cmv1 is a gate for Cucumber mosaic virus transport from bundle sheath cells to phloem in melon

cmv1-Mediated Resistance to CMV in Melon Can Be Overcome by Mixed Infections with Potyviruses

Resistance to cucumber mosaic virus (CMV) strain LS in melon is controlled by the gene cmv1, which restricts phloem entry. In nature, CMV is commonly found in mixed infections, particularly with potyviruses, where a synergistic effect is frequently produced. We have explored the possibility that this synergism could help CMV-LS to overcome cmv1-mediated resistance. We demonstrate that during mixed infection with a potyvirus, CMV-LS is able to overcome cmv1-controlled resistance and develop a systemic infection and that this ability does not depend on an increased accumulation of CMV-LS in mechanically inoculated cotyledons. Likewise, during a mixed infection initiated by aphids, the natural vector of both cucumoviruses and potyviruses that can very efficiently inoculate plants with a low number of virions, CMV-LS also overcomes cmv1-controlled resistance. This indicates that in the presence of a potyvirus, even a very low amount of inoculum, can be sufficient to surpass the resistance and initiate the infection. These results indicate that there is an important risk for this resistance to be broken in nature as a consequence of mixed infections, and therefore, its deployment in elite cultivars would not be enough to ensure a long-lasting resistance.

Mutations in CmVPS41 controlling resistance to cucumber mosaic virus display specific subcellular localization

Resistance to cucumber mosaic virus (CMV) in melon (Cucumis melo L.) has been described in several exotic accessions and is controlled by a recessive resistance gene, cmv1, that encodes a vacuolar protein sorting 41 (CmVPS41). cmv1 prevents systemic infection by restricting the virus to the bundle sheath cells, preventing viral phloem entry. CmVPS41 from different resistant accessions carries two causal mutations, either a G85E change, found in Pat-81 and Freeman's cucumber, or L348R, found in PI161375, cultivar Songwhan Charmi (SC). Here, we analyzed the subcellular localization of CmVPS41 in Nicotiana benthamiana and found differential structures in resistant and susceptible accessions. Susceptible accessions showed nuclear and membrane spots and many transvacuolar strands, whereas the resistant accessions showed many intravacuolar invaginations. These specific structures colocalized with late endosomes. Artificial CmVPS41 carrying individual mutations causing resistance in the genetic background of CmVPS41 from the susceptible variety Piel de Sapo (PS) revealed that the structure most correlated with resistance was the absence of transvacuolar strands. Coexpression of CmVPS41 with viral movement proteins, the determinant of virulence, did not change these localizations; however, infiltration of CmVPS41 from either SC or PS accessions in CMV-infected N. benthamiana leaves showed a localization pattern closer to each other, with up to 30% cells showing some membrane spots in the CmVPS41SC and fewer transvacuolar strands (reduced from a mean of 4 to 1-2) with CmVPS41PS. Our results suggest that the distribution of CmVPS41PS in late endosomes includes transvacuolar strands that facilitate CMV infection and that CmVPS41 re-localizes during viral infection.

An Evolved 5' Untranslated Region of Alfalfa Mosaic Virus Allows the RNA Transport of Movement-Defective Variants

Although the coat protein (CP) has a relevant role in the long-distance movement of alfalfa mosaic virus (AMV) and brome mosaic virus (BMV), its precise function is not fully understood. Previous results showed that a specific interaction between the C termini of the movement protein (MP) and the cognate CP is required for systemic transport. Thus, we have performed a compensatory evolution experiment using an AMV RNA3 derivative defective in long-distance transport that carries a BMV MP lacking the C-terminal 48 residues and unable to interact with the AMV CP. After several passages, five independent evolution lineages were able to move long distance. The analysis of the viral RNA of these lineages showed the presence of three different modifications located exclusively at the 5' untranslated region (5' UTR). The three evolved 5' UTR variants accumulated comparable levels of viral RNA and CP but reduced the accumulation of virus particles and the affinity between the 5' UTR and the AMV CP. In addition, the evolved 5' UTR increased cell-to-cell transport for both the AMV RNA3 carrying the BMV MP and that carrying the AMV MP. Finally, the evolved 5' UTRs allowed the systemic transport of an AMV RNA3 carrying a CP mutant defective in virus particles and increased the systemic transport of several AMV RNA3 derivatives carrying different viral MPs associated with the 30K superfamily. Altogether, our findings indicate that virus particles are not required for the systemic transport of AMV but also that BMV MP is competent for the short- and long-distance transport without the interaction with the CP. IMPORTANCE The results obtained in the present work could challenge the view of the role of the virus particle in the systemic transport of plant viruses. In this sense, we show that two different MPs are competent to systemically transport the AMV genome without the requirement of the virus particles, as reported for viruses lacking a CP (e.g., Umbravirus). The incapability of the viral MP to interact with the CP triggered virus variants that evolved to reduce the formation of virus particles, probably to increase the accessibility of the MP to the viral progeny. Our results point to the idea that virus particles would not be necessary for the viral systemic transport but would be necessary for vector virus transmission. This idea is reinforced by the observation that heterologous MPs also increased the systemic transport of the AMV constructs that have reduced encapsidation capabilities.

Purification and Proteomics Analysis of Phloem Tissues from Virus-Infected Plants

The plant phloem vasculature is crucial for plant growth and development, and is essential for the systemic movement (SM) of plant viruses. Recent transcriptomic studies of the phloem during virus infection have shown the importance of this tissue, yet transcript levels do not provide definitive answers how virus-host interactions favour successful viral SM. Proteomic analyses have been used to identify host-virus protein interactions, uncovering a variety of ways by which viruses utilize host cellular machinery for completion of the viral infection cycle. Despite this new evidence through proteomics, very few phloem centric studies during viral infection have been performed. Here, we describe a protocol for the isolation of phloem tissues and proteins and the subsequent label-free quantitation (LFQ), for identification of proteomic alterations caused by viral infection.

The m6A RNA Demethylase ALKBH9B Plays a Critical Role for Vascular Movement of Alfalfa Mosaic Virus in Arabidopsis

The N 6-methyladenosine (m6A) pathway has been widely described as a viral regulatory mechanism in animals. We previously reported that the capsid protein (CP) of alfalfa mosaic virus (AMV) interacts with the Arabidopsis m6A demethylase ALKBH9B regulating m6A abundance on viral RNAs (vRNAs) and systemic invasion of floral stems. Here, we analyze the involvement of other ALKBH9 proteins in AMV infection and we carry out a detailed evaluation of the infection restraint observed in alkbh9b mutant plants. Thus, via viral titer quantification experiments and in situ hybridization assays, we define the viral cycle steps that are altered by the absence of the m6A demethylase ALKBH9B in Arabidopsis. We found that ALKBH9A and ALKBH9C do not regulate the AMV cycle, so ALKBH9B activity seems to be highly specific. We also define that not only systemic movement is affected by the absence of the demethylase, but also early stages of viral infection. Moreover, our findings suggest that viral upload into the phloem could be blocked in alkbh9b plants. Overall, our results point to ALKBH9B as a possible new component of phloem transport, at least for AMV, and as a potential target to obtain virus resistance crops.

Imaging Techniques to Study Plant Virus Replication and Vertical Transmission

Plant viruses are obligate parasites that need to usurp plant cell metabolism in order to infect their hosts. Imaging techniques have been used for quite a long time to study plant virus-host interactions, making it possible to have major advances in the knowledge of plant virus infection cycles. The imaging techniques used to study plant-virus interactions have included light microscopy, confocal laser scanning microscopy, and scanning and transmission electron microscopies. Here, we review the use of these techniques in plant virology, illustrating recent advances in the area with examples from plant virus replication and virus plant-to-plant vertical transmission processes.

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.

Review: Plant-pathogen interactions through the plasmodesma prism

Plasmodesmata (PD) allow membrane and cytoplasmic continuity between plant cells, and they are essential for intercellular communication and signaling in addition to metabolite partitioning. Plant pathogens have evolved a variety of mechanisms to subvert PD to facilitate their infection of plant hosts. PD are implicated not only in local spread around infection sites but also in the systemic spread of pathogens and pathogen-derived molecules. In turn, plants have developed strategies to limit pathogen spread via PD, and there is increasing evidence that PD may also be active players in plant defense responses. The last few years have seen important advances in understanding the roles of PD in plant-pathogen infection. Nonetheless, several critical areas remain to be addressed. Here we highlight some of these, focusing on the need to consider the effects of pathogen-PD interaction on the trafficking of endogenous molecules, and the involvement of chloroplasts in regulating PD during pathogen defense. By their very nature, PD are recalcitrant to most currently used investigative techniques, therefore answering these questions will require creative imaging and novel quantification approaches.

CmVPS41 Is a General Gatekeeper for Resistance to Cucumber Mosaic Virus Phloem Entry in Melon

Melon production is often compromised by viral diseases, which cannot be treated with chemicals. Therefore, the use of genetic resistances is the main strategy for generating crops resistant to viruses. Resistance to Cucumber mosaic virus (CMV) in melon is scarcely described in few accessions. Until recently, the only known resistant accessions were Freeman's Cucumber and PI 161375, cultivar Songwhan Charmi (SC). Resistance to CMV in melon is recessive and generally oligogenic and quantitative. However, in SC, the resistance to CMV strains of subgroup II is monogenic, depending only on one gene, cmv1, which is able to stop CMV movement by restricting the virus to the bundle sheath cells and preventing a systemic infection. This restriction depends on the viral movement protein (MP). Chimeric viruses carrying the MP of subgroup II strains, like the strain LS (CMV-LS), are restricted in the bundle sheath cells, whereas those carrying MP from subgroup I, like the strain FNY (CMV-FNY), are able to overcome this restriction. cmv1 encodes a vacuolar protein sorting 41 (CmVPS41), a protein involved in the transport of cargo proteins from the Golgi to the vacuole through late endosomes. We have analyzed the variability of the gene CmVPS41 in a set of 52 melon accessions belonging to 15 melon groups, both from the spp melo and the spp agrestis. We have identified 16 different haplotypes, encoding 12 different CmVPS41 protein variants. Challenging members of all haplotypes with CMV-LS, we have identified nine new resistant accessions. The resistance correlates with the presence of two mutations, either L348R, previously found in the accession SC and present in other three melon genotypes, or G85E, present in Freeman's Cucumber and found also in four additional melon genotypes. Moreover, the new resistant accessions belong to three different melon horticultural groups, Conomon, Makuwa, and Dudaim. In the new resistant accessions, the virus was able to replicate and move cell to cell, but was not able to reach the phloem. Therefore, resistance to phloem entry seems to be a general strategy in melon controlled by CmVPS41. Finally, the newly reported resistant accessions broaden the possibilities for the use of genetic resistances in new melon breeding strategies.

Identification of Cucumber mosaic resistance 2 (cmr2) That Confers Resistance to a New Cucumber mosaic virus Isolate P1 (CMV-P1) in Pepper (Capsicum spp.)

Cucumber mosaic virus (CMV) is one of the most devastating phytopathogens of Capsicum. The single dominant resistance gene, Cucumber mosaic resistant 1 (Cmr1), that confers resistance to the CMV isolate P0 has been overcome by a new isolate (CMV-P1) after being deployed in pepper (Capsicum annuum) breeding for over 20 years. A recently identified Indian C. annuum cultivar, "Lam32," displays resistance to CMV-P1. In this study, we show that the resistance in "Lam32" is controlled by a single recessive gene, CMV resistance gene 2 (cmr2). We found that cmr2 conferred resistance to CMV strains including CMV-Korean, CMV-Fny, and CMV-P1, indicating that cmr2 provides a broad-spectrum type of resistance. We utilized two molecular mapping approaches to determine the chromosomal location of cmr2. Bulked segregant analysis (BSA) using amplified fragment-length polymorphism (AFLP) (BSA-AFLP) revealed one marker, cmvAFLP, located 16 cM from cmr2. BSA using the Affymetrix pepper array (BSA-Affy) identified a single-nucleotide polymorphism (SNP) marker (Affy4) located 2.3 cM from cmr2 on chromosome 8. We further screened a pepper germplasm collection of 4,197 accessions for additional CMV-P1 resistance sources and found that some accessions contained equivalent levels of resistance to that of "Lam32." Inheritance and allelism tests demonstrated that all the resistance sources examined contained cmr2. Our result thus provide genetic and molecular evidence that cmr2 is a single recessive gene that confers to pepper an unprecedented resistance to the dangerous new isolate CMV-P1 that had overcome Cmr1.

Exogenously applied dsRNA molecules deriving from the Zucchini yellow mosaic virus (ZYMV) genome move systemically and protect cucurbits against ZYMV

Zucchini yellow mosaic virus (ZYMV) causes serious damage in a large number of cucurbits, and control measures are necessary. Transgenic cucurbits expressing parts of the ZYMV genome have been shown to be resistant to the cognate virus. A non-transgenic approach involving the exogenous application of double-stranded RNA (dsRNA) has also been shown to induce resistance in tobacco against Cucumber mosaic virus (CMV) and Tobacco mosaic virus (TMV). In the present study, dsRNA molecules derived from the helper component-proteinase (HC-Pro) and coat protein (CP) genes of the ZYMV_DE_2014 isolate were produced in vitro. On exogenous dsRNA application in cucumber, watermelon and squash plants, dsRNA HC-Pro conferred resistance of 82%, 50% and 18%, and dsRNA CP molecules of 70%, 43% and 16%, respectively. On deep sequencing analysis of ZYMV-infected watermelon, hot-spot regions for viral small interfering RNAs (vsiRNAs) in the genome of ZYMV were identified. Stem-loop reverse transcription-polymerase chain reaction (RT-PCR) detection of selected 21-nucleotide-long vsiRNAs in plants that received only dsRNA molecules suggested that the dsRNAs exogenously applied onto plants were successfully diced, thus initiating RNA silencing. dsRNA molecules were found to be progressively degraded in planta, and strongly detected by semi-quantitative RT-PCR for at least 9 days after exogenous application. Moreover, dsRNA molecules were detected in systemic tissue of watermelon and squash, showing that dsRNA is transported long distances in these plants.

A mutation in the melon Vacuolar Protein Sorting 41prevents systemic infection of Cucumber mosaic virus

In the melon exotic accession PI 161375, the gene cmv1, confers recessive resistance to Cucumber mosaic virus (CMV) strains of subgroup II. cmv1 prevents the systemic infection by restricting the virus to the bundle sheath cells and impeding viral loading to the phloem. Here we report the fine mapping and cloning of cmv1. Screening of an F2 population reduced the cmv1 region to a 132 Kb interval that includes a Vacuolar Protein Sorting 41 gene. CmVPS41 is conserved among plants, animals and yeast and is required for post-Golgi vesicle trafficking towards the vacuole. We have validated CmVPS41 as the gene responsible for the resistance, both by generating CMV susceptible transgenic melon plants, expressing the susceptible allele in the resistant cultivar and by characterizing CmVPS41 TILLING mutants with reduced susceptibility to CMV. Finally, a core collection of 52 melon accessions allowed us to identify a single amino acid substitution (L348R) as the only polymorphism associated with the resistant phenotype. CmVPS41 is the first natural recessive resistance gene found to be involved in viral transport and its cellular function suggests that CMV might use CmVPS41 for its own transport towards the phloem.