Functional Analysis of V2 Protein of Beet Curly Top Iran Virus

Geminivirus beet curly top Iran virus (BCTIV) is one of the main causal agents of the beet curly top disease in Iran and the newly established Becurtovirus genus type species. Although the biological features of known becurtoviruses are similar to those of curtoviruses, they only share a limited sequence identity, and no information is available on the function of their viral genes. In this work, we demonstrate that BCTIV V2, as the curtoviral V2, is also a local silencing suppressor in Nicotiana benthamiana and can delay the systemic silencing spreading, although it cannot block the cell-to-cell movement of the silencing signal to adjacent cells. BCTIV V2 shows the same subcellular localization as curtoviral V2, being detected in the nucleus and perinuclear region, and its ectopic expression from a PVX-derived vector also causes the induction of necrotic lesions in N. benthamiana, such as the ones produced during the HR, both at the local and systemic levels. The results from the infection of N. benthamiana with a V2 BCTIV mutant showed that V2 is required for systemic infection, but not for viral replication, in a local infection. Considering all these results, we can conclude that BCTIV V2 is a functional homologue of curtoviral V2 and plays a crucial role in viral pathogenicity and systemic movement.

The coat protein p25 from maize chlorotic mottle virus involved in symptom development and systemic movement of tobacco mosaic virus hybrids

Viral coat protein (CP) has numerous critical functions in plant infection, but little is known about p25, the CP of maize chlorotic mottle virus (MCMV; Machlomovirus), which causes severe yield losses in maize worldwide. Here, we investigated the roles of p25 in pathogenicity and systemic movement, as well as potential interactions with host plants, using a hybrid tobacco mosaic virus (TMV)-based expression system. Highly conserved protein p25 is predicted to contain a membrane-anchored nuclear localization signal (NLS) sequence and an extracellular sequence. In transgenic Nicotiana benthamiana plants containing the movement protein (MP) of TMV (TMV-MP), p25 induced severe symptoms, including dwarf and foliar necrosis, and was detected in inoculated and non-inoculated leaves. After the deletion of NLS from nuclear-located p25, the protein was found throughout the host cell, and plant stunting and starch granule deformity were reduced. Systemic movement and pathogenicity were significantly impaired when the C-terminal regions of p25 were absent. Using virus-induced gene silencing (VIGS), the transcript level of heat shock protein HSP90 was distinctly lower in host plants in association with the absence of leaf necrosis induced by TMV-p25. Our results revealed crucial roles for MCMV p25 in viral pathogenicity, long-distance movement, and interactions with N. benthamiana.

Comparative Anatomical Responses of Tolerant and Susceptible European Plum Varieties to Black Knot Disease

Plums are affected by a cancerous disease called "black knot disease" caused by the fungus Apiosporina morbosa. It affects both Japanese (Prunus salicina) and European (Prunus domestica) plums equally. To understand the spread of the disease, histological analysis was performed in two different European plum cultivars (susceptible and tolerant). Light and scanning electron microscope (SEM) analyses confirmed the presence of the growing hyphae in the internal tissues of the susceptible trees. By using stereoscopic analysis with a fluorescence filter, we were able to detect the hyphae in the visible lesion area. At about 2 inches from above and below the knots, no spore or hypha were visible with the light microscope. However, SEM images showed strong evidence that the fungus is capable of migrating to adjacent vessels in the susceptible plum genotype. In fact, at that distance below and above the knots, conidia were detected inside xylem vessels suggesting a systemic movement of the fungus that has not been shown so far. No symptoms were observed in the resistant genotype. Starch granules, vessel occlusions, and lipid droplets were the main distinguishable characteristics between susceptible and tolerant varieties.

Genetic Dissection of the Erwinia amylovora Disease Cycle

Fire blight, caused by the bacterial phytopathogen Erwinia amylovora, is an economically important and mechanistically complex disease that affects apple and pear production in most geographic production hubs worldwide. We compile, assess, and present a genetic outlook on the progression of an E. amylovora infection in the host. We discuss the key aspects of type III secretion-mediated infection and systemic movement, biofilm formation in xylem, and pathogen dispersal via ooze droplets, a concentrated suspension of bacteria and exopolysaccharide components. We present an overall outlook on the genetic elements contributing to E. amylovora pathogenesis, including an exploration of the impact of floral microbiomes on E. amylovora colonization, and summarize the current knowledge of host responses to an incursion and how this response stimulates further infection and systemic spread. We hope to facilitate the identification of new, unexplored areas of research in this pathosystem that can help identify evolutionarily susceptible genetic targets to ultimately aid in the design of sustainable strategies for fire blight disease mitigation.

Negative modulation of SA signaling components by the capsid protein of tobacco mosaic virus is required for viral long-distance movement

An important aspect of plant-virus interaction is the way viruses dynamically move over long distances and how plant immunity modulates viral systemic movement. Salicylic acid (SA), a well-characterized hormone responsible for immune responses against virus, is activated through different transcription factors including TGA and WRKY. In tobamoviruses, evidence suggests that capsid protein (CP) is required for long-distance movement, although its precise role has not been fully characterized yet. Previously, we showed that the CP of Tobacco Mosaic Virus (TMV)-Cg negatively modulates the SA-mediated defense. In this study, we analyzed the impact of SA-defense mechanism on the long-distance transport of a truncated version of TMV (TMV ∆CP virus) that cannot move to systemic tissues. The study showed that the negative modulation of NPR1 and TGA10 factors allows the long-distance transport of TMV ∆CP virus. Moreover, we observed that the stabilization of DELLA proteins promotes TMV ∆CP systemic movement. We also characterized a group of genes, part of a network modulated by CP, involved in TMV ∆CP long-distance transport. Altogether, our results indicate that CP-mediated downregulation of SA signaling pathway is required for the virus systemic movement, and this role of CP may be linked to its ability to stabilize DELLA proteins.

Comparative analysis identifies amino acids critical for citrus tristeza virus (T36CA) encoded proteins involved in suppression of RNA silencing and differential systemic infection in two plant species

Complementary (c)DNA clones corresponding to the full-length genome of T36CA (a Californian isolate of Citrus tristeza virus with the T36 genotype), which shares 99.1% identity with that of T36FL (a T36 isolate from Florida), were made into a vector system to express the green fluorescent protein (GFP). Agroinfiltration of two prototype T36CA-based vectors (pT36CA) to Nicotiana benthamiana plants resulted in local but not systemic GFP expression/viral infection. This contrasted with agroinfiltration of the T36FL-based vector (pT36FL), which resulted in both local and systemic GFP expression/viral infection. A prototype T36CA systemically infected RNA silencing-defective N. benthamiana lines, demonstrating that a genetic basis for its defective systemic infection was RNA silencing. We evaluated the in planta bioactivity of chimeric pT36CA-pT36FL constructs and the results suggested that nucleotide variants in several open reading frames of the prototype T36CA could be responsible for its defective systemic infection. A single amino acid substitution in each of two silencing suppressors, p20 (S107G) and p25 (G36D), of prototype T36CA facilitated its systemic infectivity in N. benthamiana (albeit with reduced titre relative to that of T36FL) but not in Citrus macrophylla plants. Enhanced virus accumulation and, remarkably, robust systemic infection of T36CA in N. benthamiana and C. macrophylla plants, respectively, required two additional amino acid substitutions engineered in p65 (N118S and S158L), a putative closterovirus movement protein. The availability of pT36CA provides a unique opportunity for comparative analysis to identify viral coding and noncoding nucleotides or sequences involved in functions that are vital for in planta infection.

Amino acids at the exposed C-terminus of the S coat protein of cowpea mosaic virus play different roles in particle formation and viral systemic movement

The icosahedral capsid of cowpea mosaic virus is formed by 60 copies of the large (L) and small (S) coat protein subunits. The 24-amino-acid C-terminal peptide of the S coat protein can undergo proteolytic cleavage without affecting particle stability or infectivity. Mutagenic studies have shown that this sequence is involved in particle assembly, virus movement, RNA encapsidation and suppression of gene silencing. However, it is unclear how these processes are related, and which part(s) of the sequence are involved in each process. Here, we have analysed the effect of mutations in the C-terminal region of the S protein on the assembly of empty virus-like particles and on the systemic movement of infectious virus. The results confirmed the importance of positively charged amino acids adjacent to the cleavage site for particle assembly and revealed that the C-terminal 11 amino acids are important for efficient systemic movement of the virus.

A Viral Noncoding RNA Complements a Weakened Viral RNA Silencing Suppressor and Promotes Efficient Systemic Host Infection

Systemic movement of beet necrotic yellow vein virus (BNYVV) in Beta macrocarpa depends on viral RNA3, whereas in Nicotiana benthamiana this RNA is dispensable. RNA3 contains a coremin motif of 20 nucleotides essential for the stabilization of noncoding RNA3 (ncRNA3) and for long-distance movement in Beta species. Coremin mutants that are unable to accumulate ncRNA3 also do not achieve systemic movement in Beta species. A mutant virus carrying a mutation in the p14 viral suppressor of RNA silencing (VSR), unable to move long distances, can be complemented with the ncRNA3 in the lesion phenotype, viral RNA accumulation, and systemic spread. Analyses of the BNYVV VSR mechanism of action led to the identification of the RNA-dependent RNA polymerase 6 (RDR6) pathway as a target of the virus VSR and the assignment of a VSR function to the ncRNA3.

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.

Rice stripe tenuivirus p2 may recruit or manipulate nucleolar functions through an interaction with fibrillarin to promote virus systemic movement

Rice stripe virus (RSV) is the type species of the genus Tenuivirus and represents a major viral pathogen affecting rice production in East Asia. In this study, RSV p2 was fused to yellow fluorescent protein (p2-YFP) and expressed in epidermal cells of Nicotiana benthamiana. p2-YFP fluorescence was found to move to the nucleolus initially, but to leave the nucleolus for the cytoplasm forming numerous distinct bright spots there at later time points. A bimolecular fluorescence complementation (BiFC) assay showed that p2 interacted with fibrillarin and that the interaction occurred in the nucleus. Both the nucleolar localization and cytoplasmic distribution of p2-YFP fluorescence were affected in fibrillarin-silenced N. benthamiana. Fibrillarin depletion abolished the systemic movement of RSV, but not that of Tobacco mosaic virus (TMV) and Potato virus X (PVX). A Tobacco rattle virus (TRV)-based virus-induced gene silencing (VIGS) method was used to diminish RSV NS2 (encoding p2) or NS3 (encoding p3) during RSV infection. Silencing of NS3 alleviated symptom severity and reduced RSV accumulation, but had no obvious effects on virus movement and the timing of symptom development. However, silencing of NS2 abolished the systemic movement of RSV. The possibility that RSV p2 may recruit or manipulate nucleolar functions to promote virus systemic infection is discussed.

Persistence and transgenerational effect of plant-mediated RNAi in aphids

Plant-mediated RNA interference (RNAi) has been successfully used as a tool to study gene function in aphids. The persistence and transgenerational effects of plant-mediated RNAi in the green peach aphid (GPA) Myzus persicae were investigated, with a focus on three genes with different functions in the aphid. Rack1 is a key component of various cellular processes inside aphids, while candidate effector genes MpC002 and MpPIntO2 (Mp2) modulate aphid-plant interactions. The gene sequences and functions did not affect RNAi-mediated down-regulation and persistence levels in the aphids. Maximal reduction of gene expression was ~70% and this was achieved at between 4 d and 8 d of exposure of the aphids to double-stranded RNA (dsRNA)-producing transgenic Arabidopsis thaliana. Moreover, gene expression levels returned to wild-type levels within ~6 d after removal of the aphids from the transgenic plants, indicating that a continuous supply of dsRNA is required to maintain the RNAi effect. Target genes were also down-regulated in nymphs born from mothers exposed to dsRNA-producing transgenic plants, and the RNAi effect lasted twice as long (12-14 d) in these nymphs. Investigations of the impact of RNAi over three generations of aphids revealed that aphids reared on dsMpC002 transgenic plants experienced a 60% decline in aphid reproduction levels compared with a 40% decline of aphids reared on dsRack1 and dsMpPIntO2 plants. In a field setting, a reduction of the aphid reproduction by 40-60% would dramatically decrease aphid population growth, contributing to a substantial reduction in agricultural losses.

The origin and effect of small RNA signaling in plants

Given their sessile condition, land plants need to integrate environmental cues rapidly and send signal throughout the organism to modify their metabolism accordingly. Small RNA (sRNA) molecules are among the messengers that plant cells use to carry such signals. These molecules originate from fold-back stem-loops transcribed from endogenous loci or from perfect double-stranded RNA produced through the action of RNA-dependent RNA polymerases. Once produced, sRNAs associate with Argonaute (AGO) and other proteins to form the RNA-induced silencing complex (RISC) that executes silencing of complementary RNA molecules. Depending on the nature of the RNA target and the AGO protein involved, RISC triggers either DNA methylation or chromatin modification (leading to transcriptional gene silencing, TGS) or RNA cleavage or translational inhibition (leading to post-transcriptional gene silencing, PTGS). In some cases, sRNAs move to neighboring cells and/or to the vascular tissues for long-distance trafficking. Many genes are involved in the biogenesis of sRNAs and recent studies have shown that both their origin and their protein partners have great influence on their activity and range. Here we summarize the work done to uncover the mode of action of the different classes of sRNA with special emphasis on their movement and how plants can take advantage of their mobility. We also review the various genetic requirements needed for production, movement and perception of the silencing signal.

Resistance to Cucurbit yellow stunting disorder virus in Cucumber

Three hundred accessions of Cucumis sativus, including wild cucumbers, land races, traditional cultivars, and breeding lines, were evaluated under natural-infection conditions in order to identify potential sources of resistance to Cucurbit yellow stunting disorder virus (CYSDV). Although 100% of the susceptible control plants showed typical yellowing symptoms induced by CYSDV, another 24 C. sativus accessions showed partial or total absence of yellowing symptoms. In contrast, when CYSDV inoculation was carried out under controlled conditions, only two (A1 and A2) of these 24 accessions showed resistance to the virus. The nature of the resistance found in A1 and A2 plants was characterized by studying the pattern of virus accumulation and symptom development under controlled infection conditions, and by analyzing the possible nonpreference of Bemisia tabaci for these accessions under free-choice conditions. There was a delay in the establishment of the CYSDV infection in A1 and A2 plants which was evident shortly after inoculation and in apical leaves of the plants at long times after inoculation. Symptom severity was also less for A1 and A2 than for a susceptible control at 8 and 12 weeks postinoculation. Thus, delayed viral infection appeared to be associated with decreased symptom severity in A1 and A2 plants. Our results also showed nonpreference for plants of the A2 accession by B. tabaci, the CYSDV vector.

Fate of Pierce's Disease Strains of Xylella fastidiosa in Common Riparian Plants in California

The fate of strains of the bacterium Xylella fastidiosa that cause Pierce's disease of grapevines was investigated in 33 species of mostly perennial plants common in riparian habitats in northern coastal California grape-growing regions. Plants were inoculated in the field with needle puncture using cultured cells of X. fastidiosa as inoculum or inoculated in the laboratory with infective insect vectors (Graphocephala atropunctata). Populations of X. fastidiosa were highest in most plant species within 3 to 6 weeks of inoculation, followed by declines in populations of viable bacteria over the next 3 to 4 months. Homogenates of petioles of California black walnut (Juglans hindsii) and coffeeberry (Rhamnus californica) inhibited in vitro growth of X. fas-tidiosa, precluding culture of the bacterium from these plants. Big leaf maple (Acer macrophyl-lum), California buckeye (Aesculus californica), California blackberry (Rubus ursinus), coast live oak (Quercus agrifolia), elderberry (Sambucus mexicana), French broom (Genista monspessulanus), periwinkle (Vinca major), valley oak (Quercus lobata), and the grape root-stock Vitis rupestris supported systemic populations of X. fastidiosa that survived throughout the year outdoors in Napa Valley, California.