Interaction between the tobacco mosaic virus movement protein and host cell pectin methylesterases is required for viral cell-to-cell movement

Pectin-associated immune responses in plant-microbe interactions: A review

This review explores the role of pectin, a complex polysaccharide found in the plant cell wall, in mediating immune responses during interactions between plants and microbes. The objectives of this study were to investigate the molecular mechanisms underlying pectin-mediated immune responses and to understand how these interactions shape plant-microbe communication. Pectin acts as a signaling molecule, triggering immune responses such as the production of antimicrobial compounds, reinforcement of the cell wall, and activation of defense-related genes. Pectin functions as a target for pathogen-derived enzymes, enabling successful colonization by certain microbial species. The document discusses the complexity of pectin-based immune signaling networks and their modulation by various factors, including pathogen effectors and host proteins. It also emphasizes the importance of understanding the crosstalk between pectin-mediated immunity and other defense pathways to develop strategies for enhancing plant resistance against diseases. The insights gained from this study have implications for the development of innovative approaches to enhance crop protection and disease management in agriculture. Further investigations into the components and mechanisms involved in pectin-mediated immunity will pave the way for future advancements in plant-microbe interaction research.

Homogalacturonan Pectins Tuned as an Effect of Susceptible rbohD, Col-0-Reactions, and Resistance rbohF-, rbohD/F-Reactions to TuMV

The plant cell wall is an actively reorganized network during plant growth and triggered immunity in response to biotic stress. While the molecular mechanisms managing perception, recognition, and signal transduction in response to pathogens are well studied in the context of damaging intruders, the current understanding of plant cell wall rebuilding and active defense strategies in response to plant virus infections remains poorly characterized. Pectins can act as major elements of the primary cell wall and are dynamic compounds in response to pathogens. Homogalacturonans (HGs), a main component of pectins, have been postulated as defensive molecules in plant-pathogen interactions and linked to resistance responses. This research focused on examining the regulation of selected pectin metabolism components in susceptible (rbohD-, Col-0-TuMV) and resistance (rbohF-, rbohD/F-TuMV) reactions. Regardless of the interaction type, ultrastructural results indicated dynamic cell wall rebuilding. In the susceptible reaction promoted by RbohF, there was upregulation of AtPME3 (pectin methylesterase) but not AtPME17, confirmed by induction of PME3 protein deposition. Moreover, the highest PME activity along with a decrease in cell wall methylesters compared to resistance interactions in rbohD-TuMV were noticed. Consequently, the susceptible reaction of rbohD and Col-0 to TuMV was characterized by a significant domination of low/non-methylesterificated HGs. In contrast, cell wall changes during the resistance response of rbohF and rbohD/F to TuMV were associated with dynamic induction of AtPMEI2, AtPMEI3, AtGAUT1, and AtGAUT7 genes, confirmed by significant induction of PMEI2, PMEI3, and GAUT1 protein deposition. In both resistance reactions, a dynamic decrease in PME activity was documented, which was most intense in rbohD/F-TuMV. This decrease was accompanied by an increase in cell wall methylesters, indicating that the domination of highly methylesterificated HGs was associated with cell wall rebuilding in rbohF and rbohD/F defense responses to TuMV. These findings suggest that selected PME with PMEI enzymes have a diverse impact on the demethylesterification of HGs and metabolism as a result of rboh-TuMV interactions, and are important factors in regulating cell wall changes depending on the type of interaction, especially in resistance responses. Therefore, PMEI2 and PMEI3 could potentially be important signaling resistance factors in the rboh-TuMV pathosystem.

Auxin as an architect of the pectin matrix

Auxin is a versatile plant growth regulator that triggers multiple signalling pathways at different spatial and temporal resolutions. A plant cell is surrounded by the cell wall, a complex and dynamic network of polysaccharides. The cell wall needs to be rigid to provide mechanical support and protection and highly flexible to allow cell growth and shape acquisition. The modification of the pectin components, among other processes, is a mechanism by which auxin activity alters the mechanical properties of the cell wall. Auxin signalling precisely controls the transcriptional output of several genes encoding pectin remodelling enzymes, their local activity, pectin deposition, and modulation in different developmental contexts. This review examines the mechanism of auxin activity in regulating pectin chemistry at organ, cellular, and subcellular levels across diverse plant species. Moreover, we ask questions that remain to be addressed to fully understand the interplay between auxin and pectin in plant growth and development.

Molecular perspectives on age-related resistance of plants to (viral) pathogens

Age-related resistance to microbe invasion is a commonly accepted concept in plant pathology. However, the impact of such age-dependent interactive phenomena is perhaps not yet sufficiently recognized by the broader plant science community. Toward cataloging an understanding of underlying mechanisms, this review explores recent molecular studies and their relevance to the concept. Examples describe differences in genetic background, transcriptomics, hormonal balances, protein-mediated events, and the contribution by short RNA-controlled gene silencing events. Throughout, recent findings with viral systems are highlighted as an illustration of the complexity of the interactions. It will become apparent that instead of uncovering a unifying explanation, we unveiled only trends. Nevertheless, with a degree of confidence, we propose that the process of plant age-related defenses is actively regulated at multiple levels. The overarching goal of this control for plants is to avoid a constitutive waste of resources, especially at crucial metabolically draining early developmental stages.

dsRNA-induced immunity targets plasmodesmata and is suppressed by viral movement proteins

Emerging evidence indicates that in addition to its well-recognized functions in antiviral RNA silencing, dsRNA elicits pattern-triggered immunity (PTI), likely contributing to plant resistance against virus infections. However, compared to bacterial and fungal elicitor-mediated PTI, the mode-of-action and signaling pathway of dsRNA-induced defense remain poorly characterized. Here, using multicolor in vivo imaging, analysis of GFP mobility, callose staining, and plasmodesmal marker lines in Arabidopsis thaliana and Nicotiana benthamiana, we show that dsRNA-induced PTI restricts the progression of virus infection by triggering callose deposition at plasmodesmata, thereby likely limiting the macromolecular transport through these cell-to-cell communication channels. The plasma membrane-resident SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1, the BOTRYTIS INDUCED KINASE1/AVRPPHB SUSCEPTIBLE1-LIKE KINASE1 kinase module, PLASMODESMATA-LOCATED PROTEINs 1/2/3, as well as CALMODULIN-LIKE 41 and Ca2+ signals are involved in the dsRNA-induced signaling leading to callose deposition at plasmodesmata and antiviral defense. Unlike the classical bacterial elicitor flagellin, dsRNA does not trigger a detectable reactive oxygen species (ROS) burst, substantiating the idea that different microbial patterns trigger partially shared immune signaling frameworks with distinct features. Likely as a counter strategy, viral movement proteins from different viruses suppress the dsRNA-induced host response leading to callose deposition to achieve infection. Thus, our data support a model in which plant immune signaling constrains virus movement by inducing callose deposition at plasmodesmata and reveals how viruses counteract this layer of immunity.

A novel cellular factor of Nicotiana benthamiana susceptibility to tobamovirus infection

Viral infection, which entails synthesis of viral proteins and active reproduction of the viral genome, effects significant changes in the functions of many intracellular systems in plants. Along with these processes, a virus has to suppress cellular defense to create favorable conditions for its successful systemic spread in a plant. The virus exploits various cellular factors of a permissive host modulating its metabolism as well as local and systemic transport of macromolecules and photoassimilates. The Nicotiana benthamiana stress-induced gene encoding Kunitz peptidase inhibitor-like protein (KPILP) has recently been shown to be involved in chloroplast retrograde signaling regulation and stimulation of intercellular transport of macromolecules. In this paper we demonstrate the key role of KPILP in the development of tobamovius infection. Systemic infection of N. benthamiana plants with tobacco mosaic virus (TMV) or the closely related crucifer-infecting tobamovirus (crTMV) induces a drastic increase in KPILP mRNA accumulation. KPILP knockdown significantly reduces the efficiency of TMV and crTMV intercellular transport and reproduction. Plants with KPILP silencing become partially resistant to tobamovirus infection. Therefore, KPILP could be regarded as a novel proviral factor in the development of TMV and crTMV infection in N. benthamiana plants.

Sequence, structure and functionality of pectin methylesterases and their use in sustainable carbohydrate bioproducts: A review

Pectin methylesterases (PMEs) are enzymes that play a critical role in modifying pectins, a class of complex polysaccharides in plant cell walls. These enzymes catalyze the removal of methyl ester groups from pectins, resulting in a change in the degree of esterification and consequently, the physicochemical properties of the polymers. PMEs are found in various plant tissues and organs, and their activity is tightly regulated in response to developmental and environmental factors. In addition to the biochemical modification of pectins, PMEs have been implicated in various biological processes, including fruit ripening, defense against pathogens, and cell wall remodelling. This review presents updated information on PMEs, including their sources, sequences and structural diversity, biochemical properties and function in plant development. The article also explores the mechanisms of PME action and the factors influencing enzyme activity. In addition, the review highlights the potential applications of PMEs in various industrial sectors related to biomass exploitation, food, and textile industries, with a focus on development of bioproducts based on eco-friendly and efficient industrial processes.

The cotton pectin methyl esterase gene GhPME21 functions in microspore development and fertility in Gossypium hirsutum L

Pectin widely exists in higher plants' cell walls and intercellular space of higher plants and plays an indispensable role in plant growth and development. We identified 55 differentially expressed genes related to pectin degradation by transcriptomic analysis in the male sterile mutant, ms1. A gene encoding pectin methylesterase (GhPME21) was found to be predominantly expressed in the developing stamens of cotton but was significantly down-regulated in ms1 stamens. The tapetal layer of GhPME21 interfered lines (GhPME21i) was significantly thickened compared to that of WT at the early stage; anther compartment morphology of GhPME21i lines was abnormal, and the microspore wall was broken at the middle stage; Alexander staining showed that the pollen grains of GhPME21i lines differed greatly in volume at the late stage. The mature pollen surfaces of GhPME21i lines were deposited with discontinuous and broken sheets and prickles viewed under SEM. Fewer pollen tubes were observed to germinate in vitro in GhPME21i lines, while tiny of those in vivo were found to elongate to the ovary. The seeds harvested from GhPME21i lines as pollination donors were dry and hollow. The changes of phenotypes in GhPME21i lines at various stages illustrated that the GhPME21 gene played a vital role in the development of cotton stamens and controlled plant fertility by affecting stamen development, pollen germination, and pollen tube elongation. The findings of this study laid the groundwork for further research into the molecular mechanisms of PMEs involved in microspore formation and the creation of cotton male sterility materials.

Identification and Functional Analyses of Host Proteins Interacting with the P3a Protein of Brassica Yellows Virus

Viruses are obligate parasites that only undergo genomic replication in their host organisms. ORF3a, a newly identified non-AUG-initiated ORF encoded by members of the genus Polerovirus, is required for long-distance movement in plants. However, its interactions with host proteins still remain unclear. Here, we used Brassica yellows virus (BrYV)-P3a as bait to screen a plant split-ubiquitin-based membrane yeast two-hybrid (MYTH) cDNA library to explain the functional role of P3a in viral infections. In total, 138 genes with annotations were obtained. Bioinformatics analyses revealed that the genes from carbon fixation in photosynthetic, photosynthesis pathways, and MAPK signaling were affected. Furthermore, Arabidopsis thaliana purine permease 14 (AtPUP14), glucosinolate transporter 1 (AtGTR1), and nitrate transporter 1.7 (AtNRT1.7) were verified to interact with P3a in vivo. P3a and these three interacting proteins mainly co-localized in the cytoplasm. Expression levels of AtPUP14, AtGTR1, and AtNRT1.7 were significantly reduced in response to BrYV during the late stages of viral infection. In addition, we characterized the roles of AtPUP14, AtGTR1, and AtNRT1.7 in BrYV infection in A. thaliana using T-DNA insertion mutants, and the pup14, gtr1, and nrt1.7 mutants influenced BrYV infection to different degrees.

Enzymatic fingerprinting reveals specific xyloglucan and pectin signatures in the cell wall purified with primary plasmodesmata

Plasmodesmata (PD) pores connect neighbouring plant cells and enable direct transport across the cell wall. Understanding the molecular composition of these structures is essential to address their formation and later dynamic regulation. Here we provide a biochemical characterisation of the cell wall co-purified with primary PD of Arabidopsis thaliana cell cultures. To achieve this result we combined subcellular fractionation, polysaccharide analyses and enzymatic fingerprinting approaches. Relative to the rest of the cell wall, specific patterns were observed in the PD fraction. Most xyloglucans, although possibly not abundant as a group, were fucosylated. Homogalacturonans displayed short methylated stretches while rhamnogalacturonan I species were remarkably abundant. Full rhamnogalacturonan II forms, highly methyl-acetylated, were also present. We additionally showed that these domains, compared to the broad wall, are less affected by wall modifying activities during a time interval of days. Overall, the protocol and the data presented here open new opportunities for the study of wall polysaccharides associated with PD.

Leaf vein patterning is regulated by the aperture of plasmodesmata intercellular channels

To form tissue networks, animal cells migrate and interact through proteins protruding from their plasma membranes. Plant cells can do neither, yet plants form vein networks. How plants do so is unclear, but veins are thought to form by the coordinated action of the polar transport and signal transduction of the plant hormone auxin. However, plants inhibited in both pathways still form veins. Patterning of vascular cells into veins is instead prevented in mutants lacking the function of the GNOM (GN) regulator of auxin transport and signaling, suggesting the existence of at least one more GN-dependent vein-patterning pathway. Here we show that in Arabidopsis such a pathway depends on the movement of auxin or an auxin-dependent signal through plasmodesmata (PDs) intercellular channels. PD permeability is high where veins are forming, lowers between veins and nonvascular tissues, but remains high between vein cells. Impaired ability to regulate PD aperture leads to defects in auxin transport and signaling, ultimately leading to vein patterning defects that are enhanced by inhibition of auxin transport or signaling. GN controls PD aperture regulation, and simultaneous inhibition of auxin signaling, auxin transport, and regulated PD aperture phenocopies null gn mutants. Therefore, veins are patterned by the coordinated action of three GN-dependent pathways: auxin signaling, polar auxin transport, and movement of auxin or an auxin-dependent signal through PDs. Such a mechanism of tissue network formation is unprecedented in multicellular organisms.

How do plants form vein networks, in the absence of cellular migration or direct cell-cell interaction? This study shows that a GNOM-dependent combination of polar auxin transport, auxin signal transduction, and movement of an auxin signal through plasmodesmata patterns leaf vascular cells into veins.

Strawberry Vein Banding Virus Movement Protein P1 Interacts With Light-Harvesting Complex II Type 1 Like of Fragaria vesca to Promote Viral Infection

Chlorophyll a/b-binding protein of light-harvesting complex II type 1 like (LHC II-1L) is an essential component of photosynthesis, which mainly maintains the stability of the electron transport chain. However, how the LHC II-1L protein of Fragaria vesca (FvLHC II-1L) affects viral infection remains unclear. In this study, we demonstrated that the movement protein P1 of strawberry vein banding virus (SVBV P1) interacted with FvLHC II-1L in vivo and in vitro by bimolecular fluorescence complementation and pull-down assays. SVBV P1 was co-localized with FvLHC II-1L at the edge of epidermal cells of Nicotiana benthamiana leaves, and FvLHC II-1L protein expression was upregulated in SVBV-infected F. vesca. We also found that FvLHC II-1L effectively promoted SVBV P1 to compensate for the intercellular movement of movement-deficient potato virus X (PVX^ΔP25) and the systemic movement of movement-deficient cucumber mosaic virus (CMV^ΔMP). Transient overexpression of FvLHC II-1L and inoculation of an infectious clone of SVBV showed that the course of SVBV infection in F. vesca was accelerated. Collectively, the results showed that SVBV P1 protein can interact with FvLHC II-1L protein, which in turn promotes F. vesca infection by SVBV.

The Plant Invertase/Pectin Methylesterase Inhibitor Superfamily

Invertases (INVs) and pectin methylesterases (PMEs) are essential enzymes coordinating carbohydrate metabolism, stress responses, and sugar signaling. INVs catalyzes the cleavage of sucrose into glucose and fructose, exerting a pivotal role in sucrose metabolism, cellulose biosynthesis, nitrogen uptake, reactive oxygen species scavenging as well as osmotic stress adaptation. PMEs exert a dynamic control of pectin methylesterification to manage cell adhesion, cell wall porosity, and elasticity, as well as perception and signaling of stresses. INV and PME activities can be regulated by specific proteinaceous inhibitors, named INV inhibitors (INVIs) and PME Inhibitors (PMEIs). Despite targeting different enzymes, INVIs and PMEIs belong to the same large protein family named "Plant Invertase/Pectin Methylesterase Inhibitor Superfamily." INVIs and PMEIs, while showing a low aa sequence identity, they share several structural properties. The two inhibitors showed mainly alpha-helices in their secondary structure and both form a non-covalent 1:1 complex with their enzymatic counterpart. Some PMEI members are organized in a gene cluster with specific PMEs. Although the most important physiological information was obtained in Arabidopsis thaliana, there are now several characterized INVI/PMEIs in different plant species. This review provides an integrated and updated overview of this fascinating superfamily, from the specific activity of characterized isoforms to their specific functions in plant physiology. We also highlight INVI/PMEIs as biotechnological tools to control different aspects of plant growth and defense. Some isoforms are discussed in view of their potential applications to improve industrial processes. A review of the nomenclature of some isoforms is carried out to eliminate confusion about the identity and the names of some INVI/PMEI member. Open questions, shortcoming, and opportunities for future research are also presented.

With a Little Help from My Cell Wall: Structural Modifications in Pectin May Play a Role to Overcome Both Dehydration Stress and Fungal Pathogens

Cell wall structural modifications through pectin cross-linkages between calcium ions and/or boric acid may be key to mitigating dehydration stress and fungal pathogens. Water loss was profiled in a pure pectin system and in vivo. While calcium and boron reduced water loss in pure pectin standards, the impact on Allium species was insignificant (p > 0.05). Nevertheless, synchrotron X-ray microscopy showed the localization of exogenously applied calcium to the apoplast in the epidermal cells of Allium fistulosum. Exogenous calcium application increased viscosity and resistance to shear force in Allium fistulosum, suggesting the formation of calcium cross-linkages ("egg-box" structures). Moreover, Allium fistulosum (freezing tolerant) was also more tolerant to dehydration stress compared to Allium cepa (freezing sensitive). Furthermore, the addition of boric acid (H₃BO₃) to pure pectin reduced water loss and increased viscosity, which indicates the formation of RG-II dimers. The Arabidopsis boron transport mutant, bor1, expressed greater water loss and, based on the lesion area of leaf tissue, a greater susceptibility to Colletotrichum higginsianum and Botrytis cinerea. While pectin modifications in the cell wall are likely not the sole solution to dehydration and biotic stress resistance, they appear to play an important role against multiple stresses.

Plasmodesmata Structural Components and Their Role in Signaling and Plant Development

Plasmodesmata are plant intercellular channels that mediate the transport of small and large molecules including RNAs and transcription factors (TFs) that regulate plant development. In this review, we present current research on plasmodesmata form and function and discuss the main regulatory pathways. We show the progress made in the development of approaches and tools to dissect the plasmodesmata proteome in diverse plant species and discuss future perspectives and challenges in this field of research.

Two TOBAMOVIRUS MULTIPLICATION 2A homologs in tobacco control asymptomatic response to tobacco mosaic virus

The most common response of a host to pathogens is arguably the asymptomatic response. However, the genetic and molecular mechanisms responsible for asymptomatic responses to pathogens are poorly understood. Here we report on the genetic cloning of two genes controlling the asymptomatic response to tobacco mosaic virus (TMV) in cultivated tobacco (Nicotiana tabacum). These two genes are homologous to tobamovirus multiplication 2A (TOM2A) from Arabidopsis, which was shown to be critical for the accumulation of TMV. Expression analysis indicates that the TOM2A genes might play fundamental roles in plant development or in responses to stresses. Consistent with this hypothesis, a null allele of the TOM2A ortholog in tomato (Solanum lycopersicum) led to the development of bent branches and a high tolerance to both TMV and tomato mosaic virus (ToMV). However, the TOM2A ortholog in Nicotiana glauca did not account for the asymptomatic response to TMV in N. glauca. We showed that TOM2A family is plant-specific and originated from Chlorophyte, and the biological functions of TOM2A orthologs to promote TMV accumulation are highly conserved in the plant kingdom-in both TMV host and nonhost species. In addition, we showed that the interaction between tobacco TOM1 and TOM2A orthologs in plant species is conserved, suggesting a conserved nature of TOM1-TOM2A module in promoting TMV multiplication in plants. The tradeoff between host development, the resistance of hosts to pathogens, and their influence on gene evolution are discussed. Our results shed light on mechanisms that contribute to asymptomatic responses to viruses in plants and provide approaches for developing TMV/ToMV-resistant crops.

Amplification, sequencing and characterization of pectin methyl esterase inhibitor 51 gene in Tectona grandis L.f

Tectona grandis L.f. (Teak), a very important source of incomparable timber, withstands a wide range of tropical deciduous conditions. We achieved partial amplification of pectin methylesterase inhibitor 51 (PMEI) gene in teak by E. pilularis cinnamoyl Co-A reductase (CCR) gene specific primer. The amplified teak gene was of 750 bp, 79% identity and 97% query cover with PMEI of Sesamum indicum. The phylogenetic tree clustered the amplified gene with PMEI of database plant species, Erythranthe guttata and Sesamum indicum (87% bootstrap value). On conversion to amino acid sequence, the obtained protein comprised 237 amino acids. However, PMEI region spanned from 24 to 171 amino acids, 15.94 kDa molecular weight, 8.97 pI value and C₆₉₇H₁₁₁₇N₁₉₉O₂₁₁S₉ molecular formula with four conserved cysteine residues as disulfide bridges. 25.9 % protein residues were hydrophilic, 42.7% hydrophobic and 31.2% neutral. Teak 3D PMEI protein structure corresponded well with Arabidopsis thaliana and Actinidia deliciosa PMEIs. The gene maintains integrity of pectin component of middle lamella of primary cell wall and confers tolerance against various kinds of stresses. Teak conferred with overexpression of PMEI may secure a wide adaptability as well as luxuriant timber productivity and quality in adverse/ fluctuating/ scarce climatic and environmental conditions of tropical forests.

Arabidopsis thaliana Genes Associated with Cucumber mosaic virus Virulence and Their Link to Virus Seed Transmission

Virulence, the effect of pathogen infection on progeny production, is a major determinant of host and pathogen fitness as it affects host fecundity and pathogen transmission. In plant-virus interactions, ample evidence indicates that virulence is genetically controlled by both partners. However, the host genetic determinants are poorly understood. Through a genome-wide association study (GWAS) of 154 Arabidopsis thaliana genotypes infected by Cucumber mosaic virus (CMV), we identified eight host genes associated with virulence, most of them involved in response to biotic stresses and in cell wall biogenesis in plant reproductive structures. Given that virulence is a main determinant of the efficiency of plant virus seed transmission, we explored the link between this trait and the genetic regulation of virulence. Our results suggest that the same functions that control virulence are also important for CMV transmission through seeds. In sum, this work provides evidence of a novel role for some previously known plant defense genes and for the cell wall metabolism in plant virus interactions.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

Early transcriptome changes induced by the Geminivirus C4 oncoprotein: setting the stage for oncogenesis

BACKGROUND

The Beet curly top virus C4 oncoprotein is a pathogenic determinant capable of inducing extensive developmental abnormalities. No studies to date have investigated how the transcriptional profiles differ between plants expressing or not expressing the C4 oncoprotein.

RESULTS

We investigated early transcriptional changes in Arabidopsis associated with expression of the Beet curly top virus C4 protein that represent initial events in pathogenesis via a comparative transcriptional analysis of mRNAs and small RNAs. We identified 48 and 94 differentially expressed genes at 6- and 12-h post-induction versus control plants. These early time points were selected to focus on direct regulatory effects of C4 expression. Since previous evidence suggested that the C4 protein regulated the brassinosteroid (BR)-signaling pathway, differentially expressed genes could be divided into two groups: those responsive to alterations in the BR-signaling pathway and those uniquely responsive to C4. Early transcriptional changes that disrupted hormone homeostasis, 18 and 19 differentially expressed genes at both 6- and 12-hpi, respectively, were responsive to C4-induced regulation of the BR-signaling pathway. Other C4-induced differentially expressed genes appeared independent of the BR-signaling pathway at 12-hpi, including changes that could alter cell development (4 genes), cell wall homeostasis (5 genes), redox homeostasis (11 genes) and lipid transport (4 genes). Minimal effects were observed on expression of small RNAs.

CONCLUSION

This work identifies initial events in genetic regulation induced by a geminivirus C4 oncoprotein. We provide evidence suggesting the C4 protein regulates multiple regulatory pathways and provides valuable insights into the role of the C4 protein in regulating initial events in pathogenesis.

Review: Membrane tethers control plasmodesmal function and formation

Cell-to-cell communication is crucial in coordinating diverse biological processes in multicellular organisms. In plants, communication between adjacent cells occurs via nanotubular passages called plasmodesmata (PD). The PD passage is composed of an appressed endoplasmic reticulum (ER) internally, and plasma membrane (PM) externally, that traverses the cell wall, and associates with the actin-cytoskeleton. The coordination of the ER, PM and cytoskeleton plays a potential role in maintaining the architecture and conductivity of PD. Many data suggest that PD-associated proteins can serve as tethers that connect these structures in a functional PD, to regulate cell-to-cell communication. In this review, we summarize the organization and regulation of PD activity via tethering proteins, and discuss the importance of PD-mediated cell-to-cell communication in plant development and defense against environmental stress.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Divergent Symptoms Caused by Geminivirus-Encoded C4 Proteins Correlate with Their Ability To Bind NbSKη

Geminiviruses induce severe developmental abnormalities in plants. The C4/AC4 protein encoded by geminiviruses, especially those not associated with betasatellites, functions as a symptom determinant by hijacking a shaggy-related protein kinase (SKη) and interfering with its functions. Here, we report that the symptom determinant capabilities of C4 proteins encoded by different geminiviruses are divergent and tightly correlated with their abilities to interact with SKη from Nicotiana benthamiana (NbSKη). Swap of the minidomain of tomato leaf curl Yunnan virus (TLCYnV) C4 critical for the interaction with NbSKη increases the capacities of the C4 proteins encoded by tomato yellow leaf curl China virus (TYLCCNV) or tobacco curly shoot virus (TbCSV) to induce symptoms. The severity of symptoms induced by recombinant TYLCCNV C4 or TbCSV C4 correlates with the amount of NbSKη tethered to the plasma membrane by the viral protein. Moreover, a recombinant TYLCCNV harboring the minidomain of TLCYnV C4 induces more-severe symptoms than wild-type TYLCCNV. Thus, this study provides new insights into the mechanism by which different geminivirus-encoded C4 proteins possess divergent symptom determinant capabilities.IMPORTANCE Geminiviruses constitute the largest group of known plant viruses and cause devastating diseases in many economically important crops worldwide. Geminivirus-encoded C4 protein is a multifunctional protein. In this study, we found that the C4 proteins from different geminiviruses showed differential abilities to interact with NbSKη, which correlated with their symptom determinant capabilities. Moreover, a minidomain of tomato leaf curl Yunnan virus (TLCYnV) C4 that is indispensable for interacting with NbSKη and tethering it to the plasma membrane, thus leading to symptom induction, was determined. Supporting these findings, a recombinant geminivirus carrying the minidomain of TLCYnV C4 induced more-severe symptoms than the wild type. Therefore, these findings expand the scope of the interaction of NbSKη and C4-mediated symptom induction and thus contribute to further understanding of the multiple roles of C4.

Intercellular trafficking via plasmodesmata: molecular layers of complexity

Plasmodesmata are intercellular pores connecting together most plant cells. These structures consist of a central constricted form of the endoplasmic reticulum, encircled by some cytoplasmic space, in turn delimited by the plasma membrane, itself ultimately surrounded by the cell wall. The presence and structure of plasmodesmata create multiple routes for intercellular trafficking of a large spectrum of molecules (encompassing RNAs, proteins, hormones and metabolites) and also enable local signalling events. Movement across plasmodesmata is finely controlled in order to balance processes requiring communication with those necessitating symplastic isolation. Here, we describe the identities and roles of the molecular components (specific sets of lipids, proteins and wall polysaccharides) that shape and define plasmodesmata structural and functional domains. We highlight the extensive and dynamic interactions that exist between the plasma/endoplasmic reticulum membranes, cytoplasm and cell wall domains, binding them together to effectively define plasmodesmata shapes and purposes.

Conformational behavior of coat protein in plants and association with coat protein-mediated resistance against TMV

Tobacco mosaic virus (TMV) coat protein (CP) self assembles in viral RNA deprived transgenic plants to form aggregates based on the physical conditions of the environment. Transgenic plants in which these aggregates are developed show resistance toward infection by TMV referred to as CP-MR. This phenomenon has been extensively used to protect transgenic plants against viral diseases. The mutants T42W and E50Q CP confer enhanced CP-MR as compared to the WT CP. The aggregates, when examined, show the presence of helical discs in the case of WT CP; on the other hand, mutants show the presence of highly stable non-helical long rods. These aggregates interfere with the accumulation of MP as well as with the disassembly of TMV in plant cells. Here, we explored an atomic level insight to the process of CP-MR through MD simulations. The subunit-subunit interactions were assessed with the help of MM-PBSA calculations. Moreover, classification of secondary structure elements of the protein also provided unambiguous information about the conformational changes occurring in the two chains, which indicated toward increased flexibility of the mutant protein and seconded the other results of simulations. Our finding indicates the essential structural changes caused by the mutation in CP subunits, which are critically responsible for CP-MR and provides an in silico insight into the effects of these transitions over CP-MR. These results could further be utilized to design TMV-CP-based small peptides that would be able to provide appropriate protection against TMV infection.

Developmentally regulated Arabidopsis thaliana susceptibility to tomato spotted wilt virus infection

Tomato spotted wilt virus (TSWV) is one of the most devastating plant viruses and often causes severe crop losses worldwide. Generally, mature plants become more resistant to pathogens, known as adult plant resistance. In this study, we demonstrated a new phenomenon involving developmentally regulated susceptibility of Arabidopsis thaliana to TSWV. We found that Arabidopsis plants become more susceptible to TSWV as plants mature. Most young 3-week-old Arabidopsis were not infected by TSWV. Infection of TSWV in 4-, 5-, and 6-week-old Arabidopsis increased from 9%, 21%, and 25%, respectively, to 100% in 7- to 8-week-old Arabidopsis plants. Different isolates of TSWV and different tospoviruses show a low rate of infection in young Arabidopsis but a high rate in mature plants. When Arabidopsis dcl2/3/4 or rdr1/2/6 mutant plants were inoculated with TSWV, similar results as observed for the wild-type Arabidopsis plants were obtained. A cell-to-cell movement assay showed that the intercellular movement efficiency of TSWV NSm:GFP fusion was significantly higher in 8-week-old Arabidopsis leaves compared with 4-week-old Arabidopsis leaves. Moreover, the expression levels of pectin methylesterase and β-1,3-glucanase, which play critical roles in macromolecule cell-to-cell trafficking, were significantly up-regulated in 8-week-old Arabidopsis leaves compared with 4-week-old Arabidopsis leaves during TSWV infection. To date, this mature plant susceptibility to pathogen infections has rarely been investigated. Thus, the findings presented here should advance our knowledge on the developmentally regulated mature host susceptibility to plant virus infection.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Analysis of Metabolites and Gene Expression Changes Relative to Apricot (Prunus armeniaca L.) Fruit Quality During Development and Ripening

Apricot (Prunus armeniaca L.) is a valuable worldwide agronomical crop, with a delicious fruit highlighted as a functional food with both nutritional and bioactive properties, remarkably beneficial to human health. Apricot fruit ripening is a coordinated developmental process which requires change in the expression of hundreds to thousands of genes to modify many biochemical and physiological processes arising from quality characteristics in ripe fruit. In addition, enhancing fruit and nutraceutical quality is one of the central objectives to be improved in the new varieties developed by breeding programs. In this study we analyzed the contents of main metabolites linked to the nutraceutical value of apricot fruits, together with the most important pomological characteristics and biochemical contents of fruit during the ripening process in two contrasted apricot genotypes. Additionally, the gene expression changes were analyzed using RNA-Seq and real time qPCR. Results showed that genes with differential expression in the biosynthetic pathways, such as phenylpropanoids, flavonoids, starch and sucrose and carotenoid metabolism, could be possible candidates as molecular markers of fruit quality characteristics for fruit color and soluble solid content. The gene involves in carotenoid metabolism carotenoid cleavage dioxygenase 4, and the gene sucrose synthase in starch and sucrose metabolism were identified as candidate genes in the ripening process for white skin ground color and flesh color and high soluble sugar content. The application of these candidate genes on marker-assisted selection in apricot breeding programs may contribute to the early selection of high-quality fruit genotypes with suitable nutraceutical values.

Viruses Reveal the Secrets of Plasmodesmal Cell Biology

Plasmodesmata (PD) are essential for intercellular trafficking of molecules required for plant life, from small molecules like sugars and ions to macromolecules including proteins and RNA molecules that act as signals to regulate plant development and defense. As obligate intracellular pathogens, plant viruses have evolved to manipulate this communication system to facilitate the initial cell-to-cell and eventual systemic spread in their plant hosts. There has been considerable interest in how viruses manipulate the PD that connect the protoplasts of neighboring cells, and viruses have yielded invaluable tools for probing the structure and function of PD. With recent advances in biochemistry and imaging, we have gained new insights into the composition and structure of PD in the presence and absence of viruses. Here, we first discuss viral strategies for manipulating PD for their intercellular movement and examine how this has shed light on our understanding of native PD function. We then address the controversial role of the cytoskeleton in trafficking to and through PD. Finally, we address how viruses could alter PD structure and consider possible mechanisms of the phenomenon described as 'gating'. This discussion supports the significance of virus research in elucidating the properties of PD, these persistently enigmatic plant organelles.

Plasmodesmata Conductivity Regulation: A Mechanistic Model

Plant cells form a multicellular symplast via cytoplasmic bridges called plasmodesmata (Pd) and the endoplasmic reticulum (ER) that crosses almost all plant tissues. The Pd proteome is mainly represented by secreted Pd-associated proteins (PdAPs), the repertoire of which quickly adapts to environmental conditions and responds to biotic and abiotic stresses. Although the important role of Pd in stress-induced reactions is universally recognized, the mechanisms of Pd control are still not fully understood. The negative role of callose in Pd permeability has been convincingly confirmed experimentally, yet the roles of cytoskeletal elements and many PdAPs remain unclear. Here, we discuss the contribution of each protein component to Pd control. Based on known data, we offer mechanistic models of mature leaf Pd regulation in response to stressful effects.

Plant Viruses in Plant Molecular Pharming: Toward the Use of Enveloped Viruses

Plant molecular pharming has emerged as a reliable platform for recombinant protein expression providing a safe and low-cost alternative to bacterial and mammalian cells-based systems. Simultaneously, plant viruses have evolved from pathogens to molecular tools for recombinant protein expression, chimaeric viral vaccine production, and lately, as nanoagents for drug delivery. This review summarizes the genesis of viral vectors and agroinfection, the development of non-enveloped viruses for various biotechnological applications, and the on-going research on enveloped plant viruses.

Pectin Methylesterases: Cell Wall Remodeling Proteins Are Required for Plant Response to Heat Stress

Heat stress (HS) is expected to be of increasing worldwide concern in the near future, especially with regard to crop yield and quality as a consequence of rising or varying temperatures as a result of global climate change. HS response (HSR) is a highly conserved mechanism among different organisms but shows remarkable complexity and unique features in plants. The transcriptional regulation of HSR is controlled by HS transcription factors (HSFs) which allow the activation of HS-responsive genes, among which HS proteins (HSPs) are best characterized. Cell wall remodeling constitutes an important component of plant responses to HS to maintain overall function and growth; however, little is known about the connection between cell wall remodeling and HSR. Pectin controls cell wall porosity and has been shown to exhibit structural variation during plant growth and in response to HS. Pectin methylesterases (PMEs) are present in multigene families and encode isoforms with different action patterns by removal of methyl esters to influencing the properties of cell wall. We aimed to elucidate how plant cell walls respond to certain environmental cues through cell wall-modifying proteins in connection with modifications in cell wall machinery. An overview of recent findings shed light on PMEs contribute to a change in cell-wall composition/structure. The fine-scale modulation of apoplastic calcium ions (Ca2+) content could be mediated by PMEs in response to abiotic stress for both the assembly and disassembly of the pectic network. In particular, this modulation is prevalent in guard cell walls for regulating cell wall plasticity as well as stromal aperture size, which comprise critical determinants of plant adaptation to HS. These insights provide a foundation for further research to reveal details of the cell wall machinery and stress-responsive factors to provide targets and strategies to facilitate plant adaptation.

Susceptibility Genes to Plant Viruses

Plant viruses use cellular factors and resources to replicate and move. Plants respond to viral infection by several mechanisms, including innate immunity, autophagy, and gene silencing, that viruses must evade or suppress. Thus, the establishment of infection is genetically determined by the availability of host factors necessary for virus replication and movement and by the balance between plant defense and viral suppression of defense responses. Host factors may have antiviral or proviral activities. Proviral factors condition susceptibility to viruses by participating in processes essential to the virus. Here, we review current advances in the identification and characterization of host factors that condition susceptibility to plant viruses. Host factors with proviral activity have been identified for all parts of the virus infection cycle: viral RNA translation, viral replication complex formation, accumulation or activity of virus replication proteins, virus movement, and virion assembly. These factors could be targets of gene editing to engineer resistance to plant viruses.

The P1 gene of Cauliflower mosaic virus is responsible for breaking resistance in Arabidopsis thaliana ecotype Enkheim (En-2)

Arabidopsis thaliana ecotype En-2 is resistant to several strains of Cauliflower mosaic virus (CaMV), including strain W260, but is susceptible to strain NY8153. Resistance in En-2 is conditioned by a single, semi-dominant gene called CAR1. We constructed several recombinant infectious clones between W260 and NY8153 and evaluated their capability to infect En-2. This analysis showed that the capacity of NY8153 to break resistance in En-2 was conditioned by mutations within the CaMV gene 1, a gene that encodes a protein dedicated to cell-to-cell movement (P1), and conversely, that P1 of W260 is responsible for eliciting the plant defense response. A previous study had shown that P6 of W260 was responsible for overcoming resistance in Arabidopsis ecotype Tsu-0 and that P6 of CaMV strain CM1841 was responsible for triggering resistance. The present study now shows that a second gene of CaMV is targeted by Arabidopsis for plant immunity.

Hijacking of the nucleolar protein fibrillarin by TGB1 is required for cell-to-cell movement of Barley stripe mosaic virus

Barley stripe mosaic virus (BSMV) Triple Gene Block1 (TGB1) is a multifunctional movement protein with RNA-binding, ATPase and helicase activities which mainly localizes to the plasmodesmata (PD) in infected cells. Here, we show that TGB1 localizes to the nucleus and the nucleolus, as well as the cytoplasm, and that TGB1 nuclear-cytoplasmic trafficking is required for BSMV cell-to-cell movement. Prediction analyses and laser scanning confocal microscopy (LSCM) experiments verified that TGB1 possesses a nucleolar localization signal (NoLS) (amino acids 95-104) and a nuclear localization signal (NLS) (amino acids 227-238). NoLS mutations reduced BSMV cell-to-cell movement significantly, whereas NLS mutations almost completely abolished movement. Furthermore, neither the NoLS nor NLS mutant viruses could infect Nicotiana benthamiana systemically, although the NoLS mutant virus was able to establish systemic infections of barley. Protein interaction experiments demonstrated that TGB1 interacts directly with the glycine-arginine-rich (GAR) domain of the nucleolar protein fibrillarin (Fib2). Moreover, in BSMV-infected cells, Fib2 accumulation increased by about 60%-70% and co-localized with TGB1 in the plasmodesmata. In addition, BSMV cell-to-cell movement in fib2 knockdown transgenic plants was reduced to less than one-third of that of non-transgenic plants. Fib2 also co-localized with both TGB1 and BSMV RNA, which are the main components of the ribonucleoprotein (RNP) movement complex. Collectively, these results show that TGB1-Fib2 interactions play a direct role in cell-to-cell movement, and we propose that Fib2 is hijacked by BSMV TGB1 to form a BSMV RNP which functions in cell-to-cell movement.

Using genomic analysis to identify tomato Tm-2 resistance-breaking mutations and their underlying evolutionary path in a new and emerging tobamovirus

In September 2014, a new tobamovirus was discovered in Israel that was able to break Tm-2-mediated resistance in tomato that had lasted 55 years. The virus was isolated, and sequencing of its genome showed it to be tomato brown rugose fruit virus (ToBRFV), a new tobamovirus recently identified in Jordan. Previous studies on mutant viruses that cause resistance breaking, including Tm-2-mediated resistance, demonstrated that this phenotype had resulted from only a few mutations. Identification of important residues in resistance breakers is hindered by significant background variation, with 9-15% variability in the genomic sequences of known isolates. To understand the evolutionary path leading to the emergence of this resistance breaker, we performed a comprehensive phylogenetic analysis and genomic comparison of different tobamoviruses, followed by molecular modeling of the viral helicase. The phylogenetic location of the resistance-breaking genes was found to be among host-shifting clades, and this, together with the observation of a relatively low mutation rate, suggests that a host shift contributed to the emergence of this new virus. Our comparative genomic analysis identified twelve potential resistance-breaking mutations in the viral movement protein (MP), the primary target of the related Tm-2 resistance, and nine in its replicase. Finally, molecular modeling of the helicase enabled the identification of three additional potential resistance-breaking mutations.

Proteomic Analysis of Plasmodesmata From Populus Cell Suspension Cultures in Relation With Callose Biosynthesis

Plasmodesmata are channels that link adjacent cells in plant tissues through which molecular exchanges take place. They are involved in multiple processes vital to plant cells, such as responses to hormonal signaling or environmental challenges including osmotic stress, wounding and pathogen attack. Despite the importance of plasmodesmata, their proteome is not well-defined. Here, we have isolated fractions enriched in plasmodesmata from cell suspension cultures of Populus trichocarpa and identified 201 proteins that are enriched in these fractions, thereby providing further insight on the multiple functions of plasmodesmata. Proteomics analysis revealed an enrichment of proteins specifically involved in responses to stress, transport, metabolism and signal transduction. Consistent with the role of callose deposition and turnover in the closure and aperture of the plasmodesmata and our proteomic analysis, we demonstrate the enrichment of callose synthase activity in the plasmodesmata represented by several gene products. A new form of calcium-independent callose synthase activity was detected, in addition to the typical calcium-dependent enzyme activity, suggesting a role of calcium in the regulation of plasmodesmata through two forms of callose synthase activities. Our report provides the first proteomic investigation of the plasmodesmata from a tree species and the direct biochemical evidence for the occurrence of several forms of active callose synthases in these structures. Data are available via ProteomeXchange with identifier PXD010692.

Transcriptome analysis of watermelon (Citrullus lanatus) fruits in response to Cucumber green mottle mosaic virus (CGMMV) infection

Cucumber green mottle mosaic virus (CGMMV) belongs to the Tobamovirus genus and is a major global plant virus on cucurbit plants. It causes severe disease symptoms on infected watermelon plants (Citrullus lanatus), particularly inducing fruit decay. However, little is known about the molecular mechanism of CGMMV-induced watermelon fruit decay. For this study, comparative analysis of transcriptome profiles of CGMMV-inoculated and mock-inoculated watermelon fruits were conducted via RNA-Seq. A total of 1,621 differently expressed genes (DEGs) were identified in CGMMV-inoculated watermelon, among which 1,052 were up-regulated and 569 were down-regulated. Functional annotation analysis showed that several DEGs were involved in carbohydrate metabolism, hormone biosynthesis and signaling transduction, secondary metabolites biosynthesis, and plant-pathogen interactions. We furthermore found that some DEGs were related to cell wall components and photosynthesis, which may directly be involve in the development of the symptoms associated with diseased watermelons. To confirm the RNA-Seq data, 15 DEGs were selected for gene expression analysis by qRT-PCR. The results showed a strong correlation between these two sets of data. Our study identified many candidate genes for further functional studies during CGMMV-watermelon interactions, and will furthermore help to clarify the understanding of pathogenic mechanism underlying CGMMV infection in cucurbit plants.

Extracellular Matrix in Plants and Animals: Hooks and Locks for Viruses

The extracellular matrix (ECM) of animal and plants cells plays important roles in viral diseases. While in animal cells extracellular matrix components can be exploited by viruses for recognition, attachment and entry, the plant cell wall acts as a physical barrier to viral entry and adds a higher level of difficulty to intercellular movement of viruses. Interestingly, both in plant and animal systems, ECM can be strongly remodeled during virus infection, and the understanding of remodeling mechanisms and molecular players offers new perspectives for therapeutic intervention. This review focuses on the different roles played by the ECM in plant and animal hosts during virus infection with special emphasis on the similarities and differences. Possible biotechnological applications aimed at improving viral resistance are discussed.

Modulation of host plant immunity by Tobamovirus proteins

BACKGROUND

To establish successful infection, plant viruses produce profound alterations of host physiology, disturbing unrelated endogenous processes and contributing to the development of disease. In tobamoviruses, emerging evidence suggests that viral-encoded proteins display a great variety of functions beyond the canonical roles required for virus structure and replication. Among these, their modulation of host immunity appears to be relevant in infection progression.

SCOPE

In this review, some recently described effects on host plant physiology of Tobacco mosaic virus (TMV)-encoded proteins, namely replicase, movement protein (MP) and coat protein (CP), are summarized. The discussion is focused on the effects of each viral component on the modulation of host defense responses, through mechanisms involving hormonal imbalance, innate immunity modulation and antiviral RNA silencing. These effects are described taking into consideration the differential spatial distribution and temporality of viral proteins during the dynamic process of replication and spread of the virus.

CONCLUSION

In discussion of these mechanisms, it is shown that both individual and combined effects of viral-encoded proteins contribute to the development of the pathogenesis process, with the host plant's ability to control infection to some extent potentially advantageous to the invading virus.

Localization of Barley yellow dwarf virus Movement Protein Modulating Programmed Cell Death in Nicotiana benthamiana

Barley yellow dwarf virus (BYDV) belongs to Luteovirus and is limited only at phloem related tissues. An open reading frame (ORF) 4 of BYDV codes for the movement protein (MP) of BYDV gating plasmodesmata (PD) to facilitate virus movement. Like other Luteoviruses, ORF 4 of BYDV is embedded in the ORF3 but expressed from the different reading frame in leaky scanning manner. Although MP is a very important protein for systemic infection of BYDV, there was a little information. In this study, MP was characterized in terms of subcellular localization and programmed cell death (PCD). Gene of MP or its mutant (ΔMP) was expressed by Agroinfiltration method. MP was clearly localized at the nucleus and the PD, but ΔMP which was deleted distal N-terminus of MP showed no localization to PD exhibited the different target with original MP. In addition to PD localization, MP appeared associated with small granules in cytoplasm whereas ΔMP did not. MP associated with PD and small granules induced PCD, but ΔMP showed no association with PD and small granules did not exhibit PCD. Based on this study, the distal N-terminal region within MP is seemingly responsible for the localization of PD and the induction small granules and PCD induction. These results suggest that subcellular localization of BYDV MP may modulate the PCD in Nicotiana benthamiana.

The Intergenic Interplay between Aldose 1-Epimerase-Like Protein and Pectin Methylesterase in Abiotic and Biotic Stress Control

The mechanical damage that often precedes the penetration of a leaf by a pathogen promotes the activation of pectin methylesterase (PME); the activation of PME leads to the emission of methanol, resulting in a "priming" effect on intact leaves, which is accompanied by an increased sensitivity to Tobacco mosaic virus (TMV) and resistance to bacteria. In this study, we revealed that mRNA levels of the methanol-inducible gene encoding Nicotiana benthamiana aldose 1-epimerase-like protein (NbAELP) in the leaves of intact plants are very low compared with roots. However, stress and pathogen attack increased the accumulation of the NbAELP mRNA in the leaves. Using transiently transformed plants, we obtained data to support the mechanism underlying AELP/PME-related negative feedback The insertion of the NbAELP promoter sequence (proNbAELP) into the N. benthamiana genome resulted in the co-suppression of the natural NbAELP gene expression, accompanied by a reduction in the NbAELP mRNA content and increased PME synthesis. Knockdown of NbAELP resulted in high activity of PME in the cell wall and a decrease in the leaf glucose level, creating unfavorable conditions for Agrobacterium tumefaciens reproduction in injected leaves. Our results showed that NbAELP is capable of binding the TMV movement protein (MP_TMV) in vitro and is likely to affect the cellular nucleocytoplasmic transport, which may explain the sensitivity of NbAELP knockdown plants to TMV. Although NbAELP was primarily detected in the cell wall, the influence of this protein on cellular PME mRNA levels might be associated with reduced transcriptional activity of the PME gene in the nucleus. To confirm this hypothesis, we isolated the N. tabacum PME gene promoter (proNtPME) and showed the inhibition of proNtPME-directed GFP and GUS expression in leaves when co-agroinjected with the NbAELP-encoding plasmid. We hypothesized that plant wounding and/or pathogen attack lead to PME activation and increased methanol emission, followed by increased NbAELP expression, which results in reversion of PME mRNA level and methanol emission to levels found in the intact plant.

Paclobutrazol induces tolerance in tomato to deficit irrigation through diversified effects on plant morphology, physiology and metabolism

Dwindling water resources combined with meeting the demands for food security require maximizing water use efficiency (WUE) both in rainfed and irrigated agriculture. In this regard, deficit irrigation (DI), defined as the administration of water below full crop-water requirements (evapotranspiration), is a valuable practice to contain irrigation water use. In this study, the mechanism of paclobutrazol (Pbz)-mediated improvement in tolerance to water deficit in tomato was thoroughly investigated. Tomato plants were subjected to normal irrigated and deficit irrigated conditions plus Pbz application (0.8 and 1.6 ppm). A comprehensive morpho-physiological, metabolomics and molecular analysis was undertaken. Findings revealed that Pbz application reduced plant height, improved stem diameter and leaf number, altered root architecture, enhanced photosynthetic rates and WUE of tomato plants under deficit irrigation. Pbz differentially induced expression of genes and accumulation of metabolites of the tricarboxylic acid (TCA) cycle, γ-aminobutyric acid (GABA-shunt pathway), glutathione ascorbate (GSH-ASC)-cycle, cell wall and sugar metabolism, abscisic acid (ABA), spermidine (Spd) content and expression of an aquaporin (AP) protein under deficit irrigation. Our results suggest that Pbz application could significantly improve tolerance in tomato plants under limited water availability through selective changes in morpho-physiology and induction of stress-related molecular processes.

Production of Polyclonal Antibody against Grapevine fanleaf virus Movement Protein Expressed in Escherichia coli

The genomic region of Grapevine fanleaf virus (GFLV) encoding the movement protein (MP) was cloned into pET21a and transformed into Escherichia coli strain BL21 (DE3) to express the protein. Induction was made with a wide range of isopropyl-β-D-thiogalactopyranoside (IPTG) concentrations (1, 1.5, and 2 mM) each for duration of 4, 6, or 16 h. However, the highest expression level was achieved with 1 mM IPTG for 4 h. Identity of the expressed protein was confirmed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) followed by Western blotting. The expressed 41 kDa protein was purified under denaturing condition by affinity chromatography, reconfirmed by Western blotting and plate-trapped antigen enzyme-linked immunosorbent assay (PTA-ELISA) before being used as a recombinant antigen to raise polyclonal antibodies in rabbits. Purified anti-GFLV MP immunoglobulines (IgGs) and conjugated IgGs detected the expressed MP and GFLV virions in infected grapevines when used in PTA-ELISA, double antibody sandwich-ELISA, and Western blotting. This is the first report on the production of anti-GFLV MP polyclonal antibodies and application for the virus detection.

The Nucleolar Fibrillarin Protein Is Required for Helper Virus-Independent Long-Distance Trafficking of a Subviral Satellite RNA in Plants

RNA trafficking plays pivotal roles in regulating plant development, gene silencing, and adaptation to environmental stress. Satellite RNAs (satRNAs), parasites of viruses, depend on their helper viruses (HVs) for replication, encapsidation, and efficient spread. However, it remains largely unknown how satRNAs interact with viruses and the cellular machinery to undergo trafficking. Here, we show that the P20 protein of Bamboo mosaic potexvirus satRNA (satBaMV) can functionally complement in trans the systemic trafficking of P20-defective satBaMV in infected Nicotiana benthamiana The transgene-derived satBaMV, uncoupled from HV replication, was able to move autonomously across a graft union identified by RT-qPCR, RNA gel blot, and in situ RT-PCR analyses. Coimmunoprecipitation experiments revealed that the major nucleolar protein fibrillarin is coprecipitated in the P20 protein complex. Notably, silencing fibrillarin suppressed satBaMV-, but not HV-, phloem-based movement following grafting or coinoculation with HV Confocal microscopy revealed that the P20 protein colocalized with fibrillarin in the nucleoli and formed punctate structures associated with plasmodesmata. The mobile satBaMV RNA appears to exist as ribonucleoprotein (RNP) complex composed of P20 and fibrillarin, whereas BaMV movement proteins, capsid protein, and BaMV RNA are recruited with HV coinfection. Taken together, our findings provide insight into movement of satBaMV via the fibrillarin-satBaMV-P20 RNP complex in phloem-mediated systemic trafficking.

Movement Protein of Cucumber Mosaic Virus Associates with Apoplastic Ascorbate Oxidase

Plant viral movement proteins facilitate virion movement mainly through interaction with a number of factors from the host. We report the association of a cell wall localized ascorbate oxidase (CsAO4) from Cucumis sativus with the movement protein (MP) of Cucumber mosaic virus (CMV). This was identified first in a yeast two-hybrid screen and validated by in vivo pull down and bimolecular fluorescence complementation (BiFC) assays. The BiFC assay showed localization of the bimolecular complexes of these proteins around the cell wall periphery as punctate spots. The expression of CsAO4 was induced during the initial infection period (up to 72 h) in CMV infected Nicotiana benthamiana plants. To functionally validate its role in viral spread, we analyzed the virus accumulation in CsAO4 overexpressing Arabidopsis thaliana and transiently silenced N. benthamiana plants (through a Tobacco rattle virus vector). Overexpression had no evident effect on virus accumulation in upper non-inoculated leaves of transgenic lines in comparison to WT plants at 7 days post inoculation (dpi). However, knockdown resulted in reduced CMV accumulation in systemic (non-inoculated) leaves of NbΔAO-pTRV2 silenced plants as compared to TRV inoculated control plants at 5 dpi (up to 1.3 fold difference). In addition, functional validation supported the importance of AO in plant development. These findings suggest that AO and viral MP interaction helps in early viral movement; however, it had no major effect on viral accumulation after 7 dpi. This study suggests that initial induction of expression of AO on virus infection and its association with viral MP helps both towards targeting of the MP to the apoplast and disrupting formation of functional AO dimers for spread of virus to nearby cells, reducing the redox defense of the plant during initial stages of infection.

Ins and Outs of Multipartite Positive-Strand RNA Plant Viruses: Packaging versus Systemic Spread

Viruses possessing a non-segmented genome require a specific recognition of their nucleic acid to ensure its protection in a capsid. A similar feature exists for viruses having a segmented genome, usually consisting of viral genomic segments joined together into one viral entity. While this appears as a rule for animal viruses, the majority of segmented plant viruses package their genomic segments individually. To ensure a productive infection, all viral particles and thereby all segments have to be present in the same cell. Progression of the virus within the plant requires as well a concerted genome preservation to avoid loss of function. In this review, we will discuss the "life aspects" of chosen phytoviruses and argue for the existence of RNA-RNA interactions that drive the preservation of viral genome integrity while the virus progresses in the plant.