Three Types of Elicitors Induce Grapevine Resistance against Downy Mildew via Common and Specific Immune Responses

Eicosapentaenoic acid: New insights into an oomycete-driven elicitor to enhance grapevine immunity

The widespread use of pesticides in agriculture remains a matter of major concern, prompting a critical need for alternative and sustainable practices. To address this, the use of lipid-derived molecules as elicitors to induce defence responses in grapevine plants was accessed. A Plasmopara viticola fatty acid (FA), eicosapentaenoic acid (EPA) naturally present in oomycetes, but absent in plants, was applied by foliar spraying to the leaves of the susceptible grapevine cultivar (Vitis vinifera cv. Trincadeira), while a host lipid derived phytohormone, jasmonic acid (JA) was used as a molecule known to trigger host defence. Their potential as defence triggers was assessed by analysing the expression of a set of genes related to grapevine defence and evaluating the FA modulation upon elicitation. JA prompted grapevine immunity, altering lipid metabolism and up-regulating the expression of several defence genes. EPA also induced a myriad of responses to the levels typically observed in tolerant plants. Its application activated the transcription of defence gene's regulators, pathogen-related genes and genes involved in phytoalexins biosynthesis. Moreover, EPA application resulted in the alteration of the leaf FA profile, likely by impacting biosynthetic, unsaturation and turnover processes. Although both molecules were able to trigger grapevine defence mechanisms, EPA induced a more robust and prolonged response. This finding establishes EPA as a promising elicitor for an effectively managing grapevine downy mildew diseases.

Broad-Spectrum Efficacy and Modes of Action of Two Bacillus Strains against Grapevine Black Rot and Downy Mildew

Black rot (Guignardia bidwellii) and downy mildew (Plasmopara viticola) are two major grapevine diseases against which the development of efficient biocontrol solutions is required in a context of sustainable viticulture. This study aimed at evaluating and comparing the efficacy and modes of action of bacterial culture supernatants from Bacillus velezensis Buz14 and B. ginsengihumi S38. Both biocontrol agents (BCA) were previously demonstrated as highly effective against Botrytis cinerea in grapevines. In semi-controlled conditions, both supernatants provided significant protection against black rot and downy mildew. They exhibited antibiosis against the pathogens by significantly decreasing G. bidwellii mycelial growth, but also the release and motility of P. viticola zoospores. They also significantly induced grapevine defences, as stilbene production. The LB medium, used for the bacterial cultures, also showed partial effects against both pathogens and induced plant defences. This is discussed in terms of choice of experimental controls when studying the biological activity of BCA supernatants. Thus, we identified two bacterial culture supernatants as new potential biocontrol products exhibiting multi-spectrum antagonist activity against different grapevine key pathogens and having a dual mode of action.

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.

Evaluation of inducing activity of CIP elicitors from diverse sources based on monosaccharide composition and physiological indicators

Application of elicitors can greatly enhance plant immune resistance against pathogens. However, it is still obscure whether elicitor activity is influenced by diverse sources. This study investigated the effect of foliar spraying of 19 batches of Chrysanthemum indicum polysaccharides (CIPs) on the disease resistance of Atractylodes macrocephala Koidz. (A. macrocephala) and explored the main reasons for the differences of inducing activity of CIP elicitors. PCA, OPLS-DA, grey relational analysis and entropy weight method had good predictability for the activity evaluation of CIP elicitors and other plant-derived elicitors. The results showed that 19 batches of CIPs had definite regional differences in inducing activity and monosaccharide content. CIP elicitors with high inducing activity could significantly increase the accumulation of Atractylenolide Ⅱ and Atractylenolide Ⅲ, the mRNA relative transcription level of CAT, POD, PAL genes, the amount of pH change in the medium and effectively reduce the disease index of A. macrocephala. Furthermore, CIP with high inducing activity exhibited the high contents of Rha, Ara and GalA, which might be the main contributor to their high activity. The evaluation procedure developed in this work can be applied for screening CIP elicitors with high inducing activity, and it lays a foundation for identifying more functional elicitors related to plant immune resistance.

Benzothiadiazole Affects Grape Polyphenol Metabolism and Wine Quality in Two Greek Cultivars: Effects during Ripening Period over Two Years

Grape berries are one of the most important sources of phenolic compounds, either consumed fresh or as wine. A pioneer practice aiming to enrich grape phenolic content has been developed based on the application of biostimulants such as agrochemicals initially designed to induce resistance against plant pathogens. A field experiment was conducted in two growing seasons (2019-2020) to investigate the effect of benzothiadiazole on polyphenol biosynthesis during grape ripening in Mouhtaro (red-colored) and Savvatiano (white-colored) varieties. Grapevines were treated at the stage of veraison with 0.3 mM and 0.6 mM benzothiadiazole. The phenolic content of grapes, as well as the expression level of genes involved in the phenylpropanoid pathway were evaluated and showed an induction of genes specifically engaged in anthocyanins and stilbenoids biosynthesis. Experimental wines deriving from benzothiadiazole-treated grapes exhibited increased amounts of phenolic compounds in both varietal wines, as well as an enhancement in anthocyanin content of Mouhtaro wines. Taken together, benzothiadiazole can be utilized to induce the biosynthesis of secondary metabolites with oenological interest and to improve the quality characteristics of grapes produced under organic conditions.

Methyl Jasmonate and Nanoparticles Doped with Methyl Jasmonate affect the Cell Wall Composition of Monastrell Grape Skins

The structural composition of the cell wall of grape skins is related to the cell wall integrity and subsequent extraction of the different compounds that are contained inside vacuoles and also the cell wall breakdown products. Different reports have established that methyl jasmonate (MeJ) produces changes in the composition of the grape skin cell wall. The use of elicitors to promote the production of secondary metabolites in grapes has been studied in several reports; however, its study linked to nanotechnology is less developed. These facts led us to study the effect of methyl jasmonate (MeJ) and nanoparticles doped with MeJ (nano-MeJ) on the cell walls of Monastrell grapes during three seasons. Both treatments tended to increase cell wall material (CWM) and caused changes in different components of the skin cell walls. In 2019 and 2021, proteins were enlarged in both MeJ and nano-MeJ-treated grapes. A general decrease in total phenolic compounds was detected with both treatments, in addition to an increment in uronic acids when the grapes were well ripened. MeJ and nano-MeJ produced a diminution in the amount of cellulose in contrast to an increase in hemicellulose. It should be noted that the effects with nano-MeJ treatment occurred at a dose 10 times lower than with MeJ treatment.

Grapevine response to a Dittrichia viscosa extract and a Bacillus velezensis strain

The present study aims to evaluate the response of the three Mediterranean local grapevines 'Garnacha Blanca', 'Garnacha Tinta', and 'Macabeo' to treatments with biocontrol products, namely a botanical extract (Akivi, Dittrichia viscosa extract) and a beneficial microorganism (Bacillus UdG, Bacillus velezensis). A combination of transcriptomics and metabolomics approaches were chosen in order to study grapevine gene expression and to identify gene marker candidates, as well as, to determine differentially concentrated grapevine metabolites in response to biocontrol product treatments. Grapevine plants were cultivated in greenhouse under controlled conditions and submitted to the treatments. Thereafter, leaves were sampled 24h after treatment to carry out the gene expression study by RT-qPCR for the three cultivars and by RNA-sequencing for 'Garnacha Blanca'. Differentially expressed genes (DEGs) were investigated for both treatments and highly influenced DEGs were selected to be tested in the three cultivars as treatment gene markers. In addition, the extraction of leaf components was performed to quantify metabolites, such as phytohormones, organic acids, and phenols. Considering the upregulated and downregulated genes and the enhanced metabolites concentrations, the treatments had an effect on jasmonic acid, ethylene, and phenylpropanoids defense pathways. In addition, several DEG markers were identified presenting a stable overexpression after the treatments in the three grapevine cultivars. These gene markers could be used to monitor the activity of the products in field treatments. Further research will be necessary to confirm these primary results under field conditions.

Elicitor-Induced VOC Emission by Grapevine Leaves: Characterisation in the Vineyard

The present study is aimed at determining whether leaf volatile organic compounds (VOCs) are good markers of the grapevine response to defence elicitors in the field. It was carried out in two distinct French vineyards (Burgundy and Bordeaux) over 3 years. The commercial elicitor Bastid® (Syngenta, Saint-Sauveur, France) (COS-OGA) was first used to optimise the VOCs' capture in the field; by bagging stems together with a stir bar sorptive extraction (SBSE) sensor. Three elicitors (Bastid®, copper sulphate and methyl jasmonate) were assessed at three phenological stages of the grapevines by monitoring stilbene phytoalexins and VOCs. Stilbene production was low and variable between treatments and phenological stages. VOCs-particularly terpenes-were induced by all elicitors. However, the response profiles depended on the type of elicitor, the phenological stage and the vineyard, and no sole common VOC was found. The levels of VOC emissions discriminated between weak (Bastid® and copper sulphate) and strong (methyl jasmonate) inducers. Ocimene isomers were constitutively present in the overall blends of the vineyards and increased by the elicitors' treatments, whilst other VOCs were newly released throughout the growing seasons. Nonetheless, the plant development and climate factors undoubtedly influenced the release and profiles of the leaf VOCs.

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.

Integrative Analysis of Elicitor-Induced Camptothecin Biosynthesis in Camptotheca acuminata Plantlets Through a Combined Omics Approach

Treatments with abiotic elicitors can efficiently induce the accumulation of specialized metabolites in plants. We used a combined omics approach to analyze the elicitation effects of MeJa, AgNO3, and PEG on camptothecin (CPT) biosynthesis in Camptotheca acuminata plantlets. Untargeted analyses revealed that treatments with MeJa, AgNO3, and PEG significantly inhibited the photosynthetic pathway and promoted carbon metabolism and secondary metabolic pathways. The CPT levels increased by 78.6, 73.3, and 50.0% in the MeJa, AgNO3, and PEG treatment groups, respectively. Using C. acuminata plantlets after elicitation treatment, we mined and characterized 15 new alkaloids, 25 known CPT analogs and precursors, 9 iridoid biosynthetic precursors, and 15 tryptamine biosynthetic precursors based on their MS/MS fragmentation spectra. Using 32 characterized genes involved in CPT biosynthesis as bait, we mined 12 prioritized CYP450 genes from the 416 CYP450 candidates that had been identified based on co-expression analysis, conserved domain analysis, and their elicitation-associated upregulation patterns. This study provides a comprehensive perspective on CPT biosynthesis in C. acuminata plantlets after abiotic elicitation. The findings enable us to elucidate the previously unexplored CYP450-mediated oxidation steps for CPT biosynthesis.

Biostimulants in Viticulture: A Sustainable Approach against Biotic and Abiotic Stresses

Climate change and disproportionate anthropogenic interventions, such as the excess of phytopharmaceutical products and continuous soil tillage, are jeopardizing viticulture by subjecting plants to continuous abiotic stress. One of the main physiological repercussions of abiotic stress is represented by the unbalanced redox homeostasis due to the overproduction of reactive oxygen species (ROS), ultimately leading to a state of oxidative stress (detrimental to grape quality). To these are added the direct and indirect damages caused by pathogens (biotic stresses). In light of this scenario, it is inevitable that sustainable techniques and sensitivity approaches for environmental and human health have to be applied in viticulture. Sustainable viticulture can only be made with the aid of sustainable products. Biostimulant (PB) applications (including resistance inducers or elicitors) in the vineyard have become interesting maneuvers for counteracting vine diseases and improving grape quality. These also represent a partial alternative to soil fertilization by improving nutrient absorption and avoiding its leaching into the groundwater. Their role as elicitors has important repercussions in the stimulation of the phenylpropanoid pathway by triggering the activation of several enzymes, such as polyphenol oxidase, lipoxygenase, phenylalanine ammonia-lyase, and peroxidase (with the accumulation of phenolic compounds). The present review paper summarizes the PBs' implications in viticulture, gathering historical, functional, and applicative information. This work aims to highlight the innumerable beneficial effects on vines brought by these products. It also serves to spur the scientific community to a greater contribution in investigating the response mechanisms of the plant to positive inductions.

VOCs Are Relevant Biomarkers of Elicitor-Induced Defences in Grapevine

Grapevine is susceptible to fungal diseases generally controlled by numerous chemical fungicides. Elicitors of plant defence are a way of reducing the use of these chemicals, but still provide inconsistent efficiency. Easy-to-analyse markers of grapevine responses to elicitors are needed to determine the best conditions for their efficiency and position them in protection strategies. We previously reported that the elicitor sulphated laminarin induced the emission of volatile organic compounds (VOCs) by grapevine leaves. The present study was conducted to characterise and compare VOC emissions in response to other elicitors. Bastid^® was first used to test the conditions of VOC collection and analysis. Using SBSE-GC-MS, we detected several VOCs, including the sesquiterpene α-farnesene, in a time-dependent manner. This was correlated with the induction of farnesene synthase gene expression, in parallel with stilbene synthesis (another defence response), and associated to resistance against downy mildew. The other elicitors (Redeli^®, Romeo^®, Bion^®, chitosan, and an oligogalacturonide) induced VOC emission, but with qualitative and quantitative differences. VOC emission thus constitutes a response of grapevine to elicitors of various chemical structures. Therefore, VOC analysis is relevant for studying the impact of environmental factors on grapevine defence responses and optimising the performance of elicitors in vineyards.