ABSTRACT
Sweet potato (Ipomoea batatas Lam.) root tissues accumulate bitter and oily substances in the mold-damaged regions when they are infected with pathogenic fungus such as Ceratocystis fimbriata Ell. and Halst. They are antifungal and Ehrlich’s reagent-positive furanosesquiterpenes, which consist of several major components such as ipomeamarone [3 + 4’ + 2”] and ipomeamaronol [3 + 4’ + 3”] and many kinds of minor family compounds derived from them. Ipomeamarone was first isolated and determined in the structure among phytoalexins in the plant kingdom (Hiura, 1943; Kubota and Matsura, 1953). Its chemical structure is shown in Fig. 1. These furanosesquiterpenes are produced not only by infection of pathogenic fungi but also by treatment with poisonous heavy metal ions (Uritani et al., 1960) and mycotoxins (Fujita and Yoshizawa, 1987) in sweet potato root tissues. Therefore, various kinds of poisons, including mercuric chloride, have also been used as artificial phytoalexin inducers (elicitors) for the biochemical investigation on synthesis of furanosesquiterpenes in the root tissues (Oba and Uritani, 1981; Fujita and Yoshizawa, 1989; Fujita, 1985). Using various 14C-labeled intermediates, it was elucidated that the furanosesquiterpenes 468were biosynthesized via acetyl CoA (Oba et al., 1970), 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), mevalonate (MVA) (Akazawa et al., 1962; Akazawa, 1964; Oshima and Uritani, 1968), MVA phosphate, MVA pyrophosphate, isopentenylpyrophosphate (Oshima and Uritani, 1969), geranyl pyrophosphate (Oguni and Uritani, 1969), farnesyl pyrophosphate and farnesol [1 + 1’ + 1”](Oguni and Uritani, 1970, 1971b), initiated from pyruvate, which was supplied by glycolysis. The metabolic pathway between acetyl-CoA and farnesylpyrophosphate is designated the first half-step and the pathway from farnesol to the phytoalexins the second half-step. They functionally bear their shares of responsibilities in the biosynthesis of the furanosesquiterpenes. The former plays a role in supplying farnesol as a material for carbon skeletons of ipomeamarone and ipomeamaronol. It has been clear that the amount of the supply is 469enzymatically controlled by HMG-CoA reductase as the rate-limiting enzyme in the pathway (Ito et al., 1979). Therefore, the amount of accumulation of furanosesquiterpenes is primarily controlled by that as quantitative pathway. In contrast to that, the latter contributes to the formation of the characteristic carbon skeleton of ipomeamarone and also confers the diversity of the minor family compounds which are derivatively produced by the bypath of synthesis of ipomeamarone (qualitative pathway). Chemical structures of farnesol and ipomeamarone. (I), Furan ring moiety; (II), tetrahydrofuran ring and the next ethylene moieties; (III), the rest (the isobutyl end of side chain) of the moiety. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203752647/a1a27a5a-cb92-45e1-8d4d-c3cf86d40c11/content/fig20_1_OB.tif"/>
