Higher plants secondary metabolites

Polyphenols (syn vegetable tannins - proanthocya-nidins and esters of gallic acid, vide supra) constitute one of the most distinctive groups of higher plant secondary metabolites. Their uniqueness lies not only in their molecular size and polyphenolic character but also in their ability to complex strongly with proteins, carbohydrates, nucleic acids and alkaloids. Studies of these properties are not only of intrinsic scientific interest. a scientist who confesses to an interest in polyphenols is invariably asked one question - why do plants form them ... 

Plant secondary metabolites which mimic JH activity appear to be active on a narrow range of host species. What account(s) for this effect The majority of bioassays used last larval instars of P. apterus, O. fasciatus and pupae of T. molitor to test for activity of the juvenoids. Are these the most sensitive insects Six JHs have been identified to date different homologs have been isolated from specific insect orders. Juvenile hormone III appears to be ubiquitous [12, 13] and, in most species, is the only JH present. Juvenile hormone I and II are important in the regulation of metamorphosis and ovarian maturation in Lepidoptera [5] and the bis-epoxide appears to be the principle JH in higher Diptera [20]. Therefore, the nature of the JH in the test insect and the role that it plays in development must be considered in the selection of the bioassay a compound which mimics the action of JH in P. apterus (Hemiptera) is unlikely to be active in a Lepidopteran insect. 

Although the study of plant secondary metabolites has been of value for understanding the phylogeny of higher plants, the reverse is true also. Knowledge of plant systemat-ics and phylogeny has considerable predictive value for the study of secondary metabolites. 

In higher plants aromatic amino acids are required not only for protein synthesis, but as precursors for hormones, and a vast diversity of phenylpropanoid or other secondary metabolites. Thus, the availability of aromatic amino acids in a number of the spatially separate compartments of the plant-cell microenvironment is essential. 

As Figure 1 depicts, phenylalanine ammonia-lyase (PAL), which occurs ubiquitously in higher plants and the wood-rotting Basidiomycetes (1-3), seems to play a common central role in the conversion of phenylalanine (by deamination) to a wide variety of secondary metabolites. These include lignins in higher plants (4), veratryl alcohol in the white-rot fungus Phanerochaete chrysosporium (4a), and methyl p-anisate in the brown-rot fungus... 

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