3. Dixon

NATURE | VOL 411 | 14 JUNE 2001 | www.nature.com 843 Collectively, plants produce a remarkably diverse array of over 100,000 low-molecular-mass natural products, also known as secondary metabolites. Secondary metabolites are distinct from the components of intermediary (primary) metabolism in that they are generally nonessential for the basic metabolic processes of the plant. Most are derived from the isoprenoid, phenylpropanoid, alkaloid or fatty acid/polyketide pathways (Fig. 1). This rich diversity results in part from an evolutionary process driven by selection for acquisition of improved defence against microbial attack or insect/animal predation. However, such diversity has made it difficult to apply conventional molecular and genetic techniques to address the functions of natural products in plant defence, or to improve plant disease resistance using metabolic pathway engineering. Related plant families generally make use of related chemical structures for defence (for example, isoflavonoids in the Leguminosae, sesquiterpenes in the Solanaceae), although some chemical classes are used for defensive functions across taxa (for example, phenylpropanoid derivatives). Some species produce a broad range of antimicrobial compounds. For example, cocoa, when infected by the vascular wilt fungus Verticillium dahliae, accumulates the pentacyclic triterpene arjunolic acid, two hydroxylated acetophenones and, most unusually, elemental sulphur (Fig. 1, 25), the only known inorganic antimicrobial agent produced by plants. Most antimicrobial plant natural products have relatively broadspectrum activity, and specificity is often determined by Natural products and plant disease resistance