Grapevine Phyllosphere Community Analysis in Response to Elicitor Application against Powdery Mildew

Conjoined partners: efficacy and side effects of grafting and dsRNA application on the microbial endophyte population of grapevine plants inoculated with two esca-related fungal pathogens

Grafting has been exploited since 7000 BC to enhance productivity, disease resistance, and adaptability of cultivated plants to stressful conditions especially in woody crops such as grapevine (Vitis spp.). In contrast, the application of sequence specific double-stranded RNAs (dsRNAs) to control fungal pathogens and insect pests has only been recently developed. The possibility of combining these approaches to enhance plant resilience, reducing reliance on pesticides, offers new perspectives for a more sustainable agriculture. In this study, we assessed the potential of utilizing dsRNAs to enhance resilience against esca-related wood fungal pathogens in grapevine, considering various rootstock-scion combinations. The results showed that the scion genotype modulates the ability of the rootstock to cope with the inoculated wood fungal pathogens, mainly by altering the efficacy of producing stilbene compounds. Additionally, we found that dsRNAs reduced the growth of two inoculated esca-related fungal pathogens but they did not completely stop their colonization. Furthermore, wood microbiome data showed that the scion genotype (always belonging to Vitis vinifera species) was also able to influence the rootstock-associated microbiota, with a major effect on the fungal community. Lastly, adverse effects on non-target microorganisms are reported, raising questions on the environmental fate of dsRNAs and how dsRNAs can directly or indirectly affect plant-associated microbial communities.

Carbohydrate elicitor-induced plant immunity: Advances and prospects

The perceived negative impacts of synthetic agrochemicals gave way to alternative, biological plant protection strategies. The deployment of induced resistance, comprising boosting the natural defense responses of plants, is one of those. Plants developed multi-component defense mechanisms to defend themselves against biotic and abiotic stresses. These are activated upon recognition of stress signatures via membrane-localized receptors. The induced immune responses enable plants to tolerate and limit the impact of stresses. A systemic cascade of signals enables plants to prime un-damaged tissues, which is crucial during secondary encounters with stress. Comparable stress tolerance mechanisms can be induced in plants by the application of carbohydrate elicitors such as chitin/chitosan, β-1,3-glucans, oligogalacturonides, cellodextrins, xyloglucans, alginates, ulvans, and carrageenans. Treating plants with carbohydrate-derived elicitors enable the plants to develop resistance appliances against diverse stresses. Some carbohydrates are also known to have been involved in promoting symbiotic signaling. Here, we review recent progresses on plant resistance elicitation effect of various carbohydrate elicitors and the molecular mechanisms of plant cell perception, cascade signals, and responses to cascaded cues. Besides, the molecular mechanisms used by plants to distinguish carbohydrate-induced immunity signals from symbiotic signals are discussed. The structure-activity relationships of the carbohydrate elicitors are also described. Furthermore, we forwarded future research outlooks that might increase the utilization of carbohydrate elicitors in agriculture in order to improve the efficacy of plant protection strategies.

The 'microbiome counterattack': Insights on the soil and root-associated microbiome in diverse chickpea and lentil genotypes after an erratic rainfall event

Legumes maintain soil fertility thanks to their associated microbiota but are threatened by climate change that causes soil microbial community structural and functional modifications. The core microbiome associated with different chickpea and lentil genotypes was described after an unexpected climatic event. Results showed that chickpea and lentil bulk soil microbiomes varied significantly between two sampling time points, the first immediately after the rainfall and the second 2 weeks later. Rhizobia were associated with the soil of the more productive chickpea genotypes in terms of flower and fruit number. The root-associated bacteria and fungi were surveyed in lentil genotypes, considering that several parcels showed disease symptoms. The metabarcoding analysis revealed that reads related to fungal pathogens were significantly associated with one lentil genotype. A lentil core prokaryotic community common to all genotypes was identified as well as a genotype-specific one. A higher number of specific bacterial taxa and an enhanced tolerance to fungal diseases characterized a lentil landrace compared to the commercial varieties. This outcome supported the hypothesis that locally adapted landraces might have a high recruiting efficiency of beneficial soil microbes.

Cadmium inhibits powdery mildew colonization and reconstructs microbial community in leaves of the hyperaccumulator plant Sedum alfredii

Understanding the influence of the heavy metal cadmium (Cd) on the phyllosphere microbiome of hyperaccumulator plants is crucial for enhancing phytoremediation. The characteristics of the phyllosphere of Sedum alfredii Hance, a hyperaccumulator plant, were investigated using 16S rRNA and internal transcribed spacer amplicon sequencing of powdery mildew-infected leaves treated or untreated with Cd. The results showed that the colonization of powdery mildew caused severe chlorosis and necrosis in S. alfredii leaves, and the relative abundance of Leotiomycetes in infected leaves increased dramatically and significantly decreased phyllosphere microbiome diversity. However, S. alfredii preferentially accumulated higher concentrations of Cd in the leaves of infected plants than in uninfected plants by powdery mildew, which in turn significantly inhibited powdery mildew colonization in leaves; the relative abundance of the fungal class Leotiomycetes in infected leaves decreased, and alpha and beta diversities of the phyllosphere microbiome significantly increased with Cd treatment in the infected plants. In addition, the inter-kingdom networks in the microbiota of the infected leaves treated with Cd presented many nodes and edges, and the highest inter-kingdom modularity compared to the untreated infected leaves, indicating a highly connected microbial community. These results suggest that Cd significantly inhibits powdery mildew colonization by altering the composition of the phyllosphere microbiome in S. alfredii leaves, paving the way for efficient heavy metal phytoremediation and providing a new perspective on defense strategies against heavy metals.

Holistic understanding of the response of grapevines to foliar application of seaweed extracts

Viticulture is highly dependent on phytochemicals to maintain good vineyard health. However, to reduce their accumulation in the environment, green regulations are driving the development of eco-friendly strategies. In this respect, seaweeds have proven to be one of the marine resources with the highest potential as plant protective agents, representing an environmentally-friendly alternative approach for sustainable wine production. The current work follows an interdisciplinary framework to evaluate the capacity of Ulva ohnoi and Rugulopteryx okamurae seaweeds to induce defense mechanisms in grapevine plants. To our knowledge, this is the first study to evaluate Rugulopteryx okamurae as a biostimulator . This macroalgae is relevant since it is an invasive species on the Atlantic and Mediterranean coast causing incalculable economic and environmental burdens. Four extracts (UL1, UL2, RU1 and RU2 developed from Ulva and Rugulopteryx, respectively) were foliar applied to Tempranillo plants cultivated under greenhouse conditions. UL1 and RU2 stood out for their capacity to induce defense genes, such as a PR10, PAL, STS48 and GST1, mainly 24 hours after the first application. The increased expression level of these genes agreed with i) an increase in trans-piceid and trans-resveratrol content, mainly in the RU2 treated leaves, and, ii) an increase in jasmonic acid and decrease in salicylic acid. Moreover, an induction of the activity of the antioxidant enzymes was observed at the end of the experiment, with an increase in superoxide dismutase and catalase in the RU2-treated leaves in particular. Interestingly, while foliar fungal diversity was not influenced by the treatments, alga extract amendment modified fungal composition, RU2 application enriching the content of various groups known for their biocontrol activity. Overall, the results evidenced the capacity of Rugulopteryx okamurae for grapevine biostimulation, inducing the activation of several secondary metabolite pathways and promoting the abundance of beneficial microbiota involved in grapevine protection. While further studies are needed to unravel the bioactive compound(s) involved, including conducting field experiments etc., the current findings are the first steps towards the inclusion of Rugulopteryx okamurae in a circular scheme that would reduce its accumulation on the coast and benefit the viticulture sector at the same time.

Effects of synthetic and environmentally friendly fungicides on powdery mildew management and the phyllosphere microbiome of cucumber

Modern agricultural practices rely on synthetic fungicides to control plant disease, but the application of these fungicides has raised concerns regarding human and environmental health for many years. As a substitute, environmentally friendly fungicides have been increasingly introduced as alternatives to synthetic fungicides. However, the impact of these environmentally friendly fungicides on plant microbiomes has received limited attention. In this study, we used amplicon sequencing to compare the bacterial and fungal microbiomes in the leaves of powdery mildew-infected cucumber after the application of two environmentally friendly fungicides (neutralized phosphorous acid (NPA) and sulfur) and one synthetic fungicide (tebuconazole). The phyllosphere α-diversity of both the bacterial and fungal microbiomes showed no significant differences among the three fungicides. For phyllosphere β-diversity, the bacterial composition exhibited no significant differences among the three fungicides, but fungal composition was altered by the synthetic fungicide tebuconazole. While all three fungicides significantly reduced disease severity and the incidence of powdery mildew, NPA and sulfur had minimal impacts on the phyllosphere fungal microbiome relative to the untreated control. Tebuconazole altered the phyllosphere fungal microbiome by reducing the abundance of fungal OTUs such as Dothideomycetes and Sordariomycetes, which included potentially beneficial endophytic fungi. These results indicated that treatments with the environmentally friendly fungicides NPA and sulfur have fewer impacts on the phyllosphere fungal microbiome while maintaining the same control efficacy as the synthetic fungicide tebuconazole.

New-Generation Sequencing Technology in Diagnosis of Fungal Plant Pathogens: A Dream Comes True?

The fast and continued progress of high-throughput sequencing (HTS) and the drastic reduction of its costs have boosted new and unpredictable developments in the field of plant pathology. The cost of whole-genome sequencing, which, until few years ago, was prohibitive for many projects, is now so affordable that a new branch, phylogenomics, is being developed. Fungal taxonomy is being deeply influenced by genome comparison, too. It is now easier to discover new genes as potential targets for an accurate diagnosis of new or emerging pathogens, notably those of quarantine concern. Similarly, with the development of metabarcoding and metagenomics techniques, it is now possible to unravel complex diseases or answer crucial questions, such as "What's in my soil?", to a good approximation, including fungi, bacteria, nematodes, etc. The new technologies allow to redraw the approach for disease control strategies considering the pathogens within their environment and deciphering the complex interactions between microorganisms and the cultivated crops. This kind of analysis usually generates big data that need sophisticated bioinformatic tools (machine learning, artificial intelligence) for their management. Herein, examples of the use of new technologies for research in fungal diversity and diagnosis of some fungal pathogens are reported.

Application of Elicitors in Grapevine Defense: Impact on Volatile Compounds

Elicitors as alternatives to agrochemicals are widely used as a sustainable farming practice. The use of elicitors in viticulture to control disease and improve phenolic compounds is widely recognized in this field. Concurrently, they also affect other secondary metabolites, such as aroma compounds. Grape and wine aroma compounds are an important quality factor that reflects nutritional information and influences consumer preference. However, the effects of elicitors on aroma compounds are diverse, as different grape varieties respond differently to treatments. Among the numerous commercialized elicitors, some have proven very effective in improving the quality of grapes and the resulting wines. This review summarizes some of the elicitors commonly used in grapevines for protection against biotic and abiotic stresses and their impact on the quality of volatile compounds. The work is intended to serve as a reference for growers for the sustainable development of high-quality grapes.

A Laminarin-Based Formulation Protects Wheat Against Zymoseptoria tritici via Direct Antifungal Activity and Elicitation of Host Defense-Related Genes

The present study aimed to evaluate the potential of the laminarin-based formulation Vacciplant to protect and induce resistance in wheat against Zymoseptoria tritici, a major pathogen on this crop. Under greenhouse conditions, a single foliar spraying of the product 2 days before inoculation with Z. tritici reduced disease severity and pycnidium density by 42 and 45%, respectively. Vacciplant exhibited a direct antifungal activity on Z. tritici conidial germination both in vitro and in planta. Moreover, it reduced in planta substomatal colonization as well as pycnidium formation on treated leaves. Molecular investigations revealed that Vacciplant elicits but did not prime the expression of several wheat genes related to defense pathways, including phenylpropanoids (phenylalanine ammonia-lyase and chalcone synthase), octadecanoids (lipoxygenase and allene oxide synthase), and pathogenesis-related proteins (β-1,3-endoglucanase and chitinase). By contrast, it did not modulate the expression of oxalate oxidase gene involved in the reactive oxygen species metabolism. Ultrahigh-performance liquid chromatography-mass spectrometry analysis indicated limited changes in leaf metabolome after product application in both noninoculated and inoculated conditions, suggesting a low metabolic cost associated with induction of plant resistance. This study provides evidence that the laminarin-based formulation confers protection to wheat against Z. tritici through direct antifungal activity and elicitation of plant defense-associated genes.

The hidden world within plants: metatranscriptomics unveils the complexity of wood microbiomes

The importance of plants as complex entities influenced by genomes of the associated microorganisms is now seen as a new source of variability for a more sustainable agriculture, also in the light of ongoing climate change. For this reason, we investigated through metatranscriptomics whether the taxa profile and behaviour of microbial communities associated with the wood of 20-year-old grapevine plants are influenced by the health status of the host. We report for the first time a metatranscriptome from a complex tissue in a real environment, highlighting that this approach is able to define the microbial community better than referenced transcriptomic approaches. In parallel, the use of total RNA enabled the identification of bacterial taxa in healthy samples that, once isolated from the original wood tissue, displayed potential biocontrol activities against a wood-degrading fungal taxon. Furthermore, we revealed an unprecedented high number of new viral entities (~120 new viral species among 180 identified) associated with a single and limited environment and with potential impact on the whole holobiont. Taken together, our results suggest a complex multitrophic interaction in which the viral community also plays a crucial role in raising new ecological questions for the exploitation of microbial-assisted sustainable agriculture.

Grapevine virome and production of healthy plants by somatic embryogenesis

Grapevine (Vitis spp.) is a widespread fruit tree hosting many viral entities that interact with the plant modifying its responses to the environment. The production of virus‐free plants is becoming increasingly crucial for the use of grapevine as a model species in different studies. Using high‐throughput RNA sequencing, the viromes of seven mother plants grown in a germplasm collection vineyard were sequenced. In addition to the viruses and viroids already detected in grapevine, we identified 13 putative new mycoviruses. The different spread among grapevine tissues collected in vineyard, greenhouse and in vitro conditions suggested a clear distinction between viruses/viroids and mycoviruses that can successfully be exploited for their identification. Mycoviruses were absent in in vitro cultures, while plant viruses and viroids were particularly accumulated in these plantlets. Somatic embryogenesis applied to the seven mother plants was effective in the elimination of the complete virome, including mycoviruses. However, different sanitization efficiencies for viroids and grapevine pinot gris virus were observed among genotypes. The absence of mycoviruses in in vitro plantlets, associated with the absence of all viral entities in somaclones, suggested that this regeneration technique is also effective to eradicate endophytic/epiphytic fungi, resulting in gnotobiotic or pseudo‐gnotobiotic plants.

Unlocking the Changes of Phyllosphere Fungal Communities of Fishscale Bamboo (Phyllachora heterocladae) under Rhombic-Spot Disease Stressed Conditions

As an important nonwood bioresource, fishscale bamboo (Phyllachora heterocladae Oliver) is widely distributed in the subtropical region of China. Rhombic-spot disease, caused by Neostagonosporella sichuanensis, is one of the most serious diseases that threatens fishscale bamboo health. However, there is limited knowledge about how rhombic-spot disease influences the diversity and structures of phyllosphere fungal communities. In this study, we investigated the phyllosphere fungal communities from stems, branches, and leaves of fishscale bamboo during a rhombic-spot disease outbreak using 18S rRNA sequencing. We found that only the phyllosphere fungal community from stems was significantly affected by pathogen invasion in terms of community richness, diversity, and structure. FUNGuild analysis revealed that the major classifications of phyllosphere fungi based on trophic modes in stems, branches, and leaves changed from symbiotroph-pathotroph, no obvious dominant trophic mode, and symbiotroph to saprotroph, saprotroph–pathotroph–symbiotroph, and saprotroph–symbiotroph, respectively, after pathogen invasion. The fungal community composition of the three tissues displayed significant differences at the genus level between healthy and diseased plants. The associations among fungal species in diseased samples showed more complex co-occurrence network structures than those of healthy samples. Taken together, our results highlight the importance of plant pathological conditions for the assembly of phyllosphere fungal communities in different tissues.

Comparative analyses of sooty mould communities from Brazil and Central Europe

To gain an insight into fungal sooty mould communities on leaves of trees and shrubs in the tropics and in temperate regions, 47 biofilms of the Mata Atlântica rainforest relic and the Caatinga vegetation in the state of Sergipe, Northeast Brazil, and from Central European colline and alpine zones were compared. The four sampling sites clearly differed in composition of their epiphyllous fungal communities. The fungal OTUs from all sites belonged mainly to the Ascomycota, with Dothideomycetes being the dominant class. The core community group consisted of a few site-specific representatives in co-occurrence with the ubiquitous Mycosphaerella tassiana and Aureobasidium pullulans. Most species of the core community were dark pigmented and were accompanied by facultative unpigmented or lightly pigmented species. Among the cultivable fungal species, the proportion of melanised species was significantly more abundant in samples from the two European sites, which supports the theory of thermal melanism. The identity of the host plant had a stronger impact on fungal community composition than the presence of sap-feeding insects.

The Molecular Priming of Defense Responses is Differently Regulated in Grapevine Genotypes Following Elicitor Application against Powdery Mildew

Molecular changes associated with response to powdery mildew (PM) caused by Erysiphe necator have been largely explored in Vitis vinifera cultivars, but little is known on transcriptional and metabolic modifications following application of resistance elicitors against this disease. In this study, the whole transcriptome sequencing, and hormone and metabolite analyses were combined to dissect long-term defense mechanisms induced by molecular reprogramming events in PM-infected ‘Moscato’ and ‘Nebbiolo’ leaves treated with three resistance inducers: acibenzolar-S-methyl, potassium phosphonate, and laminarin. Although all compounds were effective in counteracting the disease, acibenzolar-S-methyl caused the most intense transcriptional modifications in both cultivars. These involved a strong down-regulation of photosynthesis and energy metabolism and changes in carbohydrate accumulation and partitioning that most likely shifted the plant growth-defense trade-off towards the establishment of disease resistance processes. It was also shown that genotype-associated metabolic signals significantly affected the cultivar defense machinery. Indeed, ‘Nebbiolo’ and ‘Moscato’ built up different defense strategies, often enhanced by the application of a specific elicitor, which resulted in either reinforcement of early defense mechanisms (e.g., epicuticular wax deposition and overexpression of pathogenesis-related genes in ‘Nebbiolo’), or accumulation of endogenous hormones and antimicrobial compounds (e.g., high content of abscisic acid, jasmonic acid, and viniferin in ‘Moscato’).

Long-Term Impact of Chemical and Alternative Fungicides Applied to Grapevine cv Nebbiolo on Berry Transcriptome

Viticulture is one of the horticultural systems in which antifungal treatments can be extremely frequent, with substantial economic and environmental costs. New products, such as biofungicides, resistance inducers and biostimulants, may represent alternative crop protection strategies respectful of the environmental sustainability and food safety. Here, the main purpose was to evaluate the systemic molecular modifications induced by biocontrol products as laminarin, resistance inducers (i.e., fosetyl-Al and potassium phosphonate), electrolyzed water and a standard chemical fungicide (i.e., metiram), on the transcriptomic profile of ‘Nebbiolo’ grape berries at harvest. In addition to a validation of the sequencing data through real-time polymerase chain reaction (PCR), for the first-time the expression of some candidate genes in different cell-types of berry skin (i.e., epidermal and hypodermal layers) was evaluated using the laser microdissection approach. Results showed that several considered antifungal treatments do not strongly affect the berry transcriptome profile at the end of season. Although some treatments do not activate long lasting molecular defense priming features in berry, some compounds appear to be more active in long-term responses. In addition, genes differentially expressed in the two-cell type populations forming the berry skin were found, suggesting a different function for the two-cell type populations.

Two New Putative Plant Viruses from Wood Metagenomics Analysis of an Esca Diseased Vineyard

The concept of plant as a holobiont is now spreading among the scientific community and the importance to study plant-associated microorganisms is becoming more and more necessary. Along with bacteria and fungi, also viruses can play important roles during the holobiont-environment interactions. In grapevine, viruses are studied mainly as pathological agents, and many species (more than 80) are known to be able to replicate inside its tissues. In this study two new viral species associated with grape wood tissues are presented, one belongs to the Potyviridae family and one to the Bunyavirales order. Due to the ability of potyviruses to enhance heterologous virus replication, it will be important to assess the presence of such a virus in the grapevine population to understand its ecological role. Furthermore, the association of the cogu-like virus with esca symptomatic samples opens new questions and the necessity of a more detailed characterization of this virus.