Impact of Environmental Factors on Stilbene Biosynthesis

Stilbenes are a small family of polyphenolic secondary metabolites that can be found in several distantly related plant species. These compounds act as phytoalexins, playing a crucial role in plant defense against phytopathogens, as well as being involved in the adaptation of plants to abiotic environmental factors. Among stilbenes, trans-resveratrol is certainly the most popular and extensively studied for its health properties. In recent years, an increasing number of stilbene compounds were subjected to investigations concerning their bioactivity. This review presents the most updated knowledge of the stilbene biosynthetic pathway, also focusing on the role of several environmental factors in eliciting stilbenes biosynthesis. The effects of ultraviolet radiation, visible light, ultrasonication, mechanical stress, salt stress, drought, temperature, ozone, and biotic stress are reviewed in the context of enhancing stilbene biosynthesis, both in planta and in plant cell and organ cultures. This knowledge may shed some light on stilbene biological roles and represents a useful tool to increase the accumulation of these valuable compounds.


Introduction
Stilbenes are a small yet important class of non-flavonoid polyphenols, sharing a common structure characterized by a 14-carbon skeleton composed of two benzene rings linked by an ethylene bridge ( Figure 1). Due to the presence of the central ethylene moiety between the aromatic rings, stilbenes exist as the two possible stereoisomers cis and trans. However, the naturally occurring stilbenes are usually in the trans form [1]. Plant stilbenes, together with other polyphenols such as flavonoids, isoflavonoids, curcuminoids, and xanthones, belong to the class of polyketides. Over 400 different stilbene compounds are currently known [2], mostly derived from trans-resveratrol (3,5,4 -trihydroxy-trans-stilbene) (Figure 1), although different structures can be found in specific plant families [3].
Stilbenes have been identified in at least 72 plant species belonging to 31 genera and 12 distantly related families, including Pinaceae (e.g., Picea abies (L.) Karst. and Pinus nigra J.F. Arnold), Gnetaceae (e.g., Gnetum parvifolium (Warb.) W.C. Cheng and G. africanum Welw.), Fabaceae (Arachis hypogaea L. and Robinia pseudoacacia L.), Vitaceae (e.g., Vitis vinifera L. and V. amurensis Rupr.), Moraceae (e.g., Morus alba L. and M. macroura Miq.), and Polygonaceae (e.g., Polygonum cuspidatum Sieb. et Zucc. and P. multiflorum Thunb.) [4,5]. Given their nutraceutical value, stilbene content and composition have mainly been investigated in food plants, and the knowledge of stilbene distribution in nature is still poor. This is partially related to the complexity of the quali-quantitative analysis of stilbenes, which is in turn related to the unavailability of standards and the detection limits of analytical methods [2]. For these reasons, most of the studies carried out Stilbenes are mainly involved in constitutive and inducible protection of the plant against biotic (phytopathogenic microorganisms and herbivores) and abiotic (e.g., UV radiation and tropospheric ozone) stress [3,6]. On one side they counteract the aggression exerting a direct toxic effect on the pathogen, while on the other they act as antioxidants, protecting the cells from oxidative damage [7][8][9]. Stilbenes possess several antipathogenic properties including antibacterial, antifungal [10,11], nematocidal [12], and insecticidal [13,14]. They could also act as a deterrent towards vertebrate herbivory [15], as a possible negative effect of stilbenes has been reported on snowshoe hares (Lepus americanus Erxleben) [16,17] and field voles (Microtus agrestis L.) [18]. The role of stilbenes, among other polyphenols, in counteracting oxidative stress is just as important, as the plant response to pathogen attack involves the production of reactive oxygen species (ROS), which both act as signals for the activation of stress and defense pathways and as toxic substances capable of directly damaging the pathogen. Oxidative stress may also be induced by many abiotic conditions, such as drought, thermal stress, ultraviolet radiation, mechanical stress, heavy metals, salts, and air pollutants such as ozone [19]. Unsurprisingly, many of these factors also affect stilbene production [20].
Due to their potential use in the nutraceutical, cosmeceutical, and pharmaceutical fields, great interest is directed at the methods for large-scale production of stilbenes. For instance, it has been estimated that the global market for trans-resveratrol will almost double in the next 6 years, from 58 million USD in 2020 to 99.4 million USD by 2026 [37]. Methods for obtaining stilbenes can be grouped into three categories: direct extraction from plants, chemical synthesis, and the use of biotechnologies. The chemical synthesis of stilbenes has been reported, but this method is not economically feasible, in addition to being difficult in terms of stereospecific synthesis [38,39]. Considerable efforts have been devoted to the development of biotechnological methods for stilbene production, which broadly include tissue culture techniques [40], biotransformation [41], and metabolic engineering [42]. Nevertheless, the major way of supplying stilbenes is the direct extraction from plants such as P. cuspidatum and V. vinifera [43].
The stilbene content and profile in stilbene-producing plants vary strongly in response to a variety of environmental factors. In recent years, a considerable body of knowledge regarding the stilbene biosynthetic pathway and the impact of environmental conditions on the production of these valuable metabolites has accumulated. This review presents the recent knowledge of the stilbene biosynthetic pathway and the impact of different environmental factors on stilbene production.
Pinus species can biosynthesize two types of stilbenes, i.e., pinosylvin and dihydropinosylvin, which are biosynthetically derived from cinnamoyl-CoA and dihydrocinnamoyl-CoA, respectively ( Figure 3B,C). STS from P. strobus shows a clear preference for cinnamoyl-CoA and was therefore characterized as pinosylvin synthase [58]. Otherwise, STS from P. sylvestris shows an unusual preference for dihydro-cinnamoyl-CoA, identifying it as a dihydropinosylvin synthase [56]. STS does not exhibit absolute substrate specificity. While showing a preference for a given substrate, the same STS enzyme can accept different cinnamic acid derivatives as starting substrates catalyzing the biosynthesis of different stilbenes. For example, the enzyme responsible for the biosynthesis of piceatannol (3,5,3 ,4tetrahydroxystilbene) has not been identified yet, however, pinosylvin synthase from P. strobus proved to be active with caffeoyl-CoA in vitro ( Figure 3D), suggesting that it could be responsible for piceatannol biosynthesis in planta [58].
STS enzymes belong to the type III polyketide synthase superfamily (PKSs), which also includes chalcone synthase (CHS; EC 2.3.1.74) [59]. STS and CHS share a high degree of similarity both in their amino acid sequence identity (which reaches 75-90% depending on the species) and in their crystallographic structures [51,60]. CHS genes are present in the genome of all plants analyzed so far, while STS have been identified in a limited number of plant species, often phylogenetically unrelated. Converging lines of evidence indicate that CHS is the archetypal enzyme from which STS evolved multiple times independently in stilbene-producing plants, through gene duplication followed by functional divergence [60-62]. CHS and STS are the most investigated enzymes among PKSs and, due to their high sequence similarity, they are often referred to as the CHS/STS family [63,64].
Although it employs the same substrates as STS, CHS is responsible for the first committed step in the biosynthesis of flavonoid-type compounds. Both enzymes generate the same linear tetraketide intermediate. However, CHS catalyzes a C6→C1 Claisen condensation of the intermediate to produce naringenin chalcone, while STS catalyzes an alternative C2→C7 aldol condensation of the intermediate to form a stilbene backbone ( Figure 4) [59, 65,66].
STS was first extracted and purified from suspension cultures of peanut cells elicited with UV radiation [53]. Cloning of two peanut STS genes revealed a high sequence identity with CHS throughout the coding region and the presence of an intron at the same position as a conserved intron in CHS [67]. STS genes and cDNAs were subsequently cloned and characterized from grapevine cell suspension cultures [68] and Scots pine plantlets [56], both induced by fungal elicitors. At present, STS genes have been cloned from several plant species including mulberry (Morus notabilis C.K. Schneid and M. atropurpurea Roxb.) [42,66], Scots pine [69], white spruce (Picea glauca (Moench) Voss) [70], Norway spruce (Picea abies (L.) H. Karst.) [71], Japanese red pine [57], and sorghum (Sorghum bicolor (L.) Moench) [72]. To the best of our knowledge, sorghum is the only monocot plant in which an STS gene (SbSTS1) has been identified.
In most stilbene-producing plants, STS exists as a small family consisting of 1-10 closely related paralogs. For example, the STS multigene family is represented by two members in white spruce [70] and Norway spruce [71], three members in Japanese red pine [57], almost five members in Scots pine [69], six members in peanut [73], and ten members in mulberry [66]. Remarkable exceptions to this role are sorghum, in whose genome only one STS gene has been identified [74,75], and grapevine, which possesses an uncommonly large number of STS genes. Both grapevine and sorghum genomes have been entirely sequenced [74,76]. Early Southern-blot analysis suggested that the grapevine STS gene family consisted of 15-20 members [77]. Genome-wide analysis carried out on the V. vinifera PN40024 genome led to the identification of 48 putative STS genes, designated VvSTS1 to VvSTS48, with at least 33 potentially coding for functional STS proteins [60,62].
To date, there is no evidence regarding the different substrate specificity and enzymatic activity of different VvSTSs. Functional characterization of nine VvSTSs confirmed that they encode for functional STS enzymes [62]. Since these nine genes were specifically chosen to represent the diversity of the VvSTS gene family, it is most likely that all grapevine VvSTSs encode enzymes with similar activity and specificity. Despite the high similarity between STS genes which makes it difficult to accurately distinguish the individual transcripts, gene expression studies revealed different transcriptional responses of distinct VvSTSs during development and in response to environmental stresses [60, 78,79]. The expression of some VvSTSs was also found to be tissue-specific [60,79]. It is therefore likely that the large quantity of members in the grapevine STS gene family has evolved to allow for fine spatial and temporal regulation of stilbene biosynthesis under both normal and stress conditions.

Glucosylation/Deglucosylation
Glycosylation is one of the most common modifications of plant secondary metabolites [80,81] that can modify their physicochemical and biological properties. Water-solubility, physicochemical stability, biological half-life, compartmentalization, and biological activity of stilbenes and other phenylpropanoids can be dramatically altered by glycosylation [82][83][84].
In stilbene-producing plants, a significant fraction of stilbenes is accumulated in a glucosylated form [85]. For instance, Fallopia japonica Houtt. (formerly Polygonum cuspidatum) produces both resveratrol and resveratrol 3-O-β-glucoside (commonly known as piceid or polydatin) ( Figure 5), and the glucosylated form can reach concentrations of up to six times higher than the free aglycone [86]. Morus alba L. and Rheum undulatum L. accumulate the glucosylated stilbenes mulberroside A (a diglucoside of oxyresveratrol) and rhapontin (a monoglucoside of rhapontigenin, also known as rhaponticin) [87] (Figure 5). Significant amounts of cisand trans-piceid are accumulated in grapevine, both constitutively [40, 88,89] and in response to pathogen attack [90,91] and to environmental stresses such as UV light [88,[92][93][94], salinity [95], and drought [96,97]. Numerous glycosyltransferases that produce glucose esters of hydroxybenzoic and hydroxycinnamic acids accept a broad spectrum of structurally similar substrates [98]. A bi-functional glycosyltransferase from Concord grape (Vitis labrusca L.) (VLRSgt) that produces stilbene glucosides and glucose esters of hydroxycinnamic acids in vitro has been characterized in 2007 by Hall and De Luca [99]. The mesocarp-specific expression of VLRSgt reflected the increased accumulation of resveratrol glucosides during berry maturation, coherent with a role for this enzyme in stilbene glucosylation in the mesocarp.
It is well known that glucosylation increases resveratrol water solubility [82] and helps to protect stilbenes and other polyphenols from enzymic oxidation [100,101], which could extend their half-life in plant cells, preserve their biological properties, and assist stilbene transportation and accumulation.
Considerable levels of trans-piceid can be found in grape derivatives such as wines and juices [102,103]. Due to the β-glucosidase activity of yeasts, a decrease in trans-piceid concentration accompanied by an increase in trans-resveratrol concentration is often observed during grape fermentation [104,105]. Since trans-resveratrol has better health properties than piceid, a great deal of interest has been paid to strategies aimed to increase transresveratrol concentration during winemaking. One of these consists in the selection of yeast strains with high β-glucosidase activity, capable of efficiently converting trans-piceid into the free aglycone during the alcoholic fermentation [106,107]. Bacterial or fungal βglucosidases can also be used to obtain trans-resveratrol from plant extracts rich in piceid, such as those obtained from P. cuspidatum [108,109]. Other glycosylated stilbenes, such as mulberroside A and rhaponticin from M. alba and R. undulatum, can be enzymatically converted to their aglycones oxyresveratrol and rhapontigenin with an increase in their bioactivity [87,[110][111][112].
Methylation of hydroxyl groups alters the solubility and reactivity of stilbenes and can therefore affect their biological activity. For instance, pterostilbene has been shown to be 5-10 times more active than non-methylated resveratrol in inhibiting the germination of downy mildew (Plasmopara viticola) sporangia and grey mold (Botrytis cinerea) conidia [120]. Pinosylvin monomethyl ether has been reported to have significantly lower antifungal and antibacterial activity than pinosylvin [121], although it has shown greater activity against some brown-rot fungi [122].
Investigations on the relationship between chemical structure and biological activity revealed increased cytotoxicity and anticancer activity associated with resveratrol methylation [123,124]. The substitution of hydroxy with methoxy groups enhances the lipophilicity of pterostilbene over resveratrol, which results in high bioavailability. This difference in pharmacokinetics might explain the higher bioactivity of pterostilbene over its parental compound resveratrol. Methylated resveratrol derivatives have consequently become attractive target compounds for both bioproduction and metabolic engineering [125,126]. Several plant OMTs have been characterized in the last decades. However, the majority of them have been found to be involved in the methylation of aromatic hydroxyl groups of different compounds such as benzylisoquinoline alkaloids [127], phenylpropanoids [128], and flavonoids [129], while only a few stilbene-specific OMTs have been reported to date.
V. vinifera resveratrol OMT (VvROMT) was shown to specifically catalyze the methylation of resveratrol and pinosylvin (3,5-dihydroxystilbene) at the C-3 or C-5 positions [130]. VvROMT was shown to convert resveratrol to pterostilbene both in vitro and in planta. The transient co-expression of VvROMT and VvSTS in tobacco resulted in the accumulation of pterostilbene. VvROMT gene expression in grapevine leaves was induced by different stresses, including P. viticola infection and UV radiation, accordingly with the role of pterostilbene in chemical plant defense [130]. S. bicolor resveratrol OMT (SbOMT1) has been shown to catalyze the 4 -O-methylation of resveratrol both in vitro and in planta [125,131]. In 2019, Koeduka and colleagues [132] isolated and characterized a putative aromatic Omethyltransferase gene (AcOMT1) in Acorus calamus (Araceae) using RNA-seq analysis. Recombinant AcOMT1 expressed in Escherichia coli showed high 4 -O-methylation activity toward resveratrol and its derivative, isorhapontigenin (3,4 ,5-trihydroxy-3 -methoxystilbene).
In Scots pine, pinosylvin can be methylated by a pinosylvin O-methyltransferase (PsPMT1) to pinosylvin monomethyl ether, following ozone or fungal elicitation treatment [133]. However, it should be noted that PsPMT1 showed a relatively broad substrate specificity, methylating several compounds such as stilbene aglycones, flavonoids, and hydroxycinnamic acids, many of these even more efficiently than pinosylvin. In 2017, Paasela and co-workers [134] subsequently isolated and characterized an O-methyltransferase from P. sylvestris (PpPMT2), which is held responsible for the methylation of pinosylvin. Unlike the multifunctional PsPMT1, PsPMT2 preferentially methylated pinosylvin into its monomethyl ether, showing a high degree of specificity for stilbenes. The authors observed that PsPMT2 is co-expressed with STS in response to wounding of xylem and UV-C treatment of needles, suggesting that these two enzymes are under common regulation.
Over 45 prenylated stilbenoids have been identified in A. hypogaea [148]. The major prenylated stilbenoids accumulated in peanuts are trans-arachidin-1, trans-arachidin-2, trans-arachidin-3, and trans-3 -(3-methyl-2-butenyl)-resveratrol ( Figure 7) [148]. In accordance with their role as phytoalexins, peanut prenylated stilbenoids have been shown to accumulate upon challenge with microorganisms [142][143][144][145]149] and to possess remarkable antifungal activity [150][151][152]. Several recent studies have shown interesting therapeutic potential for peanut prenylated stilbenoids [135,136,153,154]. Prenylated compounds generally exhibit greater bioavailability than their non-prenylated counterparts, due to the increase in lipophilicity linked to the prenyl groups. Despite their biological and medical relevance, the biosynthetic pathways of prenylated stilbenoids have yet to be elucidated and the genes encoding stilbenoid-specific prenyltransferases have only recently been identified in plants. In 2018, membrane-bound stilbene-specific prenyltransferases have been described in peanut and mulberry [148,155]. Combining targeted transcriptomic and metabolomic analyses, Yang et al. [148] discovered five candidate prenyltransferase genes in elicitortreated A. hypogaea hairy root cultures. Two of these, AhR4DT-1 and AhR3DT-1, were functionally characterized in a transient expression system consisting of Agrobacteriuminfiltrated leaves of Nicotiana benthamiana Domin. The authors demonstrated that AhR4DT-1 catalyzes the prenylation of resveratrol at its C-4 position leading to arachidin-2 formation, while AhR3DT-1 is responsible for resveratrol prenylation at C-3 leading to the 3-methyl-2-butenyl-3-resveratrol formation. In 2018, Zhong and colleagues [155] identified and functionally characterized a stilbenoid-specific prenyltransferase from M. alba (MaOGT) that recognizes oxyresveratrol and geranyl diphosphate (GPP) as natural substrates and catalyzes oxyresveratrol prenylation. Both peanut and mulberry prenyltransferases have proved to be highly specific for stilbene substrates and fluorescent microscopy analysis has shown that they are localized in the chloroplast, similarly to other membrane-bound plant prenyltransferases [156].
The largest group of oligostilbenes is represented by resveratrol oligomers, arising from the polymerization of two to eight resveratrol units [169,170] (Figure 8). Among these, the most investigated are viniferins, which accumulate in V. vinifera upon abiotic stress (e.g., UV irradiation) or fungal infection (e.g., Botrytis cinerea and Plasmopara viticola) [8,120,171].
Resveratrol oligomerization has been achieved in vitro by enzymatic oxidation utilizing horseradish peroxidase [172,173] or laccase-like stilbene oxidase from B. cinerea [174,175]. It has been proposed that peroxidases (POD or POX EC 1.11.1.7) located in the plant cell wall and vacuole are responsible for oxidative polymerization of resveratrol to form its natural oligomers [157,176]. POD enzymes use hydrogen peroxide to catalyze oxidative reactions and have already been exploited for the in vitro synthesis of oligostilbenes [173,177,178]. However, it should be noted that in vitro experiments with these enzymes did not lead to the formation of oligomers with the natural configuration found in plants, and to date, there is no direct evidence of the involvement of specific peroxidases in the formation of oligomeric stilbenes in planta [5].
In recent years, considerable efforts have also been made to develop non-enzymatic methods to produce oligostilbenes [179,180].

Impact of Environmental Factors on the Biosynthesis of Stilbenes
It is known that the biosynthesis of stilbenes in plants can be triggered by a variety of biotic and abiotic environmental factors. Some of these, like fungal infection and UV-C radiation, have been under study for many years, while others like bacterial infection and ozone stress have only recently caught the attention of the research community. In Table 1 and in the following paragraphs some of the most important results coming from studies on the influence of different environmental factors on the biosynthesis of stilbenes are reported and described. Irradiation of grape berries with UV-C or inoculation of grape berries with Rhizopus stolonifer Stilbenes (resveratrol and pterostilbene) Increased stilbene accumulation, greater with UV-C compared to fungal inoculum [183] V. vinifera cv. Napoleon Irradiation of grape berries with UV-C or UV-B Stilbenes (resveratrol and piceid); anthocyanins; flavonoids; hydroxycinnamic acids (caffeoyltartaric acid and chlorogenic acid) Increased stilbene accumulation, greater with UV-C compared to UV-B (3 and 2-fold, respectively) [184] V. vinifera cv. Corvina Irradiation of grape berries with UV-B and wilting at different temperatures Stilbenes (trans and cis-resveratrol, trans and cis-piceid); total polyphenols, flavonoids, anthocyanins, catechins, and proanthocyanidins Enhanced stilbene accumulation and STS gene expression [185] V. vinifera cvs. Black Corinth and Flame seedless Irradiation of grape berries with UV-C Resveratrol; total anthocyanins Greater resveratrol increase (4-fold) in cv. Flame Seedless. Lower increase in cv. Black Corinth. Negative relationship between resveratrol synthesis and anthocyanin concentration [186] V. vinifera cv. Flame seedless, Red Globe, Crimson seedless, Napoleon, Superior seedless, Moscatel Italica, Dominga

Resveratrol
Increased resveratrol concentration, especially in UV-treated grapes stored at high temperature [195] V. vinifera cv. Redglobe Irradiation of grape berries with UV-C and storage at different temperatures Stilbenes (trans-resveratrol, cisand trans-piceid); flavonols; anthocyanins; flavanols (catechins) Increased concentration of cisand trans-piceid after UV-C treatment and cold storage [196] V. vinifera cv. Crimson Treatment of grape berries with UV-C and chitosan, followed by storage at different temperatures

Resveratrol and piceatannol
Both high temperature and UV-C strongly induce the expression of PAL, C4H-, 4CL-, and STS-like genes, but only UV-C enhance stilbene accumulation Both high temperature and UV-C strongly induce the expression of PAL, C4H, 4CL, STS, and CYP genes. High temperatures do not affect stilbene accumulation in stems but decrease stilbene concentration in roots at 3 h. UV-C irradiation induces total stilbene accumulation in stems but not in roots. [205] Pinus sylvestris Treatment of needles from 5-years-old plantlets with UV-C V. vinifera cv. Cardinal Treatment of grape berries with low temperature (0 • C) and high CO 2 levels (20%)

Trans-resveratrol; total anthocyanins
Low temperature reduces trans-resveratrol content in both treated and non-treated grapes, although the decrease is higher in CO 2 -treated grapes [208] V. vinifera cvs. Dominga, Superior seedless, Autumn Royal, Red Globe Treatment of grape berries with low temperature (0 • C) and high CO 2 levels (3 days) Resveratrol, resveratrol-glucoside, trans-piceatannol, z-miyabenol, pallidol Stilbene accumulation in response to low temperature and CO 2 is cultivar dependent. High CO 2 levels activate stilbene pathways in cv. Dominga. Low temperature increase stilbenes biosynthesis in cv. Red Globe. Stilbene accumulation is independent of the atmosphere storage in cvs. Superior Seedless and Autumn Royal [209] V. vinifera cv. Shiraz Treatment of grape berries with high light (2500 µmol m −2 s −1 ), high temperature (40 • C), oxidative stress (120 µM menadione), 3.026 mM abscisic acid, and 200 µM jasmonic acid (JA)

Resveratrol, piceid, and viniferin
At the pre-veraison stage, an increase in anthocyanins levels is accompanied by a declining stilbene accumulation in response to JA, menadione, and high light. At the veraison stage, mild change in anthocyanin levels in response to all the treatments is accompanied by stilbene accumulation [210] V. vinifera cv. Barbera Cisand trans-resveratrol, cisand trans-piceid, cisand trans-piceatannol Increased accumulation of stilbenes (mainly transand cis-piceid) and induction of stilbene biosynthetic genes in response to red and blue LED light [212,213] V. vinifera cv. Negramaro Light-exposed and dark-maintained cell cultures Inhibitory effect of high light on stilbene biosynthesis; 50 µM MeJa is optimal for efficient production of total phenols, flavonoids, and stilbenes [215] V. vinifera cv. Malvasia; V. rupestris Du Lut Light-exposed and dark-maintained cell cultures   V. vinifera cv. Barbera Elicitation of cell suspension cultures with chitosan Transand cis-resveratrol Induction of trans-resveratrol production and STS gene expression [229]  Induction of trans-resveratrol, piceid, and viniferins by jasmonates. Jasmonic acid enhances simultaneously δand ε-viniferin biosynthesis, whereas MeJa stimulates preferentially δ-viniferin production. [233] V. vinifera cv. Monastrell

Trans-resveratrol
Highest increase in trans-reveratrol production was obtained with the combined use of MeJa, cyclodextrins, and an optimal sucrose concentration. Greatest release of trans-resveratrol into the culture medium is achieved with the combined use of MeJa, cyclodextrin, and UV-C V. vinifera cv. Hongbaladuo; V. vinifera × V. amurensis cv. Beihong

Treatment of leaves and berries with CaCl 2 and UV-C used alone or in combination
Cisand trans-resveratrol Increased resveratrol content with single treatments, greater increase with combined treatment [236] V. vinifera cv. Gamay Fréaux Elicitation of cell suspension cultures with jasmonic acid, salicylic acid, β-glucan, and chitosan Stilbenes (trans-resveratrol and trans-piceid); total anthocyanins Increased resveratrol production with co-treatment with jasmonic acid and β-glucan [237]  Trans-resveratrol, trans-piceid, cis-ε-viniferin, trans-ε-viniferin, trans-δ-viniferin MeJa is the most effective in inducing trans-resveratrol the biosynthesis, while 12-oxo-phytodienoic acid, jasmonic acid, and coronatine are the most effective in inducing the biosynthesis of viniferins [214] V. vinifera cv. Monastrell Elicitation cell suspension cultures with cyclodextrins and coronatine Trans-resveratrol Induction of stilbene biosynthetic genes by cyclodextrins and/or coronatine. Maximum level of trans-resveratrol production and secretion into the culture medium with co-treatment with 50 mM cyclodextrins and 1 µM coronatine [238] V. vinifera cv. Tempranillo Foliar application of MeJa, chitosan, and yeast extract Stilbenes (trans-and cis-piceid and transand cis-resveratrol); flavonols; anthocyanins; hydroxybenzoic acids; hydroxycinnamic acids MeJa and yeast extract improve both grape and wine anthocyanin content. Stilbene content is clearly improved by yeast extract [239] Vitis vinifera L. cv. Kalecik Karası Elicitation of grape berries with ultrasound Trans-resveratrol About 20-fold increase in trans-resveratrol content in grape skin [240] Arachis hypogaea cv. Tainan No. 14

Elicitation of calli with bacteria and fungi (both viable and autoclaved) or with chitin
Trans-resveratrol and trans-piceatannol Induction of stilbene biosynthesis by fungi (both viable and autoclaved) and chitin [241] A. hypogaea cv. Hull line 3 Elicitation of hairy root cultures with MeJa and methyl-β-cyclodextrin Trans-resveratrol, trans-piceatannol, trans-arachidin-1 and trans-arachidin-3 Co-treatment with MeJa and cyclodextrin led to high levels of stilbenes in the culture medium [242] Arachis hypogaea cv. Georgia green Treatment of peanuts with ultrasonication or UV-C Trans-resveratrol, trans-piceid Increased resveratrol, piceid, and total stilbene concentration, more induced by ultrasound than UV-C [201]

UV Radiation
Multiple lines of evidence indicate that stilbenes, as well as other polyphenols, play an important role in protecting plants from the damaging effects of ultraviolet (UV) radiation [243][244][245]. Induction of polyphenolic phytoalexin biosynthesis in response to UV exposure has been observed in numerous plants [246,247]. Non-polyphenolic phytoalexins can also be elicited by UV, e.g., labdane-related diterpenoids in rice [248] and terpenoid indole alkaloids in the Madagascar periwinkle (Catharanthus roseus (L.) G.Don) [249].
The solar UV spectrum is conventionally subdivided into three wavelength ranges: UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm) [250]. UV-C is extremely harmful to organisms, but it is naturally filtered by the stratospheric ozone layer and it is consequently not relevant under natural conditions of solar irradiation. Ozone absorption coefficient drops rapidly at wavelengths longer than 280 nm, reaching zero around 330 nm, therefore UV-B and UV-A can reach Earth's surface and interact with plants [251]. However, UV harmfulness declines in a similar way, as UV-A cannot be absorbed by DNA and it is thus far less damaging for plants [252]. As a result, for a long time, the effects of UV-A on plant physiology had been underestimated and most studies have focused primarily on UV-C and subordinately on UV-B.
UV-C significantly increases stilbene production in different stilbene-producing plants including grapevine [92,184,194,253], peanut [216,243,254], Gnetum parvifolium (Warb.) W.C. Cheng [204,205], Picea jezoensis (Siebold & Zucc.) Carr. [255], and Polygonum cuspidatum Siebold and Zucc. [256] (Table 1). Recently, Vannozzi and colleagues [60] performed a genome-wide analysis of the VvSTS multigene family expression pattern on V. vinifera cv. Pinot Noir ('PN40024' genotype) leaf disks subjected to different stresses (i.e., UV-C exposure, wounding, and downy mildew infection) through a whole transcriptomic (RNA-seq) and real-time qRT-PCR approach. They observed that all stress treatments led to a significant up-regulation of at least several members of the VvSTS multigene family. However, UV-C exposure resulted in the highest induction of the majority of VvSTS members. The induction of VvSTSs was accompanied by reduced expression of VvCHS genes to lower levels compared to untreated leaf discs, suggesting that the competitive relationship between VvSTS and VvCHS may play a role in the accumulation of stilbenes in response to UV-C [60,198].
Molecular mechanisms behind the induction of stilbene biosynthesis in response to UV radiation are still not completely understood. The induction is accompanied by transcriptional activation, protein accumulation, and activation of STS and other enzymes involved in stilbene biosynthesis [5]. In 2013, Höll and co-workers [257] reported that two R2R3-MYB-type transcription factors, namely MYB14 and MYB15, control the transcriptional expression of VvSTS genes under UV-C irradiation in V. vinifera cv. Shiraz leaf discs. However, these transcription factors appear not to be specifically involved in response to UV-C, but also to other stresses including fungal infections and wounds.
In a recent study, the member of the stilbene synthase family VpSTS29 derived from Chinese wild Vitis pseudoreticulata W.T. Wang was overexpressed in V. vinifera cv. Thompson Seedless and the localization of the VpSTS29-GFP protein was investigated [247]. The accumulation of stilbenes elicited by UV-C irradiation was accompanied by the translocation of the VpSTS29-GFP protein from the cytoplasm to the chloroplast. Interestingly, transgenic plants overexpressing VpSTS29-GFP exhibited much lower H 2 O 2 content than untransformed plants and an altered expression of genes related to redox processes, stilbene biosynthesis, and light stimulus [247].
Regarding UV-B, several studies showed that these wavelengths can stimulate the biosynthesis of polyphenols, including stilbenes [184,199,200,258]. UV-B triggers a radiationspecific signaling pathway in grape skin, which activates the biosynthesis and accumulation of secondary metabolites [259]. A UV-B treatment was found to elicit a significant increase in stilbene production, although less marked than that achieved with UV-C [260]. In 2008, Berli and co-workers [258] analyzed the stilbene content in berry skins collected from plants of V. vinifera cv. Malbec cultivated under sunlight with full UV-B (+UV-B) or filtered UV-B (−UV-B) in three different locations at 500, 1000, and 1500 m above sea level (asl) and observed that different solar UV-B levels affect the accumulation of trans-resveratrol. The highest resveratrol content was detected in the berry skins from the +UV-B treatment at 1500 m asl, where the difference between +UV-B and −UV-B was statistically significant. Studies on ozone-treated Scots pine also showed a slight increase in stilbene synthase mRNA, as well as in pinosylvin and pinosylvin methyl ether contents under exposure to UV-B light [261,262].
Post-harvest treatment with UV-B and UV-C has been widely used in fruit and vegetable storage [199,263], where it delays fruit ripening and senescence [264], and activates the defenses against pathogens [265]. Post-harvest UV treatment has been exploited in grapes to increase the content in phenolic compounds including stilbenes in berries and wines [199] (Table 1).
As previously mentioned, the impact of UV-A on stilbene biosynthesis has been underestimated, however recent studies suggest that this radiation deserves more attention. An increase in stilbene content related to UV-A irradiation has been observed in Eucalyptus nitens (H. Deane and Maiden) Maiden by Close and colleagues [266]. The authors postulated that, since stilbenes have absorbance properties consistent with a function as UV-A screens, they could be part of an active UV-A response. This conclusion is consistent with the observation that adult retinal pigment epithelial cells treated with resveratrol show higher viability when exposed to UV-A [267]. A role of UV-A in stilbene biosynthesis was observed in leaves of grey alder (Alnus incana (L.) Moench) and white birch (Betula pubescens Ehrh.) trees under field conditions [268]. It was recently observed that trans-resveratrol and trans-pterostilbene biosynthesis in leaves of O'Neal high bush blueberries (Vaccinium corymbosum L.) can be enhanced by irradiation with both UV-A and UV-C [269]. The comparison of the two UV-treatments showed that UV-A is more effective in promoting trans-resveratrol production, while UV-C is better in enhancing transpterostilbene production.

Light
Light is vital for plants, representing the main energy source for these phototrophic organisms. Light plays a pivotal role in plant growth and development, but it also affects secondary metabolism [20,270]. Extensive literature shows the relationship between light and biosynthesis of polyphenol compounds such as anthocyanins and flavonols [271,272]. However, only a limited number of studies are available on the light-dependent regulation of the stilbene biosynthetic pathway.
Advancements in technology have recently brought light-emitting diodes (LEDs) to the scene of botanical research, allowing for the use of specific wavelengths at high irradiance levels, with realistic results on the study of plant physiological responses to them [273]. Although there is still scarce information about stilbene biosynthesis in response to LED lighting, several studies observed an increase in stilbene biosynthesis, as well as in the expression of stilbene biosynthetic genes, in grapevine exposed to specific wavelengths.
The influence of red LED light (625 nm) and methyl jasmonate (MeJa) on the production of phenylpropanoids in V. vinifera cv. Barbera cell suspension cultures were investigated by Tassoni and co-workers [211]. The combined treatment with red light and MeJa increased the biosynthesis of both anthocyanins and stilbenes, while also promoting the release of catechins into the culture medium. The treatment with red light alone produced a 50% increase in stilbene content in grapevine cells, accompanied by an average decrease in anthocyanin and catechin content of 10% and 18%, respectively. These results suggest a diversion of the phenylpropanoid pathway towards the production of stilbenes under red light [211].
A series of experiments on berries of V. labruscana Bailey cvs. Campbell Early and Kyoho irradiated with fluorescent white light or purple (380 nm), blue (440 nm), and red (660 nm) LED lights showed that red and blue light induces the upregulation of several STS genes and the accumulation of transand cis-resveratrol, transand cis-piceid, and piceatannol both in grape berry skin [212], and in detached leaves [213].
In 2015, Taurino and colleagues [214] observed an inhibitory effect of light on transresveratrol production and an enhancement of trans-piceid biosynthesis in cell suspension cultures of V. vinifera cv. Negramaro. Moreover, in 2018, Andi and collaborators [215] reported that high-level white light irradiation (10,000 lux) inhibits the biosynthesis of trans-piceid and trans-resveratrol in cell suspension cultures of V. vinifera cv. Shahai.
Interesting differences in the impact of light on constitutive and MeJa-induced stilbene biosynthesis emerged from the comparison of cell lines of V. vinifera cv. Malvasia and V. rupestris Du Lot [40]. In both species, the constitutive stilbene content was higher under light conditions, although V. vinifera mainly accumulated piceid, while V. rupestris accumulated trans-resveratrol, trans-δ-viniferin, and trans-ε-viniferin. Furthermore, V. vinifera cells responded to MeJa elicitation with a significant increase in stilbene production under both light and dark conditions, while V. rupestris cells were responsive to elicitation exclusively under dark conditions [40].
Research has only recently begun to investigate the relationship between light and stilbene biosynthesis in species other than grapevine. A study on A. hypogaea sprouts treated with UV-C and white LED light showed a significant response to white light in stilbene accumulation by the upregulation of genes and enzymes involved in their biosynthetic pathway, although less prominent than that observed in response to UV-C [216].

Temperature
Temperature is an environmental factor of primary importance for plants. It is among the most crucial climatic drivers of biodiversity [274,275] and significantly affects gene expression, protein synthesis, enzymatic activity, and overall primary and secondary metabolism [276][277][278]. Numerous studies have investigated the influence of temperature on the biosynthetic pathway of polyphenols, but only a small fraction of these have focused on stilbenes [204][205][206][207] (Table 1).
A recent study on Gnetum parvifolium revealed that exposure to high temperature (40 • C) enhances resveratrol and piceatannol biosynthesis in leaves of young seedlings, as well as increasing the expression of five STS-like genes in leaves of mature trees, fruit flesh, and seeds [204]. Subsequently, the same authors confirmed the increased expression of STS-like genes under high temperature; however, they did not find a significant increase in the accumulation of total stilbenes in stems and roots of one-year-old plants, suggesting an influence from post-transcriptional regulation on stilbene biosynthesis [205].
The impact of temperature on stilbene biosynthesis was also investigated in grapevines on whole plants, post-harvested fruits, and cultured cells. In 2017, Pastore and coworkers [206] analyzed the entire transcriptome of the berry skin in V. vinifera cv. Sangiovese during ripening under high temperature or low temperature regimes characterized respectively by 26 and 21 • C as average and 42 and 35 • C as maximum daily air temperature. They observed an inhibitory effect of high temperatures on stilbene biosynthesis, in contrast to low temperatures that induced the expression of several members of STS and PAL multigene families, indicating the activation of stilbene biosynthesis. A coordinated expression of STS and PAL has been often observed in grape berries, suggesting that several enzymatic steps in the stilbene biosynthetic pathway are co-regulated [279]. An inhibitory effect of high temperature on the stilbene biosynthetic pathway in grapevine was also reported by Rienth et al. [280] and Wang et al. [207].
Several studies investigated the impact of post-harvest temperature treatment on stilbene biosynthesis. Grapes stored at 0 • C showed a cultivar-dependent modulation in the expression of STS genes, as well as in the accumulation of resveratrol, resveratrol-glucoside, trans-piceatannol, z-miyabenol, and pallidol, especially in the Red Globe cultivar [208,209]. Grape berries cv. Shiraz exposed to high temperature (40 • C) showed an increase in viniferin content, which was accompanied by a decrease in the resveratrol and piceid content [210]. It was also reported that cold storage in combination with UV-C enhanced cisand trans-piceid content in cv. Red Globe berries stored at 4 • C, while no increase of piceids was obtained by UV-C postharvest treatment alone [196].
Different wilting conditions during winemaking were also investigated by Versari and co-workers [185], revealing an increase of resveratrol and STS mRNA under traditional (ambient temperature for 100 days), low temperature (28 • C for 15 days), and mixed temperature (45 • C for 36 h, ambient temperature for 94 d) wilting, especially with the latter method. No increase in resveratrol was reported with high temperature (45 • C for 110 h) wilting. Temperature-dependent resveratrol accumulation was observed by Houillé and colleagues [217], who reported an optimal range for trans-resveratrol biosynthesis at 15-20 • C, a delayed accumulation of this stilbene at 5 • C, and inhibition at -20 • C and under heat shock (65 • C for 2 h immediately after cane harvest, followed by storage at 20 • C).
Taken together, the data available to date show that temperature has a significant impact on the biosynthesis of stilbenes, which is, however, highly variable in relation to the cultivar, the biological system investigated, and the interaction with other environmental factors.

Wounding
It is well known that the biosynthesis of stilbenes in plants can be triggered by physical stimuli such as wounding stress. This abiotic stress has been shown to affect stilbene accumulation in Vitaceae [224], Fabacee [203], and Pinaceae [221].
Freshly pruned canes of V. vinifera cv. Pinot Noir showed a transient expression of PAL and STS genes, followed by a rapid increase in trans-resveratrol and trans-piceatannol, when cut in short segments (from 0.2 to 10 cm), with the highest increase in 0.5 cmlength sections [224]. According to previous studies [217], only trans-resveratrol and transpiceatannol biosynthesis was elicited in pruned grape canes, while no increase in the content of trans-ε-viniferin, ampelopsin A, trans-miyabenol C, cisand trans-vitisin B, hopeaphenol, and isohopeaphenol was recorded. The sequential induction of lipoxygenase (VvLOX, involved in jasmonic acid biosynthesis) and VvSTS genes suggested that the activation of stilbenoid metabolism in response to wounding stress involves the jasmonate signaling pathway [224].
The induction of grapevine stilbene biosynthetic genes after wounding has been reported in several studies. The genome-wide analysis carried out by Vannozzi and colleagues [60] on V. vinifera cv. Pinot Noir showed an increase in the transcription level of several VvSTS gene family members. Induction of VvSTS in canes from grapevine Cabernet Franc has been recorded during the first 4 weeks of storage by Houillé and coworkers [217], indicating that grapevine wood is still transcriptionally active after pruning. In 2017, Yin et al. [220] observed a significant induction by wounding stress of VqSTS36 promoter activity in Vitis quadrangularis L. In 2018, Vannozzi and co-workers [218] reported that in response to wounding the transcript level of VviSTS29, -41 and -48 gradually increases, coupled with the induction of WRKY and R2R3-MYB transcription factors. Four WRKY genes, namely VviWRKY03, VviWRKY24, VviWRKY43, and VviWRKY53, were thus reported for being involved in the regulation of the stilbene biosynthetic pathway.
Several studies on Scots pine have shown remarkable increases in pinosylvin and pinosylvin monomethyl ether accumulation following wounding stress, associated with overexpression of PsSTS, as well as of genes coding for O-methyltransferases involved in stilbene methylation (PsMT1 and PsMT2) [134,221,223].
Increases in trans-resveratrol accumulation coupled to overexpression of stilbene genes in response to wounding stress were also observed in A. hypogaea [202,203,225].

Biotic Stress
Stilbenes are well-known to act as chemical defense compounds against pathogen attack in plants [3]. Certain stilbene-producing plants constitutively biosynthesize high levels of stilbenes, independently from pathogen infection. However, in many species, the expression of stilbene biosynthetic genes and the production of stilbenes increase rapidly and conspicuously in response to pathogenic attack. For example, in the roots of P. cuspidatum, large amounts of resveratrol and stilbene glucosides are constitutively accumulated [281,282]. In Scots pine, high levels of pinosylvin and pinosylvin 3-O-methyl ether constitutively accumulate in the heartwood where they protect the wood against decaying fungi [283], but different stress factors including herbivore and pathogen attack can also elicit the biosynthesis of both stilbenes in sapwood and needles, where they consequently act as phytoalexins [284,285].
To date, the impact of bacterial infection on stilbene biosynthesis in grapevine has been poorly investigated. A bacterial strain belonging to the genus Bacillus has been found to elicit trans-resveratrol biosynthesis in V. vinifera cv. Chardonnay and V. rupestris in vitrogrown plantlets [296]. In 2011, Verhagen and colleagues [297] reported that different bacterial strains originating from the vineyard such as Pantoea agglomerans (Pa-AF2), Bacillus subtilis (Bs-271), Acinetobacter lwoffii (Al-113), and Pseudomonas fluorescens (Pf-CT2) can elicit trans-resveratrol and trans-ε-viniferin biosynthesis in cells and leaves of V. vinifera cv. Chardonnay. In 2015, Gruau and co-workers [298] investigated the ability of grapevine cv. Chardonnay plants to express immune responses at both above-and below-ground after interacting with the beneficial bacterium P. fluorescens (PTA-CT2). Bacterial colonization occurred exclusively in the roots and altered the plant phenotype that exhibited multiple defense responses both locally and systemically. The interaction with bacteria-induced opposite changes in stilbene levels in leaves and roots. Significant increases in the content of trans-resveratrol, trans-piceid, and trans-ε-viniferin were observed in the leaves, while the content of all three stilbenes significantly decreased in the roots. This suggests that this interaction plays a role in the transfer of stilbene phytoalexins to the shoot, contributing to the systemic immune response [298]. Both Verhagen et al. [297] and Gruau et al. [298] showed that defense responses triggered by the interaction with beneficial bacteria can greatly improve grapevine resistance against the fungal pathogen B. cinerea.
Constitutive accumulation of resveratrol has been detected in several tissues of peanut plants, albeit at extremely low concentrations. A dramatic increase in both the quantity and diversity of stilbene phytoalexins can occur in response to fungal infection in peanuts [299]. In 2008, Sobolev [300] observed that the inoculation of peanut kernels with different fungal strains belonging to the genus Aspergillus strongly elicits the biosynthesis of transresveratrol, trans-arachidin-1, trans-arachidin-2, trans-arachidin-3, trans-3-isopentadienyl-4,3 ,5 -trihydroxystilbene, and SB-1. All tested fungal strains of Aspergillus species infecting peanuts activated stilbenoid production in peanut kernel, with interesting variations among different kernel layers. After 24 h of incubation, the tissues closer to the infection site accumulated all the analyzed compounds, while tissues distant from the infected area almost exclusively contained trans-resveratrol. After 48 h of incubation, the six stilbenes were also accumulated in areas far from the infection site, suggesting that trans-resveratrol serves as the building block for other stilbenoids [300].
In peanuts, as well as in grapevine, data regarding the impact of bacterial infection on stilbene biosynthesis are still scarce. To compare the inductive effects of fungi and bacteria on stilbenoid biosynthesis, two phytopathogenic fungal strains (Botryodiplodia theobromae LBBT HC6-1 and B. cinerea FCBC TN1) and two Gram-negative phytopathogenic bacterial strains (Xanthomonas campestris pv. citri XW24 and Pseudochrobactrum asaccharolyticum) were used to treat peanut calluses [241]. The elicitation treatments were performed with either viable or non-viable (autoclaved) microorganisms. The results showed that fungal elicitation is much more effective in inducing biosynthesis of trans-resveratrol and transpiceatannol than bacterial elicitation, regardless of species and viability [241].

Elicitation
Plant cell and organ cultures obtained from stilbene-producing plants represent a reliable model system, both for basic research on plant defense mechanisms and for the biotechnological stilbene production, that can be induced and/or enhanced by a wide range of elicitors. The term elicitor, which was originally referred to as molecules capable of inducing the biosynthesis of phytoalexins in plants, is currently used to designate any physical and chemical factors that can trigger any kind of defense response in plants [301]. Elicitors can be classified, according to their nature, as "abiotic elicitors" or "biotic elicitors" and, according to their origin, as "exogenous elicitors" or "endogenous elicitors" [301,302]. Elicitation has mainly been investigated as a tool to enhance the in vitro production of stilbenes [40, 219,233,303,304]. However, in recent years increasing attention has been paid to elicitation in both pre-and post-harvest in vivo models, as a strategy to induce the natural plant defenses against pathogens and to improve the health properties of plant foods [188,189,239,[305][306][307].
Methyl jasmonate (MeJa) has been shown to be the most effective elicitor for promoting stilbene production in grapevine cell cultures [219,227,233,234,303,308,309]. However, stilbenes are mainly accumulated intracellularly and only a small fraction of them is released into the culture medium by MeJa-treated grapevine cells. Complexation with cyclodextrins (CDs) promotes the release of stilbenes into the medium [304,310]. CDs are cyclic oligosaccharides that have been reported to activate the expression of stilbene biosynthetic genes through the induction of several transcription factors in grapevine [311]. Dimethyl-β-CDs have been shown to act as elicitors capable of stimulating the biosynthesis of stilbenes, as well as promoting their release in the medium and increasing their stability [304,308]. The efficacy of the combined use of MeJa and dimethyl-β-CDs in enhancing the biosynthesis and the extracellular secretion of stilbenes in grapevine cell cultures has been demonstrated by extensive literature [228,304,[311][312][313][314]. It has been recently reported that the co-treatment with MeJa and stevioside (a diterpene glycoside extracted from leaves of Stevia rebaudiana Bertoni), elicits the production and extracellular secretion of resveratrol and viniferins in cell cultures of V. labruscana cv. Campbell Early [304].
CDs have been widely exploited to enhance the production of stilbene compounds in hairy root cultures of A. hypogaea. Co-treatment of peanut hairy roots with CDs and MeJa induced the production and secretion in the culture medium of trans-resveratrol, piceatannol, trans-arachidin-1, and trans-arachidin-3 [242]. In 2019, Somboon and colleagues [315] reported that the elicitation of A. hypogaea hairy roots with the herbicide paraquat (1,1 -dimethyl-4,4 -bipyridinium dichloride), followed by the combination of MeJa and CDs, resulted in increased amounts of stilbene compounds like trans-resveratrol, trans-arachidin-1, and trans-arachidin-3. Physical elicitors have also been widely exploited for the induction of stilbenes production in peanut, as discussed by Hasan et al. [299] and Wongshaya et al. [316].
The elicitation of white mulberry (Morus alba) callus cultures with 2-hydroxypropylβ-CD has been shown to improve resveratrol and oxyresveratrol production [317]. It has also been shown that the treatment of cell suspension cultures of M. alba with MeJa and yeast extract (YE) increases the production of oxyresveratrol and resveratrol [318]. A recent investigation has demonstrated that root cultures of M. alba co-treated with MeJa and YE produce relatively high levels of resveratrol, oxyresveratrol, and mulberroside A [319].
In peanut, different chemical elicitors proved to be effective in inducing the biosynthesis of stilbene phytoalexins. Induction of resveratrol by treating peanut cell suspension cultures with YE was reported by Lanz and co-workers [320]. A single elicitation treatment with sodium acetate of peanut hairy root cultures resulted in a 60-fold induction and secretion of trans-resveratrol into the medium after 24 h [321].
Several studies have been conducted with various elicitors to enhance stilbene production in both pre-and post-harvest grape berries. In 2015, Guerrero and colleagues [322] investigated for the first time the effect of pre-harvest UV elicitation in grapevines. Twentyfour hours after the UV-C pre-harvest treatment, they observed a 22 to 46-folds increase in trans-resveratrol content in table grapes cv. Red Globe. They subsequently reported that pre-harvest UV-C treatment repeated for 3 consecutive days resulted in an 86-fold increase in stilbenoid content (trans-resveratrol, trans-piceatannol, isorhapontigenin, cisand trans-piceid, εand ω-viniferin) in table grapes cv. Crimson seedless [323].
As previously mentioned (Section 3.1), a vast literature describes the impact of UV elicitation on stilbene accumulation in post-harvest grapes [94,324]. In 2019, Segade and co-workers [325] recently investigated the impact of post-harvest ozone elicitation on the biosynthesis of stilbenes in Moscato bianco winegrapes. They found that short-term treatment with high ozone doses (60 µL/L; 48 h exposure) not only prevented the loss of stilbenes during the dehydration process but also induced the accumulation of transresveratrol and trans-piceatannol. Surprisingly, long-term and continuous ozone treatment did not induce trans-resveratrol production, but it did not negatively affect the stilbene content in winegrapes. Consequently, the use of ozone could have important applications in winemaking to produce wines with high added value considering both the possible reduction of SO 2 addition and, under certain conditions, increased contents of stilbenes [325].
Ultrasonication is mechanical stress that can affect stilbene biosynthesis, dramatically increasing resveratrol levels in peanut kernels, as well as in grape skin and leaves. Recent reviews [20,326] thoroughly covered this topic, reporting an increase in trans-resveratrol, trans-piceid in peanuts [201,327], and an increase of resveratrol coupled with the upregulation of the resveratrol synthase (RS) gene in grape skins and leaves [328] treated with ultrasound. It was also reported that short-term exposure to low power ultrasonication is more effective in eliciting resveratrol accumulation. More recently, Yu and co-workers [329] obtained similar results in peanut sprouts, which responded to ultrasound treatment with an increase in trans-resveratrol, accompanied by a slight increase in total sugars and a remarkable decrease in crude fat and peanut allergenic proteins. Despite these promising results, the use of ultrasonication as a physical elicitor to increase the biotechnological stilbene production in vitro has been poorly investigated. The first published study on this topic dates to 2012 when Santamaria and collaborators [219] reported the accumulation of trans-resveratrol, trans-piceid, trans-ε-viniferin, trans-δ-viniferin, and cis-viniferin in grapevine cv. Alphonse Lavallée cell suspensions in response to low-energy ultrasounds.

Other Environmental Factors
Stilbene biosynthesis is affected by a wide array of abiotic environmental factors besides the ones mentioned above, and extensive literature has been produced regarding this subject. Past reviews identified, aside from UV, light, wounding and temperatures, drought, ultrasonication, ozone, salinity, pesticides, and soil nutrient content as important abiotic stress sources capable of modulating stilbene biosynthesis [3,5,6,20,330].
Water stress has been shown to produce conflicting results on stilbene biosynthesis in grapevine, with either an increase [96,331] or decrease [332,333] in stilbene content and different behaviors among resveratrol, piceid, and viniferin content in different cultivars, suggesting a pivotal role for genotype [334]. The diversity in polyphenol composition and its changes in response to drought has been recently investigated on a large panel of grapevine cultivars by Pinasseau et al. [335].
The effects of ozone on stilbene biosynthesis, on the other hand, have been known for several decades. Ozone was observed to increase STS activity, as well as the biosynthesis of pinosylvin and pinosylvin 3-methyl ether in Scots pine seedlings [336]. It was also reported to induce an augmented transcript level of the PMT gene in Scots pine needles [221]. Sitka spruce (Picea sitchensis (Bong.) Carr.) stem bark tissues have also been observed reacting to ozone treatment with an increase in resistance to pathogens, although no alterations in the levels of the stilbene glucosides astringin and isorhapontin were recorded [337].
Vitis spp. were also investigated through cell cultures [338] and post-harvest treatment [183,191], recording general increases in trans-resveratrol, piceatannol, pterostilbene, viniferins, and increases in STS expression in response to ozone (although only on ozone resistant phenotypes). Further investigations of STS expression in transgenic tobacco plants in combination with its promoter (Vst1) and the β-glucuronidase (GUS) reporter gene resulted in 11-fold GUS expression following a single ozone pulse and induction of the Vst1 promoter in several tissues [339]. Recent reviews also covered the elicitation of stilbenes due to ozone [5,20], a topic that was later expanded by Ghimire and colleagues [340] observations published in 2019 regarding the combined effects of elevated ozone, temperature, and nitrogen on stilbene concentrations in Scots pine. They reported that warming does suppress the induction of some stilbene compounds/derivatives in ambient ozone levels.
Another important abiotic stress is salinity. Salt stress represents a devastating constraint to plants' development processes and physiological homeostasis, causing membrane disorganization, cytoplasm alkalinization, ROS induction, transport perturbations, tissue proliferation, and perturbations to photosynthesis, ionic and ionic-related channels [95].
Sodium chloride (NaCl) salt stress is known to strongly delay the rapid induction of STS and resveratrol production in V. rupestris (Scheele) cell cultures, with a late increase in stilbenes. V. riparia (Michx.), instead, does not seem to respond to salt stress [341].
A recent experiment on V. vinifera plantlets subjected to NaCl treatment revealed that the salt-tolerant cv. Razegui did not show important variations in stilbenes biosynthesis, while the salt-sensitive cv. Syrah showed an increase in trans-resveratrol, trans-piceid, and cis-piceid, probably to cope with the higher oxidative disturbance [95]. Resveratrol seems to possess an important role in salt adaptation, as it was found that the application of resveratrol in combination with α-tocopherol to Citrus aurantium L. seedlings reduces NaCl membrane permeability, lipid peroxidation, and pigments degradation, besides reducing H 2 O 2 accumulation in leaves and restoring the reduction of photosynthesis induced by NaCl [342].
Different salts also affect resveratrol production in grapevine. In 2013, Cai and coworkers [343] analyzed the effects of cobalt chloride (CoCL 2 ), silver nitrate (AgNO 3 ), and cadmium chloride (CdCl 2 ) on V. vinifera cv. Gamay Fréaux cell cultures. They reported that cobalt ions at 5.0, 25, and 50 µM concentrations and silver and cadmium ions at 5.0 µM concentration stimulated a 1.6-fold increase in 3-O-glucosyl-resveratrol without suppressing cell growth or compromising cell viability. In contrast, higher concentrations of silver and cadmium remarkably reduced cell viability.
To further expand this topic, we remand to a recent excellent review from Hasan and Bae [20].

Conclusions and Future Prospects
The study of stilbenes is of crucial importance for basic research, to understand the biological role of these metabolites in plant defense from biotic and abiotic stress. The interest towards stilbenes also comes from applied research, mainly due to the numerous bioactivities of these compounds and the consequent potentials in the nutraceutical, cosmeceutical, and pharmaceutical fields. The present review arises in this context, dealing with the most recent knowledge of the stilbene biosynthetic pathway and the environmental factors that affect their biosynthesis and accumulation in different plant species, both in planta and in in vitro systems. Given the vastness of the topic and the abundance of scientific literature available to date, this review is not intended to be comprehensive. Some issues concerning the study of stilbenes, albeit of great interest, such as their biological function, pharmacological activity, and distribution in nature and in plant foods have not been explored, because they have been the subject of recent excellent reviews [2][3][4]22,170]. Another topic of great interest, extensively covered by a recent review by Dubrovina and Kiselev [5], consists in the study of the molecular mechanisms by which environmental stimuli regulate the stilbene biosynthetic genes. In recent years, increasing efforts have been devoted by different research groups to expand knowledge on this topic, which is leading to the emergence of interesting new knowledge. Therefore, we expect to have in the near future a more detailed model of the molecular mechanisms that link the perception of environmental stimuli to the expression of genes responsible for the biosynthesis of stilbenes. Furthermore, we expect research efforts to shed light on the link between the biosynthesis of stilbenes and environmental factors that have so far been poorly studied, such as viruses, bacteria, herbivores, UV-A, and ionizing radiation. Acknowledgments: Special thanks go to Alessia Massimi, who designed the graphical abstract, and Paul B. Edwards, who kindly proofread the manuscript for grammar, spelling, and punctuation.

Conflicts of Interest:
The authors declare no conflict of interest.