natural-products-in-plant-pest-management

Natural Products from Plants 493.3 Secondary Metabolites: Characteristics, Functions andPossible Applications in AgricultureExcept for primary metabolites such as carbohydrates, proteins, lipids andnucleic acids, plants also contain a large variety of secondary metabolites.Secondary metabolites are classified by different authors in different waysbut for the purpose of this précis, a simple classification into three groups issupplied: (i) isoprenoid; (ii) aromatic; and (iii) alkaloid components (Stumpfand Conn, 1981; Dey and Harborne, 1989; Salisbury and Ross, 1992).Isoprenoid componentsThis group of compounds is relatively diverse and three terms have beenallocated to it, namely isoprenoids, terpenoids and terpenes. The term‘ isoprenoid’ is relatively descriptive in the sense that the common factor thatrelates these compounds is the 5C units, called isoprene units (Fig. 3.1).However, the term ‘terpene’ is probably used more often.Under this group, known hormones such as gibberillic acid (GA), abscisicacid (ABA) and brassinosteroids as well as other components such as sterols,carotenoids, rubber and the phytol ‘tail’ of chlorophyll are classified. Allof these consist of repeating isoprene units. Isoprene units join in differentways to form components with chain, ring or combined chain–ring structures(Fig. 3.2).Terpenes are found abundantly in nature and are, as far as chemicalstructure is concerned, a diverse group of secondary metabolites. On thebasis of the number of isoprene units, terpenes are divided into seven classes.Hemiterpenes consist of one isoprene unit (C 5); they are not found in thisform (freely) in nature, but can be found in the form of alcohols or acids(examples shown in Fig. 3.3).Fig. 3.1. An isoprene unit.Repeating isoprene-units (chain) Ring Ring and chain combinedFig. 3.2. Different ways in which isoprene units can combine.