## 1 The Concept of Phytoalexins
Figure 3: _In vivo_ biosynthesis of resveratrol from phenylalanine or tyrosine via the phenylpropanol/polymalonate pathway. PAL/TAL: phenylalanine/tyrosine ammonia lyase; C4H: cinnmate-4-hydroxylase; C4L: coumarate:coenzyme A ligase; STS: stilbene (resveratrol) synthase. **Figure 4.** Scheme of biosynthetic pathways to the main phytoalexins from Leguminosae. IFS: 2-hydroxy isoflavanone synthase; DMI: 7,2'-dihydroxy-4'-methoxy-isoflavanol; DMDI: 7,2'-dihydroxy-4',5'-methylenedioxy-isoflavanol; HMM: 6\(\alpha\)-hydroxymaackiain 3-\(O\)-methyltransferase; HI4'OMT: SAM: 2,7,4'-trihydroxy-isoflavanone 4'-\(O\)-methyltransferase (adapted from [76,77]). The dashed arrows represent hypothetic steps and the solid arrows denote reactions for which the catalyzing enzymes have been cloned. **Figure 4.** Scheme of biosynthetic pathways to the main phytoalexins from Leguminosae. IFS: 2-hydroxy isoflavanone synthase; DMI: 7,2'-dihydroxy-4'-methoxy-isoflavanol; DMDI: 7,2'-dihydroxy-4',5'-methylenedioxy-isoflavanol; HMM: 6\(\alpha\)-hydroxymaackiain 3-\(O\)-methyltransferase; HI4'OMT: SAM: 2,7,4'-trihydroxy-isoflavanone 4'-\(O\)-methyltransferase (adapted from [76,77]). The dashed arrows represent hypothetic steps and the solid arrows denote reactions for which the catalyzing enzymes have been cloned. All these results show that resveratrol is a determinant factor in the expression of resistance of plants to phytopathogens and underline the above-mentioned interest for engineering this phytoalexin in plants. However, there were some cases for which no resistance was observed after transforming the plants with _STS_ genes. For example, transformation of white poplar (_Populus alba_) with the grapevine _StSy_ or with a pinosylvin-synthase-encoding gene from _Pinus sylvestris_ did not confer any increased resistance to _Melaspora pulcherrina_ (rust disease) [78, 79]. Similarly, no increased resistance against _B. cinerea_ has been observed in STS transgenic kiwi plants expressing the grapevine STS _pSV25_ under the control of pCAMV 35S [80]. These contradictory results suggest that pathogen control by transgenic STS plants can be significant but may be considered as empiric and not predictable. Generally, STS expression does not trigger any detrimental effect on plant development, growth, morphology or fertility. However, in few cases, STS overexpression can lead to morphological alterations in transgenic plants such as color modification and male sterility as observed in tobacco and petunia [81] or in conifers [82]. Such events may be explained by the possible transgene insertion site, such as, for instance, insertions within a gene involved in male fertility for sterile lines. Isoflavonoids constitute the second major phytoalexin family that has attracted considerable attention for plant engineering though this gain of interest was due mainly to the antioxidant and anticancer activities of these compounds. 4'-\(O\)-methylation of isoflavenoid-type compounds is a prerequisite for further substitutions of the isoflavenoid core, leading to the pterocarpan phytoalexin medicarpin. Genetic manipulation of the isoflavone 7-\(O\)-methyltransferase has been reported in alfalfa (_Medicago sativa_) in relation to increased resistance of that plant to _Phoma medicaginis_, the causal agent for spring black stem and leaf spot disease [83]. Gene constructs harboring the full-length isoflavone-\(O\)-methyltransferase from alfalfa (IOMT8) cDNA in the sense and antisense orientations under control of the constitutive promoter 35S were obtained. Unexpectedly, overexpression of IOMT led to 7-\(O\)-methylisoflavonoid derivatives _in vitro_ as well as _in vivo_ unchallenged leaves of transgenic alfalfa when ectopically expressed but mainly produced 4'-\(O\)-methylderivatives, such as formononentin (4'-\(O\)-methyldaidzein) and medicarpin in plants elicited with metallic cations or infected with _P. medicaginis_.