## 1 The Concept of Phytoalexins
We will describe in this review, efforts that have been done for engineering phytoalexin production in plants as well as provide information on how phytoalexin accumulation can be modulated through engineering of plant hormones, defense-related markers or elicitors. Genetic Manipulation of Phytoalexin Production and Disease Resistance Addressing Gain- or Loss-of-Function Genetic Approaches  
As surprising as it may seem, there have been not so much reports of attempts to manipulate phytoalexins by genetically modifying expression of biosynthetic pathway genes for disease resistance in plants. Works in this area address both gain- or loss-of-functions genetic approaches, with the most abundant literature concerning resveratrol, the major phytoalexin from Vitaceae (for reviews see [59, 60, 61, 62, 63]). Resveratrol is obtained through the universal phenylpropanoic acid pathway beginning with phenylalanine (via the phenylalanine ammonia lyase, PAL) or with tyrosine (via the tyrosine ammonia lyase, TAL) (Figure 3). The obtained cinnamic acid is activated by ligation to coenzyme A by 4-coumaroyl:CoA ligase (C4L). Resveratrol is then synthesized by the stilbene synthase (STS) which acts to condense three successive units of malonyl-CoA with _p_-coumaroyl-CoA. STS, the key enzyme of resveratrol synthesis, thus uses as substrates precursor molecules that are present throughout the plant kingdom. Therefore, the introduction of a single gene is sufficient to synthesize resveratrol in heterologous plant species. Transformations have been then operated to investigate the potential of stilbene biosynthetic genes to confer resistance to pathogens. Interest in resveratrol engineering has increased due to its antimicrobial properties acting as an allelochemical or a phytoalexin, but also due to its antioxidant and antitumor capabilities (for reviews see [59, 60, 61, 62, 63, 64]). Transfer experiments began with two grapevine STS genes (_Vst1_ and _Vst2_) introduced into tobacco under the control of the stress responsive promoter pVst1, conferring that plant a higher resistance to _B. cinerea_ infection [65]. This study constituted the first report of a disease resistance resulting from foreign phytoalexin expression in a novel plant. In the same way, [PERSON] and [PERSON] [66] have transformed alfalfa (_Medicago sativa_) with the _Arachis hypogea_ STS _AhRS_ gene transcriptionally regulated by an enhanced cauliflower mosaic virus (CaMV) 35S promoter, conferring increased resistance to _Phoma medicaginis_. The grapevine rootstock 41-B overexpressing the grapevine _Vst1_ stilbene synthase gene under the control of the fungus inducible promoter _ms_ PR 10.1 produced high stilbene levels and exhibited _in vitro_ resistance to _B. cinerea_[67]. Similar transformations have improved the resistance of rice to _Piricularia orizae_ (grapevine STS gene _Vst1_ with the stress responsive promoter pVst1) [68], of barley and wheat to _B. cinerea_ and to powdery mildew (grapevine STS gene _Vst1_ with the stress responsive promoter pVst1 + CaMV 35S) [69, 70], of tomato to _P. infestans_ (two grapevine STS genes _Vst1_ and _Vst2_ under the control of the stress responsive promoter pVst1) [71], of wheat to _Puccinia recordita_ and _Septoria nodorum_ (two grapevine STS genes _Vst1_ and _Vst2_ with the stress responsive promoter pVst1 + CaMV 35S) [72], of papaya to _Phyththora palmivora_ (grapevine STS gene _Vst1_ with the stress responsive promoter pVst1) [73] and of _Rehmannia glutinosa_ to _Fusarium oxysporum_ with the _Arachis hypogea_ STS _AhRS3_ gene transcriptionally regulated by the constitutive CaMV 35S promoter [74]. More recently, overexpression of a resveratrol gene _PcRS_ from _Polygonum cuspidatum_ under the control of the constitutive CaMV 35S promoter in transgenic _Arabidopsis_ was shown to result in the accumulation of _trans_-piceid (the 3-\(O\)-\(\beta\)-d-resveratrol glucoside) and an increased resistance to the fungal pathogen, _Colletotrichh higginsianum_[75].