Cross-Protection as Control Strategy Against Grapevine fanleaf virus in Naturally Infected Vineyards

Virulent strains of Zymoseptoria tritici suppress the host immune response and facilitate the success of avirulent strains in mixed infections

Plants interact with a plethora of pathogenic microorganisms in nature. Pathogen-plant interaction experiments focus mainly on single-strain infections, typically ignoring the complexity of multi-strain infections even though mixed infections are common and critical for the infection outcome. The wheat pathogen Zymoseptoria tritici forms highly diverse fungal populations in which several pathogen strains often colonize the same leaf. Despite the importance of mixed infections, the mechanisms governing interactions between a mixture of pathogen strains within a plant host remain largely unexplored. Here we demonstrate that avirulent pathogen strains benefit from being in mixed infections with virulent strains. We show that virulent strains suppress the wheat immune response, allowing avirulent strains to colonize the apoplast and to reproduce. Our experiments indicate that virulent strains in mixed infections can suppress the plant immune system, probably facilitating the persistence of avirulent pathogen strains in fields planted with resistant host plants.

Traditional Approaches and Emerging Biotechnologies in Grapevine Virology

Environmental changes and global warming may promote the emergence of unknown viruses, whose spread is favored by the trade in plant products. Viruses represent a major threat to viticulture and the wine industry. Their management is challenging and mostly relies on prophylactic measures that are intended to prevent the introduction of viruses into vineyards. Besides the use of virus-free planting material, the employment of agrochemicals is a major strategy to prevent the spread of insect vectors in vineyards. According to the goal of the European Green Deal, a 50% decrease in the use of agrochemicals is expected before 2030. Thus, the development of alternative strategies that allow the sustainable control of viral diseases in vineyards is strongly needed. Here, we present a set of innovative biotechnological tools that have been developed to induce virus resistance in plants. From transgenesis to the still-debated genome editing technologies and RNAi-based strategies, this review discusses numerous illustrative studies that highlight the effectiveness of these promising tools for the management of viral infections in grapevine. Finally, the development of viral vectors from grapevine viruses is described, revealing their positive and unconventional roles, from targets to tools, in emerging biotechnologies.

Potential Implications and Management of Grapevine Viruses in Mexico: A Review

Worldwide, virus infections in grapevines are of concern due to the potential for economic loss. Although the grape industry in Mexico is relatively small and focused mainly on the local market, production dates back to the time of the Spanish colonization. This manuscript discusses the findings on grapevine viruses in Mexico. Nine viruses have been identified in the last fifty years, including grapevine red blotch virus (GRBV), grapevine leafroll-associated virus 3 (GLRaV-3), grapevine fanleaf virus (GFLV), and grapevine virus A (GVA). Important information is provided about these viruses and viral pathogens that have not yet been reported in Mexico, but represent an ongoing threat to plant health and grapevine production in other viticultural regions of the world. Strategies for virus control in vineyards are described. The information discussed here should be shared with growers and stakeholders to prevent future negative impacts on the Mexican grapevine industry and to save ancient grapevine accessions.

Characterization of Grapevine Fanleaf Virus Isolates in ‘Chardonnay’ Vines Exhibiting Severe and Mild Symptoms in Two Vineyards

Fanleaf degeneration is a complex viral disease of Vitis spp. that detrimentally impacts fruit yield and reduces the productive lifespan of most vineyards worldwide. In France, its main causal agent is grapevine fanleaf virus (GFLV). In the past, field experiments were conducted to explore cross-protection as a management strategy of fanleaf degeneration, but results were unsatisfactory because the mild virus strain negatively impacted fruit yield. In order to select new mild GFLV isolates, we examined two old ‘Chardonnay’ parcels harbouring vines with distinct phenotypes. Symptoms and agronomic performances were monitored over the four-year study on 21 individual vines that were classified into three categories: asymptomatic GFLV-free vines, GFLV-infected vines severely diseased and GFLV-infected vines displaying mild symptoms. The complete coding genomic sequences of GFLV isolates in infected vines was determined by high-throughput sequencing. Most grapevines were infected with multiple genetically divergent variants. While no specific molecular features were apparent for GFLV isolates from vines displaying mild symptoms, a genetic differentiation of GFLV populations depending on the vineyard parcel was observed. The mild symptomatic grapevines identified during this study were established in a greenhouse to recover GFLV variants of potential interest for cross-protection studies.

Biotechnology of virus eradication and plant vaccination in phytobiome context

A plant's associated biota plays an integral role in its metabolism, nutrient uptake, stress tolerance, pathogen resistance and other physiological processes. Although a virome is an integral part of the phytobiome, a major contradiction exists between the holobiont approach and the practical need to eradicate pathogens from agricultural crops. In this review, we discuss grapevine virus control, but the issue is also relevant for numerous other crops, including potato, cassava, citrus, cacao and other species. Grapevine diseases, especially viral infections, cause main crop losses. Methods have been developed to eliminate viruses and other microorganisms from plant material, but elimination of viruses from plant material does not guarantee protection from future reinfection. Elimination of viral particles in plant material could create genetic drift, leading in turn to an increase in the occurrence of pathogenic strains of viruses. A possible solution may be a combination of virus elimination and plant propagation in tissue culture with in vitro vaccination. In this context, possible strategies to control viral infections include application of plant resistance inducers, cross protection and vaccination using siRNA, dsRNA and viral replicons during plant 'cleaning' and in vitro propagation. The experience and knowledge accumulated in human immunization can help plant scientists to develop and employ new methods of protection, leading to more sustainable and healthier crop production.

A single resistance factor to solve vineyard degeneration due to grapevine fanleaf virus

Grapevine fanleaf disease, caused by grapevine fanleaf virus (GFLV), transmitted by the soil-borne nematode Xiphinema index, provokes severe symptoms and economic losses, threatening vineyards worldwide. As no effective solution exists so far to control grapevine fanleaf disease in an environmentally friendly way, we investigated the presence of resistance to GFLV in grapevine genetic resources. We discovered that the Riesling variety displays resistance to GFLV, although it is susceptible to X. index. This resistance is determined by a single recessive factor located on grapevine chromosome 1, which we have named rgflv1. The discovery of rgflv1 paves the way for the first effective and environmentally friendly solution to control grapevine fanleaf disease through the development of new GFLV-resistant grapevine rootstocks, which was hitherto an unthinkable prospect. Moreover, rgflv1 is putatively distinct from the virus susceptibility factors already described in plants.

A New Era for Mild Strain Cross-Protection

Societal and environmental pressures demand high-quality and resilient cropping plants and plant-based foods grown with the use of low or no synthetic chemical inputs. Mild strain cross-protection (MSCP), the pre-immunization of a plant using a mild strain of a virus to protect against subsequent infection by a severe strain of the virus, fits with future-proofing of production systems. New examples of MSCP use have occurred recently. New technologies are converging to support the discovery and mechanism(s) of action of MSCP strains thereby accelerating the popularity of their use.

Grapevine fanleaf virus affects grape (Vitis vinifera) berry anthocyanin content via the transcriptional regulation of anthocyanin biosynthetic genes

Grapevine fanleaf virus (GFLV) causes grapevine fanleaf degeneration, one of the oldest known viral diseases of grapevines. The virus has been found in all winegrowing regions around the world. In the seasons 2011-12 a comparison between field grown GFLV-infected and healthy grapevines was conducted for the cultivars Schioppettino in North-Eastern Italy and Refošk in South-Western Slovenia. Our research showed that GFLV infection caused a drop of the yield due to reduction of both cluster weight and berry weight. Besides the yield, the berry composition was also affected; in detail, anthocyanin concentration increased in both varieties but significantly only in the case of Schioppettino. Upregulation of the F3'5'H gene and downregulation of F3'H gene in the berries of GFLV infected vines compared with the ones of healthy control vines resulted in modified proportions between di- and tri- hydroxylated or methylated derivatives of anthocyanins. The F3H1 gene was identified to be the most strongly regulated gene of the flavonoid biosynthetic pathway by GFLV infection, indicating its important role in increasing anthocyanin concentration in grapes of GFLV infected vines as compared with healthy controls.

Genetic structure and molecular variability of Grapevine fanleaf virus populations

To gain insights into the evolutionary mechanisms of Grapevine fanleaf virus (GFLV) from the genus Nepovirus, family Secoviridae, the sequences of the complete coding region of RNA2, including genes 2A(HP), 2B(MP) and 2C(CP), and partial sequence from the RNA1-encoded gene 1E(Pol) of 14 GFLV isolates from three naturally infected California vineyards were characterized. Phylogenetic analyses suggested two to three evolutionarily divergent lineages that did not reflect the vineyard origin of the isolates or an association with rootstock genotype or scion cultivar. Examination of the genetic variability of the California isolates alongside isolates worldwide, for which three RNA1 and 44 RNA2 coding sequences are available, revealed similar patterns of molecular evolution for the different regions within the GFLV genome but distinct selection constraints with the strongest pressure exerted on genes 2C(CP) and 2B(MP), an intermediate level of pressure exerted on gene 1E(Pol), and the weakest pressure exerted on gene 2A(HP). Some of the California isolates resulted from interspecies recombination events between GFLV and Arabis mosaic virus with crossover sites suspected in gene 1E(Pol) and identified in genes 2A(HP) and 2B(MP); and intraspecies recombination events inferred in the four target genes but most frequently observed within gene 2C(CP). This study suggested that purifying selection and recombination are important evolutionary mechanisms in the genetic diversification of GFLV.

Cross-protection: a century of mystery

Cross-protection is a phenomenon in which infection of a plant with a mild virus or viroid strain protects it from disease resulting from a subsequent encounter with a severe strain of the same virus or viroid. In this chapter, we review the history of cross-protection with regard to the development of ideas concerning its likely mechanisms, including RNA silencing and exclusion, and its influence on the early development of genetically engineered virus resistance. We also examine examples of the practical use of cross-protection in averting crop losses due to viruses, as well as the use of satellite RNAs to ameliorate the impact of virus-induced diseases. We also discuss the potential of cross-protection to contribute in future to the maintenance of crop health in the face of emerging virus diseases and related threats to agricultural production.

Genetic structure and variability of virus populations in cross-protected grapevines superinfected by Grapevine fanleaf virus

Recombination was assessed in a vineyard site in which grapevines cross-protected with mild strains GHu of Grapevine fanleaf virus (GFLV) or Ta of Arabis mosaic virus (ArMV) were superinfected with GFLV field isolates following transmission by the nematode vector Xiphinema index. The genetic structure and variability within RNA2 of isolates from grapevines co-infected with GFLV field isolates and either GFLV-GHu or ArMV-Ta were characterized to identify intra- and interspecies recombinants. Sequence analysis and phylogenetic relationships inferred intraspecies recombination among GFLV field isolates but not between field isolates and GFLV-GHu. SISCAN analysis confirmed a mosaic structure for two GFLV field isolates for which recombination sites were located in the movement protein and coat protein genes. One of the recombinants was found in eight grapevines that were in close spatial proximity within the vineyard site, suggesting its transmission by X. index. No interspecies recombination was detected between GFLV field isolates and ArMV-Ta. Altogether, our findings suggest that mild protective strains GFLV-GHu and ArMV-Ta did not assist the emergence of viable recombinants to detectable level during a 12-year cross-protection trial. To our knowledge, this is the first extensive characterization of the genetic structure and variability of virus isolates in cross-protected plants.