## 3 Genetic Manipulation of Phytoalexin Glucosylation and Disease Resistance  
Glucosylation of scopoletin, a coumarin phytoalexin from tobacco (_Nicotiana tabacum_ L.), is a particular example illustrating the crucial role of plant UDP-Glc:phenylpropanol glucosyltransferaseases (UGTs) in plant defense responses and their relevance to the more general context of plant/pathogen interactions [95,96]. Glucosylation indeed intervenes in the accumulation, storage, transport and water solubility of a wide range of compounds [97].  
Based on two different and consecutive studies including an antisense strategy, on one hand [98], and a gain-of-function approach, on the other hand [99], [PERSON] and colleagues have underlined the role of glucosyltransferaseases in scopoletin accumulation in tobacco plants following inoculation with Tobacco Mosaic Virus (TMV). In the first approach, downregulation of the expression of two tobacco salicylic acid- and pathogen-inducible UDP-glucosyltransferaseense genes _Togt 1_ and _Togt 2_ (acting on scopoletin glucosylation) was obtained by introducing the complete coding sequence of Togt 1 in the antisense orientation downstream from the constitutive CaMV 35S promoter in tobacco [98]. Expectedly, TOGT downregulation resulted in a decrease in scopoletin UGT activity together with a decline in the contents of both the aglycone and the glucoside of scopoletin named scopolin. Scopolin accumulation was reduced by 70% to 75% in tobacco reacting hypersensitivity to TMV.  
The TOGT inhibition-mediated decrease in both the scopolin and scopoletin contents could have been the result of a feedback inhibition of upstream enzymes of the scopoletin biosynthesis. Antisensing _Togt 1_ was found to have no effects on other enzymes involved in plant defense mechanisms such as _O_-methyltransferaseases of class II (COMT II) or acidic \(\beta\)-1,3-glucanases [98]. Interestingly, transgenic lines downregulated for TOGT showed a 63% increase in TMV lesion surfaces _vs._ control plants relative to changes in scopolin and the TMV-replicate inhibiting-phytoalexin, scopoletin. As the antisense strategy used previously left open the possibility of a non-specific downregulation of a similar glucosyltransferase or of enzymes involved in the biosynthetic route to scopoletin, a gain-of-function approach was needed to further confirm the role of this pathogen-inducible glucosyltransferase during the hypersensitive response of _N. tabacum_ to TMV.  
Clonage of the _togt 1_ full-length cDNA was carried out in the sense orientation downstream the 35S CaMV promoter into the pFB8 vector [99]. Basal scopoletin UGT activity in lines harboring the sense orientation construct was about 3-fold higher than in vector control plants and the resulting accumulation of scopoletin and scopolin was found to be 2-fold higher. Acceleration of the development of TMV lesions in transgenic TOGT overexpressing lines, despite increases in the phytoalexin content, was thought to result in a better restriction of TMV, thus preventing virus spread. These results thus clearly demonstrated the involvement of TOGT in scopoletin glycosylation _in planta_, controlling the accumulation of this antiviral compound during the hypersensitive reaction of tobacco to TMV; they also suggested the overall relevance of metabolic engineering of glucosyltransferaseases in the outcome of plant/pathogen interactions. Conversely, TOGT activity for scopoletin glycosylation was reported to be moderately expressed upon cytokineine-mediated resistance of tobacco to _Pseudomonas Syringae_ pv _tabaci_[100].