## 1 The Concept of Phytoalexins
Are phytoolexins active? Relatively ancient studies reporting on the direct comparison of the protectant activity of isoflavoid phytoolexins and the fungicides benomyl and mancozeb, have shown that phytoolexins are considerably less toxic than chemical fungicides [16]. Although differences in activity do occur between and within phytoolexin families, effective doses of phytoolexins generally fall within order of magnitude \(10^{-5}\) to \(10^{-4}\) M [17, 18, 19]. Beside their fungistatic action, phytoolexins can exert some fungitoxic activities as their action results in cytological abnormalities in fungal cells. Resveratrol and pterostilbene, for instance, two phytoolexins from grapevine, induce the formation of curved germ tubes, cessation of growth of some germ tubes with protoplasmic retraction in the dead hyphal tip cell, cytoplasmic granulation of the cellular content, disorganization of mitochondria and disruption of the plasma membrane in _B. cinerea_ conidia [17, 18, 19]. Phytoolexins may also act as uncoupling agents of electron transport and photophosphorylation, leading to a rapid and complete cessation of respiration in _B. cinerea_ conidia [17] and are able to induce fungal apoptotic-like programmed cell death as a result of the action of camalexin on _B. cinerea_[20]. The question as to whether phytoolexins are produced by all plants is remaining as there are few reports of thorough examinations concluding that these compounds are not synthesized by a particular plant species [3]. Even today too few families have been studied to permit the assertion that phytoolexin synthesis is a universal defense mechanism of plants [2]. Knowledge on phytoolexins from 1960 to 1980 was based essentially on extensive works in two plant families, Leguminosae or Fabaceae [21] and Solanaceae [22], and investigations of one or of a few species in each of a number of other plant families (Vitaceae, Poaceae, Malvaccae, Euphoriaceae, Moraceae, Orchidaceae, Ginkgoaceae...) [4, 23]. (Figures 1 and 2). The 3-deoxyanthocyanidins including luteolinidin (Figure 1) and apigeninidin, form an unusual group of flavonoid phytoalexins produced by members of the Poaceae, for instance, as a response of sorghum mesocotyls to infection by pathogenic and nonpathogenic fungi [51, 52, 53]. Gossypol and its congeners, that is, naphthaleneD compounds, constitute the main phytoalexins from cotton plants (_Gossypium_ spp.) of the Malvaccae family [54]. We will end this list, which is far from being exhaustive, with camalexin, the major phytoplankton from the Brassicaceae (also known as Cruciferae) [55] and resveratrol, the major phytoplankton from the Vitaceae [56] (Figures 1 and 2). How can our knowledge of phytoalgebras be used to develop new approaches to plant disease control? Phytoalexins have long been recognized to as being important in the defense mechanisms of plants against phytopathogenic microorganisms. Their potential biological properties have stimulated a ferment of activity concerning the biosynthesis and the metabolism of these compounds in the plant. Figure 2: Chemical structures of some phytoalexins cited in this work: \(\alpha\)-solanine (steroid glycoalkaloids); rishitin, lubimin, capsidiol, gossypol, kauralexin B2; zealexin B1 and momilactone B (norsesquiterpenoids/sesquiterpenoids); and phytocassanes. Research has gone forward because these compounds are thought to help agriculturally and economically important crop plants withstand colonization by pathogens. In many instances, a close correlation has been found between phytoalexin production and resistance to diseases. Use of modern molecular biology tools for elucidating the control mechanisms of phytoalexin synthesis and for engineering disease resistant plants is based on the expression of stress- or disease-related genes [57]. Genetic engineering of phytoalexins for disease resistance requires manipulation of a single or a few genes directly involved in their biosynthetic pathways or involved in their signaling/regulatory pathways [58].