beta-Aminobutyric acid-induced resistance against downy mildew in grapevine acts through the potentiation of callose formation and jasmonic acid signaling

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.

Advances in understanding grapevine downy mildew: From pathogen infection to disease management

Plasmopara viticola is geographically widespread in grapevine-growing regions. Grapevine downy mildew disease, caused by this biotrophic pathogen, leads to considerable yield losses in viticulture annually. Because of the great significance of grapevine production and wine quality, research on this disease has been widely performed since its emergence in the 19th century. Here, we review and discuss recent understanding of this pathogen from multiple aspects, including its infection cycle, disease symptoms, genome decoding, effector biology, and management and control strategies. We highlight the identification and characterization of effector proteins with their biological roles in host-pathogen interaction, with a focus on sustainable control methods against P. viticola, especially the use of biocontrol agents and environmentally friendly compounds.

Elevated CO2 alters photosynthesis, growth and susceptibility to powdery mildew of oak seedlings

Elevated CO2 (eCO2) is a determinant factor of climate change and is known to alter plant processes such as physiology, growth and resistance to pathogens. Quercus robur, a tree species integrated in most forest regeneration strategies, shows high vulnerability to powdery mildew (PM) disease at the seedling stage. PM is present in most oak forests and it is considered a bottleneck for oak woodland regeneration. Our study aims to decipher the effect of eCO2 on plant responses to PM. Oak seedlings were grown in controlled environment at ambient (aCO2, ∼400 ppm) and eCO2 (∼1000 ppm), and infected with Erysiphe alphitoides, the causal agent of oak PM. Plant growth, physiological parameters and disease progression were monitored. In addition, to evaluate the effect of eCO2 on induced resistance (IR), these parameters were assessed after treatments with IR elicitor β-aminobutyric acid (BABA). Our results show that eCO2 increases photosynthetic rates and aerial growth but in contrast, reduces root length. Importantly, under eCO2 seedlings were more susceptible to PM. Treatments with BABA protected seedlings against PM and this protection was maintained under eCO2. Moreover, irrespectively of the concentration of CO2, BABA did not significantly change aerial growth but resulted in longer radicular systems, thus mitigating the effect of eCO2 in root shortening. Our results demonstrate the impact of eCO2 in plant physiology, growth and defence, and warrant further biomolecular studies to unravel the mechanisms by which eCO2 increases oak seedling susceptibility to PM.

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.

Mechanisms associated with the synergistic induction of resistance to tobacco black shank in tobacco by arbuscular mycorrhizal fungi and β-aminobutyric acid

Tobacco black shank (TBS), caused by Phytophthora nicotianae, is one of the most harmful diseases of tobacco. There are many studies have examined the mechanism underlying the induction of disease resistance by arbuscular mycorrhizal fungi (AMF) and β-aminobutyric acid (BABA) alone, but the synergistic effects of AMF and BABA on disease resistance have not yet been studied. This study examined the synergistic effects of BABA application and AMF inoculation on the immune response to TBS in tobacco. The results showed that spraying BABA on leaves could increase the colonization rate of AMF, the disease index of tobacco infected by P.nicotianae treated with AMF and BABA was lower than that of P.nicotianae alone. The control effect of AMF and BABA on tobacco infected by P.nicotianae was higher than that of AMF or BABA and P.nicotianae alone. Joint application of AMF and BABA significantly increased the content of N, P, and K in the leaves and roots, in the joint AMF and BABA treatment than in the sole P. nicotianae treatment. The dry weight of plants treated with AMF and BABA was 22.3% higher than that treated with P.nicotianae alone. In comparison to P. nicotianae alone, the combination treatment with AMF and BABA had increased Pn, Gs, Tr, and root activity, while P. nicotianae alone had reduced Ci, H₂O₂ content, and MDA levels. SOD, POD, CAT, APX, and Ph activity and expression levels were increased under the combined treatment of AMF and BABA than in P.nicotianae alone. In comparison to the treatment of P.nicotianae alone, the combined use of AMF and BABA increased the accumulation of GSH, proline, total phenols, and flavonoids. Therefore, the joint application of AMF and BABA can enhance the TBS resistance of tobacco plants to a greater degree than the application of either AMF or BABA alone. In summary, the application of defense-related amino acids, combined with inoculation with AMF, significantly promoted immune responses in tobacco. Our findings provide new insights that will aid the development and use of green disease control agents.

Epigenetic regulation of plant immunity: from chromatin codes to plant disease resistance

Facing a deteriorating natural environment and an increasing serious food crisis, bioengineering-based breeding is increasing in importance. To defend against pathogen infection, plants have evolved multiple defense mechanisms, including pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). A complex regulatory network acts downstream of these PTI and ETI pathways, including hormone signal transduction and transcriptional reprogramming. In recent years, increasing lines of evidence show that epigenetic factors act, as key regulators involved in the transcriptional reprogramming, to modulate plant immune responses. Here, we summarize current progress on the regulatory mechanism of DNA methylation and histone modifications in plant defense responses. In addition, we also discuss the application of epigenetic mechanism-based resistance strategies in plant disease breeding.

Chemical priming of plant defense responses to pathogen attacks

Plants can acquire an improved resistance against pathogen attacks by exogenous application of natural or artificial compounds. In a process called chemical priming, application of these compounds causes earlier, faster and/or stronger responses to pathogen attacks. The primed defense may persist over a stress-free time (lag phase) and may be expressed also in plant organs that have not been directly treated with the compound. This review summarizes the current knowledge on the signaling pathways involved in chemical priming of plant defense responses to pathogen attacks. Chemical priming in induced systemic resistance (ISR) and systemic acquired resistance (SAR) is highlighted. The roles of the transcriptional coactivator NONEXPRESSOR OF PR1 (NPR1), a key regulator of plant immunity, induced resistance (IR) and salicylic acid signaling during chemical priming are underlined. Finally, we consider the potential usage of chemical priming to enhance plant resistance to pathogens in agriculture.

β-Aminobutyric acid induced phytotoxicity and effectiveness against nematode is stereomer-specific and dose-dependent in tomato

β-Aminobutyric acid (BABA) induces resistance to a/biotic stress but is associated with phytotoxicity in some plant species. There are two enantiomers of BABA, the R and S enantiomers. We evaluated the phytotoxicity caused by the RS BABA (racemic mixture of R and S BABA), evaluating the dose-response effect and different modes of application on tomato. Results show that RS BABA-induced phytotoxicity in tomato is dose-dependent and stronger with foliar applications than with soil drench. We further evaluated the phytotoxicity of the two enantiomers separately and observed that BABA-induced phytotoxicity is stereomer-specific. In comparison with less phytotoxic effects induced by S BABA, R BABA induces dose-dependent and systemic phytotoxic symptoms. To investigate the possible physiological causes of this phytotoxicity, we measured levels of oxidative stress and anthocyanins and validated the findings with gene expression analyses. Our results show that high doses of RS and R BABA induce hydrogen peroxide, lipid peroxidation, and anthocyanin accumulation in tomato leaves, while this response is milder and more transient upon S BABA application. Next, we evaluated BABA induced resistance against root-knot nematode Meloidogyne incognita in tomato. BABA-induced resistance was found to be stereomer-specific and dependent on dose and mode of application. R or RS BABA multiple soil drench application at low doses induces resistance to nematodes with less phytotoxic effects. Taken together, our data provide useful knowledge on how BABA can be applied in crop production by enhancing stress tolerance and limiting phytotoxicity.

Information on disease resistance patterns of grape varieties may improve disease management

Resistance to downy mildew (DM) and powdery mildew (PM) contributes to sustainable vineyard management by reducing the diseases and the need for fungicide applications. Resistant varieties vary in their degree of resistance to DM and PM, and in their susceptibility to other diseases. As a consequence, fungicide use may differ among varieties depending on their "resistance patterns" (i.e., the resistance level of a variety toward all of the diseases in the vineyard). The resistance patterns of 16 grapevine varieties to DM, PM, black rot (BR), and gray mold (GM) were evaluated over a 4-year period under field conditions. Disease severity was assessed on leaves and bunches, and the AUDPC (Area Under Disease Progress Curve) was calculated to represent the epidemic progress. GM was found only on bunches and only at very low levels, irrespective of the year or variety, and was therefore excluded from further analyses. The varieties were then grouped into four resistance patterns: i) low resistance to DM and PM, intermediate resistance to BR; ii) high resistance to DM, intermediate resistance to PM, low resistance to BR; iii) intermediate resistance to DM and BR, low resistance to PM; and iv) high resistance to DM, PM, and BR. AUDPC values on leaves were positively correlated with AUDPC values on bunches for susceptible varieties but not for resistant ones, with the exception of PM. Therefore, bioassays with leaves can be used to predict the resistance of bunches to DM and BR for susceptible varieties but not for resistant ones. These results may facilitate both strategic and tactical decisions for the sustainable management of grapevine diseases.

L-Aspartate and L-Glutamine Inhibit Beta-Aminobutyric Acid-Induced Resistance in Tomatoes

Plant diseases caused by pathogens lead to economic and agricultural losses, while plant resistance is defined by robustness and timing of defence response. Exposure to microbial-associated molecular patterns or specific chemical compounds can promote plants into a primed state with more robust defence responses. β-aminobutyric acid (BABA) is an endogenous stress metabolite that induces resistance, thereby protecting various plants' diverse stresses by induction of non-canonical activity after binding into aspartyl-tRNA synthetase (AspRS). In this study, by integrating BABA-induced changes in selected metabolites and transcript data, we describe the molecular processes involved in BABA-induced resistance (BABA-IR) in tomatoes. BABA significantly restricted the growth of the pathogens P. syringae pv. tomato DC3000 and was related to the accumulation of transcripts for pathogenesis-related proteins and jasmonic acid signalling but not salicylic acid signalling in Arabidopsis. The resistance was considerably reduced by applying amino acids L-Asp and L-Gln when L-Gln prevents general amino acid inhibition in plants. Analysis of amino acid changes suggests that BABA-IR inhibition by L-Asp is due to its rapid metabolisation to L-Gln and not its competition with BABA for the aspartyl-tRNA synthetase (AspRS) binding site. Our results showed differences between the effect of BABA on tomatoes and other model plants. They highlighted the importance of comparative studies between plants of agronomic interest subjected to treatment with BABA.

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.

MADS2 regulates priming defence in postharvest peach through combined salicylic acid and abscisic acid signaling

MADS-box genes play well-documented roles in plant development, but relatively little is known regarding their involvement in defence responses. In this study, pre-treatment of peach (Prunus persica) fruit with β-aminobutyric acid (BABA) activated resistance against Rhizopus stolonifer, leading to a significant delay in the symptomatic appearance of disease. This was associated with an integrated defence response that included a H2O2 burst, ABA accumulation, and callose deposition. cDNA library screening identified nucleus-localized MADS2 as an interacting partner with NPR1, and this was further confirmed by yeast two-hybrid, luciferase complementation imaging, and co-immunoprecipitation assays. The DNA-binding activity of NPR1 conferred by the NPR1-MADS2 complex was required for the transcription of SA-dependent pathogenesis-related (PR) and ABA-inducible CalS genes in order to gain the BABA-induced resistance, in which MAPK1-induced post-translational modification of MADS2 was also involved. In accordance with this, overexpression of PpMADS2 in Arabidopsis potentiated the transcription of a group of PR genes and conferred fungal resistance in the transgenic plants. Conversely, Arabidopsis mads2-knockout lines showed high sensitivity to the fungal pathogen. Our results indicate that MADS2 positively participates in BABA-elicited defence in peach through a combination of SA-dependent NPR1 activation and ABA signaling-induced callose accumulation, and that this defence is also related to the post-translational modification of MADS2 by MAPK1 for signal amplification.

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.

Glycine Betaine and β-Aminobutyric Acid Mitigate the Detrimental Effects of Heat Stress on Chinese Cabbage (Brassica rapa L. ssp. pekinensis) Seedlings with Improved Photosynthetic Performance and Antioxidant System

Heat stress is one of the major abiotic factors that limit the growth, development, and productivity of plants. Both glycine betaine (GB) and β-aminobutyric acid (BABA) have received considerable attention due to their roles in stimulating tolerance to diverse abiotic stresses. In order to understand how GB and BABA biostimulants alleviate heat stress in a cool-weather Chinese cabbage (Brassica rapa L. ssp. pekinensis) plant, we investigated the GB- and BABA-primed heat-stressed plants in terms of their morpho-physiological and biochemical traits. Priming with GB (15 mM) and BABA (0.2 mM) was conducted at the third leaf stage by applying foliar sprays daily for 5 days before 5 days of heat stress (45 °C in 16 h light/35 °C in 8 h dark) on Chinese cabbage seedlings. The results indicate that GB and BABA significantly increased chlorophyll content, and the parameters of both gas exchange and chlorophyll fluorescence, of Chinese cabbage under heat stress. Compared with the unprimed heat-stressed control, the dry weights of GB- and BABA-primed plants were significantly increased by 36.36% and 45.45%, respectively. GB and BABA priming also greatly mitigated membrane damage, as indicated by the reduction in malondialdehyde (MDA) and electrolyte leakage through the elevation of proline content, and increased activity levels of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). Taken together, GB and BABA have great potential to enhance the thermotolerance of Chinese cabbage through higher photosynthesis performance, osmoprotection, and antioxidant enzyme activity.

Diversity of Neofusicoccum parvum for the Production of the Phytotoxic Metabolites (-)-Terremutin and (R)-Mellein

Two Neofusicoccumparvum isolates and a UV mutant were characterized for their phytotoxin production in vitro, their pathogenicity on grapevine, and their genome sequenced. The isolate Np-Bt67 produced high level of (-)-terremutin, but almost no (R)-mellein, and it was the most aggressive on grapevine, triggering apoplexy. Similar symptoms were not induced by purified (-)-terremutin. The isolate Bourgogne S-116 (Np-B) produced 3-fold less (-)-terremutin and high amounts of (R)-mellein, but it was less aggressive on grapevine than Np-Bt67. The UV9 mutant obtained from Np-B (NpB-UV9) no longer produced (-)-terremutin but overproduced (R)-mellein by 2.5-fold, and it was as pathogenic as its parent. NpB-UV9 differed from its parent by simple mutations in two genes (transcription factor UCR-NP2_6692, regulatory protein UCR-NP2_9007), not located neither near (R)-mellein, nor (-)-terremutin biosynthetic genes, but likely involved in the control of (-)-terremutin biosynthesis. Grapevine immunity was disturbed upon challenge with these pathogens or purified phytotoxins, leading to an upregulation of SA-dependent defenses, while (-)-terremutin interfered with host JA/ET-dependent defenses. Our results suggest that neither (-)-terremutin nor (R)-mellein alone is essential for the pathogenicity of N. parvum on grapevine, since isolate/mutant non-producing these toxins in vitro is pathogenic. However, these phytotoxins could play a quantitative role in the infection process.

D-Tagatose-Based Product Triggers Sweet Immunity and Resistance of Grapevine to Downy Mildew, but Not to Gray Mold Disease

The use of natural bio-based compounds becomes an eco-friendly strategy to control plant diseases. Rare sugars would be promising compounds as inducers of plant “sweet immunity”. The present study aimed to investigate the induced resistance of grapevine leaves against Plasmopara viticola and Botrytis cinerea by a rare sugar-based product (IFP48) and its active ingredient D-tagatose (TAG), in order to elucidate molecular mechanism involved in defense-related metabolic regulations before and after pathogen challenge. Data showed that spraying leaves with IFP48 and TAG lead to a significant reduction of downy mildew, but not of gray mold disease. The induced protection against P. viticola relies on IFP48’s and to a lesser extent TAG’s ability to potentiate the activation of salicylic acid- and jasmonic acid/ethylene-responsive genes and stilbene phytoalexin accumulation. Most of defense responses remained upregulated in IFP48-treated plants after infection with P. viticola, but inconsistent following challenge with B. cinerea. The beneficial effects of IFP48 were associated with an enhanced accumulation of tagatose inside leaf tissues compared to TAG treatment. Meanwhile, the amounts of sugars, glucose, fructose, maltose, galactose and trehalose remained unchanged or decreased in IFP48-treated leaves after P. viticola infection, although only a few genes involved in sugar transport and metabolism showed transcriptional regulation. This suggests a contribution of sugar homeostasis to the IFP48-induced sweet immune response and priming plants for enhanced resistance to P. viticola, but not to B. cinerea.

Metabonomics analysis of drought resistance of wheat seedlings induced by β-aminobutyric acid-modified chitooligosaccharide derivative

Chitooligosaccharide grafted with β-aminobutyric acid based on the idea of bioactive molecular splicing was prepared, and the differences in drought resistance activity before and after grafting were compared. The mechanism was investigated by comparing the differences of the derivative with the Control and Drought about metabolomes. The results showed that the expected derivative was successfully synthesized, named COS-BABA, and had better drought resistance-inducing activity than the raw materials. We suggest that COS-BABA induced drought resistance through second messenger-induced activation of signaling pathways related to traumatic acid and indol-3-lactic acid, which enhanced nucleic acid metabolism to accumulate nucleotides and decreased some amino acids to facilitate protein synthesis. These proteins are regulated to strengthen photosynthesis, resulting in the promotion of carbohydrate metabolism. The accumulation of unsaturated fatty acids stabilized the cell membrane structure and prevented nonstomatal water dissipation. This study provides ideas for the development of more effective drought resistance inducers.

Cultivar Contributes to the Beneficial Effects of Bacillus subtilis PTA-271 and Trichoderma atroviride SC1 to Protect Grapevine Against Neofusicoccum parvum

Grapevine trunk diseases (GTDs) are a big threat for global viticulture. Without effective chemicals, biocontrol strategies are developed as alternatives to better cope with environmental concerns. A combination of biological control agents (BCAs) could even improve sustainable disease management through complementary ways of protection. In this study, we evaluated the combination of Bacillus subtilis (Bs) PTA-271 and Trichoderma atroviride (Ta) SC1 for the protection of Chardonnay and Tempranillo rootlings against Neofusicoccum parvum Bt67, an aggressive pathogen associated to Botryosphaeria dieback (BD). Indirect benefits offered by each BCA and their combination were then characterized in planta, as well as their direct benefits in vitro. Results provide evidence that (1) the cultivar contributes to the beneficial effects of Bs PTA-271 and Ta SC1 against N. parvum, and that (2) the in vitro BCA mutual antagonism switches to the strongest fungistatic effect toward Np-Bt67 in a three-way confrontation test. We also report for the first time the beneficial potential of a combination of BCA against Np-Bt67 especially in Tempranillo. Our findings highlight a common feature for both cultivars: salicylic acid (SA)-dependent defenses were strongly decreased in plants protected by the BCA, in contrast with symptomatic ones. We thus suggest that (1) the high basal expression of SA-dependent defenses in Tempranillo explains its highest susceptibility to N. parvum, and that (2) the cultivar-specific responses to the beneficial Bs PTA-271 and Ta SC1 remain to be further investigated.

Transcriptome analysis revealed key genes involved in flavonoid metabolism in response to jasmonic acid in pigeon pea (Cajanus cajan (L.) Millsp.)

Flavonoids are important metabolites of pigeon pea in relation to its stress resistance. However, the molecular basis and regulatory mechanisms of flavonoids in pigeon pea remain unclear. Methyl jasmonate (MeJA) is a signaling molecule associated with biosynthesis of flavonoids. In this study, after exogenous treatment of 10 mg/L MeJA, infection of pathogenic fungi to pigeon pea was alleviated and the content of flavonoids was increased. Results of gene expression and metabolic changes that were respectively analyzed by transcriptome sequencing and high performance liquid chromatography (HPLC) showed that (1) concentrations of various flavonoids, such as genistein, apigenin, vitexin and biochanin A were significantly up-regulated; (2) 13675 differentially expressed genes were produced, mainly enriched in signal transduction and isoflavone biosynthesis pathways: (3) the expression levels of key synthase genes (CcI2'H, CcHIDH, Cc7-IOMT) in the flavonoid biosynthesis pathway were significantly up-regulated; (4) Overexpression of CcbHLH35 significantly induced upregulation of flavonoid synthase genes and accumulation of genistein, vitexin and apigenin. Our findings reveals the pivotal roles of MeJA in synthesis and functioning of flavonoids in pigeon pea, which provide a basis for further studies on flavonoid-mediated defense responses.

Trichoderma spp. volatile organic compounds protect grapevine plants by activating defense-related processes against downy mildew

Volatile organic compounds (VOCs) are produced by soil-borne microorganisms and play crucial roles in fungal interactions with plants and phytopathogens. Although VOCs have been characterized in Trichoderma spp., the mechanisms against phytopathogens strongly differ according to the strain and pathosystem. This study aimed at characterizing VOCs produced by three Trichoderma strains used as biofungicides and to investigate their effects against grapevine downy mildew (caused by Plasmopara viticola). A VOC-mediated reduction of downy mildew severity was found in leaf disks treated with Trichoderma asperellum T34 (T34), T. harzianum T39 (T39), and T. atroviride SC1 (SC1) and 31 compounds were detected by head space-solid phase microextraction gas chromatography-mass spectrometry. Among the Trichoderma VOCs annotated, α-farnesene, cadinene, 1,3-octadiene, 2-pentylfuran, and 6-pentyl-2H-pyran-2-one reduced downy mildew severity on grapevine leaf disks. In particular, 6-pentyl-2H-pyran-2-one and 2-pentylfuran increased the accumulation of callose and enhanced the modulation of defense-related genes after P. viticola inoculation, indicating an induction of grapevine defense mechanisms. Moreover, 6-pentyl-2H-pyran-2-one activated the hypersensitive response after P. viticola inoculation, possibly to reinforce the grapevine defense reaction. These results indicate that Trichoderma VOCs can induce grapevine resistance, and these molecules could be further applied to control grapevine downy mildew.

Transcriptional profiling reveals multiple defense responses in downy mildew-resistant transgenic grapevine expressing a TIR-NBS-LRR gene located at the MrRUN1/MrRPV1 locus

Grapevine downy mildew (DM) is a destructive oomycete disease of viticulture worldwide. MrRPV1 is a typical TIR-NBS-LRR type DM disease resistance gene cloned from the wild North American grapevine species Muscadinia rotundifolia. However, the molecular basis of resistance mediated by MrRPV1 remains poorly understood. Downy mildew-susceptible Vitis vinifera cv. Shiraz was transformed with a genomic fragment containing MrRPV1 to produce DM-resistant transgenic Shiraz lines. Comparative transcriptome analysis was used to compare the transcriptome profiles of the resistant and susceptible genotypes after DM infection. Transcriptome modulation during the response to P. viticola infection was more rapid, and more genes were induced in MrRPV1-transgenic Shiraz than in wild-type plants. In DM-infected MrRPV1-transgenic plants, activation of genes associated with Ca²⁺ release and ROS production was the earliest transcriptional response. Functional analysis of differentially expressed genes revealed that key genes related to multiple phytohormone signaling pathways and secondary metabolism were highly induced during infection. Coexpression network and motif enrichment analysis showed that WRKY and MYB transcription factors strongly coexpress with stilbene synthase (VvSTS) genes during defense against P. viticola in MrRPV1-transgenic plants. Taken together, these findings indicate that multiple pathways play important roles in MrRPV1-mediated resistance to downy mildew.

Responses to Drought Stress Modulate the Susceptibility to Plasmopara viticola in Vitis vinifera Self-Rooted Cuttings

Climate change will increase the occurrence of plants being simultaneously subjected to drought and pathogen stress. Drought can alter the way in which plants respond to pathogens. This research addresses how grapevine responds to the concurrent challenge of drought stress and Plasmopara viticola, the causal agent of downy mildew, and how one stress affects the other. Self-rooted cuttings of the drought-tolerant grapevine cultivar Xynisteri and the drought-sensitive cultivar Chardonnay were exposed to full or deficit irrigation (40% of full irrigation) and artificially inoculated with P. viticola in vitro or in planta. Leaves were sampled at an early infection stage to determine the influence of the single and combined stresses on oxidative parameters, chlorophyll, and phytohormones. Under full irrigation, Xynisteri was more susceptible to P. viticola than the drought-sensitive cultivar Chardonnay. Drought stress increased the susceptibility of grapevine leaves inoculated in vitro, but both cultivars showed resistance against P. viticola when inoculated in planta. Abscisic acid, rather than jasmonic acid and salicylic acid, seemed to play a prominent role in this resistance. The irrigation-dependent susceptibility observed in this study indicates that the practices used to mitigate the effects of climate change may have a profound impact on plant pathogens.

Kaolin Particle Film Protects Grapevine cv. Cabernet Sauvignon Against Downy Mildew by Forming Particle Film at the Leaf Surface, Directly Acting on Sporangia and Inducing the Defense of the Plant

Downy mildew is a major threat to viticulture, leading to severe yield loss. The use of traditional copper-based fungicides is effective, but has adverse effects on the environment and human health, making it urgent to develop an environmentally friendly disease management program. Multi-functional kaolin particle film (KPF) is promising as an effective and safer treatment strategy, since this material lacks chemically active ingredients. In this study, ability of Kaolin particle film (KPF) pretreatment to protect grapevine leaves from Plasmopara viticola was tested and the mode of action of KPF was analyzed. KPF application reduced the disease severity and the development of intercellular hyphae. Additionally, there was reduced accumulation of H₂O₂ and malondialdehyde (MDA) with pretreatment. The observation of ultrastructure on the leaf surface showed KPF deposition and stomatal obstruction, indicating that KPF protected plants against disease by preventing the adhesion of pathogens to the leaf surface and blocking invasion through the stomata. KPF pretreatment also activated host defense responses, as evidenced by increased activities of anti-oxidative enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] and defense-related enzymes [phenylalanine ammonia-lyase (PAL), chitinases, and β-1,3-glucanases], increased phytohormone signals [abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA)] and the up-regulation of defense genes related to plant defense. Overall, these results demonstrate that KPF treatment counters grapevine downy mildew by protecting leaves and enhancing plant defense responses.

γ-Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

The entanglement between primary metabolism regulation and stress responses is a puzzling and fascinating theme in plant sciences. Among the major metabolites found in plants, γ-aminobutyric acid (GABA) fulfils important roles in connecting C and N metabolic fluxes through the GABA shunt. Activation of GABA metabolism is known since long to occur in plant tissues following biotic stresses, where GABA appears to have substantially different modes of action towards different categories of pathogens and pests. While it can harm insects thanks to its inhibitory effect on the neuronal transmission, its capacity to modulate the hypersensitive response in attacked host cells was proven to be crucial for host defences in several pathosystems. In this review, we discuss how plants can employ GABA's versatility to effectively deal with all the major biotic stressors, and how GABA can shape plant immune responses against pathogens by modulating reactive oxygen species balance in invaded plant tissues. Finally, we discuss the connections between GABA and other stress-related amino acids such as BABA (β-aminobutyric acid), glutamate and proline.

Oxathiapiprolin, a Novel Chemical Inducer Activates the Plant Disease Resistance

Oxathiapiprolin was developed as a specific plant pathogenic oomycete inhibitor, previously shown to have highly curative and protective activities against the pepper Phytophthora blight disease under field and greenhouse tests. Therefore, it was hypothesized that oxathiapiprolin might potentially activate the plant disease resistance against pathogen infections. This study investigated the potential and related mechanism of oxathiapiprolin to activate the plant disease resistance using the bacterium Pseudomonas syringae pv tomato (Pst) and plant Arabidopsis interaction as the targeted system. Our results showed that oxathiapiprolin could activate the plant disease resistance against Pst DC3000, a non-target pathogen of oxathiapiprolin, in Arabidopsis, tobacco, and tomato plants. Our results also showed the enhanced callose deposition and H2O2 accumulation in the oxathiapiprolin-treated Arabidopsis under the induction of flg22 as the pathogen-associated molecular pattern (PAMP) treatment. Furthermore, increased levels of free salicylic acid (SA) and jasmonic acid (JA) were detected in the oxathiapiprolin-treated Arabidopsis plants compared to the mock-treated ones under the challenge of Pst DC3000. Besides, the gene expression results confirmed that at 24 h after the infiltration with Pst DC3000, the oxathiapiprolin-treated Arabidopsis plants had upregulated expression levels of the respiratory burst oxidase homolog D (RBOHD), JA-responsive gene (PDF1.2), and SA-responsive genes (PR1, PR2, and PR5) compared to the control. Taken together, oxathiapiprolin is identified as a novel chemical inducer which activates the plant disease resistance against Pst DC3000 by enhancing the callose deposition, H2O2 accumulation, and hormone SA and JA production.

BABA application improves soybean resistance to aphid through activation of phenylpropanoid metabolism and callose deposition

BACKGROUND

Beta-aminobutyric acid (BABA) confer plant resistance to a broad spectrum of biotic and abiotic stresses. The soybean aphid (SBA), is native to eastern Asia and is a predominant insect pest of soybean. Both isoflavone and lignin pathway are important branches of the general phenylpropanoid pathway, which would be likely associated with resistance against soybean aphid. However, little is known about the role of the phenylpropanoid pathway in defense response to SBA as induced by BABA application.

RESULTS

The application of BABA effectively enhanced soybean resistance against Aphis glycines, the soybean aphid. Consistent with significantly increased content of isoflavones, especially genistein, the related biosynthetic genes were upregulated by use of BABA. Lignin, another important defense component against arthropods, accumulated at a high level and four lignin biosynthesis related genes were also activated. Additionally, BABA application augmented the expression of callose synthase genes and increased callose deposition in SBA-infested seedlings. In non-caged and caged tests, SBA numbers were significantly reduced in BABA-treated seedlings.

CONCLUSION

These results demonstrate that application of BABA has an obvious positive effect on soybean resistance to aphids, and this defense response partly depends on the potentiation of isoflavone biosynthesis and callose deposition. © 2019 Society of Chemical Industry.

Microfluidic synthesis of methyl jasmonate-loaded PLGA nanocarriers as a new strategy to improve natural defenses in Vitis vinifera

The objective of the present work was to synthesize biopolymeric nanoparticles (NPs) entrapping the resistance-inductor methyl jasmonate (MeJA) to be employed as a novel and alternative strategy in integrated pest management. NPs were prepared by using a continuous flow microfluidic reactor that allows to precisely control some features that are crucial for applications such as size, polydispersion, morphology and reproducibility. Poly(lactic-co-glycolic acid) (PLGA), a biopolymer largely studied for its use in biological applications, was chosen for the production of NPs entrapping MeJA, a biotic endogenous elicitor able to trigger plant’s defense responses. The effect of different fluid-dynamic conditions, PLGA molecular weight and concentration on NP properties (dimensions, polydispersion, morphology, stability) was evaluated. DLS and SEM were employed to characterize the obtained NPs. MeJA-loaded PLGA NPs ranging from 40 to 70 nm were administered to Vitis vinifera cell cultures, in order to evaluate the biological response in terms of stilbene biosynthesis. HPLC investigations showed a faster response when the elicitor was administered by PLGA NPs in comparison with free MeJA. This result demonstrates that the encapsulation in PLGA NPs significantly promotes MeJA cell uptake and the activation of MeJA-induced responses.

Expression of stilbene synthase VqSTS6 from wild Chinese Vitis quinquangularis in grapevine enhances resveratrol production and powdery mildew resistance

In grape (Vitis), stilbene phytoalexins can either be in situ synthesized or transported to the site of response during powdery mildew infection, enhancing disease resistance. Resveratrol is a phytoprotective stilbenoid compound that is synthesized by stilbene synthase (STS) in response to biotic and abiotic stresses, and is also known to have health benefits in the human diet. We have previously shown that transgenic Vitis vinifera cv. Thompson Seedless plants overexpressing a stilbene synthase gene, VqSTS6, from wild Chinese Vitis quinquangularis had a higher stilbenoid content, leading to an enhanced resistance to powdery mildew (Uncinula necator (Schw.) Burr). However, the biosynthesis and transportation in the plant tissue under powdery mildew infection are still unclear. Here, inhibitor and micro-grafting technologies were used to study the accumulation of resveratrol following powdery mildew infection. We observed that the levels of STS expression and stilbenoids increased in response to powdery mildew infection. Powdery mildew and inhibitor treatment on detached grape branches showed that resveratrol was in situ synthesized. Experiments with grafted plantlets showed that the abundance of stilbenoid compounds increased in the shoot during VqSTS6 overexpression in the root, while VqSTS6-Flag fusion was not tranported to the scions and only expressed in the transgenic rootstocks. Compared with wild-type Thompson Seedless plants, the non-transgenic/VqSTS6 transgenic (scion/rootstock) grafted Thompson Seedless plantlets exhibited increased resistance to powdery mildew. In addition, overexpression of VqSTS6 in roots led to increased levels of stilbenoid compounds in five other European grape varieties (V. vinifera cvs. Chardonnay, Perlette, Cabernet Sauvignon, Riesling and Muscat Hamburg). In conclusion, stilbenoid compounds can be either in situ synthesized or transported to the site of powdery mildew infection, and overexpression of VqSTS6 in the root promotes stilbenoids accumulation and disease resistance in European grapevine varieties.

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.

β-Aminobutyric Acid Priming Acquisition and Defense Response of Mango Fruit to Colletotrichum gloeosporioides Infection Based on Quantitative Proteomics

β-aminobutyric acid (BABA) is a new environmentally friendly agent to induce disease resistance by priming of defense in plants. However, molecular mechanisms underlying BABA-induced priming defense are not fully understood. Here, comprehensive analysis of priming mechanism of BABA-induced resistance was investigated based on mango-Colletotrichum gloeosporioides interaction system using iTRAQ-based proteome approach. Results showed that BABA treatments effectively inhibited the expansion of anthracnose caused by C. gleosporioides in mango fruit. Proteomic results revealed that stronger response to pathogen in BABA-primed mango fruit after C. gleosporioides inoculation might be attributed to differentially accumulated proteins involved in secondary metabolism, defense signaling and response, transcriptional regulation, protein post-translational modification, etc. Additionally, we testified the involvement of non-specific lipid-transfer protein (nsLTP) in the priming acquisition at early priming stage and memory in BABA-primed mango fruit. Meanwhile, spring effect was found in the primed mango fruit, indicated by inhibition of defense-related proteins at priming phase but stronger activation of defense response when exposure to pathogen compared with non-primed fruit. As an energy-saving strategy, BABA-induced priming might also alter sugar metabolism to provide more backbone for secondary metabolites biosynthesis. In sum, this study provided new clues to elucidate the mechanism of BABA-induced priming defense in harvested fruit.

Systemic defense activation by COS-OGA in rice against root-knot nematodes depends on stimulation of the phenylpropanoid pathway

Activation of induced plant resistance to control pests and diseases is regaining attention in the current climate where chemical pesticides are being progressively banned. Formulations of chitosan oligomers (COS) and pectin-derived oligogalacturonides (OGA), COS-OGA, have previously been described to induce resistance against fungal diseases in different crop plants. Here, we investigated their potential and mode-of-action as preventive measures to control root-knot nematode Meloidogyne graminicola infection in rice. The results show a significant reduction in root-galling and nematode development in rice plants that were treated through foliar application with the COS-OGA formulations FytoSol^® and FytoSave^® 24 h before nematode inoculation. Hormone measurements, gene expression analyses, corroborated by treatments on salicylic acid (SA) and jasmonic acid (JA)-mutants indicated that the systemic COS-OGA induced defense mechanism against nematodes is not based on SA or JA activation. However, phenylalanine ammonia lyase (PAL) gene expression in roots as well as enzymatic PAL activity in the shoots were significantly induced 24 h after foliar COS-OGA spraying in comparison with untreated plants. COS-OGA-induced systemic defense was abolished in the rice OsPAL4-mutant, demonstrating that COS-OGA-induced defense is dependent on OsPAL4 activation in rice plants.

Surfactin and fengycin contribute to the protection of a Bacillus subtilis strain against grape downy mildew by both direct effect and defence stimulation

Bacillus subtilis GLB191 (hereafter GLB191) is an efficient biological control agent against the biotrophic oomycete Plasmopara viticola, the causal agent of grapevine downy mildew. In this study, we show that GLB191 supernatant is also highly active against downy mildew and that the activity results from both direct effect against the pathogen and stimulation of the plant defences (induction of defence gene expression and callose production). High-performance thin-layer chromatography analysis revealed the presence of the cyclic lipopeptides fengycin and surfactin in the supernatant. Mutants affected in the production of fengycin and/or surfactin were thus obtained and allowed us to show that both surfactin and fengycin contribute to the double activity of GLB191 supernatant against downy mildew. Altogether, this study suggests that GLB191 supernatant could be used as a new biocontrol product against grapevine downy mildew.

Comparison of the Molecular Responses of Tolerant, Susceptible and Highly Susceptible Grapevine Cultivars During Interaction With the Pathogenic Fungus Eutypa lata

Eutypa lata is the causal agent of eutypa dieback, one of the most destructive grapevine trunk disease that causes severe economic losses in vineyards worldwide. This fungus causes brown sectorial necrosis in wood which affect the vegetative growth. Despite intense research efforts made in the past years, no cure currently exists for this disease. Host responses to eutypa dieback are difficult to address because E. lata is a wood pathogen that causes foliar symptoms several years after infection. With the aim to classify the level of susceptibility of grapevine cultivars to the foliar symptoms caused by E. lata, artificial inoculations of Merlot, Cabernet Sauvignon, and Ugni Blanc were conducted over 3 years. Merlot was the most tolerant cultivar, whereas Ugni Blanc and Cabernet Sauvignon exhibited higher and differential levels of susceptibility. We took advantage of their contrasting phenotypes to explore their defense responses, including the activation of pathogenesis-related (PR) genes, oxylipin and phenylpropanoid pathways and the accumulation of stilbenes. These analyses were carried out using the millicell system that enables the molecular dialogue between E. lata mycelium and grapevine leaves to take place without physical contact. Merlot responded to E. lata by inducing the expression of a large number of defense-related genes. On the contrary, Ugni Blanc failed to activate such defense responses despite being able to perceive the fungus. To gain insight into the role of carbon partitioning in E. lata infected grapevine, we monitored the expression of plant genes involved in sugar transport and cleavage, and measured invertase activities. Our results evidence a coordinated up-regulation of VvHT5 and VvcwINV genes, and a stimulation of the cell wall invertase activity in leaves of Merlot elicited by E. lata, but not in Ugni Blanc. Altogether, this study indicates that the degree of cultivar susceptibility is associated with the activation of host defense responses, including extracellular sucrolytic machinery and hexose uptake during the grapevine/E. lata interaction. Given the role of these activities in governing carbon allocation through the plant, we postulate that the availability of sugar resources for either the host or the fungus is crucial for the outcome of the interaction.

Bio-Sulfur Pre-Treatment Suppresses Anthracnose on Cucumber Leaves Inoculated with Colletotrichum orbiculare

Bio-sulfur can be produced in the process of desulfurization from a landfill and collected by some microorganism such as Thiobacillus sp. as a sulfur element. In order to investigate practical use of bio-sulfur as an agent for controlling plant disease, in vitro antifungal activity of bio-sulfur was tested against Colletotrichum orbiculare known to cause cucumber anthracnose. Efficacy of bio-sulfur for suppressing anthracnose disease was also evaluated in vivo using cucumber leaves. Mycelial growth of C. orbiculare on medium containing bio-sulfur was inhibited. Disease severity of cucumber leaves pre-treated with bio-sulfur was significantly decreased compared to that of untreated ones. To illustrate how bio-sulfur could suppress anthracnose disease, structures of cucumber leaves infected with C. orbiculare were observed under a fluorescent microscope and a scanning electron microscope (SEM). Cucumber leaves pre-treated with bio-sulfur showed a low rate of appressorium formation whereas untreated ones showed abundant appressoria. Shrunk fungal hyphae were mostly observed on bio-sulfur-pretreated leaves by SEM. Similar results were observed on leaves pre-treated with a commercial fungicide Benomyl®. These results suggest that inhibition of appressorium formation of C. orbiculare by bio-sulfur may contribute to its suppression of cucumber anthracnose.

Sucrose transporters of resistant grapevine are involved in stress resistance

The whole promoter regions of SUTs in Vitis were firstly isolated. SUTs are involved in the adaptation to biotic and abiotic stresses. The vulnerability of Vitis vinifera to abiotic and biotic stresses limits its yields. In contrast, Vitis amurensis displays resistance to environmental stresses, such as microbial pathogens, low temperatures, and drought. Sucrose transporters (SUTs) are important regulators for plant growth and stress tolerance; however, the role that SUTs play in stress resistance in V. amurensis is not known. Using V. amurensis Ruper. 'Zuoshan-1' and V. vinifera 'Chardonnay', we found that SUC27 was highly expressed in several vegetative organs of Zuoshan-1, SUC12 was weakly expressed or absent in most organs in both the species, and the distribution of SUC11 in source and sink organs was highest in Zuoshan-1. A search for cis-regulatory elements in the promoter sequences of SUTs revealed that they were regulated by light, environmental stresses, physiological correlation, and hormones. The SUTs in Zuoshan-1 mostly show a higher and rapid response than in Chardonnay under the induction by Plasmopara viticola infection, cold, water deficit, and dark conditions. The induction of SUTs was associated with the upregulation of key genes involved in sucrose metabolism and the biosynthesis of plant hormones. These results indicate that stress resistance in Zuoshan-1 is governed by the differential distribution and induction of SUTs by various stimuli, and the subsequent promotion of sucrose metabolism and hormone synthesis.

Early stage metabolic events associated with the establishment of Vitis vinifera - Plasmopara viticola compatible interaction

Grapevine (Vitis vinifera L.) is the most widely cultivated and economically important fruit crop in the world, with 7.5 million of production area in 2017. The domesticated varieties of grapevine are highly susceptible to many fungal infections, of which downy mildew, caused by the biotrophic oomycete Plasmopara viticola (Berk. et Curt.) Berl. et de Toni is one of the most threatening. In V. vinifera, several studies have shown that a weak and transient activation of a defense mechanism occurs, but it is easily overcome by the pathogen leading to the establishment of a compatible interaction. Major transcript, protein and physiologic changes were shown to occur at later infection time-points, but comprehensive data on the first hours of interaction is scarce. In the present work, we investigated the major physiologic and metabolic changes that occur in the first 24 h of interaction between V. vinifera cultivar Trincadeira and P. viticola. Our results show that there was a negative modulation of several metabolic classes associated to pathogen defense such as flavonoids or phenylpropanoids as well as an alteration of carbohydrate content after inoculation with the pathogen. We also found an accumulation of hydrogen peroxide and increase of lipid peroxidation but to a low extent, that seems to be insufficient to restrain pathogen growth during the initial biotrophic phase of the interaction.

Single-dose β-aminobutyric acid treatment modifies tobacco (Nicotiana tabacum L.) leaf acclimation to consecutive UV-B treatment

β-Aminobutyric acid (BABA) pre-treatment has been shown to alter both biotic and abiotic stress responses. The present study extends this observation to acclimative UV-B-response, which has not been explored in this context so far. A single soil application of 300 ppm BABA modified the non-enzymatic antioxidant capacities and the leaf hydrogen peroxide levels in tobacco (Nicotiana tabacum L.) leaves in response to a 9-day treatment with 5.4 kJ m-2 d-1 biologically effective supplementary UV-B radiation in a model experiment that was performed in a growth chamber. BABA decreased leaf hydrogen peroxide levels both as a single factor and in combination with UV-B, but neither BABA nor UV-B affected leaf photochemistry significantly. The total antioxidant capacities were increased by either BABA or UV-B, and this response was additive in BABA pre-treated leaves. These results together with the observed changes in hydroxyl radical neutralising ability and non-enzymatic hydrogen peroxide antioxidant capacities show that BABA pre-treatment (i) has a long-term effect on leaf antioxidants even in the absence of other factors and (ii) modifies acclimative readjustment of prooxidant-antioxidant balance in response to UV-B. BABA-inducible antioxidants do not include phenolic compounds as a UV-B-induced increase in the adaxial leaf flavonoid index and total leaf extract UV absorption were unaffected by BABA.

Bacillus subtilis PTA-271 Counteracts Botryosphaeria Dieback in Grapevine, Triggering Immune Responses and Detoxification of Fungal Phytotoxins

Plant pathogens have evolved various strategies to enter hosts and cause diseases. Particularly Neofusicoccum parvum, a member of Botryosphaeria dieback consortium, can secrete the phytotoxins (-)-terremutin and (R)-mellein during grapevine colonization. The contribution of phytotoxins to Botryosphaeria dieback symptoms still remains unknown. Moreover, there are currently no efficient control strategies of this disease, and agro-environmental concerns have raised increasing interest in biocontrol strategies to limit disease spread in vineyards, especially by using some promising beneficial bacteria. Here, we first examined in planta the biocontrol capacity of Bacillus subtilis PTA-271 against N. parvum Np-Bt67 strain producing both (-)-terremutin and (R)-mellein. We then focused on the direct effects of PTA-271 on pathogen growth and the fate of pure phytotoxins, and explored the capacity of PTA-271 to induce or prime grapevine immunity upon pathogen infection or phytotoxin exposure. Results provided evidence that PTA-271 significantly protects grapevine cuttings against N. parvum and significantly primes the expression of PR2 (encoding a β-1,3-glucanase) and NCED2 (9-cis-epoxycarotenoid dioxygenase involved in abscisic acid biosynthesis) genes upon pathogen challenge. Using in vitro plantlets, we also showed that PTA-271 triggers the expression of salicylic acid- and jasmonic acid-responsive genes, including GST1 (encoding a glutathione-S-transferase) involved in detoxification process. However, in PTA-271-pretreated plantlets, exogenous (-)-terremutin strongly lowered the expression of most of upregulated genes, except GST1. Data also indicated that PTA-271 can detoxify both (-)-terremutin and (R)-mellein and antagonize N. parvum under in vitro conditions. Our findings highlight (-)-terremutin and (R)-mellein as key aggressive molecules produced by N. parvum that may weaken grapevine immunity to promote Botryosphaeria dieback symptoms. However, PTA-271 can efficiently attenuate Botryosphaeria dieback by enhancing some host immune responses and detoxifying both phytotoxins produced by N. parvum.

Strengthening Grapevine Resistance by Pseudomonas fluorescens PTA-CT2 Relies on Distinct Defense Pathways in Susceptible and Partially Resistant Genotypes to Downy Mildew and Gray Mold Diseases

Downy mildew caused by the oomycete Plasmopara viticola and gray mold caused by the fungus Botrytis cinerea are among the highly threatening diseases in vineyards. The current strategy to control these diseases relies totally on the application of fungicides. The use of beneficial microbes is arising as a sustainable strategy in controlling various diseases. This can be achieved through the activation of the plants' own immune system, known as induced systemic resistance (ISR). We previously showed that bacteria-mediated ISR in grapevine involves activation of both immune response and priming state upon B. cinerea challenge. However, the effectiveness of beneficial bacteria against the oomycete P. viticola remains unknown, and mechanisms underpinning ISR against pathogens with different lifestyles need to be deciphered. In this study, we focused on the capacity of Pseudomonas fluorescens PTA-CT2 to induce ISR in grapevine against P. viticola and B. cinerea by using two grafted cultivars differing in their susceptibility to downy mildew, Pinot noir as susceptible and Solaris as partially resistant. On the basis of their contrasting phenotypes, we explored mechanisms underlying ISR before and upon pathogen infection. Our results provide evidence that in the absence of pathogen infection, PTA-CT2 does not elicit any consistent change of basal defenses, while it affects hormonal status and enhances photosynthetic efficiency in both genotypes. PTA-CT2 also induces ISR against P. viticola and B. cinerea by priming common and distinct defensive pathways. After P. viticola challenge, PTA-CT2 primes salicylic acid (SA)- and hypersensitive response (HR)-related genes in Solaris, but SA and abscisic acid (ABA) accumulation in Pinot noir. However, ISR against B. cinerea was associated with potentiated ethylene signaling in Pinot noir, but with primed expression of jasmonic acid (JA)- and SA-responsive genes in Solaris, together with downregulation of HR-related gene and accumulation of ABA and phytoalexins.

Abscisic Acid as a Dominant Signal in Tomato During Salt Stress Predisposition to Phytophthora Root and Crown Rot

Salt stress predisposes plants to Phytophthora root and crown rot in an abscisic acid (ABA)-dependent manner. We used the tomato-Phytophthora capsici interaction to examine zoospore chemoattraction and assessed expression of pathogenesis-related (PR) genes regulated by salicylic acid (SA) and jasmonic acid (JA) following a salt-stress episode. Although salt treatment enhances chemoattraction of tomato roots to zoospores, exudates from salt-stressed roots of ABA-deficient mutants, which do not display the predisposition phenotype, have a similar chemoattraction as exudates from salt-stressed, wild-type roots. This suggests that ABA action during predisposing stress enhances disease through effects on plant responses occurring after initial contact and during ingress by the pathogen. The expression of NCED1 (ABA synthesis) and TAS14 (ABA response) in roots generally corresponded to previously reported changes in root ABA levels during salt stress onset and recovery in a pattern that was not altered by infection by P. capsici. The PR genes, P4 and PI-2, hallmarks in tomato for SA and JA action, respectively, were induced in non-stressed roots during infection and strongly suppressed in infected roots exposed to salt-stress prior to inoculation. However, there was a similar proportional increase in pathogen colonization observed in salt-stressed plants relative to non-stressed plants in both wild-type and a SA-deficient nahG line. Unlike the other tomato cultivars used in this study that showed a strong predisposition phenotype, the processing tomato cv. 'Castlemart' and its JA mutants were not predisposed by salt. Salt stress predisposition to crown and root rot caused by P. capsici appears to be strongly conditioned by ABA-driven mechanisms in tomato, with the stress compromising SA-and JA-mediated defense-related gene expression during P. capsici infection.

Phytohormone and genome variations in Vitis amurensis resistant to downy mildew

Downy mildew (DM) resistance is a highly desirable agronomic trait in grape breeding. High variation in Plasmopara viticola resistance was found in Vitis cultivars. Some accessions show high P. viticola resistance even under conditions highly conducive to DM. Here, leaf disc inoculation experiments revealed that Vitis amurensis 'Zuoshaner' exhibited DM resistance with necrotic spots, whereas the V. amurensis × V. vinifera hybrid cultivar 'Zuoyouhong' was susceptible. Changes in plant hormones accumulation profiles differed between the cultivars. To investigate the genetic mechanisms related to DM resistance, we performed genome-wide sequencing of 'Zuoshaner' and 'Zuoyouhong' and identified cultivar-specific single-nucleotide polymorphisms, insertions/deletions (indels), structural variations (SVs), and copy number variations (CNVs), identifying 5399 SVs and 191 CNVs specific for 'Zuoshaner'. Genes affected by these genetic variations were enriched in biological processes, including defense response and response to stress and stimulation, and were associated with sesquiterpenoid and triterpenoid biosynthesis, ABC transporters, and phenylalanine metabolism pathways. Additionally, indels and SVs were detected in six NBS-LRR disease resistance genes, and a CNV was mapped to the Rpv8 locus responsible for downy mildew resistance. These findings further our understanding of the genetic mechanisms underlying grape mildew resistance, and will facilitate genomic marker-assisted breeding for improved V. amurensis cultivars.

Jasmonate-mediated defence responses, unlike salicylate-mediated responses, are involved in the recovery of grapevine from bois noir disease

BACKGROUND

Bois noir is an important disease of grapevine (Vitis vinifera L.), caused by phytoplasmas. An interesting, yet elusive aspect of the bois noir disease is "recovery", i.e., the spontaneous and unpredictable remission of symptoms and damage. Because conventional pest management is ineffective against bois noir, deciphering the molecular bases of recovery is beneficial. The present study aimed to understand whether salicylate- and jasmonate-defence pathways might have a role in the recovery from the bois noir disease of grapevine.

RESULTS

Leaves from healthy, bois noir-diseased and bois noir-recovered plants were compared, both in the presence (late summer) and absence (late spring) of bois noir symptoms on the diseased plants. Analyses of salicylate and jasmonate contents, as well as the expression of genes involved in their biosynthesis, signalling and action, were evaluated. In symptomatic diseased plants (late summer), unlike symptomless plants (late spring), salicylate biosynthesis was increased and salicylate-responsive genes were activated. In contrast, jasmonate biosynthesis and signalling genes were up-regulated both in recovered and diseased plants at all sampling dates. The activation of salicylate signalling in symptomatic plants might have antagonised the jasmonate-mediated defence response by suppressing the expression of jasmonate-responsive genes.

CONCLUSIONS

Our results suggest that grapevine reacts to phytoplasma infection through salicylate-mediated signalling, although the resultant full activation of a salicylate-mediated response is apparently ineffective in conferring resistance against bois noir disease. Activation of the salicylate signalling pathway that is associated with the presence of bois noir phytoplasma seems to antagonise the jasmonate defence response, by failing to activate or suppressing both the expression of some jasmonate responsive genes that act downstream of the jasmonate biosynthetic pathway, as well as the first events of the jasmonate signalling pathway. On the other hand, activation of the entire jasmonate signalling pathway in recovered plants suggests the potential importance of jasmonate-regulated defences in preventing bois noir phytoplasma infections and the subsequent development of bois noir disease. Thus, on one hand, recovery could be achieved and maintained over time by preventing the activation of defence genes associated with salicylate signalling, and on the other hand, by activating jasmonate signalling and other defence responses.

Specific adjustments in grapevine leaf proteome discriminating resistant and susceptible grapevine genotypes to Plasmopara viticola

Grapevine downy mildew is an important disease affecting crop production leading to severe yield losses. This study aims to identify the grapevine cultivar-specific adjustments of leaf proteome that allow the discrimination between resistance and susceptibility towards P. viticola (constitutive (0h) and in after inoculation (6, 12 and 24h). Leaf proteome analysis was performed using 2D difference gel electrophoresis followed by protein identification via mass spectrometry. In addition, we analysed ROS production, antioxidant capacity, lipid peroxidation and gene expression. Proteins related to photosynthesis and metabolism allowed the discrimination of resistant and susceptible grapevine cultivars prior to P. viticola inoculation. Following inoculation increase of hydrogen peroxide levels, cellular redox regulation, establishment of ROS signalling and plant cell death seem to be key points differentiating the resistant genotype. Lipid associated signalling events, particularly related to jasmonates appear also to play a major role in the establishment of resistance. The findings from this study contribute to a better understanding of genotype-specific differences that account for a successful establishment of a defence response to the downy mildew pathogen.

BIOLOGICAL SIGNIFICANCE

Here, we present for the first time grapevine cultivar-specific adjustments of leaf proteome that allow the discrimination between resistance and susceptibility towards P. viticola (constitutive (0h) and in after inoculation (6, 12 and 24h). We have highlighted that, following inoculation, the major factors differentiating the resistant from the susceptible grapevine cultivars are the establishment of effective ROS signalling together with lipid-associated signalling events, particularly related to jasmonates. It is believed that plants infected with biotrophic pathogens suppress JA-mediated responses, however recent evidences shown that jasmonic acid signalling pathway in grapevine resistance against Plasmopara viticola. Our results corroborate those evidences and highlight the importance of lipid- signalling for an effective resistance response against the downy mildew pathogen.

Beta-aminobutyric acid priming of plant defense: the role of ABA and other hormones

Plants are exposed to recurring biotic and abiotic stresses that can, in extreme situations, lead to substantial yield losses. With the changing environment, the stress pressure is likely to increase and sustainable measures to alleviate the effect on our crops are sought. Priming plants for better stress resistance is one of the sustainable possibilities to reach this goal. Here, we report on the effects of beta-aminobutyric acid, a priming agent with an exceptionally wide range of action and describe its way of preparing plants to defend themselves against various attacks, among others through the modulation of their hormonal defense signaling, and highlight the special role of abscisic acid in this process.

Thiamine-induced priming against root-knot nematode infection in rice involves lignification and hydrogen peroxide generation

Thiamine (vitamin B1, VB1) can act as a plant defence trigger, or priming agent, leading to a rapid counterattack on pathogen invasion. In this study, the priming effect of thiamine on rice (Oryza sativa cv. Nipponbare) and its activity against root-knot nematode (Meloidogyne graminicola) infection were evaluated. Thiamine treatment and subsequent nematode inoculation activated hydrogen peroxide (H2O2) accumulation and lignin deposition in plant roots, and this correlated with enhanced transcription of OsPAL1 and OsC4H, two genes involved in the phenylpropanoid pathway. The number of nematodes in rice roots was slightly but significantly reduced, and the development of the nematodes was delayed, whereas no direct toxic effects of VB1 on nematode viability and infectivity were observed. The combined application of thiamine with l-2-aminooxy-3-phenylpropionic acid (AOPP), an inhibitor of phenylalanine ammonia-lyase (PAL), significantly hampered the VB1-priming capacity. These findings indicate that thiamine-induced priming in rice involves H2O2 and phenylpropanoid-mediated lignin production, which hampers nematode infection. Further cellular and molecular studies on the mechanism of thiamine-induced defence will be useful for the development of novel nematode control strategies.

Comparative proteomic analysis of β-aminobutyric acid-mediated alleviation of salt stress in barley

The non-protein amino acid β-aminobutyric acid (BABA) is known to induce plant resistance to a broad spectrum of biotic and abiotic stresses. This is the first study describing the effect of BABA seed priming on physiological and proteomic changes under salt stress conditions in barley (Hordeum vulgare). The aim of our study was to investigate the changes of fresh weight, dry weight and relative water content (RWC) as well as root proteome changes of two barley lines contrasting in salt tolerance (DH14, DH 187) in response to salt stress after seed priming in water or in 800 μM BABA. Seed priming with BABA significantly increased (p ≤ 0.05) RWC in both barley lines, which indicates considerably lower water loss in BABA-primed plants than in the non-primed control plants. Dry and fresh matter increased significantly in line DH 187, whereas no changes were detected in line DH14. BABA-primed plants of both lines showed different proteomic patterns than the non-primed control plants. The root proteins exhibiting significant abundance changes (1.75-fold change, p ≤ 0.05) were separated by two-dimensional polyacrylamide gel electrophoresis (2D- PAGE). Thirty-one spots, representing 24 proteins, were successfully identified by MALDI TOF/TOF mass spectrometry. The most prominent differences include the up-regulation of antioxidant enzymes (catalase, peroxidase and superoxide dismutase), PR proteins (chitinase, endo-1,3-β-glucosidase), and chaperones (cyclophilin, HSC 70). Our results indicate that BABA induces defence and detoxification processes which may enable faster and more effective responses to salt stress, increasing the chances of survival under adverse environmental conditions.

Leaf Treatments with a Protein-Based Resistance Inducer Partially Modify Phyllosphere Microbial Communities of Grapevine

Protein derivatives and carbohydrates can stimulate plant growth, increase stress tolerance, and activate plant defense mechanisms. However, these molecules can also act as a nutritional substrate for microbial communities living on the plant phyllosphere and possibly affect their biocontrol activity against pathogens. We investigated the mechanisms of action of a protein derivative (nutrient broth, NB) against grapevine downy mildew, specifically focusing on the effects of foliar treatments on plant defense stimulation and on the composition and biocontrol features of the phyllosphere microbial populations. NB reduced downy mildew symptoms and induced the expression of defense-related genes in greenhouse- and in vitro-grown plants, indicating the activation of grapevine resistance mechanisms. Furthermore, NB increased the number of culturable phyllosphere bacteria and altered the composition of bacterial and fungal populations on leaves of greenhouse-grown plants. Although, NB-induced changes on microbial populations were affected by the structure of indigenous communities originally residing on grapevine leaves, degrees of disease reduction and defense gene modulation were consistent among the experiments. Thus, modifications in the structure of phyllosphere populations caused by NB application could partially contribute to downy mildew control by competition for space or other biocontrol strategies. Particularly, changes in the abundance of phyllosphere microorganisms may provide a contribution to resistance induction, partially affecting the hormone-mediated signaling pathways involved. Modifying phyllosphere populations by increasing natural biocontrol agents with the application of selected nutritional factors can open new opportunities in terms of sustainable plant protection strategies.

Laser Microdissection of Grapevine Leaves Reveals Site-Specific Regulation of Transcriptional Response to Plasmopara viticola

Grapevine is one of the most important fruit crops in the world, and it is highly susceptible to downy mildew caused by the biotrophic oomycete Plasmopara viticola. Gene expression profiling has been used extensively to investigate the regulation processes of grapevine-P. viticola interaction, but all studies to date have involved the use of whole leaves. However, only a small fraction of host cells is in contact with the pathogen, so highly localized transcriptional changes of infected cells may be masked by the large portion of non-infected cells when analyzing the whole leaf. In order to understand the transcriptional regulation of the plant reaction at the sites of pathogen infection, we optimized a laser microdissection protocol and analyzed the transcriptional changes in stomata cells and surrounding areas of grapevine leaves at early stages of P. viticola infection. The results indicate that the expression levels of seven P. viticola-responsive genes were greater in microdissected cells than in whole leaves, highlighting the site-specific transcriptional regulation of the host response. The gene modulation was restricted to the stomata cells and to the surrounding areas of infected tissues, indicating that the host response is mainly located at the infection sites and that short-distance signals are implicated. In addition, due to the high sensitivity of the laser microdissection technique, significant modulations of three genes that were completely masked in the whole tissue analysis were detected. The protocol validated in this study could greatly increase the sensitivity of further transcriptomic studies of the grapevine-P. viticola interaction.

Callose Synthase Family Genes Involved in the Grapevine Defense Response to Downy Mildew Disease

The deposition of callose is a common plant defense response to intruding pathogens and part of the plant's innate immunity. In this study, eight grapevine callose synthase (CalS) genes were identified and characterized. To investigate biological function of CalS in grapevine against the infection of Plasmopara viticola, expression patterns of grapevine CalS family genes were analyzed among resistant/susceptible cultivars. After P. viticola infection, expression of CalS1, 3, 7, 8, 9, 10, and 11 were significantly modified among the grapevine cultivars. For example, the expression of CalS1 and CalS10 were greatly increased in downy mildew (DM)-immune Muscadinia rotundifolia 'Carlos' and 'Noble'. Transient expression assay with promoters of the CalS1 and CalS10 genes confirmed that they were regulated by the oomycete pathogen P. viticola. CalS1 promoter activity was also significantly up-regulated by ABA in DM-immune M. rotundifolia 'Noble', but down-regulated in DM-susceptible Vitis vinifera 'Chardonnay'. The CalS1 promoter, however, was also down-regulated by GA in 'Chardonnay', but not affected in 'Noble'. The promoter activity of CalS10 was significantly up-regulated by GA in 'Chardonnay', but not regulated by ABA at all. It is proposed that CalS1 and CalS10 were involved in grapevine defense against DM disease.

Linking Jasmonic Acid to Grapevine Resistance against the Biotrophic Oomycete Plasmopara viticola

Plant resistance to biotrophic pathogens is classically believed to be mediated through salicylic acid (SA) signaling leading to hypersensitive response followed by the establishment of Systemic Acquired Resistance. Jasmonic acid (JA) signaling has extensively been associated to the defense against necrotrophic pathogens and insects inducing the accumulation of secondary metabolites and PR proteins. Moreover, it is believed that plants infected with biotrophic fungi suppress JA-mediated responses. However, recent evidences have shown that certain biotrophic fungal species also trigger the activation of JA-mediated responses, suggesting a new role for JA in the defense against fungal biotrophs. Plasmopara viticola is a biotrophic oomycete responsible for the grapevine downy mildew, one of the most important diseases in viticulture. In this perspective, we show recent evidences of JA participation in grapevine resistance against P. viticola, outlining the hypothesis of JA involvement in the establishment of an incompatible interaction with this biotroph. We also show that in the first hours after P. viticola inoculation the levels of OPDA, JA, JA-Ile, and SA increase together with an increase of expression of genes associated to JA and SA signaling pathways. Our data suggests that, on the first hours after P. viticola inoculation, JA signaling pathway is activated and the outcomes of JA-SA interactions may be tailored in the defense response against this biotrophic pathogen.