# Impact of Environmental Factors on Stilbene Biosynthesis
## 2 Biosynthesis of Stilbenes and Stilbenoids
### 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-\(\beta\)-glucose (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 oxyreservatrol) and rhapontin (a monoglucoside of rhapontin, also known as rhapontin) [87] (Figure 5). Significant amounts of _cis-_ and _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].  
Figure 5: Common examples of stilbene glucosylation.  
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 [PERSON] and [PERSON] [99]. The mesocarp-specific expression of VLRSgt reflected the increased accumulation of resveratrolol 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 \(\beta\)-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 _trans_-resveratrol concentration during winemaking. One of these consists in the selection of yeast strains with high \(\beta\)-glucosidase activity, capable of efficiently converting _trans_-piceid into the free aglycone during the alcoholic fermentation [106; 107]. Bacterial or fungal \(\beta\)-glucosidase 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].