1. Introduction

2. Chemistry and antifungal activity of some stilbenes of grapevine: A brief overview

3. Induction of stilbene phytoalexin synthesis by chemicals and possible applications in the vineyard

3.1. Organic and Inorganic Metallic Salts

Among the chemicals which can induce the production of phytoalexins, numerous metallic salts have been described as having the ability to trigger the synthesis of these compounds with more or less applications in the vineyard (Table 1), in particular. Fairly old studies have reported the use of heavy metallic salts such as, among the most active, silver (AgCl), mercury (HgCl₂) and copper (CuCl₂) salts for the induction of pisatin synthesis in pea (Pisum sativum) [47]. Also, in pea, copper chloride at a concentration of 5 mM was found to induce the activity of a methyltransferase catalyzing the methylation of (+)-6α-hydroxymaackiain into (+)-pisatin [48].

Copper chloride and other metallic salts such as copper sulfate (CuSO₄), which is widely used in viticulture, have been recognized as inducers of the synthesis of several stilbene phytoalexins and flavonoid glucosides in Veratrum grandiflorum [49] and the synthesis of resveratrol, ε-viniferin and piceid in grapevine plantlets in vitro [50]. Organic or inorganic aluminum-based metallic salts have also been employed in grapevine with potential applications for the protection of this plant in the field.

The specific mechanisms of activation of the grapevine defenses by aluminium, particularly implementation of the phytoalexin system, have been deciphered very recently as we will see below. Adrian et al. [51] reported a phenomenon of hyper-elicitation of the resveratrol synthesis in isolated grapevine shoots with aluminum chloride (AlCl₃) at a concentration of 7 mM in V. rupestris. In a similar way to what has been described for the biosynthesis of pisatin in pea, methylation of resveratrol to its dimethylated derivative, pterostilbene, was observed on grapevine leaf discs by induction of a resveratrol-O-methyltransferase in the presence of 1% AlCl₃ [21]. Among the organic aluminum salts, fosetyl-Al has been the subject of numerous studies in plants with applications in the vineyard.

Fosetyl^®-Al (Aluminum Tris-O-ethylphosphonate) is a well-known fungicide (Aliette, Bayer Crop Science) which has shown in practice a fair efficacy against downy mildews and, particularly, grapevine downy mildew (P. viticola) [52,53,54,55,56] but which was also applied for the treatment of Esca [56]. Very rapidly, works sought to highlight a relationship between the protective action of fosetyl-Al and a stimulation of the defense mechanisms in tobacco [57,58] and more specifically of the phytoalexin system in tobacco [59], citrus [60] and in grapevine [61,62]. These latest works, carried out on detached grapevine leaves or on leaf discs, have reported that it was difficult to distinguish which part could be attributed to the stimulation of phytoalexin synthesis following infection by sporangial suspensions of P. viticola and which part was due to the sole application of the fungicide. For Raynal et al. [61], fosetyl-Al alone did not induce phytoalexin synthesis in grapevine (resveratrol and cis-ε-viniferin), whereas these compounds saw their concentration increasing in the tissues when this fungicide was applied simultaneously at—or 24 h after—infection (priming effect) (Table 1). For Dercks and Creasy [62], interpretation of the results was even more complex. The sole application of fosetyl-Al induced resveratrol production but at modest levels (4.7 μg/g fresh weight). Post-infectional applications of fosetyl-Al, that is, 22 h after inoculation with P. viticola, led to a significant rise in resveratrol amounts (19.1 μg/g fresh weight in the susceptible variety, Riesling; 69 μg/g fresh weight in the mid-resistant species, V. rupestris and 107.38 μg/g fresh weight in the resistant variety, Castor). On the other hand, the 2 to 4 times increase in the content of resveratrol compared to the control during the pre-infectional application of fosetyl-Al (24 h before inoculation by P. viticola), was particularly interesting since it suggested that prior application of the fungicide displayed a priming effect on the synthesis of resveratrol and implementation of the phytoalexin response [62]. Fosetyl-Al efficacy towards the sporulation of P. viticola depended on the date of application of the treatment, the dose used and above all, the grape vine variety. The doses of fosetyl-Al required to totally suppress sporulation at the pre-infectional stage varied from 200 μg/mL for V. rupestris (mid-tolerant and the cv Castor (resistant) and 400 μg/mL for the cv Riesling (susceptible). At the post-infectional stage, the sensitive variety was no longer protected by fosetyl-al regardless of the dose, V. rupestris was protected only with the highest dose while the resistant variety (Castor) remained protected by both treatment doses.

To appreciate, the capacity of aluminium to trigger stilbene phytoalexin synthesis in grapevine, tests were carried out in the 90 s jointly by the University of Dijon and Goemar Laboratories on a fungicide called, Synermix (Table 1). This contained aluminum chloride in combination with a seaweed extract. Previously, trials conducted in compliance with regional treatment programs for the fight against gray mold in the vineyards of Bordeaux, Champagne and the Val de Loire over an eight-year period revealed an average efficacy of this fungicide of 33.6% against B. cinerea [63,64].

Table 1. Induction of stilbene phytoalexin synthesis by chemicals associated with resistance of grapevine to diseases.

Table 1. Induction of stilbene phytoalexin synthesis by chemicals associated with resistance of grapevine to diseases.

Chemicals	Plant Material	Biological Inputs	References
Fosetyl-Al	Potted grapevine (cv Carignan)	Protection towards P. viticola infection. No effect of fosetyl-Al alone on stilbene accumulation but active when applied at the time of infection or shortly after	[61]
Fosetyl-Al	cv Riesling (susceptible), V. rupestris (mid-tolerant) and cv Castor (resistant) leaf discs	Total suppression of P. viticola sporulation for pre-infectional applications. No protection towards the susceptible cultivar with post-infectional applications. Induction of stilbene production and priming effect	[62]
Synermix (aluminum chloride in combination with a seaweed extract)	Eight-year period experiments in the vineyard (V. vinifera)	Efficacy of 33.6% towards B. cinerea infection vs. Iprodione (42.2%). 70.7% efficacy for the combination Synermix/Iprodione. High AlCl3-induced resveratrol production	[51,64]
Copper sulfate	In vitro plantlets and potted grapevine (cv Chardonnay)	Protection against B. cinerea infection (> 60%) and only 38% towards P. viticola associated with an induction of chitinase and β-1,3-glucanases along with stilbene production	[50]
Fosetyl-Al	Nine-year period experiments in the vineyard (cvs Albana and Sangiovese)	Reduction of the foliar symptoms of esca decorrelated with stilbene production	[56]

At the same time, application of a fungicide known for the treatment of gray mold, iprodione, demonstrated an effectiveness close to 42.2%. Interestingly, use of the two fungicide products in combination showed an efficacy in the vineyard of 70.7%, that is, a synergistic effect of +9.1% compared to the calculated theoretical efficacy of the combination of the two fungicides according to Colby’s formula [65], where TE is the theoretical efficacy of the combination of the two fungicides; α and β are respectively the efficacy of fungicides A and B measured in the fields. The synergy is deduced by subtracting the efficacy of the fungicide combination in the fields to the theoretical efficacy of the fungicide combination:

$$\mathrm{T}\mathrm{E}=\mathsf{\alpha }+\mathsf{\beta }-\left[\frac{\mathsf{\alpha }\times \mathsf{\beta }}{100}\right]$$

(1)

In attempting to explain the effectiveness of Synermix in the control of gray mold in the vineyard, the hypothesis of a stimulation by this fungicide of the natural defenses of grapevine (phytoalexin system) has been put forward. On the basis of previous works carried out with metallic salts (salts of copper and mercury), the inducing effect of aluminum chloride, one of the components of Synermix, was evaluated on the synthesis of phytoalexins of this plant. A study published in 1996 clearly demonstrated a hyper-elicitation of resveratrol production, resulting in quantities of the order of a few hundred micrograms per gram of fresh weight, on grapevine shoots (V. rupestris) dipped in tubes containing solutions of AlCl₃.6H₂O ranging from 7 to 90 mM within 15 h of incubation [51]. In contrast, no eliciting activity was observed on resveratrol synthesis using the seaweed extract. Synermix was marketed as an adjuvant for fungicidal products and not as a fungicide but was withdrawn from the market probably due to the possible toxicity of the aluminum salt present in the formulation.

Wang et al. [66] recently elucidated the mechanisms by which the aluminum cation Al³⁺ triggered phytoalexin production in grapevine cell cultures. Aluminum initiated intracellular actin remodeling (Al-actin bundling) through the activation of a NADPH oxidase located at the plasma membrane level, the membrane-associated NADPH oxidase Reactive burst Oxidase Homologue, RbOH. An induction of two subfamilies of genes encoding resveratrol synthase (STS47 and STS27) as well as the gene coding for the transcription factor MyB14 involved in the transcription of the stilbene synthase promoter, was observed through aluminum action, a phenomenon which was RbOH-actin-dependent. This resulted in resveratrol synthesis. Interestingly, rigorously similar observations have been reported in planta in two cultivars of V. vinifera and V. sylvestris demonstrating that these mechanisms are validated from the cell to the whole plant. The fact that RbOH mediates the remodeling of actin in response to aluminum involving the activation of several genes coding for the synthesis of stilbene phytoalexins in grapevine, would make this enzyme a privileged target to prime host defenses upstream of the infection process. The authors therefore suggested looking for ways other than aluminum, whose toxicity is proven, to stimulate the RbOH protein as alternative approaches for plant protection [66].

With copper sulfate (CuSO₄) applied at a concentration of 50 μg/mL to in vitro grapevine plantlets or potted plants (cv Chardonnay), a protective effect of more than 60% in the extension of B. cinerea necrosis and only 38% in the case of downy mildew development, was respectively observed [50]. This protection was associated with an induction of chitinase and β-1,3-glucanase activities of the plant as well as a significant rise in stilbene synthesis in the leaves (resveratrol: 20 μg/g FW; ε-viniferin: 45 μg/g FW and piceid: 25 μg/g FW). These experiments showed that pre-treatment with non-toxic concentrations of CuSO₄ led to increased protection towards grey mold and, to a lesser extent, against downy mildew, correlated with a stimulation of the plant defense mechanisms. This effect can be potentiated by a co-treatment with chitosan (see Section 3.3).

Application of fosetyl-Al for the control of the main pathogens responsible for esca in grapevine showed a protective activity of this treatment against the incidence of the disease foliar symptoms of esca disease in an experiment carried out in the vineyard from 1999 to 2007 on cvs Albana and Sangiovese [56]. However, the protective effect of fosetyl-Al appeared to be decorrelated from the production of stilbenes (resveratrol and ε-viniferin), the application of fosetyl-Al alone not inducing their synthesis.

3.2. Phytohormone derivatives with a stimulating effect on grapevine natural defenses

A certain number of phytohormones along with their derivatives (salicylic acid, SA, jasmonic acid, JA, and its methylated derivative, methyljasmonate, MeJA, ethylene, etc…) display an inducing effect on plant systemic acquired resistance (SAR) including active responses such as accumulation of pathogenesis-related proteins, phytoalexins or plant cell wall strengthening. It is beyond the scope of this review to deal with all the trials that have been carried out in grapevine. Some of these molecules (benzothiadiazole, metyljasmonate alone or in combination) have been used in particular with the aim of modifying the metabolism of polyphenols in grapes, increasing the biosynthesis of stilbenes through induction of the genes encoding enzymes of the phenypropanoid/stilbene pathway [67,68]. Only works linking protection against pathogens and stimulation of the phytoalexin system will be discussed here (also excluding those carried out on cell suspensions).

Benzothiadiazole (S-methyl benzo [1–3]thiadiazole-7-carbothioate, Bion, Syngenta) (BTH), is a salicylic acid (SA) mimic which provides plants with protection against pathogens without possessing a direct antifungal activity, but which acts on the induction of systemic acquired resistance (SAR) by SA-dependent pathways [69,70]. In the study conducted by Iriti et al. [71], BTH (0.3 mM) was applied in three different treatments within one week to detached grape bunches (V. vinifera cv Merlot) at veraison time (Table 2).

Subsequent infections of the bunches by B. cinerea carried out at the fourth week after treatment with BTH clearly showed a protective effect of the treatment regarding contamination of the grape clusters by gray mold since 87% of them only exhibited between 0 and 10% infection while 92% of the untreated ones showed a contamination rate of between 50 and 100%. The authors then attempted to explain the observed protective effect of BTH by an increase, particularly, in the synthesis of resveratrol in the berries. A 40% increase in the resveratrol content of the berries treated with BTH compared to the control (0.546 mg/kg FW versus 0.390 mg/kg FW) was effectively observed, but these very low levels in absolute values were in this case not high enough to support the hypothesis of a stimulation of the phytoalexin system by BTH [71]. This study therefore remained inconclusive as to a possible inducing effect of BTH on phytoalexin synthesis. Another work has reported the protective effect of BTH used at a dose of 0.05% on the grapevine V. vinifera cv Grüner Veltiner, a variety susceptible to downy mildew (P. viticola), against the infection by this pathogen, resulting in a reduction in the incidence of the disease to only 15.8% in greenhouses [72]. The priming effect of BTH was linked to the activation of various genes coding for PR proteins, peroxidase, but also PAL (phenylpropanoid pathway) and STS (resveratrol synthesis) (Table 2).

Application of BTH (1.9 mM) to the leaves of grapevine plants of V. vinifera cv Cabernet Sauvignon was carried out prior to inoculation with different isolates of downy mildew (P. viticola) and powdery mildew (Erysiphe necator) [73]. BTH exhibited a growth inhibition for all the tested isolates ranging from 60 to 98% (P. viticola) and from 65 to 75% (E. necator), thus conferring grapevine a very interesting level of protection. Besides an overexpression of a number of genes coding for PR proteins, lipoxygenase (LOX-9) and glutathione-S-transferase (GST) following pretreatment with BTH and at different times following inoculation by the two isolates of each of the two pathogens, an overexpression of the STS gene was observed as well as a significant accumulation of piceid (up to 60 μg/g dry weight (DW)) whereas the levels of resveratrol remained very low (2 to 8 μg/g DW). Quite surprisingly and taking account of the fact that pterostilbene is generally only a minor component in quantitative terms of the phytoalexin response of grapevine [74,75], the latter accumulated significantly in the leaves. under BTH treatment (whether contaminated or not) reaching, for example, 16 to 21 μg/g DW in plants treated with BTH and inoculated with E. necator. The high fungistatic and fungitoxic activities of this compound compared to its non-methylated derivative, resveratrol, partly explained the protective effect exerted by BTH against subsequent contaminations by P. viticola and E. necator [73]. These experiments thus clearly confirmed that the protection provided by BTH treatment involved an activation of SAR (Table 2).

Very interesting experiments were conducted in the vineyard for two consecutive years (2014 and 2015) comparing two protection treatments, one with a classic fungicide, pyrimethanil, and the other with the chemical elicitor BTH on grapevine cv Sémillon, which is very susceptible to gray mold (B. cinerea) [76]. In vitro activity tests have shown that pyrimethanil had an IC₅₀, i.e., 51-times lower than BTH (67.5 mg/L vs. 3450 mg/L). In the vineyard, the differences observed regarding the protective effects of these two compounds were considerably reduced: 35% reduction of disease incidence for pyrimethanil and 20% for BTH in 2014; 29% and 25% respectively in 2015, demonstrating the relative efficacy of the two treatments. The correlation between the reported protection activities with an overexpression of the grapevine defense genes was even trickier. If the genes coding for several PR proteins were up-regulated as well as those encoding flavonoid biosynthesis, VvCHS and VvF3H resulting in a 42% increase in the total polyphenol content 48 h after the treatments’ application, an up-regulation of the genes encoding stilbene biosynthetic pathways, VvSTS and VvROMT, was delayed seven days after the treatment. Overexpression of these two genes was not accompanied by a significant production of stilbenes (piceid and pterostilbene) making it difficult to correlate the protective activity of BTH in the vineyard and the activation of phytoalexin biosynthesis [76].

The eliciting effect of methyljasmonate, a derivative of jasmonic acid, a phytohormone implicated in the regulation of plant defense responses as well as possessing regulatory functions in plant growth and development, has been the subject of numerous studies on grapevine cell cultures which have been reviewed in other articles [5,77]. We will limit ourselves here to works carried out on grapevine plants. V. vinifera cv Cabernet Sauvignon grapevine plants placed in a confined atmosphere and subjected to methyljasmonate (MeJa) vapors at a concentration of 400 nmol/L, 15 and 30 days after veraison, saw the piceid concentration increasing in the leaves for the two phenological stages (1200 and 990 nmol/g DW) but without any effect on resveratrol concentration which remained at very low levels (21 and 74 nmol/DW) [78]. In the berries, stilbenes were almost undetectable or in very low amounts whatever the phenological stage. Interestingly, MeJa showed a potentiating effect on the production of resveratrol in the leaves in response to UV stress (1632 and 2658 nmol/g DW) but without any significant effect on the resveratrol berry content. Treatments with airborne MeJa could have an interest in the vineyard for the protection of the leaves against the attacks of pathogenic agents at the last stages of ripening but would not produce notable effects on that of the berries. This type of treatment would find, on the other hand, an application for the protection of flowers and young berries whose synthesis of stilbene phytoalexins is very active [79]. Vezzuli et al. [80] reported similar experiments by spraying MeJa at different phenological stages in potted grapevine of the Barbera variety (fruit set, veraison and maturity or by combining these treatments over the entire ripening period). The activating effect of MeJa on stilbene production (resveratrol + ε-viniferin) was mainly observed in ripe berries, but the levels of phytoalexins attained were not high enough (< μg/g DW) to inhibit further pathogens’ development (Table 2).

Pre-treatment of foliar cuttings (cv Cabernet Sauvignon) or in the vineyard (cv Merlot) with MeJa used at doses of 5 mM (in a single application for potted plants or in several applications in the vineyard every 7–10 days from bloom to veraison) conferred a 75 and 73% reduction in the symptom incidence of powdery mildew (E. nector), respectively, while treatment with fosetyl-Al provided with an 83% protection [81]. However, no protective effect against the development of downy mildew (14%) was reported. Rise in the resistance to powdery mildew was correlated with a preinfectional increase in the transcript levels of the class 4 acid chitinase (25 times) as well as PAL and STS (44 and 11 times, respectively). Activation of the PAL and STS genes was accompanied by an increase in the stilbene content of the leaves (244 nmol/g DW for piceid; 176 nmol/g DW for resveratrol; 10 nmol/g DW for pterostilbene; 80 and 8 nmol/g DW for the dimers ε- and δ-viniferin, respectively). These data are very interesting insofar as they showed a correlation between pre-treatment with MeJa, stimulation of grapevine defense mechanisms including phytoalexin response, and protection against powdery mildew in the vineyard [81].

Three different elicitors [82] were tested as regard to the protection of potted grapevines (cv Cabernet Sauvignon) against contamination by downy mildew (P. viticola), MeJa, BTH and phosphonates (PHOS), a constituent of fosetyl-Al [83], registered as a fungicide for the treatment of downy mildew in grapevine. Pretreatment with the three elicitors at the respective doses of 2 g/L, 1.5 g/L and 1.09 g/L (5 mM) applied six days before inoculation provided a protective effect of 98.5, 97 and 85.8%, respectively towards contamination by downy mildew. At the gene level, BTH induced the overexpression of many PR proteins unlike leaves treated with MeJa and PHOS. The three treatments caused an increase in the concentration of flavanols, which are antifungal compounds (MeJa>BTH>PHOS) as well as a significant induction of stilbene biosynthesis (piceid up to 8 μg/mg DW; ε-viniferin: 9 μg/mg DW) and, to a lesser extent, of resveratrol and pterostilbene (< 1 μg/mg DW) for MeJa. This strong stilbene accumulation appeared, however, to be decorrelated from the expression of the VvPAL and VvSTS genes (Table 2).

Ethylene is a gaseous phytohormone that plays a crucial role in plant development and growth, plant stress responses, flowering, maturation as well as senescence and gemination [84]. Ethephon (2-chloroethylphosphonic acid) is an ethylene-releasing releasing compound. Used at a dose of 0.5 g/L (Sierra, Bayer, Cropscience), it showed a protective effect against E. necator both on leaf discs and grapevine cuttings of Cabernet Sauvignon with disease control rates ranging respectively from 64 to 70% [85]. Alongside providing protection against powdery mildew, there was a significant induction in the expression levels of PR proteins as well as those of PAL and STS genes by a factor of 10 and 67, respectively associated with a correlative increase in the synthesis of piceid (312 nmoles/g DW), resveratrol (133 nmoles/g DW), ε-viniferin (58 nmoles/g DW) as well as δ-viniferin and pterostilbene (12 and 10 nmoles/g DW). While MeJa or ethephon applied individually to foliar cuttings (cv Cabernet Sauvignon) at the respective doses of 5 nM and 6.94 nM reduced powdery mildew (E. necator) colonization by 60%, fosetyl-Al showing an 85% effectiveness under the same conditions, the MeJa/ethephon combination only reduced the incidence of the disease by 30% [86]. The effect of the combination of the two chemicals on the stimulation of defense genes (GLU, β-1,3-glucanase, PAL1 and STS) and the production of stilbenes was difficult to interpret since gene expression analysis was carried out on cell suspensions, a model very different from that used for the protection tests. The MeJa/ethephon co-treatment, although not conferring a similar protection level compared to each of the treatments, led to an overexpression of the PAL1, STS and GLU genes (respectively 120, 125 and 4500 times) as well as an increase in the contents of resveratrol (10.8 nmol/mg DW) and piceid (18.3 nmol/mg DW). The antagonistic effect observed during the application of the two products in combination at the plant protection level, could be due to their different actions on signaling pathways and probably linked to suppression of the MeJa-induced expression of some PR proteins and LOX genes, by ethephon [86].

Table 2. Induction of grapevine disease resistance linked with stilbene phytoalexin synthesis by phytohormone derivatives.

Table 2. Induction of grapevine disease resistance linked with stilbene phytoalexin synthesis by phytohormone derivatives.

Compounds	Plant Material	Biological Inputs	References
Benzathiadiazole	Detached grape bunches (cv Merlot)	Significant decrease in B. cinerea incidence of the grape clusters (87% of them exhibited between 0 and 10% infection). Low stilbene accumulation	[71]
Benzathiadiazole	Potted grapevine (cv Grïuner Veltiner)	Reduction of downy mildew disease incidence to 15.8%. Strong activation of PAL and STS genes as well as peroxidases and PR-proteins	[72]
Benzathiadiazole	Leaves of grapevine plants (cv Cabernet Sauvignon)	Growth inhibition of P. viticola (60–98% and E. necator (65–75%). Overexpression of the STS gene with high piceid accumulation and significant accumulation of pterostilbene. Overexpression of several genes coding for PR proteins, LOX and GST	[73]
Benzathiadiazole vs. Pyrimethanil	Two-year period experiments in the vineyard (cv Semillon)	Decreases of grey mold incidence of respectively 35 and 20% for pyrimethanil and benzathiadiazole in 2014; −29 and −25% in 2015. Upregulation of VvSTS and VvROMT genes with no correlated accumulation of stilbenes	[76]
Methyljasmonate	Grapevine plants placed in a confined atmosphere with methyljasmonate (cv Cabernet Sauvignon)	High increase in leaf piceid content and little or no effect on the resveratrol content of grape berries. No test of disease pathogen control	[78]
Methyljasmonate	Potted grapevine of the Barbera variety	Low increase in stilbene phytoalexin potential	[80]
Methyljasmonate	Plant cuttings (one application); multiple applications in the vineyard (from floraison to veraison) (cv Cabernet Sauvignon)	Decreases of powdery mildew incidence of 75 and 73% in plant cuttings or in the vineyard, but not for downy mildew. Up-regulation of genes coding for PR-proteins as well as PAL and STS along with increase in stilbene accumulation	[81]
Methyljasmonate (MeJa), Benzathiadiazole (BTH), Phosphonates (PHOS)	Plant cuttings (cv Cabernet Sauvignon)	Respectively 98.5, 97.3 and 85.8% protection towards downy mildew infection with MeJa, BTH and PHOS. High induction of stilbene production decorrelated from VvPAL and VvSTS expression. High overexpression of PR proteins with BTH, lower with MeJa and PHOS	[82]
Ethephon (ethylene)	Plant cuttings and leaf discs (cv Cabernet Sauvignon)	Increases in protection (64–70%) against powdery mildew. Up-regulation of defense genes (PAL and STS) along with strong stilbene accumulation (piceid, resveratrol and dimers)	[85]
Methyljasmonate/Ethephon	Plant cuttings and cell suspensions (cv Cabernet Sauvignon)	Decreases of 60% of the colonization by downy mildew by methyljasmonate or ethephon alone (only 30% when applied in combination; 85% with fosetyl-Al). Increases in PAL1 and STS along with increase in piceid and resveratrol production in cell suspensions. Inhibition of some PR proteins and LOX gene-induced expression by ethephon	[86]

3.3. Bio-Elicitors

In this section will be treated what we have chosen to arbitrarily qualify as bio-elicitors such as, among others, laminarin, chitosan, fungal sterols, bacterial rhamnolipids, β-amino-butyric acid and tagatose, which are able to induce stilbene synthesis [87,88] (Table 3).

Among polysaccharides, laminarin, a β-1,3-glucan extracted from a brown alga, Laminaria digitata, has shown protective effects against B. cinerea and P. viticola by reducing the symptoms of these two diseases by respectively 55% and 75% on leaves of grapevine in vitro plantlets of the Chardonnay and Gamay varieties, both susceptible to gray mold and downy mildew [89]. Although carried out on a different plant model, parallel experiments realized with cell suspensions, showed an induction of responsive defense genes (preceded by an oxidative burst), particularly the genes coding for PR proteins as well as a strong overexpression (20 times higher than that of the controls) of the PAL and STS1 genes with a concomitant accumulation of the phytoalexins resveratrol and ε-viniferin (Table 3).

Chitosan is also a polysaccharide composed of D-glucosamine and of N-acetyl-D-glucosamine units partially deacetylated and obtained from the chitin of crabs, shrimps and other crustaceans, known for its bio-stimulant and elicitor properties [90]. The protective properties of chitosan alone or in combination with copper sulfate have been studied towards gray mold and downy mildew as well as its action on the stimulation of grapevine defense mechanisms [50,91]. Pretreatment of grapevine plantlets (cv Chardonnay) with chitosan at a dose of 150 mg/L resulted in the total suppression of B. cinerea lesions on leaves, demonstrating the protection effect of chitosan regarding gray mold [91]. Interestingly, this protective effect was observed after removal of the elicitor, by washing the leaves, and before inoculation by B. cinerea, seeming to indicate that the action of this elicitor was not only linked to its direct antifungal activity but to the activation of some plant defense mechanisms (overexpression of chitinase, PAL and LOX activities), even if parallel works seemed to question the direct antifungal activity of chitosan [50]. Similarly, pretreatment with chitosan with a molecular mass of 1500 Da and partially deacetylated (20%) at a dose of 200 μg/mL induced 60 to 71% protection towards, respectively, the contamination of grapevine in vitro plantlets or potted plants by gray mold and downy mildew [50]. This protective effect was associated with an induction of plant chitinase and β-1,3-glucanase activities and an increase in stilbene contents (resveratrol: 18 μg/g FW; ε-viniferin: 50 μg/g FW and piceid: 40 μg/g FW). The protection rate increased to 85% in the case of downy mildew upon combination of chitosan (200 μg/mL) and copper sulfate (50 μg/mL) with potentiation of the phytoalexin response [50] (Table 3).

Ergosterol is a major sterol found in fungi that plays a role in the fluidity and permeability of cell membranes. Pretreatment with ergosterol administered at a concentration of 200 μM on grapevine plants (cv Ugni Blanc) conferred a protective effect of 75% against contamination by B. cinerea [92]. Conversely, only 55% of protection were achieved in case of pretreatment with BTH. The resistance provided by ergosterol was accompanied by an induction, mediated by the mobilization of the trans-acting elements of the transcription factor WRKY, of stilbene synthase gene (VST1) expression and incidentally by the overexpression of the VvLTP1 gene coding for the lipid transfer protein 1. Overexpression of the VST1 gene was correlated with low resveratrol synthesis in the plants treated with ergosterol (227 ng/g FW) (Table 3).

β-amino-butyric acid (BABA) is a non-protein amino acid known to induce resistance against oomycetes in plants [93,94]. Pretreatment with aqueous solutions of BABA (1 mM) prior to contamination by downy mildew (P. viticola) on two cultivars (plant cuttings), one susceptible (Chasselas), the other resistant (Solaris), induced a strong reduction in sporulation as well as the growth of P. viticola hyphae [95]. The priming effect exerted by BABA resulted in an increase in the overall amounts of stilbenes (resveratrol, piceid and ε-viniferin) alongside stilbenes displaying strongly fungicidal properties such as δ-viniferin and pterostilbene. Priming activity of BABA on the expression of PAL and C4H was mainly observed in the downy mildew-resistant variety, Solaris, whereas STS was up-regulated in both the BABA-primed cultivars (Table 3).

Works have reported that natural rhamnolipids can potentially be used for the protection of plants against bacterial and fungal diseases, protecting for example, oilseed rape and tomato from B. cinerea attacks [96,97]. In grapevine (V. vinifera), pretreatment with a mixture of rhamnolipids (RLs) extracted from the bacterium Pseudomonas aeruginosa at concentrations of 0.1 and 1 mg/L showed a protective effect on in vitro whole plantlets against contamination by B. cinerea [98]. RL activity resulted in a strong increase in the transcript accumulation of genes encoding various PR proteins, LOX as well as PAL and STS genes. Because of their lipo-solubility, RLs exerted a direct antifungal action and were capable to potentiate the action of other bio-elicitors such as chitosan.

The role of sugars in the interactions between plants and microbes and in the resistance of the former to numerous pathogens in connection with the concept of sweet immunity is well established [99,100]. In this context, rare sugars have been used to stimulate the defenses of plants and thus increase their resistance to phytopathogenic agents [101]. Among these, D-tagatose has shown protective activity especially against members of the class of Oomycetes [102,103]. Root or aerial pretreatments of grapevine in vitro plantlets (cv Chardonnay) by IFP48, an 80% D-tagatose-based product currently under registration on the European market and used at a dose of 5 g/L, showed an anti-oomycete activity, reducing sporangia density of P. viticola by 35% [104]. This protective effect was correlated with an overexpression of genes coding for PR proteins, LOX9 as well as PAL and STS genes (with a respective induction of 10 and 8 times). Overexpression of these responsive defense genes in grapevine explained, at least in part, the protection conferred by IFP48 as the level of overexpression of those genes remained high after inoculation with downy mildew. It should also be noted that IFP48 exhibited a direct anti-oomycete activity, reducing by 100% sporangia density of P. viticola. A similar experiment carried out by the same group confirmed the protective role of IFP48 with a 50% reduction of P. viticola sporangia density but showed no effect, on the other hand, of this product on the control of gray mold. (B. cinerea) [105]. The protection mechanisms of IFP48 resulted in modulation of the expression of some responsive defense genes controlled by the salicylic acid pathway (PR1 and PR2 genes) and that of jasmonic acid (ERF1, Ethylene Response Factor 1 and PR3c genes) in the absence of pathogen contamination. In both studies, only a low accumulation of stilbenes was reported (3–5 μg/g FW for resveratrol and a few micrograms per grams of FW for viniferins ε and δ).

Table 3. Use of bio-elicitors for grapevine protection and stimulation of the phytoalexin response.

Table 3. Use of bio-elicitors for grapevine protection and stimulation of the phytoalexin response.

Elicitor	Plant Material	Biological Inputs	References
Laminarin	Grapevine in vitro plantlets (cv Chardonnay and Gamay)	55 to 75% protection against respectively gray mold and downy mildew. Overexpression of genes coding for PR proteins as well as PAL and STS1 genes with concomitant accumulation of phytoalexins (cell suspensions)	[89]
Chitosan partially deacetylated	Grapevine in vitro plantlets (cv Chardonnay)	Total suppression of B. cinerea necrosis on leaves. Overexpression of chitinases, PAL and LOX activities. Direct antifungal activity. No mention of stilbene production	[91]
Chitosan partially deacetylated (20%)	Grapevine in vitro plantlets and potted plants (cv Chardonnay)	60 to 70% protection against respectively gray mold and downy mildew linked with an increase in chitinase, β-1,3-glucanase activities and augmentation of stilbene content (resveratrol, piceid and ε-viniferin)	[50]
Chitosan + copper sulfate	Potted plants (cv Chardonnay)	83% protection against downy mildew associated with a potentiation of the phytoalexin response	[50]
Ergosterol	Grapevine plants(cv Ugni Blanc)	75% protection towards B. cinerea contamination (55% with benzothiadiazole). Overexpression of the WRKY transcription factor, VvST1 and VvLTP1 with correlated resveratrol production	[92]
β-amino-butyric acid	Plant cuttings of cvs Chasselas andSolaris	Strong inhibition of P. viticola sporulation and hyphal growth linked with a priming effect on stilbene accumulation correlated with overexpression of PAL and C4H genes in Solaris and STS in both cultivars	[95]
Bacterial rhamnolipidss	Grapevine in vitro plantlets	Protection of leaves against B. cinerea infection. Overexpression of PR proteins, LOX, PAL and STS genes	[98]
D-Tagatose (IFP48)	Grapevine in vitro plantlets (cv Chardonnay)	Reduction by 35% of sporangia density of P. viticola associated with a direct anti-oomycete activity. Overexpression of genes encoding PR proteins, LOX9, PAL and STS along with low stilbene production (resveratrol, piceid and dimers)	[104]
D-Tagatose (IFP48)	Grapevine in vitro plantlets (cv Chardonnay)	Reduction by 50% of sporangia density of P. viticola but no effect towards B. cinerea. Overexpression of PR1 and PR2 genes (SA pathway), ERF1 and PR3c genes (JA pathway). Low stilbene production (resveratrol, piceid and dimers)	[105])

¹ SA, salicylic acid; JA, jasmonic acid.

4. Control of grapevine diseases by beneficial organisms involving stimulation of phytoalexin synthesis

Table 4. Control of grapevine diseases by beneficial organisms involving stimulation of phytoalexin synthesis.

5. Conclusions

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