Foliar treatments with Gaultheria procumbens essential oil induce defense responses and resistance against a fungal pathogen in Arabidopsis

Multi-omics analyses of sid2 mutant reflect the need of isochorismate synthase ICS1 to cope with sulfur limitation in Arabidopsis thaliana

The SID2 (SA INDUCTION-DEFICIENT2) gene that encodes ICS1 (isochorismate synthase), plays a central role in salicylic acid biosynthesis in Arabidopsis. The sid2 and NahG (encoding a bacterial SA hydroxylase) overexpressing mutants (NahG-OE) have currently been shown to outperform wild type, presenting delayed leaf senescence, higher plant biomass and better seed yield. When grown under sulfate-limited conditions (low-S), sid2 mutants exhibited early leaf yellowing compared to the NahG-OE, the npr1 mutant affected in SA signaling pathway, and WT. This indicated that the hypersensitivity of sid2 to sulfate limitation was independent of the canonical npr1 SA-signaling pathway. Transcriptomic and proteomic analyses revealed that major changes occurred in sid2 when cultivated under low-S, changes that were in good accordance with early senescence phenotype and showed the exacerbation of stress responses. The sid2 mutants displayed a lower sulfate uptake capacity when cultivated under low-S and lower S concentrations in their rosettes. Higher glutathione concentrations in sid2 rosettes under low-S were in good accordance with the higher abundance of proteins involved in glutathione and ascorbate redox metabolism. Amino acid and lipid metabolisms were also strongly modified in sid2 under low-S. Depletion of total fatty acids in sid2 under low-S was consistent with the fact that S-metabolism plays a central role in lipid synthesis. Altogether, our results show that functional ICS1 is important for plants to cope with S limiting conditions.

Tetraclinis articulata essential oil emulsion use as alternative to chemical fungicide to control tomato grey mould disease

Tetraclinis articulata essential oil proved to be effective in controlling tomato grey mould, so we would investigate its effect on some tomato defense mechanisms. The pretreatment of Botrytis cinerea infected tomato plants with TAEO emulsion enhanced the activity of antioxidant enzymes activity SOD, CAT, APX, and GPX, and total polyphenols content and it decreased IC50 of free radical-scavenging activity and H2O2 content. Results showed amelioration in antioxidant status in TAEO emulsion treated and B. cinerea infected plants indicating that treatment decreased infection in tomato plants. The qRT-PCR analysis of defense genes expression Chitinase SlChi, transcription factors SlWRKY and SlAP2/ERF, Lipoxygenase SlLOX, and Thioredoxin SlTRX showed that they were up-regulated as early as 12 hpi sustained with a second increase at 48 hpi in TAEO emulsion pretreated and infected plants. These results suggest the potential use of TAEO emulsion as natural product to induce tomato antioxidant status and activate defense genes.

Phytochemistry and Biological Profile of Gaultheria procumbens L. and Wintergreen Essential Oil: From Traditional Application to Molecular Mechanisms and Therapeutic Targets

Gaultheria procumbens L. is a medicinal plant whose aerial parts (leaves, stems, and fruits) and methyl salicylate-rich essential oil (wintergreen oil) are used in phytotherapy to treat inflammation, muscular pain, and infection-related disorders. This overview summarises the current knowledge about ethnobotany, phytochemistry, pharmacology, molecular mechanisms, biocompatibility, and traditional use of G. procumbens and the wintergreen oil distilled from different plant organs. Over 70 hydrophilic compounds, including methyl salicylate glycosides, flavonoids, procyanidins, free catechins, caffeoylquinic acids, and simple phenolic acids, have been identified in G. procumbens plant parts. Moreover, aliphatic compounds, triterpene acids, and sterols have been revealed in lipophilic fractions. Furthermore, over 130 volatile compounds have been detected in wintergreen oil with dominating methyl salicylate (96.9-100%). The accumulated research indicates that mainly hydrophilic non-volatiles are responsible for the pharmacological effects of G. procumbens, primarily its potent anti-inflammatory, antioxidant, and photoprotective activity, with mechanisms verified in vitro and ex vivo in cellular and cell-free assays. The biological effectiveness of the dominant methyl salicylate glycoside-gaultherin-has also been confirmed in animals. Wintergreen oil is reported as a potent anti-inflammatory agent exhibiting moderate antioxidant and antimicrobial activity in vitro and significant insecticidal and larvicidal capacity. Together, G. procumbens accumulate a diverse fraction of polyphenols, triterpenes, and volatiles with validated in vitro and ex vivo biological activity but with the absence of in vivo studies, especially clinical trials concerning effective dose determination and toxicological verification and technological research, including drug formulation.

Roles of the EPS66A polysaccharide from Streptomyces sp. in inducing tobacco resistance to tobacco mosaic virus

EPS66A was derived from an unidentified Streptomyces sp. HL-66 by chemical fraction and disease-resistance assays. It was identified as a polysaccharide through a series of chemical characterization, including infrared spectrum analysis, methylation, gas chromatography-mass spectrometry, nuclear magnetic resonance, and high-performance gel permeation chromatography. To determine its effect in plant, EPS66A was applied to tobacco leaves infected with TMV, resulting in the plant with enhanced systemic resistance with a significant reduction of TMV severity. Plant defense was confirmed by early responses, including hypersensitive response (HR) indicated by programed cell death, moderate alkalization, oxidative burst, increase in nitric oxide (NO) and salicylic acid (SA). Furthermore, EPS66A induced callose deposition to form defense barriers against pathogen invasion and the expression of pathogenesis-related (PR) genes, which confirmed the second level of plant defense. Therefore, EPS66A served as a resistance inducer, which was reorganized by tobacco cells that triggered the production of signal molecules. The signals moved in long distance and systemically in plant, which coordinated the expression of defense responses. The study provided a new perspective in understanding the mechanism of EPS66A in regulating plants on environmental adaptability and provided a theoretical foundation for designing safe and sustainable pesticides.

Genetic Analysis of the Grapevine GATA Gene Family and Their Expression Profiles in Response to Hormone and Downy Mildew Infection

Grapevine (Vitis. vinifera L.) is one of the most economically important fruit crops throughout the world. However, grape production is increasingly impacted by numerous diseases, including downy mildew, caused by the oomycete Plasmopara viticola. In grapevine and other plants, members of the GATA family of transcription factors play key roles in light and phytohormone signaling. However, little is known about their potential roles in biotic defense responses. As a first step, we identified 27 GATA transcription factors in grapevine and defined their transcriptional responses to three biotic stress-related phytohormones (SA, MeJA, and BR) in callus cells, and challenge with P. viticola in a downy mildew-sensitive cultivar, V. vinifera ‘Pinot noir’, and a resistant cultivar, V. piasezkii ‘Liuba-8′. Many of the VvGATA genes had higher expression at 0.5 h after hormones treatments. Moreover, a group of VvGATAs was dramatically induced in ‘Liuba-8′ at 24 post infection by P. viticola. However, the same genes were significantly repressed and showed low expression levels in ‘Pinot noir’. Additionally, VvGATA27 was located in the nucleus and had transcriptional activity. Taken together, the study identified the GATA full gene families in grapes on phylogenetic analysis and protein structure. Moreover, this study provided a basis for discussing the roles of VvGATAs in response to hormones and P. viticola infection. Our results provided evidence for the selection of candidate genes against downy mildew and lay the foundation for further investigation of VvGATA transcription factors.

Grapevine Stimulation: A Multidisciplinary Approach to Investigate the Effects of Biostimulants and a Plant Defense Stimulator

The increasing use of plant defense stimulators (PDS) and biostimulants (BS) to make agriculture more sustainable has led to questions about their action on plants. A new PhysBioGen approach is proposed with complementary tools: PHYSiological (root weight); BIOchemical and BIOlogical (secondary metabolite quantification and Plasmopara viticola development) and expressions of 161 GENes involved in metabolic plant functions. The proposed approach investigated the effects of three phytostimulants on Vitis vinifera: one PDS (ASM) and one BS chelated (CH) and another enriched with seaweed (SW). Distinct responses were obtained between the PDS and the two BS. In particular, we observed the persistence of anti-mildew efficacy over time, correlated with differentiated expressions of defense genes (VvROMT, VvSAMT, VvPR8). As expected, the two BS displayed more similarities to each other than to the PDS (flavonols, anthocyanins, free salicylic acid). However, the two BS revealed differences in the modulation of genes involved in defense and primary metabolism and some genes were identified as potential markers of their action (VvWRKY1, VvLOX9, VvPOD, VvPDV1, VvXIP1, VVDnaJ). Our results highlight the common and the specific effects of the two BS and the PDS. These new tools could help in understanding the mode of action of phytostimulants in order to achieve better quality and production yield and/or as a way to limit chemical inputs in the vineyard.

Phytotoxicity of Essential Oils: Opportunities and Constraints for the Development of Biopesticides. A Review

The extensive use of chemical pesticides leads to risks for both the environment and human health due to the toxicity and poor biodegradability that they may present. Farmers therefore need alternative agricultural practices including the use of natural molecules to achieve more sustainable production methods to meet consumer and societal expectations. Numerous studies have reported the potential of essential oils as biopesticides for integrated weed or pest management. However, their phytotoxic properties have long been a major drawback for their potential applicability (apart from herbicidal application). Therefore, deciphering the mode of action of essential oils exogenously applied in regards to their potential phytotoxicity will help in the development of biopesticides for sustainable agriculture. Nowadays, plant physiologists are attempting to understand the mechanisms underlying their phytotoxicity at both cellular and molecular levels using transcriptomic and metabolomic tools. This review systematically discusses the functional and cellular impacts of essential oils applied in the agronomic context. Putative molecular targets and resulting physiological disturbances are described. New opportunities regarding the development of biopesticides are discussed including biostimulation and defense elicitation or priming properties of essential oils.

Phyllosphere Colonization by a Soil Streptomyces sp. Promotes Plant Defense Responses Against Fungal Infection

Streptomycetes are soil-dwelling, filamentous actinobacteria and represent a prominent bacterial clade inside the plant root microbiota. The ability of streptomycetes to produce a broad spectrum of antifungal metabolites suggests that these bacteria could be used to manage plant diseases. Here, we describe the identification of a soil Streptomyces strain named AgN23 which strongly activates a large array of defense responses when applied on Arabidopsis thaliana leaves. AgN23 increased the biosynthesis of salicylic acid, leading to the development of salicylic acid induction deficient 2 (SID2)-dependent necrotic lesions. Size exclusion fractionation of plant elicitors secreted by AgN23 showed that these signals are tethered into high molecular weight complexes. AgN23 mycelium was able to colonize the leaf surface, leading to plant resistance against Alternaria brassicicola infection in wild-type Arabidopsis plants. AgN23-induced resistance was found partially compromised in salicylate, jasmonate, and ethylene mutants. Our data show that Streptomyces soil bacteria can develop at the surface of plant leaves to induce defense responses and protection against foliar fungal pathogens, extending their potential use to manage plant diseases.

Oregano essential oil vapour prevents Plasmopara viticola infection in grapevine (Vitis Vinifera) and primes plant immunity mechanisms

The reduction of synthetic fungicides in agriculture is necessary to guarantee a sustainable production that protects the environment and consumers' health. Downy mildew caused by the oomycete Plasmopara viticola is the major pathogen in viticulture worldwide and responsible for up to 60% of pesticide treatments. Alternatives to reduce fungicides are thus utterly needed to ensure sustainable vineyard-ecosystems, consumer health and public acceptance. Essential oils (EOs) are amongst the most promising natural plant protection alternatives and have shown their antibacterial, antiviral and antifungal properties on several agricultural crops. However, the efficiency of EOs highly depends on timing, application method and the molecular interactions between the host, the pathogen and EO. Despite proven EO efficiency, the underlying processes are still not understood and remain a black box. The objectives of the present study were: a) to evaluate whether a continuous fumigation of a particular EO can control downy mildew in order to circumvent the drawbacks of direct application, b) to decipher molecular mechanisms that could be triggered in the host and the pathogen by EO application and c) to try to differentiate whether essential oils directly repress the oomycete or act as plant resistance primers. To achieve this a custom-made climatic chamber was constructed that enabled a continuous fumigation of potted vines with different EOs during long-term experiments. The grapevine (Vitis vinifera) cv Chasselas was chosen in reason of its high susceptibility to Plasmopara viticola. Grapevine cuttings were infected with P. viticola and subsequently exposed to continuous fumigation of different EOs at different concentrations, during 2 application time spans (24 hours and 10 days). Experiments were stopped when infection symptoms were clearly observed on the leaves of the control plants. Plant physiology (photosynthesis and growth rate parameters) were recorded and leaves were sampled at different time points for subsequent RNA extraction and transcriptomics analysis. Strikingly, the Oregano vulgare EO vapour treatment during 24h post-infection proved to be sufficient to reduce downy mildew development by 95%. Total RNA was extracted from leaves of 24h and 10d treatments and used for whole transcriptome shotgun sequencing (RNA-seq). Sequenced reads were then mapped onto the V. vinifera and P. viticola genomes. Less than 1% of reads could be mapped onto the P. viticola genome from treated samples, whereas up to 30% reads from the controls mapped onto the P. viticola genome, thereby confirming the visual observation of P. viticola absence in the treated plants. On average, 80% of reads could be mapped onto the V. vinifera genome for differential expression analysis, which yielded 4800 modulated genes. Transcriptomic data clearly showed that the treatment triggered the plant's innate immune system with genes involved in salicylic, jasmonic acid and ethylene synthesis and signaling, activating Pathogenesis-Related-proteins as well as phytoalexin synthesis. These results elucidate EO-host-pathogen interactions for the first time and indicate that the antifungal efficiency of EO is mainly due to the triggering of resistance pathways inside the host plants. This is of major importance for the production and research on biopesticides, plant stimulation products and for resistance-breeding strategies.

Priming of Plant Resistance to Heat Stress and Tomato Yellow Leaf Curl Thailand Virus With Plant-Derived Materials

Plants are often simultaneously exposed to diverse environmental stresses, and can tune suitable responses to them through hormones. Salicylic acid (SA) and jasmonic acid (JA) signaling pathways are known to enhance resistance against heat stress and tomato yellow leaf curl Thailand virus (TYLCTHV) infection. However, there is limited information regarding alternative natural priming agents against heat stress and viruses. In this study, two plant-derived priming agents, eugenol and anise oil, were tested for their roles in conferring thermotolerance and virus resistance in tomato plants. Under heat stress, the survival rates and average fresh weight were higher in plants treated with eugenol or anise oil than in control plants. These two priming agents were further tested for antiviral activities. After TYLCTHV infection, the disease incidence and relative abundance of TYLCTHV were lower in anise oil- and eugenol-treated plants than in control plants. Further analyses revealed that a few SA, JA, and RNA silencing genes were enhanced in the former. Moreover, SA, JA, and H2O2 contents increased considerably after eugenol and anise oil treatments. Our findings imply that anise oil and eugenol initiated SA- and JA-mediated defenses to promote thermotolerance and antiviral responses of tomato plants.

MtNF-YA1, A Central Transcriptional Regulator of Symbiotic Nodule Development, Is Also a Determinant of Medicago truncatula Susceptibility toward a Root Pathogen

Plant NF-Y transcription factors control a wide array of biological functions enabling appropriate reproductive and developmental processes as well as adaptation to various abiotic and biotic environments. In Medicago truncatula, MtNF-YA1 was previously identified as a key determinant for nodule development and establishment of rhizobial symbiosis. Here, we highlight a new role for this protein in compatibility to Aphanomyces euteiches, a root pathogenic oomycete. The Mtnf-ya1-1 mutant plants showed better survival rate, reduced symptoms, and increased development of their root apparatus as compared to their wild-type (WT) background A17. MtNF-YA-1 was specifically up-regulated by A. euteiches in F83005.5, a highly susceptible natural accession of M. truncatula while transcript level remained stable in A17, which is partially resistant. The role of MtNF-YA1 in F83005.5 susceptibility was further documented by reducing MtNF-YA1 expression either by overexpression of the miR169q, a microRNA targeting MtNF-YA1, or by RNAi approaches leading to a strong enhancement in the resistance of this susceptible line. Comparative analysis of the transcriptome of WT and Mtnf-ya1-1 led to the identification of 1509 differentially expressed genes. Among those, almost 36 defense-related genes were constitutively expressed in Mtnf-ya1-1, while 20 genes linked to hormonal pathways were repressed. In summary, we revealed an unexpected dual role for this symbiotic transcription factor as a key player in the compatibility mechanisms to a pathogen.

Bio-based resistance inducers for sustainable plant protection against pathogens

An increasing demand for environmentally acceptable alternative for traditional pesticides provides an impetus to conceive new bio-based strategies in crop protection. Employing induced resistance is one such strategy, consisting of boosting the natural plant immunity. Upon infections, plants defend themselves by activating their immune mechanisms. These are initiated after the recognition of an invading pathogen via the microbe-associated molecular patterns (MAMPs) or other microbe-derived molecules. Triggered responses inhibit pathogen spread from the infected site. Systemic signal transport even enables to prepare, i.e. prime, distal uninfected tissues for more rapid and enhanced response upon the consequent pathogen attack. Similar defense mechanisms can be triggered by purified MAMPs, pathogen-derived molecules, signal molecules involved in plant resistance to pathogens, such as salicylic and jasmonic acid, or a wide range of other chemical compounds. Induced resistance can be also conferred by plant-associated microorganisms, including beneficial bacteria or fungi. Treatment with resistance inducers or beneficial microorganisms provides long-lasting resistance for plants to a wide range of pathogens. This study surveys current knowledge on resistance and its mechanisms provided by microbe-, algae- and plant-derived elicitors in different crops. The main scope deals with bacterial substances and fungus-derived molecules chitin and chitosan and algae elicitors, including naturally sulphated polysaccharides such as ulvans, fucans or carageenans. Recent advances in the utilization of this strategy in practical crop protection are also discussed.