Natural products as leads for new herbicides 1441 Introduction

In the plant kingdom there is an incredible diversity of what are termed secondary metabolites, that is, those molecules which are not absolutely required for normal cell function. At least 10- active plant metabolites are now known and the reader is directed to the NAPRALERT website (www.napralert.org) for a comprehensive database, covering the literature for natural products.

Why do so many natural molecules exist and what is their physiological function in plants? They were first thought to exist as metabolic waste products but, since the metabolic cost to the plant is high, there must be clear benefits to the plant for producing them. It is now thought that secondary metabolism has evolved with the co-evolution of micro-bial and insect parasites, and animal herbivores, largely as defence and protection mechanisms. Since specialist organisms have evolved that have succeeded in overcoming plant poisons and chemical defences, new metabolic pathways and chemical modifications have evolved and continue to evolve accordingly. The earliest defence molecules were the resins, lignins, condensed tannins and flavonoids. The evolution of the angiosperms and herbaceous plants was marked by the proliferation of further secondary metabolites as products of the acetate-mevalonate pathway.

During steady-state photosynthesis, at least 20% of the carbon fixed in green plants is directed, via triose phosphates, towards synthesis of an impressive array of biologically important end-products. These include lignins, alkaloids, tannins, isoprenoids and a wide range of phenolic compounds, including the 15-carbon flavonoids. Indeed, it is estimated that about 2% of all carbon fixed, equivalent to 109 tonnes each year, is converted to flavonoids alone. The precise end products formed depend on the metabolic needs of the plant at any given time, which are highly dependent on the prevailing environmental conditions and growth stage. Furthermore, plants under stress and those with plentiful supplies of nitrogen, are preferentially attacked by insects, and the plant responds by increasing the biosynthesis of defence molecules. During pathogen attack, the concentrations of flavonoids and related compounds greatly increase at the site of infection, to concentrations that are toxic to pathogens in in vitro assays. Wounding and feeding by herbivores can induce the biosynthesis of toxic coumarins and tannins, and phenolic acids as precursors to lignins and suberins in wound healing. The accumulation of flavonols, especially kaempferol at wound sites, may also prevent microbial infection. Recent research indicates that stresses such as infection and exposure to ultraviolet light, that are perceived in one part of the plant, can be communicated to the rest of the plant and elicit systemic effects. This communication is thought to be mediated by salicylic acid, which shares part of its biosynthetic pathway with the flavonoids.

Photosynthetic carbon reduction

Hydrolysable tannins

Polysaccharides <-Hexose and triose phosphates

Shikimic acid

Lignins

Isopentenyl pyrophosphate

Acety

Aromatic amino acids Alkaloids

Fatty acids fats waxes

Polyketides Flavonoids polyphenols flavones quinones flavonols

Isoprenoids terpenes steroids phytol carotenoids

Fatty acids fats waxes

Polyketides Flavonoids polyphenols flavones quinones flavonols anthocyanidins condensed tannins

Figure 14.2 A simplified scheme showing the derivation of the main groups of plant secondary metabolites (modified from Duke et al., 2000).

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