Novel Aspects on The Interaction Between Grapevine and Plasmopara viticola: Dual-RNA-Seq Analysis Highlights Gene Expression Dynamics in The Pathogen and The Plant During The Battle For Infection

CRISPR/Cas9 editing of Downy mildew resistant 6 (DMR6-1) in grapevine leads to reduced susceptibility to Plasmopara viticola

Downy mildew of grapevine (Vitis vinifera), caused by the oomycete Plasmopara viticola, is an important disease that is present in cultivation areas worldwide, and using resistant varieties provides an environmentally friendly alternative to fungicides. DOWNY MILDEW RESISTANT 6 (DMR6) from Arabidopsis is a negative regulator of plant immunity and its loss of function confers resistance to downy mildew. In grapevine, DMR6 is present in two copies, named VvDMR6-1 and VvDMR6-2. Here, we describe the editing of VvDMR6-1 in embryogenic calli using CRISPR/Cas9 and the regeneration of the edited plants. All edited plants were found to be biallelic and chimeric, and whilst they all showed reduced growth compared with non-transformed control plants, they also had reduced susceptibility to P. viticola. Comparison between mock-inoculated genotypes showed that all edited lines presented higher levels of salicylic acid than controls, and lines subjected to transformation presented higher levels of cis-resveratrol than controls. Our results identify VvDMR6-1 as a promising target for breeding grapevine cultivars with improved resistance to downy mildew.

Novel loci associated with resistance to downy and powdery mildew in grapevine

Among the main challenges in current viticulture, there is the increasing demand for sustainability in the protection from fungal diseases, such as downy mildew (DM) and powdery mildew (PM). Breeding disease-resistant grapevine varieties is a key strategy for better managing fungicide inputs. This study explores the diversity of grapevine germplasm (cultivated and wild) from Caucasus and neighboring areas to identify genotypes resistant to DM and PM, based on 13 Simple Sequence Repeat (SSR) loci and phenotypical (artificial pathogen inoculation) analysis, and to identify loci associated with DM and PM resistance, via Genome-Wide Association Analysis (GWAS) on Single Nucleotide Polymorphism (SNP) profiles. SSR analysis revealed resistant alleles for 16 out of 88 genotypes. Phenotypic data identified seven DM and 31 PM resistant genotypes. GWAS identified two new loci associated with DM resistance, located on chromosome 15 and 16 (designated as Rpv36 and Rpv37), and two with PM resistance, located on chromosome 6 and 17 (designated as Ren14 and Ren15). The four novel loci identified genomic regions rich in genes related to biotic stress response, such as genes involved in pathogen recognition, signal transduction and resistance response. This study highlights potential candidate genes associated with resistance to DM and PM, providing valuable insights for breeding programs for resistant varieties. To optimize their utilization, further functional characterization studies are recommended.

The Interactions between Two Fungal Endophytes Epicoccum layuense R2-21 and Alternaria alternata XHYN2 and Grapevines (Vitis vinifera) with De Novo Established Symbionts under Aseptic Conditions

In this study, we focused on grapevine-endophyte interactions and reprogrammed secondary metabolism in the host plant due to defense against the colonization of endophytes. Thus, the transcriptional responses of tissue cultured grapevine seedlings (Vitis vinifera L. cv.: Cabernet Sauvignon) to two fungal endophytes Epicoccum layuense R2-21 (Epi R2-21) and Alternaria alternata XHYN2 (Alt XHYN2) at three different time points (6 h, 6 d, 15 d) were analyzed. As expected, a total of 5748 and 5817 differentially expressed genes (DEGs) were separately initiated in Epi R2-21 and Alt XHYN2 symbiotic tissue cultured seedlings compared to no endophyte treatment. The up-regulated DEGs at all time points in Epi R2-21- or Alt XHYN2-treated seedlings were mainly enriched in the flavonoid biosynthesis, phenylpropanoid biosynthesis, phenylalanine metabolism, stilbenoid, diarylheptanoid and gingerol biosynthesis, and circadian rhythm-plant pathways. In addition, the up-regulated DEGs at all sampling times in Alt XHYN2-treated tissue cultured seedlings were enriched in the plant-pathogen interaction pathway, but appeared in Epi R2-21 symbiotic seedlings only after 15 d of treatment. The down-regulated DEGs were not enriched in any KEGG pathways after 6 h inoculation for Epi R2-21 and Alt XHYN2 treatments, but were enriched mainly in photosynthesis-antenna proteins and plant hormone signal transduction pathways at other sampling times. At three different time points, a total of 51 DEGs (all up-regulated, 1.33-10.41-fold) were involved in secondary metabolism, and 22 DEGs (all up-regulated, 1.01-8.40-fold) were involved in defense responses in endophytic fungi symbiotic tissue cultured seedlings. The protein-protein interaction (PPI) network demonstrated that genes encoding CHS (VIT_10s0042g00920, VIT_14s0068g00920, and VIT_16s0100g00910) and the VIT_11s0065g00350 gene encoding CYP73A mediated the defense responses, and might induce more defense-associated metabolites. These results illustrated the activation of stress-associated secondary metabolism in the host grapevine during the establishment of fungi-plant endophytism. This work provides avenues for reshaping the qualities and characteristics of wine grapes utilizing specific endophytes and better understanding plant-microbe interactions.

Unravelling molecular mechanisms involved in resistance priming against downy mildew (Plasmopara viticola) in grapevine (Vitis vinifera L.)

Downy mildew (DM; Plasmopara viticola) is amongst the most severe fungal diseases in viticulture and the reason for the majority of fungicide applications. To reduce synthetic and copper-based fungicides, there is an urgent need for natural alternatives, which are being increasingly tested by the industry and the research community. However, their mode of action remains unclear. Therefore, our study aimed to investigate the transcriptomic changes induced by oregano essential oil vapour (OEOV) in DM-infected grapevines. OEOV was applied at different time points before and after DM infection to differentiate between a priming effect and a direct effect. Both pre-DM treatment with OEOV and post-infection treatment resulted in a significant reduction in DM sporulation. RNA-seq, followed by differential gene expression and weighted gene co-expression network analysis, identified co-expressed gene modules associated with secondary metabolism, pathogen recognition and response. Surprisingly, the molecular mechanisms underlying the efficiency of OEOV against DM appear to be independent of stilbene synthesis, and instead involve genes from a putative signalling pathway that has yet to be characterized. This study enhances our understanding of the molecular regulation of innate plant immunity and provides new insights into the mode of action of alternative natural antifungal agents.

Looking beyond Virus Detection in RNA Sequencing Data: Lessons Learned from a Community-Based Effort to Detect Cellular Plant Pathogens and Pests

High-throughput sequencing (HTS), more specifically RNA sequencing of plant tissues, has become an indispensable tool for plant virologists to detect and identify plant viruses. During the data analysis step, plant virologists typically compare the obtained sequences to reference virus databases. In this way, they are neglecting sequences without homologies to viruses, which usually represent the majority of sequencing reads. We hypothesized that traces of other pathogens might be detected in this unused sequence data. In the present study, our goal was to investigate whether total RNA-seq data, as generated for plant virus detection, is also suitable for the detection of other plant pathogens and pests. As proof of concept, we first analyzed RNA-seq datasets of plant materials with confirmed infections by cellular pathogens in order to check whether these non-viral pathogens could be easily detected in the data. Next, we set up a community effort to re-analyze existing Illumina RNA-seq datasets used for virus detection to check for the potential presence of non-viral pathogens or pests. In total, 101 datasets from 15 participants derived from 51 different plant species were re-analyzed, of which 37 were selected for subsequent in-depth analyses. In 29 of the 37 selected samples (78%), we found convincing traces of non-viral plant pathogens or pests. The organisms most frequently detected in this way were fungi (15/37 datasets), followed by insects (13/37) and mites (9/37). The presence of some of the detected pathogens was confirmed by independent (q)PCRs analyses. After communicating the results, 6 out of the 15 participants indicated that they were unaware of the possible presence of these pathogens in their sample(s). All participants indicated that they would broaden the scope of their bioinformatic analyses in future studies and thus check for the presence of non-viral pathogens. In conclusion, we show that it is possible to detect non-viral pathogens or pests from total RNA-seq datasets, in this case primarily fungi, insects, and mites. With this study, we hope to raise awareness among plant virologists that their data might be useful for fellow plant pathologists in other disciplines (mycology, entomology, bacteriology) as well.

The Combination of Both Heat and Water Stresses May Worsen Botryosphaeria Dieback Symptoms in Grapevine

(1) Background: Grapevine trunk diseases (GTDs) have become a global threat to vineyards worldwide. These diseases share three main common features. First, they are caused by multiple pathogenic micro-organisms. Second, these pathogens often maintain a long latent phase, which makes any research in pathology and symptomatology challenging. Third, a consensus is raising to pinpoint combined abiotic stresses as a key factor contributing to disease symptom expression. (2) Methods: We analyzed the impact of combined abiotic stresses in grapevine cuttings artificially infected by two fungi involved in Botryosphaeria dieback (one of the major GTDs), Neofusicoccum parvum and Diplodia seriata. Fungal-infected and control plants were subjected to single or combined abiotic stresses (heat stress, drought stress or both). Disease intensity was monitored thanks to the measurement of necrosis area size. (3) Results and conclusions: Overall, our results suggest that combined stresses might have a stronger impact on disease intensity upon infection by the less virulent pathogen Diplodia seriata. This conclusion is discussed through the impact on plant physiology using metabolomic and transcriptomic analyses of leaves sampled for the different conditions.

Influence of Nitrogen on Grapevine Susceptibility to Downy Mildew

Downy mildew, caused by the obligate parasite Plasmopara viticola, is one of the most important threats to viticulture. The exploitation of resistant and susceptibility traits of grapevine is one of the most promising ways to increase the sustainability of disease management. Nitrogen (N) fertilization is known for influencing disease severity in the open field, but no information is available on its effect on plant-pathogen interaction. A previous RNAseq study showed that several genes of N metabolism are differentially regulated in grapevine upon P. viticola inoculation, and could be involved in susceptibility or resistance to the pathogen. The aim of this study was to evaluate if N fertilization influences: (i) the foliar leaf content and photosynthetic activity of the plant, (ii) P. viticola infectivity, and (iii) the expression of the candidate susceptibility/resistance genes. Results showed that N level positively correlated with P. viticola infectivity, confirming that particular attention should be taken in vineyard to the fertilization, but did not influence the expression of the candidate genes. Therefore, these genes are manipulated by the pathogen and can be exploited for developing new, environmentally friendly disease management tools, such as dsRNAs, to silence the susceptibility genes or breeding for resistance.

Hyperspectral mapping of the response of grapevine cultivars to Plasmopara viticola infection at the tissue scale

Resistance of grapevine to Plasmopara viticola is associated with the hypersensitive reaction, accumulation of stilbenoids, and formation of callose depositions. Spectral characterization of infected leaf tissue of cvs 'Regent' and 'Solaris' with resistance genes Rpv 3-1 and Rpv 10 and Rpv 3-3, respectively, suggested that resistance is not dependent on large-scale necrotization of host tissue. Reactions of the resistant cultivars and a reference susceptible to P. viticola were studied using hyperspectral imaging (range 400-1000 nm) at the tissue level and microscopic techniques. Resistance of both cultivars was incomplete and allowed pathogen reproduction. Spectral vegetation indices characterized the host response to pathogen invasion; the vitality of infected and necrotic leaf tissue differed significantly. Resistance depended on local accumulation of polyphenols in response to haustorium formation and was more effective for cv. 'Solaris'. Although hypersensitive reaction of some cells prevented colonization of palisade parenchyma, resistance was not associated with extensive necrotization of tissue, and the biotrophic pathogen survived localized death of penetrated host cells. Hyperspectral imaging was suitable to characterize and differentiate the resistance reactions of grapevine cultivars by mapping of the cellular response to pathogen attack on the tissue level and yields useful information on host-pathogen interactions.

Pathogen Adaptation to American (Rpv3-1) and Eurasian (Rpv29) Grapevine Loci Conferring Resistance to Downy Mildew

Durable resistance is a key objective in genetic improvement for disease resistance in grapevines, which must survive for years in the field in the presence of adaptable pathogen populations. In this study, the adaptation of 72 Northern Italian isolates of Plasmopara viticola, the downy mildew agent, has been investigated into Bianca, possessing Rpv3-1, the most frequently exploited resistance locus for genetic improvement, and Mgaloblishvili, a Vitis vinifera variety possessing the newly discovered Rpv29 locus. Infection parameters (latency period, infection frequency, and disease severity) and oospore production and viability were evaluated and compared to those of Pinot noir, the susceptible reference. The expected levels of disease control were achieved by both resistant cultivars (>90% on Bianca; >25% on Mgaloblishvili), despite the high frequency of isolates able to grow on one (28%) or both (46%) accessions. The disease incidence and severity were limited by both resistant cultivars and the strains able to grow on resistant accessions showed signatures of fitness penalties (reduced virulence, infection frequency, and oospore density). Together, these results indicate an adequate pathogen control but suitable practices must be adopted in the field to prevent the diffusion of the partially adapted P. viticola strains to protect resistance genes from erosion.

Pathogenesis of Plasmopara viticola Depending on Resistance Mediated by Rpv3_1, and Rpv10 and Rpv3_3, and by the Vitality of Leaf Tissue

Grapevine cultivars vary in their resistance to Plasmopara viticola, causal agent of downy mildew. Genes from various Vitis species confer pathogen resistance (Rpv), resulting in reduced compatibility of the host-pathogen interaction and partial disease resistance that may become apparent at different stages of pathogenesis. This study describes the pathogenesis of P. viticola on the partially resistant cultivars Regent (Rpv3-1) and Solaris (Rpv3-3, Rpv10) as compared with the susceptible cultivar Mueller-Thurgau using various microscopic techniques, visual disease rating as well as qPCR. Host plant resistance had no effect on the initial steps of pathogenesis outside the host plant cells (zoospore attachment, formation of substomatal vesicle) and became detectable only after the formation of primary haustoria. The restricted compatibility resulted in reductions in haustorium size and in the number of secondary haustoria and was associated with callose depositions around haustoria and stomatal guard cells, collapsed mesophyll cells (hypersensitive reaction), and additional production of an amorphous substance in the intercellular space of cultivar Solaris. Resistance mechanisms reduced the efficiency of P. viticola haustoria and largely restricted tissue colonization to the spongy parenchyma; resistance of cultivar Solaris having thicker leaves was more effective than that of cultivar Regent. Despite of the effects of resistance genes, P. viticola was able to complete its life cycle by forming sporangiophores with sporangia through the stomata on both resistant cultivars indicating partial resistance. Differences in the pathogenesis on detached and attached grapevine leaves highlighted the impact of host tissue vitality on both resistance and susceptibility to the biotrophic pathogen.

Plasmopara viticola the Causal Agent of Downy Mildew of Grapevine: From Its Taxonomy to Disease Management

Plasmopara viticola (P. viticola, Berk. & M. A. Curtis; Berl. & De Toni) causing grapevine downy mildew is one of the most damaging pathogens to viticulture worldwide. Since its recognition in the middle of nineteenth century, this disease has spread from America to Europe and then to all grapevine-growing countries, leading to significant economic losses due to the lack of efficient disease control. In 1885 copper was found to suppress many pathogens, and is still the most effective way to control downy mildews. During the twentieth century, contact and penetrating single-site fungicides have been developed for use against plant pathogens including downy mildews, but wide application has led to the appearance of pathogenic strains resistant to these treatments. Additionally, due to the negative environmental impact of chemical pesticides, the European Union restricted their use, triggering a rush to develop alternative tools such as resistant cultivars breeding, creation of new active ingredients, search for natural products and biocontrol agents that can be applied alone or in combination to kill the pathogen or mitigate its effect. This review summarizes data about the history, distribution, epidemiology, taxonomy, morphology, reproduction and infection mechanisms, symptoms, host-pathogen interactions, host resistance and control of the P. viticola, with a focus on sustainable methods, especially the use of biocontrol agents.

Dissecting the susceptibility/resistance mechanism of Vitis vinifera for the future control of downy mildew

The Eurasian grapevine (Vitis vinifera), a species cultivated worldwide for high-quality wine production, is extremely susceptible to the agent of downy mildew, Plasmopara viticola. Nevertheless, germplasm from Georgia (Southern Caucasus, the first grapevine domestication centre), characterized by a high genetic variability, showed resistance traits to P. viticola. The cultivar Mgaloblishvili exhibited the most promising phenotype in terms of resistance against P. viticola. Its defence response results in: i) low disease intensity; ii) low sporulation; iii) damaged mycelium; iv) production of antimicrobial compounds such as volatile organic compounds (VOCs), whose effectiveness on the pathogen was evaluated by leafdisc assays. At the transcriptomic level, its resistance mechanism is determined by the differential expression of both resistance and susceptible genes. The resistance genes are related to: i) pathogen recognition through PAMP, DAMP and effector receptors; ii) ethylene signalling pathway; iii) synthesis of antimicrobial compounds (VOCs) and fungal wall degrading enzymes; iv) development of structural barriers (cell wall reinforcement). The first putative susceptible gene was the transcription factor VviLBDIf7 gene, whose validation was carried out by dsRNA (double-stranded RNA) assay. In this work, these unique results on plant-pathogen interaction are reviewed with the aim of developing new strategies to control the disease.

Modifications of Grapevine Berry Composition Induced by Main Viral and Fungal Pathogens in a Climate Change Scenario

The grapevine is subject to high number of fungal and viral diseases, which are responsible for important economic losses in the global wine sector every year. These pathogens deteriorate grapevine berry quality either directly via the modulation of fruit metabolic pathways and the production of endogenous compounds associated with bad taste and/or flavor, or indirectly via their impact on vine physiology. The most common and devastating fungal diseases in viticulture are gray mold, downy mildew (DM), and powdery mildew (PM), caused, respectively by Botrytis cinerea, Plasmopara viticola, and Erysiphe necator. Whereas B. cinerea mainly infects and deteriorates the ripening fruit directly, deteriorations by DM and PM are mostly indirect via a reduction of photosynthetic leaf area. Nevertheless, mildews can also infect berries at certain developmental stages and directly alter fruit quality via the biosynthesis of unpleasant flavor compounds that impair ultimate wine quality. The grapevine is furthermore host of a wide range of viruses that reduce vine longevity, productivity and berry quality in different ways. The most widespread virus-related diseases, that are known nowadays, are Grapevine Leafroll Disease (GLRD), Grapevine Fanleaf Disease (GFLD), and the more recently characterized grapevine red blotch disease (GRBD). Future climatic conditions are creating a more favorable environment for the proliferation of most virus-insect vectors, so the spread of virus-related diseases is expected to increase in most wine-growing regions. However, the impact of climate change on the evolution of fungal disease pressure will be variable and depending on region and pathogen, with mildews remaining certainly the major phytosanitary threat in most regions because their development rate is to a large extent temperature-driven. This paper aims to provide a review of published literature on most important grapevine fungal and viral pathogens and their impact on grape berry physiology and quality. Our overview of the published literature highlights gaps in our understanding of plant-pathogen interactions, which are valuable for conceiving future research programs dealing with the different pathogens and their impacts on grapevine berry quality and metabolism.

Construction of a high-density genetic map and detection of a major QTL of resistance to powdery mildew (Erysiphe necator Sch.) in Caucasian grapes (Vitis vinifera L.)

BACKGROUND

Vitis vinifera L. is the most cultivated grapevine species worldwide. Erysiphe necator Sch., the causal agent of grape powdery mildew, is one of the main pathogens affecting viticulture. V. vinifera has little or no genetic resistances against E. necator and the grape industry is highly dependent on agrochemicals. Some Caucasian V. vinifera accessions have been reported to be resistant to E. necator and to have no genetic relationships to known sources of resistance to powdery mildew. The main purpose of this work was the study and mapping of the resistance to E. necator in the Caucasian grapes 'Shavtsitska' and 'Tskhvedianis tetra'.

RESULTS

The Caucasian varieties 'Shavtsitska' and 'Tskhvedianis tetra' showed a strong partial resistance to E. necator which segregated in two cross populations: the resistant genotypes delayed and limited the pathogen mycelium growth, sporulation intensity and number of conidia generated. A total of 184 seedlings of 'Shavtsitska' x 'Glera' population were genotyped through the Genotyping by Sequencing (GBS) technology and two high-density linkage maps were developed for the cross parents. The QTL analysis revealed a major resistance locus, explaining up to 80.15% of the phenotypic variance, on 'Shavtsitska' linkage group 13, which was associated with a reduced pathogen infection as well as an enhanced plant necrotic response. The genotyping of 105 Caucasian accessions with SSR markers flanking the QTL revealed that the resistant haplotype of 'Shavtsitska' was shared by 'Tskhvedianis tetra' and a total of 25 Caucasian grape varieties, suggesting a widespread presence of this resistance in the surveyed germplasm. The uncovered QTL was mapped in the region where the Ren1 locus of resistance to E. necator, identified in the V. vinifera 'Kishmish vatkana' and related grapes of Central Asia, is located. The genetic analysis conducted revealed that the Caucasian grapes in this study exhibit a resistant haplotype different from that of Central Asian grape accessions.

CONCLUSIONS

The QTL isolated in 'Shavtsitska' and present in the Caucasian V. vinifera varieties could be a new candidate gene of resistance to E. necator to use in breeding programmes. It co-localizes with the Ren1 locus but shows a different haplotype from that of grapevines of Central Asia. We therefore consider that the Caucasian resistance locus, named Ren1.2, contains a member of a cluster of R-genes, of which the region is rich, and to be linked with, or possibly allelic, to Ren1.

RNAi of a Putative Grapevine Susceptibility Gene as a Possible Downy Mildew Control Strategy

Downy mildew, caused by the oomycete Plasmopara viticola, is one of the diseases causing the most severe economic losses to grapevine (Vitis vinifera) production. To date, the application of fungicides is the most efficient method to control the pathogen and the implementation of novel and sustainable disease control methods is a major challenge. RNA interference (RNAi) represents a novel biotechnological tool with a great potential for controlling fungal pathogens. Recently, a candidate susceptibility gene (VviLBDIf7) to downy mildew has been identified in V. vinifera. In this work, the efficacy of RNAi triggered by exogenous double-stranded RNA (dsRNA) in controlling P. viticola infections has been assessed in a highly susceptible grapevine cultivar (Pinot noir) by knocking down VviLBDIf7 gene. The effects of dsRNA treatment on this target gene were assessed by evaluating gene expression, disease severity, and development of vegetative and reproductive structures of P. viticola in the leaf tissues. Furthermore, the effects of dsRNA treatment on off-target (EF1α, GAPDH, PEPC, and PEPCK) and jasmonic acid metabolism (COI1) genes have been evaluated. Exogenous application of dsRNA led to significant reductions both in VviLBDIf7 gene expression, 5 days after the treatment, and in the disease severity when artificial inoculation was carried out 7 days after dsRNA treatments. The pathogen showed clear alterations to both vegetative (hyphae and haustoria) and reproductive structures (sporangiophores) that resulted in stunted growth and reduced sporulation. Treatment with dsRNA showed signatures of systemic activity and no deleterious off-target effects. These results demonstrated the potential of RNAi for silencing susceptibility factors in grapevine as a sustainable strategy for pathogen control, underlying the possibility to adopt this promising biotechnological tool in disease management strategies.

Proteomic analysis of early-stage incompatible and compatible interactions between grapevine and P. viticola

Wild grapevines can show strong resistance to the downy mildew pathogen P. viticola, but the associated mechanisms are poorly described, especially at early stages of infection. Here, we performed comparative proteomic analyses of grapevine leaves from the resistant genotype V. davidii "LiuBa-8" (LB) and susceptible V. vinifera "Pinot Noir" (PN) 12 h after inoculation with P. viticola. By employing the iTRAQ technique, a total of 444 and 349 differentially expressed proteins (DEPs) were identified in LB and PN, respectively. The majority of these DEPs were related to photosynthesis, respiration, cell wall modification, protein metabolism, stress, and redox homeostasis. Compared with PN, LB showed fewer downregulated proteins associated with photosynthesis and more upregulated proteins associated with metabolism. At least a subset of PR proteins (PR10.2 and PR10.3) was upregulated upon inoculation in both genotypes, whereas HSP (HSP70.2 and HSP90.6) and cell wall-related XTH and BXL1 proteins were specifically upregulated in LB and PN, respectively. In the incompatible interaction, ROS signaling was evident by the accumulation of H₂O₂, and multiple APX and GST proteins were upregulated. These DEPs may play crucial roles in the grapevine response to downy mildew. Our results provide new insights into molecular events associated with downy mildew resistance in grapevine, which may be exploited to develop novel protection strategies against this disease.

From plant resistance response to the discovery of antimicrobial compounds: The role of volatile organic compounds (VOCs) in grapevine downy mildew infection

The discovery of new mechanisms of resistance and natural bioactive molecules could be two of the possible ways to reduce fungicide use in vineyard and assure an acceptable and sustainable protection against Plasmopara viticola, the grapevine downy mildew agent. Emission of volatile organic compounds (VOCs), such as terpenes, norisoprenoids, alcohols and aldehydes, is frequently induced in plants in response to attack by pathogens, such as P. viticola, that is known to cause a VOCs increment in cultivars harboring American resistance traits. In this study, the role of leaf VOCs in the resistance mechanism of two resistant cultivars (Mgaloblishvili, a pure Vitis vinifera cultivar, and Bianca, an interspecific hybrid) and the direct antimicrobial activity of four selected VOCs have been investigated. The leaf VOCs profiles, analyzed through solid-phase microextraction gas chromatography-mass spectrometry analysis, as well as the expression of six terpene synthases (TPSs), were determined upon pathogen inoculation. In both cultivars, the expression pattern of six TPSs increased soon after pathogen inoculation and an increment of nine VOCs has been detected. While in Mgaloblishvili VOCs were synthesized early after P. viticola inoculation, they constituted a late response to pathogen in Bianca. All the four terpenes (farnesene, nerolidol, ocimene and valencene), chosen according to the VOC profiles and gene expression analysis, caused a significant reduction (53-100%) in P. viticola sporulation. These results support the role of VOCs into defense mechanisms of both cultivars and suggest their potential role as a natural and eco-friendly solution to protect grapevine from P. viticola.

Fungicide Resistance Evolution and Detection in Plant Pathogens: Plasmopara viticola as a Case Study

The use of single-site fungicides to control plant pathogens in the agroecosystem can be associated with an increased selection of resistance. The evolution of resistance represents one of the biggest challenges in disease control. In vineyards, frequent applications of fungicides are carried out every season for multiple years. The agronomic risk of developing fungicide resistance is, therefore, high. Plasmopara viticola, the causal agent of grapevine downy mildew, is a high risk pathogen associated with the development of fungicide resistance. P. viticola has developed resistance to most of the fungicide classes used and constitutes one of the most important threats for grapevine production. The goals of this review are to describe fungicide resistance evolution in P. viticola populations and how to conduct proper monitoring activities. Different methods have been developed for phenotyping and genotyping P. viticola for fungicide resistance and the different phases of resistance evolution and life cycles of the pathogen are discussed, to provide a full monitoring toolkit to limit the spread of resistance. A detailed revision of the available tools will help in shaping and harmonizing the monitoring activities between countries and organizations.

Georgian Grapevine Cultivars: Ancient Biodiversity for Future Viticulture

Grapevine (Vitis vinifera) is one of the most widely cultivated plant species of agricultural interest, and is extensively appreciated for its fruits and the wines made from its fruits. Considering the high socio-economic impact of the wine sector all over the world, in recent years, there has been an increase in work aiming to investigate the biodiversity of grapevine germplasm available for breeding programs. Various studies have shed light on the genetic diversity characterizing the germplasm from the cradle of V. vinifera domestication in Georgia (South Caucasus). Georgian germplasm is placed in a distinct cluster from the European one and possesses a rich diversity for many different traits, including eno-carpological and phenological traits; resistance to pathogens, such as oomycetes and phytoplasmas; resistance to abiotic stresses, such as sunburn. The aim of this review is to assess the potential of Georgian cultivars as a source of useful traits for breeding programs. The unique genetic and phenotypic aspects of Georgian germplasm were unraveled, to better understand the diversity and quality of the genetic resources available to viticulturists, as valuable resources for the coming climate change scenario.

Rpv29, Rpv30 and Rpv31: Three Novel Genomic Loci Associated With Resistance to Plasmopara viticola in Vitis vinifera

Plasmopara viticola (Berk. et Curt.) Berl. and de Toni, the agent of downy mildew, is one of the most important pathogens of European grapevine (Vitis vinifera L.). Extensive evaluation of cultivated grapevine germplasm has highlighted the existence of resistant phenotypes in the Georgian (Southern Caucasus) germplasm. Resistance is shown as a reduction in disease severity. Unraveling the genetic architecture of grapevine response to P. viticola infection is crucial to develop resistant varieties and reduce the impact of disease management. The aim of this work was to apply a genome-wide association (GWA) approach to a panel of Georgian-derived accessions phenotyped for P. viticola susceptibility and genotyped with Vitis18kSNP chip array. GWA identified three highly significant novel loci on chromosomes 14 (Rpv29), 3 (Rpv30) and 16 (Rpv31) associated with a low level of pathogen sporulation. Rpv29, Rpv30, and Rpv31 loci appeared to be associated with plant defense genes against biotic stresses, such as genes involved in pathogen recognition and signal transduction. This study provides the first evidence of resistant loci against P. viticola in V. vinifera germplasm, and identifies potential target genes for breeding P. viticola resistant grapevine cultivars.