt stress in a genotype- and developmental stage-dependent manner. The root and shoot biomasses decrease in the presence of NaCl during vegetative growth and several stress-responsive genes are activated. Finding environmentally friendly ways to increase plant health and resilience to exogenous stresses is important for a sustainable agriculture. In this context, the use of beneficial bacteria, collectively referred to as plant growth-promoting bacteria (PGPB), is becoming an attractive and emergent agricultural strategy. In this study, data are provided on the effects of a Pseudomonas isolate (Pseudomonas sp. SVB-B33) phylogenetically closely related to P. psychrotolerans applied via roots to salt-stressed hemp. The application of both living and dead bacteria impacts the fresh weight of the root biomass, as well as the expression of several stress-related genes in roots and leaves. These results pave the way to future investigations on the use of Pseudomonas sp. SVB-B33 in combination with silica to mitigate stress symptoms and increase the resilience to other forms of exogenous stresses in textile hemp.

Monitoring Pseudomonas syringae Growth in Agroinfiltrated Leaves: The "Agromonas" Assay

We recently developed a simple method called "agromonas" which facilitates the detection of Pseudomonas syringae growth in agroinfiltrated leaves expressing genes of interest. This protocol consists of agroinfiltration followed by inoculation of Pseudomonas syringae by either infiltration or spray inoculation. Next, bacterial growth is measured a few days later by plating leaf extracts out on P. syringae-selective medium. In this protocol, we describe all the steps required for agroinfiltration, inoculation of P. syringae by both injection and spray infection and explain how to quantify in planta live bacteria.

Engineering Approaches in Plant Molecular Farming for Global Health

Since the demonstration of the first plant-produced proteins of medical interest, there has been significant growth and interest in the field of plant molecular farming, with plants now being considered a viable production platform for vaccines. Despite this interest and development by a few biopharmaceutical companies, plant molecular farming is yet to be embraced by ‘big pharma’. The plant system offers a faster alternative, which is a potentially more cost-effective and scalable platform for the mass production of highly complex protein vaccines, owing to the high degree of similarity between the plant and mammalian secretory pathway. Here, we identify and address bottlenecks in the use of plants for vaccine manufacturing and discuss engineering approaches that demonstrate both the utility and versatility of the plant production system as a viable biomanufacturing platform for global health. Strategies for improving the yields and quality of plant-produced vaccines, as well as the incorporation of authentic posttranslational modifications that are essential to the functionality of these highly complex protein vaccines, will also be discussed. Case-by-case examples are considered for improving the production of functional protein-based vaccines. The combination of all these strategies provides a basis for the use of cutting-edge genome editing technology to create a general plant chassis with reduced host cell proteins, which is optimised for high-level protein production of vaccines with the correct posttranslational modifications.

Agroinfiltration Mediated Scalable Transient Gene Expression in Genome Edited Crop Plants

Agrobacterium-mediated transformation is one of the most commonly used genetic transformation method that involves transfer of foreign genes into target plants. Agroinfiltration, an Agrobacterium-based transient approach and the breakthrough discovery of CRISPR/Cas9 holds trending stature to perform targeted and efficient genome editing (GE). The predominant feature of agroinfiltration is the abolishment of Transfer-DNA (T-DNA) integration event to ensure fewer biosafety and regulatory issues besides showcasing the capability to perform transcription and translation efficiently, hence providing a large picture through pilot-scale experiment via transient approach. The direct delivery of recombinant agrobacteria through this approach carrying CRISPR/Cas cassette to knockout the expression of the target gene in the intercellular tissue spaces by physical or vacuum infiltration can simplify the targeted site modification. This review aims to provide information on Agrobacterium-mediated transformation and implementation of agroinfiltration with GE to widen the horizon of targeted genome editing before a stable genome editing approach. This will ease the screening of numerous functions of genes in different plant species with wider applicability in future.

Biological Phase Separation and Biomolecular Condensates in Plants

A surge in research focused on understanding the physical principles governing the formation, properties, and function of membraneless compartments has occurred over the past decade. Compartments such as the nucleolus, stress granules, and nuclear speckles have been designated as biomolecular condensates to describe their shared property of spatially concentrating biomolecules. Although this research has historically been carried out in animal and fungal systems, recent work has begun to explore whether these same principles are relevant in plants. Effectively understanding and studying biomolecular condensates require interdisciplinary expertise that spans cell biology, biochemistry, and condensed matter physics and biophysics. As such, some involved concepts may be unfamiliar to any given individual. This review focuses on introducing concepts essential to the study of biomolecular condensates and phase separation for biologists seeking to carry out research in this area and further examines aspects of biomolecular condensates that are relevant to plant systems.

Agromonas: a rapid disease assay for Pseudomonas syringae growth in agroinfiltrated leaves

The lengthy process to generate transformed plants is a limitation in current research on the interactions of the model plant pathogen Pseudomonas syringae with plant hosts. Here we present an easy method called agromonas, where we quantify P. syringae growth in agroinfiltrated leaves of Nicotiana benthamiana using a cocktail of antibiotics to select P. syringae on plates. As a proof of concept, we demonstrate that transient expression of PAMP receptors reduces bacterial growth, and that transient depletion of a host immune gene and transient expression of a type-III effector increase P. syringae growth in agromonas assays. We show that we can rapidly achieve structure-function analysis of immune components and test the function of immune hydrolases. The agromonas method is easy, fast and robust for routine disease assays with various Pseudomonas strains without transforming plants or bacteria. The agromonas assay offers a reliable approach for further comprehensive analysis of plant immunity.

Agromonas: a rapid disease assay for Pseudomonas syringae growth in agroinfiltrated leaves

The lengthy process to generate transformed plants is a limitation in current research on the interactions of the model plant pathogen Pseudomonas syringae with plant hosts. Here we present an easy method called agromonas, where we quantify P. syringae growth in agroinfiltrated leaves of Nicotiana benthamiana using a cocktail of antibiotics to select P. syringae on plates. As a proof of concept, we demonstrate that transient expression of PAMP receptors reduces bacterial growth, and that transient depletion of a host immune gene and transient expression of a type-III effector increase P. syringae growth in agromonas assays. We show that we can rapidly achieve structure-function analysis of immune components and test the function of immune hydrolases. The agromonas method is easy, fast and robust for routine disease assays with various Pseudomonas strains without transforming plants or bacteria. The agromonas assay offers a reliable approach for further comprehensive analysis of plant immunity.

From macro to micro: a combined bioluminescence-fluorescence approach to monitor bacterial localization

Bacterial bioluminescence is widely used to study the spatiotemporal dynamics of bacterial populations and gene expression in vivo at a population level but cannot easily be used to study bacterial activity at the level of individual cells. In this study, we describe the development of a new library of mini‐Tn7‐lux and lux::eyfp reporter constructs that provide a wide range of lux expression levels, and which combine the advantages of both bacterial bioluminescence and fluorescent proteins to bridge the gap between macro‐ and micro‐scale imaging techniques. We demonstrate that a dual bioluminescence‐fluorescence approach using the lux operon and eYFP can be used to monitor bacterial movement in plants both macro‐ and microscopically and demonstrate that Pseudomonas syringae pv phaseolicola can colonize the leaf vascular system and systemically infect leaves of common bean (Phaseolus vulgaris). We also show that bacterial bioluminescence can be used to study the impact of plant immune responses on bacterial multiplication, viability and spread within plant tissues. The constructs and approach described in this study can be used to study the spatiotemporal dynamics of bacterial colonization and to link population dynamics and cellular interactions in a wide range of biological contexts.

Agrobacterium strains and strain improvement: Present and outlook

Almost 40 years ago the first transgenic plant was generated through Agrobacterium tumefaciens-mediated transformation, which, until now, remains the method of choice for gene delivery into plants. Ever since, optimized Agrobacterium strains have been developed with additional (genetic) modifications that were mostly aimed at enhancing the transformation efficiency, although an optimized strain also exists that reduces unwanted plasmid recombination. As a result, a collection of very useful strains has been created to transform a wide variety of plant species, but has also led to a confusing Agrobacterium strain nomenclature. The latter is often misleading for choosing the best-suited strain for one's transformation purposes. To overcome this issue, we provide a complete overview of the strain classification. We also indicate different strain modifications and their purposes, as well as the obtained results with regard to the transformation process sensu largo. Furthermore, we propose additional improvements of the Agrobacterium-mediated transformation process and consider several worthwhile modifications, for instance, by circumventing a defense response in planta. In this regard, we will discuss pattern-triggered immunity, pathogen-associated molecular pattern detection, hormone homeostasis and signaling, and reactive oxygen species in relationship to Agrobacterium transformation. We will also explore alterations that increase agrobacterial transformation efficiency, reduce plasmid recombination, and improve biocontainment. Finally, we recommend the use of a modular system to best utilize the available knowledge for successful plant transformation.

Rpa1 mediates an immune response to avrRpm1Psa and confers resistance against Pseudomonas syringae pv. actinidiae

The type three effector AvrRpm1Pma from Pseudomonas syringae pv. maculicola (Pma) triggers an RPM1-mediated immune response linked to phosphorylation of RIN4 (RPM1-interacting protein 4) in Arabidopsis. However, the effector-resistance (R) gene interaction is not well established with different AvrRpm1 effectors from other pathovars. We investigated the AvrRpm1-triggered immune responses in Nicotiana species and isolated Rpa1 (Resistance to Pseudomonas syringae pv. actinidiae 1) via a reverse genetic screen in Nicotiana tabacum. Transient expression and gene silencing were performed in combination with co-immunoprecipitation and growth assays to investigate the specificity of interactions that lead to inhibition of pathogen growth. Two closely related AvrRpm1 effectors derived from Pseudomonas syringae pv. actinidiae biovar 3 (AvrRpm1Psa ) and Pseudomonas syringae pv. syringae strain B728a (AvrRpm1Psy ) trigger immune responses mediated by RPA1, a nucleotide-binding leucine-rich repeat protein with an N-terminal coiled-coil domain. In a display of contrasting specificities, RPA1 does not respond to AvrRpm1Pma , and correspondingly AvrRpm1Psa and AvrRpm1Psy do not trigger the RPM1-mediated response, demonstrating that separate R genes mediate specific immune responses to different AvrRpm1 effectors. AvrRpm1Psa co-immunoprecipitates with RPA1, and both proteins co-immunoprecipitate with RIN4. In contrast with RPM1, however, RPA1 was not activated by the phosphomimic RIN4T166D and silencing of RIN4 did not affect the RPA1 activity. Delivery of AvrRpm1Psa by Pseudomonas syringae pv. tomato (Pto) in combination with transient expression of Rpa1 resulted in inhibition of the pathogen growth in N. benthamiana. Psa growth was also inhibited by RPA1 in N. tabacum.

Engineering bacteriocin-mediated resistance against the plant pathogen Pseudomonas syringae

The plant pathogen, Pseudomonas syringae (Ps), together with related Ps species, infects and attacks a wide range of agronomically important crops, including tomato, kiwifruit, pepper, olive and soybean, causing economic losses. Currently, chemicals and introduced resistance genes are used to protect plants against these pathogens but have limited success and may have adverse environmental impacts. Consequently, there is a pressing need to develop alternative strategies to combat bacterial disease in crops. One such strategy involves using narrow-spectrum protein antibiotics (so-called bacteriocins), which diverse bacteria use to compete against closely related species. Here, we demonstrate that one bacteriocin, putidacin L1 (PL1), can be expressed in an active form at high levels in Arabidopsis and in Nicotiana benthamiana in planta to provide effective resistance against diverse pathovars of Ps. Furthermore, we find that Ps strains that mutate to acquire tolerance to PL1 lose their O-antigen, exhibit reduced motility and still cannot induce disease symptoms in PL1-transgenic Arabidopsis. Our results provide proof-of-principle that the transgene-mediated expression of a bacteriocin in planta can provide effective disease resistance to bacterial pathogens. Thus, the expression of bacteriocins in crops might offer an effective strategy for managing bacterial disease, in the same way that the genetic modification of crops to express insecticidal proteins has proven to be an extremely successful strategy for pest management. Crucially, nearly all genera of bacteria, including many plant pathogenic species, produce bacteriocins, providing an extensive source of these antimicrobial agents.

The transcriptome, extracellular proteome and active secretome of agroinfiltrated Nicotiana benthamiana uncover a large, diverse protease repertoire

Infiltration of disarmed Agrobacterium tumefaciens into leaves of Nicotiana benthamiana (agroinfiltration) facilitates quick and safe production of antibodies, vaccines, enzymes and metabolites for industrial use (molecular farming). However, yield and purity of proteins produced by agroinfiltration are hampered by unintended proteolysis, restricting industrial viability of the agroinfiltration platform. Proteolysis may be linked to an immune response to agroinfiltration, but understanding of the response to agroinfiltration is limited. To identify the proteases, we studied the transcriptome, extracellular proteome and active secretome of agroinfiltrated leaves over a time course, with and without the P19 silencing inhibitor. Remarkably, the P19 expression had little effect on the leaf transcriptome and no effect on the extracellular proteome. 25% of the detected transcripts changed in abundance upon agroinfiltration, associated with a gradual up-regulation of immunity at the expense of photosynthesis. By contrast, 70% of the extracellular proteins increased in abundance, in many cases associated with increased efficiency of extracellular delivery. We detect a dynamic reprogramming of the proteolytic machinery upon agroinfiltration by detecting transcripts encoding for 975 different proteases and protease homologs. The extracellular proteome contains peptides derived from 196 proteases and protease homologs, and activity-based proteomics displayed 17 active extracellular Ser and Cys proteases in agroinfiltrated leaves. We discuss unique features of the N. benthamiana protease repertoire and highlight abundant extracellular proteases in agroinfiltrated leaves, being targets for reverse genetics. This data set increases our understanding of the plant response to agroinfiltration and indicates ways to improve a key expression platform for both plant science and molecular farming.

Comparative Membrane-Associated Proteomics of Three Different Immune Reactions in Potato

Plants have evolved different types of immune reactions but large-scale proteomics about these processes are lacking, especially in the case of agriculturally important crop pathosystems. We have established a system for investigating PAMP-triggered immunity (PTI) and two different effector-triggered immunity (ETI; triggered by Avr2 or IpiO) responses in potato. The ETI responses are triggered by molecules from the agriculturally important Phytophthora infestans interaction. To perform large-scale membrane protein-based comparison of these responses, we established a method to extract proteins from subcellular compartments in leaves. In the membrane fractions that were subjected to quantitative proteomics analysis, we found that most proteins regulated during PTI were also regulated in the same way in ETI. Proteins related to photosynthesis had lower abundance, while proteins related to oxidative and biotic stress, as well as those related to general antimicrobial defense and cell wall degradation, were found to be higher in abundance. On the other hand, we identified a few proteins—for instance, an ABC transporter-like protein—that were only found in the PTI reaction. Furthermore, we also identified proteins that were regulated only in ETI interactions. These included proteins related to GTP binding and heterotrimeric G-protein signaling, as well as those related to phospholipase signaling.

Artificial Agrobacterium tumefaciens strains exhibit diverse mechanisms to repress Xanthomonas oryzae pv. oryzae-induced hypersensitive response and non-host resistance in Nicotiana benthamiana

Xanthomonas oryzae pv. oryzae (Xoo) rapidly triggers a hypersensitive response (HR) and non-host resistance in its non-host plant Nicotiana benthamiana. Here, we report that Agrobacterium tumefaciens strain GV3101 blocks Xoo-induced HR in N. benthamiana when pre-infiltrated or co-infiltrated, but not when post-infiltrated at 4 h after Xoo inoculation. This suppression by A. tumefaciens is local and highly efficient to Xoo. The HR-inhibiting efficiency of A. tumefaciens is strain dependent. Strain C58C1 has almost no effect on Xoo-induced HR, whereas strains GV3101, EHA105 and LBA4404 nearly completely block HR formation. Intriguingly, these three HR-inhibiting strains employ different strategies to repress HR. Strain GV3101 displays strong antibiotic activity and thus suppresses Xoo growth. Comparison of the genotype and Xoo antibiosis activity of wild-type A. tumefaciens strain C58 and a set of C58-derived strains reveals that this Xoo antibiosis activity of A. tumefaciens is negatively, but not solely, regulated by the transferred-DNA (T-DNA) of the Ti plasmid pTiC58. Unlike GV3101, strains LBA4404 and EHA105 exhibit no significant antibiotic effect on Xoo, but rather abolish hydrogen peroxide accumulation. In addition, expression assays indicate that strains LBA4404 and EHA105 may inhibit Xoo-induced HR by suppression of the expression of Xoo type III secretion system (T3SS) effector genes hpa1 and hrpD6. Collectively, our results unveil the multiple levels of effects of A. tumefaciens on Xoo in N. benthamiana and provide insights into the molecular mechanisms underlying the bacterial antibiosis of A. tumefaciens and the non-host resistance induced by Xoo.

An Agrobacterium tumefaciens Strain with Gamma-Aminobutyric Acid Transaminase Activity Shows an Enhanced Genetic Transformation Ability in Plants

Agrobacterium tumefaciens has the unique ability to mediate inter-kingdom DNA transfer, and for this reason, it has been utilized for plant genetic engineering. To increase the transformation frequency in plant genetic engineering, we focused on gamma-aminobutyric acid (GABA), which is a negative factor in the Agrobacterium-plant interaction. Recent studies have shown contradictory results regarding the effects of GABA on vir gene expression, leading to the speculation that GABA inhibits T-DNA transfer. In this study, we examined the effect of GABA on T-DNA transfer using a tomato line with a low GABA content. Compared with the control, the T-DNA transfer frequency was increased in the low-GABA tomato line, indicating that GABA inhibits T-DNA transfer. Therefore, we bred a new A. tumefaciens strain with GABA transaminase activity and the ability to degrade GABA. The A. tumefaciens strain exhibited increased T-DNA transfer in two tomato cultivars and Erianthus arundinacues and an increased frequency of stable transformation in tomato.

Differences in LEA-like 11-24 gene expression in desiccation tolerant and sensitive species of Linderniaceae are due to variations in gene promoter sequences

Many desiccation induced late embryogenesis abundant (LEA) protein encoding genes have been identified from Craterostigma plantagineum Hochst. In the desiccation tolerant plants C. plantagineum (Cp) and Lindernia brevidens Skan (Lb) transcripts encoding LEA-like 11-24 protein are abundantly expressed during desiccation whereas in Lindernia subracemosa De Wild. (Ls), a desiccation sensitive plant, the LEA-like 11-24 transcripts are expressed at a low level. Since promoters determine gene expression, a comparative promoter analysis was carried out to decipher the underlying mechanisms of differential gene expression. Two transient transformation methods (particle bombardment and optimised Agrobacterium co-cultivation) were used to analyse the promoter activities of the Cp, Lb and Ls LEA-like 11-24 gene in homologous and heterologous systems. Minimal promoters were isolated from all three species and their promoter activities were assessed in response to mannitol or ABA. Particle bombardment or Agrobacterium co-cultivation yielded similar results. Site-directed mutagenesis was used to identify which cis-acting elements in the LEA-like 11-24 promoter fragments are crucial during mannitol and ABA treatments. The presence of these promoter cis-elements explains the differences in transcript abundance in the desiccation tolerant and desiccation sensitive species. Results indicated the importance of the drought responsive elements (DRE) element for promoter activity.

Salivary proteins of spider mites suppress defenses in Nicotiana benthamiana and promote mite reproduction

Spider mites (Tetranychidae sp.) are widely occurring arthropod pests on cultivated plants. Feeding by the two-spotted spider mite T. urticae, a generalist herbivore, induces a defense response in plants that mainly depends on the phytohormones jasmonic acid and salicylic acid (SA). On tomato (Solanum lycopersicum), however, certain genotypes of T. urticae and the specialist species T. evansi were found to suppress these defenses. This phenomenon occurs downstream of phytohormone accumulation via an unknown mechanism. We investigated if spider mites possess effector-like proteins in their saliva that can account for this defense suppression. First we performed an in silico prediction of the T. urticae and the T. evansi secretomes, and subsequently generated a short list of candidate effectors based on additional selection criteria such as life stage-specific expression and salivary gland expression via whole mount in situ hybridization. We picked the top five most promising protein families and then expressed representatives in Nicotiana benthamiana using Agrobacterium tumefaciens transient expression assays to assess their effect on plant defenses. Four proteins from two families suppressed defenses downstream of the phytohormone SA. Furthermore, T. urticae performance on N. benthamiana improved in response to transient expression of three of these proteins and this improvement was similar to that of mites feeding on the tomato SA accumulation mutant nahG. Our results suggest that both generalist and specialist plant-eating mite species are sensitive to SA defenses but secrete proteins via their saliva to reduce the negative effects of these defenses.

Transcriptome Analysis of Plant Hormone-Related Tomato (Solanum lycopersicum) Genes in a Sunlight-Type Plant Factory

In plant factories, measurements of plant conditions are necessary at an early stage of growth to predict harvest times of high value-added crops. Moreover, harvest qualities depend largely on environmental stresses that elicit plant hormone responses. However, the complexities of plant hormone networks have not been characterized under nonstress conditions. In the present study, we determined temporal expression profiles of all genes and then focused on plant hormone pathways using RNA-Seq analyses of gene expression in tomato leaves every 2 h for 48 h. In these experiments, temporally expressed genes were found in the hormone synthesis pathways for salicylic acid, abscisic acid, ethylene, and jasmonic acid. The timing of CAB expression 1 (TOC1) and abscisic acid insensitive 1 (ABA1) and open stomata 1 (OST1) control gating stomata. In this study, compare with tomato and Arabidopsis thaliana, expression patterns of TOC1 have similarity. In contrast, expression patterns of tomato ABI1 and OST1 had expression peak at different time. These findings suggest that the regulation of gating stomata does not depend predominantly on TOC1 and significantly reflects the extracellular environment. The present data provide new insights into relationships between temporally expressed plant hormone-related genes and clock genes under normal sunlight conditions.

The 'prime-ome': towards a holistic approach to priming

Plants can be primed to respond faster and more strongly to stress and multiple pathways, specific for the encountered challenge, are involved in priming. This adaptability of priming makes it difficult to pinpoint an exact mechanism: the same phenotypic observation might be the consequence of unrelated underlying events. Recently, details of the molecular aspects of establishing a primed state and its transfer to offspring have come to light. Advances in techniques for detection and quantification of elements spanning the fields of transcriptomics, proteomics, and metabolomics, together with adequate bioinformatics tools, will soon allow us to take a holistic approach to plant defence. This review highlights the state of the art of new strategies to study defence priming in plants and provides perspectives towards 'prime-omics'.

Unmasking host and microbial strategies in the Agrobacterium-plant defense tango

Coevolutionary forces drive adaptation of both plant-associated microbes and their hosts. Eloquently captured in the Red Queen Hypothesis, the complexity of each plant-pathogen relationship reflects escalating adversarial strategies, but also external biotic and abiotic pressures on both partners. Innate immune responses are triggered by highly conserved pathogen-associated molecular patterns, or PAMPs, that are harbingers of microbial presence. Upon cell surface receptor-mediated recognition of these pathogen-derived molecules, host plants mount a variety of physiological responses to limit pathogen survival and/or invasion. Successful pathogens often rely on secretion systems to translocate host-modulating effectors that subvert plant defenses, thereby increasing virulence. Host plants, in turn, have evolved to recognize these effectors, activating what has typically been characterized as a pathogen-specific form of immunity. Recent data support the notion that PAMP-triggered and effector-triggered defenses are complementary facets of a convergent, albeit differentially regulated, set of immune responses. This review highlights the key players in the plant's recognition and signal transduction pathways, with a focus on the aspects that may limit Agrobacterium tumefaciens infection and the ways it might overcome those defenses. Recent advances in the field include a growing appreciation for the contributions of cytoskeletal dynamics and membrane trafficking to the regulation of these exquisitely tuned defenses. Pathogen counter-defenses frequently manipulate the interwoven hormonal pathways that mediate host responses. Emerging systems-level analyses include host physiological factors such as circadian cycling. The existing literature indicates that varying or even conflicting results from different labs may well be attributable to environmental factors including time of day of infection, temperature, and/or developmental stage of the host plant.

Transient plant transformation mediated by Agrobacterium tumefaciens: Principles, methods and applications

Agrobacterium tumefaciens is widely used as a versatile tool for development of stably transformed model plants and crops. However, the development of Agrobacterium based transient plant transformation methods attracted substantial attention in recent years. Transient transformation methods offer several applications advancing stable transformations such as rapid and scalable recombinant protein production and in planta functional genomics studies. Herein, we highlight Agrobacterium and plant genetics factors affecting transfer of T-DNA from Agrobacterium into the plant cell nucleus and subsequent transient transgene expression. We also review recent methods concerning Agrobacterium mediated transient transformation of model plants and crops and outline key physical, physiological and genetic factors leading to their successful establishment. Of interest are especially Agrobacterium based reverse genetics studies in economically important crops relying on use of RNA interference (RNAi) or virus-induced gene silencing (VIGS) technology. The applications of Agrobacterium based transient plant transformation technology in biotech industry are presented in thorough detail. These involve production of recombinant proteins (plantibodies, vaccines and therapeutics) and effectoromics-assisted breeding of late blight resistance in potato. In addition, we also discuss biotechnological potential of recombinant GFP technology and present own examples of successful Agrobacterium mediated transient plant transformations.

Agroinfiltration by cytokinin-producing Agrobacterium sp. strain GV3101 primes defense responses in Nicotiana tabacum

Transient infiltrations in tobacco are commonly used in plant studies, but the host response to different disarmed Agrobacterium strains is not fully understood. The present study shows that pretreatment with disarmed Agrobacterium tumefaciens GV3101 primes the defense response to subsequent infection by Pseudomonas syringae in Nicotiana tabacum. The presence of a trans-zeatin synthase (tzs) gene in strain GV3101 may be partly responsible for the priming response, as the tzs-deficient Agrobacterium sp. strain LBA4404 only weakly imparts such responses. Besides inducing the expression of defense-related genes like PR-1 and NHL10, GV3101 pretreatment increased the expression of tobacco mitogen-activated protein kinase (MAPK) pathway genes like MEK2, WIPK (wound-induced protein kinase), and SIPK (salicylic acid-induced protein kinase). Furthermore, the GV3101 strain showed a stronger effect than the LBA4404 strain in activating phosphorylation of the tobacco MAPK, WIPK and SIPK, which presumably prime the plant immune machinery. Lower doses of exogenously applied cytokinins increased the activation of MAPK, while higher doses decreased the activation, suggesting a balanced level of cytokinins is required to generate defense response in planta. The current study serves as a cautionary warning for plant researchers over the choice of Agrobacterium strains and their possible consequences on subsequent pathogen-related studies.

Simultaneous analysis of apolar phytohormones and 1-aminocyclopropan-1-carboxylic acid by high performance liquid chromatography/electrospray negative ion tandem mass spectrometry via 9-fluorenylmethoxycarbonyl chloride derivatization

A strategy to detect and quantify the polar ethylene precursor 1-aminocyclopropan-1-carboxylic acid (ACC) along with the more apolar phytohormones abscisic acid (ABA), indole-3-acetic acid (IAA), jasmonic acid (JA), jasmonic acid-isoleucine conjugate (JA-Ile), 12-oxo-phytodienoic acid (OPDA), trans-zeatin, and trans-zeatin 9-riboside using a single extraction is presented. Solid phase resins commonly employed for extraction of phytohormones do not allow the recovery of ACC. We circumvent this problem by attaching an apolar group to ACC via derivatization with the amino group specific reagent 9-fluorenylmethoxycarbonyl chloride (Fmoc-Cl). Derivatization in the methanolic crude extract does not modify other phytohormones. The derivatized ACC could be purified and detected together with the more apolar phytohormones using common solid phase extraction resins and reverse phase HPLC/electrospray negative ion tandem mass spectrometry. The limit of detection was in the low nanomolar range for all phytohormones, a sensitivity sufficient to accurately determine the phytohormone levels from less than 50mg (fresh weight) of Arabidopsis thaliana and Nicotiana benthamiana tissues. Comparison with previously published phytohormone levels and the reported changes in phytohormone levels after stress treatments confirmed the accuracy of the method.

Agrobacterium-derived cytokinin influences plastid morphology and starch accumulation in Nicotiana benthamiana during transient assays

BACKGROUND

Agrobacterium tumefaciens-based transient assays have become a common tool for answering questions related to protein localization and gene expression in a cellular context. The use of these assays assumes that the transiently transformed cells are observed under relatively authentic physiological conditions and maintain 'normal' sub-cellular behaviour. Although this premise is widely accepted, the question of whether cellular organization and organelle morphology is altered in Agrobacterium-infiltrated cells has not been examined in detail. The first indications of an altered sub-cellular environment came from our observation that a common laboratory strain, GV3101(pMP90), caused a drastic increase in stromule frequency. Stromules, or 'stroma-filled-tubules' emanate from the surface of plastids and are sensitive to a variety of biotic and abiotic stresses. Starting from this observation, the goal of our experiments was to further characterize the changes to the cell resulting from short-term bacterial infestation, and to identify the factor responsible for eliciting these changes.

RESULTS

Using a protocol typical of transient assays we evaluated the impact of GV3101(pMP90) infiltration on chloroplast behaviour and morphology in Nicotiana benthamiana. Our experiments confirmed that GV3101(pMP90) consistently induces stromules and alters plastid position relative to the nucleus. These effects were found to be the result of strain-dependant secretion of cytokinin and its accumulation in the plant tissue. Bacterial production of the hormone was found to be dependant on the presence of a trans-zeatin synthase gene (tzs) located on the Ti plasmid of GV3101(pMP90). Bacteria-derived cytokinins were also correlated with changes to both soluble sugar level and starch accumulation.

CONCLUSION

Although we have chosen to focus on how transient Agrobacterium infestation alters plastid based parameters, these changes to the morphology and position of a single organelle, combined with the measured increases in sugar and starch content, suggest global changes to cell physiology. This indicates that cells visualized during transient assays may not be as 'normal' as was previously assumed. Our results suggest that the impact of the bacteria can be minimized by choosing Agrobacterium strains devoid of the tzs gene, but that the alterations to sub-cellular organization and cell carbohydrate status cannot be completely avoided using this strategy.

Optimizing the transient Fast Agro-mediated Seedling Transformation (FAST) method in Catharanthus roseus seedlings

An Agro-mediated transformation method has been adapted in Catharanthus roseus seedlings for transient overexpression. Our results suggest that Agro-mediated methods may induce defense-related genes, which should be considered in its application. The Fast Agro-mediated Seedling Transformation (FAST) method, which involves the co-cultivation and transient transformation of young seedlings with Agrobacterium, was adapted and optimized in Catharanthus roseus. We investigated the optimal conditions for Gus expression by varying the Agrobacterium density (OD600 = 0.29 and 0.50), A. rhizogenes strain (15834 and R1000), and co-cultivation time in liquid (2, 12, or 24 h) followed by incubation time on solid media (1 or 2 days). Transformation efficiency was assessed quantitatively in terms of average GUS intensity per cotyledon surface area and percentage of cotyledons transformed. GUS staining was observed in 100% of cotyledons co-cultivated with A. rhizogenes (OD600 = 0.50) co-transformed with the Mas promoter-driven Gus and pSoup helper plasmids, in the presence of 0.01% v/v Silwet L-77 for 24 h in liquid followed by 2-days on solid media. In addition, we observed that co-cultivation with Agrobacterium strongly induced Zct1 and Orca3, two transcription factors known to regulate defense-related alkaloid biosynthesis in C. roseus. Homologous transcription factors regulate defense responses in many plant species. Therefore, possible induction of defense-related genes by Agro-mediated transformation should be a consideration in experimental design.

Mp10 and Mp42 from the aphid species Myzus persicae trigger plant defenses in Nicotiana benthamiana through different activities

Aphids are phloem-feeding insects that, like other plant parasites, deliver effectors inside their host to manipulate host responses. The Myzus persicae (green peach aphid) candidate effectors Mp10 and Mp42 were previously found to reduce aphid fecundity upon intracellular transient overexpression in Nicotiana benthamiana. We performed functional analyses of these proteins to investigate whether they activate defenses through similar activities. We employed a range of functional characterization experiments based on intracellular transient overexpression in N. benthamiana to determine the subcellular localization of Mp10 and Mp42 and investigate their role in activating plant defense signaling. Mp10 and Mp42 showed distinct subcellular localization in planta, suggesting that they target different host compartments. Also, Mp10 reduced the levels of Agrobacterium-mediated overexpression of proteins. This reduction was not due to an effect on Agrobacterium viability. Transient overexpression of Mp10 but not Mp42 activated jasmonic acid and salicylic acid signaling pathways and decreased susceptibility to the hemibiotrophic plant pathogen Phytophthora capsici. We found that two candidate effectors from the broad-host-range aphid M. persicae can trigger aphid defenses through different mechanisms. Importantly, we found that some (candidate) effectors such as Mp10 interfere with Agrobacterium-based overexpression assays, an important tool to study effector activity and function.

Increased 1-aminocyclopropane-1-carboxylate deaminase activity enhances Agrobacterium tumefaciens-mediated gene delivery into plant cells

Agrobacterium-mediated transformation is a useful tool for the genetic modification in plants, although its efficiency is low for several plant species. Agrobacterium-mediated transformation has three major steps in laboratory-controlled experiments: the delivery of T-DNA into plant cells, the selection of transformed plant cells, and the regeneration of whole plants from the selected cells. Each of these steps must be optimized to improve the efficiency of Agrobacterium-mediated plant transformation. It has been reported that increasing the number of cells transformed by T-DNA delivery can improve the frequency of stable transformation. Previously, we demonstrated that a reduction in ethylene production by plant cells during cocultivation with A. tumefaciens-expressing 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase resulted in increased T-DNA delivery into the plant cells. In this study, to further improve T-DNA delivery by A. tumefaciens, we modified the expression cassette of the ACC deaminase gene using vir gene promoter sequences. The ACC deaminase gene driven by the virD1 promoter was expressed at a higher level, resulting in a higher ACC deaminase activity in this A. tumefaciens strain than in the strain with the lac promoter used in a previous study. The newly developed A. tumefaciens strain improves the delivery of T-DNA into Solanum lycopersicum (tomato) and Erianthus ravennae plants and thus may be a powerful tool for the Agrobacterium-mediated genetic engineering of plants.

An efficient Agrobacterium-mediated transient transformation of Arabidopsis

Agrobacterium tumefaciens-mediated transient transformation has been a useful procedure for characterization of proteins and their functions in plants, including analysis of protein-protein interactions. Agrobacterium-mediated transient transformation of Nicotiana benthamiana by leaf infiltration has been widely used due to its ease and high efficiency. However, in Arabidopsis this procedure has been challenging. Previous studies suggested that this difficulty was caused by plant immune responses triggered by perception of Agrobacterium. Here, we report a simple and robust method for Agrobacterium-mediated transient transformation in Arabidopsis. AvrPto is an effector protein from the bacterial plant pathogen Pseudomonas syringae that suppresses plant immunity by interfering with plant immune receptors. We used transgenic Arabidopsis plants that conditionally express AvrPto under the control of a dexamethasone (DEX)-inducible promoter. When the transgenic plants were pretreated with DEX prior to infection with Agrobacterium carrying a β-glucuronidase (GUS, uidA) gene with an artificial intron and driven by the CaMV 35S promoter, transient GUS expression was dramatically enhanced compared to that in mock-pretreated plants. This transient expression system was successfully applied to analysis of the subcellular localization of a cyan fluorescent protein (CFP) fusion and a protein-protein interaction in Arabidopsis. Our findings enable efficient use of Agrobacterium-mediated transient transformation in Arabidopsis thaliana.

Transient silencing of the grapevine gene VvPGIP1 by agroinfiltration with a construct for RNA interference

Grapevine is an economically important crop, and the recent completion of its genome makes it possible to study the function of specific genes through reverse genetics. However, the analysis of gene function by RNA interference (RNAi) in grapevine is difficult, because the generation of stable transgenic plants has low efficiency and is time consuming. Recently, transient expression of genes in grapevine leaves has been obtained by Agrobacterium tumefaciens infiltration (agroinfiltration). We therefore tested the possibility to silence grapevine genes by agroinfiltration of RNAi constructs. A construct to express a double strand RNA (dsRNA) corresponding to the defense-related gene VvPGIP1, encoding a polygalacturonase-inhibiting protein (PGIP), was obtained and transiently expressed by agroinfiltration in leaves of grapevine plants grown in vitro. Expression of VvPGIP1 and accumulation of PGIP activity were strongly induced by infiltration with control bacteria, but not with bacteria carrying the dsRNA construct, indicating that the gene was efficiently silenced. In contrast, expression of another defense-related gene, VST1, encoding a stilbene synthase, was unaffected by the dsRNA construct. We have therefore demonstrated the possibility of transient down-regulation of grapevine genes by agroinfiltration of constructs for the expression of dsRNA. This system can be employed to evaluate the effectiveness of constructs that can be subsequently used to generate stable RNAi transgenic plants.

Reactive oxygen and oxidative stress tolerance in plant pathogenic Pseudomonas

Reactive oxygen species (ROS) are a key feature of plant (and animal) defences against invading pathogens. As a result, plant pathogens must be able to either prevent their production or tolerate high concentrations of these highly reactive chemicals. In this review, we focus on plant pathogenic bacteria of the genus Pseudomonas and the ways in which they overcome the challenges posed by ROS. We also explore the ways in which pseudomonads may exploit plant ROS generation for their own purposes and even produce ROS directly as part of their infection mechanisms.

Cytokinins mediate resistance against Pseudomonas syringae in tobacco through increased antimicrobial phytoalexin synthesis independent of salicylic acid signaling

Cytokinins are phytohormones that are involved in various regulatory processes throughout plant development, but they are also produced by pathogens and known to modulate plant immunity. A novel transgenic approach enabling autoregulated cytokinin synthesis in response to pathogen infection showed that cytokinins mediate enhanced resistance against the virulent hemibiotrophic pathogen Pseudomonas syringae pv tabaci. This was confirmed by two additional independent transgenic approaches to increase endogenous cytokinin production and by exogenous supply of adenine- and phenylurea-derived cytokinins. The cytokinin-mediated resistance strongly correlated with an increased level of bactericidal activities and up-regulated synthesis of the two major antimicrobial phytoalexins in tobacco (Nicotiana tabacum), scopoletin and capsidiol. The key role of these phytoalexins in the underlying mechanism was functionally proven by the finding that scopoletin and capsidiol substitute in planta for the cytokinin signal: phytoalexin pretreatment increased resistance against P. syringae. In contrast to a cytokinin defense mechanism in Arabidopsis (Arabidopsis thaliana) based on salicylic acid-dependent transcriptional control, the cytokinin-mediated resistance in tobacco is essentially independent from salicylic acid and differs in pathogen specificity. It is also independent of jasmonate levels, reactive oxygen species, and high sugar resistance. The novel function of cytokinins in the primary defense response of solanaceous plant species is rather mediated through a high phytoalexin-pathogen ratio in the early phase of infection, which efficiently restricts pathogen growth. The implications of this mechanism for the coevolution of host plants and cytokinin-producing pathogens and the practical application in agriculture are discussed.