## 7 Conclusions  
Starting in the 1950's, research on phytoalgebras has begun with biochemistry and bio-organic chemistry, resulting in the determination of their structure, their biological activity as well as mechanisms of their synthesis and their catabolism by microorganisms. Elucidation of the biosynthesis of numerous phytoalexins has permitted the use of molecular biology tools for the exploration of the genes encoding enzymes of their synthesis pathways and their regulators. Genetic manipulation of photolexins requires _a priori_ a sound knowledge of the genes involved in their biosynthesis and how accumulation of a given phytoalexin can be modulated. Success of the transformation will also depend on the inability of the pathogen to counteract the phytoalexin action. It is well known that phytopathogenic fungi, particularly, are able to metabolize the phytoalexins to which they are exposed. Engineering of fungal genes responsible for detoxification of phytoalexins in plants has pointed out their role in the interactions between plants and pathogens. For example, overexpression in hairy roots of pea (_Pisum sativum_ L.) of a pisatin demethylating activity (PDA) from the pea-pathogen fungus _Nectria haematococcca_ reduced the amounts of pisatin. As a result, transgenic plant tissues with reduced capability to accumulate pisatin were found to be less resistant to fungal infection [85]. Also, it appears clearly evident that, in phytopathogenice fungi, ATP-binding cassette (ABC) transporters, which may extrude plant defense products as well as fungicides, act as virulence factors, providing protection against defense compounds produced by the host. Many factors thus interplay which could affect the outcome of the interaction between plants and pathogens. Unexpectedly, engineering phytoalexins for disease resistance in plants seems to have been limited to exploiting only a few phytoalexin biosynthetic genes, especially those encoding stilbenes and some isoflavoids. The first example of a disease resistance resulting from foreign phytoalexin expression in a novel plant was published only in 1993 [65]. One can imagine that such a success would have opened the way for a ferment of activity in this area, but, if transformations were then operated to investigate the potential of stilbene biosynthetic genes to confer resistance to pathogens [59, 60, 62], strategy in engineering phytoalexins from other plant species did not receive as much applications as might be expected. As previously stated in this review, interest in secondary metabolite engineering deals with their implications in human health and disease. The extraordinary success obtained with resveratrol, the phytoalexin of Vitaceae, is linked to the fact that in this case, the engineered system requires a relatively simple genetic construct. Resveratrol is indeed obtained in one single step from _p_-coumaroyl-CoA and three malonyl-CoA units catalyzed by stilbene synthase. As these substrate precursor molecules are present throughout the plant kingdom, the introduction of a single gene is therefore sufficient to synthesize resveratrol in heterologous plant species. Except the case of resveratrol biosynthesis which appears to be very simple, other phytoalexins (isoflavonoids, terpenoids) are formed through very complex biosynthetic pathways. Engineering the entire pathway is not feasible and the problem is to choose the right enzyme catalyzing the limitant reaction of the given pathway. Moreover, modalities of the expression of genes encoding a given enzyme can be unpredictable. In fact, overexpression of 7-_O_-methyltransferase transferase in alfalfa led to different regiospecificities, producing mainly 4'-_O_-methyl derivatives instead of the expected 7-_O_-methyl derivatives [83]. New techniques for metabolic engineering have thus to be exploited in the next years to come.